Specifications

F4
GE Limitamp
®
Medium Voltage Motor Control
Protection & Control
F
The cost of this type of relaying is justified by the size of the
investment to be protected. Large motors (usually above
1500 hp) that are expensive to repair or replace often employ
differential relays.
Specifically, differential relays accomplish the following:
1. Provide for power interruption to a motor in the event of a
phase-to-phase insulation failure in the motor windings.
2. Provide for power interruption to a motor in the event of a
phase-to-ground fault in the motor winding.
The primary use of differential relays in Limitamp Controllers
is to give fast, sensitive protection for faults in the end turn
outside the stator punchings. Such faults are relatively rare
compared with ground faults. However, when they do occur,
the presence of differential relays would probably mean the
difference between minor and extensive damage.
Two methods of motor differential protection are available.
One uses six identical current transformers: three located in
the motor leads and three located in the wye points of the
motor windings, usually at the motor. In conjunction with
these six current transformers, a Multilin SR469 or similar
relay is used to detect the difference in current in the current
transformer (CTs). The other method, known as self-balancing,
uses three donut-type CTs. Both the motor leads and the
wye connections are brought back through the holes in the
donut CTs. For this system, Multilin SR469 relay can be used
Ground-Fault Relays
Ground fault relays are justified economically for all motors
rated 2300 to 7200 volts, 150 horsepow
er and above. The
purpose is to provide interruption of power to the motor as
rapidly as is practical after positive indication that a ground
fault has occurred. Most multifunction relays such as Multilin
SR369/469 offer ground fault protection as a standard. Refer
to component brochures for further details.
The time of interruption of ground-fault current is dependent
on several factors:
1. Sensitivity of the ground-fault relay.
(a) Instantaneous type
(b) Time-delay type
2. Magnitude of ground current.
3. Clearing time of the power interrupter.
The importance of clearing ground-fault current rapidly can-
not be overstressed. Ground current inside rotating machines
causes damage to the lamination which, if not interrupted
rapidly, necessitates complete disassembly and repair of the
motor.
Although most ground-fault relays are now of the instanta-
neous type, few applications do require inverse-time current
relays for coordination and selectivity reasons. The use of
instantaneous-type relays is made possible through the
employment of a zero-sequence window-type current
transformer installed in the starter in such a way as to permit
all three conductors of the three-phase line to be used as the
current-transformer primary.
Phase currents add to algebraic zero, regardless of magnitude,
and no secondary current flows except that induced by the
primary current going to ground. This system gives positive
indication of ground current, eliminates false tripping and
permits instantaneous relaying.
If time coordination with other ground-fault relays is neces-
sary, time overcurrent relays may be used in the current-
transformer arrangement.
For certain sized motors where the power system permits,
ground-fault relays may be used as a less expensive alternative
to differential relays. Most phase-to-phase winding faults
detected by differential relays result in a simultaneous
phase-to-ground fault, thereby operating the ground fault
relay. For that reason, ground fault relays may be used as a
less expensive alternative to differential relays.
Another method of detecting ground currents in a three-phase
system employs three separate line-current transformers,
one in each phase, with the secondaries fed through a single
current relay. In this system, the secondary currents should
sum to zero just as they do in the primary of the window
type current transformer. And, with no ground current flowing,
the three secondary currents do add and cancel each other
out. Ground current only will cause the relay to operate. For
currents of large magnitude, however, such as motor locked-
rotor current, current-transformer saturation becomes a
problem, causing residual current to flow in the relay coil...
resulting in false tripping. To prevent false tripping with the
residual connection, time-delay relays are necessary to permit
riding over the starting period of the motor. This fact makes
instantaneous relays impractical in the residual system.
Instantaneous ground-fault relays may be applied to Limitamp
(NEMA Class E2) controllers without limitation on available
ground current. The fuse and relay-contactor clearing times
are such that ground-fault currents up to and including the
fuse rating will be cleared without damage to the controller.
Standard ground-fault relay used in Vacuum Limitamp
Control is a solid-state relay which operates on approximately
4 to 12 amperes ground-fault current. If greater sensitivity is
required, other solid-state ground-fault relays may be furnished
which can be adjusted to trip as low as 1 ampere. However,
extreme care must be exercised in applying ground-fault
relays of such low pick up. They could trip falsely on system-
charging current. A magnetic ground-fault relay can be
provided on request.