A.1.1 Equipment description
A power plant has 2 condensing steam turbine generator sets installed, with a single unit capacity of 300MW. Before the accident, the #1 high-voltage standby transformer had 6kV1A and 1B sections and served as the backup power supply for the #2 unit.
The boiler ash discharge method is pneumatic ash conveying, and the ash removal system is equipped with 5 screw air compressors, with air compressor specifications of 220kW, air-cooled units, and exhaust volume of 36m³/min.
A.1.2 Accident description
Before the accident, the #1 unit was under overhaul, the #2 unit was operating normally, and the #1, #2, and #5 ash removal air compressors were standby. The maintenance personnel required to deal with the defect of the drain valve of the #4 ash removal air compressor. During the process of stopping the #4 ash removal air compressor, the operator was not familiar with the operation of the equipment. When the air compressor switch refused to move, he made an incorrect operation, which eventually led to the overstepping of the 6kV transformer switch in operation, causing the protection of multiple operating equipment to be actuated, causing the boiler MFT to actuate and extinguish the fire, and a feed water pump to be seriously damaged. During the emergency shutdown process, the main engine #4 bearing and journal were worn.
A.1.3 Event Process
At 10:11 a.m. on a certain day, the maintenance personnel required to deal with the defect of the drain valve of the #4 ash removal air compressor. The operator stopped the #4 ash removal air compressor (the upper computer showed that it had been stopped, but it was still running after investigation). The maintenance personnel opened the soundproof cover and found that the cooling fan of the #4 ash removal air compressor was still running. The operator was notified. The operator pressed the emergency stop button, but the cooling fan of the #4 ash removal air compressor still did not stop. In order to stop the cooling fan of the #4 ash removal air compressor, the operator mistakenly cut off the power supply of the oil cut-off solenoid valve. At 10:18, the cooling fan was found to be smoking and on fire. We immediately tried our best to put out the fire after 5 minutes. However, at 10:21, the control personnel found that the switch 1, 6101, 6102, and 6208 of the #1 high-voltage transformer were tripped, the 6kV1A, 6kV1B, and 6kV2B sections were depressurized, and the zero-sequence overcurrent protection of the #1 high-voltage transformer protection branch was activated. Subsequently, the two air preheaters of the #2 furnace tripped (operating at the 380V2B2 section at the same time), the induced draft fan, the MFT of the #2 furnace, and the "low voltage protection" of the feedwater pump tripped. The temperature of the #4 bearing of the turbine was found to be rising, with the highest temperature being 137°C. The high-voltage transformer 1 switch, 6101 switch, and 6102 switch were forced to close, but the forced closing was unsuccessful; the control personnel went to 6kV2B to check and found that the switch of the #4 dust removal air compressor was tripped, and it was immediately tripped. The high-voltage transformer 1 switch, 6101 switch, and 6102 switch were normal. The switch of #4 dust removal air compressor was released and it was found that the primary fuses of phases B and C were blown, and phase A was not blown.
After the accident, it was found that: #4 dust removal air compressor and motor were seriously damaged (later confirmed by the manufacturer on site that they can still be used after repair), 2A water pump core package was seriously damaged, 2A water pump hydraulic coupler was close to scrap, some pipes around 2A water pump were damaged to varying degrees, and the main engine #4 bearing and shaft diameter were worn.
A.1.4 Cause Analysis
When the #4 ash removal air compressor was shut down, the switch refused to operate due to poor contact of its normally open auxiliary contact. The #4 ash removal air compressor was not shut down. The ash removal operator mistakenly cut off the power supply of the oil cut-off solenoid valve during the handling, causing the #4 ash removal air compressor to run without oil. The oil temperature gradually rose to 109℃, and the high oil temperature protection switch was tripped. Since the #4 ash removal air compressor switch refused to operate, the air compressor continued to operate, the motor overheated, smoked and caught fire, and the insulation was damaged and grounded. After grounding, the #4 ash removal air compressor switch still refused to operate, and the 6kV2B section working power supply 6208 switch was tripped. After the quick cut start, the high standby transformer 1 switch was tripped, and the 6kV2B section lost pressure, causing the main auxiliary equipment to lose power #2 furnace MFT; Later, due to improper operation of the operator, the 2A feedwater pump reversed, and then caused the coupler to overspeed and be seriously damaged; The drum water level could not be maintained and the machine was forced to shut down. Impurities or hard particles remaining in the #4 bearing oil inlet pipe entered the bearing, causing wear on the shaft diameter. The lubricating oil film was damaged, resulting in temperature rise and damage to the tungsten metal.
A.1.5 Problems exposed by the accident
(1) Before the accident, the work ticket was not issued to deal with the defect of the #4 dust removal air compressor drain valve.
(2) The air compressor was not properly maintained, and the contactor switch had not been checked and cleaned for a long time, resulting in poor contact.
(3) The attendant was not familiar with the air compressor equipment. After discovering that the #4 dust removal air compressor had not stopped, he mistakenly cut off the power supply of the oil cut-off solenoid valve, causing the accident.
(4) The 2A and 2B air preheaters were running at the same time in the 380V2B2 section. The operating personnel did not discover this abnormal operating mode in time, resulting in the complete shutdown of the two air preheaters during the accident, the MFT was activated, and the accident expanded.
(5) During the accident, the reverse rotation of the feed water pump was not discovered in time, and no correct judgment was made, resulting in improper operation.
(6) There is a problem with the thermal interlocking protection logic. The air preheater is completely stopped and the induced draft fan is tripped without delay.
A.1.6 Preventive measures
(1) Strengthen the maintenance and management of equipment, assign equipment to specific people, and report and handle abnormalities in a timely manner.
(2) Strengthen the inspection of equipment hidden dangers and conduct a hidden danger inspection activity for all equipment of the machine, boiler and electricity.
(3) Inspect and replace the same type of bearings of the turbine separately.
(4) Strengthen the technical training of operating personnel.
