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Ground Detection Considerations - GE AKD-10 Application Manual

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Ground detection considerations

High resistance pulsing ground detection system
This system provides a means for grounding the neutral of
an ungrounded power system, utilizing the "high-resistance"
method. It allows the switchgear to operate as an
"ungrounded" system but eliminates the danger of high
transient overvoltage during certain types of ground faults.
For delta systems, a set of grounding transformers is provided
for connection of the grounding resistor.
The high-resistance pulsing ground detection system uses a
voltmeter relay with an adjustable set point to detect abnormal
ground current through the grounding resistor. A green indi-
cating light shows normal conditions, and a red indicating
light indicates the presence of a phase-to-ground fault. Alarm
contacts allow remote indication of the ground condition.
The location of the fault is quickly determined using a pulsing
current in conjunction with a sensitive clamp-on ammeter.
This permits clearing of the ground fault before a second
phase-to-ground fault causes an outage. After the fault is
located and cleared, the system is reset and ready to detect
the next ground fault.
Ground detection on ungrounded systems
This system provides visual indication of the presence of a
phase-to-ground condition on a delta ungrounded system.
Ground detection on ungrounded systems consists of one set
of three potential transformers rated for full phase-to-phase
voltage on the primary winding and 120-volt secondary
winding. The primary is connected wye. The secondary con-
nection is dependent on the type of ground indicators and
alarm devices used. A loading or stabilizing resistor in the
potential transformer primary connection to ground is used
to prevent ferroresonance with the distributed capacitance of
the system. Ground indication and alarm can be accomplished
as described in the following tables:
Table 29.1 Operation with lights or voltmeters
Standard
Option
Three 120V indicating
Three voltmeters
lights with clear lenses
instead of indicating
(one per phase).
lights. Potential
Potential transformers
transformers with
with wye connected
wye connected
secondaries.
secondaries.
Operational description
Assuming rated system voltage on
the primary of the potential trans-
formers, the three lamps would glow
about equally at subnormal brilliancy
because the voltage across each lamp
is 69.3 volts. Similarly, if voltmeters
are used instead of lamps, each
would read 69.3 volts.
If one phase of the system becomes
grounded, the potential transformer
on the grounded phase would be
short-circuited, and the voltage on
the other two transformers would
rise to approximately full phase-to-
phase voltage. The lamp on the
grounded phase would be dark and
the other two lamps would glow at
normal brilliancy. Similarly, the
voltmeter on the grounded phase
would read zero and the other two
voltmeters would read 120 volts.
29

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