Modern concept of building power supply systems.

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Power supply systems

Leonid Fedorovich Zakharov, Candidate of Technical Sciences

MODERN CONCEPT OF CONSTRUCTING POWER SUPPLY SYSTEMS.

Modern power supply systems (MPS) are designed to convert, regulate, distribute electric power and ensure uninterrupted supply of various DC and AC voltages necessary for the normal operation of communication equipment, radio engineering devices, computing systems, personal computers, protection and signaling equipment.

The power supply system may include rectifiers, batteries, uninterruptible power supply units for direct and alternating current, converters and voltage stabilizers, switching equipment and current distribution networks that connect power supply equipment and power consumers.

The power supply system may be designed according to a centralized structure, decentralized or mixed.

With a centralized structure, power sources are grouped in separate cabinets or blocks, and power is supplied to various loads through one or more power circuits from a single power supply unit (SPU).

The advantage of a centralized power supply system is the convenience of its maintenance and operation. Disadvantages: difficulty in transmitting large currents through wires, the need for redundancy of units, as well as difficulties in miniaturizing secondary power sources (SPS) included in the power supply system.

With a decentralized structure, individual loads are powered through one or more power circuits from individual power supply units.

Moreover, achievements in the development of power semiconductor converters and sealed batteries make it possible to implement decentralized systems, the EPU of which can be located in the same rooms (or racks) with the powered equipment. Bringing the EPU closer to the powered equipment improves the quality of the supply voltage, saves non-ferrous metal (required for the current distribution network), increases the efficiency and reliability of the EPU, and reduces mutual influences between different powered equipment.

A mixed power supply system contains both centralized and decentralized power supply devices. The advantage of a mixed power supply system is the ability to make the required changes to the power supply circuit with minimal time and material resources during unification of the power supply system.

For the rational construction of a power supply circuit, a technically and economically sound choice of the number, power and type of power supply devices, as well as the number of power supply circuits, is of significant importance.

One of the most important features characterizing the operation and reliability of the power supply system is the presence of a battery in its composition and the method of its operation.

According to this feature, power supply units can be divided into: buffer (with a battery connected to the powered equipment), with a battery separated from the powered equipment; dual-beam (battery-free) and combined.

Buffer power supply units (Fig. 1) have become widely used for powering communication equipment.

Fig. 1. Buffer power supply system.

The advantage of the buffer power supply system is: providing the equipment with uninterruptible power supply; providing the AB with the role of a dynamic filter; the ability to increase the system capacity due to the parallel connection of converter devices.

EPUs with a battery separated from the powered equipment are widely used in uninterruptible power supply devices (UPS) of personal computers (PCs) and computing systems.

Such UPSs are the only protection for a computer and peripheral equipment from network interference.

Depending on the operating principle, there are three types of UPS:

1. 1. UPS with off-line architecture. In network mode, the off-lineUPS powers The PC through a branch containing only an input filter (Fig. 2). At the same time, the UPS charger recharges the batteries. If the power supply is interrupted or the voltage in the network falls below a certain permissible value, the UPS switches on the power from the battery. The PC and peripheral equipment are powered by the voltage of the industrial alternating current network. The direct voltage of the battery must be converted into alternating voltage with a value corresponding to the nominal value of the network voltage. For this, a special device is used in the UPS — inverter. Among the advantages of off-line UPS, it is worth noting the simplicity of the circuit design, low cost, minimal dimensions and weight.

Fig. 2 Structural diagram of off-line UPS

1. 1. On-line architecture UPS. This type of UPS is also called a double-conversion source. In them, the input alternating voltage is converted into direct voltage using a rectifier and fed to a high-frequency (HF) converter (Fig. 3). From the output of the HF converter, the high-frequency voltage is fed to the inverter and from it to the output of the device. The need to use a HF converter is due to the fact that significant changes in the network voltage are converted into relatively small changes in the frequency of the HF signal at its output. The fact is that PC electronics are more critical to changes in the level of the supply network voltage than to its frequency. The charger and battery are connected directly to the UPS output. In addition, the design of the on-line type UPS provides galvanic isolation between the industrial network and the PC power supply. Uninterruptible power supplies of the on-line architecture are more expensive and are used when reliable and high-quality protection of vital equipment is required, often operating around the clock (network servers, medical equipment, personal computers performing especially important functions, etc.).

Fig. 3 Structural diagram of the on-line UPS

1. Hybrid architecture UPS (line interactive). These UPS are essentially an improvement on off-line UPS. In such sources, the inverter is continuously connected to the output, which ensures galvanic isolation. Such power sources can, in principle, be used to protect equipment in both categories described above. Often, the choice between on-line and line interactive UPS is determined not so much by their functional characteristics as by their price.

Based on the considered structural diagrams of UPS, small-sized uninterruptible power supplies with an intelligent control circuit are currently being implemented, capable of smoothly regulating the output voltage and perfectly isolating the load from noise, pulses and sine wave distortion.

In a two-beam (battery-free) system (Fig. 4), the power supply of individual groups of consumers of the same voltage rating is carried out directly from two independent AC networks through rectifier (stabilizing) devices.

Fig. 4. Two-beam (battery-free) power supply system

In this case, the rectifiers of each beam are loaded by no more than 50% of their rated power. And when one of the AC power sources is disconnected, the load is powered by the remaining beam.

The disadvantages of the two-beam power supply system include:

• low quality of generated electricity in transient operating modes of the power supply unit;
• the need for reliable grid power supply.

A promising direction in the field of development and creation of efficient power supply systems are combined systems, one of the variants of the structural diagram of which is presented in Fig. 7.

According to this structure (Fig. 5), for example, uninterruptible power supply units (UPS) of the world-famous company BENNING are implemented.

The operating principle of the system (Fig. 5) is as follows.

The alternating voltage of the network is supplied to a phase-controlled rectifier with a valve converter. The rectifier converts the voltage of the network into a direct voltage supplied to the inverter, and simultaneously charges the battery.

Fig. 5. Combined power supply system

The IU operating characteristic of the rectifier complies with DIN 41773.

The inverter converts the DC voltage into an AC voltage using pulse width modulation, producing an optimized sine wave. High-frequency conversion and optimal regulation in the power supply system achieve a low nonlinear distortion coefficient with low filtering costs.

This, in turn, improves the dynamic characteristics of the system when the load changes.

In case of interruptions or damage to the network, the battery connected to the input at constant voltage automatically and without delay takes over the power supply of the inverter. The degree of battery discharge is monitored. If the battery discharge exceeds the limit value, the inverter is automatically switched off.

Automatic switching of the consumer to the power grid is performed by an electronic switching unit (EUE) if, for example, the inverter overload exceeds the permissible value.

The electronic switching device EUE allows for uninterruptible switching of consumers to direct power supply from the bypass line network while maintaining permissible parameter deviations. Switching can be performed automatically using a control signal or manually. Each uninterruptible switching, automatically or manually, is possible only if the voltage, frequency and phase position of the inverter are synchronized with the bypass line parameters. Deviations in the network frequency (outside the permissible limits) cause the switching to be blocked.

Reverse switching can only be performed after the inverter fault has been eliminated. It occurs in any case without interrupting the power supply to the consumer, even if a network fault is simulated during the switching test.

The standard equipment of the UPS is a manually switched service bypass line for maintenance and repair work.

One of the most difficult development tasks is the selection (from the planned options) of a power supply system with the best technical and economic indicators.

In this case, to select the optimal option for a power supply system, it is necessary to solve three interrelated technical and economic problems:

• determining the reliability of the considered options for the power supply system (reliability requirements mainly depend on the category of consumers);
• determination of capital costs and annual operating costs corresponding to each variant of the EPS;
• assessment of consumer losses from power supply interruptions depending on the reliability of the power supply.

The expression for the reduced annual costs for each variant of the analyzed systems can be presented as follows:

,

Where: — annual capital costs of the i-th option, taking into account the standard coefficient ; — operating costs of the i-th option; — losses of the consumer of electric energy from power supply interruptions; ; — loss from the very fact of power outage; — loss per unit of power outage duration; — number of power outages per year; — total duration of power outages of the i-th variant during the year. Typically, capital expenditures and annual operating costs compared to the annual loss are inversely related. In Fig. 6 shows the dependence curves of annual costs (curve 1), losses (curve 2) and the total (+) characteristic (curve 3), as a function of power supply reliability.

Fig. 6 Dependence curves of annual present costs , losses , and () on the reliability of the power supply R.

As follows from Fig. 6, with the growth of costs, the reliability of the power supply system increases and, consequently, the annual loss from power supply interruptions decreases.

The reliability of the power supply system depends on the redundancy ratio and can be determined by a certain number of parallel-connected power supply circuits, power supply units, power supply units, switching devices, etc.

For the considered variants of power supply systems (centralized, decentralized and mixed), the equivalent circuits for reliability calculations can be presented as follows (Fig. 7).

Fig. 7 a, b, c. Equivalent circuits of centralized, decentralized and mixed PSS for reliability calculations.

The diagrams (Fig. 7 a, b, c) indicate: n — the number of elements in each PSS circuit; , — the probability of failure-free operation of the power supply system.

It should be noted that if the voltage in the power supply system network (Fig. 7 a) disappears, there may be an immediate or delayed interruption in the power supply to consumers, and in the power supply system (Fig. 7 b), if there is no voltage in one of the networks, the remaining power supply system can provide (with overload) power to all (or some) consumers.

The overall probability of failure-free operation of the backup system will be as follows:.

Where: m is the number of backup power supply circuits; p is the probability of reliable operation of each of the n elements of the power supply circuit of the SPS.

The overall probability of failure-free operation of the system is usually known when selecting a power supply system, since it is a given value.

For a mixed power supply system, the following condition must be met: .

If the probability of reliable operation of each of the n elements of the power circuit of the EPS is known (equal to p), then it is possible to find such a number (m+1) of backup power circuits, at which the overall reliability of the power supply system will be no less than the specified value .

;
;
, since

Thus, it is possible to find the required number (m+1) of backup circuits of the EPS for a given number of n-elements in each circuit and a known probability of reliable operation of all elements satisfying the condition: .

The dependence of the probability of failure-free operation of a redundant system on the average time of failure-free operation of this system can be determined by the formula: , where P(t) is the probability of reliable operation of the system as a function of time t.

Modern computing technology and electronic communication equipment implemented on low-voltage integrated circuits impose increased requirements on the quality of the supply voltage. Therefore, along with the assessment of the quantitative indicators of the power supply system equipment, it is necessary to consider the operation of the power supply system in static and dynamic modes, i.e. to assess its qualitative characteristics.

Moreover, the analysis of the construction of reliable, economical and efficient power supply systems allows us to conclude that the tasks of optimizing the power supply system are reduced to the following:

• minimizing the number of stages of electric power conversion;
• reducing power losses in individual units and in the system itself, as well as in the power distribution network;
• using new circuit solutions and energy conversion methods that improve not only the energy but also the quality characteristics of secondary power sources included in the EPS;
• using reactive power correctors;
• rationalizing the EPS and the current distribution network, leading to a reduction in material and non-productive costs;
• integration of EPS elements and units as the main way to improve the reliability of power supply systems;
• unification and functional-modular design.

A promising direction for the development and creation of reliable, economical power supply systems are EPS made on the basis of combined EPUs according to a decentralized structure with intelligent” control.