GE P643 User Manual

Note:
Is1 and Is2 ar
e relative to the line CT, which is always the reference CT.
3.3.2 DELAYED BIAS
To provide further stability when external faults are being cleared, the protection checks for the highest value of
bias curr
ent calculated during the previous cycle. If that value is higher than the present value, it is used to restrain
the tripping decision. This is referred to as the Residual Bias. The Residual Bias technique maintains through fault
stability for clearance of external faults.
3.3.3 TRANSIENT BIAS
If there is a sudden increase in the mean-bias measurement, an additional bias quantity is introduced in the bias
calculation. Transient Bias pr
ovides stability for external faults where CT saturation might occur.
The transient bias function enhances the stability of the differential element during external faults and allows for
the time delay in CT saturation caused by small external fault currents and high X/R ratios.
No transient bias is produced under load switching conditions, or when the CT comes out of saturation.
3.3.4 RESTRICTED EARTH FAULT LOGIC
REF HV Start
REF Trip HV
&
REF IH2 Start HV
IH2 REF Block HV
Enabled
I
d
i
f
f
H
V
CTS HV
&
Is1
Is2
I
diff
I
bias
I
bias
HV
HV-LZREF sf OOR
Blk REF HV
1
DT
V00708
Note: Same principle applies for LV and TV windings
Figure 79: Low impedance restricted Earth Fault logic
3.4 HIGH IMPEDANCE REF
A high impedance restricted earth fault protection function is available for up to three windings. An external
r
esistor is required to provide stability in the presence of saturated line current transformers.
TN1 CT is associated with the HV winding high impedance REF, or with the autotransformer high impedance REF.
TN2 CT is associated with the LV winding high impedance REF and TN3 CT is associated with the TV winding high
impedance REF.
Current transformer supervision, HV StubBus Act, LV StubBus Act or TV StubBus Act signals do not block the high
impedance REF protection. You must configure logic in the PSL to block the high impedance REF when any of the
above signals are asserted.
P64x Chapter 8 - Restricted Earth Fault Protection
P64x-TM-EN-1.3 175

GE P643 Specifications

General

Brand	GE
Model	P643
Category	Relays
Language	English