GE P54C User Manual

|I
bias
|= 0.5 (|I
L
OCAL
| + |I
REMOTE1
| + |I
REMOTE2
|)
Assume a load current of I
L
flowing from end LOCAL to REMOTE1 and REMOTE2. Assume also a high resistance
single-end fault of current I
F
, being fed from end LOCAL. In the worst case, we can also assume I
F
to be in phase
with I
L
:
I
LOCAL
= I
L
+ I
F
I
REMOTE1
= -y*I
L
where 0<y<1
I
REMOTE2
= - (1-y) I
L
|I
di
|= |I
F
|
|I
bias
| = |I
L
| + 0.5 |I
F
|
Phase curr
ent differential protection sensitivity when |I
bias
| < Phase Is2:
The phase current differential protection would operate if |I
diff
| > Phase k1 |I
bias
| + Phase Is1
therefore:
|I
F
| > (Phase k1 |I
L
| + Phase Is1) / (1 - 0.5 Phase k1)
For Phase Is1 = 0.2 pu, Phase k1 = 30% and Phase Is2 =2.0 pu, then
● for |I
L
| = 1.0 pu, the phase current differential protection would operate if |I
F
| > 0.59 pu
● for |I
L
| = 1.59 pu, the phase current differential protection would operate if |I
F
| > 0.80 pu
If |I
F
| = 0.80 pu and |I| = 1.59 pu, then |I
bias
| = 1.99 pu, which reaches the limit of the low percentage bias curve.
Phase current differential protection sensitivity when |
bias
| > Phase Is2:
The phase current differential protection would operate if |I
diff
| > Phase k2 |I
bias
| - (Phase k2 - Phase k1) Phase Is2 +
Phase Is1
therefore:
|I
F
| > (Phase k2 |I
L
| - (Phase k2 - Phase k1) Phase Is2 + Phase Is1) / (1 - 0.5 Phase k2)
For Phase Is1= 0.2 pu, Phase k1 = 30%, Phase Is2 = 2.0 pu and Phase k2 = 100%, then,
● for |I
L
| = 2.0 pu, the phase current differential protection would operate if |I
F
| > 1.6 pu
● for |I
L
| = 2.5 pu, the phase current differential protection would operate if |I
F
| > 2.6 pu
Fault resistance coverage
Assuming the fault resistance R
F
is much higher than the line impedance and source impedance, then for a 33 kV
system and 400/1 CT:
|I
F
| = (Vph-n /R
F
)(1/CT ratio) pu = 47.63/R
F
pu
Based on the analysis above, the phase current differential protection detects a fault current in excess of 0.59 pu
with a load current of 1 pu flowing. The fault resistance is less than 47.63/0.59 = 81 ohms in this case.
With a short time overload current of 2.0 pu, the phase current differential protection can detect a fault resistance
of 47.63/1.6 = 30 ohms or lower.
Chapter 6 - Current Differential Protection P54A/B/C/E
120 P54xMED-TM-EN-1

GE P54C Specifications

General

Brand	GE
Model	P54C
Category	Relays
Language	English