Page 311 - Practical Power System and Protective Relays Commissioning

Protection Relays Settings Chapter | 19 305

16 3 500 3 10 3
5 p
V S2 LV side ffiffiffi ð R 3 1 R 4 1 R CT2 Þ
1000 3 3 3 220
5 20:994 3 1 3 1 2:5Þ
ð
5 20:994 8:5Þ 5 178:449 V
ð
For an autotransformer:
I Neutral5 I LV side I HV side
ð
ð
V S neutral side 5 20:994 15:396Þ R 5 1 R 6 1 R CT3 Þ
ð
5 5:598 1:5 1 1:5 1 2:5Þ
5 5:598 5:5Þ
ð
5 30:789 V
Then, V S2 . V S1 . V S3 .
Choose V s2 as V S 5 180 V.
Use voltage relay 20 200 V with 20-V taps.
Set V S 5 180 V, burden 5 1 VA.
Then V K min should be 2V S .
V K min 5 2 3 180 5 360 V.
Use CT with V K 5 400 V ,a current relay with burden 5 500 Ω, and
current 5 30 mA.
The voltage across the current relay R I will be as follows:
V 5 500 3 0.03 5 15 V
Then a series resistor is required to adjust the setting of the current relay
R I calculated as follows (as shown in Fig. 19.5.11):
180 2 15
R S setting series resistorð Þ 5
0:03
R S 5 5500 Ω

Check if shunt resistance is needed or not.

FIGURE 19.5.11 Relay circuit.