Corner delta 3 phase transformers 1

[ttind]

We ahve three banks of these and muy motor current are far from ballanced


I have a grounded delta transformer system. 3 transformers, 3 phase 480 volts. My question is shouldn't my current readings for my motors be balanced? I have a 200 HP AC motor driving a lightly loaded generator. The current reading for the three phases is 120, 90,90.

 

[dpc]

You need to measure the voltage of each phase and check the voltage balance. A small voltage imbalance will cause a much larger current imbalance.

Yes, your motor currents should be very close to balanced.

Any capacitors at the motor?

 

[ttind]

It is a 480 volt system. Voltages phase to phase range as much as 15 volts.

 

[dpc]

Compute the voltage imbalance by calculating the average voltage, then compute the maximum deviation of any one voltage from the average. NEMA MG1 motor standard says that the maximum voltage imbalance should not exceed 5%. For any voltage imbalance over 1%, the motor must be de-rated.

Current imbalance will be 6 to 10 times the voltage imbalance.

This will cause overheating in the motor.

 

[ttind]

My voltage readings are 471 474 454 Phase to phase
Phase to ground is 470 453 4
Current on motor is 135 104 and 90
This is an average volts of 466
So 23 volts would be 5 %. Max difference is 8 volts. Or 2%.
All of my motors reguardless of size show this inbalance.
If the power company had a transformer with an impedance unbalance will it cause this?

 

[waross]

Are you on an open delta?
Voltage imbalance with an impedance mismatch with normal impedances will be much less. The full effect of transformer impedance is only felt during a short circuit. With normal loads, the regulation of the transformer will dominate and it will result in even less voltage imbalance. Transformers on different taps will only make about 5% difference. You may have a heavily loaded primary phase with low primary voltage.
How are your transformer primaries connected?
respectfully

 

[ttind]

7200 primarys are Y and secondary is corner ground delta

 

[waross]

ttind;
Is the primary wye point connected to the primary neutral or is it floating?
respectfully

 

[ttind]

Each of the 7200 lines feeds an individual transformer and the other tap is tied to all three. Not sure if the center is tied to ground or not. It is a Y connection

 

[dpc]

You could ask the utility to check the primary voltages to see if the imbalance is on the primary, or is being caused by imbalanced loading on your system.

 

[ttind]

I have had the utility company out to check. They are stumped ?

 

[dpc]

What did the utility tell you the primary voltage was? The voltage imbalance either exists on the primary side or it doesn't. If it does, the problem is up the line, probably some single-phase regulators that have a problem.

If the primary voltage is balanced, the problem is either in the transformers, the transformer connection/grounding, or due to voltage drop caused by unequal loading on the three phases, or a bad connection somewhere. With three single-phase transformers, it is possible that the three transformers are not the same size and/or have different impedances.

You just need to isolate where the voltage imbalance starts and focus your attention there.

 

[waross]

If you are downstream from a bank of voltage regulators the problem may start there.If one phase voltage is low because of heavy loading on that phase, the corresponding neutral currrent will cause a phase shift. The regulators will correct the voltages to neutral but cannot correct the phase shift. After the voltage regulators, you can end up with balanced line to neutral voltages but unbalanced line to line voltages.
Your delta secondary may have a ohase shift and unequal voltages.
When the utility checks the voltage they may find the line to neutral voltages to be balanced, but the line to line voltages unbalanced. Such is the nature of a phase displaced neutral after voltage regulators have corrected the line to neutral voltages.
respectfully

 

[jghrist]

Isn't the concern with the voltage unbalance misplaced? It is only 2%, but the current unbalance is very high at 120, 90, 90 (20% unbalance). This much unbalanced current could cause the observed unbalanced voltage. It seems that the problem is with the load, not the source.

 

[anandmoh]

This case is very interesting .Can you paste the single line diagram of the interconnection between the Transformer,motor and generator.Such an unbalance is not normal.It would then be easier to analyse.

 

[waross]

Induction motors generate a back EMF when running. Due to the physical characteristics of the motor, the back EMF is three phase, displaced the normal 120 deg. When the feed voltages are phase displaced, the motors will be extracting power from one or two phases and supplying power to the other phase(s).
This contributes to a heavy current unbalance.
When the voltages on a closed delta system are not equal, you have a phase shift on one or two phases.
Draw the vector diagram and measure the angles.
respectfully

 

[ttind]

Someone wrote "Isn't the concern with the voltage unbalance misplaced? It is only 2%, but the current unbalance is very high at 120, 90, 90 (20% unbalance). This much unbalanced current could cause the observed unbalanced voltage. It seems that the problem is with the load, not the source."

We have 3 transformer banks at this facility. All of these are doing the same thing. We have another plant one mile down the road that had a wye secondary [2 banks] that are doing the same thing.

 

[waross]

Are both plants fed from the same primary circuit?
Are you a long way from the substation?
Are there any voltage regulators between the substation and your plants?
respectfully

 

[ttind]

Are both plants fed from the same primary circuit? YES
Are you a long way from the substation? 3/4 MILE
Are there any voltage regulators between the substation and your plants? Not between that I know of but at the sub. The utility people say they have regulators for each transformer that they try to maintain between 120 and 125 volts. I do not understand this regulator but I believe it does it by changing taps. They have one transformer that runs hotter than the others and said they have had several SPECIALIST check this unit. One of their linemen told me that sometimes you couldn't touch it. I ask them if maybe the winding laminations were breaking down and causing a phase shift or unbalance.
What effect does this unbalance have on my utility bill ?

 

[waross]

You will be paying for the extra heat that the motors generate when they run hotter than normal.
It sounds as if your problem may be the problem transformer at the sub.
I wonder if there is a problem with the tap changer? If the taps on one phase were inoperative or were out of step with the other phases it may cause these problems.
respectfully