Alstom SHPM 101 - 19.20 Sketching relay characteristics; 19.21 Circuit transformer requirements; a) Phase fault condition

SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Page 26 of 27
19.20 Sketching relay characteristics
Using the methods described in the Appendix B the relay impedance
characteristics can be sketched.
19.21 Circuit transformer requirements
The requirements of the Class X CT are given in Section 19.
a) Phase fault condition:
Minimum positive sequence source impedance behind relay (neglecting small
infeed via parallel FEB – MARCH circuit from AUG source) is the minimum JAN
source impedance plus the impedance of the two JAN - FEB circuits in parallel.
= 3.48
/88° +
20
x 0.44 /69°
2
= (0. 12 + j3.48) + (1.58 + j4.11)
= (1.7 + j7.59) = 7.78
/77.4° ohms primary
Impedance to Zone 1 reach point (already calculated)
= 20.96
/70° = (7.17 + j19.7) ohms primary
Source to fault impedance = (1.7 + j7.59) + (7.17 + j19.7)
= (8.87 + j27.29) = 28.7
/72°
Maximum relay current for a phase fault at the Zone 1 reach point
=
132 x 10
3
3 x 28.7
= 2655A primary
= 2655 x
1
500
= 5.31A secondary
X
/
R
ratio =
27.29
= 3.08
8.87
Relay burden for a phase fault is 0.26 ohms.
Assuming: CT resistance = 2.5 ohms
CT load resistance = 15 m of 2.5 mm
2
copper
(7.41 ohms/km)
= 0. 11 ohms
CT kneepoint voltage requirement for a 3 phase fault is given by:
VK ≥ 5.31 (1 + 3.08) (0.26 + 2.5 + 0.11)
VK ≥ 62.2 volts