Neutral Inversion in Ungrounded Three phase systems

Neutral Inversion is a phenomenon in which the neutral of an ungrounded three phase system falls out of the voltage triangle

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. 

Hence, if the exciting impedance is more it the ratio of the capacitive impedance to the inductive impedance decreases causing a higher voltage in the neutral.  This high neutral can sometimes shift outside the triangle.

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.

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