Application Notes P64x/EN AP/C63

MiCOM P642, P643, P645

(AP) 6-21

AP

To ensure stability of the protection, the LV zero sequence current must be eliminated from the differential current. Traditionally this has been achieved by either delta connected line CTs or by the inclusion of a delta winding in the connection of an interposing current transformer.

In accordance with its definition, the zero-sequence current is determined as follows from vector and amplitude matched phase currents: � � � �� � � � � ��� � � �� � � � � �� � � �� � � � � � ����

_,_,_,03

1

The current that is used in the differential equation is the filtered current per phase:

0_,,

0_,,

0_,, ��� ��� ��� � � �� � � � ��� � � � � � � �� � � �� � � � ��� � � � � � � �� � � �� � � � ��� � � � � � � ��

Setting the zero-sequence current filtering function is very simple and does not require any calculations. Zero-sequence current filtering should only be activated for those ends where there is operational earthing (grounding) of a neutral point:

Figure 11: Zero sequence current filtering

Figure 12 shows the current distribution for an AN fault on the delta side of a Yd1 transformer with a grounding transformer inside the protected zone.