This usually occurs in a three phase system which is not earthed and where a single transformer is used in a voltage-based earth-fault sensing system. The transformer may be connected to one of the phases and the ground. This transformer needs to be provided without sufficient resistance loading. The no-load or exciting impedance of the transformer is in parallel with the line capacitance of the phase to which the transformer is connected.
This situation can be avoided by providing sufficient resistance loading to the secondary of the voltage transformer. This reduces the inductive impedance of the transformer and limits the shifting neutral voltage.