Page 300 - Practical Power System and Protective Relays Commissioning

294 Practical Power System and Protective Relays Commissioning

Z PSB 5 1:3 Z 3F
5 1:3 3 16:042
ð
5 20:854 Ω secondary valueÞ
5 20:854 Ω
Z PSB
where Z PSB 5 power swing blocking impedance setting, Z 3 5 relay third zone.
Load encroachment into Z 3 reach
0:8 rated voltage
5
Z Load min
1:2 rated current O3
0:8 3 220000
5
1:2 3 800 3 O3
5 105:85 Ω
Z Load min secondary 5 105:85 3 0:363 5 38:422 Ω

Z 3 Reach , 0:9 Z Load min
16:041 , 0:9 3 38:422
16:041 , 34:58; the above criteria are correct:

Subchapter 19.5

Differential Protection Setting

19.5.1 INTRODUCTION
Transformer protection can be high-impedance differential protection or low-
impedance differential protection. Both have setting calculations as shown in
the following examples.

19.5.2 LOW-IMPEDANCE DIFFERENTIAL PROTECTION
19.5.2.1 Worked Example 1

(Fig. 19.5.1)
Tr 66/11 kV 25MVA Dy1
Tap changer 56 10% of 11 kV side nominal voltage.
Calculation:
HV side:
25 3 10 3
I 1 5 p ffiffiffi 5 219 A
3 3 66
Choose CT ratio 5 250/1