SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Page 17 of 27
Select K4 = 1
K5 = 0.4
K6 = 0.00
θ
N
≥ /KN.Z
Ph
θ
N
≥ 67.3°
Select θ
N
= 70°
Actual Z
N
= 0.28 /70° ohms
Coarse ground loop impedance = Z
Ph
+ Z
N
= 0.28 /70° + 0.28 /70°
= 0.56
/70° ohms
19.8 Setting restrictive reach of ground fault comparators
Required primary resistive coverage for ground faults = 40 ohms
40 ohms primary = 2.92 ohms secondary
Minimum load impedance likely to be seen by ground fault distance comparators
=
60 x 10
3
3 x 200
= 173 ohms primary
=12.6 ohms secondary
There will, therefore, be no problems of load encroachment on the ground fault
comparator operating regions, if the comparators are set to see 40 ohms primary
fault resistance.
Select nearest resistive reach setting above that required.
R
S
=
K3 = K3
= 2.92 ohms
I
n
5
Select K3 = 16
Actual resistive reach = 3.2 ohms secondary = 43.8 ohms primary
Check on ratio of resistive reach/Zone 1 forward ground loop reach
=
3.2
= 5.49
(0.56
/70°) x 1.04
The above ratio is within the limit of 15 (see Section 9)
19.9 Checking minimum relay voltage for a fault at the Zone 1 reach point
Maximum positive sequence impedance behind relay = 23.8
/76.4º ohms
Impedance to Zone 1 reach point = 4
/70°
Overall source to fault impedance = (5.6 + j23.1) + (1.37 +j3.76)
= (6.97 + j26.9) = 27.8
/75.5°
Relay voltage for a phase fault at the Zone 1 reach point
=
4.0 x 110 = 15.8 volts
27.8
Maximum zero sequence impedance to the Zone 1 reach point is 16 /68° ohms
Source ground loop impedance = 2 x (5.6 + j23.1) + (22.94 + j64.4)
3
= (11.34 + j36.87) = 38.6
/72.9°
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