For lines with a voltage of 110 kV and above, relay protection devices against multi-phase short circuits and earth faults must be provided. The type of main line protection is determined based on the requirements for maintaining the stability of the power system. It is believed that the requirements for stable operation of the power system, as a rule, are satisfied if three-phase short circuits on the lines, accompanied by a decrease in voltage on the supply buses below (0.6 ... ... 0.7) Unom, are disconnected without time delay (at provided that stability calculations do not impose other, more stringent requirements). In addition, the use of fast protection may be necessary when faults switched off with a time delay can lead to a disruption in the operation of critical consumers or to unacceptable heating of conductors, as well as when it is necessary to implement high-speed automatic reclosure.
On dead-end lines with a voltage of 110-220 kV, step current protection or step current and voltage protection should be installed. If such protections do not meet the requirements of sensitivity or fault disconnection speed, stepwise distance protection is provided. In this case, instantaneous current cutoff is recommended as an additional protection.
To protect against earth faults, a stepped residual current protection (directional or non-directional) is provided.
To protect lines with a voltage of 110-220 kV from a short circuit to earth, as a rule, stepwise zero-sequence current protection is provided. The current relays of all protection stages are switched on for the sum of three phases, which ensures the zero-sequence current flow through them in case of single-phase short circuits to earth. The calculation of the zero-sequence stepped current protection is reduced to determining the tripping current and the time delays of the individual protection stages; the need to use a power direction relay in protection; protection sensitivity.
Rice. 1. Calculation schemes for determining the protection operation current
zero sequence of a dead-end overhead line 110-220 kV according to conditions 1 and 2: a - initial; b - substitution - to determine the equivalent resistance of transformers and lines with a single-phase connection (one of the targets is disabled); i1l1 - inductive resistance of the line section l1; xt1 and xt2 - inductive resistances of transformers tl and t2 when one phase is energized
On the example of a scheme typical for the power supply of industrial enterprises (Fig. 1, a) (dead-end line with one-sided power supply), the method for choosing the parameters for the protection of lines for which a long-term operation of two phases is not provided is considered. Protection can be performed in one or two stages.
Taking into account the presence of standard panels, on lines supplying substations with grounded neutral, it is recommended to perform two-stage protection with a directional second stage, which makes it possible to increase its sensitivity and reduce the short circuit disconnection time. The operation current of the first stage of protection when it is performed without time delay is selected according to the following conditions.
1. Detuning from the magnetizing current surge of transformers with dead-earthed neutrals and switched on when the line is turned on. For circuit breakers with a three-phase drive, this condition is not taken into account when choosing the protection operation parameters. It is also not taken into account if the first stage of protection is tuned in time to prevent non-simultaneous closing of the circuit breaker phases. At the same time, for circuit breakers with single-phase drives, the operation time of the first stage must be at least 0.1-0.2 s (the lower limit is for air circuit breakers, the upper limit is for oil circuit breakers).
Substations of industrial enterprises are performed, as a rule, according to simplified schemes with short circuits in the transformer circuit. When determining the sensitivity of the zero sequence protection of the lines to which such substations are connected, one should take into account the decrease in current 3/0min and power (3/03 £/n) min due to a possible simultaneous three-phase short circuit behind the transformer and a single-phase short circuit to earth on the high side of the transformer when the short circuit is turned on.
The ratio of zero-sequence currents in line protection in case of a ground fault of one phase at the high voltage terminals of a transformer with a short circuit between three phases on the low voltage side (mode 1.3) and in case of a ground fault of one phase (mode 1) can be determined from Table.
Current protection against phase-to-phase short circuits
Current stepped protection against phase-to-phase short circuits is widely used on dead-end lines 110-220 kV. As the first stage, performed, as a rule, without time delay, current cutoff is used. The primary tripping current of the current cutoff installed on the line (Fig., a) and performed without time delay is determined by the following conditions:
Detuning from the current passing at the place of installation of protection, with three-phase short circuits behind the transformers fed by the line in question. The detuning according to this condition is carried out according to the expression (11), where /£3) is the maximum current in the protection with a three-phase short circuit after the transformers in the maximum mode of the system and at the minimum resistance of the transformers, taking into account the on-load tap-changer; kH~ 1.3...1.4. In the presence of branch substations with switches on the HV side, the current cutoff protecting the line, to ensure selectivity, must be detuned from the maximum short-circuit current on the HV side of the nearest substation with switches.
Detuning from the current of the load motors in case of a three-phase short circuit on the buses of the substation where this protection is installed (point K\ in Fig. 37, a). In this case, the expression (7.5) is calculated, where /i, ™ is the maximum current sent by the motors of the load fed from the line in question, with a three-phase short circuit on the buses of the substations to which the line is connected; kH - 1.3.,1.4,
Detuning from the self-starting current of the load motors fed from the line in question. Expression (7.2) is calculated according to this condition.
Detuning from magnetizing current surges of transformers connected to the line when it is turned on. The calculation is made for three types of switching: single- and two-phase (simultaneous switching on of two phases, then with some delay switching on the third phase), as well as three-phase (simultaneous switching on of all three phases). The calculated expression has the form 
where хг equiv is the equivalent resistance of the transformers and the line to the place of installation of protection for the calculated type of inclusion. The determination of LT equiv is carried out similarly to expression (15). When calculating for a single-phase connection, only transformers with a grounded neutral are taken into account, which are introduced into the equivalent circuit with resistances xy, calculated according to the calculation expressions on p. 143. When calculating for a two-phase connection, all transformers fed from the line in question are introduced into the equivalent circuit with resistances xf, regardless of the neutral grounding mode. When calculating for a three-phase connection, all transformers are also taken into account. In this case, the transformers are introduced into the equivalent circuit with resistances, the values of which are equal to 1.35 * for transformers and 1.3 for autotransformers. The value of the coefficient Сb is determined according to the table. 3.
3. The value of the coefficient Sat
The value of the coefficient Cg |
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Type of relay used in protection | Estimated inclusion | Transformer magnetic core steel - cold rolled | Steel of magnetic cores of transformers - hot-rolled |
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Unom = 110 kV | Unom = 220 kV | Unom = 110 kV | Unom = 220 kV |
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Single and three phase | |||||
two-phase | |||||
Single and three phase | |||||
It should be noted that when choosing the cutoff actuation current protecting the line shown in Fig., a, it is necessary to take into account the mode of disconnecting one of the circuits and connecting all transformers to the circuit remaining in operation.
The sensitivity of the current cutoff is checked in the minimum mode of the supply system with a two-phase short circuit on the buses of substations connected to the protected line. The minimum current cutoff sensitivity factor when it performs the functions
the main protection should be of the order of 1.5. If the current cutoff without time delay performs the functions of additional line protection, then the sensitivity coefficient should be about 1.2 in case of short circuit at the place of protection installation in the most favorable mode according to the sensitivity condition. In cases where simple current cutoffs do not meet the sensitivity requirements, it may be appropriate to use a combined current and voltage cutoff.
The operation current of the combined cutoff is selected from the condition of ensuring sufficient sensitivity in case of a two-phase metal short circuit at the end of the protected zone in the minimum mode of the supply system: 
where k4 y is the current cutoff sensitivity factor (k4 t = 1.5).
In addition to the condition (7.17), /c 0 k must satisfy the condition of reliable detuning from the self-starting currents in the automatic reclosure mode in case of faults in the voltage circuits (calculated expression (2)). The primary voltage for the operation of the voltage relay is selected according to the condition of detuning from a short circuit on the low (medium) voltage buses of the substation, in which, in case of damage behind the transformer, accompanied by a current equal to / s 0 k, the residual voltage at the place of installation of the protection will be the smallest: 
where hl is the resistance of the line section from the busbars of the supply substation, on which the protection in question is installed, to the busbars of the HV substation, the damage behind the transformer of which is calculated; rm - the smallest (taking into account the on-load tap-changer) resistance of the transformer, the damage behind which is calculated; kn- 1,2 - reliability factor.
The operation voltage of the combined cut-off must be within (0.15 ... 0.65) Unom, which is determined by the minimum setting of standard voltage relays (lower limit) and the condition for ensuring detuning from a possible decrease in voltage in the network (upper limit).
The sensitivity of the combined voltage cutoff is checked by the residual voltage Uocr at the place of protection installation at phase-to-phase voltages at the end of the protected line in the maximum operating mode of the system: 
The sensitivity factor of the combined voltage cutoff must be at least 1.5.
Overcurrent protection with time delay is used, as a rule, as the second stage of protection of dead-end lines with a voltage of 110-220 kV. Calculation expressions for calculating the maximum current protection of lines with a voltage of 6-10 kV are also valid for lines with a voltage of 110-220 kV.
To increase the sensitivity, the protection can be performed with a voltage start.
Distance protection
The calculation of protection is reduced to the determination of the operation resistances and time delays of individual stages, as well as its sensitivity. To protect dead-end lines with a voltage of 110-220 kV, remote protection is performed in two stages when using the EPE-1636 panel or one-stage - when using a simplified protection panel.
The response resistance of the first stage of protection is selected according to the condition of detuning from the short circuit behind the transformers that are fed from the line in question. To protect the line shown in Fig. a, the calculation expressions have the form 
where hl1 and hl2 are the resistances of the line sections; rt1 and rt3 are the minimum values of the resistances of transformers T1 and TK, taking into account the on-load tap changer (if different transformers are installed at the substations, then expressions (18) and (19) take into account transformers with lower resistances); kT Tl, kgt3 - current distribution coefficients equal to the ratios of the current at the place of installation of the protection and, accordingly, the currents in the transformers T1, TZ and in the section of the line L2 in case of short circuit behind the transformers. If there are circuit breakers on the HV side of the branch substations, then the first stage of protection, to ensure selectivity, is detuned from the resistance of the line section to the nearest substation with circuit breakers.
Obviously, when determining r \ 3, the modes corresponding to the maximum values of the current distribution coefficients should be taken as calculated. In the absence of power from the low (medium) voltage side of the transformers /gt, t1 = kr r3 = = 1. As the response resistance of the first stage of distance protection, the smaller of the values obtained by formulas (18) and (19) is taken.
The selected operation resistance is checked according to the condition of detuning from the inrush current of the magnetizing transformers when the line is turned on under voltage according to the expression
(notation - see expression (14)). The value of the coefficient Sb is taken according to the work and data of the manufacturer.
The primary response resistance of the second stage of protection (starting element) is selected according to the condition of detuning from the minimum resistance in the conditions of self-starting of the load motors after the external short circuit is turned off:
where UUKa sz - the minimum value of the primary voltage at the place of installation of protection in the conditions of self-starting of electric motors, determined by calculation (approximately can be taken equal to 80-90% of the minimum operating voltage of the network); kB = 1.05 ... 1.1 - relay return factor; kH = 1.2 - reliability factor; kC3 - coefficient of self-starting of motors in the mode after switching off the external short circuit, determined by calculation (approximately kC3 = 1.5 . . . 2); /slave mzhs - the maximum value of the operating current of the protected line;<рм_ ч - угол макси- мальвой чувствительности реле сопротивления", <рраб- угол полного сопротивления нагрузки в рассматриваемом режиме после отключения внешнего КЗ.
When choosing the operation parameters of the starting elements of the distance protection of lines with branches, in addition, one should also take into account the condition of detuning from the self-starting mode of the load of substations powered by the line in question when the line is turned on. The response resistance according to the specified condition is determined by expression 7.20. In this case, the coefficient kB is not taken into account, and kC3 and frab are determined in the self-starting mode of the retarded load when the line is turned on. 
The operation resistance of the relay of the first and second stages of protection is determined by the expressions
where pt and pc are the transformation ratios of the current and voltage transformers, respectively; £cx is the coefficient of the relay switching circuit.
According to the found values of the operation resistance, the catalog settings of the relay are selected. The sensitivity coefficient of protection is determined by the expression k4 = g® /2protect, where gprotect is the maximum resistance value supplied to the protection in case of short circuit at the design point. To check the sensitivity of protection, the calculated point is the one characterized by the highest value of gzach, for the one considered in Fig. 37, and the lines are point K2: 
where kt2 is the current distribution coefficient corresponding to the mode in which it takes the minimum value. To increase the protection sensitivity factor, you can use the elliptical characteristic of the starting element. The use of the elliptical characteristic of the relay of the starting element often makes it possible to provide a reliable redundancy of the protection of the transformers of the receiving substations. The lowest allowable protection sensitivity factor is approximately 1.5.
Selected relay settings must be checked for current sensitivity of fine operation /tr (given in the protection catalog data depending on the setting of the protection relay). The sensitivity of the relay to the current of precise operation is estimated by the sensitivity coefficient for a short circuit at the calculated point.
In accordance with the requirements of the PUE, the volume of relay protection devices for power transmission lines is determined by the level of rated voltage.
Lines of 110 kV and above are made with a grounded neutral. For the 110-500 kV line, relay protection devices against multi-phase and single-phase earth faults should be provided.
To protect against multi-phase short circuits, remote protection is installed, and TO is installed as a backup.
Fault protection is performed using a zero-sequence current transformer and acts on the signal from capacitive current.
Block BMRZ-KL
Purpose of the BMRZ-KL block.
The BMRZ-KL digital relay protection unit is designed to perform the functions of relay protection, automation, control, measurement and signaling of cable and overhead power lines, distribution substations and power plants, protection of electric motors. The function of determining the location of the fault (FLO) has been implemented - calculating the distance in kilometers to the place of a two-phase or three-phase short circuit on power lines. The presence of branches on a multi-terminal line leads to an increase in the error of the fault. The following parameters are used to calculate the distance to the fault location:
· specific reactive resistance of the line (Ohm/km), which is set by the consumer in the form of a setting when setting the BMRZ-KL;
· current and voltage values of the short circuit loop, obtained from the oscillograms of the emergency process.
The current and voltage in the short circuit loop is recorded in the section of the oscillogram with steady electrical quantities. If during an accident a two-phase short circuit turns into a three-phase one, the average distances to the short circuit point are calculated. At the same time, the decrease in the reliability of the OMP result is reflected on the display of the BMRZ-KL in the form of the message "The result is unstable." The accuracy of calculating the distance to the fault location is proportional to the errors of the measuring current and voltage transformers and the accuracy of setting the parameters of the protected line. The result of the OMA does not depend on the contact resistance at the fault location. The inaccuracies in determining the line parameters have a much greater influence on the MLR. If OMT is impossible, for example, when protections are triggered without time delay, the distance to the damage site is not displayed.
The BMRZ-KL block provides free assignment of reserve discrete inputs and outputs. The unit implements two options for protection against OZZ:
· directional protection with zero sequence power direction control (analogue of ЗЗП - 1М and ЗНЗ);
· registration of the effective value of the sum of higher harmonics in the current 3 Io (analogue USZ-3M).
The second method is effective in networks with compensated neutral and can be used to automatically or manually disconnect a damaged feeder, drastically reducing the troubleshooting time. When BMRZ-KL blocks are combined into an automated control system, information about the values of higher harmonics 3Io in all feeders of the switchgear section appears on the computer of the relayer or substation manager 1-2 s after the occurrence of the fault.
The BMRZ-KL block is produced in four versions, differing in the communication channel and in the operating current voltage.
Functions of the BMRZ-KL block.
· Directional three-stage overcurrent protection (MTZ) with combined voltage start. For any step, the settings are selected individually.
· Directional protection against single-phase short circuits on the ground (OSZ) with start-up on current and zero sequence voltage. Registration of higher current harmonics 3Iо.
· Undervoltage protection (ZMN) with control of two linear voltages and negative sequence voltage, with the possibility of blocking when starting the first and second stages of overcurrent protection.
· Protection against unbalance and phase failure of the supply feeder (ZOF) with negative sequence current control, as well as I 2 /I 1 .
· Redundancy in case of circuit breaker failure.
· Automatic restart.
· Execution of commands for automatic load shedding and automatic reclosing by frequency.
· Automatic oscillography of accident processes. (63 waveforms)
· Memory of emergency events.
· Counting impulses from active and reactive electricity meters (technical accounting).
· Measurement of network parameters.
· Self-diagnosis.
· Two setting programs.
Remote protection BMRZ-LT
Three-stage distance protection (DZ) with a quadrangular response zone for all three stages (or a quadrangular response zone for the first two stages and triangular for the third) is designed to protect overhead lines (overhead line unit - transformer) from phase-to-phase short circuits without ground faults and is made with three relays resistance in each stage, included in the circuits AB, BC, CA.
Four-stage residual current protection with definite time delays is designed to operate in case of single-phase and two-phase earth faults. The first three steps can be performed with a detuning from the power transformer magnetizing current surge. Any stage can be configured by the user using software keys:
non-directional;
Directional, with control by a zero-sequence power direction permissive relay;
Directional, with zero-sequence power direction blocking relay control;
Overcurrent protection
Three-stage current protection can be configured by the user using software keys: - non-directional; - directional with enable or blocking by signals from the power direction relay; - with combined start by (U and U2) voltage; The overcurrent protection stage with phantom voltage start is designed for long-range redundancy in case of a short circuit on the low voltage side downstream of transformers and monitoring the successful self-start of the remaining load after the short circuit protection downstream of the transformer is turned off.
Phase failure protection
Unbalance and phase failure protection can be configured by the user using software keys:
non-directional;
With negative sequence power direction control;
With zero sequence power direction control.
Breaker Failure Redundancy (CBF)
The "breaker failure" signal is issued after a specified time after the issuance of a signal to open the circuit breaker while maintaining the current through the connection disconnected by the protection. The breaker failure algorithm is made with control of the switch position. Time settings: 0.10 to 1.00 s, step 0.01 s.
Automatic reclosing (AR)
The block provides a two-time AR. The first and second AR cycles can be deactivated independently from each other by software keys. The automatic reclosure can be blocked when the cutoff is triggered and there is a voltage of 3Uo (ground in the network).
Multi-phase protection
We use TO as the main protection
Protection trip current
Relay operation current
Sensitivity factor
Therefore, the protection does not satisfy the sensitivity conditions
According to the PUE, stepwise current protections should be installed on single lines with one-way supply from multi-phase faults. If such protections do not satisfy the requirements for sensitivity or breaking speed, stepwise distance protection shall be provided. In the latter case, as an additional protection, it is recommended to use current cutoff without time delay.
Distance protection
Stage I
We find the response resistance of the I stage of protection
Line resistance (90%)
Transformer resistance
Relay trip resistance
II Stage
Line resistance (10%)
Motor resistances:
where is the supertransient resistance, 0.2.
Protection response time
III Stage
Protection trip resistance
Relay operation resistance according to the formula (3.7)
Protection sensitivity coefficient as the main
Ground fault protection
Done with TTNP
We find the capacitive current of the overhead line
Specific capacitive current of wire AC 70 - 0.045A / km
Earth fault trip current
Earth fault current for overhead lines
Checking the sensitivity
Therefore, the protection satisfies the sensitivity conditions
Selection of the auxiliary current source
We use rechargeable batteries as a source of operational current, i.e. we use direct current sources. The main advantage of which is independence from the mode of operation and the state of the primary network. Therefore, direct operating current is more reliable during a network failure.
Networks, as a rule, work with a dead-earthed neutral.
Therefore, protections are carried out both from multi-phase (with the exception of double earth faults at different points), and from single-phase short circuits. Networks often have a complex configuration, multiple power sources. Therefore, to protect against multi-phase short circuits (including double earth faults at one point), remote step protections with different characteristics of resistance elements are often used, equipped with blocking against swings and violations of secondary circuits. From earth faults, not distance protections are used, but current multi-stage directional zero-sequence protections.
In cases where, according to the conditions for ensuring the stability of the system and responsible consumers, protection is required over the entire length of the protected section without time delay (on the buses of stations and hub substations Ures with a 3-phase short circuit< 0,6-0,7Uном), возможны два решения вопроса: дополнение ступенчатых защит устройствами ВЧ блокировки или передачи отключающих сигналов и использование в качестве основной отдельной продольной защиты с абсолютной селективностью, предпочтение отдается второму варианту, обеспечивающему независимость в эксплуатации и более совершенное ближнее резервирование. На тупиковых линиях иногда удается использовать и более простые токовые ступенчатые защиты.
Topic 8. Protection of lines with a voltage of 110-220 kV
Lecture 12. Protection of lines with a voltage of 110-220 kV
Remote protections.
3. Purpose and principle of operation d distance protections.
Time delay characteristics of distance protections.
5. Principles for the implementation of selective protection of lines using Dz. The structure of line protection using distance protection.
6. Swing blocking device (UBK)
7. Schemes for switching on remote organs for current and voltage. Requirements for switching circuits
8. Technical characteristics of digital protections
9. Acceleration of remote protections on the RF channel.
General information about the protection of lines with a voltage of 110-220 kV
Networks with a voltage of 110 - 220 kV operate in a mode with an effective or solidly grounded neutral. Therefore, any earth fault in such networks is a short circuit with a current sometimes exceeding the current of a three-phase short circuit. Such a short circuit must be disconnected with the minimum possible time delay.
High voltage lines operate with high load currents, which requires the use of protections with special characteristics. On transit lines that can be overloaded, distance protections are used to effectively detune from load currents. On dead-end lines, in many cases, current protection can be dispensed with. Current and distance protections are performed stepwise. The number of steps should be at least 3, in some cases 4-5 steps are necessary.
According to the PUE, overload prevention devices should be used in cases where the duration of the overload current flow for the equipment is more than 10 ... 20 minutes. Overload protection should act on unloading equipment, breaking the transit, disconnecting the load, and only last but not least, disconnecting overloaded equipment.
High voltage lines are of considerable length, which complicates the search for the fault location. Therefore, the lines must be equipped with devices that determine the distance to the fault site (DFL). According to the directive materials of the CIS, lines with a length of 20 km or more should be equipped with weapons of mass destruction. Line protections on digital relays allow you to simultaneously perform the function of OMP.
A delay in opening the short circuit can lead to a violation of the stability of the parallel operation of power plants. Due to a long voltage drop, the equipment of power plants may stop and the technological process of generating electricity may be disrupted, additional damage may occur to the line on which the short circuit occurred. Therefore, on such lines, protections are used that turn off the short circuit at any point without time delay. Such protections include differential protections installed at the ends of the line and connected by a high-frequency, wire or optical communication channel, or conventional protections accelerated when an enabling or blocking signal is received from the opposite side.
All required protections are performed on the basis of one digital device. However, the failure of this one device leaves the equipment unprotected, which is unacceptable. Therefore, it is advisable to carry out protection of high voltage lines from two sets: main and backup. The reserve set can be simplified in comparison with the main one: it does not have automatic reclosure, OMA, it has fewer stages, etc. The redundant set must be powered by another breaker, other sets of current transformers and voltage transformers and act on a separate circuit breaker trip solenoid.
High-voltage line protection devices must take into account the possibility of circuit breaker failure and therefore must have a breaker failure.
To analyze an accident and the operation of relay protection and automation, it is necessary to register signals in case of emergency events.
Thus, for high-voltage lines, protection and automation kits must perform the following functions:
Protection against phase-to-phase short circuits and short circuits to earth.
Three-phase or single-phase automatic reclosure.
Overload protection.
Determining the location of damage.
Oscillography of currents and voltages in the event of a short circuit, as well as registration of discrete signals of protection and automation.
Protective devices must be redundant or duplicated.
For lines with switches with phase-by-phase control, it is necessary to have protection against open-phase mode, since a long-term open-phase mode in networks with a voltage of 110 - 220 kV is not allowed.
Distance protection (Dz)
Purpose and principle of action. Distance protections are complex directional or non-directional protections with relative selectivity, made using minimum resistance relays.
Dz react to the value of the line resistance to the place of the short circuit, which is proportional to the distance, i.e. distances. This is where the name distance protection comes from. For distance protection to work, it is necessary to have current circuits from the CT connection and voltage circuits from the VT.
Rice. 12.1. Ring network with two power supplies. O - maximum current directional protection; ∆ - distance protection
To protect dead-end cable or overhead lines with one-sided supply, overcurrent protection or current cutoff is sufficient. But, if these lines are connected in series one after another or interconnect several power sources, it is impossible to make such protections selective.
Let's imagine that a line departs from the buses of substation No. 1, feeding another substation - No. 2. And another line goes off the buses of this next substation.
When using overcurrent protection at substation No. 1, it should operate at the first line, but allow the protection of substation No. 2 to operate at the next.
But at the same time, it must also reserve the protection of the second, for which it must also act at line 2. To do this, the protection duration must be set so that the shutter speed at the first substation is longer. In addition, it will be necessary to divide the logic of the overcurrent protection into two or more stages, setting for the first of them the tripping current equal to the rated current at the end of the first line.
And now suppose that on the opposite side line No. 2 is fed by another source of energy, independent of the first. Now the task becomes more complicated: the short-circuit currents change. In addition, the lines will need to be directed.
There is another type of protection that can help to effectively disable exactly the line with damage - differential protection. But for a long-distance transmission line, it is very difficult to fulfill it.
When using overcurrent protection and current cutoffs, the protection devices turn out to be complex, and besides, they are not effective enough. The way out of the situation is the use of remote protection.
Protection principle
Distance protection (DZ) - a name that says that it reacts to the distance to the short circuit point. And to be more precise: the logic of its work depends on the location of the closure point, which determines the protection.
She does this with the help of devices called resistance relays.
Their task: to indirectly measure the resistance from the location of the protection to the point of short circuit. And for this, according to Ohm's law, it requires not only current, but also the voltage received from the voltage transformer installed on the substation busbars.
The resistance relay operates under the condition:
Here Zust– relay actuation resistance setting. The measured value is fictitious, since in some operating modes (for example, during swings), its physical meaning, as resistance, is lost.
The operation settings, and, consequently, the resistance relay for DZ, as a rule, are at least three.
The protected area is divided into areas called zones. The response time for each of the zones is different. And the setting of the resistance relay is equal to the resistance up to the point at the end of the corresponding zone. For clarification, let's recall an example with substations and lines.
Setting of the first zone remote sensing
It is calculated so that it protects only its outgoing line. But not to the very end, but taking into account the error in measuring the resistance - 0.7-0.85 of its length. When the first zone of remote sensing is triggered, the line is switched off with the minimum possible time delay, since it is guaranteed to be on it.
The second zone of remote sensing
Reserves the protection failure of the next substation. Why does she react to at the end of line #2. And the first DZ zone for the second line circuit breaker from substation No. 2 is set to resistance up to the same short circuit point, but already from the buses of this substation. But the time delay of the 2nd zone of the remote sensing of substation No. 1 is greater than that of the 1st zone of the remote sensing of substation No. 2.
This ensures the required selectivity: the switch of the second line from substation No. 2 will turn off before the protection time relay at substation No. 1 operates.
The third zone of remote sensing
Required to reserve the protection of the next line, if available. Additional zones are not provided.
An interesting video about setting up distance protection, see below:
The device and operation of the remote protection kit.
However, on some resistance relays and time relays, such protection cannot be performed. In practice, it includes several functional blocks.
Starting bodies DZ
These are current relays or impedance relays. Their task is to determine the presence in the protected circuit and start the operation of other protection devices.
remote organs.
A set of resistance relays for determining the operation zone and the distance to the short circuit. A device that generates time delays for protection zones. This is ordinary.
Power Direction Relay
In fact, it is rarely used, since resistance relays structurally have their own radiation pattern, which does not allow the protection to be triggered when “behind the back”. As a result, protection operation is excluded in case of short circuits in the direction opposite to the protected line.
Blocking Bodies
One of which is power failure protection. In case of malfunctions of the circuits, the TN DZ is taken out of action. The next lock works when swinging in the system. When they occur, there is usually a decrease in the voltage on the buses and an increase in the current in the protected lines. These changes are perceived by the remote protection units as a decrease in , due to which false operation of the protection is also not excluded.
Application of distance protection
Distance protection is used in networks powered by two or more sources.
These are communication lines with a voltage of 35, 110 kV and higher, through which the transit of electricity is carried out.
Remote sensing is especially effective and indispensable in ring power supply schemes, the use of which is very common for the country's unified energy system.
For all networks where remote sensing is installed, it is basic protection.
The design of the DZ on an electromechanical basis implies the presence of a large number of elements: transformers. An entire panel is allocated for its placement. Modern versions of microprocessor protections fit in one terminal, adjacent to their other types, as well as the ability to record protection operations, blocking operations, and record oscillograms of emergency processes. The combination of several devices in one terminal provides not only compactness, but also ease of use of the relay protection of the line.
Another interesting short video about the analysis of the work of distance protection:
