Alstom SHPM 101 - Section 19. CURRENT TRANSFORMER REQUIREMENTS; 19.11 Sketching relay characteristics; 19.12 Circuit transformer requirements

SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Page 19 of 27
For the low set phase current level detectors in this application:
Sensitivity =
4.8
x 5% = 17.1%
1.4
The low set phase current level detectors, therefore, have adequate sensitivity for
faults at the Zone 3 reach point.
19.11 Sketching relay characteristics
Using the methods described in Appendix B, the relay characteristics can be
sketched. It is unlikely that the power swing blocking facility would be used in this
particular application, but the power swing blocking characteristics have been
sketched for completeness.
19.12 Circuit transformer requirements
The requirements for the Class X CTs are given in Section 19.
a) Phase fault condition
Minimum positive sequence source impedance behind relay
= (0.287 + j3.52) = 3.53
/85.3 ohms
Positive sequence impedance to Zone 1 reach point
= 4.0
/70° ohms primary
= (1.37 + j3.776) ohms primary
Overall minimum source to Zone 1 reach point phase fault impedance
= (0.287 + j3.52) + (1.37 + j3.76) ohms
= (1.66 + j7.28) ohms
= 7.47
/77.2° ohms
X
/
R
ratio = 7.28
–
1.66 = 4.39
Maximum 3 phase fault current at Zone 1 reach point
=
60 x 10
3
3 x 7.47
= 4637A primary
= 4637 x
5
= 116A secondary
200
Relay burden for a phase fault is 0.03 ohms
Assuming: CT resistance = 0.06 ohms
CT lead resistance = 15m of 6.0 mm
2
copper
(3.09 ohms/km)
= 0.046 ohm
CT kneepoint voltage requirement for a 3 phase fault is given by:
V
K
≥ 116 (1 + 4.39) (0.03 + 0.06 + 0.046)
V
K
≥ 85 volts