Dear Experts,

We have 6.6KV wound rotor motor, 5MW, 500A FLA, cement milling motor. For 2 years operating, about 1 month to 2 months operating, motor is trip by single phase overload (always random 2 phase ~ 450A, 1 phase ~ 1000A). 

We have changed the new motor, new cables (power circuit), but the symptom is always happening.

We have been checking switchgear, PT, CT, circuit breaker, but they are not problem.

 

Do you have opinion about that? 

Thank you

Lampard Nguyen

Original Post

That's a headscratcher all right.





quote:
For 2 years operating, about 1 month to 2 months operating, motor is trip by single phase overload (always random 2 phase ~ 450A, 1 phase ~ 1000A). 




Are you saying that the motor has operated for 2 years normally and then the problem only showed up in the last 1-2 months?

 

I can appreciate the logic of why a ground fault would be suspected, but…. 500A+ ground fault jumping among phases and doesn't leave any evidence (burn marks) or trip any protective relaying other than overload? That seems unlikely, although I don't have another ready explanation.

 

If the machine were delta-wound and there were for some reason open circuits in one leg, then we see one leg carries the vector sum of the other two leg currents.    But why would an open circuit jump among phases?

 

Some unusual things might happen during transient. Is this trip occuring during a start?    During some other transient? Or during steady state?

 

How are these reported currents measured / determined? Read from a digital relay? Measured from a clamp-on?

 

What is the grounding configuration of the supply to this bus? Solid grounded? Resistance Grounded? Ungrounded? Any ground protection for the bus?

 

 

Some troubleshooting to consider:

Check for three-phase current balance during steady state operation.

Get oscilloscope traces of currents if transient or oscillation is suspsected.

 

Megger and bridge stator.   Include as much of the supply circuit as possible.

Megger and bridge rotor circuit.

Check resistor switches.

Investigate the ct secondary circuit.  Typically has only one ground at center of a wye connection on the ct secondary. Lift the ground and megger remainder of circuit to ground at 250vdc (well at least we do that with our electromagnetic relays… you might want to take any electronic relays out of the circuit before that test).

Originally Posted by Edwin de B:

Looks like a short circuit to ground in some place. All other problems will show an increase in current on 2 or 3 phases.

Are there any parts of the power circuit that were not exchanged?

Hello Edwin,

Thank you for your comment. We had replaced power cables for that motor, and also new motor was replaced. But, problem still present.

 

I am wondering the problem come from MV Switchgear. We have not exchange MV Switchgear.

Hello Electripete,

Thank you for your comment.

The problem have been occuring since 2013, every 1-2 month, the problem occrued again. Sometimes high current (1000A) is at phase 1, sometimes phase 2, sometimes phase 3, It's random. 

 

New motor, new power cables, but problem still occurs. The trip is not occuring during starting, just occurs when operating.

 

We also checked secondary circuit, no problem.

I too agree this is a head scratcher....

 

The cables you changed, are they for the stator or rotor circuit? I am assuming you have a resistor bank that controls the speed of the motor, see attached

 

With the amps being so high, this would typically go BOOM! I agree with epete, the meters/CT/PT that you are using need to be verified that you are truly seeing these high of current levels

 

The rotor circuit, this could explain why the amps are random and so high. But for this to occur you would have to have a large resistance change to equate for some many amps, hence potentially the rotor circuit is to blame.

 

The rotor has multiple poles, each pole pieces has the same resistance, if a pole was to short internally, it typically reduces that poles resistance. If you performed a voltage drop on each pole on the rotor, again the drop should be the same on each pole. What I am getting at is the resistance is balanced, in order for your amps to truly get that high, resistance needs to be added.

 

If a breaker/s were seeing this large of current deviation it should trip...but is it the stator or rotor circuit that is tripping? For the cables to short together or to ground to create the large amp difference, they too would fail

 

Hope this helps

Dave

Attachments

Photos (1)

 

quote:
New motor, new power cables, but problem still occurs. The trip is not occuring during starting, just occurs when operating.



I agree with previous poster that the rotor circuit now seems like a prime suspect.    The influence of a heavy imbalance on rotor circuit will move among all three stator phases at pole pass frequency to influence all three stator phase currents similarly, but at different times.   When you say one stator phase current reads higher than the others, that may just be a snapshot in time… a few seconds later a different stator phase current would read higher.   

 

Swapping out the motor you eliminate the rotor itself, but you still have slip rings, cabling, switches to consider in the rotor circuit (I assume no resistors inserted while running).   First simple check with the motor running - see if the three rotor currents are balanced (and watch them for a few minutes). Off-line checks would focus on resistance of rotor circuit from brushes to resistor bank (with brushes rings lifted). Also inspect the brushes / slip rings while you're lifting the brushes.

 

I'd vote rotor circuit and current sensing circuits/relays are the two prime suspects.  Distant third is some severe anomaly in the power supply to the stator… you haven't told us anything about it.

As per my experience with these things the 500A ground fault jumping among phases and trip any protective relaying other than overload?

If the machine were delta-wound and there were for some reason open circuits in one leg, then we see one leg carries the vector sum of the other two leg currents.

Some unusual things might happen during transient.

 pcb cost

If I were you I should check voltage ratio of motor in transformer mode . Here you have to be careful because if you have 6kV on stator connection it is also probably  high voltage on  rotor connections * I expect more than 2 kV  and that information you will have on the signboard * .

We used safer version respectively to give 3 x 380 V to the stator and measure  current on every  stator phases and  rotor voltage at each resistor shortening  .

It can give information on whether any step you  have a problem with asymmetry and cause for concern .

Good luck 

 

 

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