GE L90 Instruction Manual page 507

CHAPTER 5: SETTINGS GROUPED ELEMENTS
Figure 5-154: RGF and percent differential zones of protection
This protection is often applied to transformers having impedance-grounded wye windings. The element can 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 following figure shows the basic
application and wiring rules.
Figure 5-155: Typical applications of RGF protection
5
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 can 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 can cause problems.
The UR uses a novel definition of the restraining signal to cope with the above stability problems while providing fast and
sensitive protection. Even with the improved definition of the restraining signal, the breaker-and-a-half application of the
restricted ground fault must be approached with care, and is not recommended unless the settings are carefully selected
to avoid maloperation due to CT saturation.
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 5-283