667 / 2022-03-31 11:26:53

On-line monitoring of reactive power switching in substations based on mining of high-frequency characteristics of secondary voltage of substation bus PT

Reactive power switching, re-ignition, vacuum circuit breaker, online monitoring, PT secondary voltage, high frequency characteristic quantity

Aiming at the problem of frequent faults of 10kV or 35kV reactive power compensation reactors in substations, this paper conducts a comprehensive study on the transient voltage and current characteristics combined with on-site switching tests, and obtains the basic characteristics of the re-ignition process and the free oscillation attenuation process, and analyzed the influence of different transient processes on reactors, busbars and cables and other equipment, The results show that when re-ignition does not occur, the transient overvoltage level is within 4 times of the normal operating voltage. Once re-ignition occurs, the transient overvoltage will be greatly increased to 5-7 times.Re-ignition causes the insulation between turns of the equipment to drop, and eventually short-circuit turns are produced on the reactor coil, which develops into fire burning and three-phase tripping. It can be seen that the high-amplitude and high-frequency transient voltage generated by the re-ignition of the circuit breaker during opening is one of the main reasons for equipment failure, and the re-ignition rate can be used as an important evaluation index for the operation of reactive power equipment. Whether the switching reactive equipment of circuit breakers (especially for vacuum short-circuit breakers) produces re-ignition is related to many factors, including the opening characteristics of the circuit breaker equipment itself, as well as the parameters of the field cable, length, and reactor parameters.At present, in the design of substations, the selection of equipment specifications does not consider the re-ignition rate limit requirements，there are no factory and new installation test requirements for the reactive power switching characteristics of the circuit breaker itself.The lack of control over the operating conditions under the combined effect of the above two factors is the main reason for the frequent failure of reactive power equipment.

In response to this problem, this paper proposes a new online monitoring method for evaluating the characteristics of circuit breakers for reactive power switching by judging the re-ignition rate. Obtain the broadband transient signal from the existing PT secondary voltage of the 10kV or 35kV bus in the substation,make full use of the advantage of PT's amplitude-frequency characteristics to reignite high-frequency signal amplification characteristics,fill in the gaps in the evaluation methods of operating quality and transient characteristics of newly installed or in-service reactive power equipment. Further, based on the extraction of characteristic quantities such as free oscillation frequency and attenuation constant in the decay process of open free oscillation in the PT secondary signal, and the comparison with historical values, the identification of early turn-to-turn insulation defects in reactive equipment can be realized. When the inter-turn insulation defect of the reactor occurs, the resonant oscillation frequency and attenuation constant change compared with the historical value due to the change of the inductance parameters.By building a defect reactor test platform in the laboratory and conducting systematic tests, it is found that the oscillation frequency changes before and after a turn-to-turn short circuit occurs in the reactor, which verifies the correctness of the method.

Compared with the previous reactive power switching test method, this method does not need to change the primary wiring of the power grid, which makes up for the shortcomings of the existing method, which can only perform limited switching tests, cannot obtain complete statistical laws, and cannot perform historical data comparison，thus, the long-term stable monitoring and defect early warning of reactive power switching state are realized for the first time.The device can be combined with the switching signal of the Reactive Integrated Control (RIC) of the substation to distinguish the transient process of different reactive devices on the same bus, so as to realize the integrated test of the switching process of all reactive devices on the same bus.The device has high economy, and a large number of applications are expected to achieve a systematic improvement of the operating state of reactive power equipment, and guide the power grid design and equipment selection design，improve the operating quality of reactive equipment.