Motor Starting Ground Fault 2

[mhuckaby]

I have run across a couple of instances where ground fault relays trip when induction motors are starting.

For example, a 75hp, 480V, 3ph motor (113 FLA) would trip the main breaker on a ground fault condition during startup. The ground fault trip level was set at 160 Amps.

We raised the trip level to 320 Amps and the breaker no longer tripped.

In another instance, a 500hp, 480V, 3ph (approx 600FLA) motor would trip on ground fault when the ground fault was set below 400A.

Is there an acceptable level of ground fault current to expect when an induction motor starts?

 

[jbartos]

Suggestions marked ///\\I have run across a couple of instances where ground fault relays trip when induction motors are starting.
///Yes, especially when there are single phase induction motors.\\For example, a 75hp, 480V, 3ph motor (113 FLA) would trip the main breaker on a ground fault condition during startup. The ground fault trip level was set at 160 Amps.
///There may be various causes, e.g. not all contacts close at the same time, the stator windings may have interturn shorts, there may be larger leaks to ground due to motor inrush currents causing larger stresses on insulated conductors touching grounded frame, etc.\\We raised the trip level to 320 Amps and the breaker no longer tripped.
///That is possible.\\In another instance, a 500hp, 480V, 3ph (approx 600FLA) motor would trip on ground fault when the ground fault was set below 400A.
///See my comments above.\\Is there an acceptable level of ground fault current to expect when an induction motor starts?
///Yes, if indicated by the motor manufacturer.\\\

 

[buzzp]

I have not experienced these types of problems with three phase motors. Seems there is something wrong with the motor or feeder system. Perhaps the supply and/or switchgear can not handle the starting current of these motors. A grossly imbalanced supply might cause a GF trip, this unbalance could result from an undersized system. If possible, look at the motor terminal voltage at start-up, phase to phase. In any case, there is definately something wrong with the applications mentioned, at least in my opinion.

 

[dpc]

What type of ground fault detection are you using? If it based on residual secondary current in the three CTs, there can be some error caused by unequal saturation of the three CTs.

If you're using a flux summation CT, then the error should be less.

But your initial setting of 160A should have easily been high enough to avoid nuisance tripping. If you have ability to adjust the time delay, you might increase it a bit and see if that allows you to lower the pickup setting back to a reasonable level.

I would megger the motor and starter circuit just for peace of mind.

 

[rbulsara]

A few things comes to mind:

1. Perhaps adding a time delay of 1 or 2 seconds to the trip of GFP will help . Specially if this due to initial imbalance in currents or capacitors in the circuit.'

2. Do these motros have PFC capacitors? I am not suggesting that PFC will trip on ground fault all the time, but just curious and something to think about. Capacitos are seen as short circut during initial charging. Some imbalanced or partially faulty caps may do it. Or may be check fuses of the capacitors, if individually fused.


3. As for GF current setting, I would say 10 to 20% of FLA is a good number.

4. As dpc said, I would double check CT wiring and polarity.

If you post what kind of GFP relay or scheme is employed might enable some one to comment further.

 

[GusD]

Hello mhuckaby.

Without knowing what type of GF relays or CT types being used in your systems,one can only speculate on the possible causes.
As you mention ,when relay trip levels are increased the
system operates well.
As previously noted ,an increase in the Time to trip Delay might be a safer option than an increase of trip current settings.
As for what are acceptable GF trip levels it is normally a function of your electrical system rather then your electric motor.
Levels are normally set as low as the system will allow,without causing nuisance trips.Minimizing the destructive Fault currents is most important.



GusD

 

[Marke]

Hello mhuckaby

I have seen a similar situation occur when using some form of sold state controller with the motor. Some ground fault systems are sensitive to harmonics and charging currents and this can cause a problem.
For example, a soft starter, during start is causing harmonic currents to flow in the stator. Additionally, because the SCRs are phase controlled, there is a moderately high charging current for the winding to ground capacitance every time the SCRs turn ON. This current is for a very short period of time, but if the ground protection system is very fast, it can pick this up. A slower responding relay will overcome the problem.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[RalphChristie]

Your groundfault settings is very high, I agree with dpc and rbulsara's comments. (10 to 20 % of FLA depending on type of protection)

I would suggest that you first check your CT polarity and wiring.

PS: Can't you measure the ground current on the relay with a clamp-on meter? (When motor is running) When value is high, there have to be a problem on your CT's (I don't think there is problem on your motor, otherwise you should have detected the fault already) If the value is just high when you start the motor, then a time delay or a slower responding relay will be an option.










RCC

 

[buzzp]

I dont believe it would be the CT polarity or it would trip all the time since the FLA is 113 and the setting is 160 or higher.

 

[rhatcher]

I don't think the problem is with the CT's. This is because the ground fault pickup setting for industrial breakers is usually adjustable in the range of 5-50% of the breaker continuous current rating. This would imply that the 'main' breaker feeding the 75 hp motor would have to be at least 800 amps to be able to have a GF pickup of 320 amps. So, saturation should not be an issue and reverse polarity would cause a trip at normal load current.

I gave dpc and rbulsara stars for good answers, but before I adjusted the time delay of the breaker I would consider checking to see what the existing ground fault current is (if any). It is possible that an actual ground fault exists somewhere else and that whatever contribution the motor makes is enough to trip the breaker. After all, like rbusara said, the normal setting for ground fault pickup would be 10-20% of the motor FLA for a breaker dedicated to that motor. The trips in question are taking place at 141% FLA.

If no other faults exist and the motor, controller, and cables are good, then you should increase the time delay and decrease the pickup level to get the maximum protection. As well, if your ground fault delay has I2T as an option then you should select that. This would allow a long time delay at whatever the minimum pickup setting you can use is but would have time delay that is reduced proportional to increasing fault currents. For example, twice the current would trip in 1/4 of the time (as in I2T, the heating equation for cables, etc.). This would allow you to ride through a motor starting transient of say 0.1-0.5 second at 100 amps (if that is really the case), but would still minimize damage caused from true faults by tripping much faster at higher currents.

I hope this helps.

Merry Christmas

 

[electricpete]

Ray - Welcome back!
The board will be a better place if you stick around.

Merry Xmas

 

[RalphChristie]

It depend on what type of grounding detection scheme and what type of protective device is been used on the breaker.

If the residual connection is been used, and there is a loose conection on one of the ct's, or a ct with a wrong polarity, then a high starting current can cause the breaker to trip on ground fault during start-up. (How big is the laod on the motor?) The higher setting of the groundfault setting (160A ground fault setting compare to 113A FLA) it is well above normal running amps of the motor, and it would not trip anymore on groundfault if it is running normal.

With this high groundfault setting, much higher than FLA, I'm almost sure the problem is somewhere on the CT's (I can be wrong) and I would suggest taking a current measurement on the groundfault element of the protective device and to check for any existing ground fault (what I don't think there is)



RCC

 

[jbartos]

Suggestion: Some postings above outlined troubleshooting the high ground fault current origin. Beside, the clamp on ammeter, it may be a good idea to check waveforms of voltage and current by oscilloscope. Shape of waveforms reveals whether or not there are peaks, distortions, transients, etc. Considering so many motors involved, it would be prudent to have a generic cause of the problem tackled to avoid potential impacts on motors and associated electrical equipment in the long run.