250 HP Current Imbalance Troubleshooting?

[Ceast]

I'm troubleshooting a late 50's generation delta connected 2400Vac engine blower motor. I recorded FLA of Aph=23 Bph=41 Cph=41. I thought the megger tests would indicate an insulation problem but all results were 3.5G ohm-4.1G ohm. PI is 1.1.

The motor is direct coupled to the generator windings. The phase voltages are balanced. The motor has nearly 100k hrs of operation.

Any recommendation? I plan on a winding resistance test.

 

[edison123]

If it is a 6 leads motor, then probably one phase leads are reversed. Normal sequence of connecting six leads into three is A1B2, B1C2 AND C1A2. Your probably has A2B2, B1C2 AND C1A1. Is the motor turning at normal speed ? It would not, if the one of the leads is reversed.

If it is a 3 leads motor (permanent delta), then you may have a turn short in the winding.

And finally, MV motors are usually wye connected. Recheck the nameplate for actual connection.

Muthu

http://www.edison.co.in

 

[edison123]

A surge comparison test would also reveal turn shorts, reversed phase etc.

Muthu

http://www.edison.co.in

 

[Ceast]

6 lead motor reconnectable delta/wye. The gen is delta and the blower mot (combustion air) is direct coupled to the gen terminals. Motor is delta

Can you explain what's going on with a winding reversal? I have limited motor experience.

Thanks

 

[waross]

The first question is:
Is there a problem.
The second question is:
How are you measuring the current?
Third:
Have you taken a second set of current measurements with different measuring equipment.
When I was working with a lot of motors I saw a lot of measurement errors. I didn't worry about current unbalance unless I also had bad heating. Then I took as many current measurements as I could in as many locations. With a clamp-on meter, the there is often interference from nearby magnetic fields. You can often see this effect by moving the meter while taking the measurement. The problems are often with the one lead in the awkward position so that there is only one possible location for the meter and interference from the fields surrounding the other cables.
PS; How are the bearings? On a two pole machine an uneven air gap may cause uneven currents. Less gap means less magnetizing current.
Can you check the generator. A generator problem may cause unequal currents. How are the open circuit voltages of the generator phases. The motor will try to correct unbalanced voltages and a large current unbalance may be an indication of a small voltage unbalance. The motor may correct the voltage unbalance to the point that it is difficult to see with the motor connected particularly if the motor is on a generator which will have much more source impedance than the grid. The motor will pay a price with unbalanced currents.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Are you sure you have a measurement point that include the total current. If you are reading a delta motor and you measure at a point where you only capture one of the two connected legs, then current will read sqrt(3) low. The ratio of your currents looks like sqrt(3).

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[Ceast]

Bill this is the same unit from my power eng post. Funny coincidence the I have failed Aph VT's and regulator on the same phase that I am seeing low current on?

See the attach load study doc. Dranetz PP.

 

[Ceast]

Pete I recorded the currents off of the bar type CT's in the OC protection cabinet which is located 4' from the motor. See the attachment.

 

[Ceast]

What do you think of the starting current signature!?!?!?!

 

[electricpete]

In your attachment, I see what looks to me like many PT's and no CT's. Maybe I am missing something. I recall a 351 is an overcurrent device and should have current inputs, but I don't see any CT's here. Maybe someone can help straighten me out as to where the CT's are.

If CT's are on the 12kv side of that transformer, then how are we separating generator current from motor current. FLA for 250hp motor on 12kv side would be about 12A.

Starting current signnture is quite bizarre. Again is this soley motor current - what was generator condition at the time?

I am quite confused.

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[electricpete]

Looking at the current siganture again, it looks like starting current was drawn in about a 0.3 sec period centered around 11:29:00. Everything later I guess was change in load. The big question why the unnatural dip in the middle of the starting current period.

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[electricpete]

Also, assuming we are looking at amps measured on the 2400 volt side, nameplate FLA should be in the neighborhood of 60A, starting current should be in the neighborhood of 300A. At first glance your measured current seems low. But I have suspicioun about the processing of the signal. For example if you try to take a simple "average" of an ac signal over something like 1.5 cycles, you will get garbage with a value much less than the actual rms value. If it current signal was properly processed and represents an rms value, could perhaps suggest low voltage or... something else?

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[waross]

I would not assume that the problem is the motor, it may be a reflection of a failing generator or failing measuring instruments or CTs or PTs.
Does the unbalance change going from islanding to parallel connections?
Can you roll the CT secondary connections? That is a quick and usually easy way to check instrumentation quality.
Rolling the CT primaries is better but is more difficult.
Is the motor overheating? What is the operating history of the motor. Have there been any incidents like a bad synchronism or a fault?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Ceast]

Sorry I haphasardly assembled the drawing. Yes there are 2 sets of CT's. One bar type (mot prot) set directly in front of the motor and the second set (gen prot) on the 15kV gen bus.

Keep in mind the motor is DIRECT wired (refer to dwg) to the generator stator. The motor begins to rotate as the voltage builds in the gen. So gen rotation & excitation = motor starting. Look at the Aph current at start-up. Almost nothing. What gives?

 

[electricpete]

I misread the time scale. Current slowy rising for 5 sec (preumably toward LRC although never got that high). Perhaps voltage was increasing during that period. Then drops to 0 for reasons unknown.

The ratio of C/A = B/A ~ 6 during the intiial period around 11:29 (presumarly starting)

The ratio of C/A = B/A ~ 1.73 during the final period from 11:30 to the end (presumably running). It looks to maintain a relative constant ratio even as current increases (presumably load is increasing?... or voltage decreasing?... do you know why current is increasing during the final period?)

Do you have a plot of voltage vs time?


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[electricpete]

I meant to say:
Do you have a plot of voltages vs time?

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[Ceast]

The voltage ramps slowly. Consider the engine cranking at the same time the generator is building voltage. The motor rpm has to be high enough to force air into the combustion chambers.

The engine kind of stumbles and stalls momentarily until the fuel system gets control. The gen voltage is unstable at lower hz. That's why you see the strange current signature.

I suppose a gen problem is possible but odds are against one phase voltage magnitude lower of higher than the other two are remote. Not impossible though.

 

[waross]

#1 You have already had PT failures. Check your CTs and PTs.
#2 You have about a 10:1 or 12:1 ratio between the generator size and the motor size. A small voltage problem in the generator will translate into a large current problem in the motor as the motor, acting as an induction generator, tries to correct the voltage problem.
You have a system comprised of a motor, a generator and the instrumentation. You have a problem that is indicated by a low phase current. That indication may be caused by the motor, the generator or the instrumentation.
Given the good megger readings and the history of PT failures, it may be well to focus on the entire system rather than just the motor.
Remember that current errors tend to be very much larger than the voltage errors that caused them.
By the way, do you have any capacitors connected? The result of one failed fuse in a capacitor bank would be interesting

Bill
--------------------
"Why not the best?"
Jimmy Carter