GROUNDING TRANSFORMER DIFFERENTIAL RELAYING

Configuration
To establish a grounded system, a grounding transformer is frequently tapped on the low-side leads of the supply transformer and is thereby included in the transformer differential zone.


Problem
Zero sequence current supplied by the grounding transformer may cause differential relay operation during an external ground fault.

Solution
Since external ground faults cause zero sequence current to flow in the CT secondary circuits, a zero sequence filter is provided for the low-side differential Y-connected transformers. This filter is composed of three auxiliary CTs and can be formed in several ways.

The simplest form is to connect the primaries in Y and the secondaries in Δ. In Figure below, the ratio of the auxiliary CTs is not critical, but a 5:5 ratio is suggested.

Grounding transformer in differential zone (external fault condition shown)

The alternative filter connection in Figure 16 requires a 1:3 ratio for the auxiliary CTs. The primaries are connected in Y and the junction or sum of the primaries is wired to the secondaries connected in series.

Thus the secondaries carry three times the primary current. Both of these connections present relatively high magnetizing impedance to all but zero sequence current.

However, modern differential relays are of even lower burden than the usual auxiliary CTs. Thus, the common point of the relay connections should not be connected to the common point of the Y-connected transformers (a connection that is necessary without the zero sequence filter).

Only the filter neutral should be connected to the CT common point. The figure also shows the primary current and CT secondary current for an external ground fault.

The zero sequence filter prevents a relay imbalance. A 1:1 overall voltage ratio is assumed in the figure above with 1 PU fault current flowing.


An application example using an inverse relay has relay pickup at 30% of transformer rating and a time of 1.7 s at 300% of setting. The pickup depends on the effective contribution of the equivalent tertiary of the case.

The transformer manufacturer should be consulted.