Page 314 - Practical Power System and Protective Relays Commissioning

308 Practical Power System and Protective Relays Commissioning

2. 100% stator earth fault protection
R 2 and R 3 will be two relays as follows:

1. Undervoltage relay 27 works on third harmonic (R 2 ).
2. Overvoltage relay (R 3 ) works on fundamental frequency.
The overlap between 27 and 59 covers 100% of the stator winding.
3. Stator OC protection
I set 5 1.05 I N P 5 333 MW then S 5 417 MVA at a power factor of 0.8.

417 3 10 3
I N 5 5 10; 943 A
O3 3 22
1
5 10; 943 3 5 0:842 A
I N secondary
13; 000
I set 1:05 I N
1:05 3 0:842
5 0:884 A

5 0:9A
SetI set
t 5 6 seconds
Action: trip of CB of transformer (T 1 ) 220 kV side.
4. Negative-phase sequence current I 2 protection (46)
Second stage (trip):
I set 5 0:15 I N
5 0:15 3 0:842 5 0:1263 A
I set 5 0:127 A
Set I set 5 12%
2
2
I 2 .t 5 10 where K 5 I 2 .t, K factor 5 10.
First stage (alarm):
Set I alarm 5 0:9 I set 5 0:9 3 0:12
5 0:108
5 10:8%

Set t alarm 5 3.0 seconds
Action: trip 220 kV side CB of T 1 second stage.
Alarm in first stage.
5. Loss of excitation protection
Loss of excitation leads to loss of synchronism and running at higher
than synchronous speed.
The following impedance relay is used (Fig. 19.6.2).
X 1 1 x 2 5 xd
X 1 5 xd x 2
xd’ 5 33% on 417 MVA base.