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