Almost 80 % of the electrical faults inlow voltage distribution systems are

line to ground faults. In a solidlygrounded distribution system the faultcurrent is very high and if there is an arc-ing type fault then the fault energy causescatastrophic damage. In industrial powersystems the bulk of the load consists ofmotors. In order to avoid severe damagethe faults need to be detected before theyevolve in to major problems.

Ground fault relays, when applied tomonitor each motor, can be set to trip at5-10 A. They can be mounted in thestarter bucket in the motor control cen-ter and the relay trip contact wired tooperate the shunt trip to open thebreaker or the contactor. In this casethe relay should be time coordinated sothat in the event of a large magnitudefault it does not open the contactorbeyond its interrupting ability.

Converting to high resistance ground-ing will lower the fault current and thusthe fault will not cause damage and themotor with the ground fault need not betripped. The fault current can remain inthe ground return path continuously. Thefault needs to be detected and an alarmneeds to be provided. It is most useful toknow which motor has the fault.

The relay set at 500 ma to 2.5 A. can

provide an alarm and energize a pilotlight on the MCC bucket door indicat-ing a fault in the motor while the loadkeeps running. The process or batch isnot interrupted and no production lossoccurs. A maintenance shutdown cannow be scheduled to repair the motor.

When the motor control center has alarge number of starters an alternativemethod of detection is to use a multi-circuit alarm system (Type DSA),which can handle up to 192 motor cir-cuits. Zero sequence sensors aremounted in the MCC, one per motorcircuit, with all three-phase load con-ductors passing through the window.The DSA unit indicates which motorhas a ground fault.

Yet another approach is to use an inte-grated high resistance ground with puls-ing system on the main supply trans-former to detect, indicate and alarm,when the system has a ground fault. Thefault can then easily be located by initi-ating the pulsing function. A flex coresensor is wrapped around load cables ofthe feeders to find the one that has thefault. The faulty motor is identified atthe MCC when the flex sensor,wrapped around the load conductors ofthe motor, indicates pulsing.

Monitoring leakage to ground is a

Ground FaultDetection &ProtectionFor MotorsBy Ajit Bapat, M.Tech., MBA, P.Eng.

March / April 2003 ELECTRICAL LINE 53

predictor of an impending ground fault.Most faults start with leakage to

ground. IPC type GM indicators are aneffective means to show when theseleakage currents begin to increasewhich eventually will lead to a groundfault. Early detection keeps the damageat the fault point very low and thereforethe repair cost and time to repair theequipment is also significantly reduced.These indicators can be group mountedin a compartment of the MCC or indi-vidually mounted in starter buckets.After a ground fault alarm is indicatedby the substation ground fault detectionsystem then a simple visual inspectionat the MCC identifies which motor hasthe ground fault by noting the indicatorwith the maximum deflection.

Most motor failures occur in the motorterminal box and are caused by moisture,aging, vibration, etc. These faults arenormally of low impedance and the GMindicator shows full scale deflection.

Internal faults in the motor windingare rare but they do occur. For a deltaconnected winding the smallest amountof ground current will occur for faultsat the mid point of the winding and fora solid fault it will indicate 50 % orhigher deflection on the indicator.

The current will be higher for fault loca-tions closer to the ends of the winding.

For star connected motor windingsthe fault current reduces as the faultlocation gets closer to the star point. Ifthe fault is at the star point then there isno fault current and the fault cannot be

detected by zero sequence current orphase to ground voltage measurement.The insulation impedance of the wind-ing to the grounded steel core needs tobe measured.

Periodically noting the deflectionshown by the ground current indicatoris a recommended practice that willshow a trend. An increasing trend notedon a motor is a definite indicator thatthe insulation is going to break downand a fault is going to happen. Thisadvance warning is tremendously use-ful in planning preventive maintenanceand scheduling an outage for the motor.

Application With Drives Ground fault sensing is normally

applied on the line side of the drive.Ground faults on the load side of thesensor location are detected. Isolationtransformers, if used, prevent detectionof ground faults on the load side of thesecondary winding. Ground faults insidethe drive on the DC bus will cause a nonsinusoidal DC current flow which willnot be detected by the AC zero sequencesensing circuit. Ground faults on the ACside of the drive, including the motorwinding, will be seen

Application On Wound Rotor Motors

A wound rotor motor behaves like atransformer and ground faults in therotor winding and circuit are isolatedand not seen by the ground fault relayprotecting the motor.

This same comment applies to syn-chronous motors. The ground faults inthe DC excitation circuit are not seenby the motor ground fault relay.

Application On Large MV MotorsQuite frequently the MV systems are

low resistance grounded and the groundfault relays applied for motor protec-tion must trip on the occurrence of aground fault. A sensitive ground differ-ential scheme is often convenientlypossible when all winding terminationsare available in the motor terminal box,so that all conductors pass through azero sequence current sensor providingground differential sensing. All othercomments noted above apply.apply.

SummaryConverting to high resistance ground-

ing will control and reduce the groundfault current. It will allow you to maintainmanufacturing process continuity even ifan insulation failure and a ground faultoccurs on the distribution system operat-ing the process load. The fault damagewill be limited. The fault can be easilylocated and repairs can be scheduled.Periodic monitoring of ground leakagecurrent can be a predictor of an incipientfailure and can be used effectively in apreventive maintenance program. Ω

Ajit Bapat is Vice President, Technologyfor IPC Resistors in Mississauga, Ontario.

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