Example: Battery leakage causes the DC system to have low ground voltage and the unit trips

1. Process and reasons

In May 2010, the load of a certain plant's D unit was 300MW, and the 640 switch of the unit's plant power supply tripped. The fault recording showed that when the 640 switch tripped, the unit's operating signal was normal, and the 640 switch tripped first; there was no fault signal in the generator-transformer protection, and no protection action record; the power grid system voltage was normal, and there was no signal for the bus differential, failure protection, and high-frequency switching, and no protection action record. After the trip, the insulation of the 640 switch control circuit was checked, and the trip coil and trip intermediate relay action voltage were normal. According to the above situation, combined with the actual analysis of the on-site equipment: Since the operating power of the tripping relay TJ in the 640 switch tripping circuit is too small (measured to be 2W); the length of the control cable in the circuit exceeds 400m, and the long cable has a capacitance effect to the ground, when there is leakage in the battery pack causing the DC system to have a low voltage to the ground (measured to be 52V), when a large interference occurs in the DC system (such as a high-power load starting, or a momentary grounding), it causes a large instantaneous fluctuation or impact in the DC system voltage, and generates an impact current in the capacitor circuit in the long control cable, which in turn causes the tripping relay TJ to operate.

2. Take measures:

3.1 Further find out the source of DC system interference, and organize a DC system interference simulation test during the unit shutdown period to verify the cause of the 640 switch tripping.

3.2 Contact the manufacturer to complete the transformation of the relevant switch tripping circuit during the unit shutdown period to ensure that the starting power of the heavy-duty relay is not less than 5W under rated voltage and the relay action time is about 10ms.

3.3 Take advantage of the unit standby time to increase the protection logic of the main steam valve of the turbine through the DEH when the main switch at the generator-transformer group outlet trips, so as to prevent the turbine from overspeeding when fully unloading.

3.4 Strengthen the monitoring and maintenance of the DC system, measure the voltage of the DC system regularly, and deal with grounding defects as soon as possible to ensure the stability of the DC system power supply and eliminate the inducing factors of DC system grounding.

3.5 Before the batteries of Units E and D are replaced, strengthen the inspection of the system batteries, check the battery leakage, and replace them in time if leakage is found. Start the battery replacement work as soon as possible. Learn from past experience, compare with standards and regulations, conduct a comprehensive inventory of the secondary system, and rectify problems in time if found.

Analysis of the accident of the #4 unit tripping caused by the loss of 110V DC voltage in the power grid of a certain plant 1. Accident phenomenon and process:

At 10:00 am on September 19, 2010, the electrical test team applied for the work ticket YB2010-09-149 "#3 network control transformer 6905 switch control cabinet installation measurement unit" to apply for the start of work. At 15:40 pm, when the electrical duty officer was switching the network control transformer in the control room, the network control 380V professional panel lost power, and the 110V DC charger on the special panel lost power. The plant power was lost and the unit tripped. The #3 and #4 diesel generators started the units and the public security section and operated normally. Nearly 50 minutes after the accident, the second phase network control 110V DC system was restored by manual wiring (the second phase network control 110V DC busbar was powered by the first phase network control 110V DC system, the #3 network control transformer and the #4 network control transformer were successfully charged, and the second phase 380V network control special panel was charged normally). At 5:40 am on September 20, after emergency repairs, the tripped unit was connected to the grid and resumed operation.

2. Analysis of the cause of the accident:

Upon inspection, it was found that the #42 and #46 battery poles of the Phase II network control 110V DC (00LBB) battery pack were corroded and broken, and the battery pack had an open circuit. When the #3 network control transformer switched to the #4 network control transformer for power supply, the moment the #3 network control transformer switch was disconnected, the Phase II network control 380V dedicated disk bus lost power for a short time, and the corresponding #21 charger and #22 charger on the same section lost power. Due to the open circuit of the Phase II network control 110V DC battery pack, the Phase II network control 110V DC bus lost voltage; the #4 network control transformer low-voltage side switch lost operating power and could not be closed, resulting in continuous power loss of the Phase II network control 380V dedicated disk.

Due to the loss of voltage of the 380V special panel, the two corresponding chargers lost power, and the battery pack was open, causing the 110V DC busbar of the second phase network control to lose voltage forever; the loss of voltage of the DC busbar also caused the working power supply of the heavy relay (110V DC) of the auxiliary contact of the second phase 220kV system switch and knife switch to be lost (equivalent to the change of the state of the outlet switch of the #4 machine and the high-voltage side switch of the second phase standby transformer, that is, the original closing switch became tripping), and the closing position signal sent to the host DCS was lost due to the loss of the 110V DC power supply of the heavy relay. The host DCS mistakenly believed that the outlet switch of the 4# machine was disconnected, and the #4 steam turbine OPC overspeed protection action was issued, closing the high-voltage main and regulating valves of the #4 machine, closing the medium-voltage regulating valve, and the #4 generator tripped. Similarly, the DCS mistakenly judged that the high-voltage side switch of the second phase starting transformer was disconnected, and the 6kV switch on the low-voltage side of the starting transformer was tripped. Although the operating unit with the common section was linked, the 6kV factory power was completely lost due to the tripping of the #4 unit.

3. Prevention and improvement measures

Carry out comprehensive cleaning and inspection of the battery packs in the whole plant. Carry out comprehensive maintenance as soon as possible for the battery packs with acid creep and serious corrosion, and perform full charge and discharge tests to prevent the battery packs from being open-circuited. Organize professional and technical personnel to optimize the secondary system, take targeted measures for the correctness of the secondary circuit, the anti-interference problem of the long cable direct jump circuit, and the safety of the DC system, prevent the protection from false operation or refusal to operate, and ensure the safety, reliability and accuracy of the protection device. Carefully manage the protection of the secondary system, especially the maintenance of the primary components. Strengthen the protection of indoor and outdoor electrical and thermal control equipment, and eliminate the hidden dangers of water leakage nearby. In view of the characteristics of the rapid replacement of secondary system equipment and the large number of imported equipment, strengthen the training of secondary system personnel and improve the technical level of secondary system maintenance personnel.