In the power system, the DC power supply system is used as a power source for relay protection, automatic devices, control operation loops, etc. It is the basic guarantee for the correct operation of relay protection and automatic devices. The DC circuit breaker is the most commonly used protection component in the DC system of substations and power plants.
The current DC feed network mostly adopts a tree structure. Generally, at least three levels of power distribution go through from the storage battery to the electrical equipment in the station. As power plants and substations control loads and power loads have higher and higher requirements for DC power supplies, DC circuit breakers have more and more stringent requirements for overload protection and short-circuit protection of electrical appliances. It does not allow any refusal or misoperation. , Especially leapfrogging misoperation, will cause damage to power equipment, system failures, enlarged accidents, and even large-scale blackouts, which will seriously endanger the safe operation of the power grid. In order to prevent the occurrence of such skip-level trip accidents, it is very necessary to test the ampere-second characteristic curve of each DC circuit breaker in the DC system, analyze and determine that the tripping time, dispersion, and internal resistance of the circuit breaker are inconsistent under different short-circuit currents. And the protection level selection coordination, so as to check whether the level difference coordination between the upper and lower circuit breakers of the DC system is reasonable. Therefore, it is very important to carry out the characteristic test of the DC circuit breaker.
It can provide the ampere-second characteristics of the circuit breaker with a maximum rated current of 1000A, and discover in time the hidden dangers of the DC system such as the dispersion and internal resistance of the DC system circuit breaker, and the unreasonable selection of the protection level. Misoperation provides a reliable basis for the level difference coordination between the upper and lower circuit breakers of the DC system, and ensures the safe operation of the DC system of the substation.
Ampere characteristics:
The action of the fuse is realized by the fusing of the melt. The fuse has a very obvious characteristic, which is the ampere-second characteristic.
For the melt, its operating current and operating time characteristics are the ampere-second characteristics of the fuse, also called the inverse time delay characteristics, that is: when the overload current is small, the fusing time is long; when the overload current is large, the fusing time is short.
For the understanding of ampere-second characteristics, we can see from Joule's law that Q=I2*R*T. In the series circuit, the R value of the fuse is basically unchanged, and the heat generation is proportional to the square of the current I, and it is proportional to the heating time T It is proportional, that is to say: when the current is large, the time required for the melt to fuse is shorter. When the current is small, the time required for the melt to fuse is longer. Even if the rate of heat accumulation is less than the rate of heat diffusion, the temperature of the fuse will not rise to the melting point, and the fuse will not even be blown. Therefore, within a certain range of overload current, when the current returns to normal, the fuse will not be blown and can be used continuously.
Therefore, each melt has a minimum melting current. Corresponding to different temperatures, the minimum melting current is also different.