Case analysis of all 9 air compressors tripping in a power plant
It is not uncommon for the air compressor MCC to malfunction and all air compressor stations to stop.
Equipment overview:
The main engines of the 2×660MW supercritical unit of XX Power Plant are all selected from Shanghai Electric Equipment. The steam turbine is Siemens N660-24.2/566/566, the boiler is SG-2250/25.4-M981, and the generator is QFSN-660-2. The unit is equipped with The steam-driven induced draft fans, water supply pumps, and 9 air compressors are all produced by XX Co., Ltd., which meet the compressed air requirements for instrumentation, ash removal and miscellaneous use in the entire plant.
Prior working conditions:
At 21:20 on August 22, 2019, unit #1 of XX Power Plant was operating normally with a load of 646MW, coal grinders A, B, C, D, and F were operating, and the air and smoke system was operating on both sides, using the standard method of power consumption in the plant. The load of unit #2 is running normally, coal grinders A, B, C, D, and E are running, the air and smoke system is running on both sides, and the factory uses standard electricity. #1~#9 air compressors are all running (normal operation mode), among which #1~#4 air compressors provide compressed air for #1 and #2 units, and #5~#9 air compressors provide dust removal and ash transportation When using the system, the instrument and miscellaneous compressed air contact doors are opened 10%, and the compressed air main pipe pressure is 0.7MPa.
#1 unit 6kV factory-used section 1A is connected to the power supply of #8 and #9 air compressors; Section 1B is connected to the power supply of #3 and #4 air compressors.
#2 unit 6kV factory-used section 2A is connected to the power supply of #1 and #2 air compressors; section 2B is connected to the power supply of #5, #6 and #7 air compressors.
process:
At 21:21 on August 22, the operator found that the #1~#9 air compressors tripped at the same time, immediately closed the instrument and miscellaneous compressed air contact doors, stopped the ash transport and dust removal system compressed air, and on-site inspection found that 380V The MCC section of the air compressor lost power.
21:35 Power is supplied to the MCC section of the air compressor, and the #1~#6 air compressors are started in sequence. After 3 minutes, the air compressor MCC loses power again, and the #1~#6 air compressors trip. The instrument uses compressed air pressure dropped, the operator sent power to the MCC section of the air compressor four times, but the power was lost again a few minutes later. The started air compressor immediately tripped, and the pressure of the compressed air system could not be maintained. We applied for dispatching approval to transfer units #1 and #2 The load dropped to 450MW.
At 22:21, the instrument compressed air pressure continued to drop, and some pneumatic adjustment doors failed. The main and reheat steam desuperheating water adjustment doors of unit #1 were automatically closed. The main steam temperature increased to 585°C, and the reheat steam temperature increased to 571°C. ℃, the boiler end wall temperature exceeds the limit alarm, and the boiler manual MFT and the unit are immediately disconnected.
At 22:34, the instrument compressed air pressure dropped to 0.09MPa, the shaft seal steam supply regulating door of unit #2 automatically closed, the shaft seal steam supply was interrupted, the unit back pressure increased, and the “low pressure exhaust steam temperature is high” protection action (see attached picture 3), the unit is detached.
22:40, slightly open the high bypass of unit #1 with auxiliary steam.
At 23:14, boiler #2 is ignited and turned on to 20%. At 00:30, I continued to open the high side valve, and found that the instructions increased, the feedback remained unchanged, and the local manual operation was invalid. It was confirmed that the high side valve core was stuck and needed to be disassembled and inspected. Manual MFT of the #2 boiler.
At 8:30, the #1 boiler is ignited, at 11:10 the steam turbine is rushed, and at 12:12 the #1 unit is connected to the grid.
Processing
At 21:21 on August 22, air compressors #1 to #9 tripped simultaneously. At 21:30, electrical maintenance and thermal maintenance personnel went to the site for inspection and found that the working power switch of the MCC section of the air compressor tripped and the bus lost power, causing all 9 air compressors to lose PLC power and all air compressors tripped.
21:35 Power is supplied to the MCC section of the air compressor, and air compressors #1 to #6 are started in sequence. After 3 minutes, the MCC of the air compressor loses power again, and air compressors #1 to #6 trip. Subsequently, the air compressor MCC working power switch and the backup power switch were tried several times, and the air compressor MCC section busbar tripped after a few minutes after charging.
Checking the ash removal remote DCS control cabinet, it was found that the switch input A6 module was igniting. The input quantity (24V) of the 11th channel of the A6 module was measured and the 220V alternating current entered. Further check that the access cable of the 11th channel of the A6 module was the cloth bag on the top of the #3 fine ash warehouse. Dust collector exhaust fan operation feedback signal. On-site inspection #3 The operation signal feedback loop in the dust exhaust fan control box of the fine ash bag dust collector is incorrectly connected to the 220V AC control power supply in the box, causing the 220V AC power to flow into the A6 module through the fan operation feedback signal line. Long-term AC voltage effects, As a result, the card failed and burned out. The maintenance personnel judged that the power supply and switching output module of the card module in the cabinet may malfunction and cannot operate normally, resulting in frequent abnormal tripping of the power supply I and power supply II switches of the MCC section of the air compressor.
The maintenance personnel removed the secondary line that caused the AC to flow in. After replacing the burned A6 module, the frequent tripping of the power supply I and power II switches of the MCC section of the air compressor disappeared. After consulting the technical personnel of the DCS manufacturer, it was confirmed that this phenomenon exists.
22:13 Power is supplied to the MCC section of the air compressor and the air compressors are started in sequence. Start unit start-up operation
Exposed issues:
1. The infrastructure construction technology is not standardized. XX Electric Power Construction Company did not construct the wiring according to the drawings, the debugging work was not carried out in a strict and detailed manner, and the supervision organization failed to complete the inspection and acceptance, which laid hidden dangers for the safe operation of the unit.
2. The control power supply design is unreasonable. The design of the air compressor PLC control power supply is unreasonable. All air compressor PLC control power supplies are taken from the same section of busbar, resulting in a single power supply and poor reliability.
3. The compressed air system design is unreasonable. During normal operation, all 9 air compressors must be running. There is no backup air compressor and the air compressor operation failure rate is high, which poses a great safety hazard.
4. The MCC power supply method of the air compressor is imperfect. The working power supply and backup power supply from sections A and B of the 380V ash removal PC to the MCC of the air compressor cannot be interlocked and cannot be restored quickly.
5. The DCS does not have the logic and screen configuration of the air compressor PLC control power supply, and the command output DCS has no records, which makes fault analysis difficult.
6. Insufficient investigation and management of hidden dangers. When the unit entered the production stage, the maintenance personnel failed to check the local control loop in time, and the incorrect wiring in the dust collector exhaust fan control cabinet was not found.
7. Lack of emergency response capabilities. The operating personnel lacked experience in dealing with compressed air interruptions, had incomplete accident predictions, and lacked emergency response capabilities. They still significantly adjusted the operating conditions of the unit after all air compressors tripped, resulting in a rapid drop in compressed air pressure; When all the compressors tripped after running, the maintenance personnel failed to determine the cause and location of the fault as soon as possible, and failed to take effective measures to restore the operation of some air compressors in a timely manner.
Precautions:
1. Remove the incorrect wiring and replace the burned DI card module of the ash removal DCS control cabinet.
2. Inspect distribution boxes and control cabinets in areas with harsh and humid working environments throughout the plant to eliminate the hidden danger of AC power flowing into DC; investigate the reliability of the power supply mode of important auxiliary machine control power supplies.
3. Take the air compressor PLC control power supply from different PC sections to improve power supply reliability.
4. Improve the power supply method of the air compressor MCC and realize the automatic interlocking of the air compressor MCC power supply one and two.
5. Improve the logic and screen configuration of the DCS air compressor PLC control power supply.
6. Formulate a technical transformation plan to add two spare air compressors to improve the operational reliability of the compressed air system.
7. Strengthen technical management, improve the ability to troubleshoot hidden dangers, draw inferences from one example and conduct regular wiring inspections on all control cabinets and distribution boxes.
8. Sort out the operation conditions of on-site pneumatic doors after losing compressed air, and improve the emergency plan for compressed air interruption in the entire plant.
9. Strengthen employee skills training, organize regular accident drills, and improve emergency response capabilities.
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