Post on 19-Jan-2016
description
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