Transformer differential protection
Transformer differential protection
Transformer differential protection protects against short-circuits between turns of a winding and between windings that correspond to phase-to-phase or threephase type short-circuits.
If there is no earthing connection at the transformer location point, this protection can also be used to protect against earth faults. If the earth fault current is limited by an impedance, it is generally not possible to set the current threshold to a
value less than the limiting current. The protection must be then carried out by a high impedance differential protection.
Transformer differential protection operates very quickly, roughly 30 ms, which allows any transformer deterioration in the event of a short-circuit between windings to be avoided.
Transformers cannot be differentially protected using high impedance differential protection for phase-to-phase short-circuit due to the natural differential currents that occur:
– the transformer inrush currents. The operating speed required means that a time delay longer than the duration of this current cannot be used (several tenths of a second);
– the action of the on-load tap changer causes a differential current.
The characteristics of transformer differential protection are related to the transformer specifications:
– transformation ratio between the current entering Iin and the current leaving Iout ;
– primary and secondary coupling method;
– inrush current;
– permanent magnetizing current.
The block diagram is shown below:
Source : Protection of electrical networks / Christophe Prévé.
