How Uninterruptible Power Supplies (UPS) Work and Work

Uninterruptible Power Supplies Off-Line

Sources uninterruptible power supply Off-Line type is defined by the standard as passive, standby action (UPS -PSO). In the normal mode of operation, the standard power supply of the load is the filtered voltage of the primary network with permissible deviations of the input voltage and frequency. In cases when the parameters of the input voltage go beyond the values \u200b\u200bof the configured ranges, the inverter of the uninterruptible power supply is turned on, ensuring the continuity of power supply to the load. The inverter is powered by batteries.

These are the simplest UPSs (Figure 1), and therefore the cheapest. The uninterruptible power supply consists of two parallel branches:
... filter load;
... rectifier-battery-inverter-load.

Fig. 1. Stand-By type uninterruptible power supply circuits

Under normal network characteristics, the voltage is supplied to the load through a filter that filters all kinds of interference. This is usually a surge suppressor, although it can be a line conditioner or a combination of both, and a static switch.

The batteries are charged at the same time through the rectifier battery... When the input voltage is lost, overestimated or lowered, the power supply of the load by an electronic switch is switched to the battery through the inverter (the inverter converts the constant voltage into alternating voltage). The switch provides switching times from 2 to 15 ms. Note that power outages during this time do not have any noticeable effect on computer systems that can easily tolerate a 10-20ms power outage. Considering that almost all modern equipment has impulse power supplies, switching is performed imperceptibly for the user. Uninterruptible power supplies of this type can support operation personal computer in the course of 5-10 minutes.

Main Disadvantages of Off-Line UPS

The main disadvantages of off-line UPS are:
... poor performance of power supplies of this type in networks with poor quality electrical network: poor protection against voltage dips (sags), overvoltage, changes in frequency and waveform of the input voltage;
... impossibility of timely restoration of battery capacity with frequent switching to battery power;
... non-sinusoidal output voltage when powered by a battery.

Line-Interactive uninterruptible power supplies

Uninterruptible power supplies of the line-interactive type (Line-Interactive, sometimes Ferroresonant) combine the advantages of the On-line type with the reliability and efficiency of the standby ones. In uninterruptible power supplies of this type, in contrast to the Off-line technology, a step automatic voltage regulator (booster), built on the basis of an autotransformer (transformer with switching windings), is included in the direct circuit. Some models use a network voltage stabilizer.

The inverter is connected to the load. In operation, it supplies the load in parallel with the conditioned AC mains voltage. The load is only fully connected when the mains input voltage is lost.

Fig. 2. Line-Interactive type uninterruptible power supply circuits

Because of this interaction with the input mains voltage, this architecture got its name. In a certain range of mains voltage variation, the output voltage is maintained within the specified limits by switching the transformer windings or by a stabilizer. The inverter typically operates at low voltage, regulates the output voltage and recharges the batteries until it needs to be turned on to fully power the load in the event of a power failure. Line-interactive uninterruptible power supplies are most widely used in computer network protection systems.

The transformer, made according to a special so-called ferro-technology, smoothes out voltage surges, while the uninterruptible power supply switches to battery operation less often, and therefore increases the battery life. Usually, these uninterruptible power supplies are equipped with sophisticated filters to provide protection against interference of various origins. Typical switching times to or from battery power are 2 ms.

Structurally, the transformer does not have several additional taps in the secondary winding (it can be an autotransformer with a single winding), the controller (microprocessor) controls the switching of the taps of the transformer when the input voltage changes, maintaining the output voltage in the required range. So, Line-Interactive uninterruptible power supply works on the principle of controlled LATR and really less often switches to battery power during input voltage surges. In this scheme charger structurally combined with the converter.

One of the advantages of this type of UPS is the wide range of permissible input voltages.

In some line-interactive models, there is a shunt circuit between the mains input and the load, such UPSs are called line-interactive shunt UPS (UPS -LIB, Reversible + Bypass). In shunt mode, the supplied load is not protected. When working with sources based on ferro technologies, you need to keep in mind:

Uninterruptible Power Supplies On-Line Type

On-Line technology allows you to implement the most reliable type of uninterruptible power supply. From the rectifier (Figure 3), the mains voltage is supplied to the DC voltage converter high level to low ПН1, and then - to the converter of constant voltage to alternating output voltage (ПН2). Converter PN2 is an inverter, the power to which is supplied both from the batteries and from the network through the voltage rectifier-converter PN1, connected in parallel:

. at normal input AC voltage, the PN2 inverter is powered by a rectifier;
... in case of deviations in the supply network from the norm, the input voltage for PN2 is removed from the storage battery.

Fig. 3. On-Line Type Uninterruptible Power Supply Circuits

Most UPS systems up to 5 kVA have a backup converter instead of a continuous battery direct current (DC-DC converter), which turns on in case of network failures and duplicates the DC bus from the low-voltage battery.

Conclusion: even in cases of slight deviations of the input voltage parameters from the norm, On-Line devices provide a rated output voltage in the range of ± 1-3%. The presence of a bypass circuit allows the load to be connected directly to the mains. The quality of power and reliability of power supply provided by devices with this type of architecture is significantly higher than that of the previous ones.

Disadvantages of On-line uninterruptible power supplies: low efficiency (85 -90%) compared to the previously discussed types due to double conversion (in relation to Standby and Line-Interactive) and high price. However, the level of protection of the load and the stability of the UPS output parameters are a reasonable compromise between safety, efficiency and cost of the device. Losses in a 4000VA UPS do not exceed 380W and can be incommensurate with the task that such a power source solves.

New modifications of uninterruptible power supplies

Now there are several new modifications of uninterruptible power supplies:
. by-pass;
... triple-conversion;
... ferrups.

The first modification (by-pass), as in Figure 3, is an additional channel for transmitting electricity to the load, its presence allows for high reliability of the device. Switching to the On-line mode is performed automatically when the parameters of the output network deviate from the norm or in emergency conditions. Thus, this mode helps to increase the reliability of the device. The second modification (triple-conversion) contains a power factor corrector. In the third modification (ferrups), a ferroresonant transformer is used, which provides high reliability rates and a wide range of input voltages.

New approaches in the construction of uninterruptible power supplies are based on the use of redundant power systems, which have a higher reliability of the output network, so that a failure of one of the elements does not lead to failure of the entire system. Typically, these are modular systems designed either to increase load power, or to improve system reliability, or use both principles together. The simplest system has an auxiliary module in the structure of the uninterruptible power supply, "isolated in hot standby mode". There are several options for technical solutions for such UPS.

The first option is to use an automatic switch (Figure 4). The inputs of one or more power supplies are connected to a single network, and are connected to the load through an automatic switch. Information about the state of operation of the units, control commands are received via the communication channel of the integrating UPS.

Fig. 4. Parallel circuit using automatic switch

The second option contains a “load distributor” (Figure 5), which evenly distributes the load between the individual sources of the system.

Fig. 5. Parallel circuit using automatic switch

The third embodiment of the parallel structure (Figure 6) uses the principle of a two-tier system. In this method, one of the "master" modules controls the load sharing between the other "slave" modules.

Fig. 6. Parallel circuit based on two-level Master-Slave system

The fourth option, with a redundant parallel architecture, looks the most promising. In such a scheme (Figure 7), not only modules are reserved, but also connections between them, and if necessary, any module can perform the functions of a master. Only such a scheme is characterized by an increase in power, the absence of shunt circuits, while continuous protection of the load using a UPS is guaranteed.

Fig. 7. Redundant Parallel System Diagram

Main technical characteristics of uninterruptible power supplies

Supply voltage form

It is this characteristic of the uninterruptible power supply that is important for the load. In the UPS operation mode, the load can be supplied with an output AC voltage close to a rectangular shape (meander), due to the smoothing properties of filters, an approximated sine wave and a pure sine wave. The closest to a sinusoidal output voltage waveform is obtained by applying pulse width modulation. Obtaining a sinusoid as a supply voltage is typical only for On-line UPS and some Line-Interactive power supplies.

Power

Full or output power.It is designated by the letter S, the unit is VA or Volt-Ampere. It is the geometric sum of active and reactive power. The parameter is calculated as the product of the rms (rms) values \u200b\u200bof current and voltage. Its value is indicated by the manufacturer of the power supply.

Active power consumption of the load. It is denoted by the letter P, the unit is watt (W). In the absence of a reactive component in the network, it coincides with the total power. It is defined as the product of the total power and the cosine of the angle φ, where φ is the phase angle of the linear voltage and current vectors, i.e. P \u003d S. cos (φ). Typical cos (φ) value for personal computers is about 0.6-0.7. This quantity is called power factor. Obviously, to select the required power for an uninterruptible power supply, the load power in watts must be divided by the value of cos (φ).

Reactive - denoted by the letter Q and is calculated as the product of the total power S by the sine of the angle φ (Q \u003d S. Sin (φ)). The unit of measurement is reactive volt-ampere (var). It characterizes the losses in the supply wires due to the loading reactive current... At cos (φ) \u003d 1, there are no losses, all the power generated by the power supply goes to the load. This is achieved through the use of passive compensating devices or active power factor correction.

Input supply voltage range

Input voltage range - defines the range of acceptable voltage values \u200b\u200bin the network, at which the uninterruptible power supply is still able to maintain the output voltage without switching to battery power. For some models, this range is load dependent. For example, at 100% load, the input voltage range can be 15-20% of the nominal, at 50% load - this range is 20-27% of the nominal, and at 30% load - 40% of the nominal. Battery life depends on this parameter, the wider the range, the longer the batteries will last, all other things being equal.

Input voltage frequency

Input frequency - characterizes the range of deviation of the mains frequency. Under normal operating conditions, the frequency deviation from the nominal value usually does not exceed 1 Hz.

Output voltage waveform distortion factor

The total harmonic distortion (THD) characterizes the deviation of the output voltage from the sinusoid, measured as a percentage. Small values \u200b\u200bof the coefficient correspond to the shape of the output voltage, approaching sinusoidal.

Mode switching time

The time of switching modes (transfer time) characterizes the inertia of an uninterruptible power supply, for different sources it is approximately up to 2-15 ms.

Battery life

Battery life is determined by battery capacity and load size. For typical low-power uninterruptible power supplies supplying personal computers, it is 5-10 minutes. This time is calculated so that the user can close all running applications while saving the information and turn off the PC in normal mode.

Crest factor

The crest factor is the ratio of the peak value of the consumed current to the average current. The value depends on the shape of the supply voltage.

Battery life

The service life of rechargeable batteries is 4-5 years, but the real one strongly depends on the operating conditions: the frequency of switching to autonomous mode, charging conditions, and the environment.

Cold start

The presence of a cold start is the ability to turn on an uninterruptible power supply in the absence of voltage in the supply network. This function is useful when you need to urgently perform any action, regardless of the presence of voltage in the mains.

UPS batteries

General information

The source, the energy of which is used to power the load in critical operating modes, is the storage battery. In uninterruptible power supplies with a capacity of up to 20 kW, as a rule, sealed lead-calcium batteries with a suspension type electrolyte are used. In batteries of this type, the electrolyte is immobilized, either by silica gel or by glass fiber, which makes them leakproof. This property of the electrolyte allows the batteries to be operated in any position, in addition, they do not need periodic electrolyte replenishment and other maintenance.

The electrodes are made of a lead-calcium alloy, which provides a long service life and a wide range of battery applications, the operating temperature range is from minus 20 to plus 50 ° C (for some types of batteries). Batteries do not suffer from the so-called "memory effect", they can be stored for a long time in a charged state (up to a year), while the self-discharge current is insignificant.

Battery design

The design of the batteries is traditional - the shock-resistant plastic case is divided into sections - "banks". The sets of cathode and anode plates are separated by spacers - fiberglass separators. The active part of the electrolyte is sulfuric acid. The cover is sealed to the body, without the possibility of disassembling the battery. In the upper part of the cover there are valves (one for each section), which ensure the release of gas in the event of its excessive formation during operation, and plate outlets. The valves are closed with an additional removable cover.

Battery storage

The battery life is approximately 5 years. With daily use of an uninterruptible power supply, the inherent charging capabilities guarantee operation during this period. Batteries self-discharge if left unused. For YUASA batteries, the self-discharge rate is approximately 3% per month at an ambient temperature of approximately 20 ° C. If the batteries are not charged over a long period of time, lead sulfates form on the negative plates of the battery. This phenomenon is known as "sulfation". Lead sulfate acts as an insulator, preventing the battery from accepting charge. The deeper the plate is softened, the less charge the battery can take.

In order to exclude irreversible consequences during storage, the charge must be carried out after a period corresponding to the ambient temperature conditions. Long-term storage batteries should be recharged periodically to ensure optimal life.

UPS battery charging methods

Battery charging is a major part of its maintenance. The battery life depends on the efficiency of the selected charging method. The following charging methods are available:
- charging at constant voltage;
- charging at constant amperage;
- two-stage constant voltage charging.

The preferred method is constant voltage charging. In this case, the battery is connected to an energy source, the charging voltage of which is kept constant during the entire charging process. In the course of charging, the current decreases and becomes much less than when charging with a constant current method, and at the end of the charge it drops to almost zero. In this case, the battery is charged to 90-95% of its nominal capacity.

Choosing an uninterruptible power supply

The range of types of uninterruptible power supplies as a means of protecting equipment and computer systemsis wide enough. The choice of the required power source is very difficult. To decide the issue of choosing one or another UPS, you need to try to analyze the factors that affect the operating conditions of the power source.

First, one should try to assess the significance of the system being fed. It is quite possible that an Off-line or Line-interactive type uninterruptible power supply will be sufficient for a home or office option. On-line UPS is more suitable for a server computer and other types of load that have increased requirements for the quality and reliability of power supply.

Secondly, it is necessary to assess the quality of the power grid: the probability and frequency of voltage outages, the presence of voltage fluctuations and various interference.

Third, you need to evaluate the capacity of the uninterruptible power supply. In order to roughly imagine what power of the UPS is required, it is necessary to determine the protected equipment and calculate the total value of the consumed power for it. Then, the watts obtained must be converted to VA, divided by the power factor. For computer equipment, the power factor is 0.5-0.6.

Manufacturers do not recommend loading an uninterruptible power supply more than 80% of the maximum load. It should be noted that it is not recommended to connect laser printers to an uninterruptible power supply due to the high power consumption of the heating element.

Magazine " Electronic components" №9,2008

Valery Klimov, Ph.D., Technical Director, Ruselt

When comparing uninterruptible power supplies (UPS) from different manufacturers, one should, first of all, pay attention to their technical characteristics, which reflect consumer properties and qualities. The article discusses the important energy indicators of the UPS and its overload characteristics. The dynamic characteristics reflect the reliable operation of the UPS during load switching, line voltage surges, overloads and other disturbances that occur in the "network - UPS - load" system. The results of an experimental study of the dynamic modes of single-phase UPS with double conversion, considered in part 1 ("EK" 6, 2008) are presented.

UPS Electrical Classification

UPS requirements and the classification of electrical characteristics of modern UPSs are most fully represented in the new international standard. The standard that was previously in force in our country does not reflect the full completeness of the requirements for modern UPS structures. The list of UPS electrical parameters proposed by the author is supplemented with a number of energy indicators:

Input specifications include: rated values \u200b\u200bof powers, voltages, currents and their permissible deviations, starting currents, input power factor, harmonic composition of input current;

Input characteristics reflect: static and dynamic indicators of accuracy, sinusoidal distortion factor, efficiency, output power factor, UPS overload capacity;

Transient (systemic) indicators characterize: frequency synchronization, standby time, recovery time of the battery (AB) charge, generalized energy coefficient;

DC circuit parameters characterize requirements for nominal values \u200b\u200bof AB voltage;

Operational requirements (environmental conditions) reflect the effects of temperature, humidity, altitude, etc. on the performance of the UPS.

Let's consider in more detail the main electrical characteristics of the UPS.

UPS input characteristics

Input voltage ratings adopted in our country: for single-phase UPS - 220 V; for three-phase UPS - 220/380 V, 50 Hz.

* The first, second and third parts of the article were published in "EK" 6, 8, 9, 2008.

Tolerances input voltage characterizes the limits of input voltage variation, at which the UPS continues to operate in the network mode without switching to autonomous mode from the AB. Modern UPS structures with booster provide a range of +/– 20% or more. It should be noted that for some single-phase UPS models, the lower input voltage limit expands with decreasing load.

Rated input apparent power (Sin.nom) - full power loading the network at 100% load factor and standard operating conditions. A distinction is made between the input power consumed when the battery is charged (Sin.min) and the power at forced battery charging (Sin.max), which exceeds the first value by 25 - 30%, depending on the size of the battery capacity and the degree of its discharge. For example, for a UPS with a rated output power of 30 kVA and an input power factor of 0.8 we have: Sin.min \u003d 32.8 kVA and Sin.max \u003d 41 kVA.

Rated input active power (Rin.nom) characterizes the power consumption at the UPS input at rated load:

Pvx.nom \u003d KrvxSin.nom

Input power factor (Крвх) characterizes the ratio of active input power to full power at rated input voltage and 100% load.

Kpvx values \u200b\u200bfor different models and capacities of the UPS can vary from 0.8 to 0.99. The higher the value of Крвх, the lower the sinusoidal distortion of the input current. In this case, the input resistance of the UPS in relation to the network will be purely active. The highest value of Крвх \u003d 0.99 is achieved in UPS structures with an input PWM converter on IGBT transistors.

The components of reactive power currents and distortion power in the input circuit of the converter (bridge circuit of a three-phase rectifier) \u200b\u200bwill close in the input circuit of the system and depend on the parameters of the input filter, the reactive parameters of the DC link (since this affects the shape of the current consumed from the network) and the degree system load.

Maximum input current - a parameter defining the choice of an external UPS circuit breaker. The value of the maximum current is determined at 100% load factor, the minimum input voltage in the forced battery charging mode:

Iin.max \u003d Sin.max / Uin.min

Starting current - characterizes the inrush of the input current due to the charging of the storage capacitors when the UPS is turned on. To limit the current surge in modern UPSs, starting circuits or the UPS soft start algorithm are used.

UPS output characteristics

Static accuracy the output voltage for single-phase low-power double conversion UPS is +/– 2%, for medium power and three-phase UPS it reaches +/– 1%, which allows parallel operation of 4 to 8 units for a total load. The dynamic accuracy of modern UPSs is +/– 5% at 100% load surges.

The external characteristic of the UPS characterizes the degree of static accuracy of the output voltage. In the general case, the rigidity of the external characteristic is determined by the internal resistance of the power circuit, which includes a rectifier, a power factor corrector (PFC), a DC voltage converter (DCV) and an inverter. KKM - PPN have stabilizing properties. Due to this, the supply voltage of the inverter is also stable, so it can be considered that the main parameter that determines the external characteristics of the UPS is the output resistance of the inverter. Modern IGBT inverters with pulse width modulation (PWM) output voltage have low internal resistance. Compared to power transformers, the inverter has a 5 times lower internal resistance, which provides not only a high stabilization accuracy of the output voltage (1 - 2)%, but also low values \u200b\u200bof the sinusoidal distortion factor of the output voltage (less than 3%) at currents in nonlinear loads with crest factor up to 3.

Rated Apparent Output Power (Sout.nom) - the maximum apparent power that the inverter can supply to a linear load with a power factor (Krn) equal to the output power factor of the UPS (Krout) under standard operating conditions (temperature, humidity, altitude).

Output power factor (Krvoh) specified by the manufacturer corresponds to the value of the load power factor at which the maximum efficiency of power consumption from the UPS is ensured. Krvyh values \u200b\u200bfor modern UPS are 0.7 ... 0.9.

Rated active output power (Rout.nom) - maximum active power supplied to the load:

Pvx.nom \u003d KrvxSin.nom

Efficiency and heat loss

Efficiency characterizes the efficiency of the UPS and presents the ratio of the output active power consumed by the load to the input active power consumed by the UPS from the network. Active power losses (heat losses) in a UPS are characterized by a number of components:

∆P \u003d Pin Pout \u003d ∆Pхх + ∆Pst + ∆Padm

∆Pхх is the constant component of losses (UPS idle loss) does not depend on the load factor and is determined by the energy required to service the control system of power units, supply cooling fans and other auxiliary units. In a small and medium power UPS 1-10 kVA, no-load losses are 20-30% of the total losses. As the power of the UPS increases, the relative proportion of no-load losses decreases.

∆Psc - variable component of losses, which depends on the load factor

∆Pst \u003d ∆P1 + ∆P2 + ∆P3 + ∆P4

∆P1 - losses in the rectifier power circuit;

∆P2 - losses in the power circuit of the power factor corrector;

∆P3 - losses in the power circuit of the DC voltage converter;

∆P4 - losses in the power circuit of the inverter.

The technical data of UPS manufacturers contains the values \u200b\u200bof the efficiency of individual power units of the UPS (mainly the rectifier and inverter) and the values \u200b\u200bof the overall (system) efficiency, which is 85 - 88% for low power UPS and 90 - 94% for medium and high power UPS;

∆Padd - additional losses for the battery charge, which are variable in time and depend on the degree of discharge of the battery and its capacity. The greatest additional losses occur when the battery is boosted. For example, the losses at rated load in a 30 kVA UPS are 2.8 kW when the battery is boosted and 2.2 kW when the battery is charged.

UPS load characteristic represents the non-linear dependence of the total power transfer factor on the load power factor FORMULA

Let us introduce the concepts of the total power transfer coefficient into the load and the load characteristics of the inverter.

Full power to load transfer ratio - the ratio of the maximum permissible load power to the nominal apparent power of the equipment: FORMULA The K5 coefficient correlates with the concept of the derating factor Kd, which indicates the percentage of the active component of the load power that can be connected to the inverter.

The derating factor depends on the nature of the load. Table 1 shows an example of the power derating factor values \u200b\u200bfor an inverter output power factor of 0.8 and various load power factor values.

Table 1. Dependence of the power reduction factor on the nature of the load.

The output filter capacitor current is added to the capacitive component of the load, which reduces the maximum permissible load at the inverter output. The reactive power component and high frequency harmonic distortion power at the inverter output will be exchanged between the load, the inverter output filter and the DC link filter capacitance. Closing in the indicated circuit of the converter power circuit, their values \u200b\u200bwill depend on the power factor of the load. Moreover, the output power factor may differ from the load power factor. The value of the coefficient of transfer of full power to the load reaches 100% when the power factor of the linear load of inductive nature is equal to the output power factor of the UPS. Figure 1a shows the load characteristics at different types linear load RL, RC and non-linear load RCD. With non-linear load, the power transfer coefficient decreases. The most common single-phase non-linear loads of the RCD type are uncontrolled rectifiers with a capacitive filter. The crest factor of the current of such a load reaches 2.5 - 3 with a power factor of 0.7 - 0.6. Figure 1b

shows the dependences of the power factor and the crest factor of the RCD load as a function of the current pulse duration at the half-cycle of the mains voltage. When the UPS is operating on different types of loads, the sum of the loads is taken as an equivalent non-linear load: 50% - RL - linear load with Crn \u003d 0.8 and 50% - RCD - load - uncontrolled rectifier with a filter capacity of 2.5 μF / W. The power transfer coefficient to a nonlinear load at a current with a crest factor Ka \u003d 3 does not exceed the value of Ks \u003d 70 - 80%.

Vector diagram of the power of the inverter (see Fig. 2) clearly reflects the load capacity of the UPS, and has recently been listed in the catalogs of a number of the world's leading UPS manufacturers. The upper quadrant of the diagram characterizes the power at active-capacitive load (kVAr-С), in the lower one - at active-inductive load (kVAr-L). Here the notation is accepted:

• the horizontal axis corresponds to the relative values \u200b\u200bof the active power P ;
• О - the center of the circle of the maximum total power with an inductive nature of the load;
• ОВ is the vector of the relative maximum total power supplied to the load of an inductive nature ( S max) at rated active power;
• About 1 -the center of the circle of the maximum total power with a capacitive nature of the load;
• ВС - the value of the rated active power at the output of the converter ( P number) ;
• ОА is the limiting value of the relative apparent power delivered to the inductive load at a reduced active power;
• OD is the limit value of the relative apparent power delivered to the capacitive load at reduced active power.

The cosines of the angles of rotation of the vectors of the total powers relative to the real coordinate axis will correspond to the power factors of the loads at the output of the inverter. The position of the vertical line of the rated output active power (P number ) is determined by the output power factor of the inverter K Pout \u003d P nom / S nom.

With a capacitive nature of the load, the center of the maximum apparent power shiftsAbout 1 downward relative to the origin of coordinates O and decrease in the total power boundary CD Exceeding the specified limits on the power vector diagram (A-B-C-D-O) means an overload of the inverter. Modern systems inverter controls in the UPS analyze the values \u200b\u200bof the full and active components of the power, fixing exceeding the limit values.

Reactive power factors of the inverter output filter

When choosing filter parameters, it is recommended to take:Kc \u003d Qc / Snom \u003d 0.25 - 0.5; Kl \u003d Ql / Snom \u003d 0.07 - 0.2. Smaller values \u200b\u200bof the coefficients can be taken for lower power inverters. An increase in the capacitive power factor leads to a decrease in the design power of the inverter, which provides nominal operating modes in the safe area of \u200b\u200bthe power vector diagram.

UPS overload characteristics and short-circuit current of the inverter Distinguish between the overload capacity of the inverter and the "bypass" circuit. In the event of significant and prolonged overloads, the UPS transfers to automatic bypass mode, which is characterized by a large overload capacity. However, modern inverters based on IGBT transistors with PWM control also have rather high overload characteristics and short-circuit currents (Isc), reaching 200 - 300% of the rated output current. At overloads not exceeding 5-10% of the rated power, the UPS can operate in inverter mode for a long time without going into bypass mode. Figure 3 shows typical UPS overload characteristics. Permitted areas of UPS operation: 1– inverter mode; 2 - automatic bypass mode; 3 - UPS shutdown area. It should be borne in mind that the quantitative indicators of the given current-time dependencies may differ for different UPS models. Knowing the overload characteristics allows you to optimally select the required UPS power rating for loads with high inrush currents, eliminating the low load factor of the UPS in static mode at rated load currents.

The issue of limiting the inverter current in overload mode is important for understanding the overload properties of the UPS. When the load current rises above the rated value, the inverter switches to the current generator mode, limiting the maximum current value at a certain value of Ilim. So that the distortion of the sinusoidality of the output voltage does not exceed 5%, it is necessary to set the threshold for limiting the maximum (amplitude) value of the output current 1.5 times greater than the amplitude value of the rated current of the inverter with a linear load:
I limit \u003d 1.5√2i out.nom
Accordingly, the crest factor of the limiting current is:
Co.lim \u003d Ilim

An inverter with PWM output voltage regulation is capable of responding to changes in load current, limiting it in amplitude. This increases the duration of the current pulse in the half-cycle of the output voltage. For example, an inverter with a rated power of 5 kVA is capable of delivering 4 kW of active power to an RCD load with a sinusoidal distortion of the output voltage of no more than 5%. Thus, the output power factor of such an inverter is Krout \u003d 0.8.

Table 2 lists the typical overload ratings of small and medium-sized UPSs.

Table 2. Typical overload characteristics of a small and medium UPS

UPS transient response

These characteristics are also called system or "input - output". These include parameters such as power factor, timing performance, UPS runtime and battery recovery.

The energy factor determines the ratio of the apparent powers - consumed by the UPS from the network and delivered by the UPS to the load:

FORMULA

If the condition Ke ≥ Krn is met, then the UPS consumes from the network full power equal to or less than that that the UPS delivers to the load:

FORMULA

This provision applies to UPSs with high input power factor when operating on non-linear loads with low power factor. This phenomenon is explained by the fact that with a non-linear load, the reactive power current and high-frequency harmonics of the distortion power current are closed in the "inverter-load" circuit and do not appear in the UPS input circuit. It can be shown that for a given load power factor Krn and efficiency, the active power at the UPS input will be:

FORMULA 9

The apparent power input to the UPS will be determined by the input power factor:

FORMULA 10

Under the condition Uin \u003d Uout, we have:

FORMULA 11

Consider an example of using a UPS with the following indicators: Krvh \u003d 0.95, efficiency \u003d 90%, when operating on a non-linear load with a power factor Krn \u003d 0.63.

From relation (11) we have: Iin \u003d 0.74 Iout. A decrease in the effective value of the input current of the PDS relative to the output current leads to a decrease in the load on the network compared to when the load is connected directly to the network. Since the power losses are proportional to the square of the current, the power losses in the power lines using a UPS in our example will be 54% of the losses when the same load is supplied from the network without a UPS. This circumstance is especially important in the presence of so-called "soft" power lines. Thus, the generalized energy coefficient is one of critical indicatorsdetermining the feasibility of using a double-conversion UPS not only to ensure uninterrupted power supply to the load in the event of a loss or distortion of the network, but also to optimize energy consumption at loads with a low power factor.

UPS battery life characteristics show the maximum operating times of the UPS from the AB energy in the absence or unacceptable deviations of the network, depending on the load factor. A significant increase in standby time is achieved by external connection of additional battery modules. Attention should be paid to the non-linear dependence of the time characteristics on the value of the load factor.

Battery charge recovery time AB characterizes the ability of the UPS to operate in repeated autonomous modes and depends on the used battery capacity. The battery charge time from 20% to 90% of the capacity is 6 - 8 hours on average.

Synchronization rates characterize the synchronous operation of the inverter and the "bypass" circuit, which must be maintained at frequency deviations within +/– 8% of the nominal with a frequency change rate within 1–4 Hz / s. During autonomous operation, the output frequency of the inverter must be maintained with an accuracy of +/– 0.1% of the nominal.

UPS dynamic performance and spectral performance

This section is devoted to the results of an experimental study of the dynamic modes and spectral characteristics of double conversion UPSs with a power of 1 - 3 kVA. These studies determined:

· Dips and bursts of instantaneous values \u200b\u200bof the output voltage and current and the time to return to the steady-state mode of UPS operation after load surges;

· UPS response to input voltage surges;

· Overload and protection capacity of UPS;

· The harmonic composition of the output voltage and current in steady-state processes with different types of loads and the shape of the input voltage.

The named list of dynamic characteristics reflects the general requirements for the UPS set forth in the standards. The results of the study of transient processes during load jumps are shown in Figures 4 a, b. Analysis shows that with a linear load jump to 100%, the output voltage drops by 3.5% of the steady-state value and then recovers to its original level in 60 ms (see Fig. 4a). Note that the static stabilization accuracy of the UPS is +/– 2%. With an abrupt drop of 100% of the linear load, an increase in the output voltage by 4% and a return to the steady-state value within 100 ms (see Fig. 4b) were recorded.

Figure 5a shows the oscillograms of the output voltage and current when the motor load is turned on, the total power of which is 150% of the rated power of the UPS. Due to the overload, the UPS automatically went into bypass mode, and then, after the end of the starting mode of the UPS motor, it went back to double conversion mode. It can be seen that the transition from double conversion mode to bypass and vice versa occurs instantly, without distortion of the voltage and current curves.

The process of transferring to bypass and returning to double conversion mode was shown in Figure 5a. When the load exceeds 110%, the inverter continues to run for 30 seconds and then the UPS transfers to bypass. If the load rises to 150%, the inverter continues to run for 0.2 s before transferring to bypass.

Figure 5b shows oscillograms of the output voltage and current of a 3 kVA UPS when a nonlinear load is turned on, the crest factor (crest factor) of which is 2.84, and the apparent power is 1.8 kVA. The initial surge current exceeded 2.4 times the steady state peak current. In this case, the output voltage decreased by 9% of the steady-state value and then recovered to its original level within 40 ms.

When investigating the behavior of the UPS during input voltage surges, it was noted that it provides almost instantaneous response to disturbances, and the stability of the output voltage remains within the static accuracy of +/– 2%. Efficiency electronic protection The inverter was tested in the autonomous operation of the UPS by turning on the motor load in excess of 150% of the rated load (starting the motor). 0.22 s after the motor was turned on, the UPS was shut down by electronic overload protection (see Fig. 6). The experiment confirmed the passport data on the overload capacity of the inverter (200 ms) and the reliability of the UPS electronic protection operation.

The study of the harmonic composition of the output voltage and current at linear and nonlinear loads showed that the distortion factor of the sinusoidal form of the output voltage does not exceed the permissible values \u200b\u200bfor any type of load, both in the network and in the autonomous modes.

Table 3 shows the results of testing a 3 kVA UPS for the composition of higher harmonics in the output and input voltages and currents at a non-linear load of 1.8 kVA.

Table 3. Spectral composition of currents and voltages at nonlinear load

As follows from the analysis of the harmonic composition of the output voltage when using a double conversion UPS, we have an insignificant sinusoidal distortion factor Ki \u003d 3.8% with a significantly nonlinear load and with an allowable content of higher harmonics of the inverter output voltage of no more than 10%. With a substantially non-sinusoidal input voltage, corresponding to a sinusoidal distortion factor of 36 - 41% (rectangular voltage with a significant third harmonic coefficient), the output voltage of the UPS has a sinusoidal shape Ki out \u003d (0.6 - 1)%. This circumstance is especially important when the UPS is powered from a low power diesel generator set (DGS), when the DGS voltage has significant distortions from a sinusoidal form.

Literature:
1. Klimov V. Modern uninterruptible power supplies: classification and structure of single-phase IDP. Part 1 // Electronic components, No. 6, 2008.
2. Klimov V. Structures of power circuits of three-phase UPS. Part 2 // Electronic components, No. 8, 2008.
3. International Standard IEC 62040-3.1999, Uninterruptible Power Systems (UPS), part 3: Method of Specifying the Performance and Test Requirements.
4. GOST 27699-88. Uninterruptible power supply systems for receivers alternating current... General technical conditions.
5. Jean N. Fiorina Inverters and Harmonics, MGE UPS Systems, MGE 159, 1993
6. Klimov V., Moskalev A. Power factor and load characteristic of a PWM inverter in uninterruptible power supply systems // Power Electronics, No. 3, 2007.
7. Klimov V., Smirnov V. Power factor of a single-phase transformerless switching power supply // Practical power electronics, issue 5, 2002.
8. Klimov V., Klimova S. Energy indicators of uninterruptible power supplies of alternating current, Electronic components, No. 4, 2004.
9. Klimov V. et al. Single-phase uninterruptible power supplies of the DPK series: dynamic and spectral characteristics // Power Electronics, No. 2, 2007.
10. Klimov V. Multi-module UPS structures and organization of parallel operation of single-module UPS. Part 3 // Electronic components, No. 9, 2008.
11. GOST 13109-97. Electricity quality standards in general-purpose power supply systems.

Requirements for the quality of electricity are legally prescribed by state standards and rather strict regulations. Electricity supplying organizations make a lot of efforts to comply with them, but they are not always implemented.

In our apartments, and in production, periodically there are:

complete power outages for an indefinite time;

aperiodic short-time (10 ÷ 100 ms) high-voltage (up to 6 kV) voltage pulses;

surges and voltage drops with different duration;

high-frequency noise pads;

frequency drifts.

All these malfunctions negatively affect the work of residential and office electricity consumers. Particularly affected by the quality of power supply are microprocessor and computer devices, which not only fail, but can also completely lose their performance.

Purpose and types of uninterruptible power supplies

To reduce the risks from the occurrence of faults in the power supply network, backup devices are used, which are commonly called uninterruptible power supplies (UPS) or UPS (derived from the abbreviation of the English phrase "Uninterruptible Power Supply").

They are manufactured in different designs to meet specific customer needs. For example, powerful UPSs with helium batteries are able to keep an entire cottage on power for several hours.

Their batteries receive charge from a power line, wind generator, or other carriers of electricity through an inverter rectifier device. They also feed the electrical consumers of the cottage.

When the external source is disconnected, the batteries are discharged to the load connected to their network. The larger the battery capacity and the lower their discharge current, the longer they work.

Medium power uninterruptible power supplies can back up indoor climate control systems and similar equipment.

At the same time, the simplest UPS models can only complete the computer emergency shutdown program. Moreover, the duration of the entire process of their work will not exceed 9-15 minutes.

Computer uninterruptible power supplies are:

built into the device case;

external.

The first designs are common in laptops, netbooks, tablets and the like. mobile devicespowered by a built-in battery, which is equipped with a power and load switching circuit.

Laptop battery with built-in controller is an uninterruptible power supply. Its circuit automatically protects operating equipment from power failures.

UPS external structuresdesigned for the normal completion of programs stationary computerare manufactured in a separate block.

They are connected via network adapter plug into an electrical outlet. Only those devices that are responsible for the operation of programs are powered from them:

system unit with a connected keyboard;

monitor displaying ongoing processes.

The rest of the peripheral devices: scanners, printers, speakers and other equipment are not powered by UPS. Otherwise, in the event of an emergency termination of programs, they will take on a part of the energy accumulated in the batteries.

Options for building UPS operating circuits

Computer and industrial UPS are manufactured in three main options:

power supply redundancy;

interactive scheme;

double conversion of electricity.

In the first method backup scheme, denoted by the English terms "Standby" or "Off-Line", the voltage is supplied from the network to the computer through the UPS, in which electromagnetic interference is eliminated by built-in filters. It is also installed here, the capacity of which is maintained by the charge current regulated by the controller.

When the external power supply disappears or goes beyond the established standards, the controller directs the battery energy to power the consumers. A simple inverter is connected to convert DC to AC.

UPS Standby Benefits

Uninterruptible power supplies of the Off-Line scheme have high efficiency, when the voltage is applied to them, they work quietly, generate little heat and are relatively cheap.

disadvantages

UPS Standby stand out:

long transition to battery power 4 ÷ 13 ms;

a distorted form of the output signal produced by the inverter in the form of a square wave, and not a harmonic sinusoid;

lack of voltage and frequency adjustment.

Such devices are most common on personal computers.

UPS interactive circuit

They are designated by the English term "Line-Interactive". They are performed according to the previous, but more complicated scheme by switching on a voltage stabilizer using a step-controlled autotransformer.

This provides a correction for the value of the output voltage, but they are not able to control the signal frequency.

Noise filtering in normal mode and switching to inverter power supply in case of emergencies occurs according to the UPS Standby algorithms.

The addition of a voltage stabilizer of various models with control methods made it possible to create inverters with a waveform of not only a square wave, but also a sinusoid. However, a small number of control stages based on relay switching does not allow full stabilization functions.

This is especially typical for cheap models, which, when switching to battery power, not only overestimate the frequency above the nominal, but also distort the shape of the sinusoid. Interference is introduced by the built-in transformer, in the core of which hysteresis processes occur.

In expensive models, inverters operate on semiconductor switches. UPS Line-Interactive have a faster transition to battery power than Off-Line UPS. It is provided by the operation of synchronization algorithms between the input voltage and the output signals. But at the same time, there is some underestimation of the efficiency.

Line-Interactive UPS cannot be used to power induction motors, which are massively installed throughout household appliances, including heating systems. They are used to operate devices with where the power is filtered and rectified at the same time: computers and consumer electronics.

Double conversion UPS

This UPS scheme is named after the English phrase "On-line" and operates on equipment that requires high quality power supply. It performs a double conversion of electricity, when the sinusoidal harmonics of the alternating current are constantly converted by the rectifier into a constant value, which is passed through the inverter to create a repeated sinusoid at the output.

Here the battery is permanently connected to the circuit, which eliminates the need for its commutation. This method practically eliminates the period of preparation of the uninterruptible power supply for switching.

The operation of the UPS On-line according to the battery condition can be divided into three stages:

charging stage;

waiting state;

discharge on the computer.

Charge period

The sine wave input and output circuits are interrupted by an internal UPS switch.

The battery connected to the rectifier receives charge energy until its capacity is restored to optimal values.

Ready period

After the end of the battery charging, the automatic power supply unit closes the internal switch.

The battery maintains a standby state.

Discharge period

The battery is automatically transferred to the power supply of the computer station.

Uninterruptible power supplies using the double conversion method have a lower efficiency when powered from the line than other models due to the energy consumption for generating heat and noise. But in complex designs, techniques are used to increase efficiency.

UPS On-line is able to correct not only the voltage value, but also its oscillation frequency. This distinguishes them favorably from previous models and allows them to be used to power various complex devices with asynchronous motors... However, the cost of such devices is much higher than previous models.

UPS composition

Depending on the type of working scheme, the uninterruptible power supply kit includes:

accumulators for accumulating electricity;

Ensuring maintenance of the battery performance;

inverter for sine wave formation,

process control scheme;

software.

For remote access to the device can be used the local network, and it is possible to increase the reliability of the circuit due to its redundancy.

In separate uninterruptible power supplies, the "Bypass" mode is used, when the load is powered by the filtered mains voltage without the operation of the main circuit of the device.

Part of the UPS has a “Booster” step voltage regulator, controlled by automatic equipment.

Depending on the need to carry out complex technical solutions, uninterruptible power supplies can be equipped with additional special functions.

An uninterruptible power supply is an important element in the construction of complex systems that need continuous operation and guarantee the safety of equipment from possible problems in the power grid. Now the market offers a wide variety of products of different categories of price, quality and production geography. It is difficult to decide, especially if you do not have the necessary experience behind you. Finances suggest that it is worth approaching the issue of choice with an eye to your own budget. Therefore, before investing in the purchase of an uninterruptible power supply, several important questions should be answered:

• How critical are you going to protect equipment?
• What is the optimal battery life of the equipment in the event of a power failure?

To answer the above questions, it is necessary to delve into the classes of uninterruptible power supplies on the market today. And also to determine the main criteria that must be taken into account in order to make a balanced choice.

UPS classes

All the variety of modern uninterruptible power supplies on the market today can be divided into several classes that differ from each other in schematics, as well as in behavior both in normal operation and in battery operation.

Allocate:

• Backup or (BackUp),
• Line-interactive UPS (),
• UPS with double conversion (, double-conversion).

The most simple and unpretentious are considered. During normal operation of the network, electricity enters the input of the UPS and, passing through it, is supplied to the main load. In the event of losses and voltage drops in the network, the UPS automatically switches to the battery. The main disadvantages of this scheme are that it takes between 4 and 10 milliseconds to transfer power from the UPS to batteries. When operating in battery mode, the output of the UPS is not the sine usual for the network, but an approximated sine.

An uninterruptible power supply with built-in batteries will be the right solution when, in case of voltage problems in the network, it is only important to correctly shutdown the equipment, which takes from 5 to 10 minutes.

If a longer operating time of the equipment is required, the required battery discharge current must be calculated. This can be done as follows:

From all of the above, it becomes clear that when choosing an uninterruptible power supply, it is necessary to take into account a lot of both technical and purely physical nuances, which are determined both by the specific location of the UPS and the equipment connected to it, and by a number of other factors.

To facilitate calculations when choosing a UPS, NAG has a convenient tool - with which you can determine all the necessary parameters.