GE 750 User Manual

GE Power Management 750/760 Feeder Management Relay 12-
37
12 S5 PROTECTION 12.5 SENSITIVE GROUND CURRENT
12
12.5.6 RESTRICTED EARTH FAULT
Restricted Earth Fault protection is often applied to transformers having grounded wye windings, to provide
sensitive ground fault detection for faults near the transformer neutral. The Sensitive Ground Input (terminals
G3, H3) can be used.
Although the 750/760 is designed for feeder protection, it can provide Restricted Earth Fault protection on
transformers that do not have dedicated protection. To use the 750/760 for this type of protection, a stabilizing
resistor and possibly a non-linear resistor will be required.
The inclusion of a stabilizing resistor encourages the circulating fault current to flow via the magnetizing imped-
ance of the saturated current transformer thus minimizing spill current in the REF relay. A non-linear resistor
will be required where the voltage across the inputs would be greater than 2000 volts. Refer to Section 3.2.7:
RESTRICTED EARTH FAULT INPUTS on page 3–10 for the connections required to use the 750/760 to per-
form Restricted Earth Fault protection.
a) STABILIZING RESISTOR
To determine the appropriate value for the Stabilizing Resistor, use the following equation:
where:
R
S
= resistance value of the stabilizing resistor
V
S
= voltage at which the 750/760 will operate
I
S
= current flowing through the stabilizing resistor and the 750/760
I
F
= maximum secondary fault current magnitude
R
CT
= internal resistance of the current transformer
R
L
= resistance of attached wire leads
b) NON-LINEAR RESISTOR
A non-linear resistor is recommended if the peak fault voltage may be above the relays maximum of 2000 V.
The following calculation is done to determine if a non-linear resistor is required. When required, this should be
provided by the end-user.
It is assumed that the ratio of the CT kneepoint (
V
K
) V
S
is to 2 for stability. Thus,
Next, the voltage that would result from a fault must be determined, neglecting saturation,
The peak value of this fault voltage would be:
If V
P
is greater than 2000V, then a non-linear resistor must be used.
R
s
V
s
I
s
------
I
F
R
CT
2 R
L
+
()⋅
I
s
------------------------------------------
==
V
K
2 V
S
=
V
f
I
f
R
CT
2 R
L
R
S
++
()⋅
=
V
P
22 V
k
V
f
V
K
–
()⋅⋅
=

GE 750 Specifications

General

Brand	GE
Model	750
Category	Relays
Language	English