AC UNINTERRUPTIBLE POWER SUPPLY
CHAPTER 5 CHAPTER 5 INTRODUCTION
Some services such as hospitals and high priority computers provide vital functions where no disruption of power is acceptable.
Such systems usually rely on Uninterruptible Power Supplies (UPS) to ensure a continuous flow of power. To provide such service, these supplies incorporate some means of energy storage. The stored energy supports the load at times of main line power failure. The amount of the stored energy depends on the load and the particular application.
Sometimes the supply must power the load just long enough to allow an orderly shutoff of t~e different stages of the load (normally in small computer systems). On the other hand, there are cases where the stored energy must be large enough to power the load during the time required to start and warm up an auxiliary diesel engine which, with an
associated generator, will take over the task of energizing the load.
When the power line recovers, by some preset procedure various parts of the load return to normal operation while the drained storage medium is recharged from the line.
Figure 5.1 demonstrates a typical ac uninterruptible power supply in which the storage medium consists of a bank of batteries. In normal line condition, the batteries are charged from the line by the battery charger, and the batteries in turn power an inverter which supplies the
load with ac power of suitable voltage and frequency. In case of a power failure, the charging current to the batteries is removed and the energy stored in the batteries keeps the inverter and the load running.
The switches are for repair and are applicable in cases where the input and output voltages and frequencies are the same. The switches are normally open; in case of a failure in the UPS, the switches are closed to effectively remove the system from the line. In this condition, the load is of course unprotected.
So far in this work, only de-to-de conversion has been examined.
However, the field of power electronics covers a much broader range:
dc-to-ac, ac-to-dc, and finally ac-to-ac conversion. More specifically, these techniques are treated here as extensions of de-to-de conversion methods.
The first chapter of this part generalizes de-to-de converters in the "quadrant" sense, and various ways of generation of ac from de
sources are mentioned. The selected result is a four-quadrant converter hereafter referred to as an "amplifier", capable of delivering bipolar voltages and current to the load. It can well satisfy the requirements of the inverter part of the UPS of Fig. 5.1. Furthermore, the output voltage of a switched-mode amplifier is controllable, and by proper feedback, low distortion ac can be generated. However, the task of the battery charger is left unaltered. The battery charger is usually composed of a set of rectifier diodes or silicon controlled rectifiers
(SCRs)_ and some large reactive elements to control the current. The combination, owing to the nonlinear nature of the devices, draws
LINE
Fig. 5.1
BATTERY BANK
BATTERY CHARGER
>
POWER FLOW
Fig .. 5. 2
BATTERY BANK
SWITCHING AMPLIFIER
control
CONTROL
POWER FLOW
INVERTER
>
POWER FLOW
LINE
AUVU1a.Uve. UPS.
LOAD
LOAD
non-sinusoidal currents. Correction for this nonunity-power-factor load on the line requires low frequency filtering, which tends to be heavy, bulky, and expensive.
A more elegant and efficient solution is to use a unity power factor ac-to-dc battery charger. One implementation uses the above- described amplifier operating in the reverse direction: the switching amplifier, being a four-quadrant converter, is capable of processing power in both directions. In 11forward11 dc-to-ac operation, this permits reactive loads to be used where, in some part of the ac cycle, the
voltage and current are out of phase and power is returned back to the de source. However, if the load is replaced by an ac voltage source, the current waveforms become distorted due to lack of a proper control scheme. Such a scheme is based on current control to interface the two ac and de voltage sources. Furthermore, with such control on current, it can be regulated to be proportional to the voltage, so it is
sinusoidal and in phase with the voltage. Therefore, the amplifier functions as a unity power factor battery charger.
An alternative uninterruptible power supply is introduced here.
The scheme is demonstrated in Fig. 5.2, which relies on the four- quadrant capability of the amplifier to permit bidirectional power processing. Notice that in Fig. 5.1 each unit processes power only in one direction, while in the UPS of Fig. 5.2 the switching amplifier is fully utilized to process power in both directions, since at any moment only one of the two functions is required. The system works as follows:
when the line is up, the switches - controlled by the control circuit -
are closed. The load is then powered by the line. The control circuitry causes the amplifier to be a unity power factor battery charger, by
constraining the ac input current to be of the same shape as and in phase with the ac input voltage. The battery bank is thus charged from the line, and the level of power flow depends on the amplitude of the current only since its shape and phase are predetermined. For a
minimal charging time, the current is held at a maximum value until the batteries reach a preset voltage level, and operation is then switched to a low current level.
In case of a power failure, the control circuit immediately opens the switches while the amplifier converts to an inverter. The load is then energized from the battery bank to maintain a continuous operation.
When the line power is restored, the control circuitry matches the phase and amplitude of the inverter to those of the line. The switches are then closed and the amplifier is converted back to a unity power factor battery charger to replenish the charge withdrawn, and the cycle repeats.
This alternative UPS reduces the number of power stages of the conventional UPS to one unit, since at any moment only one function is required. This reduction makes the new UPS simple, more reliable, and more economical. Furthermore, when the line is up, its unity power factor operation results in a more efficient line operation than does the ordinary UPS. However, there are some disadvantages associated with this technique: the load must have the same voltage and frequency ratings as the line, and the switches between the load and the line must respond quickly in case of power failure. It was mentioned that
after restoration of power, the battery bank will be charged at maximum level to get to the fully charged state, after which the operation is switched to a low-current mode. In the worst case of occurrence of a double failure at the voltage peak, it is required that the current going into the amplifier change sign immediately.
Owing to the presence of reactive elements in the amplifier, an instantaneous reversal of current is not possible and will result in a "dip'l in the voltage waveform. Nonetheless, the size and duration of this transition can be minimized by proper design. Also, the load is usually capable of recovering.
Chapter 6 extends the area of attention to ac generation. Two methods, the synthesis and the PWM methods, are reviewed. The synthesis method is primarily used in large power systems and the PWM method is more suitable for small to medium sized systems. Then, de converters are classified according to their quadrant capability. De-to-de
converters, owing to the limitations set by the semiconductor switches, are usually capable of delivering only one polarity of voltage and one polarity of current~ namely, one quadrant. By proper modification of the switch, current-bidirectionality and two-quadrant operation is obtained. Two such current-bidirectional de converters may be used to provide bipolar currents and voltages. Thus, a true four-quadrant converter is obtained to permit generation of ac power from a single de source.
Chapter 7 treats the dc-to-ac operation of push-pull power
amplifiers. Owing to continuous large signal variation of the states
of the amplifier, the design techniques and criteria are different from those of conventional de converters - constituent parts of the amplifier. Control of the output with feedback loop(s) requires
careful attention to continuous change of dynamics due to large-signal modulation. The push-pull method is extensively examined for de and ac characteristics, and the results are experimentally verified on an amplifier utilizing two Cuk converters. A feedback algorithm is designed and the predicted and measured responses are compared.
The reverse mode of operation, ac-to-dc operation, is the subject of Chapter 8. Here the roles of input and output are also reversed
sue~ that input to the system is the ac source (line) while the de source (battery) becomes the output. Unlike the dc-to-ac operation, this mode is forced upon the amplifier and application of feedback is mandatory. This is because the input impedance of the amplifier must be shaped to look ~e~L6Ltve at low frequencies. Also, the input is
connected to the line which completely changes the dynamics of the system. The control is performed on current, whose waveshape determines the quality of the charger. Again, the result of calculations on the fuk amplifier with multiple-loop feedback is compared to measurement data.
The same four-quadrant switching amplifier is used for both dc-to- ac inverter or ac-to-dc charger modes. Chapter 9 examines the process of switching between the two modes. In this region, the system may exercise a large variation of current while the voltages remain almost constant. The duty ratio saturates and the system behaves as an
open-loop converter. Finally the results are checked on the Cuk UPS.