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Medium Voltage Motor Failures


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#1 jackson8000

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Posted 18 January 2003 - 07:40 PM

Submersible pump motors thru 450hp, 2300V, fail along a group of pumping stations each supplied by one 12470-2300V transformer [some ungrounded] and each with a power factor correction capacitor switched with the load.

Measured tests [across multiple sites]show normal voltages prior to motor start, yet unbalanced current / voltages [as large as 25% current unbalance] during operation.

Motor protection relays are inadequate.

Q: Can the capacitors be ruled out as a potential cause of the unbalance problem.

Q: If the system is ungrounded, then it should still require a fault to occur to throw the voltages out of balance [and damage the motors]. However, the same phenomenon is occuring on multiple sites. What else could cause this??? [Motor leads were rotated between tests and phase results did not change thereby ruling out motor windings]

#2 marke

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Posted 18 January 2003 - 08:19 PM

Hello jackson8000

This is an interesting (and expensive problem!! Have you tried measuring the voltages and currents without the correction connected? The current imbalance that you have measured, is that current into the motor, or current into the motor and correction (i.e. line current)
I take it that there is a separate transformer per pumping station. Is this correct?
What is happening on the 12470 volt supply when the motor is running. Is this remaining balanced?

Best regards,

#3 jackson8000

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Posted 19 January 2003 - 06:40 PM

Marke,

The current has not been measured without capacitors and the current that was was measured included capacitor current also.

There is one transformer per pumping station, typically 300-500KVA, about 4%Z.

Amazingly enough, the original design engineer doesn't know how the transformers were intended to be grounded or the winding configuration. But, test results indicate both delta-delta and delta-wye configurations, we believe most secondaries are ungrounded. [Multiple transformers have failed also.]

The information on the supply [which I incorrectly stated as 12470 is actually 13,200V] is only a strip chart which shows no abnormalities. A main 13,200V substation feeds 4 or 5 overhead transmission line sections, each running a couple of miles and feeding 5 or 6 pumping stations [26 total]. Another interesting fact is that the most failures are occurring on one section of transmission line.

The only other device connected to the distribution is a surge protector at each motor controller.

#4 marke

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Posted 20 January 2003 - 09:12 AM

Hello jackson8000

From your information, it would appear to me that you have a problem on the 13,200V line that is common to the pump stations. I would guess that you have a high impedance in one leg, quite possibly a faulty isolation contact or fuse, or just a simple bad jiont somewhere. If you have a high impedance leg, this will certainly throw every thing out when you try to run motors. The thing to remember with motors is that a very small imbalance in voltage can lead to a large imbalance in current. An impedance imbalance in the supply can lead to a drop in voltage and a change in angle depending on the actual impedance in the supply. I doubt that it is related to the individual transformers used as it would then only affect one station. The only other thing to be carefull of, is the static correction applied to each motor. It is imperative that this correction is less than 80% of the magnetising current of the motor. If one or more machines are over corrected, that can cause problems to that machine and cpacitors which could then reflect back into the supply.

Best regards,




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