Motor Trips on Current Imbalance During Startup

[Solman]

We have three phase induction 160kW each motors driving pumps to pump water. One of the motors is tripping on current imbalance during startup. The motor is supplied from the switchboard by the transformer. We have changed the motor have changed the CT's we have changed the relay, we have changed the supply contactor .... what could be causing current inbalance at startup???

 

[rbulsara]

Is this a new installation? Has the sytem worked OK for a while and the problem just started? Any recent changes made?

Consider adding a time delay to override imbalance trip during the start up. If it runs OK after the dealy, then all wiring and CT are OK. If not, check the wiring.

The tranformer may have unequal impedance between the phases. What type of transformer is it?

Check for source of voltage or current imbalance, inclduing tranformer taps etc. See if all parallel condudtors in all phases are good and sharing the current equally.

 

[buzzp]

Good suggestions from rbulsara.
What level of ub is set to trip? 5% is a typical setting for this. This can be changed depending on how loaded the motor is.

I think the time delay is key here. Generally, UB trips occur in no less than 2 seconds but I normally set them to ten seconds. Verify the delay is something reasonable and measure your supply voltage as close to the tripping motor as possible to watch for voltage dips at start-up and, ideally, read all three voltages.
If the installation is an existing one that has ran in the past then the time delay should be okay but still verify the supply voltage is acceptable on all three phases at startup.
If the supply is ok, look at the contacts on the contactor for pitting, wear. I have measured the voltage drop across the line and load side of these sometimes just to see the voltage drop across them. Sometimes this can help troubleshoot worn contacts other than a visual inspection. Loose connections may also cause this. Just think, any added resistance will reduce the voltage at the motor terminals. Hopefully this gets you started.

 

[Solman]

The system has worked ok for a very long time and there has been changes on some components on the switchboard but that was long b4 the problem started. Like I said there are three motors of the same rating and every thing is the same on all three of the except they are supplied from different swichboards and only one is tripping. The last time this problem started was when the system was off for some time and when it was switched back on the motor supply tripped. The CT was moved to a different position along the cable and is seemed to have worked for a while. But now again the system was switched off and when switched back on it keeps on tripping. As for the transformer it is a dry-type Dyn11 630kVA 6,6kV / 390V (HV winding taps +-2,5%+-5% of bolted copper links). We've done couple of tests and from the analysis it is evident that the phase imbalance only happen during startup of the motor. with the 100milliseconds from the on the phases goes back to 120 deg. Thanks for the tip but if you or anyone have something to add don't hesitate. Thanx

 

[buzzp]

"The CT was moved to a different position along the cable and is seemed to have worked for a while"
I cant imagine this affecting anything unless leads were shortened or lengthened, if so, better look at burdens on the CT. Seems marginal, I know, but still possible.

What is your current unbalance set at? What about the time delay?

Is this the last motor on the feed? If so, I strongly believe that it is just a nuisance trip due to the reduced voltage at start-up. There is nothing that guarantees that all three phases will drop equally. Voltage imbalance causes a current unbalance. Say VUB is 5% then CUB could be 6-10 times this or up too 50%.

Your last sentence is not clear but if the motor is up to speed in less than 2 seconds, I still think your trip time/curve for CUB is set to low. No motor is going to damaged in less than two seconds as a result of current unbalance (I do not consider loss of phase to be unbalance).

 

[aolalde]

An induction motor 160 kW should have around 305 Ampres full load and 1500 Amperes at start when it is operated at 390 Volts.
I am suspicious of high resistance points in your motor feeding line. Make an infrared checking looking for hot spots on all connections and contactors, this problem is more critical under the high starting currents since the losses are proportional to the current squared.

 

[jraef]

Is this DOL start? You say that you replaced the "contactor", which would imply this because it wasn't plural, but I just wanted to be sure.

Assuming DOL, I'm thinking along the lines of aolalde. Since you say that "some components" were changed in the switchboard, I wopuld start there. A slightly higher resistance at a bus bar connection point may not show much at normal currents, but at high starting currents it becomes rapidly worse. That can take place anywhere in the circuit, but since you mentioned that some changes had taken place in the switchboard prior to this occurring, that would be the likely starting place. Aolalde is right, a good IR thermal scan should show it.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Solman]

Thank you all for the input. We've been working on this problem again today. First of all just to answer some of your questions, the motor is a DOL start, the current unbalance relay is set at 3Amps there is no time delay on the relay it is instantaneous and this is applicable to all three motors, like I said the other 2 are working perfectly. As for my last statement I would repeat," We've done couple of tests and from the analysis it is evident that the phase unbalance only happen during motor startup, within 100milliseconds period, from then on the phases goes back to 120 deg. Today we were busy testing the cable (just so you know this is at a nuclear power plant) we’ve got switchgear room situated at the electrical building which is at 7 meter level. The motor is in the turbine hall –6 meter level, and there are two 150mm^2 cable thickness and about 120meters long (for conversions 1m = 3.28ft = 39,4in). The test on the cable shows there is 11% difference on impedance but it was ruled out as the cause of trip because the acceptable standard is 10% for new cable. So, we tightened some connection that we suspected and cleaned them and even swoped the contactor again, then we tested to motor. Three times in the row the motor did not trip, just before we finalize the findings we decided to run it another three time… guess what, three times in the row it tripped. We looked at the results again and it shows that for few milliseconds during startup red and white phases are 180 degrees apart. What could be the cause of this???? What will shift the angle for few seconds????

 

[rbulsara]

you are just lucky not to trip other motors on instataneous current imbalance, imho. There should be some time delay in most current sensing protections.

There could be any one or moer of numerous causes but I dont think this is a serious issue. One probability may be some deformity in rotor?

 

[NI008]

We had faced similar problems on few of our motors. I would suggest:

1. Checking the percentage voltage unbalance. should be between 1-1.5 %. Limit is perhaps 2 %. At startup, it could be more pronounced with some inherent phase resistance unbalance.

2. Check the motors phase resistances with a DLRO. Unbalance between resistances should be less than 0.5 % , i think i remembered the figure correctly.

3. Note the motor acceleration time and check it against that of other similar application motor. It should give you some idea for the curve setting.

 

[edison123]

Is there any capacitor connected across the motor for pf improvement ? If so, may be you can disconnect it and test the motor again.

 

[jghrist]

Red and white phases being 180° apart would indicate that the other phase is open. One breaker or contactor phase probably closes 100 ms after the others. Either put in a time delay or adjust the contacts.

 

[Solman]

To answer edison123 question, there is no capacitor connected. I have a theory and I'd like to hear your opinion on it.: Could it be the interturn fault on the transformer that causes this, suppose at high starting current you could be having curculating current on the winding which result in un-equal voltages, I haven't got much on this theory but how possible is it???.

 

[Marke]

Solman

Every supply has an inherent imbalance in impedance and in voltage vectors. Every motor has an inhgerent imbalance due to the grain orientation of the steel used for the laminations.
Often the imbalance can be reduced by rotating the three phases around so that the motor and supply imbalances complement each other. This may help in this instance, however it does sound as though you have an intermittant source impedance somewhere.
I am curious about your comment that two phases become 180 degress for a few milliseconds. This would be very hard to determine when you are dealing with fractions of a cycle. If you mean tens of milliseconds, then fair enough, I would be looking at a supply issue. What does the current on the third phase do during this period? If you are referring to fractions of a cycle, then this could be an indication of additional current going somewhere that it should not, and I would be looking at an intermittent shorted turn in the motor stator.
I have had situations before with such intermittants that would not show up under any normal testing, but did under controlled heating of the motor.

One suggestion, a difficult one I know, is to swap motors between a working installation and the faulty one and see if the problem moves or remains. That is one way to split the field.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Solman]

Thanks Marke, We have swapped the motor twice, swapped the CT's three times, swapped the contactor twice, swapped the cable twice, We used the similar motor's supply to run the tripping one and the motor did not trip, when we connect it back and supply another motor with a different cable from tripping motor's supply, it trips. This can lead us to believe the transformer is our possible cause. I'd like to hear your opinion about what on the transformer could be causing this....???

 

[buzzp]

The transformer is not causing this, unless there is one transformer for each motor. Otherwise, seems all three would trip.

Check the wiring of your metering equipment that you used to make this determination.

If your worried about the transformer, do a doble or similar test on it. Any issues will be quickly identified especially if it is causing the strange phase relationships.

I still think the thing is nuisance tripping but the 180 deg shift has me puzzled if it the equipment was hooked up right.

I am not familiar with nuke specs but outside of that, I would never set unbalance trip to be instantaneous.

 

[edison123]

Looks to me that your UB relay is misbehaving since the supply from tripping motor, trips another motor even when connected with a different cable. You have swapped so much, why not the UB relay ?

 

[mc5w]

I had instance where a motor burned up because a corroded fuse clip caused the motor to run partially single phase. I was not called in until after the motor had been rewound and it would not come up to full speed and would trip out the motor overloads when idling. Applying the silicon carbide paper - elbow grease - electrical grease method fixed the problem.

That 180 degree phase sounds to me like a similar problem on the primary side of the transformer. Metal oxides and sulfides make for tremendously nonlinear semiconductors especially because of the high impurities in naturally occurring corrosion. You probably have a connection that turns into an effective open circuit at high current and then recloses at lower current. Essentially a Mickey Mouse self resetting circuit breaker.

 

[laundry]

HELLO SOLMAN
I had a problem some years ago when Iwas chief engineer in a paper mill,I worked my way back to the switch room removed the rear of the cubicle and gues what, one copper bar of the red phase in to the back of the isolater had come loose,it was pitted around the washer.I did a resistance check on it,it was ok but when a large current was taken you could see a slight arking. we made a new copper conducter fitted it and the system ran ok.