Abstract: This paper analyzes the "sudden tripping" phenomenon that occurs in some hydraulic circuit breakers under strong electromagnetic interference in substations due to the low minimum operating voltage on the tripping coil. A modification scheme is proposed. Keywords: Circuit breaker, tripping coil, sudden tripping Introduction DL/T596—1996 "Preventive Test Procedures for Power Equipment" stipulates that "the minimum operating voltage on the terminals of the tripping and closing electromagnets or closing contactors of the operating mechanism of high-voltage circuit breakers should be between 30% and 65% of the rated operating voltage." GB50190—91 "Standard for Acceptance Testing of Electrical Equipment in Electrical Installation Engineering" also stipulates that "when the voltage measured at the coil terminal of a DC or AC tripping electromagnet is greater than 65% of the rated value, the circuit breaker should reliably trip; when this voltage is less than 30% of the rated value, it should not trip." In some SF6 circuit breakers, the hydraulic mechanism's tripping circuit has a coil connected in series with a resistor. For example, the LW6-500 type, a 110VDC operating power supply, has a tripping coil (18Ω) with a 32Ω resistor connected in series in the tripping circuit. It is generally considered that the minimum operating voltage is the total voltage of the tripping circuit (including the series resistor). If the minimum operating voltage is 30% Un, the operating voltage across the tripping coil is only 11.88V, which is clearly insufficient to meet the anti-interference requirements. This could lead to the circuit breaker "tripping unexpectedly" under strong electromagnetic interference due to the excessively low minimum operating voltage of the tripping coil. According to incomplete statistics, the Gezhouba Dajiang Power Plant switchyard has experienced a total of 17 instances of unexpected tripping over the past 10 years. During the repair and commissioning of the 5061 circuit breaker at the Gezhouba converter station, it was found to have tripped unexpectedly 5 times within 3 days. 1. Analysis If the minimum operating voltage on the trip coil is appropriately increased without changing the operating characteristics of the circuit breaker, the anti-interference capability of the circuit breaker trip circuit can be enhanced, so that it can meet the following requirements: 1) Increase the minimum operating voltage across the trip coil, ideally to around 30% Un (Un is the rated voltage); 2) After the trip coil is connected in series with a resistor, the minimum operating voltage across the trip circuit should not exceed 65% Un; 3) After replacing the trip coil, the tripping time and other operating characteristics of the circuit breaker should meet the requirements of the regulations; 4) The trip coil meets the requirements of the tripping current carrying capacity, and the anti-pumping relay (TBJ starting current is 1A) operates reliably. When the tripping valve of the operating mechanism is not changed, the minimum starting force F and mechanical work W required for its operation will remain unchanged [1]. F = 6.4(IN)2[S/δ + 0.8g(z/l)2] × 10-8, kg (1) When the electromagnet parameters remain unchanged, we have: F∝(IN)2 (2) W = (IN)2(Gδ0-Gδm)/2＝(IN)2Gδ0/2 (3) Where Gδ0 is the magnetic permeability of the electromagnet in the attracted position; Gδm is the magnetic permeability of the electromagnet in the released position; z,l are the structural parameters of the electromagnet. From equations (1) and (3), it can be seen that if the operating characteristics of the operating mechanism are not changed, the IN (ampere-turns) of the trip coil must be kept constant. Under this premise, there are two ways to increase the minimum operating voltage on the trip coil: 1) Change the valve structure to increase the force F and mechanical work W required to start the trip valve. Thus, from equations (1) and (3), it can be seen that if the number of coil turns N and other parameters remain unchanged, the starting current Id required for the trip electromagnet to start will be increased, thereby increasing the minimum operating voltage on the trip coil; 2) Replace the coil. According to equations (1), (2), and (3), if the force F and static mechanical work W required to start the trip valve remain unchanged, and the trip coil is changed, then the IN of the two trip coils before and after replacement should remain unchanged. Therefore, I′dN1＝I″dN2 (4) The minimum operating voltage on the two different trip coils R′2 and R″2 is: UR′2＝I′dR′2， UR″2＝I″dR″2。 (5) From equations (4) and (5), we have: UR″2＝I″dR″2=（I′dN1／N2）R″2=UR′2（N1/N2）（R″2／R′2） （6） Therefore, under the premise of ensuring the current carrying capacity of the coil, the coil wire diameter can be appropriately reduced to keep the number of turns of the trip coil small and increase the resistance value of the trip coil, thereby increasing the minimum operating voltage on the trip coil and meeting the requirements of the operating mechanism for the trip coil. The first method is more effective, but it is more difficult to implement because it requires changing the valve structure; the second method requires replacing the trip coil, which is easier to implement, but because the resistance of the entire trip circuit increases, the circuit current decreases, and the input power P of the trip circuit also decreases, from the perspective of the entire dynamic trip process [2], it will reduce the trip work of the operating mechanism, thereby prolonging the trip time td, and the effect may be worse. For this reason, it is necessary to appropriately increase the number of coil turns to increase the attraction force and mechanical work of the trip electromagnet in order to improve its performance. The second method will be discussed now. Taking the 5061 (LW6-500 type) circuit breaker in the AC field of the Gezhouba converter station as an example, its operating voltage is 110VDC, and the specifications of the trip coil are: 0.35mm² wire diameter, 1000 turns, 18Ω, with an external 32Ω resistor. Before replacing the trip coil, tests showed that the operating voltage U2 across the auxiliary trip coil of the 5061's tripping phase (phase A) was only about 15V, while the operating voltages U1 and U2 across the main and auxiliary trip coils of the other two phases were also around 20V. The measurement results are shown in Table 1. It was determined that the circuit breaker might malfunction due to the low minimum operating voltage across the trip coil, caused by strong electromagnetic interference in the substation. Therefore, it was decided to modify the 5061's trip coil, initially selecting a 0.31mm² wire diameter, 1300 turns, 30Ω coil with an external 30Ω resistor. As can be seen from equation (6), the minimum operating voltage on the trip coil will increase, but due to the increased resistance of the trip circuit, the tripping time td will also become longer. [align=center]Table 1 Minimum operating voltage V of the trip coil before replacing the 5061[/align] For ease of analysis, we consulted with the manufacturer and proposed six configuration schemes for replacing the coil (the main and auxiliary trip coils are the same) for comparative testing. Pingdingshan High Voltage Switch Factory conducted operating characteristic research tests on them respectively, and assessed their tripping time, minimum operating voltage, and whether the high oil pressure low voltage switch could operate reliably. Finally, it was confirmed that for the LW6-500 SF6 circuit breaker with an operating power supply of 110VDC, the scheme of 0.31mm2 wire diameter, 1300 turns, 30Ω coil, and an external 30Ω resistor is more suitable (the factory's minimum operating voltage test is shown in Table 2). [align=center]Table 2 Minimum Operating Voltage V of the Improved Tripping Circuit[/align] The test results of the 5061 circuit breaker at the Gezhouba Converter Station after replacing the tripping coil are shown below: The coil test shows that after replacing the tripping coil, the circuit breaker's tripping time, synchronization, metallic short-circuit time, and operation under high oil pressure and low voltage all meet the requirements of the regulations; the minimum operating voltage on the tripping coil has been increased; the circuit breaker's anti-interference capability has been enhanced; and the circuit breaker's tripping time has become longer than before (see Table 3). Since its modification and commissioning in June 1998, the 5061 circuit breaker has not experienced any "sudden tripping." [align=center]Table 3 Breaking Time (ms) of 5061 Circuit Breaker Before and After Replacing the Breaking Coil[/align] [align=center]Table 4 Minimum Operating Voltage (V) of 5061 Breaking Coil After Replacing the Breaking Coil[/align] 2. Conclusions and Recommendations For high-voltage circuit breakers with resistors in series in the breaking circuit, under the condition of meeting the operating characteristic requirements of the regulations, appropriately increasing the resistance value of the breaking coil will make the minimum operating voltage across the breaking coil as close as possible to 30%Un. The minimum operating voltage across the breaking circuit (including the series resistor) should be greater than 30%Un and less than 65%Un. In this way, while meeting the current carrying capacity requirements of the breaking circuit and other circuit breaker operating characteristic requirements, the minimum operating voltage of the breaking coil will be greatly improved, which not only meets the requirements of the regulations but also strengthens the anti-interference capability. References: [1] Tsinghua University High Voltage Teaching and Research Group. High Voltage Circuit Breakers (Volume 2). Beijing: Electric Power Industry Press. [2] DL/T405-1996. Technical Specifications for Imported 252 (245) - 550kV AC High Voltage Circuit Breakers and Disconnecting Switches.