Motor Protection

[jplinn]

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

 

[dpc]

For induction motors, NEMA MG-1 suggests a voltage imbalance limit of 1% (max deviation from the **average** voltage).

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.

 

[jraef]

While that may be the NEMA recommendation (I'm taking it at your word dpc, I didn't look it up), I find that in most cases I would be unable to keep a motor on-line with a 1% current imbalance trip setting. Utility voltages just are not that tight. They are allowed to deviate +-2% on voltage and that produces a current imbalance far greater than 1%. I set it for 5% and will maybe go to 10% if everyone acknowledges that there are possible consequences to that in terms of motor insulation life. If I still get nuisance trips I recommend that the utility be called in.

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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[Marke]

Hi jraef
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

 

[jraef]

Oops, I read it more carefully and he did say voltage imbalance, not current. My mistake. Thats what happens in a 4 day work week with five days worth of work!

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