Abstract: This paper addresses the frequent burnout of the closing and opening coils in the older ZN5-10 circuit breaker model. The causes of this phenomenon are analyzed, and effective measures are taken to promptly modify the operating circuit, resolving the previously troublesome secondary wiring problem and ensuring the normal power supply of the system. Keywords : Closing and opening coil, time relay, operating circuit, circuit breaker, auxiliary contacts. The 35kV substation is the power supply center of our mine, responsible for the power supply of the entire mine. Due to their long service life and outdated equipment such as switchgear, the 47 6kV feeder cabinets are model G 1A, equipped with the relatively outdated ZN5-10 type circuit breakers. While the ZN5 series indoor high-voltage vacuum circuit breakers are widely used in our mine due to their advantages such as easy operation, simple maintenance, long service life, minimal pollution, and very low noise, in actual use, external faults such as the unreliable operation of the auxiliary contacts of the circuit breakers frequently cause the closing and tripping coils to burn out, posing a significant threat to the safe and reliable operation of the power supply system. Solving this problem has become a key focus of our next phase of work. From June 2002 to the end of 2003, the 35kV substation adopted the RCS-9000 integrated automation microcomputer protection device from NARI Group Corporation. Although the performance and reliability of the protection improved, the problem of the vacuum switch auxiliary contacts failing to reliably operate when the vacuum circuit breaker tripped remained unresolved, resulting in the burnout of 5 closing coils and 2 opening coils. Meanwhile, this fault also frequently occurs in other mine substations, such as the Beisu Coal Mine, which are equipped with the same equipment. This not only increases maintenance workload but also seriously affects mine production and safety. Therefore, we modified the operating circuit of the opening and closing coil protection for this type of high-voltage vacuum circuit breaker. 1. Working Principle of the Opening and Closing Circuit (See Figure 1) 1.1 Control Gate When manually closing, pressing the closing button SB1, or during remote control or protection closing, the microcomputer issues a closing command. HJ closes, connecting the closing circuit. The closing holding relay HBJ is energized, and the self-holding contact HBJ closes for self-holding. Simultaneously, the closing contactor HC coil is energized, the main contacts HC1 and HC2 close, and the closing coil HQ is energized and engaged, driving the mechanism to move the circuit breaker to the closing position. After closing, the circuit breaker auxiliary contact DL1 opens, and the closing holding relay HBJ and the closing contactor HC coil are de-energized. The main contacts HC1 and HC2 open, and the closing coil HQ is de-energized. The closing process ends. 1.2 Tripping When manually tripping, press the tripping button SB2. When tripping via remote control or protection, the microcomputer issues a tripping command, TJ closes, connecting the tripping circuit. The tripping holding relay TBJ is energized, and the self-holding contact TBJ closes to self-hold. At the same time, the tripping contactor TQ coil is energized, driving the mechanism to move and causing the circuit breaker to the tripping position. After tripping, the circuit breaker auxiliary contact DI2 is disconnected, and the tripping holding relay TBJ and the tripping coil I1Q are de-energized. The tripping process ends. 2 Fault Analysis During the tripping and closing operation, there may be two fault causes: (1) Due to aging and deformation of the mechanism, wear and loosening, or improper adjustment, the mechanism is not in position, and the circuit breaker auxiliary contact cannot be disconnected. (2) Although the mechanism is in position, the circuit breaker auxiliary contact cannot be reliably disconnected. Both of these causes will cause the tripping and closing operation to be not completed correctly, resulting in HC (when closing) or TQ (when tripping) being energized for a long time and burning out. 3. Modification Plan In response to the above-mentioned fault phenomena, we adopted a combination of hardware replacement and electrical protection modification, the measures are as follows: (1) Adjust and replace the auxiliary switch of the vacuum switch. (2) Adjust and replace the operating mechanism. (3) Modify the operating circuit, and connect a time relay in parallel at both ends of the opening and closing coils to protect them. See the wiring diagram of the modified opening and closing circuit 2. [b]4 Working Principle of the Modified Opening and Closing Circuit 4.1 Closing[/b] When manually closing, press the closing button SB1. When closing by remote control or protection, the microcomputer issues a closing command, HJ closes, the normally closed (normally closed) contact KT1 of the time relay closes after a delay, connecting the closing circuit. The closing holding relay HBJ is energized, and the self-holding contact HBJ closes to self-hold. At the same time, the closing contactor HC coil is energized, the main contacts HC1 and HC2 close, the closing coil HQ and the time relay KT1 are energized and attracted, driving the mechanism to move, so that the circuit breaker is switched to the closing position. In abnormal circumstances, if the vacuum switch auxiliary contact DL1 fails to reliably disconnect, the normally closed (closed) contact KT1 of the time relay opens after a delay, de-energizing the coils of the closing holding relay HBJ and the closing contactor HC. The main contacts HC1 and HC2 open, de-energizing the closing coil HQ. 4.2 Tripping When manually tripping, pressing the trip button SB2, or during remote or protection tripping, the microcomputer issues a tripping command. rJ closes, the normally closed (closed) contact KT2 of the time relay closes after a delay, connecting the tripping circuit. The trip holding relay TBJ is energized, and its self-holding contact TBJ closes for self-holding. Simultaneously, the coils of the tripping contactor TQ and the time relay KT2 are energized, driving the mechanism to trip the circuit breaker to the tripped position. In abnormal circumstances, if the vacuum switch auxiliary contact DL2 fails to reliably disconnect, the normally closed (closed) contact KT2 of the time relay opens after a delay, de-energizing the tripping coil TO. 5. Selection and Setting of Modification Equipment When using parallel time relays to protect the closing coil, the following three points should be noted: (1) Selection of time relay KT: The voltage level of KT should be determined according to the opening and closing voltage, and the contact capacity of KT should be determined according to the rated current of the DC contactor coil. (2) Time setting of time relay KT: The setting time of KT should be greater than the closing time of the vacuum switch. The time setting should not be too long to prevent the coil from overheating and causing insulation degradation. (3) Pay attention to the commissioning after installation: During commissioning, short-circuit the DL contacts and check whether the time relay KT operates reliably. Conclusion In early 2004, we modified the operation circuits of 47 high-voltage vacuum switches in the 35kV substation one by one. Due to the double protection of the time relay contacts and the circuit breaker auxiliary contacts disconnecting the coil power supply, so far, no burning of the opening and closing coils has occurred. This modification has been recognized by the relevant departments of the mine and has achieved good results.