A protection scheme against DC faults VSC based DC systems with bus capacitors

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Abstract
This paper describes a novel protection method to limit the fault currents caused by short circuit in a voltage source converter (VSC) based DC systems with capacitors connected to the DC bus. Analyzing the development of DC fault current in such DC systems shows that capacitor discharge dominates the very rapid fault current rising at the instance of faults. If this high slope fault current surge is not limited, stored energy in the capacitor will cause hazard to personals as well as connected equipment. If the discharging current from the DC bus capacitor is not limited, it will require much higher break capability of the DC breakers. Therefore a novel protection method is proposed to limit the DC fault currents by detecting and interrupting the DC capacitor discharge using a solid state circuit breaker (capacitor SSCB) which is connected in series with the DC bus capacitor. Implementing this new method will effectively reduce peak value of the fault current, alleviate current stress of diodes in VSC bridges, in a way that only local current information is required. It needs no external control signals, and power loss of this capacitor SSCB is relatively low. Both simulations and tests have proved this method to be effective.

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