GE G60 UR Series, Instruction Manual

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5-166 G60 Generator Protection System GE Multilin
5.6 GROUPED ELEMENTS 5 SETTINGS
5
Figure 5–87: 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.
The UR uses a novel definition of the restraining signal to cope with the above stability problems while providing for 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.
The differential current is produced as an unbalance current between the ground current of the neutral CT (IG) and the neu-
tral current derived from the phase CTs (IN = IA + IB + IC):
(EQ 5.18)
The relay automatically matches the CT ratios between the phase and ground CTs by re-scaling the ground CT to the
phase CT level. The restraining signal ensures stability of protection during CT saturation conditions and is produced as a
maximum value between three components related to zero, negative, and positive-sequence currents of the three phase
CTs as follows:
(EQ 5.19)
The zero-sequence component of the restraining signal (IR0) is meant to provide maximum restraint during external ground
faults, and therefore is calculated as a vectorial difference of the ground and neutral currents:
(EQ 5.20)
842732A1.CDR
IG
IG
IA
IA
IB
IB
IC
IC
2
1
2
1
2
1
IG
IG
Transformer Winding
Transformer Winding
(A) Transformer
(B) Transformer in a Breaker-and-a-Half
(C) Stator
(D) Stator without a Ground CT
Stator Winding
Stator Winding
IA
IA
IA
IA
IB
IB
IB
IB
IC
IC
IC
IC
Igd IG IN + IG IA IB IC + + + = =
Irest max IR0 IR1 IR2
, , ( )
=
IR0 IG IN – IG IA IB IC + +
( )
– = =