**1. Functions of the Auxiliary Diesel Generator** The auxiliary diesel generator is primarily configured to provide the following functions: a) Replacement function: When any normal emergency diesel generator in the power plant becomes unavailable, the auxiliary diesel generator can replace that unavailable generator and perform all its functions. b) Station black-out (SBO) protection function: A station black-out occurs when the power plant loses both internal and external power sources, and the normal emergency diesel generators also become unavailable. In this case, the auxiliary diesel generator can be started independently to provide the power necessary for the safe shutdown of the nuclear reactor. **2. Main Wiring Method of the Auxiliary Diesel Generator** From a nuclear safety perspective, each unit in a nuclear power plant is equipped with two electrically independent series, A and B. Each series' 1E-class medium-voltage bus is equipped with an emergency diesel generator as the emergency power supply for that 1E-class medium-voltage bus. Generally, the auxiliary diesel generator can simultaneously serve as a backup emergency power supply for 2, 4, or 6 units, depending on the electrical configuration of the nuclear power plant in each country and region. The following briefly describes the main wiring method of the auxiliary diesel generator using two units as an example. 2.1 Method 1: As shown in Figure 1, the principle of this main wiring method is that the outlet of the auxiliary diesel generator is connected to the intermediate-voltage control panel (CBSWG) via a circuit breaker. This CBSWG has five outgoing lines. One outgoing contactor, through a 6.6 kV/380 V transformer, is electrically or mechanically interlocked with the plant's 380 V AC power supply to supply power to the auxiliary equipment of the diesel generator. The other four outgoing lines are connected to four 1E-class intermediate-voltage busbars (1 LHA, 1 LHB, 2 LHA, and 2 LHB) respectively after being interlocked by a key (i.e., only one line can be connected at a time). Due to its simple wiring and convenient operation, this method is widely used in some nuclear power plants abroad, such as those in South Africa and South Korea. Figure 1: Main Wiring Diagram 1 for Auxiliary Diesel Generator 2.2 Method 2: As shown in Figure 2, this main wiring method is widely used in 900 MW pressurized water reactor nuclear power plants (such as those in France). Its main principle is that the outlet of the auxiliary diesel generator is connected to four 1E-class medium-voltage busbars via connecting plates in a medium-voltage conversion panel (9 LHT 003 AR). This wiring method is also currently used in the Daya Bay Nuclear Power Plant in China. Its main feature is that in order to avoid common-mode faults caused by electrical interlocking, it puts forward higher requirements for the 1E-class system. For example, the connecting plates in the medium-voltage conversion panel ensure the physical separation of equipment on the A and B series. The key interlocking (i.e., two-out-of-one) of the circuit breaker between compartments 002 JA and 003 JA ensures that the emergency diesel generator and the auxiliary diesel generator cannot be powered at the same time (a requirement of nuclear safety). Figure 2 Main wiring diagram of auxiliary diesel generator II. [b]3 Power supply mode of auxiliary diesel generator[/b] Generally speaking, when the unit is running normally, the auxiliary diesel generator is in hot standby mode and is completely electrically isolated from the 1E-class medium-voltage busbars. If any emergency diesel generator fails and is not expected to be repaired within 72 hours (as stipulated by the local nuclear safety authority, this time includes unit downtime), the 1E-class medium-voltage bus connected to the failed emergency diesel generator will be switched to the auxiliary diesel generator, which will then perform all the functions of the emergency diesel generator, such as self-starting and load-carrying capacity under LOP, LOP+SI, and LOP+SI+HHCP conditions. Furthermore, in the event of a plant-wide power outage (SBO), the auxiliary diesel generator plays a crucial backup protection role to ensure the safe shutdown of the nuclear reactor. [b]4 Testing Methods for Auxiliary Diesel Generators[/b] Because auxiliary diesel generators play a vital role in nuclear power plants , their availability must be verified through regular testing to ensure their usability in emergency situations (such as emergency diesel generator failure or SBO). Currently, the testing methods, content, and cycles vary among nuclear power plants worldwide due to differences in the main wiring configuration of auxiliary diesel generators, but generally, startup tests, low-power tests, and full-power tests are required. The following section uses a South Korean nuclear power plant as an example. As shown in Figure 1, the test circuit of the auxiliary diesel generator in the Korean nuclear power plant is exactly the same as the power supply circuit. A 100% power supply connection test with the plant's power grid is conducted monthly, and a start-up test and a 50% power supply connection test with the external power grid are conducted during major overhauls. During the tests, the normal emergency diesel generator is in standby mode. The main characteristics of this test method are: from an operational perspective, it is easy to operate and can realistically simulate the actual operating conditions of the diesel generator; from a safety perspective, through electrical interlocking, the normal emergency diesel generator will only automatically start when a power failure is detected on the 1E-class medium-voltage bus. Another test method uses a pure resistance device as the test load for the auxiliary diesel generator, as shown in Figure 2. Since the test circuit and the power supply circuit are independent of each other, the test does not affect the normal operation of the unit, and no special grid connection device is required, thus the risk is lower. This method is widely used in French nuclear power plants. However, this scheme can only test the load-carrying capacity of the diesel generator and cannot realistically simulate the actual operating conditions after the diesel generator is connected to the system or verify the adjustment capability of its excitation system. Only by adding equipment such as reactors to the test load device can this test method be further improved. [b]5. Auxiliary Diesel Generator Motor Control Center[/b] The main power supply load of the auxiliary diesel generator motor control center is for diesel engine auxiliary equipment, fire protection system, lifting equipment, 48V/125V DC power supply, and ventilation and air conditioning. Under normal standby conditions, its power supply is generally taken from one common medium-voltage switchboard in the plant (such as the South African nuclear power plant, as shown in Figure 1) or two emergency low-voltage switchboards in the A row of two units (such as the Taiwan nuclear power plant , as shown in Figure 3). Figure 3 Main wiring diagram of auxiliary diesel generator. With the above two power supply wiring methods, when the diesel engine starts, the motor control center (MCC) is powered by the diesel generator. This can ensure the power needs of certain auxiliary equipment of the diesel engine (such as the preheating system and prelubrication system) under standby conditions, and also meet the requirements of the DC power system, ventilation system and fire protection system for power supply continuity. The author believes that the latter method has higher safety performance than the former method. [b]6 Conclusion[/b] The installation of auxiliary diesel generators is not only a requirement for the nuclear safety performance of nuclear power plants, but also an important indicator for the International Atomic Energy Agency and national nuclear safety agencies to measure the safe, reliable, and stable operation of nuclear power plants. In order to align with international practices and reach the world's advanced technological level as soon as possible, Guangdong Nuclear Power Joint Venture Co., Ltd. has decided to add auxiliary diesel generators as a shared backup emergency power source for Units 1 and 2 of the Daya Bay Nuclear Power Plant and Units 1 and 2 of the Ling'ao Nuclear Power Plant. The author believes that this is not only a pursuit of a higher level of nuclear power development, but also a responsible attitude towards the peaceful use of atomic energy by mankind. 7 Appendix Glossary : ​​a) LOP (Loss of Power): Loss of AC power. b) SI (Safety Injection): Safety injection signal. c) HHCP (High High Containment Pressure): High-high pressure in the reactor vessel. d) MCC (Motor Control Center): Motor control center. e) Class 1E: System or equipment related to nuclear safety. f) 1LHP, 1LHQ, 2LHP, 2LHQ: Emergency diesel generators. g) DG (diesel generator): Emergency diesel generator. h) MV (medium voltage): Medium voltage. i) CBSWG (circuit breaker switchgear): Circuit breaker switchgear. j) NPS (normal power supply): Normal medium voltage power supply for internal and external use. k) 1LGB, ALGC, 2LGB, 2LGC: Normal medium voltage power supply for internal and external use. l) 1LHA, 1LHB, 2LHA, 2LHB: Emergency medium voltage busbars. m) Pr: Rated power. ■