SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Page 27 of 27
b) Ground faults:
Minimum ground loop impedance behind relay is minimum JAN source ground
loop impedance plus ground loop impedance of two JAN – FEB circuits in
parallel.
Minimum JAN source ground loop impedance
= 3.48
/88° = (0. 12 + j3.48) ohms primary
Ground loop impedance of two JAN – FEB circuits
=(
4.33 + j12.4) = (2.17 + j6.2) = 6.57 /70.7°ohms primary
2
Ground loop impedance behind relay
= (0. 12 + j3.48) + (2.17 + j6.21)
= (2.29 + j9.68) = 9.95
/76.7° ohms primary
Ground loop impedance to Zone 1 reach point
= (9.84 + j29.6) = 31.16
/71.6° ohms primary
Source to Zone 1 reach point ground loop impedance
= (2.29 + j9.68) + (9.84 + j29.6)
= (12.13 + j39.3) = 41.1
/72.8° ohms primary
Maximum fault current for a ground fault at the Zone 1 reach point
=
132 x 10
3
3 x 41.1
= 1854A primary
= 1854 x
1
= 3.71A secondary
500
X
/
R
ratio
=
39.3
= 3.24
12.13
From Chapter 2 page 158 of manual relay burden for a ground fault is 0.53
ohms.
Assuming: CT resistance = 2.5 ohms
CT load resistance = 0. 11 ohms
CT kneepoint voltage requirement for a ground fault is given by:
VK ≥ 3.71 (1 + 3.24) (0.53 + 2.5 + 0.22)
VK ≥ 51.1 volts
It is also required that at the CT kneepoint voltage (which should be at least 63
volts according to preceding calculations) the CT exciting current should be less
than 100mA.
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