Such uncertainty and unreliability of the network, in other words, the uncertainty of its parameters, can be reduced by parallel connection of a backup source that powers the load during interruptions in the external power supply. The role of a backup source can be performed by diesel generator sets, powerful flywheels (storing kinetic energy), fuel cells, etc., but rechargeable batteries are the most widely used. The battery, in the process of discharging, releasing the chemical energy stored in it in the form of electrical energy, is able to stabilize the voltage. The discharge curve of a battery has a flat section (unlike, for example, a capacitor, whose discharge curve is exponential). And since the battery is a device designed for direct current, to match it with the alternating current network it is necessary to connect one or more rectifiers in series. The result is a buffer power circuit, the most widely used today due to its simplicity and reliability.

EPU provides constant power

An uninterruptible power supply is a set of equipment for the production or conversion and storage of electrical energy, designed to supply power to the load with the required quality from independent energy sources and ensure uninterrupted power supply when switching from one energy source to another. A special case of an uninterruptible power supply is a power supply unit, or EPU, designed to power a load with direct current. A modern EPU is a buffer power supply system without voltage regulation during the process of discharging and charging the battery: the battery is connected in parallel with the rectifiers and the load and provides power to the load during interruptions in the external power supply. This scheme is the most reliable due to its simplicity and today has no alternative.

Principles of EPU construction

Almost all EPUs have a modular design and can be equipped with a different number of rectifiers, depending on the load size, type and capacity of batteries, reliability requirements, and installation design. Power supply devices of the same power - 15 kW (48 V, 300 A) - can be composed of rectifiers of 100 A (3 - 5 pcs.), 25 A (12 - 16 pcs.) and 1.5 kW (10 – 14 pcs.), etc., have the ability to further change the power (the type of controller and distribution devices may depend on this), etc.

Basic principles for constructing a power supply installation:

• modularity(i.e., the ECU configuration is selected taking into account the power requirements of a specific load);
• scalability(the power level of the EPU is regulated by installing additional rectifiers);
• reservation, thanks to which the failure of one or even two rectifiers does not lead to failure of the electronic control unit;
• monitoring and fault diagnosis.

• low-frequency, which include diode-thyristor and thyristor rectifiers operating at the industrial network frequency;
• high-frequency, otherwise called rectifiers with transformerless input and high-frequency conversion.

Modern telecommunications equipment, including power supply equipment, is characterized by a reduction in expected service life. The reason is rapid moral aging. Even in the recent past, the average service life of equipment was 20 years and determined the time during which it was advisable to repair equipment. Now, when choosing power supply equipment, especially those located outside large switching centers, in highly developed countries they focus on 5 years. This is due to the ever-accelerating development of technology, the emergence of more efficient components, and changing operating requirements. More frequent replacement of equipment can be economically justified only if it increases its reliability, reduces operating costs and improves ease of maintenance.

Currently, the equipment offered by manufacturers makes it possible to provide power to any facility with the required quality. It is only necessary to take a responsible approach to choosing an uninterruptible power supply and consider the task of ensuring reliability throughout the entire power supply system.

EPU structure

In all operating modes of the power supply unit, the parallel connection of the rectifiers, battery and load is maintained. In normal mode, rectifiers provide power to communication equipment and keep the battery in constant recharge mode. If the AC mains voltage fails or deviates beyond acceptable limits, the operation of the rectifiers stops - the load is switched to power from the battery operating in discharge mode. When the AC voltage is restored, the rectifiers resume operation, providing power to the load and charging the battery.

Due to significant changes in the voltage at the output of the electronic control unit, the range of which is determined by the minimum permissible battery discharge voltage and the maximum voltage of its operational charge, this circuit can only be used for equipment designed for wide ranges of supply voltage changes.

If part of the load is designed for a narrow range of changes in the supply voltage or requires a voltage of a different rating for its supply, a boost converter or voltage converter can be included in the power supply installation.

EPUs often include inverters to power some consumers that require alternating current for their operation. The total power of alternating current consumers should not exceed 10% of the power of the electric control unit.

High-frequency rectifiers are the basis of the electronic control unit

Currently, high-frequency rectifiers with a transformerless input are used for new and modernization of old power supply installations, so we will consider them in more detail.

A modern rectifier can be divided into two components connected in series: a transformerless mains voltage rectifier with a power factor correction unit and a voltage converter. The power factor corrector minimizes input current distortion by ensuring constant power consumption of the external network (power factor is close to 1.0) and increases the output rectified voltage to the level of 350 - 400 V. The rectified voltage is supplied to the converter, which consists of an inverter (produces a rectangular voltage high frequency), step-down transformer and rectifier.

Based on the conversion frequency, rectifiers can be divided into groups:

30 – 50 kHz. The first rectifiers, which appeared 25–30 years ago, operated at these frequencies. The operating principle is pulse width modulation (PWM). The advantages include the high maintainability of the devices, the disadvantages include relatively low reliability (mean time between failures, MTBF, less than 100 thousand hours);

60 – 120 kHz. The operating principle is PWM. Rectifiers with an input power corrector do not introduce distortion into the supply network. Such conversion frequencies are used in most modern single-phase rectifiers;

300 – 400 kHz. The operating principle is phase resonance correction. A power corrector is installed at the input.

Currently, at frequencies less than 50 kHz, as a rule, either powerful three-phase rectifiers or, conversely, low-power, cheap rectifiers without power correctors operate. Rectifiers with a power of less than 2 kW are usually single-phase, while those with a power of more than 2 kW are three-phase. As part of the EPU, single-phase rectifiers can be connected to different phases of the power supply network, which makes it possible to increase the stability of the EPU in the event of an unreliable power supply and the possible loss of one phase.

Most modern loads are powered, as a rule, through DC/DC converters, which maintain their output parameters (voltage and current) with a high degree of accuracy regardless of changes in input voltage, i.e. the load consumes constant power. Especially for powering such a load, there are rectifiers with limited output power, the use of which in buffer power systems makes it possible to reduce the number of rectifiers in the system by 20% compared to electronic control units based on rectifiers with limited output current.

An important characteristic of rectifiers, especially for power supply installations of rural telephone exchanges, is their ability to remain operational even with significant deviations in the input mains voltage. For such conditions, you can find single-phase rectifiers that remain operational in the input voltage range from 100 to 300 V.

It should be noted that when the input voltage decreases, the current consumed from the external network increases, and this in turn requires changing the parameters of protection devices (fuses or circuit breakers) at the rectifier input, which can reduce their operating efficiency. To avoid this, equipment manufacturers limit the minimum permissible voltage at the rectifier input (at which all its parameters are maintained) to approximately 175 V. As the input voltage is further reduced, the output power of the rectifier is proportionally reduced.

EPU controller –

an important element of modern installations. In addition to monitoring the current parameters of the EPU equipment, controlling the temperature compensation of the battery charging voltage and storing in memory all changes in operating modes and equipment failures, the controller can control the sequential shutdown of secondary loads in the event of loss of external power supply and when operating on battery power, ensuring longer operation of priority consumers. Some controllers allow you not only to control the operation of the power supply installation itself, but also to monitor the entire building - from electrical equipment to the security system.

The reliability of the power system can be dramatically improved by expanding the ability to diagnose equipment faults. During the diagnostic process, a signal is transmitted remotely not about a failure of one or another element that has already occurred (for example, the rectifier is not working or the battery has turned off), but only about symptoms of malfunctions: about a violation of the operating mode of the rectifier elements (although the rectifier itself is still working) or about a change voltage distribution across battery cells.

The reliability of the buffer power circuit is determined by the reliability of the battery. This statement may seem controversial, but the failure of any rectifier should not lead to failure of the electronic control unit, since there are backup rectifiers; at the same time, battery failure, which usually occurs in the absence of external power supply, leads to a disruption in power supply to the load. Additionally, battery voltage is usually determined by the rectifiers rather than the battery itself. In this way, the rectifiers are constantly monitored, and the battery is monitored mainly during discharge. Therefore, an important function of the controller is battery monitoring, which includes:

• temperature compensation of charging voltage;
• assessment of battery charge level;
• control of battery shutdown at the end of discharge with protection against false alarms;
• charge current limitation;
• periodic battery testing.

Main functions of the electronic control unit

Forced load sharing

The nominal value of the rectifier output voltage is changed automatically by the load sharing circuit regulator (active division) or by tilting the output characteristic of the rectifier (passive division) in such a way that when several rectifiers operate in parallel, they all have similar output current values.

Switching output voltage settings

Operating mode without battery (2.06 V per 1 cell) used to power loads with narrow supply voltage limits (for example, in systems with a battery separated from the load or in systems without batteries). In addition, this mode is used when testing batteries. All parallel operating rectifiers are hard switched to 2.06 V/el. The rectifiers switch to this mode automatically, simultaneously with the start of battery testing.

Recharge mode (containment mode) (from 2.21 to 2.30 V/cell) used for normal operation of all rectifiers. The required output voltage depends on the type of battery used.

Battery charging mode (from 2.31 to 2.40 V/cell). To reduce battery charging time, all rectifiers can be switched to 2.31 – 2.40 V/cell mode. The required charging voltage depends on the type of battery used.

Temperature compensation of charging voltage. The voltage in containment mode varies inversely with the battery temperature according to the temperature coefficient. The output voltage decreases as the battery temperature increases and increases as it decreases. The temperature coefficient is set by the battery manufacturer and should be set according to the type of battery used.

Battery monitoring

Deep discharge protection is done by disconnecting the battery from the system when the voltage drops below a set threshold. For this purpose, a control unit with a powerful contactor that disconnects the load (Low Volt Disconnect - LVD) is installed in the DC circuit in series with the battery or load. The battery is disconnected when the battery and load voltages drop below the set value. The battery is connected and charged as soon as voltage appears at the output of the rectifiers.

Battery test performed when switching rectifiers to mode 2.06 V/el. They are ready to work, but do not power the load - the load receives all the current from the battery. The system remains in this state until the test control time has expired or until the battery voltage drops to the set value. After this, the rectifiers restore normal battery charging mode. The battery test can be turned on automatically (at the controller's command) or manually. Additionally, the test can be run once a battery asymmetry is detected.

Measuring asymmetry. The voltage at the midpoint of the battery is compared to half the voltage across the load. In cases where the difference between these two indicators exceeds the set value, an appropriate alarm is issued.

Battery charging current limitation. Some manufacturers include in their rectifiers a function to limit the maximum battery charging current to a value corresponding to the recommended charging current (indicated by the battery manufacturer), usually 0.1C10 (maximum 0.3C10).

The development of telecommunications equipment poses new challenges in the field of power supply for both equipment in telecommunication centers and active elements of the access network. The new generation of power supply equipment is designed to meet these challenges.

However, we must not forget that uninterruptible power supplies are only part of a single system, which includes grounding, current distribution network, protection, automation and switching devices in AC and DC circuits, filters, remote control systems, process equipment, i.e. everything that is called the power supply system - PDS. The reliability of the PDS depends not only on the type of equipment used, but also on the competent construction and qualified maintenance of the entire power supply system.

Uninterruptible power supplies.

General information about uninterruptible power supplies

In complex systems of guaranteed power supply, in addition to diesel power plants, so-called uninterruptible power supplies (UPS) of alternating and direct current are also used as sources of energy.

UPSs are designed to supply power to a load, the operation of which, firstly, is possible only with high quality and “purity” of the electrical energy, and, secondly, does not allow interruption of the supply electric current. This load is called critical. This mainly includes devices with complex electronic equipment.

Any UPS includes (as a mandatory component) a rechargeable battery (AB) - an autonomous source of direct current energy.

UPSs are classified as secondary sources of energy. In general, they convert EE from an external power supply with a relatively low quality (or battery EE) into high quality EE or another type of current.

The UPS is characterized by the following two operating modes:

· normal- the load is powered by mains energy, and the battery is in recharging mode;

· emergency

AC UPS.

UPS of this type are designed to supply AC power to consumers (loads). In our opinion, UPS manufactured by the American company Liebert are the most preferable for use in KSGEPP. This company is generally recognized as a world leader in the production of UPSs made using the most advanced On-Line technology, which allows you to completely protect the critical load along the power circuit from any malfunctions in the external power supply network.

Liebert UPS power ranges operating in On-Line mode: from 700 VA to 800 kVA.

On-line - This is a technology or scheme for constructing a UPS, characterized by the presence of double conversion of the input voltage and a constantly operating inverter.

IN normal mode operation, the input alternating voltage (Uin) of the external power supply is rectified (Fig. 25), and then, using an inverter, it is again converted into alternating voltage (Uout), supplied to the load.

Rice. 25. UPS according to the ON-Line scheme)

When the input voltage disappears the inverter, permanently connected to the battery, instantly switches to power from it, continuing to supply the load with alternating current without breaking the output voltage sinusoid, without distorting its shape and in the absence of any transient (switching) processes.

Double conversion of the input voltage in the On-Line system completely protects the UPS output (and therefore its load) from any interference from the input side - the power supply network. This is well illustrated by the table above (see last column).

All On-Line UPSs are equipped with a bypass - a device that bypasses the double voltage conversion circuit and supplies the load directly with the external power supply voltage (in some UPSs, through a filter). A distinction is made between automatic and manual switching to bypass mode. Auto- triggers when the UPS is overloaded or there is a malfunction in its components, manual transition Bypass mode is used for maintenance.

The presence of a bypass mode expands the functionality of the UPS and increases the reliability of the power supply to the load.

All UPS manufactured by Liebert are based on semiconductor electronics.

Rice. 26. AP 4300 series UPS power circuit diagram

In Fig. As an example, Fig. 26 shows a diagram of the power circuit of a UPS of the AR 4300 series, with a power of 10 kVA. Three-phase voltage (380 V, 50 Hz) of the external power supply is supplied to a low-frequency (50 Hz) 3-phase rectifier 1. The rectified and smoothed voltage is converted by the auxiliary inverter 2 into rectangular alternating voltage with a frequency of 17 kHz. The latter, through transformer Tr, is supplied to a single-phase high-frequency rectifier 3, to the output of which a 3-phase inverter 4 is connected. With its help, the strictly constant voltage at the output of rectifier 3 is converted into a three-phase alternating voltage with a frequency of 50 Hz, which powers the critical load. Between all devices (1, 2, 3, 4) and at the output there are inductive-capacitive filters (not shown in Fig. 26).

The transistors of both inverters are controlled using special master oscillators (not shown in the diagram). Moreover, the transistors of the output three-phase inverter are controlled using a PWM (pulse width modulator), which ensures an almost purely sinusoidal output voltage. The stability of the magnitude of this voltage and its frequency is achieved due to the high stability of the frequency of the control signals of the master oscillators. The diagram shows where the battery is connected to the power circuit. The charger that recharges the battery during normal operation of the UPS is not shown in the diagram.

Additional features of the Liebert-Hiross On-Line UPS

Microprocessor control and diagnostics.

Protection of the load from static discharges, atmospheric electricity discharges and impulse noise in accordance with the EN 500 82-1 standard.

Protection of the load from high-frequency harmonics and transients from the external power supply in accordance with the EN 55022-A standard.

Possibility of normal operation in a wide range of changes in output voltage without switching to battery power due to the presence of a smooth voltage stabilizer at the UPS input (for example, in the UPS model AR 4300 this range is from -18% to +26% of a voltage of 380 V).

The input/output of the UPS is galvanically isolated using an isolating transformer in the main circuit (see Fig. 26 - transformer Tr).

Using advanced technologies to extend battery life, namely: temperature compensation technology for battery charging current and virtual battery technology.

Possibility of increasing battery life by connecting external battery packs. For example, a UPS model AR 4300 with a built-in battery can operate autonomously for 12 ... 19 minutes, and with external batteries - up to 3.5 - 4.5 hours.

Increased overload capacity, high reliability. So the UPS model AR 4300 can operate with loads: 110% - 1 hour, 125% - 10 minutes, 150% - 1 minute, 200% - 5 s.

Advanced communication capabilities, reliable software.

Low level of acoustic noise (for AR 4300 - less than 56 ... 58 dB).

Other Types of AC UPS

For local (separate) use of computers and related auxiliary electronic devices, low-power UPSs are used. They are carried out both according to the On-Line scheme and other simplified schemes, namely: Off-Line and Line-Interactive schemes.

Rice. 27. Off-Line UPS

Off-Line UPS also includes an inverter (Fig. 27). However, the latter comes into operation from the battery only when the external power supply voltage disappears. A special feature of the Off-Line circuit is the presence of an automatic switch that switches the load power circuit.

Rice. 28. UPS scheme Line Interactive

The advantages of the circuit are simplicity and efficiency: the disadvantage is that there is no stabilization of the input voltage when operating in normal mode and the relative length (- 4 ms) of the process of switching to emergency mode (from the battery).

UPS type Line-Interactive the circuit is similar to Off-Line UPS. The only difference is that at the input there is a step stabilizer (booster), built on the basis of an autotransformer (Fig. 28). As a result, the UPS is able to withstand long-term deep “sags” of the input mains voltage without switching to the battery. The advantages and disadvantages are the same as the Off-Line scheme.

UPSs of the Off-Line and Line-Interactive types are used, as a rule, to power personal computers, small network nodes, workstations and peripheral devices.

DC UPS.

UPS of this type are designed to supply DC power to consumers (loads). Therefore, they are also called DC power systems.

DC UPSs (DC power supply systems) are used in telecommunications and telemetry technology, in mobile telephone systems, radio and space communications, in medical equipment, alarm and protection devices, etc.

Currently, one of the competitive manufacturers and suppliers of DC UPS is the Norwegian company Tellus Emi AS. The products of this company satisfy the most stringent requirements currently imposed on power electrical equipment.

In general, the required structural elements of a DC UPS are: a rectifier, a rechargeable battery (AB) and a device for controlling and distributing energy.

Rice. 29. Block diagram of DC UPS

UPS rectifiers from Tellus Emi AS are equipped (Fig. 29) with individual unified rectifier modules (URM), each of which has its own control and monitoring device. Several such modules, connected in parallel in the N + 1 configuration, connected to their output AB and the load distribution module MRN, together with the general control and monitoring module MUK, form a complete DC power supply system - DC UPS.

In normal operation, i.e. When the AC power supply is in good working order, the UPS rectifier unit supplies power to DC consumers and recharges the battery. When the voltage in the power supply network disappears, uninterrupted power supply to consumers continues due to the battery without any interruption of the load current.

The modular design principle of the UPS allows, by selecting various combinations of modules and batteries, to implement a power supply system with any output parameters and characteristics.

Tellus Emi AS produces DC UPS in two versions:

· In the form of racks (cabinets) with power from 500 W to hundreds of kW with built-in batteries (SKB series);

· In a wall-mounted version with power from 500 W to 4.5 kW with external batteries (ASL series).

Features of these UPS:

Output voltage: 12, 24, 36, 48, 60, 110 V (SKB); 12, 24, 36, 48 V (ESL).

Load current: 10...1000 A (SKB); 6 ... 60 V (ESL).

The stability of the output voltage in static and dynamic modes is + 0.5%.

Voltage ripple - less than 50 mV. Efficiency - more than 88...90%.

The performance of the UPS is maintained in the temperature range - 20 ... + 45 o C.

The UPSs are controlled and monitored by a microprocessor module.

Possibility of expanding all elements of the UPS structure by connecting other units in parallel to increase the charging current of the battery, the number of outputs for the load, and the capacity of the battery.

Below is the appearance of the rack-mount and wall-mounted DC UPS options from Tellus Emi AS.