Date: 2019-07-24 12:35:08 Views: 15
GCB is special circuit-breaker for generator， it is produced totally aimed at special technical requirement for directly connect to generator circuit，it has strong ability of breaking the DC component of the short-circuit current and step-breaking ability, extremely long life for mechanical and electric operation, its three-phase linkage operating mechanism can provide safely synchronous operation, it also can reduce malfunction average recovery time of step-up transformer and generator if apply GCB, and increase availability of power station, thus, improve plant efficiency, especially improve the ability of GCB breaking generator exit delay zero passage short-circuit current, it is not possessed by common distribution type circuit breaker.
Each type of AC circuit-breaker breaking need a current zero passage, arc will separate by itself on current zero passage. Under certain conditions, short circuit current come from generator may has 100ms or longer delay current zero. Under no-load state of generator, if short circuit occurred on the state of lowest voltage, then, it will appear asymmetrical short-circuit current with a DC component. Especially before malfunction, when the generator is in the state of less Shelly magnetic and with leading power factor, it will appear asymmetric highest value. On this condition, the DC component of short-circuit current will be higher than symmetrical components, and will keep to the delay current zero.
The AC component and DC component of short circuit current have different attenuation law, AC component decrease progressively according to the exponent along with generator short-circuit sub-transient and transient time constant, DC component attenuate according to the exponent law along with short circuit time constant. In accordance with different size and design structure of generator, these time constant values change greatly, when AC component of short circuit current attenuate faster than DC component, it will produce delay current zero in a certain period of time. As for small capacity generator, because of the small ratio of the inductance and resistance, short circuit time constant is small, DC component attenuate quickly, short circuit usually take tens of milliseconds, namely through zero, arc extinction is easier. While, short circuit time constant of 35MW generator is larger, exceeds 150ms, DC component attenuate slowly, there are conditions that short circuit current do not go through the zero point after hundreds of milliseconds. If generator exit select conventional power distribution circuit breakers, it will produce abnormal over-voltage when breaker action cut short circuit fault, arc extinction is not easy. When need to select GCB, take advantage of circuit breaker contacts separating produce a high arc voltage, to increase the arc resistance in series with Ra, make DC component of short circuit currentattenuate rapidly, thereby forcing pass zero. That is to say, only GCB can satisfy cut short circuit current when do not pass zero, without producing dangerous overvoltage.
Besides GCB apply to breaking the current do not pass zero, it also has high insulation level, strong step-breaking capacity, capacity of breaking small inductors and small capacitors current etc. The effect of protecting generator and main transformer is irreplaceable by normal distribution circuit breaker. In recent years, there has been some small hydropower in succession selected set of general distribution type vacuum circuit breaker cabinet substitute for GCB use, although these users had installed over-voltage protection devices (some users even just equip zinc oxide arrester on both sides of vacuum circuit breaker), but there is a hidden danger in the long term. First, when the new unit is put into operation, it has a relatively high level of insulation, Insulation can still fit, but the unit insulation level decline gradually along with run time increased, insulation coordination became a problem. Second, after all, the performance of general distribution type breaker has a large difference with the requirement for performance of the generator exit circuit breaker, it is difficult to qualify for harsh conditions of use. Therefore, there may be random damage to the unit and insecurity factors.