Earth Fault Compensation & Current Injection

How Is Earth Fault Compensation Calculated?

Petersen coils (also known as earth fault suppression coils) are used in medium and high-voltage networks so that in the event of a single-pole earth fault, the capacitive current across the fault location is compensated by an inductive current of approximately the same magnitude but in the opposite direction. For this purpose, the coil must be set to an inductive resistance XL in the healthy state of the system, which corresponds approximately to the capacitive resistance XC of the system.

Earth fault compensation in the three-phase system is shown in the following example analogous to the example for the isolated system. With full compensation by the compensation coil, the capacitive earth fault current Ice is fully compensated and in the event of an earth fault, the current at the fault location becomes I_F=0.

Real grids are generally not fully compensated, but usually operated slightly overcompensated, as the zero sequence voltage is highest at full compensation (at the resonance point) and therefore one of the conductor voltages also assumes the most excessive value.

The degree of over- or undercompensation is indicated by the so-called detuning factor v.

v = (I_L – I_C) / I_L

The resonance point and therefore the ideal tuning point for the earth fault suppression coil changes in real grids because the grid capacity changes due to switching. Automatic control of the coil is important to ensure that it is always tuned as appropriately as possible for the current grid status. This can be done with the REG-DP compensation coil regulator from A. Eberle. The REG-DP calculates the correct earth fault compensation automatically and continuously by measuring the resonance curve.

The required inductance of the coil is selected so that it resonates with the capacitance of the network. The resonance condition can be described mathematically by the following equation:

L = 1/ ω² * C

Here, L is the inductance of the coil, C is the capacitance of the network and ω is the angular frequency of the network (ω = 2πf, where f is the network frequency).

In practice, the coil is set so that it compensates the grid capacitance as accurately as possible. This means that the resulting residual current in the event of an earth fault is minimal. The exact setting can also be adjusted on site to take account of fluctuations in the grid capacity.

In extinguished networks, it is particularly important that the earth fault compensation is calculated and set precisely, as the safety and stability of the network depend heavily on the effectiveness of the resonant star earthing.

This earth fault compensation achieves a high degree of operational safety, as earth faults can be compensated without immediate grid disconnection, which is particularly advantageous in complex and large-scale supply grids.