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5-246 T60 Transformer Protection System GE Multilin

5.6 GROUPED ELEMENTS 5 SETTINGS

and the percentage of shorted turns on the secondary. Therefore, the resultant differential currents may be below the slope

threshold of the main differential element and the fault could go undetected. Application of the restricted ground fault pro-

tection extends the coverage towards the neutral point (see the RGF and Percent Differential Zones of Protection diagram).

Figure 5–117: RGF AND PERCENT DIFFERENTIAL ZONES OF PROTECTION

This protection is often applied to transformers having impedance-grounded wye windings. The element may also be

applied to the stator winding of a generator having the neutral point grounded with a CT installed in the grounding path, or

the ground current obtained by external summation of the neutral-side stator CTs. The Typical Applications of RGF Protec-

tion diagram explains the basic application and wiring rules.

Figure 5–118: TYPICAL APPLICATIONS OF RGF PROTECTION

The relay incorporates low-impedance restricted ground fault protection. This low-impedance form of protection faces

potential stability problems. An external phase-to-phase fault is an ultimate case. Ideally, there is neither ground (IG) nor

neutral (IN = IA + IB + IC) current present. If one or more of the phase CTs saturate, a spurious neutral current is seen by

the relay. This is similar to a single infeed situation and may be mistaken for an internal fault. Similar difficulties occur in a

breaker-and-a-half application of the restricted ground fault, where any through fault with a weak infeed from the winding

itself may cause problems.

842731A1.CDR

35%

RGF

ZONE

DIFFERENTIAL ZONE

WINDING

842732A1.CDR

Transformer Winding

(A) Transformer

(B) Transformer in a Breaker-and-a-Half

(D) Stator without a Ground CT

Stator Winding

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