This paper analyses the transient characteristics and breaking performance of direct-current\n(DC) forced-interruption vacuum interrupters in 270 V power-supply systems. Three stages\nare identified in forced interruption: the DC-arcing stage, current-commutation stage, and\nvoltage-recovery stage. During the current-commutation stage, the reverse peak-current coefficient\nk, which is a key design factor, is used to calculate the rate of current at zero-crossing (di/dt).\nMATLAB/Simulink simulation models are established to obtain the transient characteristics\ninfluenced by the forced-commutation branch parameters and the coefficient k. To study the breaking\nperformance of spiral-type contacts, experiments are conducted for different contact materials and\narcing times for currents less than 3.5 kA. During the DC-arcing stage, a locally intensive burning arc\nis observed in the CuW80 contact; however, it is not observed in the CuCr50 contact. On examining\nthe re-ignition interruption results of the CuW80 contact, the intensive burning arc is found to be\npositioned within a possible re-ignition region. When the arcing time is longer than 1 ms, the intensive\nburning arc occurs and affects the breaking performance of the spiral-type contacts. If the DC-arcing\nstage is prolonged, the total arcing energy increases, which leads to a lower breaking capacity.
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