Alstom SHPM 101 - Section 19. CURRENT TRANSFORMER REQUIREMENTS; 19.1 Worked example No. 1

SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Page 12 of 27
Section 19. CURRENT TRANSFORMER REQUIREMENTS
Class X CT are required to meet the following specification:
a) V
K
≥ I
F
(1 + X/
R
) (Z
R
+ R
CT
+ R
L)
for phase faults
≥
I
Fe
(1 + Xe/
Re
) (Z
Re
+ R
CT
+ 2R
L
) for ground faults
where V
K
= required CT kneepoint voltage
I
F
= maximum secondary fault current for a 3 phase fault at the
Zone 1 reach point
I
Fe
= maximum secondary fault current for a ground fault at the
Zone 1 reach point
X/
R
= system reactance/resistance ratio between source and a three
phase fault
Xe
/Re
= system reactance/resistance ratio between source and a
ground fault
Z
R
= relay burden for a phase
Z
Re
= relay burden for a ground fault
R
CT
= CT secondary winding resistance
R
L
= resistance of one lead between CT and relay
b)
I
e
≤ 0.1 I
n
at V
K
where I
e
= CT exciting current
I
n
= CT rated secondary current
Where necessary, ALSTOM T&D Protection & Control Ltd, can supply details of
reduced CT requirements on some systems.
19.1 Worked example No. 1
To protect a 10km single circuit, wooden pole, 60kV overhead line using a
permissive overreach transfer tripping scheme or Zone 1 extension scheme.
EXAMPLE SYSTEM
April 60kV
May 60kV
June 60kV
July 60kV
30km
150/60kV
50MVA
12.5%
20km10km
R
2 x 150/60kV
50MVA
12.5%
150/60kV
10MVA
10%