1. Accident Overview
At 16:52 on XXXX-0X-15, due to a rare tornado, a phase A ground fault occurred on the 500 kV 5304 line of a substation. The line's dual high-frequency and ground distance zone 1 protections operated to trip phase A of circuit breakers 5043 and 5042. Circuit breaker 5043 reclosed onto the faulty line first, then the accelerated protection operated to trip circuit breakers 5043 and 5042. Circuit breaker 5043 tripped all three phases, while circuit breaker 5042 refused to operate.Microcomputer relay protection tester summary of the advantages and disadvantages of similar domestic products, using advanced modern microelectronic technology and devices to realize a new type of small-sized microcomputer relay protection tester. After 1,174 ms, a phase C fault occurred on the 5304 line again. The breaker failure protection of circuit breaker 5042 operated to trip circuit breaker 5041 and sent a remote trip signal to the opposite side of the 5302 line running in the same breaker string, cutting off the operating 5302 line.
2. Cause Analysis of the Accident
Upon inspection, the negative power wire terminal on the coil A2 of the non-full-phase intermediate relay inside the mechanism of circuit breaker 5042 was loose (the non-full-phase function inside the mechanism of circuit breaker 5042 was not used, but only the wires connecting the two ends of its time relay contacts were removed). When phase A of the circuit breaker tripped during the first fault, the normally open auxiliary contacts of phases B and C and the normally closed auxiliary contact of phase A closed, activating the pressure lockout trip/close re-lay 2ZJ on the protection panel along the direction indicated by the arrow in the diagram. The operation of 2ZJ opened the circuit breaker trip/close circuit, causing the circuit breaker to refuse to operate during the second fault.
3. Problems Exposed by the Accident
(1) Maintenance personnel did not strictly implement the relevant provisions on secondary circuit modifications in the "Safety Regulations for On-site Work of Relay Protection and Safety Automatic Devices". Unused circuits were not completely removed, resulting in parasitic circuits, which affected the operating secondary circuits and directly led to this accident.
(2) The manufacturer's design of the secondary circuits inside the circuit breaker mechanism was not standardized and reasonable. The negative power terminals of the intermediate relay coils in different circuits were cross-connected in parallel, laying hidden dangers for the existence of parasitic circuits.
(3) Due to vibration during opening and closing operations, the wiring of the secondary circuits in the circuit breaker mechanism is very prone to loosening, but maintenance work was not adequate.
4. Preventive Measures for Accidents
(1) For circuits that exist in both the circuit breaker and the protection (such as non-full-phase and anti-pump circuits), only one set should be retained, and the other set should be completely removed to eliminate parasitic circuits.
(2) The manufacturer should lead the negative power terminals of the intermediate relay coils in each circuit inside the circuit breaker mechanism to the terminal block separately, powered by the same DC power supply, and should not arbitrarily cross-connect to obtain power internally.
(3) Remove the starting circuit of the pressure lockout re-lay 2ZJ inside the protection operation box and short-circuit its contacts, using only the pressure lockout relay inside the circuit breaker mechanism to lock the trip/close circuit, thereby simplifying the secondary circuit and improving reliability.
(4) Draw inferences from this accident, learn lessons, conduct targeted inspections of similar circuit breaker secondary circuits, and take corresponding preventive measures.
Strengthen equipment maintenance management, clarify the division of labor among maintenance personnel of various specialties, vigorously promote the implementation of on-site maintenance work instruction books, and promote the standardization of maintenance work.
A loose wire terminal nearly caused a serious accident of grid collapse and large-scale power outage. It can be seen that both maintenance personnel and designers should cultivate a strict, meticulous, and practical work style, not letting go of any small problems in work, so as to effectively prevent various accidents at the source and ensure the stable operation of the power grid.
