SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Page 18 of 27
Ground loop impedance to Zone 1 reach point
= 2 x (1.37 + j3.76) + (5.99 + j14.8)
3
= (2.91 + j7.44) = 7.99
/68.6°
Source to fault ground loop impedance
= (11.34 + j36.87) + (2.91 + j7.44)
= (14.25 + j44.31) = 46.55
/72.2°
Relay voltage for a ground fault at the Zone 1
Reach point =
7.99
x 63.5 = 10.91 volts
46.55
For ±5% reach accuracy with the Zone 1 multiplier set to unity Quadramho
requires at least 2.05 volts for the ground fault measurement or at least 3.55 volts
for phase fault measurement. For ±10% accuracy the required voltages are 1.04
and 1.8 volts respectively. For Zone 1 multipliers greater than unity, the required
relay voltages for accuracy vary linearly with the multiplier setting.
In this, the Zone 1 multiplier = 1.04. Thus the required voltages for ±5% reach
accuracy are:
1.04 x 2.05 = 2.13 volts for ground faults
1.04 x 3.55 = 3.69 volts for phase faults
Both voltage requirements are met in this application.
19.10 Checking relay current sensitivity for faults at the Zone 3 reach point
The worst case will be for an ground fault at the Zone 3 reach point.
Maximum source ground loop impedance behind relay.
= 38.6
/72.9° = (11.35 + j36.9)
Relay Zone 3 earth fault setting = (Z
Ph
+ Z
N
) x Z3 multiplier
=
(
1.4
+
1.4)
/70° x 4.6 = 2.58 /70° ohms secondary
55
= 35.3
/70° ohms primary
= (12.1 + j33.2) ohms primary
Overall source to fault ground loop impedance (including 40 ohms fault resistance)
= (11.35 + j36.9) + (12 .1 + j33.2) +40
= (63.5 + j70.1) ohms
= 94.6
/47.8°
Current for a fault at the Zone 3 reach point
=
60 x 10
3
3 x 94.6
= 366A = 183% of CT rating (200/5A)
Relay current level detector sensitivities =
Z
Ph
Z
Ph
max x base level
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