Ground Fault Detection by hamada1331

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									Ground
Fault
Detection &
Protection
For Motors
  By Ajit Bapat, M.Tech., MBA, P.Eng.


A    lmost 80 % of the electrical faults in
     low voltage distribution systems are
line to ground faults. In a solidly
                                               provide an alarm and energize a pilot
                                               light on the MCC bucket door indicat-
                                               ing a fault in the motor while the load
grounded distribution system the fault         keeps running. The process or batch is
current is very high and if there is an arc-   not interrupted and no production loss
ing type fault then the fault energy causes    occurs. A maintenance shutdown can
catastrophic damage. In industrial power       now be scheduled to repair the motor.
systems the bulk of the load consists of          When the motor control center has a
motors. In order to avoid severe damage        large number of starters an alternative
the faults need to be detected before they     method of detection is to use a multi-
evolve in to major problems.                   circuit alarm system (Type DSA),
   Ground fault relays, when applied to        which can handle up to 192 motor cir-
monitor each motor, can be set to trip at      cuits. Zero sequence sensors are
5-10 A. They can be mounted in the             mounted in the MCC, one per motor
starter bucket in the motor control cen-       circuit, with all three-phase load con-
ter and the relay trip contact wired to        ductors passing through the window.
operate the shunt trip to open the             The DSA unit indicates which motor
breaker or the contactor. In this case         has a ground fault.
the relay should be time coordinated so           Yet another approach is to use an inte-
that in the event of a large magnitude         grated high resistance ground with puls-
fault it does not open the contactor           ing system on the main supply trans-
beyond its interrupting ability.               former to detect, indicate and alarm,
   Converting to high resistance ground-       when the system has a ground fault. The
ing will lower the fault current and thus      fault can then easily be located by initi-
the fault will not cause damage and the        ating the pulsing function. A flex core
motor with the ground fault need not be        sensor is wrapped around load cables of
tripped. The fault current can remain in       the feeders to find the one that has the
the ground return path continuously. The       fault. The faulty motor is identified at
fault needs to be detected and an alarm        the MCC when the flex sensor,
needs to be provided. It is most useful to     wrapped around the load conductors of
know which motor has the fault.                the motor, indicates pulsing.
   The relay set at 500 ma to 2.5 A. can          Monitoring leakage to ground is a
                                                                                           March / April 2003   ELECTRICAL LINE       53

predictor of an impending ground fault.         detected by zero sequence current or           This same comment applies to syn-
   Most faults start with leakage to            phase to ground voltage measurement.         chronous motors. The ground faults in
ground. IPC type GM indicators are an           The insulation impedance of the wind-        the DC excitation circuit are not seen
effective means to show when these              ing to the grounded steel core needs to      by the motor ground fault relay.
leakage currents begin to increase              be measured.
which eventually will lead to a ground            Periodically noting the deflection         Application On Large MV Motors
fault. Early detection keeps the damage         shown by the ground current indicator           Quite frequently the MV systems are
at the fault point very low and therefore       is a recommended practice that will          low resistance grounded and the ground
the repair cost and time to repair the          show a trend. An increasing trend noted      fault relays applied for motor protec-
equipment is also significantly reduced.        on a motor is a definite indicator that      tion must trip on the occurrence of a
These indicators can be group mounted           the insulation is going to break down        ground fault. A sensitive ground differ-
in a compartment of the MCC or indi-            and a fault is going to happen. This         ential scheme is often conveniently
vidually mounted in starter buckets.            advance warning is tremendously use-         possible when all winding terminations
After a ground fault alarm is indicated         ful in planning preventive maintenance       are available in the motor terminal box,
by the substation ground fault detection        and scheduling an outage for the motor.      so that all conductors pass through a
system then a simple visual inspection                                                       zero sequence current sensor providing
at the MCC identifies which motor has           Application With Drives                      ground differential sensing. All other
the ground fault by noting the indicator           Ground fault sensing is normally          comments noted above apply.apply.
with the maximum deflection.                    applied on the line side of the drive.
   Most motor failures occur in the motor       Ground faults on the load side of the        Summary
terminal box and are caused by moisture,        sensor location are detected. Isolation         Converting to high resistance ground-
aging, vibration, etc. These faults are         transformers, if used, prevent detection     ing will control and reduce the ground
normally of low impedance and the GM            of ground faults on the load side of the     fault current. It will allow you to maintain
indicator shows full scale deflection.          secondary winding. Ground faults inside      manufacturing process continuity even if
   Internal faults in the motor winding         the drive on the DC bus will cause a non     an insulation failure and a ground fault
are rare but they do occur. For a delta         sinusoidal DC current flow which will        occurs on the distribution system operat-
connected winding the smallest amount           not be detected by the AC zero sequence      ing the process load. The fault damage
of ground current will occur for faults         sensing circuit. Ground faults on the AC     will be limited. The fault can be easily
at the mid point of the winding and for         side of the drive, including the motor       located and repairs can be scheduled.
a solid fault it will indicate 50 % or          winding, will be seen                        Periodic monitoring of ground leakage
higher deflection on the indicator.                                                          current can be a predictor of an incipient
   The current will be higher for fault loca-   Application On                               failure and can be used effectively in a
tions closer to the ends of the winding.        Wound Rotor Motors                           preventive maintenance program. Ω
   For star connected motor windings               A wound rotor motor behaves like a
the fault current reduces as the fault          transformer and ground faults in the         Ajit Bapat is Vice President, Technology
location gets closer to the star point. If      rotor winding and circuit are isolated       for IPC Resistors in Mississauga, Ontario.
the fault is at the star point then there is    and not seen by the ground fault relay
no fault current and the fault cannot be        protecting the motor.                             Circle 52 on Reader Service Card

								
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