CASE STUDY OF A MCC SWITCHGEAR PANEL (MOTOR CONTROL CENTRE) TRIPPING DUE TO SHORTED MOTOR.

7/29/2019 CASE STUDY OF A MCC SWITCHGEAR PANEL (MOTOR CONTROL CENTRE) TRIPPING DUE TO SHORTED MOTOR.

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REPORT 4: CASE STUDY OF A MCC SWITCHGEAR PANEL (MOTOR

CONTROL CENTRE) TRIPPING DUE TO SHORTED MOTOR.

TABLE OF CONTENT

CHAPTER PAGE

4.1 CASE STUDY OF THE MOTOR 1

4.2 CASE STUDY ON THE SWITCHGEAR PANEL 1

4.3 RECOMMENDATION 9



1

4.1 Case study of the motor.

On 9/01/2011, at around 0309hrs the paper stock preparation plant tripped.

Checked at the switchgear room and found the MCC (Motor Control Centre) panel

tripped at the main incoming air circuit breaker (ACB). The breaker tripped with

alarm no.6 indicated on the protection relay (SPAJ 140C), which is the earth fault.

The alarm was acknowledged and the breaker was switched on back. Then all

the motors were started from the control room but at around 0418hrs the MCC panel

tripped again when the faulted motor was started.

Further inspected and found the motor was shorted to earth. The motor

winding measurement was checked and found that the motor stator winding has

shorted to earth. The motor stator winding insulation resistance and the resistanceresult is as below:

The insulation resistance test showed one phase of the stator winding has

shorted to earth.

Insulation Phase to Earth (Mega Ohm) – U: 800 V: 0 W: 500

Insulation Phase to Phase (Mega Ohm) - U-V: 800 V-W: 500 W-U: 500

The winding resistance test showed the winding resistance was also unbalance.

Stator Coil resistance (Low Ohm) - U: 0.124 V: 0.063 W: 0.063

The faulty motor was dismantled and replaced with the spare motor. At

around 0650hrs motor replacement job was completed and tested the motor on the

rotation. The motor running amps was 120A (red phase), 119A (yellow phase), 123A

(blue phase) whereas the full load current of the motor is 143A.

After dismantling the motor, found one of the stator bar at the bottom part of

the motor (in between 2 foot mountings) was broken due to burnt and the paper

insulation was also found damaged. From this evident the suspected root cause is due

to hotspot problem. It was confirmed after performing an IR thermographic analysis

on the motor’s stator core lamination. Please refer to figure 4.1 and 4.2 for the

thermographic results. Further inspected in the motor terminal box, the winding cable



2

condition, the cable lug condition and the terminals, all were found in good condition

and no burnt marks.

The upper and side part of the motor was found clean except for the bottom

part of the motor (near to the mounting) was found dirty. From the data collected, it is

suspected the stock which covered the motor fins at the bottom part of the motor,

where the stator bar was found broken which had caused high temperature at that

particular area. The motor insulation was Class F, which the permissible temperature

was 155 degree Celsius.

When the motor continuously operates in this condition, it had caused the

insulation at the area deteriorate rapidly and resulted in insulation failure and lead to

motor burnt. Increasing the temperature by 10°C will reduce the life expectancy of the

insulation into half.

Figure 4.1: Thermal Analysis – Before Rewinding



3

Figure 4.2: Thermal Analysis – After Rewinding

4.2 Case Study on Switchgear Panel.

Referring to the insulation resistance measurement the motor’s stator ‘V’

windings has the lowest resistance value when measured with respect to earth. The

‘V’ phase winding which failed on the insulation has caused the ‘V’ phase winding to

be shorted to the stator core. The stator core is physically connected to the motor

frame which has been terminated to the earth.

The MCC panel cabling arrangement and protection co-ordination are well

explained in Figure 3. From the incident on 9/1/2010 the earth fault current from the

motor has triggered the protection relay to trip the main incoming Air Circuit Breaker

which consequently shuts off the entire panel. An earth fault current of 29% was

recorded on the relay. The 29% of earth fault current would yield around 1160A from

the existing earth fault protection setting. The calculation is as shown below.

29

1004000 ) =



4

The protection relay is an IDMT type of relay and configured to be a standard

inverse tripping curve characteristics. Figure 4 shows the typical time current curve

for a standard inverse IDMT curve. Based from the previous relay calibration settings,

the earth fault current was set for 15% which yields 600A and the time multiplier was

set for 0.1 whereas for the over-current, it was set for 87% which yields 3480A and

the time multiplier was set for 0.1. Both the calculation is as shown below.

15

1004000 ( ) =

87

100

4000 ( ) =

It can also be noted from the curve, for a current level of 1.93 which is the

ratio between the fault current level (1160A) and relay earth fault current setting

(600A), the tripping time would be approximately between 1.0 to 1.1 seconds from

the curve for TMS of 0.1. However for more accuracy, the tripping time could be

determined by using the standard inverse relay characteristics equation. The equation

is as stated below.

= 0.14

(

). 1

From the equation, for an earth fault current of 1160A, the time taken for the

air circuit breaker to trip is 1.054 seconds. Although the motor is protected by an

individual branch MCCB, yet from its time-current curve characteristics, the time that

will be taken for the MCCB to trip is from 30s to 300s. Referring to the time current

curve of the MCCB in figure 5, for a current rate of 1160A the MCCB is still in the

thermal characteristics region.



5

The thermal characteristics of a MCCB are also referred as a slow response

region. Most of the times, the upstream relay settings had been chosen by considering

the time grading co-ordination with the downstream breaker. However, in our case the

experience has shown that there has been a total lack of earth fault protection co-

ordination. The upstream breaker tends to trip in 1.054s for a fault level of 1160A

whereas the downstream breaker will take 20s to 300s to trip for the same fault

current level. Numerous incidents have been reported where breakers have tripped in

an uncoordinated manner leading extensive plant disruption causing longer down

times and extended restoration time.



6

Figure 4.3: Normal inverse-time characteristics of the Over-current and earth-fault

unit SPCJ 4D29.

If = 1.93



7

Figure 4.4: Time Current characteristics of SN250 ABB MCCB.

If = 1.93



8

`

Figure 4.5: MCC protection co-ordination.

Air Circuit Breaker

Make = OttermillRated voltage = 500VRated Current = 4000A

Breaking Capacity = 50kA

ProtectionRelay

Protection RelayMake = ABBModel = SPAJ 140CType = Standard InverseOver-Current setting =87%

Earth Fault setting = 15%

MCC1 transformer

Power = 2500kVAVoltage Ratio = 6.6/0.415 kV

Current Transformer

Ratio = 4000/5 A

MCCB

Make = ABBRated Voltage = 440V

Rated Current = 250ABreaking Capacity = 35kA

Motor

Fault Current =1160A

Earthing bond at MCC panel

M



9

4.3 Recommendation

Faults, which occur within the motor windings, are mainly earth faults and

phase to phase faults caused by breakdown in the winding insulation. The earth faults

can be very easily detected by means of an instantaneous earth fault relays, usually

with a setting of approximately 500mA which is connected to the zero current

transformer. The advantageous of earth fault relay are generally no need for time

delay as there isn’t any downstream protection. The sensitivity is also considered to

avoid nuisance tripping and the input to the relay is from a zero current transformer

(ZCT). The typical connection for an earth fault motor protection is shown below.

Figure 4.6: MCC protection co-ordination.

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CASE STUDY OF A MCC SWITCHGEAR PANEL (MOTOR CONTROL CENTRE) TRIPPING DUE TO SHORTED MOTOR.

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