Motor Protection
Motor Protection
(OP)
What is the typical setpoint and basis for unbalance protection on an induction motor. Specifically, magnitude differences between the 3 phase currents, negative sequence current and negative sequence voltage.
Background - Induction generator when fully loaded trips off due to current imbalance. The generator is at the end of a long line with a voltage imbalance (within the utility voltage acceptable range). When operating at lower loads - no problem
jpl
Background - Induction generator when fully loaded trips off due to current imbalance. The generator is at the end of a long line with a voltage imbalance (within the utility voltage acceptable range). When operating at lower loads - no problem
jpl





RE: Motor Protection
For synchronous generators, the IEEE/ANSI recommendations for negative sequence current are more complicated, based on I2(pu)^^2*t. Typical maximum is 30, but this can vary depending on the type and size of generator. The standards generally also provide for a maximum continuous I2 of 5 to 10%. This should be obtained from the generator supplier.
The concern in both cases is rotor overheating due to negative sequence current induced in the rotor. Due to the phase sequence, this current appears at a double frequency in the rotor.
The negative sequence current is the problem, but for the typical motor, it's much easier to measure the input voltages and compute the imbalance.
RE: Motor Protection
The best method I believe in for negative sequence protection however, is to use a sophisticated Motor Protection Relay such as a GE/Multilin 369 or 469, or a Schweitzer SEL-701, or any one of several other good quality relays, as long as they have a feature that will bias the Thermal Model for unbalanced current. This means that they will tweak the overload protection to compensate for the additional heating effect of the unbalance, but if the motor is lightly loaded and not even close to overheating, it allows operation without nuisance tripping even if the current is slightly unbalanced. In that case, I then set the Imbalance Trip setting at 10% or higher with a 5 second delay so that I catch a severe imbalance faster than the OL Thermal Model will.
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RE: Motor Protection
I believe that dpc was referring to a 1% imbalance in voltage, not current and that would result in a much higher imbalance in current.
Figures that I have seen suggest that at full load, the current imbalance should be less than 5% and as the load is reduced, the current imbalance can be increased. Most protection is set well above 5% imbalance because it does not change with load. If you had imbalance current set to 5% at light load, it would trip continuously due to the imbalance in the magnetising current.
Ideally, the imbalance would be a setting that alters with load, tripping at around 5% (or less) at full load but allowing for an imbalance of up to perhaps 20% under open shaft conditions.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Motor Protection
Eng-Tips: Help for your job, not for your homework Read FAQ731-376