Voltage fluctuations

[Thedroid]

I was called out to the plant today for a Phase Loss trip on one of the MV synchronous mill motors. Upon arriving I checked the event recorders in the MVC soft start and the 469 hooked up to this motor and found that the voltage at the time of the trip was 3950, 3962, 3700. I pulled a trend from our ECS and noticed that the voltage had dipped on that incoming line also. I checked the incoming line voltage on the power monitor and it read 4255, 4251, 4217.

I pulled a trend for the three phases, and noticed for about the last three months the third phase has been a little lower than the other two, and the also the graph was a little choppy compared to the other two, wich remain smooth and stable. It seems like this problem is originating from the utilities. We were experiencing a snowstorm at the time this motor tripped.

I decided to reset and try again. Everything started up and ran just fine, but we still have this unbalance. Where should I start my investigation on Monday?

 

[edison123]

Loose contact in that phase ? Use a temp gun to check the temperatures of all the terminations.

 

[itsmoked]

If you check your voltage at the entry and the currents are similar, then you should ask your utility what the problem is. They could have an external imbalance or a loose connection.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

I'd call the utility. They may be having an issue with a voltage regulator,or maybe just unbalanced loading.

It might be worth your time to drive the line the serves your facility all the way back to the substation and see if there any pole-mounted regulators, switched capacitors or other little surprise packages on your feeder. It's always good to know what's between you and the substation transformer.

 

[Thedroid]

I'm thinking that the problem originates before our facility. We have three transformers where the utility enters at 46kv and is stepped down to 4160. The power monitor on one of these lines is showing one of the phases about 30 volts lower than the other two. Is this something to be concerned about? I'm thinking that this was the cause of the phase loss trip today.

 

[jghrist]

The voltage unbalance is a lot more at the motor starter than at the incoming line. This would indicate a problem in the plant. First thing to check is for current unbalance that might be causing more voltage drop in the plant distribution on the third phase.

 

[alehman]

It seems like you have a fair amount of voltage drop on all three phases between your incommer and the motor. Is there a transformer between the service and the motor?

 

[ScottyUK]

Don't trust your VTs until you have proven them. Do you have another VT on the same bar which you can use for correlation?


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Good point Scotty.
Possible not good contact too.
Regards.
Slava

 

[Thedroid]

During normal running conditions the voltage at the motor is almost identical to the voltage in the substation. The voltage readings that I put in my post were from the event recorder at the time of trip. I have no way of getting an accurate reading of the incoming line at the moment of the fault. Today we called the utility and they said that as long as the unbalance doesn't exceed 100V then there is no problem. They did acknowledge that there was some issues yesterday from the snowstorm we were having.

I don't think that we have any voltage drop issues in the plant. I think that it originates from the incoming lines or the 46/4.16kV transformer. We own the transformers, but they are serviced by the utility.

 

[Thedroid]

Yes we do have different VT's to compare against, and they are all real close to each other. I can only pull up an accurate trend on one, to look at the conditions at the time of the fault.

Mills been running fine since yesterday, no other issues so far.

 

[alehman]

4255, 4251, 4217 is not at all unreasonable. You say the motor voltage normally tracks the incommer closely. So this is the typical level of unbalance you see at the starter?

 

[rockman7892]

I agree that the incoming voltages only appear to show about a 0.5% unbalance which is not all that bad.

 

[Thedroid]

Yes it is the typical unbalance seen at the starter. When other large motors start or there is some kind of disturbance, the lower of the 3 phases drops more than the other 2. If A and B drop 30 volts the C will drop 90.