9-5.3 Rectifier—Battery—Static Inverter Systems (a) Basic Continuous-Inverter System. The basic continuous-inverter system (Fig 9.3) consists of a static rectifier, battery charger, battery, and static inverter The inverter carries the a-c load at all times Under normal

 

A-C LOAD

 

The following various combinations of the basic power — supply building blocks are representative of the power — supply systems in use. In any system different possibilities exist for improving one characteristic at the expense of others. However, the vast majority of practical systems incorporate the principles and characteristics represented by the power supplies described in this section. 9-5.1 Simple A-C/D-C System The basic a-с/d-c system (Fig. 9.1) consists of a single voltage-regulating and harmonic-filtered stepdown trans­former and a static rectifier This system is usually fed from the facility essential auxiliary-power buses and, of course, is subject to interruptions in the order of 15 sec before the large standby diesel generators can reestablish power to the essential buses At best the simple a-c/d-c system provides filtering and voltage regulation for noncrmcal instrumenta­tion loads. When the system is required to supply only d-c power, the regulating transformer is usually incorporated within the static rectifier

 

circumstances the charger provides the d-c load plus the input to the inverter and floats the battery or recharges it as required. The charger in turn is fed from the plant auxiliary source of a-c power. If the plant auxiliary power fails, the inverter continues to run on the battery for a period of time dependent upon the reserve battery capacity. In this mode of operation, as long as the charger is functioning properly, there is no drain on the battery. Input current for the inverter is derived from the charger and does not pass through the battery. If the a-c line fails, the inverter drain is transferred to the battery without any interruption or disturbance in the inverter output Thus this simple system functions as a complete no-break backup power source Some capacitance is generally required in the input circuit of the inverter, and this provides a small amount of energy that, because there is a fast voltage regulator in the inverter, enables the system to handle the input transient from float voltage on the battery to discharge voltage without a significant transient in the output Loss of voltage on the input of the rectifier has no effect on the inverter output The advantages of this system are offset by the complete loss of power if the inverter fails. This disadvantage can be eliminated by using redundant equipment (see Fig 9 7) (b) Continuous-Inverter System with Direct A-C Feed. The inverter system shown in Fig 9 4 is a more sophisticated version of the basic system shown in Fig 9 3 The normal a-c line feeds into the inverter directly at all times and is rectified. The resulting value of the d-c voltage is compared to the d-c voltage from the battery Solid-state circuitry within the inverter determines the highest d-c source, battery or rectified a-c line, and then inverts it to supply the a-c load. This is commonly referred to as

 

D-C LOAD

 

A-C INPUT

 

A-C LOAD

 

Fig 9.1—Simple a-c/d-c system

 

9-5.2 Rectifier—Battery System The basic system (Fig. 9.2) consists of a static rectifier, a battery charger, and a battery. The normal d-c load is supplied from the rectifier. If the a-c line fails, the load is automatically transferred to the battery without interrup­tion. Although the system is highly reliable, the capacity of the batteries limits the length of possible emergency operation. Of course, the system can only serve d-c loads

 

Fig. 9.4—Continuous-inverter system with direct a-c feed.

auctioneering. The internal transfer between the two sources of d-c power occurs instantaneously and is virtually undetectable. In this way the a-c load never sees the incoming a-c line, only the a-c output from the static inverter. Line synchronization problems, which are often a cause of difficulty (when external switching methods are used to effect the transfer between the inverter output and plant auxiliary a-c line), are avoided. Reliability is enhanced because there are no switching operations to cause transient voltages, and fewer critical components are needed The same result can be achieved by bringing the a-c line to the inverter through the battery charger (see Eig. 9.3), but then the battery charger must be large enough to handle the entire a-c load in addition to the d-c load and battery loads. This would necessarily increase system cost

Another inherent advantage of the system where the a-c line is fed into the inverter directly is built-in stabilization of line frequency and voltage Since the incoming a-c supply is rectified at once, input frequency is of little concern. The output oscillator of the inverter can have frequency stability to almost any accuracy desired, being only a function of inverter design The typical 60-Hz system maintains frequency to a tolerance of ±1.0%. The addition of an oscillator standard in the inverter can reduce the variation m output frequency to ±0.01%. The input frequency does not necessarily have to define the inverter output frequency, and therefore the same power supply can be used for frequency conversion. Similarly, phase changing can also be accomplished since the number of phases in does not determine the number of phases out

(c) Continuous-Inverter System with Electromechanical Transfer Switch. In certain applications it is desirable to feed the a-c load from a source other than the inverter This can be accomplished by switching from inverter to line (see Fig 9.5) or switching from line to inverter The two methods differ in the length of interruption of a-c power to the load and should be used where short-term interruptions can be tolerated. Conventional transfer switches are used and are generally supplied as electrically held, mechanically interlocked contactors. In the commoner mode of opera­tion, the inverter is considered as the normal source The inverter carries the a-c load until it is manually transferred by an operator or until an inverter failure occurs The chief advantage of this arrangement is that the inverter failure rate is substantially lower than that of the plant auxiliary- power source. Thus transfers occur substantially less often than they would if the a-c line were considered the normal source. Since the inverter is operating continuously, there is assurance that both sources are available as long as the a-c line is present

A disadvantage of the inverter-to-line switching mode of operation is that the charger must have sufficient capacity not only to feed any d-c loads and recharge the battery but also to supply the input current to the inverter. In the alternate arrangement, where the plant auxiliary a-c line is considered the normal source and transfers are made to the inverter on-line failure, the charger capacity need only be sufficiently greater to recharge the battery and feed any d-c loads

In most transfer arrangements of this type, a delay is provided to prevent the emergency from transferring to the normal source after an outage until the normal source has been present for some period of time This time interval ranges from a few seconds to as much as several minutes, depending on the application

With standard contactors operated in a conventional manner, outage times on transfer are of the order of 0 1 sec. The outage time depends primarily on the time for failure sensing and transfer of the contactor In switching from line to inverter, however, an additional period of reduced output voltage is caused by the response of the inverter to a step-load change. For most commercially available inverters, the time to respond to a full-load step change is less than 0.04 sec Because of the relatively long transfer time, it is generally not necessary for the inverter to be synchronized in phase with the a-c power line Frequency synchronization may be desirable where the inverter carries the load continuously to keep certain classes of loads, such as clocks and chart drives, in step with local time.

In certain applications the transfer from normal to emergency a-c power source need not be automatic By substituting a make-before-break manual transfer switch for the automatic transfer switch indicated in Fig 9.5, the load can be successfully removed from the inverter without loss of potential The manual make-before-break transfer is an inexpensive concept of switching to the plant auxiliary source of power while still maintaining the continuity of power flow to the load. This system suffers the same problems as the system shown m Fig 9.5 The load experiences complete loss of power should the inverter fail (One could, however, manually transfer to the plant auxiliary a-c source after detection of the inverter failure )

(d) Continuous-Inverter System with High-Speed Trans­fer Switch. In certain cases extremely sensitive a-c in­strumentation and control systems cannot tolerate the finite outage time given by the transfer arrangement of Fig. 9.5. The system shown in Fig. 9.6 uses special transfer- switch drive circuitry that allows transfer to be effected in less than 1 cycle, or 0.016 sec, in a standard 60-Hz system. Depending on the sensitivity of the sensing circuitry and

the size of the transfer switch, transfer times below 0.008 sec can be obtained. In all short-time transfer schemes, special attention must be given to line-phase synchroniza­tion and transformer saturations.

Because of the short transfer time, the inverter must be operated in phase synchronism with the commercial power line. It must also be recognized that the transfer time from line to inverter is increased by the load response time of the inverter. For these reasons it is generally preferable to use the inverter as the normal source of power and transfer to the line only if an inverter fails. Retransfer, after correction of the inverter failure, can be made either by allowing for the increased transfer time or by providing an auxiliary manual make-before-break switch which momentarily parallels the inverter output with the plant auxiliary-power system on retransfer.

To achieve a high-speed transfer, the sensing circuit must detect departure of the source from its standard value in periods as short as a millisecond. Most plant auxiliary — power systems experience short-term transients. It is extremely difficult to design transfer-sensing circuits that avoid transferring on a momentary line transient that is not necessarily followed by a complete line failure. The inverter output is relatively free of such spikes unless they are generated by the load, and therefore the likelihood of false transfer is avoided when operating with the inverter as the normal source. However, most inverters have a current — limited output characteristic; so any overload exceeding the output capability of the inverter is regarded as a failure and causes transfer to the plant auxiliary-power system.

In several available commercial systems of the type shown in Fig. 9.6, the high-speed electromechanical transfer switch is replaced by a solid-state silicon-controlled rectifier (SCR) a-c switch. The advantages gained include a decrease in transfer time down to 0.002 sec or less and, since the switch has no moving parts, a virtual elimination of maintenance requirements. A disadvantage is that the static transfer switch does not provide the same degree of positive isolation from the plant auxiliary system as does the mechanical transfer switch. Therefore, in a situation where a plant auxiliary-power-system failure is accompanied by a high-voltage transient, the static switch could fail and cause a complete system breakdown involving both a portion of the plant auxiliary power and the inverter. An important consideration when assessing the relative merits of a static vs. electromechanical transfer switch is that the inverter response time must be added to the transfer time when transferring from line to inverter.

The system shown in Fig. 9.6 uses the stored energy of the output transformer to overcome the relatively slow response of the inverter and provides improved transfer­time characteristics. By transferring on the primary of the ferroresonant output transformer with a high-speed transfer switch, one can achieve transfer times in the range of 0.008 to 0.016 sec. Because of the stored energy in the output transformer, there is no interruption of supply to the load during the transfer period. As previously mentioned, the inverter must be operated in phase synchronism with the plant auxiliary-power system to effect the desired uninter­rupted transfer.

The system indicated in Fig. 9.6 requires the inverter to be the normal source of power. Several advantages result from this mode of operation. The frequency of transfer during operation is substantially reduced since the inverter output is comparatively clean. During plant-auxiliary — power-system fault conditions, when transfers from the line normally take place, the plant auxiliary power is charac­terized by erratic phase shifts and voltage excursions, and, if the transfer from source to inverter is to be successful at these times, the inverter must be maintained in phase with the faulted line. It is questionable if the inverter could respond properly to this mode of operation. Either the transfer would be made out of phase or the inverter would malfunction. In addition, the voltage transients charac­terized by the failing a-c line may make clearing of the transfer-switch contacts difficult. For example, a suf­ficiently large voltage transient on the line prior to switching could cause an arc to persist in the opening contact of the transfer switch for more than 0.008 sec. Since this would, in essence, short the inverter output to the failing line, an inverter malfunction would be likely. Using the inverter as the normal source avoids these undesirable possibilities.

(e) Continuous-Inverter System with Redundant In­verter and Transfer Switch. In the systems shown in Figs. 9.5 and 9.6, the switching is from the inverter back to the commercial a-c line to provide a backup source of power. The system shown in Fig. 9.7 represents a significant

improvement with respect to isolation from the plant auxiliary a-c line. During normal operation both of the inverters are operated in parallel and are sized so that either

could carry the entire a-c load. The two inverters are

connected through normally closed transfer switches to the common a-c load. The logic and synchronizing circuits ensure that under all circumstances the inverters are

operating in phase synchronism with each other and with the commercial a-c line if required. As long as both units are operating in phase with each other, the load is shared. Should either inverter fail, by either a reduction in output voltage or a shift in phase, the logic circuit disconnects the faulty inverter from the system, thereby transferring the entire load to the remaining inverter. The transfer switch

need not operate with extreme rapidity since the surviving inverter can drive the output transformer of the failing inverter without adverse effects. The failure of either inverter would cause some load disturbance attributable to

the response of the surviving inverter to the 50% step-load increase (assuming each inverter to be normally 50% loaded). The overall output performance of this system is, therefore, essentially identical to that of the system represented by Fig. 9.6. The principal advantage is that both sources of power can be considered extremely reliable. Since there is no connection to the commercial power line except that provided by the charger, the possibility of introducing large voltage transients from external sources is minimal.

It is important to realize there is a substantial increase in cost as the complexity of the transfer circuits is increased. The transfer-circuit complexity increases as the permissible load-interruption time decreases. Usually the power source for the instrumentation and control system in a nuclear reactor facility is only one part of a larger system. In fact, because redundancy is necessary in the instrumenta­tion and control system, several separate isolated power systems may be required. Therefore, in overall operation an inverter failure may be indistinguishable from a failure in a portion of the instrumentation and control system that the inverter feeds. In such circumstances justification of an elaborate (and expensive) power supply must take into account that the additional cost might better be used to improve some part of the system other than the power source.