Hi
Can someone explain me what is the negative phase sequence current?
What is its effect on motors?
How is it measured?

I am asking this because I have 1700kW, 315A motor protected by REF542+ tripped on spurious Earth fault and after studying the current trend it appeared that the current when the motor tripped was 270A(<315A). I suspect that the motor is tripped due to Unbalance Load protection which evaluates the measured amount of negative phase sequence current at fundamental frequency.

Knowing that the settings are:

1. Earth Fault Protection : 0.2In, 0.1s
2. Unbalance Load Protection: 0.09In,

Can somebody help??


Thanks

Hello Riyad

Sorry for the slow response to your question.

We can analyse changes in waveforms in terms of harmonic currents and using the harmonics, we can then explain aditional heating in the rotor of a motor. i.e. if we have some third harmonic present, the ther willbe a torque field operating at three time the fundemental in addition to the fundimental torque field. This third harmonic filed will cause third harmonic currents to flow in the rotor and effectively there is a very high slip between the rotor operating at the fundiemntal frequency and the torque field operating at three times the fundimental frequency. This high slip results, as in the case of any high slip, in high slip losses in the rotor and therefore an increase in the rotor losses.

If we now consider an phase imbalance, we get a similar result only instead of a torque field that is operating at a multiple of the base frequency, we now have a torque filed that is rotating in reverse. If we consider the case of a total phase loss on one phase, then we effectively have a lingle phase supply on the motor, and a single phase supply is effectively two conter rotating torque fields of equal magnitude but opposite directions. The resultant torque is zero and the slip losses from each torque filed are high due to the 100% slip in both cases. If we apply some thrid phase voltage, we are in effect, reduceing the counter rotating torque field. This results in a reduced positive torque field. If the motor is rotating forwards, and there is an imbalance in the phases, then we cna analyse this as effectively being a full amplitude positive torque field and a reduced amplitude reverse torque field. This will result in 200% slip for the negative rotating torque field and consequent increase in losses.
The negative torque field is due to the negative sequence currents.

Best regards,