Technical safety analysis of additional diesel generator sets for nuclear power plants

Since the Japanese nuclear accident, people have paid more and more attention to the safety issues of nuclear power plants. Recently, many media have paid attention to the safety of many domestic and foreign nuclear power plants.Among them, emergency diesel generator sets can increase the nuclear safety margin of nuclear power plants and ensure the safe shutdown of nuclear reactors when the entire plant loses normal and emergency AC power.Everyone knows that the main reason for the nuclear accident in Japan was the backup dieselGenerator setIt takes all the damage to cause a nuclear leak.Therefore, Jiangsu Haixing Power Technology Co., Ltd. collected an article 'Additional DieselGenerator setApplication in Nuclear Power Plants' article.
At present, nuclear power plants in most countries and regions in the world, such as France, the United States, South Africa, South Korea and Taiwan, have installed additional diesel generators.China is no exception.Let's listen to the Haixing generator technician's explanation of the function, wiring method, power supply mode, test method, and power supply wiring method of the motor control center of the additional diesel generator.
1. Functional function of additional diesel generator
a) Replacement function.When any normal emergency diesel engine in the power station is unavailable, the additional diesel generator set can replace the unavailable diesel engine and perform all its functions.
b) Resist plant-wide power outage (stationblack-outSBO) function.The so-called power outage of the whole plant means that when the power station loses power inside and outside the plant, the normal emergency diesel engine is also unavailable.At this point the additional diesel generator can be started independently and provide the power necessary to safely shut down the nuclear reactor.
2. Wiring method
From the perspective of nuclear safety, each unit of the nuclear power plant is equipped with two electrically independent series A and B, and the 1E-level medium voltage busbar of each series is equipped with an emergency diesel generator as the 1E-level medium voltage busbar. Emergency power supply for the busbar.Generally speaking, additional diesel engines can be used as backup emergency backup power for 2 units, 4 units, and 6 units at the same time, which mainly depends on the electrical configuration of nuclear power plants in each country and region.The following takes two units as an example to briefly describe the main wiring method of the additional diesel engine.
method one,

(Figure 1, Additional diesel generator main wiring diagram 1)
As shown in Figure 1, the principle of this main wiring is that the outlet of the additional diesel generator is connected to the medium voltage panel (CBSWG) through a circuit breaker. The medium voltage panel has 5 outlets, one of which outlet contactor passes through 6.6kV/ The 380V transformer and the 380VAC power supply in the factory are electrically or mechanically interlocked to supply power to the diesel engine auxiliary equipment; the other four outlets are connected to four 1E-class medium-voltage busbars (1LHA, 1LHA, 1LHB, 2LHA and 2LHB).Because of its simple wiring and convenient operation, this method is widely used in some foreign nuclear power plants, such as South Africa and South Korea.

Method 2.

(Figure 2, additional diesel generator main wiring diagram 2)
As shown in Figure 2, this main wiring method is widely used in 900MW pressurized water reactor nuclear power plants (such as France).The main principle is that the outlet of the additional diesel generator is connected to four 1E-level medium-voltage busbars through the connecting piece in a medium-voltage conversion panel (9LHT003AR). This wiring method is also currently used in Daya Bay Nuclear Power Station in my country.Its main feature is to avoid common mode failures due to electrical interlocking, which puts forward higher requirements for Class 1E systems. For example, the connecting piece in the medium voltage conversion panel ensures the physical separation of equipment on the A and B series. , the key interlocking of the circuit breaker between the 002JA and 003JA positions (i.e. one out of two) ensures that the emergency diesel engine and the additional diesel engine cannot be powered at the same time (nuclear safety requirements).

3. Power supply mode
Generally speaking, when the unit is running normally, the additional diesel engine is in a hot standby state and is completely electrically isolated from the Class 1E medium-voltage busbar.If any emergency diesel engine fails and it is expected to be difficult to repair within 72 hours (as specified by the local Nuclear Safety Administration, this time includes the downtime of the unit), the Class 1E medium voltage bus connected to the failed emergency diesel engine will be switched. To the additional diesel engine, it simultaneously performs all the functions of a fault emergency diesel engine, such as self-starting and load-carrying capabilities under LOP, LOPSI, and LOPSIHHCP conditions.In addition, if a plant-wide power outage (SBO) occurs, in order to ensure the safe shutdown of the nuclear reactor, the additional diesel engine plays a very important backup protection role.

4. Test methods
Since additional diesel engines play a crucial role in nuclear power plants, in order to ensure their availability in emergency situations (such as emergency diesel engine failure, SBO), regular tests are required to verify their availability.
At present, nuclear power plants in various countries or regions around the world have different test methods, test contents and test cycles due to different main wiring methods of additional diesel engines, but basically they all need to do start-up tests, low-power tests and full-power tests.Let’s take the Korean nuclear power plant as an example.
As shown in Figure 1, the test circuit of the additional diesel engine of the Korean nuclear power plant is exactly the same as the power supply circuit. A Pr and on-site power grid connection test is conducted once a month. During the overhaul period, startup and 50% Pr and off-site power supply grid connection tests are performed.During the test, the normal emergency diesel engine is in standby mode.The main features of this test method are: from an operational perspective, it is easy to operate and can truly simulate the actual operating conditions of the diesel engine; from a safety perspective, through electrical interlocking, the normal emergency diesel engine can only detect 1E-level medium pressure It will start automatically when the bus loses power.
Another test method is to use a pure resistance device as the test load of the additional diesel engine, as shown in Figure 2.Since the test circuit and the power supply circuit are independent of each other, the normal operation of the unit will not be affected during the test, and there is no need to set up a special grid-connected device. Therefore, the risk is small, and it is widely used in French nuclear power plants.However, this solution can only test the load capacity of the diesel engine, and cannot truly simulate the actual operating conditions of the diesel engine after it is connected to the system and verify the adjustment ability of its excitation system.Only by adding equipment such as reactance to the test load device can the test method be more perfect.

5. Motor control center
The main power supply loads of the additional diesel engine motor control center are diesel engine auxiliary equipment, fire protection systems, lifting equipment, 48V/125V DC power supply, ventilation and air conditioning, etc.Under normal standby conditions, its upper-end power supply generally comes from one channel from the public medium-voltage distribution board in the plant (such as the South African nuclear power plant, as shown in Figure 1) or two channels from the emergency low-voltage distribution board in column A of the two units (such as the Taiwan Nuclear Power Plant, as shown in Figure 3 Show).

(Figure 3, additional diesel generator set main wiring diagram 3)
With the above two power supply wiring methods, when the diesel engine is started, the motor control center (MCC) is self-powered by the diesel generator, which can ensure that some ancillary equipment of the diesel engine (such as preheating system, prelubrication system) will be available in standby conditions. It can meet the power supply needs of DC power supply system, ventilation system and fire protection system.The author believes that the latter method has higher safety performance than the former method.

6. Summary
The installation of additional diesel engines is not only a requirement for the nuclear safety performance of nuclear power plants, but also an important indicator for the IAEA and national nuclear safety agencies to measure the safety, reliability, and stable operation of nuclear power plants.In order to be in line with the practice and reach the world's advanced technology level as soon as possible, Guangdong Nuclear Power Joint Venture Co., Ltd. has decided to add additional diesel engines as public backup emergency power sources for Units 1 and 2 of Daya Bay Nuclear Power Station and Units 1 and 2 of Ling'ao Nuclear Power Plant.The author believes that this is not only the pursuit of a higher level of nuclear power development, but also a responsible attitude towards the peaceful use of atomic energy by mankind.