High Voltage on Hi Leg 2

[bytebyte]

I hope I can get some assistance on this problem....

A customer with 3-ph open delta 120/240 V is having high voltage on the hi leg of 220 volts, which exceeded the +5% limit of 208V. The two primary side phases of the transformer are balanced. The primary voltage is fairly balanced at 123 volts on both phases. The load and reactive power on the primary side are balanced as well on the two phases.

What could be causing the high voltage on the hi leg?

Based on the primary voltage, the high leg votlage should be 123*root3= 213v. But from the high leg voltage, 220/root3=127v, but the voltage measured on the primary side is not 127 but 123v.

thanks for your help!

 

[davidbeach]

So you don't have exactly 120 degrees between the two legs. Rarely do in an open delta. But, since there should never be a single phase load connection between the Hi (Wild) leg and neutral it doesn't matter any way.

 

[bytebyte]

thanks davidbeach.

Unfortunately, I don't think the customer will be too happy if I told him the high voltage on the hi leg doesn't matter. Do you have any suggestions on how to achieve a 120 degree separation between the two transformers?

 

[davidbeach]

A third transformer.

 

[itsmoked]

Agreed, it really doesn't matter at all.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[bytebyte]

The service is fed from 35 kV so closing the delta is not an option. Any other suggestions? Thanks!!

 

[dpc]

The service is fed from 35 kV so closing the delta is not an option

Care to elaborate?

Did you make these measurement with the transformer bank loaded or unloaded? Is there a specific customer complaint?

Honestly, the voltage from the high leg to ground is of interest only to engineers. The NEC requires explicit marking of high leg systems and does not allow any loads to be connected between the high leg and ground. The 5% voltage range only applies to a voltage that is going to be used for something and this isn't.

As David Beach indicated, you have an inherently unbalanced system.





David Castor

http://www.cvoes.com

 

[waross]

Try another meter.
What are the three phase to phase voltages?
Are the transformer primaries connected phase to phase or phase to neutral?

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

 

[bytebyte]

dpc, closing the delta at high voltages have proven to cause ferroreasonance; therefore, from my understanding, most utilities do not close the delta at higher than 25 kv. The measurements were based on a recorder that is currently installed at the service and the bank is loaded. No specific complaints from the customer regarding equipment issues, but that their engineer noticed the high voltage. If the voltage is above +5% then I would think it's the utility's responsibility to correct it whether the hi leg is used or not.
Can you please guide me to the section of the NEC that applies: "The NEC requires explicit marking of high leg systems and does not allow any loads to be connected between the high leg and ground. The 5% voltage range only applies to a voltage that is going to be used for something.."


Waross, the phase-to-phase wasn't measured because the recorder was set to measure l-n. The transformer primaries are connected phase to neutral.

 

[dpc]

See NEC 110.15 and 230.56.

most utilities do not close the delta at higher than 25 kv.

There seems to be no end to the variations and permutations on ferroresonance concerns.

 

[HornTootinEE]

Is your primary Y point grounded or ungrounded? Have you considered wiring the primary in open delta as well?


If the primary Y point is ungrounded, could be feasible that the neutral is shifted

 

[waross]

There is so much information that we don't have that analysis of your issue is challenging.

"Unfortunately, I don't think the customer will be too happy if I told him the high voltage on the hi leg doesn't matter."
Well it rained here on Tuesday. I didn't like it and some canola growers were devastated. But that's life.
And by the way, many customers would be horrified to know what the voltage actually was at their service.
I have had some experience with marginal services. Things like long distribution circuits on soft systems, long, unregulated rural distribution circuits, unbalanced distribution voltages and displaced neutrals. On some rural distribution circuits it is common to have phase to neutral voltages equal but phase to phase voltages quite a bit different indicating phase angle errors and a displaced neutral.
In many areas your voltages would be considered a good balance.

The slightly high voltage may be a problem but the biggest problem may be that the customer knows that the voltage is a little high.

Your 120/240 voltage is a little high, about 2.5%. Nothing to worry about.
Why is the voltage high? There may be a good reason. Until you know don't worry about it or try to change it. Leave well enough alone.

Wild leg voltage: With the 120:240 Volt leg running at 123:246 Volts we would expect a wild leg to be at 213 Volts. 220 Volts is only 3% high.

How to make it balance? Motor loads. By the way it is a fact of life that the cheapest, simplest and easiest way to correct voltages and phase angle errors on an open delta is with slightly over-sized motors. Three phase motors will inherently tend to make corrections. This will cause the motors to run hot if they do not have some reserve capacity. Normally stator current is a good reflection of motor heating, but when a motor is used on unbalance voltages and subject to phase angle errors, there is additional rotor heating that may not be accurately or completely reflected in the stator currents. Often one size larger motor will be enough that you will not have problems.

Back to the issue at hand. If you want to pursue the reasons for the slightly high wild leg, be prepared to supply a whole lot more information.

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

 

[davidbeach]

be prepared to supply a whole lot more information

A complete phase-by-phase plus neutral model of the entire distribution circuit from the substation out to the end of every branch would be necessary to get at exactly what's wrong and how it might be corrected. Let it go.