IEE Transactions on Power Apparatus and Systems, Vol. PAS-95, no. 4, JulyIAugust 1976
STANDBY AND EMERGENCY POWER SUPPLY OF GERMAN NUCLEAR POWER PLANTS Alexander Borst
Kraftwerk Union AG Erlangen, West Germany
In ensuring nuclear safety by the utmost reliable supply of power to all safety related systems under all emergency conditions, U.S. power plants rely heavily on redundant offsite power sources. For years German plant designers have been and are still placing a greater emphasis on onsite power sources in the form of highly redundant sophisticated Diesel generating sets. This difference in fundamental design philosophy and the concepts underlying the Diesel power systems incorporated into German nuclear stations are diSCUSSed.
It has long been realized in Germany that it is either impossible or economically not feasible at most locations, where large nuclear power plants would be permitted in the country, to provide two adequate power line connections from truly independent electrical systems. Two incoming feeders from a common grid, especially if they are at the same voltage level and coupled at the station bus, are considered prone to simultaneous failures even if they are physically segregated from one another. They are equally vulnerable to electrical system faults and all external disturbances, especially lightning, storms and earthquakes.
Historically, Diesel generators have wide usage in Germany wherever emergency power is required, e.g. hospitals, airports, large office buildings, department stores, etc. Consequently, many highly reliable designs have matured over the years. Diesel standby power systems can be provided and maintained in nuclear power plants with almost any degee of operating availability that is required and usu- ally at far less cost than duplicate transmission circuits. They can be designed to render them impervious to the many external distur- bances that can jeopardize transmission lines, to suit the emergency cooling requirements of any type of reactor and to provide the de- sired operating redundancy.
In recognition of the fact that a reliable second offsite source of ample power which is adequately independent of the main power system, both physically and electrically, is not always available, the German standards also allow the concept of installing circuit break- ers at the generator terminals. Regardless of whether the combina- tion of low-voltage generator breakers and only one external feeder or no low-voltage generator breakers and two external feeders is selected, the Diesel power system must be designed in the same way as an independent and reliable standby power supply with a redun- dancy supplementing that of the multi-train reactor emergency cool- ing system.
Paper C 75 1284, recommended and approved by the IEEE Power Generation Cormnittee of the IEEE Power Engjneering Society for presentation at the IEEE PES Winter Meeting, New York, N.Y., January 26-31, 1975. This paper was u p graded to transactions status, PC 75 652-8, for presentation by title for written dis- cussion at the IEEE 1975 Joint Power Generation Conference. Manuscript sub- mitted September 23,1974;made available for printing July 28,1975.
COMPARISON OF POWER CIRCUITRY IN U.S. AND GERMAN NUCLEAR STATIONS
Figure 1 illustrates a typical electrical power flow design for a U.S. nuclear power plant connected to the power system by two transmission circuits (T1 and T2). The offsite power system consists of the connections from the standby bus through the unit auxiliary and starting transformers up to the switchyard (Circuits C1 and C2). The full lines indicate the permanently connected supply through the starting transformer, the so-called preferred power system. The
t t 1 SWITCHYARD 51
tl I C2
I A - I NORMAL 1
Eg. 1. Power circuit design for nuclear station in accordance with AEC General Design Criterion 17.
unit auxiliary transformer provides an alternate source of power supply via the broken lines to the standby bus, to which the Diesel generators are also connected.2
In contrast, German practice is to supply the standby bus from the unit auxiliary transformer through the bus for normal unit auxil- iary services (Figure 2). The operating agreements by European util- ities and the measures adopted within each generating unit to handle serious system disturbances, by disconnecting power plants once the system frequency falls below a certain value, render the supply from the unit itself even more reliable than from the system. Furthermore, the voltage of the alternate offsite power connection must be dif- ferent from that of the main power line. V1 is typically 380 kV and V2 either 110 or 220 kV.
I uhu T
T T T T I t !---I--* I I
4 STANDBY 6
0 I I
Fig. 2. Typical power circuit design for German nuclear station with two external feedem
Figure 3 is an electrical power flow schematic that has been adopted in a number of German nuclear installations since originally permitted for the 1200 MW/l500 MVA Bibb A unit.3 In place of the isolating links commonly provided in U.S. power plants between the generator and its transformer (Figure 1) to establish a second power system feeder to the standby bus within six hours of a pro- longed unit outage, circuit breakers are installed and integrated into the phaseisolated generator bus ducts. In addition, the Diesel power
system is divided into four strictly segregated highly redundant trains. Comparative reliability analyses reveal that this design concept is equivalent to that shown in Figure 2.
U L J
+ +STAND BY+ +
Fig. 3. Dpical power circuit design for German nuclear station with a single external feeder.
BASIC DESIGN CONCEPT FOR STANDBY POWER SYSTEMS
Figure 4 depicts the overall layout of the Biblis A unit which has gained increasing acceptance with subsequently designed German nuclear stations. The unit auxiliaries and emergency services are supplied from normal and standby 10 kV busses that are each di- vided into four independent sections. This ensures that the redun- dancy built into the mechanical systems associated with a four-loop reactor and four trains of emergency cooling equipment, is fully preserved at the electrical power supply end. Each of the four stand- by bus sections has its own Diesel generator.
The segregation of the 10 kV busses is carried through to the 380V level. Just as mathematical analyses show that the availability of redundant and physically independent mechanical equipment is greatly reduced when an equal number of segregated electrical power supplies is not provided, so also is it only marginally enhanced when a greater number of power supplies is made available. Consequently, duplicate drives should only be connected to any two, not to all four standby bus sections. This also is advantageous in minimizing the number of bus couplers and cross connections, reducing the pos- sibility of common faults and limiting the load that can be imposed under any circumstances on each Diesel set. Consequently, the extra 380V standby busses marked A in Figure 4, which are reserved for motor drives that are provided in sets of only two or three, and the associated bus couplers are no longer employed.
Y 2-725 MVA A 27/420kV+ll%
2-60138138 MVA 27?5%/10.5 kV
Rg. 4. Electridschematic of the Bibb Aunit, showing the arrangement of the 380 kV, 27 kV, I O kVand 380 V busses.
Design of Diesel Engines
As unit capability rises and safety requirements are increased, the standby power needed also becomes greater. Dividing the emer- gency cooling and aftercooling services into separate trains, possesses the advantages of reducing the ratin@ of the individual motor drives and of limiting the standby power requirements. Each of the four 5WAuty reactor cooling trains associated with a 1300 MWe FWR unit like those in the Biblis nuclear station, constitutes a load that can be met by a high-speed ldcylinder Diesel engine with a con- tinuous rating of about 3,000 kW. Some European manufacturers have worked hard at extending this capability range and are now able to offer 20cylinder engines rated up to 3,600 kW.
The large induction motors rated more than 5 0 0 kW driving the pumps associated with the emergency cooling and aftercooling sys- tems, have to be started and must overcome an approx. 200% pull- out torque. High-speed turbocharged Diesel engines are preferred due to their favorable power/weight ratio, excellent regulation character- istics and short loading times. However, they possess a relatively small flywheel mass (Wr2) and only develop about half their rated power at the moment of initial loading. An obvious fmt step in solving this problem is to divide the total load into several groups of motors that are switched onto the standby bus in sequence.
Whether flywheels should be added to the Diesel engines so as to flatten their inherently drooping loadlspeed characteristics, is less obvious. Experience shows increasingly clearer that the criteria of a practically constant frequency in the standby power system should not be applied to the Diesel engines. By allowing dropping load/ speed characteristics, it is usually possible to dispense with flywheels which slow down the acceleration and the loading times of the Diesel sets. Loads requiring constant frequency, e.g. instrumentation and controls, are supplied in nuclear stations not from such Diesel sources, but from precisely regulated inverters.
Design of Diesel Generators and Motor Starters
The ratings of Diesel generators in nuclear power plants gener- ally lie in the range of 2 to 4 MVA at either 6 or 10 kV. The robust design of these highly conventional machines allows them to with- stand the severe stresses of starting large induction motors.
Despite the fast response of these self-excited generators, it takes about 200 ms to restore the generator voltage to its normal value whenever voltage dips occur as the inevitable result of switch- ing in low-voltage transformers and starting up induction motor drives (Figure 5 ) . These transient voltage drops can cause contactors to be released if their solenoid voltage falls below certain limits.
VOLTAGE 6.3kV 3 m s
Fig. 5. Traces of genemtor voltage, frequency and current as tmnrformers and induction motor pump drives are switchedon to 2,600 kWj2.950 kVA, 6.3 kV, 1500 rpm Diesel set.
Attention must also be paid to the fact that the torque of running motors decreases as the square of the voltage. Consequently, the effect of such transient reductions in voltage on the motor drives and their controls must be carefully considered.
The European system interconnection authority recommends specifying that power plant motor drives operate stably down to 70% rated voltage since a 30% voltage drop is considered to be the abso- lute maximum that can occur. German standards4 for Diesel power systems in nuclear stations stipulate the design criterion of a maxi- mum voltage drop of 25% at the loads. After initial difficulties, manufacturers succeeded in fulfilling the requirement of supplying contactors with specially designed solenoids to ensure that release is only possible when the energizing voltage drops below 70% of the nominal value.
Another solution to this problem is to energize the contactors from either a dc or safe ac bus. This has the indisputable advantage of holding the contactors closed regardless of variations in the main ac voltage. However, connecting the contactor solenoids to the main ac voltage within the individual motor feeders allows the following even more important advantages to be gained:
- the failure of a control circuit only affects the associated feeder,
- no lengthly control wiring between feeders with associated potential failure possibility and excessive voltage drops, and
- simple complete disconnection of voltage within any feeder.
Generator kVA ratings must be sufficiently large to e n m that the greatest loading of the preloaded generators that can occur when transformers or motors are switched in, does not result in transient voltage dips in excess of 25%. This is the essential prerequisite to maintaining the highly reliable practice of tapping the main ac within each motor feeder to obtain a control voltage that is independent of any other feeders.
Design of Diesel Peripheral Equipment
Each Diesel set requires numerous supply and auxiliary systems for start-up and operation (Figure 6) . Experience and sophisticated reliability studies reveal that this peripheral equipment can be the source not only of numerous single faults, but also of particularly serious common faults. Apart from improving the reliability of in- dividual components, the decisive step in minimizing such hazards is to abolish all sharing of equipment by a group of Diesel sets, e.g. fuel tank, starting air compressor, cooling water and dc supplies. Dividing all the peripheral equipment and systems into strictly separate trains, so that each Diesel set becomes absolutely self-sufficient, also greatly simplifies the connections between the Diesel sets and their support systems. In particular, the complex pipework systems that result from starting several Diesel engines from a common air compressor or supplying them with fuel from a common storage tank, are re- placed by a number of highly streamlined piping layouts.
The complete segregation of self-sufficient Diesel sets was accelerated by the requirements for physical separation and pro- tection against external disturbances, e.g. earthquakes, aircraft accidents and explosion waves. Each Diesel set complete with all its own supply and auxiliary equipment from the fuel tank to the exhaust silencer is contained within a single reinforcedsoncrete fue- proofed cell.
The storage capacity of the fuel tank associated with each Diesel set is frequently the subject of debate. Demands for very large tanks are usually based on exaggerated predictions of the difficulties
and delays in replacing the contents, and on unduly lengthy esti- mates of the time periods that standby power will be required. Such estimates usually cannot be justified either by experience or proba- bility studies of the outage times of the normal power supply. The fuel oil quantity needed to generate standby power for a single day is roughly sufficient to allow the Diesel sets to be run for the specified trial periods over an entire year. Excessively large fuel tanks result in long storage with the concomitant danger of the oil being deterie rated by aging.
Compressed air equipment has proven itself superior to...