| DC power systems are developed in many countries of the world, which are applied widely in trolleybuses, ships and subways, etc.. Traditional DC transmission system works always by one terminal to the other, and the protection of short faults is depended on controlling converters and inverters or opening system by AC circuit breakers(CB) in AC side. For the sake of improving the flexibility of DC transmission systems, multi-terminal systems are expected to be adopted, and DC-CB becomes a "bottle-neck" in power system.In order to break short faults, the commutation principle is used and a novel structure DC-CB with permanent magnetic actuator (PMA) is introduced, which puts two vacuum interrupters on each end of a rod with a fixed-point in the middle of it. In this way, we can realize the CB's opening and the current's transferring synthetically, make sure the moving electrode has reached safe distance to withstand the recovery voltage. It presents a new scheme for the further development of DC power system.The commutation principle's realizing is finished by two vacuum interrupters. A transfer loop is paralleled to main interrupter, which consists of a pre-energizing capacitance and an inductance. When the fault appears, the main interrupter (in main line) opens and the sub-interrupter closes at the same time. So the oscillatory current is brought in the capacitance, inductance and arc of main interrupter. The oscillatory current is added to the short current, which forces the current in main interrupter to zero, so the opportunity of extinguishing arc appears. Current commutation can be divided into three stages: 1.short fault happens, and main interrupter opens; 2.sub-interrupter closes, and transfer current is devoted, which forces current to zero; 3.after zero, the arc in main interrupter is extinguished, and the capacitance is energized by system. Mathematics expressions of the three stages above are given and simulated. The influence of system inductance to capacitance voltage is analyzed, and the method of fast opening odd current is given in the paper.Electronic-driven system is put forward firstly in design of DC-CB, and the PMA is used to analyze the precision and reliability of electronic-driven. Single-chip micro-computer is used to perform sampling, judging, and â–³I and di/dt are used to estimate whether fault has happened, then the electronic-driven system will output orders to control CB based on estimation result.Two kinds of fast actuators are analyzed in their magnetic fields. Dynamic analysis and design is done based on the mathematic modules set in the paper. The state equation group is solutioned by means of Runge-kutta method. Compared with the experiment, the simulation result is credible, and it can guide the design of PMA. At the same time, the optimal electrode distance and the lowest prime move-velocity of contact are analyzed in the paper.Finally, breaking experiments of the sample DC-CB' are done on LC oscillatory circuit in our lab, and the largest test current reaches 20 kA. From the experiment results we know that, for the sake ofusing DC-CB with commutation principle, the main interrupter's opening and transfer current's devoting must be cooperated precisely; At the same time, the transfer current must be big enough to force fault current to zero for DC-CB's reliable work (redundancy rate K>1.5). To satisfy it, the measures of enhancing capacitance energizing voltage level or enlarge actual capacitance should be done in the scheme.
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