Abnormal Voltages on distribution Feeders

[csteele]

Is there anyone out there that might be experiencing this problem. The problem is on our 13 kV distribution system, our C Phase bus voltage is significantly lower than both A and B phase. Initial investigation found that A phase had slightly leading power factor. However all capacitor banks were taken off of this particular phase and the problem was not resolved. Then the problem is starting to show up on other parts of the system and consistently A phase is higher than both B and C by approximately 3-5 volts on a 120V base. The laods are generally balanced between all three phases. The issue is that when C phase voltage is at 120, my A phase voltage is at the top of the Tariff. Unfourtenately, I do not then have room for the LTC on the transformer to increase witout having high voltage. Is there anyone out there that has ever heard of a similar problem and what they have found

Thanks
Chris

 

[wareagle]

I would lower the regulator to the neutral position. If A phase is still 3-5 volts high then I think the transmission line voltage may need attention.

 

[dpc]

Is this an industrial facility? Have you asked your serving utility to check the incoming voltage on the service?

 

[csteele]

The Tapchanger is a 3-phase piece of equipment and when you move one phase, the entire voltage fluctuates. To address the second question. We are the utility. We have looked at the Transmission voltage also and it is very much balanced. the 69 kV voltage is within 100 volts of each other.

What does not make sense is that in other substations where the load is imbalanced, the voltage will be almost perfectly balanced.

Thanks for your quick response. I do appreciate it. If you do know anything, please pass along
Thanks
Chris

 

[davidbeach]

Are the fixed taps set correctly? Sounds kind of like different phases have different ratios, differing by about 2.5%.

 

[wfowfo]

I assume that when you say the problem is cropping up on other parts of the system, you are reading it at several different locations and not relying soley on one set of PT's in the sub.

Have you confirmed that the ammeter readings are correct and that the system is genuinely balanced?

As mentioned above, what is the nuetral position reading?

Is the LTC operating in boost or buck (raise or lower)?If there was a shorted turn in the regulator coil, it seems like the condition would reverse in the opposite direction. Does it?

Last resort would be to take the xfmr out of service and do a transformer turns ratio test on each phase.

 

[matwong]

It seems like the source of supply to the sub has got problem already. Have you traced the line upto the source for the incidents like slack connection or lost of few wire strands, especially at jumper connection, contact point of equipment, etc? The voltage normally is OK when it is at no-load.

 

[RRaghunath]

csteele,

What is the type of earthing adopted in your 13kV system. Is it high impedance grounded or isolated system!! Even if not, do you think there could be some problem in the transformer neutral - to - earth circuit, such as loose connection or open circuit!!

How much are you sure that the problem is in 13kV system and not in the VT / VT secondary circuit!! More often, this kind of things happen because of some trouble in the VT windings. If your system is old, you should especially suspect this.

 

[csteele]

I apologize for my delay in response, I have been in Davant, LA on restoration. To address the last threads, all of the fixed taps are set correctly. I have not verified the neutral tap voltage reading. That is a possibility that I will have tried. But something interesting that I have learned is that typically our system A phase runs higher than B or C. This is an inherent system performance. The only other interesting thing is that when we take the 69 kV voltage and turn it down to 13 kV, we change the A phase incoming from being on H1 to H3 on the 69/13 Transformers. On the transformer system, we use H1,H2,H3 A,B,C respectively. MAybe this could be part of the problem, but I think that I am reaching.

The last thread posted...we have a multigrounded distribution system. Also the problem is system wide, not isolated to just one or two substations.

Thanks for the help.

Also, am I way off on the possibility of the change of A phase from H1 to H3?

Thanks
Chris