PERFORMANCE COMPARISON OF SF6 COMBINATION APPLIANCE AND CIRCUIT BREAKER

Performance Comparison of SF6 Combination Appliance and Circuit Breaker

Breaking no-load transformer performance

Most of the load properties of the ring main unit in the power distribution system are the splitting and closing of the distribution transformer. The general capacity is 315-1250 kVA, and the no-load current of the distribution transformer is about 2% of the rated current. When breaking the no-load transformer, if the circuit breaker has strong arc extinguishing ability, and the arc extinguishing method is external energy type, the phenomenon of forced arc extinguishing current cut-off before zero will occur, resulting in cut-off overvoltage. If a circuit breaker is used, because of its arc extinguishing capability, it is designed to break a large short-circuit current. When breaking a small current of a no-load transformer, forced arc extinguishing is likely to occur, resulting in a cut-off overvoltage. In this regard, it is not desirable to use circuit breakers with strong arc extinguishing capabilities. And SF6 load switch, due to the electrical characteristics of SF6 gas, when the arc is cooled in SF6 gas, it still conducts electricity until the temperature is quite low, and the current cutoff before the current zero crosses is small, thus avoiding high overvoltage. Therefore, the performance of using SF6 load switch to break the no-load transformer is better than that of SF6 circuit breaker, and SF6 circuit breaker is better than vacuum circuit breaker.

Protect the performance of distribution transformers

For the protection of distribution transformers, the use of SF6 load switch + fuse combination is more effective than circuit breakers, and sometimes the latter cannot even play an effective protective role. The short-circuit test proves that when the oil-immersed transformer is short-circuited, in order to prevent the oil tank from exploding, the fault must be removed within 20 milliseconds. Limiting the short-circuit current value can effectively protect the transformer. The full breaking time of the circuit breaker consists of three parts, the relay protection action time + the inherent action time of the circuit breaker + the arcing time, which generally requires three cycles (60 milliseconds), so it cannot effectively protect the transformer. Even dry-type transformers are better than using circuit breakers because of the fast protection action of fuses.

Relay protection coordination performance

The protection of the circuit breaker at the head end of the ring network distribution network (the 10kV feed-out circuit breaker of the substation) is generally set to be 0 seconds for quick break, 0.5 seconds for overcurrent, and 0.5 seconds for zero sequence. If a circuit breaker is used in the ring main unit, even if it is set to 0.5 seconds, it is difficult to ensure that the first-end circuit breaker does not act first due to the dispersion of the inherent action time of the circuit breaker. The current-limiting fuse is not affected by the voltage drop caused by the short circuit, and can remove the fault in a very short time without causing a leapfrog trip.