"SMART" MOTOR CONTROL SYSTEM (MCS-300)

Considering the above details, it can be easily concluded that a motor protection relay will provide an effective protection to the motor only when it is equipped to measure the negative sequence component level in the motor feeder.

From the above table , we can see that in a motor running at 50% load, on single phasing in L2,
A thermal over load relay in this case , will not trip since it is seeing only 80% current in healthy lines L1 & L3 while the winding C is already over stressed. This situation gets worse with increasing loads at the time of single phasing.

A negative sequence calculation would have helped in this case.

108

50

80

50

Winding C

Winding A&B

Lines L1 & L3

3 phase current

Current in windings when single phasing has occurred

Motor Current (% of full load)

a) broken or burn out of connecting leads to motor
b) blown fuses
c) faulty contact in switching element
To understand the effect of single phasing in motors, consider the condition shown below

4.0 Reasons for Unbalance :Voltage unbalance can be due to many reasons. These include unsymmetrical loading, blown fuses in capacitor banks and single phasing.

5.0 Single phasing :

****The negative sequence impedance of the motor is approximately same as the locked rotor impedance which in turn is approximately one sixth of normal motor impedance. Due to this even small voltage unbalance can produce large negative sequence current in the motor.

a) Negative Sequence component : This component has a phase sequence opposite to that of the motor current hence the name negative sequence. It represents the amount of unbalance in the feeder . All its three phases are perfectly balanced - they are equal in magnitude and are displaced by 120 degrees. This component does not produce useful power - however by being present it contributes to the losses and causes temperature rise.

b) Zero Sequence component : This , if present, represents extent of earth fault in the feeder. All its three phases are in the same direction.
3.0 Effect of Unbalance in Motors :

1.0 Need for Negative Sequence Protection
Primary cause of motor failure is excessive heating, which if sustained over long time periods will result in motor burn out. Over heating also reduces the life of motor. If a motor is continuously over heated by just 10 degrees, its life can get reduced by almost 50%.
****Over heating normally occurs due to over current, which in turn may be due to over loads or locked rotor condition or low voltage or phase failure or repeat starts or phase unbalance.
****Current unbalance in a motor is best represented by the presence of excessive negative sequence component in the motor current. Consequently it is necessary to protect motors against negative sequence .
2.0 Supply Unbalance & Sequence components