HT MOTOR PROTECTIONHT MOTOR PROTECTION

MOTOR FAILURE RATESMOTOR FAILURE RATES

Electrical related failures – 33%

Mechanical Related failures - 31%

Environmental maintenance & other

related problems – 36%

Electrical related failures – 33%

Mechanical Related failures - 31%

Environmental maintenance & other

related problems – 36%

ELECTRICAL PROTECTIONELECTRICAL PROTECTION

Overload protection

Earth fault protection

Locked rotor protection

Start/Stall protection

Number of starts limitation

Overload protection

Earth fault protection

Locked rotor protection

Start/Stall protection

Number of starts limitation

Protection In Stage-III Mill PA FD

Inverse time over load relay Y Y Y

Instantaneous Over Current Y N Y

Definite Time Overload N N N

Earth Fault N N N

Sensitive Earth Fault Y Y Y

Locked rotor protection N N Y

Differential relay N Y N

50

R

Y

B

TO DDCMIS

SPARE

51I1 51i1

51i2

50LR

50A

50N2

T

RYB

SURGE ARRESTER

VACCUM CIRCUIT BREAKER

When a motor is started, it draws a current well in excess of full load rating throughout the period that the motor takes to run-up to speed. While the motor starting current reduces somewhat as motor speed increases.

When ever a motor stall whilst running, or fail to start, due to excessive loading, the motor will draw a current equal to its’ locked rotor current. It is not therefore possible to distinguish between a stall condition and a healthy start solely on the basis of the current drawn. Discrimination between the two conditions must be made based on the duration of the current drawn.

LOCKED ROTOR PROTECTION

STALL PROTECTION STALL PROTECTION

Start time => safe stall time: since the time delay required is longer than the maximum time that the motor can be allowed to carry starting current safely. An additional means of detection of rotor movement, indicating a safe start, is required. A speed-sensing switch usually provides this function.

SENSITIVE/ INSTANTANEOUS EARTH FAULT RELAY

SENSITIVE/ INSTANTANEOUS EARTH FAULT RELAY

Here we calculate fault between one phase and ground Fault Impedance =11.2ohm Fault current = 3E/(X1+X2+X0+3Xf)= 3(6.6K)/(8.6+8.6+3(11.2)) = 390A If = 300 app Since earth fault current must be less than 300A CT secondary = 300/150 = 2A

 

INSTANTANEOUS E/F RELAY INSTANTANEOUS E/F RELAY

SENSITIVE E/F RELAYSENSITIVE E/F RELAY

INVERSE TIME OVER CURRENT RELAYINVERSE TIME OVER CURRENT RELAY

Alarm at 105% And tripping at 110% Rated current = 126.5A Trip at = 126.5*110% =139.15A CT primary = 139.15A CT secondary = 139.15/150 =0.92A Current setting = 92%

INSTANTANEOUS O/C RELAYINSTANTANEOUS O/C RELAY

Short circuit current = 40KA app. CT secondary = 40KA/150 = 267A Normal current = 126.5A Current setting =126.5*700% = 885A

CT secondary = 885/150 = 5.9A Current setting = 600% but it is set for 800% 

NUMBER OF STARTS LIMITATIONNUMBER OF STARTS LIMITATION

Any motor has a restriction on the number of starts that are allowed in a defined period.

Starting should be blocked if the permitted number of starts is exceeded.

The situation is complicated by the fact the number of permitted ‘hot’ starts in a given period is less than the number of ‘cold’ starts, due to the differing initial temperatures of the motor.

To allow the motor to cool down between starts, a time delay may be specified between consecutive starts

DIFFERENTIAL PROTECTIONDIFFERENTIAL PROTECTION

THNK UTHNK U