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CHAPTER 4: PROGRAMMING
On the occurrence of a ground fault caused by insulation breakdown, a motor will
usually have to be taken out of service and rewound. However an unprotected motor
could suffer mechanical damage to the stator slots making repair impossible. The
fault could also cause the power supply bus to which the faulty motor is connected to
trip in order to clear the fault resulting in unnecessary process shutdowns. Ground
faults can occur in otherwise good motors because of environmental conditions.
Moisture or conductive dust, which are often present in mines, can provide an
electrical path to ground thus allowing ground current to flow. In this case, ground
fault protection should shut down the motor immediately so that it can be dried or
cleaned before being restarted.
On low resistance or solidly grounded systems, sensing of the ground current is done
using the phase CTs wired in a residual connection. For more sensitive ground current
detection, a separate CT, referred to as a core balance or zero sequence CT, encircles
the three motor conductors. Ground fault detection is only suitable for systems that
have a path from the supply to ground either through a resistance or by direct
connection. Ungrounded systems require an artificial ground to be created through
use of a device like a zig-zag transformer if ground fault protection is to be used.
In systems with several levels of ground fault detection, time co-ordination is required
for satisfactory operation. If ground fault protection is used on a bus, each motor must
have a shorter ground fault trip time delay than the bus ground fault detector or a
fault in any motor will shut down the whole bus. In a solidly grounded system, time
delays as short as possible should be used to prevent system damage unless the
contactor is not capable of breaking the fault current in which case a backup
detection system of sufficient interrupting capacity should be allowed to operate first.
When contactors are used in solidly grounded systems, the ground fault trip time
should be longer than the fuse interrupt time.
On resistance grounded systems, where the ground current is limited to safe levels
longer time delays can be used subject to co-ordination constraints. Too short time
delays may cause nuisance tripping due to transients or capacitive charging currents
and should be avoided if possible. Time delays of several hundred milliseconds are
suitable for applications where the relay has to be coordinated with other protective
devices or a long delay is desired because of transients. Time delays of several
seconds are suitable for use on high resistance grounded systems where nuisance
tripping may be a problem from capacitive or induced currents during the starting
inrush. Ground currents limited by the supply ground resistance can flow for longer
periods without causing any damage.
The relay(s) selected in this setpoint along with the respective status indicator(s) on the
front panel of the 239 will be active upon a ground fault trip.
•
GROUND PRIMARY TRIP PICKUP: Ground fault trip when enabled in
S4:
, will signal a trip condition
PROTECTION\GROUND CURRENT\GROUND TRIP
when the ground current becomes equal to or exceeds the value set in this setpoint.
The amount of current that will flow due to a ground fault depends on where the fault
occurs in the motor winding. High current flows if a short to ground occurs near the
end of the stator winding nearest to the terminal voltage. Low ground fault currents
flow if a fault occurs at the neutral end of the winding since this end should be a
virtual ground. Thus a low level of ground fault pickup is desirable to protect as much
of the stator winding as possible and to prevent the motor casing from becoming a
shock hazard. In resistance grounded systems the ground fault trip level must be set
4–40
239 MOTOR PROTECTION RELAY – INSTRUCTION MANUAL
Courtesy of NationalSwitchgear.com
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