SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Page 20 of 27
b) Ground fault condition
Minimum zero sequence source impedance behind relay
= (0.32 + j4.05) = 4.06
/85.5° ohms
Source ground loop impedance =
Minimum source ground loop impedance
= 2(0.287 + j3.52) + ( 0.32 + j4.05)
3
= (0.298 + j3.7)
= 3.71
/85.4°
Ground loop impedance to Zone 1 reach point
= (2.91 + j7.44 = 7.99
/68.6° ohms
Primary overall minimum source to Zone 1 reach point
Ground loop impedance = (0.298 + j3.7) + (2.91 + j7.44)
= (3.21 + j11.14)
= 11.6
/74°
Xe
/
Re
ratio +
11.14
= 3.47
3.21
Maximum ground fault current at Zone 1 reach point
=
60 x 10
3
3 x 11.6
= 2986A primary
= 2986 x
5
= 74.7A secondary
200
Relay burden for a ground fault is 0.056 ohms
Assuming: CT resistance = 0.06 ohms
CT lead resistance = 0.046 ohms
V
K
≥ 4.7 (1 + 3.47) (0.056 + 0.06 + 0.092)
V
K
≥ 69.5 volts
It is also required that at the CT kneepoint voltage (which according to preceding
calculations should be at least 85V) the CT exciting current should be less than
500mA.
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