Grounding and fusing

Each converter is connected to the DC ring bus through a cable node. The cable node contains grounding, fuses, circuit breakers and current transducers required for protection (Figure 5.1). The protection capacitors C, or more correct, grounding capacitors are selected so that the time constant of ground fault currents is sufficiently long for proper detection. The idea of the fault detection capacitors is that they should provide a minimum ground fault current, independent of the cable capacitance. Since the fault current is a quantity mainly determined by the cable properties, the capacitance of the fault detection capacitors is determined by the cable impedance and not the rated power and voltage of the converter connected to the cable node. This makes sense, since the same type of cable is used throughout the entire ring bus due to the fact that the cable rating should not limit the power flow in the case of a broken ring bus.

1 dcbpR L,

1 dcbpR L,

Figure 5.1: Cable node with two T-model cable segments and one converter connected. The interfaces to the cable segments are denoted A (left) and B (right).

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