As of today, there are still many customers in the choice of AC and DC universal products, has been the AC protection circuit breaker for DC system protection, due to the particularity of DC system, such application will make the system is difficult to split the potential risk and the obvious error of advance action or delay action.

DC circuit breaker has the characteristics of convenient installation and maintenance, flexible operation and high stability, and has been widely used in power DC system. It is responsible for relay protection, self-actuating device, control operation circuit in the production of fault current protection. The DC system of the substation adopts a variety of power supply modes, such as radial, radial and ring mixing, and ring, etc. The protective electrical appliances are generally used in series of three or four levels.

A typical three-stage series system in a substation is as follows: the first stage is the battery outlet circuit breaker or fuse, the second stage is the power bus and control bus feeder circuit breaker, and the third stage is the measurement and control protection screen circuit breaker or the high-voltage circuit breaker operating circuit breaker. The third level of the four-level difference system is the power screen circuit breaker, the fourth level is the measurement and control protection screen circuit breaker or the operation circuit breaker of the high-voltage circuit breaker, the first and second level protection appliances are the same as the three-level difference system.

The correct selection of the upper and lower levels of the DC circuit breaker and whether the fault current of the DC power supply is limited in a small range are related to the safety of the power system operation, and are important to prevent system damage, accident expansion and equipment damage.

1. Factors affecting differential fit characteristics

In dc system, due to the local power supply bureau of each substation dc screen and battery capacity selection, measurement and control panel power supply mode, connecting method and protect important degree of different technical requirements, such as the distribution location corresponding to different, lead to wire, conductor selection of cross-sectional area, length is different, the difference of these factors, will make the loop resistance produces change, The change of the resistance value causes the short-circuit current value to change accordingly, so that the short-circuit current of each station is different. Therefore, the circuit breaker level difference matching scheme design of substation DC system, because of the above factors change too much, has not been a good way to solve.

2. Existing pitfalls

In order to meet the selective requirements of level difference coordination and realize the superior circuit breaker does not operate by mistake, or even if the superior circuit breaker operates by mistake, it will not cause the accident diffusion. Now, in many DC systems, the one-to-one power supply mode is adopted between the circuit breaker on the feed screen (or power distribution screen) and the circuit breaker on the measurement and control protection screen. The problems caused by this mode are as follows: There are two wires between each pair of circuit breakers, the distance between the feed screen (or power screen) and the measurement and control protection screen is small, more than a dozen meters, more than a hundred meters, this and many long wires bundled together, after many years of operation and use will produce complex and chaotic problems, such as: wire and circuit breaker connection loose; The insulation of the wire drops; Accidental injury, bite; The interwoven wires hide the hazards of multi-channel mutual short circuit or arc discharge and fire, which will lead to a major hidden danger of the complete loss of power of the measurement and control protection screen.

3. The importance of differential fit characteristics

Whether the upper and lower levels of a DC circuit breaker can achieve the level difference depends on the short circuit current value and the operation time difference between the upper and lower levels of the circuit breaker. The fundamental application is to improve the current limiting performance and breaking speed of 2A~6A circuit breaker on the measurement and control protection screen, and to increase the threshold value of short-circuit instantaneous tripping device action multiple of DC circuit breaker on the power screen and feed screen to delay its action. Only when the short-circuit current value in the circuit is lower than the transient value of the superior circuit breaker, or although it has reached the transient value of the superior circuit breaker but the inferior circuit breaker is broken before the superior, can the differential fit between the circuit breakers be realized. Therefore, in order to achieve differential coordination, one way is to realize circuit breaker action time lag between the superior and the subordinate, another way is to reduce the circuit short circuit current value, current limiting type small constant value of dc circuit breaker has significant effect to reduce the circuit short circuit value, therefore the bottom level of small fault current limiting performance is selective with the key elements of the dc system.