SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Page 25 of 27
b) Ground faults:
Maximum ground loop impedance behind relay is maximum JAN source
ground loop impedance plus ground loop impedance of a JAN – FEB circuit.
Ground loop impedance of JAN source = 24.9
/88° ohms max
Ground loop impedance of JAN – FEB circuit
= 20 x (2 x 0.44
/69° + 1.09 /72°
3
= 20 x (0.32 + j0.82) + (0.33 + j1.04)
3
= (4.33 + j12.4) = 13.13
/70.8° ohms primary
Ground loop impedance behind relay
= 24.9 /88° + 13.13 /70.8°
= 37.6
/82.1°
= (5.17 + j37.2) ohms primary
Ground loop impedance to Zone 1 reach point
= (K11 + K12 + K13) K14 x (Z
Ph
+ Z
N
)
= 1.82 x 7.14
/71.6°
= 13.0
/71.6° ohms secondary
= 31.16
/71.6° ohms primary
= (9.84 + j29.6) ohms primary
Source to Zone 1 reach point ground loop impedance
= (5.17 + j37.2) + (9.84 + j29.6)
= (15.01 + j66.8)
= 68.5
/77.3° ohms
For a ground fault voltage seen by relay
= 31.16 /71.6° x 63.5
68.5 /77.3°
Magnitude = 28.9 volts
For ±5% reach accuracy with the Zone 1 multiplier set to unity, Quadramho
requires at least 2.05V for ground fault measurement, or at least 3.55V for phase
fault measurement. For ±10% accuracy the required voltages are 1.04 and 1.8V
respectively. For Zone 1 multipliers greater than unity, the required relay voltages
for accuracy vary linearly with the multiplier setting.
In this case the Zone 1 multiplier = 1.82. Thus the required voltages for ±5% reach
accuracy are:
1.82 x 2.05 = 3.73 volts for ground faults
1.82 x 3.55 = 6.46 volts for phase faults
Both voltage requirements will be met in this application.
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