Fault detection

Fault detection and clearance are important for any electrical distribution system. For the investigated DC power distribution system, grounding capacitors are required due to the fact that the converter AC sides should be galvanically separated to avoid ground current flowing between the converters through protective earth and the DC bus.

A grounding scheme and an algorithm for detection of ground faults and short circuits in DC distributed power systems, are presented for a system of sources and loads with high impedance or no connection to ground. A grounded system is preferable due to reasons of personal safety and protection of equipment. For DC distributed power systems operated with power electronic converters, the common reference between sources and loads provides a path for the common mode or neutral current. The neutral current appears since the converters have no AC side terminal connected to ground, which means that ground currents cannot be controlled. Also, zero-sequence currents, in the form of third order harmonics, are often included in the modulation signals to utilise the DC side voltage more effectively and reduce the harmonic content (positive- and negative-sequence) of the AC side currents.

The scheme presented here detects ground faults both on the AC and DC sides of the distributed power system. In the investigation, ground faults are focused, but a similar algorithm can be used to detect short circuits by measuring two of the currents flowing into the cable node. The selected detection method relies on communication between nearby cable nodes for fault localisation, since this gives possibility to operate the DC distributed power system even if it is highly segmented due to multiple faults.

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