GE 750 Instruction Manual page 200

S5 PROTECTION
5 - 74
The inclusion of a stabilizing resistor encourages the circulating fault current to flow via the
magnetizing impedance 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 V. Refer to Restricted Earth Fault Inputs on page 3–12 for the
connections required to use the 750/760 to perform Restricted Earth Fault protection.
To determine the appropriate value for the Stabilizing Resistor, use the following equation:
where:R = resistance value of the stabilizing resistor,
S
V = voltage at which the 750/760 will operate
S
I = current flowing through the stabilizing resistor and the 750/760,
S
I = maximum secondary fault current magnitude
F
R = internal resistance of the current transformer, and R
CT
leads
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
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 is greater than 2000 V, then a non-linear resistor must be used.
P
Sample Application:
The CTs used in this example are 3000/1, 10P10, 15 VA, and the transformer used in the
example is an 11 kV / 400 V, 2000 kVA. At 10P10 the voltage at which the CT will saturate
will be 10 x 15 = 150 V. An equivalent IEEE description for this CT would be 3000/1, C150.
V I ⋅ ( R 2R )
+
s F CT L
R
---- - ------------------------------------ -
= =
s
I I
s s
) V is to 2 for stability. Thus,
K S
V 2V
=
K S
⋅ ( )
V I R 2R R
= + +
f f CT L S
⋅ ⋅ ( )
V 2 2 V V V
= –
P k f K
750 FEEDER MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETPOINTS
(EQ 5.5)
= resistance of attached wire
L
(EQ 5.6)
(EQ 5.7)
(EQ 5.8)