Analysis of short-circuit current interruption by SF6 high-voltage circuit breaker

SF6 high-voltage circuit breakers play an important role in the distribution network. As electricity enters the market, the requirements for power supply reliability are getting higher and higher, and the status inspection of electrical equipment is gradually carried out. For SF6 high-voltage circuit breakers, many manufacturers can achieve maintenance-free sealing, mechanism transmission, etc. within the mechanical life. The main basis for determining the power outage maintenance or replacement of high-voltage circuit breakers is the number of short-circuit currents they break. Therefore, in the bidding of high-voltage circuit breaker equipment, the number of times the circuit breaker breaks the rated short-circuit current has become an important indicator.

1. Main arc extinguishing forms of SF6 circuit breakers

1. Characteristics of variable distance arc extinguishing structure

In recent years, a large number of imported and joint venture circuit breakers have been introduced into the power system, mainly products of SIEMENS, ABB, ALSTOM, AEG and other companies. This type of circuit breaker uses SF6 as the insulating medium. The arc extinguishing chamber structure of the circuit breakers of 110 kV and above produced by most manufacturers adopts a variable-distance arc extinguishing structure. The arc extinguishing principle is nothing more than self-energy type, compressed air type, and self-energy compressed air type. The compressed air type has a fast voltage build-up, the contacts are not easy to wear, and it is easy to cut off the current when breaking a small current; the self-energy type has a slow voltage build-up, the arc contacts are easy to burn, and the reliability is not high when breaking a small current; the self-energy compressed air type combines the two methods, using the self-energy principle to break large currents and the blowing principle to break small currents. In addition, some domestic circuit breaker manufacturers have also introduced foreign technology to develop some products, such as the LW14 (15) of Xi'an High Voltage Switch Factory and the LW6 of Pingdingshan High Voltage Switch Factory.

2. Characteristics of fixed-distance arc extinguishing structure

The arc extinguishing chamber structure of 220 kV and above circuit breakers produced by SIEMENS adopts the principle of fixed-distance compressed air bidirectional internal injection arc extinguishing. Two static contacts equipped with arc-resistant graphite contacts are fixed, and the connecting rod drives the bridge-type moving contact with arc-strike graphite rings in the arc-strike position to move, realizing the opening and closing of the circuit breaker. The graphite nozzle of the fixed-distance arc extinguishing chamber is its main feature, with precise processing, uniform electric field, and excellent arc resistance, thus extending the electrical life. At present, SIEMENS has also begun to produce 220 kV circuit breakers with variable-distance arc extinguishing principle.

2. Short circuit breaking test

The short-circuit breaking test of the circuit breaker must have a large-capacity power supply, that is, it can provide a capacity equivalent to that of the power system short-circuit in a short time. When the rated parameters of the circuit breaker are increased, due to the limitations of the test level and conditions, even if the synthetic test method (two power supplies are used during the test, one for current and the other for voltage) is selected, it is difficult to meet the test requirements. A reasonable and effective test method must be selected.

At present, the breaking test of domestic circuit breakers consists of three methods: overall test, single-phase test and unit test. The overall test is to conduct a three-phase test on a fully assembled three-phase circuit breaker according to the rated voltage and rated short-circuit breaking current specified by the circuit breaker. This test is closest to the actual breaking situation and is the most realistic way to assess the performance of the product. However, the required test capacity is equal to the rated short-circuit breaking capacity of the circuit breaker, and the test investment is quite large, which is almost impossible for large-capacity circuit breakers. Generally, the overall test is limited to circuit breakers of 35 kV and below and rated breaking current of 31.5kA and below.

For circuit breakers of 110 kV and above, the single-phase test method is used. If the rated voltage of the circuit breaker is U and the rated short-circuit breaking current is Is, the capacity required for the overall test is P=√3UIs. When testing single-phase, the first-opening phase coefficient of 1.3 is considered, so the capacity of the test equipment is P1=1.3UP IS=1.3U/√3×IS=0.43P. Therefore, most of the three-phase circuit breakers with separate phases use single-phase tests to assess the breaking capacity of the circuit breakers. The rated short-circuit breaking of SW6, SW2 and other models of circuit breakers that were widely used in the past can assess each phase. For example, the SW6 manual of Xigao Factory clearly states that without any maintenance, SW6 can break the rated short-circuit current 13 times in a row. Our understanding is that each phase can break the rated short-circuit current 13 times.