a) Types of circuits and their purpose
The main electrical connection diagram of a power plant (substation) is a set of main electrical equipment (generators, transformers, lines), busbars, switching and other primary equipment with all connections made between them in kind.
The choice of the main circuit is decisive in the design of the electrical part of a power plant (substation), since it determines the full composition of elements and connections between them. The selected main circuit is the initial one for drawing up schematic diagrams of electrical connections, auxiliary circuits, secondary connection diagrams, wiring diagrams, etc.
In the drawing, the main diagrams are shown in a single-line design with all elements of the installation disconnected. In some cases, it is allowed to depict individual elements of the circuit in a working position.
All elements of the diagram and the connections between them are depicted in accordance with the standards of the unified system for design documentation (ESKD).
In operating conditions, along with the basic, basic diagram, simplified operating diagrams are used, in which only the main equipment is indicated. The duty personnel of each shift fills out the operating diagram and makes the necessary changes to it in terms of the position of switches and disconnectors that occur during duty.
When designing an electrical installation, before the development of the main circuit, a block diagram of the output of electricity (power) is drawn up, which shows the main functional parts of the electrical installation (switchgear, transformers, generators) and the connections between them. Structural diagrams serve for the further development of more detailed and complete circuit diagrams, as well as for general acquaintance with the operation of an electrical installation.
b) Basic requirements for the main wiring diagrams
When choosing wiring diagrams, the following factors should be taken into account:
the importance and role of a power plant or substation for the power system. Power plants operating in parallel in the power system differ significantly in their purpose. Some of them, basic ones, carry the main load, others, peak ones, work incomplete 24 hours during maximum loads, while others carry the electrical load determined by their heat consumers (CHP). The different purpose of power plants determines the advisability of using different wiring diagrams even in the case when the number of connections is the same.
Substations can be designed to power individual consumers or a large area, to connect parts of the power system or different power systems. The role of substations determines its layout;
position of the power plant or substation in the power system, diagrams and voltages of adjacent networks. High voltage buses of power plants and substations can be the nodal points of the power system, bringing together several power plants for parallel operation. In this case, power flows through the buses from one part of the power system to another - power transit. When choosing the schemes of such electrical installations, first of all, the need to maintain power transit is taken into account.
Substations can be dead-end, walk-through, tap-off; the schemes of such substations will be different even with the same number of transformers of the same power.
6-10 kV switchgear diagrams depend on consumer power supply schemes: power supply via single or parallel lines, availability of reserve inputs for consumers, etc .;
All consumers from the point of view of the reliability of power supply are divided into three categories.
Category I electrical receivers are electrical receivers, the power supply interruption of which may entail danger to human life, significant damage to the national economy, damage to expensive basic equipment, massive product defects, disruption of a complex technological process, disruption of the functioning of especially important elements of the communal services.
A special group of electrical receivers is distinguished from the composition of category I electrical receivers, whose uninterrupted operation is necessary for a trouble-free shutdown of production in order to prevent threats to human life, explosions, fires and damage to expensive equipment.
For the power supply of a special group of category I electrical consumers, additional power is provided from a third independent power source. Independent power supplies can be local power plants, power grid power plants, special uninterruptible power supplies, storage batteries, etc.
Category II electrical receivers are electrical receivers, the power supply interruption of which leads to a massive undersupply of products, massive downtime of workers, mechanisms and industrial transport, disruption of the normal activities of a significant number of urban and rural residents. It is recommended to provide these electrical receivers with power from two independent sources, mutually reserving each other; breaks for the time required for turning on the backup power by the actions of the personnel on duty or the mobile operational team are permissible.
It is allowed to supply power to category II electrical consumers through one overhead line, if it is possible to carry out emergency repairs of this line in a time not exceeding 1 day. Power supply is allowed through one cable line, consisting of at least two cables connected to one common device. In the presence of a centralized reserve of transformers and the possibility of replacing the damaged transformer in a time period not exceeding 1 day, power supply from one transformer is allowed.
Category III electrical receivers - all other electrical receivers that do not fit the definitions of categories I and II.
For these electrical receivers, the power supply can be performed from one power source, provided that the power supply interruptions required for repair and replacement of a damaged element of the power supply system do not exceed 1 day.
Expansion prospects and intermediate stages of development of the power plant, substation and the adjacent section of the network. The layout and layout of the switchgear should be selected taking into account the possible increase in the number of connections during the development of the power system. Since the construction of large power plants is carried out in stages, when choosing an electrical installation scheme, the number of units and lines introduced into the first, second, third stages and during its final development is taken into account.
To select a substation scheme, it is important to take into account the number of high and medium voltage lines, the degree of their responsibility, and therefore at different stages of the development of the power system, the Substation scheme may be different.
The phased development of the switchgear circuit of a power plant or substation should not be accompanied by radical alterations. This is possible only if the prospects of its development are taken into account when choosing a scheme.
When choosing wiring diagrams, the permissible level of short-circuit currents is taken into account. If necessary, the issues of sectioning networks, dividing an electrical installation into independently operating parts, installing special current-limiting devices are solved.
From the complex set of conditions imposed that affect the choice of the main circuit of an electrical installation, the main requirements for the circuits can be distinguished:
reliability of power supply to consumers; adaptability to repair work; operational flexibility of the electrical circuit; economic expediency.
Reliability is the property of an electrical installation, a section of an electrical network or an energy system as a whole to ensure uninterrupted power supply of consumers with electricity of standardized quality. Equipment damage in any part of the circuit, if possible, should not interfere with the power supply, the delivery of electricity to the power system, the transit of power through the buses. The reliability of the circuit must correspond to the nature (category) of consumers receiving power from this electrical installation.
Reliability can be assessed by the frequency and duration of power outages to consumers and the relative emergency reserve, which is necessary to ensure a given level of trouble-free operation of the power system and its individual nodes.
The suitability of an electrical installation to carry out repairs is determined by the ability to carry out repairs without disrupting or limiting the power supply to consumers. There are schemes in which to repair the switch it is necessary to disconnect this connection for the entire period of repair, in other schemes only temporary disconnection of individual connections is required to create a special repair scheme; thirdly, the breaker is repaired without interrupting the power supply, even for a short time. Thus, the suitability for repairs of the considered scheme can be quantified by the frequency and average duration of outages of consumers and power sources for equipment repairs.
The operational flexibility of an electrical circuit is determined by its suitability for creating the necessary operating conditions and performing operational switching.
The greatest operational flexibility of the circuit is provided if operational switching in it is performed by switches or other switching devices with a remote drive. If all operations are carried out remotely, and even better by means of automation, then the elimination of the emergency condition is significantly accelerated.
Operational flexibility is measured by the number, complexity and duration of operational switches.
The economic feasibility of the scheme is assessed by the reduced costs, which include the costs of building the installation - capital investments, its operation and possible damage from a power failure.
c) Structural diagrams of power plants and substations
The structural electrical diagram depends on the composition of the equipment (the number of generators, transformers), the distribution of generators and the load between switchgears (switchgear) of different voltages and the connection between these switchgear.
In fig. 1 shows the structural diagrams of the CHP. If a CHP is built near consumers of electricity U \u003d 6 ÷ 10 kV, then it is necessary to have a generator voltage switchgear (GRU). The number of generators connected to the GRU depends on the load of 6-10 kV. In fig. 1, and two generators are connected to the GRU, and one, usually more powerful, is connected to a high voltage switchgear (HV switchgear). Lines 110 - 220 kV, connected to this switchgear, communicate with the power system.
If the construction of energy-intensive production facilities is envisaged near the CHPP, then their power supply can be carried out via overhead lines 35 - 110 kV. In this case, a medium voltage switchgear (RU MV) is provided at the CHPP (Fig. 1, b). The connection between switchgears of different voltages is carried out using three-winding transformers or autotransformers.
With an insignificant load (6-10 kV), it is advisable to block connection of generators with step-up transformers without transverse connection at the generator voltage, which reduces short-circuit currents and allows, instead of an expensive GRU, to use a complete switchgear to connect 6-10 kV consumers (Fig. 1, c). Powerful power units of 100 - 250 MW are connected to the HV switchgear without a tap to power consumers. Modern powerful CHP plants usually have a block scheme.
Picture 1. Structural diagrams of CHP
Figure 2. Structural diagrams of IES, HPP, NPP
Figure 3. Block diagrams of substations
In fig. 2 shows the structural diagrams of power plants with predominant distribution of electricity at increased voltage (IES, HPP, NPP). The absence of consumers near such power plants makes it possible to abandon the GRU. All generators are connected in blocks with step-up transformers. Parallel operation of the blocks is carried out at high voltage, where the switchgear is provided (Fig. 2, a).
If electricity is supplied at high and medium voltage, then the connection between the RU is carried out by a communication autotransformer (Fig. 2, b) or an autotransformer installed in a unit with a generator (Fig. 2, c).
In fig. 3 shows the block diagram of the substations. At a substation with two-winding transformers (Fig. 3, a), electricity from the power system enters the HV switchgear, then it is transformed and distributed among consumers in the LV switchgear. At the nodal substations, communication is carried out between individual parts of the power system and consumers are supplied with power (Fig. 3, b). It is possible to build substations with two medium voltage switchgear, HV switchgear and LV switchgear. At such substations, two autotransformers and two transformers are installed (Fig. 3, c).
The choice of one or another structural diagram of a power plant or substation is made on the basis of a technical and economic comparison of two or three options.
WIRING DIAGRAMS ON THE 6-10 kV SIDE
a) Scheme with one busbar system
The simplest diagram of electrical installations on the 6-10 kV side is a diagram with one non-partitioned busbar system (Fig. 4, a).
The diagram is simple and intuitive. Power supplies and 6-10 kV lines are connected to the busbars using switches and disconnectors. One switch is required for each circuit, which serves to turn off and on this circuit in normal and emergency modes; If it is necessary to disconnect the W1 line, it is enough to open the Q1 switch. If the Q1 breaker is taken out for repair, then after it is turned off, the disconnectors are turned off: first the linear QS1, and then the busbar QS 2.
Thus, operations with disconnectors are necessary only when withdrawing the connection in order to ensure safe work performance. Due to the uniformity and simplicity of operations with disconnectors, the accident rate due to improper actions with them by the on-duty personnel is small, which refers to the advantages of the scheme under consideration.
Figure 4. Diagrams with one busbar system, unsectioned (a) and sectioned by switches (b)
The scheme with one busbar system allows the use of complete switchgears (KRU), which reduces the cost of installation, allows widespread use of mechanization and reduces the construction time of an electrical installation.
Along with the advantages, the scheme with one non-partitioned bus system has several disadvantages. To repair the busbars and busbar disconnectors of any connection, it is necessary to completely remove the voltage from the busbars, i.e. disconnect the power supplies. This leads to an interruption in the power supply to all consumers during the repair.
In the event of a short circuit on the line, for example at point K1 (Fig. 4, a), the corresponding switch (Q4) must open, and all other connections must remain in operation; however, if this switch fails, the power supply switches Q5, Q6 will open, leaving the busbars without voltage. A short circuit on the busbars (point K2) also causes disconnection of power supplies, i.e., interruption of power supply to consumers. These disadvantages are partially eliminated by dividing the busbars into sections, the number of which usually corresponds to the number of power supplies.
In fig. 4, b shows a diagram with one busbar system. sectioned with a circuit breaker. The circuit retains all the advantages of single busbar circuits; in addition, an accident on the busbars leads to the disconnection of only one source and half of the consumers; the second section and all connections to it remain in operation.
The advantages of the circuit are simplicity, clarity, efficiency, sufficiently high reliability, which can be confirmed by the example of connecting the main step-down substation (GPP) to the buses of the electrical installation by two lines W3, W4 (Fig. 4, b). If one line is damaged (short circuit at point K2), the switches Q2, Q3 are turned off and QB2 is automatically turned on, restoring the power supply to the first section of the main gearbox along the line W4.
In case of a short circuit on the buses at point K1, switches QB1, Q6, Q3 open and QB2 automatically turns on. When one power supply is disconnected, the remaining power supply takes over the load.
Thus, the power supply of the GPP in the considered emergency modes is not disturbed due to the presence of two supply lines connected to different sections of the station, each of which must be designed for full load (100% reserve over the network). In the presence of such a reserve in the network, the scheme with one partitioned busbar system can be recommended for responsible consumers.
However, the scheme also has a number of disadvantages.
In the event of damage and subsequent repair of one section, responsible consumers, normally powered from both sections, remain without a reserve, and consumers that are not redundant through the network are turned off for the entire period of repair. In the same mode, the power supply connected to the section being repaired is disconnected for the entire duration of the repair.
The last drawback can be eliminated by connecting the power supplies to two sections simultaneously, but this complicates the design of the switchgear and increases the number of sections (two sections for each source).
In the considered circuit (Fig. 4, b), the sectional switch QB1 is switched on in normal mode. This mode is usually adopted in power plants to provide parallel operation of generators. In substations, the sectional switch is normally off to limit short-circuit currents.
The scheme with one busbar system is widely used for substations at a voltage of 6-10 kV and for powering the station's own needs, where its advantages can be fully used, especially due to the use of switchgear.
It is possible to use a circuit with one bus system connected in a ring on the generator voltage of power plants that supply most of the electricity to nearby consumers (Fig. 5). The busbars are divided into sections according to the number of generators. The sections are interconnected by means of sectional switches QB and sectional reactors LRB, which serve to limit the short-circuit current on the busbars. Lines 6-10 kV are connected to the switchgear busbars, receiving power through the group double reactors LR1, LR2, LR3 from the corresponding sections of the main switchgear. The number of group reactors depends on the number of lines and the total load of 6-10 kV consumers. Due to the low probability of accidents in the reactor itself and the busbars from the reactor to the main busbars and to the switchgear assemblies, the group reactor is connected without a switch, only a disconnector is provided for repair work in the reactor cell. For lines in these cases, switchgear cells are used.
Figure 5. Diagram with one busbar system connected in a ring
Each branch of the double reactor can be rated for a current from 600 to 3000 A, i.e. it is possible to connect several 6 kV lines to each assembly. In the diagram (Fig. 5), eighteen lines are connected through three group reactors; Thus, the number of connections to the main busbars is reduced in comparison with the scheme without group reactors by 15 cells, which significantly increases the reliability of the main busbars of the power plant, reduces the cost of building the reactor plant by reducing the number of reactors and reduces the installation time due to the use of complete cells for connection lines 6-10 kV.
Responsible consumers are powered by at least two lines from different twin reactors, which ensures the reliability of power supply.
If the generator voltage buses are divided into three or four sections, not connected in a ring, then it becomes necessary to equalize the voltage between the sections when one generator is turned off. So, when the generator G1 is turned off, the load of the first section is powered by the generators G2 and G3 that remain in operation, while the current from G2 passes through the reactor LRB1, and the current from G3 passes through two reactors - LRB2 and LRB1. Due to the voltage loss in the reactors, the voltage level in the sections will be different: the highest on the VZ section and the lowest on the B1 section. To increase the voltage on section B1, it is necessary to bypass the LRB1 reactor, for which a bypass disconnector QSB1 is provided in the circuit. In the mode under consideration, the second shunt disconnector does not turn on, since this will lead to parallel operation of the G2 and G3 generators without a reactor between them, which is unacceptable under the conditions of short circuit disconnection.
The procedure for the shunt disconnectors should be as follows: open the QB section switch, turn on the QSB shunt disconnector, turn on the QB section switch.
The more sections in a power plant, the more difficult it is to maintain the same voltage level, therefore, with three or more sections, the busbars are connected in a ring. In the diagram in Fig. 5, the first section can be connected to the third section breaker and the reactor, which creates a busbar ring. Normally all section breakers are on and the generators are running in parallel. In case of a short circuit on one section, the generator of this section and two sectional switches are turned off, however, the parallel operation of other generators is not disturbed.
When one generator is disconnected, the consumers of this section receive power from both sides, which creates a smaller voltage difference across the sections and allows section reactors to be selected for a lower current than in a circuit with an open bus system.
In the ring circuit, the rated current of sectional reactors is assumed to be approximately equal to 50-60% of the rated current of the generator, and their resistance is 8-10%.
b) Scheme with two busbar systems
Taking into account the peculiarities of electrical receivers (I, II categories), their power supply schemes (no reserve in the network), as well as a large number of connections to the busbars for the main switchgear of the CHPP, a feasibility study can provide for a scheme with two busbar systems (Fig. 6), in which each element is connected through a fork of two busbar disconnectors, which allows operation on one or the other busbar system.
Figure 6. Scheme fromtwo busbar systems
In fig. 6 the diagram is shown in working condition: generators G1 and G2 are connected to the first busbar system A1, from which the group reactors and communication transformers T1 and T2 are powered. The busbar system is partitioned with a QB breaker and an LRB reactor, which function is the same as in the single busbar arrangement. The second bus system A2 is back-up and normally has no voltage. Both bus systems can be interconnected with bus-coupled switches QA1 and QA2, which are normally open.
Another mode of operation of this circuit is also possible, when both bus systems are energized and all connections are evenly distributed between them. This mode, called fixed wiring operation, is commonly used on overvoltage rails.
The scheme with two busbar systems allows one busbar system to be repaired while keeping all connections in operation. So, when repairing one section of the working A1 bus system, all its connections are transferred to the A2 bus backup system, for which the following operations are performed:
turn on the bus-connection switch QA2 and remove the operating current from its drive;
check the on position QA2;
include disconnectors of all transferred connections on the A2 busbar system;
disconnect disconnectors of all connections from the A1 bus system, except for the QA2 disconnectors and the voltage transformer;
switch the power supply of voltage circuits of relay protection, automation and measuring instruments to the voltage transformer of the A2 bus system;
check by the ammeter that there is no load on QA2;
an operating current is supplied to the drive and QA2 is turned off;
prepare for the repair of the A1 tire section.
In the event of a short circuit on the first section of the operating system of buses A1, the generator G1, the sectional switch QB and the communication transformer T1 are turned off.
To restore the work of consumers in this case, it is necessary to perform switching:
turn off all switches not disabled by relay protection (dead-end line switches);
disconnect all disconnectors from the damaged section;
switch on disconnectors of all connections of the first section to the backup bus system;
turn on the switch of the communication transformer T1, thereby supplying voltage to the backup bus system to check its serviceability;
turn on the switches of the most responsible consumers;
turn generator G1 and, after synchronization, turn on its switch;
turn on the switches of all disconnected lines.
In this arrangement, a busbar breaker can be used to replace a breaker of any connection.
The considered scheme is flexible and reliable enough. Its disadvantages include a large number of disconnectors, insulators, current-carrying materials and switches, a more complex switchgear design, which leads to an increase in capital costs for the construction of a GRU. A significant disadvantage is the use of disconnectors as operational devices. A large number of disconnector operations and complex interlocking between switches and disconnectors lead to the possibility of erroneous disconnection of the load current by disconnectors. The likelihood of accidents due to improper operation of the service personnel in schemes with two busbar systems is greater than in schemes with one busbar.
The dual busbar arrangement can be applied to expandable CHP plants that have previously performed this arrangement.
WIRING DIAGRAMS ON THE SIDE OF 35 kV AND ABOVE
a) Simplified reactor plant diagrams
With a small number of connections on the 35-220 kV side, simplified circuits are used, in which busbars are usually absent, the number of switches is reduced. In some schemes, high voltage switches are not provided at all. Simplified circuits allow to reduce the consumption of electrical equipment, building materials, reduce the cost of the switchgear, and speed up its installation. Such schemes are most widespread at substations.
One of the simplified diagrams is the transformer - line block diagram (Fig. 7, a). In block diagrams, electrical installation elements are connected in series without cross-links with other blocks.
Figure 7. Simplified circuits on the HV side:
a - transformer block - line with HV circuit breaker; b - block transformer - line with a separator; c - two blocks with separators and a non-automatic jumper; d - bridge with switches
In the circuit under consideration, the transformer is connected to line W by a switch Q2. In case of an emergency in the line, the Q1 switch is turned off at the beginning of the line (at the regional substation) and Q2 from the HV side of the transformer, with a short circuit in the transformer, Q2 and Q3 are turned off. In the generator - transformer - line blocks, the Q2 switch is not installed, any damage in the block is turned off by the generator switches Q3 and at the district substation Q1.
In transformer-line blocks at substations (Fig. 7, b), QR separators and QN short-circuits are installed on the high voltage side. To turn off the transformer in normal mode, it is enough to turn off the load with the Q2 switch on the 6-10 kV side, and then turn off the magnetizing current of the transformer with the QR separator. The permissibility of the latter operation depends on the power of the transformer and its rated voltage.
In the event of a fault in the transformer, relay protection opens the Q2 switch and sends a pulse to open the Q1 switch at the power system substation. The shutdown pulse can be transmitted over a specially laid cable, over telephone lines or over a high-frequency channel of a high voltage line. Having received a tele-cut-off pulse (TO), the Q1 switch is turned off, after which the QR separator is automatically turned off. The transit line, to which the transformer is connected, must remain energized, therefore, after the QR is triggered, the Q1 switch is automatically closed. The pause in the automatic reclosing (AR) scheme must be coordinated with the QR trip time, otherwise the line will be switched on for an unrepaired fault in the transformer.
Q1 can be tripped without transmitting a telecripping pulse. For this, a QN shorting plug is installed on the HV side. The transformer protection, when triggered, gives a pulse to the QN drive, which, turning on, creates an artificial short circuit. Relay protection of line W1 picks up and disconnects Q1. The need to install a short-circuit breaker arises from the fact that the relay protection of the W1 line at the power system substation may be insensitive to damage inside the transformer. However, the use of short-circuits creates difficult conditions for the circuit breaker at the supply end of the line (Q1), since this circuit breaker has to disconnect the unremoved faults.
The main advantage of the circuit (Fig. 7, b) is its efficiency, which has led to the widespread use of such circuits for single-transformer substations, connected with a blind tap to the transit line.
The reliability of the considered circuit depends on the clarity and reliability of the separators and short-circuits, therefore, it is advisable to replace open-type short-circuits with SF6 ones. For the same reasons, a QW load break switch can be installed instead of the separator.
At two-transformer substations 35-220 kV, a scheme of two transformer-line blocks is used, which, for greater flexibility, are connected by a non-automatic jumper from two QS3, QS4 disconnectors (Fig. 7, c). In normal operation, one of the jumper disconnectors must be open. If this is not done, then in case of a short circuit in any line (W1 or W2), relay protection disconnects both lines, disrupting the power supply of all substations connected to these lines.
Disconnections of transformers (operational and emergency) occur in the same way as in the scheme of a single unit (Fig. 7, b). A jumper of two disconnectors is used when disconnecting lines.
In case of sustained damage on the W1 line, Q1, Q3 are turned off and the action of the automatic transfer switch on the 6-10 kV side turns on the sectional switch QB, providing power to consumers from T2. If the line is taken out for repair, then by the actions of the substation duty personnel or the operational field crew, the QS1 line disconnector is turned off, the disconnector in the jumper is turned on and the T1 transformer is put under load by closing the circuit breaker from the LV side (Q3), followed by opening the sectional switch. Power supply is possible in this scheme T1from the W2 line when repairing the W1 line (or T2 power from the W1 line).
Disconnectors are installed at 220 kV substations in front of the QR1 and QR2 separators.
On the HV side of power plants at the first stage of its development, it is possible to use a bridge circuit with switches (Fig. 7, d), with the possibility of later switching to circuits with busbars.
In the scheme for four connections, three switches Q1, Q2, Q3 are installed (Fig. 7, d). Normally, the Q3 switch on the jumper between the two lines W1 and W2 (in the bridge) is closed. In the event of a fault on the W1 line, the Q1 switch is turned off, the transformers T1 and T2 remain in operation, and communication with the power system is carried out via the W2 line. If damaged in the transformer T1 open the Q4 switch on the 6-10 kV side and the Q1 and Q3 switches. In this case, the W1 line turned out to be disconnected, although there was no damage on it, which is a drawback of the bridge circuit. If we take into account that emergency shutdown of transformers is rare, then such a shortcoming of the circuit can be tolerated, especially since after disconnecting Q1 and Q3 and, if necessary, removing the damaged transformer for repair, disconnect the QS1 disconnector and turn on Q1, Q3, restoring the operation of the W1 line.
To keep both lines in operation during revision of any breaker (Q1, Q2, Q3), an additional jumper is provided from two disconnectors QS3, QS4. Normally, one QS3 jumper disconnector is open, all breakers are closed. To revise the Q1 breaker, first turn on QS3, then turn off Q1 and disconnectors on both sides of the breaker. As a result, both transformers and both lines remained in operation. If in this mode a short circuit occurs on one line, then Q2 will turn off, i.e. both lines will remain without voltage.
To revise the Q3 switch, also pre-switch on the jumper and then disconnect Q3. This mode has the same drawback: with a short circuit on one line, both lines are turned off.
The probability of coincidence of an accident with a revision of one of the switches is the greater, the longer the duration of the breaker repair, therefore, as a final development option, this scheme is not applied at power plants.
On the side of 35 - 220 kV substations, it is allowed to use a bridge circuit with switches in the transformer circuit instead of separators and short-circuits, if the installation of the latter is unacceptable due to climatic conditions.
b) Ring schemes
In ring circuits (polygon circuits), the switches are interconnected to form a ring. Each element - a line, a transformer - is connected between two adjacent switches. The simplest ring circuit is the triangle circuit (Fig. 8, a). Line W1 is connected to the circuit by switches Q1, Q2, line W2 - by switches Q2, Q3, transformer - by switches Ql, Q3. Multiple connection of an element to the general scheme increases the flexibility and reliability of operation, while the number of switches in the considered scheme does not exceed the number of connections. In a triangle circuit for three connections, there are three switches, so the circuit is economical.
In ring circuits, any circuit breaker is revised without interrupting the operation of any element. So, when revising the Q1 switch, it is turned off and the disconnectors installed on both sides of the switch are turned off. In this case, both lines and the transformer remain in operation, however
Figure 8. Ring circuits
the circuit becomes less reliable due to ring rupture. If in this mode a short circuit occurs on line W2, then switches Q2 and Q3 will open, as a result of which both lines and the transformer will remain without voltage. A complete disconnection of all elements of the substation will also occur in the event of a short circuit on the line and a failure of one switch: for example, in the event of a short circuit on the W1 line and a failure of the Q1 switch, the Q2 and Q3 switches will open. The probability of coincidence of damage on the line with the revision of the breaker, as mentioned above, depends on the duration of the breaker repair. An increase in the overhaul period and the reliability of the circuit breakers, as well as a decrease in the repair time, significantly increase the reliability of the circuits.
In ring circuits, the reliability of the circuit breakers is higher than in other circuits, since it is possible to test any circuit breaker during the normal operation of the circuit. Testing the circuit breaker by opening it does not disrupt the operation of the connected elements and does not require any switching in the circuit.
In fig. 8, b shows a diagram of a quadrangle (square). This scheme is economical (four switches for four connections), allows testing and revision of any switch without disrupting the operation of its elements. The circuit is highly reliable. Disconnection of all connections is unlikely, it can occur if the revision of one of the switches coincides, for example Q1, the W2 line is damaged and the second circuit breaker Q4 fails. In the connection circuits, disconnector lines are not installed, which simplifies the design of the outdoor switchgear. When repairing the W2 line, turn off the Q3, Q4 switches and disconnectors installed towards the lines. The connection of the connections W1, T1 and T2 remaining in operation is carried out through the switches Q1, Q2. If during this period T1 is damaged, then the Q2 switch will open, the second transformer and line W1 will remain in operation, but the power transit will be disrupted.
The advantage of all ring circuits is the use of disconnectors only for repair work. The number of disconnector operations in such circuits is small.
The disadvantages of ring circuits include a more complex choice of current transformers, switches and disconnectors installed in the ring, since, depending on the operating mode of the circuit, the current flowing through the apparatus changes. For example, when revising Q1 (Fig. 8, b) in the Q2 circuit, the current doubles. Relay protection should also be selected taking into account all possible modes when revising the ring switches.
The quadrangle scheme is used in switchgears of 330 kV and above power plants as one of the stages in the development of the scheme, as well as at substations with a voltage of 220 kV and above.
The hexagon scheme (Fig. 8, c), which has all the features of the schemes discussed above, has received a fairly widespread use. Switches Q2 and Q5 are the weakest elements of the circuit, since their failure leads to the disconnection of two lines W1 and W2 or W3 and W4. If power transit occurs along these lines, then it is necessary to check whether this will lead to a violation of the stability of the parallel operation of the power system.
In conclusion, it should be noted that the design of switchgears in ring circuits makes it relatively easy to move from a delta circuit to a quadrangle circuit, and then to a transformer-busbar block diagram or to busbar circuits.
c) Schemes with one working and bypass bus systems
One of the important requirements for circuits on the high voltage side is to create conditions for revision and testing of circuit breakers without interruption of operation. These requirements are met by a circuit with a bypass bus system (Fig. 9). In normal operation, the AO bypass bus system is de-energized, the QSO disconnectors connecting lines and transformers to the bypass bus system are disabled. The scheme provides for a QO bypass switch, which can be connected to any section using a fork of two disconnectors. The sections in this case are parallel to each other. The QO switch can replace any other switch, for which it is necessary to perform the following operations: turn on the QO bypass switch to check the health of the bypass bus system, turn off QO, turn on QSO, turn on QO, turn off the Q1 switch, turn off the QS1 and QS2 disconnectors.
After these operations, the line receives power through the bypass bus system and the Q0 switch from the first section (9, b). All these operations are carried out without interrupting the power supply along the line, although they are associated with a large number of switching.
In order to save money, the functions of the bypass and sectional switches can be combined. The diagram in Fig. 9, and besides the Q0 switch, there is a jumper of two disconnectors QS3 and QS4. In normal mode this jumper is on, the bypass switch is connected to section B2 and also on. Thus, sections B1 and B2 are interconnected
Figure 9. Scheme with one working and bypass bus systems:
a - a diagram with a combined bypass and sectional switch and separators in transformer circuits; b - mode of replacement of a line switch by a bypass one; c - circuit with bypass and sectional switches
through QO, QS3, QS4, and the bypass switch acts as a section switch. When replacing any line breaker with a bypass switch, you must turn off the QO, turn off the jumper disconnector (QS3), and then use the QO as intended. For the entire duration of the repair of the line switch, the parallel operation of the sections, and therefore the lines, is violated. In the transformer circuits in the considered scheme, separators are installed (QW load break switches can be installed). In case of damage in the transformer (for example, T1), the switches of the lines W1, W3 and the switch QO are turned off. After disconnecting the QR1 separator, the switches turn on automatically, restoring the operation of the lines. Such a scheme requires precise work of the automation.
The diagram in Fig. nine, andrecommended for HV substations (110 kV) with the number of connections (lines and transformers) up to six inclusive, when disruption of parallel operation of lines is permissible and there is no prospect of further development. If the switchgear expansion is expected in the future, then switches are installed in the transformer circuits. Circuits with transformer switches can be used for voltages of 110 and 220 kV on the HV and MV side of substations.
In both considered schemes, the repair of a section is associated with the disconnection of all lines connected to this section and one transformer, therefore, such schemes can be used with paired lines or lines that are redundant from other substations, as well as radial, but not more than one per section.
At power plants, it is possible to use a scheme with one sectioned busbar system according to Fig. 9, c, but with separate bypass switches for each section.
d) Scheme with two working and bypass bus systems
For switchgear 110 - 220 kV with a large number of connections, a scheme with two working and bypass bus systems with one switch per circuit is used (Fig. 10, a). As a rule, both bus systems are in operation with a corresponding fixed distribution of all bays: lines W1, W3, W5 and transformer T1 are connected to the first bus system A1, lines W2, W4, W6 and a transformer T1 connected to the second busbar A2, busbar switch QA is on. Such a distribution of connections increases the reliability of the circuit, since in case of a short circuit on the buses, the bus connection switch QA and only half of the connections are turned off. If the damage on the tires is persistent, then the disconnected connections are transferred to a serviceable bus system. Interruption of power supply for half of the connections is determined by the duration of the switching. The considered scheme is recommended for switchgears 110 - 220 kV on the HV and MV side of substations with the number of connections 7-15, as well as at power plants with the number of connections up to 12.
Figure 10. Scheme with two working and bypass bus systems:
a - the main scheme; b, c - options for schemes
For switchgear 110 kV and above, the disadvantages of this scheme become significant:
failure of one circuit breaker in an emergency leads to disconnection of all power supplies and lines connected to the given bus system, and if one bus system is in operation, all connections are disconnected. The elimination of the accident is delayed, since all operations for the transition from one bus system to another are performed by disconnectors. If the power sources are powerful turbine-generator-transformer units, then their start-up after load shedding for more than 30 minutes may take several hours;
damage to the busbar switch is equivalent to a short circuit on both busbar systems, i.e., it leads to disconnection of all connected ones;
a large number of operations by disconnectors during revision and repair of switches complicates the operation of the switchgear;
the need to install bus connection, bypass switches and a large number of disconnectors increases the cost of building the switchgear.
Some increase in the flexibility and reliability of the circuit can be achieved by partitioning one or both bus systems.
At TPPs and NPPs, with the number of connections 12-16, one bus system is sectioned, with a larger number of connections - both bus systems.
At substations, one busbar system is sectioned at U \u003d 220 kV with the number of connections 12-15 or when installing transformers with a capacity of more than 125 MBA; both 110 - 220 kV bus systems are sectioned with more than 15 connections.
If the busbars are sectioned, then in order to reduce capital costs, it is possible to use combined bus-connecting and bypass switches QOA (Fig. 10, b). In normal mode, the QS1, QSO, QS2 disconnectors are on and the bypass switch acts as a bus-connection switch. If it is necessary to repair one breaker, turn off the QOA switch and the QS2 disconnector and use the bypass switch for its intended purpose. In circuits with a large number of lines, the number of such switching per year is significant, which leads to complications in operation, therefore, there are tendencies to abandon the combination of bus-connecting and bypass switches.
In a circuit with sectioned buses, in case of damage on the buses or in case of a short circuit in the line and a breaker failure, only 25% of the connections are lost (for the duration of switching), however, in case of damage in the sectional switch, 50% of the connections are lost.
For power plants with powerful power units (300 MW and more), the reliability of the circuit can be increased by connecting sources or autotransformers of communication through a fork of two switches (Fig. 10, c). These switches function as a busbar switch in normal operation. In the event of damage to any bus system, the autotransformer remains in operation, eliminating the possibility of losing both bus systems.
e) Scheme with two bus systems and three switches for two circuits
Switchgears 330 - 750 kV use a scheme with two busbars and three circuit breakers for two circuits. As seen from Fig. 11, six connections require nine switches, that is, for each connection there is a "one and a half" switch (hence the second name of the circuit: "one and a half", or "circuit with 3/2 switch per circuit").
Figure 11. Diagram with 3/2 switch for connection
Each connection is switched on via two switches. To disconnect the W1 line, you must open the switches Q1, Q2, to disconnect the transformer T1 - Q2, Q3.
In normal operation, all switches are on and both bus systems are energized. To audit any switch, disconnect it and disconnectors installed on both sides of the switch. The number of operations for revision is minimal, disconnectors serve only to separate the circuit breaker during repair, they do not perform any operational switching. The advantage of the circuit is that during the revision of any switch, all connections remain in operation. Another advantage of the one-and-a-half circuit is its high reliability, since all circuits remain in operation even if the busbars are damaged. So, for example, in the event of a short circuit on the first bus system, switches Q3, Q6, Q9 will turn off, the buses will remain without voltage, but all connections will remain in operation. With the same number of power supplies and lines, the operation of all circuits is maintained even if both bus systems are disconnected, while parallel operation on the overvoltage side can only be disrupted.
The circuit allows testing the switches in the operating mode without operating the disconnectors. Busbar repair, insulator cleaning, revision of busbar disconnectors are carried out without disrupting the operation of the circuits (the corresponding row of busbar switches is disconnected), all circuits continue to operate in parallel through the busbar system remaining energized.
The number of necessary operations by disconnectors during the year for the revision of all switches, disconnectors and busbars one by one is much less than in the scheme with two working and bypass busbar systems.
To increase the reliability of the circuit, elements of the same name are connected to different bus systems: transformers T1 , ТЗ and line W2 - to the first bus system, lines W1, W3 - transformer Т2 - to the second bus system. With this combination, in the event of damage to any element or busbars with simultaneous failure of one switch and repair of the switch of the other connection, no more than one line and one power source are disconnected.
So, for example, when repairing Q5, short circuit on line W1 and failure of switch Q1, switches Q2, Q4, Q7 are turned off, as a result of which, in addition to the damaged line W1, another element, T2, will be disconnected. After opening these switches, the W1 line can be disconnected by the line disconnector and the transformer T2 closed by the Q4 switch. Simultaneous emergency shutdown of two lines or two transformers in the considered scheme is unlikely.
In the diagram in fig. 11 three chains are connected to the busbars. If there are more than five such chains, it is recommended to section the busbars with a switch.
The disadvantages of the considered scheme are:
switching off the short circuit on the line by two switches, which increases the total number of revisions of the switches;
rise in the cost of the switchgear structure with an odd number of connections, since one circuit must be connected through two switches;
decrease in the reliability of the circuit if the number of lines does not match the number of transformers. In this case, two elements of the same name are connected to one chain of three switches, therefore, emergency shutdown of two lines at the same time is possible;
complication of relay protection circuits;
an increase in the number of switches in the circuit.
Due to its high reliability and flexibility, the circuit is widely used in switchgear 330 - 750 kV at powerful power plants.
At nodal substations, such a scheme is used when the number of connections is eight or more. With fewer connections, the lines are included in a chain of three switches, as shown in fig. 11, and the transformers are connected directly to the buses, without switches, forming a transformer-bus unit.
MAIN DIAGRAMS OF CHP
and) SchemeCHP with generator voltage busbars
At CHPPs with 63 MW generators, electricity consumers located at a distance of 3 - 5 km can receive electricity at generator voltage. In this case, a 6-10 kV GRU is built at the CHPP, as a rule, with one bus system. The number and capacity of the generators connected to the GRU are determined on the basis of the power supply project for consumers and must be such that when one generator stops, the remaining ones fully provide power to the consumers.
Communication with the power system and the delivery of excess power are carried out via 110 and 220 kV lines. If it is envisaged to connect a large number of 110, 220 kV lines, then at the CHP plant a reactor plant is constructed with two working and bypass bus systems.
With an increase in thermal loads, turbine generators with a capacity of 120 MW and more can be installed at the CHPP. Such turbogenerators are not connected to the busbars of the generator voltage (6-10 kV), since, firstly, this will sharply increase the short-circuit currents, and secondly, the nominal voltages of these generators are 15.75; 18 kV is different from the voltage of distribution networks. Powerful generators are connected in blocks operating on 110-220 kV buses.
b) Block schemesCHP
The growth of the unit capacity of turbine generators used at CHPPs (120, 250 MW) led to the widespread use of block schemes. In the diagram shown in fig. 12, consumers of 6-10 kV are powered by reacted taps from generators G1, G2; more distant consumers are powered through deep input substations from 110 kV buses. Parallel operation of generators is carried out at a higher voltage, which reduces the short-circuit current on the 6-10 kV side. Like any block scheme, such a scheme saves equipment, and the absence of a bulky GRU allows you to speed up the installation of the electrical part. The consumer switchgear has two sections with ATS on the sectional switch. For greater reliability of power supply, switches Q1, Q2 are installed in generator circuits. Communication transformers T1, T2 must be designed to supply all excess active and reactive power and must be supplied with an on-load tap-changer.
On the transformers of blocks G3, G4, an on-load tap-changer can also be provided, which makes it possible to provide an appropriate voltage level on the 110 kV buses when issuing the reserve reactive power of the CHPP operating according to the thermal schedule. The presence of on-load tap-changers in these transformers allows to reduce voltage fluctuations in MV installations.
With the further expansion of the CHP, turbogenerators G5, G6 are installed, connected in blocks. The 220 kV lines of these units are connected to the nearby regional substation. On the side of the 220 kV CHPP, no switches are installed, the line is disconnected by the switch of the district substation. In case of insufficient sensitivity of the substation relay protection to damage in transformers T5, T6, the transmission of a tele-disconnecting pulse (TO) is provided or short-circuits and separators are installed. The generators are disconnected by switches Q3, Q4.
There is no connection between 110 and 220 kV switchgears, which greatly simplifies the 220 kV switchgear scheme. As noted above, this is permissible if the connection of 110 and 220 kV networks is carried out at the nearest regional substation.
Modern powerful CHPPs (500-1000 MW) are built in a block type. A generator switch is installed in the generator-transformer blocks, which increases the reliability of the supply of MV and high-voltage switchgear, since this excludes numerous operations in the MV switchgear to transfer power from the operating to the standby transformer. at each shutdown and start-up of the power unit and operations with high voltage switches are excluded. It should not be forgotten that power units are switched on and off at CHPPs much more often than at IES or NPPs.
Figure 12. Scheme of block CHP
MAIN SCHEMES OF IES
a) Requirements for the schemes of powerful thermal power plants
The power of generators installed in thermal power plants is steadily increasing. Power units of 500, 800 MW were mastered in operation, units of 1200 MW are being mastered. The installed capacity of modern IESs reaches several million kilowatts. On the buses of such power plants, communication is carried out between several power plants, there is a flow of power from one part of the power system to another. All this leads to the fact that large IESs play a very important role in the energy system. The following requirements are imposed on the IES electrical connection diagram:
1. The main circuit must be selected on the basis of the approved project for the development of the power system, that is, the voltages at which electricity is supplied, the load curves at these voltages, the network diagram and the number of outgoing lines, the permissible short-circuit currents at high voltages, the requirements for stability and sectioning of networks, the greatest permissible loss of power in reserve in the power system and the capacity of power transmission lines.
2. At power plants with power units of 300 MW and more, damage or failure of any switch, except for the bus-connection and sectional, should not lead to the shutdown of more than one power unit and one or more lines, if the stability of the power system is maintained. In case of damage to the sectional or bus-connecting switch, the loss of two power units and lines is allowed, if the stability of the power system is maintained. If the damage or failure of one switch coincides with the repair of the other, the loss of two power units is also allowed.
3. Damage or failure of any switch must not lead to disruption of transit through the busbars of the power plant, that is, to disconnection of more than one transit circuit if it consists of two parallel circuits.
4. Units should generally be connected via separate transformers and switches on the high voltage side.
5. Power transmission lines should be disconnected by no more than two switches, and power units, auxiliary transformers - by no more than three switchgear switches of each voltage.
6. Repair of circuit breakers with voltage 110 kV and above should be possible without disconnecting the connection.
7. High voltage switchgear circuits should provide for the possibility of sectioning the network or dividing the power plant into independently operating parts in order to limit short-circuit currents.
8. When two starting-backup transformers for auxiliary needs are powered from this switchgear, the possibility of losing both transformers in the event of damage or failure of any switch must be excluded.
The final choice of the circuit depends on its reliability, which can be estimated by the mathematical method according to the specific damageability of the elements. The main circuit must meet the regime requirements of the power system, ensure the minimum design costs.
b) Block diagrams generator - transformer and generator - transformer - line
In a unit with a two-winding transformer, switches on generator voltage, as a rule, are absent (Fig. 13, a). Turning on and off the power unit in normal and emergency modes is performed by the Q1 switch on the side of the increased voltage. Such a power unit is called a monoblock. The connection of the generator to the block transformer and the tap to the MV transformer are carried out at modern power plants with closed complete conductors with separated phases, which ensure high reliability of operation, practically eliminating phase-to-phase short circuits in these connections. In this case, no switching equipment between the generator and the step-up transformer, as well as at the branch to the transformer c. n. not provided. The absence of a switch on the branch to the MV leads to the need to turn off the entire power unit in case of damage in the MV transformer (Q1 is turned off, switches on the 6 kV side of the MV transformer and the generator AGP).
Figure 13. Schemes of generator-transformer power units:
a, d - blocks with two-winding transformers; b - block with autotransformer; c - combined block; g - block with generator 1200 MW
With the high reliability of the transformers and the presence of the necessary power reserve in the power system, this scheme is adopted as a typical one for power units with a capacity of 160 MW and more.
In fig. 13, b shows a diagram of a generator block with an autotransformer. Such a scheme is used in the presence of two increased voltages at the IES. In case of damage in the generator, the Q3 switch is turned off, the connection between the two overvoltage switchgear is maintained. In case of damage on buses with a voltage of 110 - 220 kV or 500 - 750 kV, Q2 or Q1 will turn off, respectively, and the unit will remain operating on buses with a voltage of 500-750 or 110 - 220 kV. Disconnectors between the Q1, Q2, Q3 switches and the autotransformer are necessary for the possibility of taking out the switches for repair while maintaining the unit or autotransformer in operation.
In some cases, in order to simplify and reduce the cost of the design of switchgear with a voltage of 330 - 750 kV, the combination of two blocks with separate transformers for a common switch Q1 is used (Fig. 13, c). Switches Q2, Q3 are necessary to connect the generators to parallel operation and provide greater reliability, since in case of damage in one generator, the second generator remains in operation.
It should be noted that the presence of generator switches makes it possible to start the generator without using the MV start-up transformer. In this case, when the generator circuit breaker is off, the power supply to the d.s. buses. is fed through a block transformer and a working transformer s.n. After all starting operations, the generator is synchronized and closed with breaker Q2 (Q3).
Load break switches can be installed instead of bulky and expensive air circuit breakers on generator voltage. In this case, damage in any power unit will open the Q1 breaker. After separation of the damaged power unit, the serviceable one is put into operation.
The use of combined power units is permissible in powerful power systems that have sufficient reserve and throughput of intersystem connections, in case of layout difficulties (limited area for the construction of switchgear with a voltage of 500-750 kV), as well as in order to save switches, air and cable connections between transformers and switchgear of increased voltage.
Generators 1200 MW, having two independent stator windings (six-phase system), are connected to a block with a step-up transformer with two LV windings: one connected in a triangle, and the other in a star to compensate for a shift of 30 ° between the voltage vectors of the stator windings (Fig. 13, d).
In some cases, blocks with a generator switch are used (Fig. 13, e). The generator is switched off and on using the Q switch (or the QW load break switch), without affecting
Figure 14. Scheme of IES (8x300 + 1 x 1200) MW
Figure 15. Scheme of the IES (6x800) MW
MAIN DIAGRAMS OF NPP
and) Special requirements for NPP layouts
Like the diagrams of other power plants (CHPP, IES), NPP diagrams must be carried out in accordance with the requirements set forth earlier in terms of reliability, flexibility, ease of use, and efficiency.
Features of the NPP technological process, high power of reactor power units, reaching 1500 MW at modern power plants, delivery of all power to the power system via 330-1150 kV lines impose a number of special requirements on NPPs:
the main scheme of the NPP is selected on the basis of the diagram of the power system networks and the section to which this power plant is connected;
the scheme for connecting the NPP to the power system should ensure, in normal initial modes, at all stages of the NPP construction, the delivery of the full input power of the NPP and the preservation of the stability of its operation in the power system without the impact of emergency automation when any outgoing line or communication transformer is disconnected;
in repair modes, as well as in the event of failure of switches or relay protection devices, the stability of the NPP should be ensured by the action of the emergency automation system to unload the NPP. Taking into account these requirements, at the NPP, starting from the first commissioned power unit, communication with the power system is carried out by at least three lines.
When choosing the main scheme of a nuclear power plant, the following are taken into account: the unit capacity of the units and their number; voltages at which power is supplied to the power system; the amount of overflows between switchgears of different voltages; short-circuit currents for each switchgear and the need to limit them; the value of the highest power that can be lost if any breaker is damaged; the possibility of connecting one or several power units directly to the switchgear of the nearest regional substation; the use, as a rule, of no more than two switchgears of increased voltages and the possibility of refusing to use autotransformers for communication between them.
Switchgears 330-1150 kV at NPPs must be made extremely reliable:
damage or failure of any switch, except for a sectional or bus-connecting one, should not, as a rule, lead to the shutdown of more than one reactor unit and the number of lines that is permissible under the condition of the stability of the power system;
in case of damage or failure of a sectional or bus-connecting switch, as well as in the event of a coincidence of damage or failure of one switch with the repair of another, it is allowed to disconnect two reactor units and such a number of lines that is permissible under the condition of stability of the power system;
disconnection of lines, as a rule, should be carried out by no more than two switches;
disconnection of step-up transformers, transformers c. n. and communications - no more than three switches.
Such requirements are met by 4/3, 3/2 circuit breaker circuits for connection, generator - transformer - line block circuits, circuits with one or two polygons.
Switchgear 110 - 220 kV NPP is designed with one or two working and bypass bus systems. The working bus system is sectioned with more than 12 connections.
b) Typical NPP schemes
Taking into account the high requirements for NPP diagrams, design organizations develop the main electrical connection diagrams for each specific NPP. Let us consider the most typical scheme of a nuclear power plant with a 1500 MW channel boiling point reactor (RBMK-1500) and 800 MW turbine generators (Fig. 16). The power output of the NPP is carried out at a voltage of 750 and 330 kV. Switchgear 330 kV is constructed according to the scheme of 4/3 circuit breaker for connection. The switchgear 750 kV is made according to the scheme of two connected quadrangles with switches in the jumpers. Generators G3, G4 and G5, G6 form enlarged power units, which makes it possible to apply an economical quadrangle scheme after the commissioning of the third reactor power unit. The fourth reactor power unit with generators G7, G8 are connected to the second quadrangle 750 kV. With the further expansion of the nuclear power plant and the installation of the fifth reactor power unit, the generators G7, G8 and the newly installed G9, G10 will be combined into enlarged power units. The 750 kV lines have a throughput capacity of about 2000 MW, therefore, three lines will fully ensure the delivery of the entire power of the connected power units, taking into account possible expansion.
Shunt reactors LR1 to LR3 are connected to the lines via separate switches. Communication between switchgear 330 and 750 kV is carried out by a group of three single-phase autotransformers (provision is made for the installation of a backup phase). Standby transformers c. n. RT1 was connected - to the district substation 110 kV; RT2 - for switchgear 330 kV; RTZ - to the average voltage of the communication autotransformer with the possibility of switching to switchgear 330 kV; RT4 - to the LV autotransformer winding.
Figure 16. Scheme of NPP with 1500 MW reactor power units
MAIN DIAGRAMS OF SUBSTATIONS
General information
The main electrical connection diagram of the substation is selected taking into account the development scheme of the electrical networks of the power system or the district power supply scheme.
According to the method of connection to the network, all substations can be divided into dead-end, branch-off, checkpoint, nodal.
A dead-end substation is a substation that receives electricity from one electrical installation through one or more parallel lines.
The tap-off substation is connected with a blind tap to one or two passing lines.
The pass-through substation is included in the cut of one or two lines with two-way or one-way power supply.
A nodal substation is a substation to which more than two power lines are connected, coming from two or more electrical installations.
Consumer and system substations are distinguished by purpose.
The substation scheme is closely linked with the purpose and method of connecting the substation to the supply network and must:
ensure the reliability of power supply to consumers of the substation and power flows through inter-system or trunk connections in normal and post-emergency modes;
take into account the development perspective;
allow the possibility of gradual expansion of the RU of all voltages;
take into account the requirements of emergency automation;
to provide the ability to carry out repair and maintenance work on individual elements of the circuit without disconnecting neighboring connections.
The number of simultaneously triggered switches should be no more than:
two - in case of line damage;
four - in case of damage to transformers with voltage up to 500 kV, three - 750 kV.
In accordance with these requirements, typical schemes of switchgears for 6-750 kV substations have been developed, which should be used in the design of substations.
A non-standard main scheme should be justified by a technical and economic calculation.
Dead-end and branch substation schemes
Dead-end single-transformer substations on the 35-330 kV side are made according to the transformer block diagram - a line without switching equipment or with one disconnector (Fig. 17, a), if the line protection from the supply end has sufficient sensitivity to damage in the transformer. Such a scheme can also be used if the transmission of a tele-disconnecting signal is provided for 330 kV substations with transformers of any power, and for 110 - 220 kV substations with transformers over 25 MB A. When cable entry into the transformer, disconnectors are not installed.
Fuses on the 35, 110 kV side of power transformers are not used. At dead-end and branch substations only 110 kV, it is allowed to use circuits with separators (Fig. 17, b), with the exception of: substations located in cold climate zones, as well as in an especially icy area; if the actions of the separators and short-circuits lead to a loss of synchronism of synchronous motors at the consumer; at substations for transport and oil and gas production; for connecting transformers with a capacity of more than 25 MBA; in the circuits of transformers connected to the lines with the OAPV.
In the substation diagram in Fig. 17, b, on the 110 kV side, a QS disconnector, a QR separator and a QN short-circuit breaker are installed in one phase, and a Q2 switch on the 6-10 kV side.
In cases where the above schemes are not recommended, a typical scheme with a circuit breaker on the 35-500 kV side is used (Fig. 17, c).
Figure 17. Diagrams of blocks transformer - line:
a - without HV switch; b - with HV separator; c - with HV switch
Pass-through substations schemes
If it is necessary to section lines, transformer capacity up to 63 MB A inclusive and voltage 35 - 220 kV, bridge circuits are recommended (Fig. 18). The circuit shown in Fig. 18, a, is used on the 110 kV side with transformer power up to 25 MB A inclusive. The repair jumper with disconnectors QS7, QS8 is normally disconnected by one disconnector (QS7).
Switch Q1 in the bridge is turned on if power is passing through the lines W1, W2. If it is necessary to exclude the parallel operation of lines W1, W2 from the point of view of limiting short-circuit currents, the Q1 switch is open. If the transformer (T1) is damaged, the switch on the 6 (10) kV Q4 side is opened, the QN1 short-circuit breaker is turned on, the Q2 switch at the supply end of the W1 line is turned off, and the QR1 separator and then the QS1 disconnector are turned off.
Figure 18. Bridge schemes:
a - with a switch in the jumper and separators in the transformer circuits; b - with switches in the circuit of lines and a repair jumper on the side of the lines
If, according to the mode of operation of the network, it is necessary to restore the line W1 in operation, then the switch at the supply end of this line and the switch of the bridge Q1 are automatically turned on, thus, the transit along the lines W1, W2 is restored. The repair jumper is used when revising the Q1 switch, for this QS7 is turned on, Q1 and QS3, QS4 are turned off. Transit along lines W1, W2 is carried out via a repair bulkhead, transformers T1, T2 in operation.
In 220 kV networks and transformers up to 63 MB And inclusive, to increase the reliability of operation, the separators are replaced with switches Q1, Q2 (Fig. 18, b).
The repair jumper is open with the QS9 disconnector. The switch Q3 in the bridge is on, which provides power transit on lines W1 and W2. In the event of an accident in the transformer T1the circuit breaker on the 6 (10) kV side and switches Q1 and Q3 are opened. After disconnecting the QS3 disconnector, Q1 and Q3 close and the transit is restored. To repair Q1, turn on the repair jumper (disconnector QS9), disconnect Q1 and disconnectors QS1 and QS2. If a fault occurs in T2 in this mode, then Q2 and Q3 are disconnected and both transformers remain without power. You need to turn off QS6 and turn on Q3 and Q2, then T1connects to both lines. This drawback can be eliminated if the bridge and the repair bulkhead are reversed. In this case, in case of damage in the transformer, one switch on the HV side of the transformer is turned off, the switch in the bridge remains on, which means that the power transit along W1, W2 is preserved.
If the project of system automation in the 220 kV lines provides for an OAPV, then instead of the considered scheme, a quadrangle scheme is recommended.
The quadrangle scheme is used for two lines and two transformers when it is necessary to section transit lines, with responsible consumers and the power of transformers at a voltage of 220 kV 125 MB A or more and any power at a voltage of 330 - 750 kV.
Powerful nodal substation schemes
On the buses 330 - 750 kV of nodal substations, the connection of individual parts of the power system or the connection of two systems is carried out, therefore, increased requirements for reliability are imposed on the circuits on the HV side. As a rule, in this case, circuits with multiple line connections are used: ring circuits, 3/2 circuit breaker circuits and transformer-bus circuits with lines connected through two switches (with three and four lines) or with one and a half line connection (with five- six lines).
In fig. 19 shows a diagram of a powerful nodal substation. On the 330 - 750 kV side, a bus circuit is used - an autotransformer. In the circuit of each line there are two switches, autotransformers are connected to the buses without a switch (disconnectors with a remote drive are installed). If damaged T1all switches connected to K1 are turned off, the operation of 330-750 kV lines is not disturbed. After disconnecting T1the disconnector QS1 is remotely disconnected from all sides and the circuit on the HV side is restored by closing all the switches connected to the first bus system K1.
Depending on the number of 330-750 kV lines, it is possible to use ring circuits or a 3/2 circuit breaker per circuit.
On the medium voltage side of 110-220 kV power substations, a scheme with one working and one bypass bus system or with two working and one bypass bus systems is used.
When choosing a circuit on the LV side, first of all, the issue of limiting the short-circuit current is solved. For this purpose, you can use transformers with an increased value of u to, transformers with split winding LV, or install reactors in the transformer circuit. In the circuit shown in Fig. 19, dual reactors are installed on the LV side. Synchronous compensators with starting reactors are connected directly to the LV terminals of autotransformers. Connecting powerful GCs to 6-10 kV buses would lead to an unacceptable increase in short-circuit currents.
JIPT linear regulating transformers can be installed in the autotransformer circuits on the LV side for independent voltage regulation.
Figure 19. Scheme of the nodal substation
Subject 7 corrected Main schemes of ES and PS.doc
MAIN DIAGRAMS of switchgears of POWER PLANTS AND SUBSTATIONSAn electrical connection diagram of an electrical installation is a drawing in which the main elements (generators, transformers, as well as motors, disconnecting devices, instrument transformers) are shown in the legend, connected in the same sequence as in reality.
The diagrams are executed in a one-line and three-line image. For simplicity and clarity, single-line diagrams are more often used, where they show connections for one phase.
Primary circuit diagrams (main diagrams) show the circuits through which electricity is transmitted from sources to consumers.
In addition to the electrical equipment of the primary circuits at power plants and substations, auxiliary equipment (measuring instruments, relay protection and automation devices) is used to control and monitor the operation of the primary equipment. Secondary circuit diagrams are the wiring diagrams of secondary (auxiliary equipment). All connections in the secondary circuits are made with insulated wires and control cables.
When choosing the main switchgear circuits of stations or substations, the following factors are taken into account:
The value and role of a power plant or substation in the power system (power plants - basic or peak, close to industrial nodes or remote, connected to other power plants through high voltage or medium voltage buses; substations - dead-end, branch, through or distribution;
Expansion prospects;
Short-circuit current level
The main schemes of power plants must meet the basic requirements:
Reliability, i.e. the ability of the circuit to provide uninterrupted power supply to consumers, power supply or power transit in the event of equipment damage;
The ability to carry out repairs of the main equipment without limiting the power supply to consumers;
Operational flexibility, i.e. adaptability to carry out operational switching with a minimum number of operations in a minimum time and with a minimum risk;
Profitability.
Structural diagrams (block diagrams) of power plants and substations reflect the connections of generators and transformers with switchgears (RU) of different voltages. Switchgear is a set of equipment of the same voltage, connected according to a certain scheme and embodies this scheme in nature.
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Types of main schemes
One working busbar system, sectioned by circuit breaker
This scheme is used for RU - 6.10, 35 kV power plants and substations. In normal operation, the sectional switch (CB) is open. When the voltage disappears on one section, the CB is automatically switched on by the action of the ATS device (automatic transfer switch). The section switch can be turned on by the operator if, for any reason, one input from the source is taken out of service. The circuit allows maintaining the power supply of all connected lines to consumers. Since consumers are connected by paired lines to different sections, the withdrawal of one section for repair also does not lead to a disruption in the power supply to consumers.
^ Block diagrams
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Block diagrams (two line-transformer blocks with switches or isolators in the transformer circuits and a repair jumper on the side of the lines)
They are used for high-voltage switchgears of dead-end and tap-off substations 35 - 220 kV. Circuits with separators are used for switchgear 110 kV, if the power of the transformers is not
Exceeds 25 MVA. The no-load current of such transformers is small and, if necessary, is disconnected by the separator. With a large no-load current, to disconnect the transformer, one would have to go to the supplying power plant or substation.
The repair jumper is used when one of the supply lines is taken out for repair. Two disconnectors are installed in the repair bulkhead. If only one disconnector were installed in the jumper, its repair would cause a complete shutdown of the substation.
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Bridge circuits
Bridge circuits are used for high voltage switchgear of 35 - 220 kV pass-through (transit) substations. There are two variants of the bridge circuit with switches in the transformer circuits (a, b) and the bridge circuit with separators in the transformer circuits (c), which is used for 110 kV pass-through substations with transformers up to 25 MVA.
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In bridge circuits, power is transmitted through a working jumper with a switch. The repair jumper serves to preserve the transit when the operating jumper is taken out for repair of the breaker.
In circuit a), the power transit is interrupted if a fault occurs in the transformer. Sometimes this is necessary and the use of the scheme is reasonable. In diagram b), if the transformer is damaged, only the closest switch is turned off. Power transit through the working jumper is preserved. Therefore, scheme b) is used in cases where the transmission of transit through the substation is of great importance for the power system.
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The scheme is used for switchgear of high voltage of 220 kV kV bushing substations. All switches are closed during normal operation. Repair of any switch can be carried out without disrupting the transit of power through the substation and disconnecting the transformers. Damage to transformers and switches will also not disrupt transit. Therefore, the scheme is used with increased requirements for transit reliability.
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One working bus system with bypass
The scheme is an improvement of the scheme with one bus system by adding a special bypass (SNR) to the working bus system (RSB).
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The scheme is used for switchgear of higher voltage of distribution substations 110 - 220 kV. The bypass bus system is used when one of the connection breakers is taken out for repair without disconnecting the lines to consumers. For this, a bypass switch (OB) is turned on, which replaces the switch being repaired. In case of repair of one of the sections of the working bus system, disconnection of the connections connected to it is inevitable.
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Two working bus systems with bypass
The scheme is used for switchgear of high voltage of nodal substations and power plants of 110 - 220 kV. When repairing one busbar system, the connections are transferred to another.
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The bus connection switch (SHSV) in normal operation can be turned on and off. When transferring connections from one bus system to another, the AL should be in the on position. In normal operation, the individual connections can be connected to one or both of the operating busbar systems. The bypass bus system is used - as in the previous diagram - to repair the breaker of one of the connections.
For switchgear of generator voltage of power plants (6, 10, 20 kV), a scheme with two working busbar systems without a bypass is used.
^ Schemes 3/2 and 4/3
One-and-a-half scheme (a) or 3/2 scheme is used for switchgear 330-500 kV power plants and substations. This scheme uses three switches for two connections. In this case, the repair of any switch and any bus system is carried out without disconnecting the connections. The circuit does not require installation of ShSV.
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The 4/3 scheme is also used for switchgear 330-500 kV power plants and substations. In it, four switches are used to connect three connections (b).
Sectionalized busbar system with bypass
The bypass bus system allows you to replace it with the bypass switch during the repair of a circuit breaker of any connection.
It is applied at voltages of 110 - 500 kV. The OV allows the breaker of any connection to be removed for repair without interruption of power supply. ШСВ (busbar connection switch) - without interruption of power supply, transfer connections from one bus system to another and take out one of the busbars for repair.
Advantages:
1. With a short circuit on one bus system, only half of the connections are lost.
2. When one bus system is taken out for repair, the power supply of the connections is transferred to the second one without power interruption.
3. If it is required to take out the breaker for repair of one of the connections, replace it with a bypass one without interrupting the power supply.
Disadvantages:
1. In the event of a short circuit on the line and a failure of its breaker, the breaker failure protection device (breaker failure redundancy device) must operate and disconnect all switches of the bus system to which the damaged connection is connected.
2. In the event of a short circuit on one of the SS, half of the connections are lost, and if the SHSV fails, then all connections are lost.
One and a half busbar diagram
The scheme is also called “3/2” - 3 switches for 2 connections.
a) one-and-a-half scheme of busbars without alternating connections
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Advantages:
1. In the event of a short circuit on one of the SS, the switches of the 1st or 3rd row are turned off, and all connections remain in operation.
2. When taking out I or II SSh for repair, no complicated switching is required. The switches of the 1st or 3rd row must be turned off.
3. In case of a short circuit on the line, 2 of its switches are turned off and in case of failure of one of them, either the bus system is extinguished without loss of connections, or one line or one generator is lost.
4. When repairing one of the secondary and short circuits to the other, there is no loss of connection power. However, each block is allocated to its own line.
Disadvantages:
1. More expensive than all previous schemes, because contains one and a half times more switches.
2. Large operating costs due to a large amount of repair work, since each time the connection is disconnected, 2 switches are disconnected - a large wear of the switches.
3. If one of the switches of the 1st or 3rd row is under repair and there is a short circuit at one of the connections, then we lose the second connection of this field.
4. Great complexity of relay protection.
b) one and a half scheme with alternating connections
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The advantage of this scheme over the previous one is that during repairs of switches of the 2nd row and in case of failure of switches of the 1st or 3rd row with a short circuit on the line, the amount of unit losses will be 2 times less. If the circuit breaker fails, the bus system will be extinguished and the connection will be lost, the circuit breaker of which will be repaired. However, the damaged line can be disconnected by the disconnector and the power supply to the busbar system along with the lost connection can be restored.
If the number of switch chains in the circuit is more than 5, then it is recommended to section the busbars with a switch.
Due to its high reliability and flexibility, the circuit is widely used in switchgears (RU) 330 - 750 kV at powerful power plants.
At nodal substations, such a scheme is used when the number of connections is eight or more. With a smaller number of connections, the lines are included in chains of three switches, and the transformers are connected to the buses without switches, forming a transformer-bus unit.
Scheme with two busbars and four switches for three connections (scheme 4/3)
The scheme is most effective if the number of lines is 2 times less or more than the number of sources.
It has all the advantages of a one-and-a-half scheme, and in addition:
1. More economical (1.33 switches per connection instead of 1.5);
2. Busbar sectioning is required when the number of connections is 15 or more;
3. 
The reliability of the circuit is practically not reduced if two lines and one transformer are connected in the chain instead of one line and the spirit of transformers.
Disadvantages:
1. All the disadvantages that are inherent in the 3/2 scheme;
2. Due to the fact that in this circuit there are 2 times more middle row switches than in 3/2 circuit, then if these switches fail, the probability of losing the second connection will be higher.
The circuit can be carried out with 1, 2, 3 or 4 rows of switches. The most successful is a two-row arrangement of switches:
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LR are installed to compensate for the capacitive current generated by power lines of 500 kV and above.
Description of the main circuit
The main circuit of electrical substations is a set of main electrical equipment: transformers, lines, switches, busbars, disconnectors and other switching equipment with all electrical connections made between them.
The main substation layouts are subject to the same basic requirements for reliability, service safety, durability, maintainability, efficiency and maneuverability as for the main power plant layouts.
Depending on the position of the substation in the system, these requirements, in particular the requirements for reliability and maneuverability, may be less stringent in some cases.
The number of transformers at the substation has a certain value for the choice of the scheme. According to current practice, no more than two transformers are usually installed in substations.
According to the PUE, when developing the main circuit of electrical power circuits, it is necessary to take into account the categories of consumers to ensure the reliability of power supply. Installation of one transformer at a substation is allowed in cases where the consumers of the district belong to the 2nd and 3rd categories, allowing short interruptions in the power supply necessary to turn on the backup power from the network.
At a 500 kV substation. used one and a half scheme (3 switches and 2 connections). The connections are not fixed on any one SS, but are included in the gap between the circuit breakers. The choice of this scheme is based on its advantages over other and not so critical disadvantages.
The advantages of the one-and-a-half scheme include the following: revision of any switch or bus system is performed without disrupting the operation of the connections and with a minimum number of operations when these elements are taken out for repair; disconnectors are used only for repairs (ensuring a visible rupture to the energized switchgear elements); both bus systems can be switched off simultaneously without disrupting the connections. The one-and-a-half scheme combines the reliability of a busbar scheme with the maneuverability of a polygon scheme.
The disadvantages of the one-and-a-half scheme include a large number of switches and current transformers, the complication of relay protection of connections and the choice of switches and all other equipment for doubled rated currents.
The increased number of switches in the one-and-a-half scheme is partially compensated by the absence of busbar switches.
Description of the main equipment of the 500 kV substation
At the 500 kV substation there are two incoming and two outgoing lines of 500 kV, as well as two autotransformers that convert the voltage from 500 kV to 330 kV. ... Measuring current and voltage transformers. Numerous connecting buses and busbars for connecting equipment to each other. There is also a technical building at the substation, where there is a permanent duty personnel who monitor the performance of the substation, as well as all relay protection and automation boards are located.
Page 17 of 111
The simplest type of the main circuit, which appeared earlier than others, is a circuit with one non-partitioned bus system (Fig. 2-1, a); the advantages of the scheme are in extreme simplicity, clarity in nature and minimal costs for the construction of the reactor plant. However, such a scheme does not provide sufficient reliability of power supply. Damage to busbars, busbar disconnectors or any circuit breaker will cause complete extinguishment of all connections. Tire repair requires a power cut to all consumers. Revision of any switch is also associated with the repayment of its connection for the entire period of work.
It is possible to reduce the volume of repayment for one bus system by sectioning it (Fig. 2-1, b). However, a significant reduction in the volume of repayments in such a scheme during accidents can be achieved only with its deep sectioning, when the number of sections is equal to the number of connections.
Figure: 2-1. Single bus system: a - unsectioned; b - sectioned; c - annular; d - with a bypass disconnector 
Figure: 2-2. Bridge schemes; a - simple; b-with two disconnectors in the jumper; в- with three switches; g- double
This makes the scheme uneconomical, and the need to repay the connections when repairing their switches remains.
Replacing part of the circuit breakers with sectional disconnectors to reduce the cost of the sectioned circuit significantly reduces its reliability and can be allowed only in small, unreliable installations in cases where repair conditions are decisive.
An increase in the reliability of a circuit with one bus system can be achieved by converting it into a circular one by connecting the ends of the buses to each other (Fig. 2-1, c). However, the advantages of the ring circuit, consisting in the bi-directional supply of the connections, are realized only with its deep sectioning. Revision of the connection switch here also leads to the repayment of this connection during the repair.
The addition of a bypass disconnector /, which allows revisions of the connection switch without interrupting the power supply to consumers, increases the maintainability of the ring circuit (Fig. 2-1, d).
The bypass disconnector can be installed on a jumper between adjacent lines (Fig. 2-2, a). The resulting bridge scheme has a noticeably greater flexibility and maintainability, since it can revise any line switch without extinguishing the connection, so that revision of the jumper disconnector is not demanded disconnecting both lines, it is enough to install a second bypass disconnector in series with it (Fig. 2-2, b). However, the best results are obtained by combining the bridge scheme with busbar sectioning between the bays. The resulting bridge circuit with three switches (Fig. 2-2, c) is very convenient for powering a two-transformer substation with a transit line, as well as for infusing the power of a small two-unit power plant with a block circuit. 
Rice, 2-3. Polygon schemes: a - simple; b - connected; c - with a diagonal link; d - with paired connections
The double bridge circuit (Fig. 2-2, d) allows you to have an extra connection at increased voltage. Diagrams Fig. 2-2, c and d are used in the first stage of the construction of a power plant or with a small number of connections. These schemes are quite economical, since the number of switches in them is one less than the number of connections.
The desire to increase the efficiency of the ring scheme while maintaining their technical advantages has led to the creation of polygon-type schemes. As seen from Fig. 2-3, a, the polygon scheme differs from the ring in the absence of connection switches. In this scheme, the switches are installed in the split of the busbars, closed in a ring. The connections are connected to the buses between the switches through disconnectors. Thus, each connection turns out to be connected to the circuit through two switches at once, which must be switched on or off both when the connection is switched. After disconnecting the feeder, the ring - appears to be open, and it can be closed again only after disconnecting the feeder disconnector. The number of switches in a polygon is equal to the number of connections, that is, the same as in a non-partitioned ring, however, due to the placement of the switches in the corners of the polygon, the circuit has all the advantages of a deeply partitioned circuit. Another advantage of the polygon scheme is a small amount of compensation even in the case of the most severe damage to one of the switches (no more than two connections are lost). The revision of any circuit breaker requires a minimum of operations and can be done without disrupting the connection.
The disadvantages of the polygon circuit include the complexity of the relay protection of connections and the choice of current transformers, in which it is necessary to provide for the possibility of repairing any of the three switches of the common chain.
Another disadvantage of the circuit is the need for more frequent revision of the switches, since any short-circuit disconnection is made in it by two switches at once.
Finally, a short circuit during the revision of one of the switches can lead to serious difficulties, when the breakdown of the circuit into unconnected parts is likely to cause a power imbalance (in part of the circuit there will be a shortage or even complete absence of power supplies, at the same time in the other parts of the power cannot be used).
To mitigate these shortcomings, the number of joins, and hence the number of sides of the polygon, is limited to six; with a larger number of connections, they are divided between two or even three interconnected polygons (Fig. 2-3, b). In some cases, the number of sides of the polygon is allowed, more than six, but at the same time diagonal ties are carried out (Fig. 2-3, c).
If it is possible to provide a reserve power supply of the connections through the network, the polygon scheme can be made even more economical by pairing the connections (Fig. 2-3, d). In this case, the number of switches is halved, for example, in a square scheme, it is possible to connect eight connections. In the event of a short circuit on one of the connections, both are temporarily disconnected, but the power supply to the undamaged one can be quickly restored. In the event of a short circuit in the bus section, the line connections must receive a backup power supply from the network. Of course, in this case, the generator connection will be disconnected for the entire period of restoration of damaged buses, which will also take place in circuits with unpaired connections.
The paired circuit can be significantly improved by adding a disconnector (1 in Fig. 2-3, d) between the paired connections. In this case, either of the two connections can be disconnected and reconnected without temporarily disconnecting the other. In case of disconnection of the connection, it is enough to turn off the disconnector 1 first, and when connecting - turn on this disconnector last. 
Figure: 2-4. Triangle (a) and square (b) schemes
Examples of the simplest schemes of polygons are the schemes of a triangle (Fig. 2-4, a) and a square (Fig. 2-4, b), which can be successfully used with a small number of connections.
An improvement to the single busbar arrangement is to add a dedicated bus bypass to the operating system (Figure 2-5). In this case, each connection can be connected to the bypass bus system through its own bypass disconnector, and the bypass system itself is connected to the operating system using a bypass switch. The conclusion of the connection switch for repair is simple and is performed as follows: 1) the bypass switch is turned on; 2) the bypass disconnector of the connection is turned on, the switch of which must be revised; 3) the connection switch is turned off, and its circuit is disassembled. After the earthing has been applied, the circuit breaker is ready for repair. 
Figure: 2-6. Double busbar
The scheme with one working and one bypass bus system has advantages: revision of any switch can be performed without interrupting the connection; there are no bus junction disconnectors (personnel errors are excluded). 
Figure: 2-5. Single bus system with bypass bus
Figure: 2-7. Scheme with two main and one bypass bus systems 
Figure: 2-8. Scheme with two switches per circuit
The scheme has the following disadvantages: it is necessary to install bypass and sectional switches; revision of the main working bus system is impossible without repayment of connections; a short circuit on the working bus system leads to extinguishing all connections of one section; damage to the sectional switch leads to extinguishing all connections of both sections.
A natural development of a single busbar arrangement is a dual busbar arrangement (Figure 2-6). The busbar switch allows for arbitrary separation of connections between bus systems, while creating various options for operating the network, depending on the requirements of the system and the operating conditions of the power plant. Sectional switches reduce the amount of bus short circuit suppression.
The advantages of the scheme with two working bus systems are, firstly, in the quick restoration of the supply of the connections in case of a short circuit in one of the sections by switching them to an intact bus system and, secondly, in facilitating the repair of busbars and busbar disconnectors.
Repair of the connection switch is possible here only when installing removable bypass jumpers and transferring the operation of the connection protection relay to the busbar switch, which in this scheme replaces the audited switch. Since the installation of jumpers instead of the switch is performed when the voltage is removed from the connection, the preparation of the switch for repair inevitably causes an interruption in the operation of this connection.
This drawback can be eliminated by adding bypass buses to the two operating systems (Figure 2-7). The resulting circuit with two main and one bypass bus systems with one switch per circuit, having all the advantages of a simple circuit with two systems, has a higher maintainability. 
Figure: 2-9. Fixed connection diagram: transformer - busbars (a); line - tires (b)
Over the past 20 years, it has become widespread in our country at powerful block stations due to the fact that it makes it possible to revise any bus system and any switch without interrupting the work of connections, and also allows you to group these connections in an arbitrary way.
However, in modern conditions, with an increase in voltages to 750-1150 kV and an increase in the unit capacities of blocks up to 1.2 GW, and individual stations up to 4-6 GW, this system becomes insufficiently reliable and economical. A large power loss (2-3 GW) in the event of failure of any 750 kV auxiliary switch and the significant cost of installing these switches (6-8 million rubles) limit the scope of application of circuits with 110-220 kV bypass buses.
The two-breaker-per-circuit (dual circuit) circuit is a variation on the dual busbar circuit (Figure 2-8). The increase in reliability and maintainability in it is achieved by installing sequentially with each disconnector forks of switches.
The advantages of such a scheme lie in the ease of repairs of any bus system and in the possibility of taking the circuit breakers out for repair without operations with energized disconnectors. Damage to the tires does not lead to the termination of the connections here.
However, if a bus short circuit occurs during the revision of one of the bus systems, it will be accompanied by a complete redemption of all connections.
The disadvantages of the double circuit should also include the need for more frequent revisions of the switches, since damage on the lines is turned off by two switches at once. But the main disadvantage of the circuit is its excessive cost due to the large number of switches and current transformers. Therefore, its use is currently not recommended.
A variant of the double circuit is a circuit with fixed transformer-bus connections (Fig. 2-9, a) or a line-bus (Fig. 2-9, b). The revision of any switch is possible here without disrupting the operation of the connections with a minimum of switching in the circuit. However, the circuit also has major drawbacks: damage to the tires means the loss of a block or line; line fault is canceled by all breakers 
Figure: 2-10, One-and-a-half scheme (a) and scheme 4/3 (b)
(more often revisions of switches); when the number of connections is more than five, the circuit requires the installation of a large number of switches; revision of tires requires block repayment or line disconnection; Damage to a busbar system during the revision of another system leads to the complete repayment of the entire installation.
Taking into account all these disadvantages, the use of schemes with fixed connections is allowed only with a small number of connections in some rare cases.
For powerful block power plants, one-and-a-half schemes and 4/3 schemes, as well as schemes of "clean" blocks generator-transformer-line (G-T-L), are increasingly being used.
The one-and-a-half scheme (Fig. 2-10, a) has the following advantages: revision of any switch or bus system is performed without disrupting the operation of the connections and with a minimum number of operations when these elements are taken out for repair; disconnectors are used only for repairs (ensuring a visible rupture to the energized switchgear elements); both bus systems can be switched off simultaneously without disrupting the connections. As can be seen, the one-and-a-half scheme combines the reliability of a busbar scheme with the maneuverability of a polygon scheme.
The disadvantages of the one-and-a-half scheme include a large number of switches and current transformers, the complication of relay protection of connections and the choice of switches and all other equipment for doubled rated currents.
The increased number of switches in the one-and-a-half scheme is partially compensated by the absence of busbar switches.
The 4/3 circuit (Fig. 2-10, b) is similar to one and a half, but more economical, since it does not have 1/2 switch per circuit more than in a circuit with a double bus system, but only 1/3 ...
Schemes of the "clean" G-T-L unit are used only at voltages of 110-220 kV and a relatively short length of block lines, since these schemes poorly use the capabilities of block lines: their throughput at voltages of 330-750 kV significantly exceeds the power of block generators , and when the generator is stopped for repair, the unit line cannot be used to reduce losses in the network (Fig. 2-11). 
Figure: 2-11. "Clean" blocks G-T-L
Much better is the G-T-L circuit proposed by L.I.Dvoskin with an equalizing polygon and a bypass bus system, in which the number of switches is greater than the number of connections only by one, and bypass buses and an equalizing polygon allow maneuvering lines in normal mode, in case of accidents and repairs , avoiding power imbalances and interruptions in the operation of connections. It should only be noted the complexity of the relay protection of the block transformer connected to the circuit with three switches,
