Corner delta 3 phase transformers
Corner delta 3 phase transformers
(OP)
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.
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.






RE: Corner delta 3 phase transformers
Yes, your motor currents should be very close to balanced.
Any capacitors at the motor?
RE: Corner delta 3 phase transformers
RE: Corner delta 3 phase transformers
Current imbalance will be 6 to 10 times the voltage imbalance.
This will cause overheating in the motor.
RE: Corner delta 3 phase transformers
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?
RE: Corner delta 3 phase transformers
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
RE: Corner delta 3 phase transformers
RE: Corner delta 3 phase transformers
Is the primary wye point connected to the primary neutral or is it floating?
respectfully
RE: Corner delta 3 phase transformers
RE: Corner delta 3 phase transformers
RE: Corner delta 3 phase transformers
RE: Corner delta 3 phase transformers
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.
RE: Corner delta 3 phase transformers
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
RE: Corner delta 3 phase transformers
RE: Corner delta 3 phase transformers
RE: Corner delta 3 phase transformers
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
RE: Corner delta 3 phase transformers
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.
RE: Corner delta 3 phase transformers
Are you a long way from the substation?
Are there any voltage regulators between the substation and your plants?
respectfully
RE: Corner delta 3 phase transformers
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 ?
RE: Corner delta 3 phase transformers
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
RE: Corner delta 3 phase transformers
The local power co. put in a temp transformer to try to correct my unbalanced current but it didn't help. I told them they need to look at the incoming power that was feeding the transformer.
RE: Corner delta 3 phase transformers
RE: Corner delta 3 phase transformers
If one of the transformers at the sub is running a little high on the voltage, a wye delta bank with the primary neutral connected will be pulling current from the "high" transformer to try to balance the voltages. This can result in overheating of your transformer, and also in the substation transformer.
Please check and let us know if the primary neutral point is connected to the system neutral or floating.
respectfully
RE: Corner delta 3 phase transformers
RE: Corner delta 3 phase transformers
Ask the utility to check the primary voltages and correct them if needed.
You can only get a proper phase relationship on your delta if the voltages are equal. Sketch the vectors and check the angles. If the three voltages are not equal your delta will not be equallateral, and the angles will not be equal. Motors don't like this phase shift. You may be able to compensate by changing the tap settings on your transformers but this is not recommended. The proper solution is to correct the problem with the utility transformer. It is possible that the tap changer on one of their transformers is malfunctioning or hanging up.
respectfully
RE: Corner delta 3 phase transformers
One mill is corner ground delta secondary and the other mill is Y. 7200/480.
Our local Rural Elect Co engineer thinks this is normal. With all equiptment shut off and only a 70 HP air compressor running the current is like 50 60 and 75 amps.
Now he writes the Plant Manager and says that he is still working on the problem but the funky amp readings are because of harmonics on the lines are messing up our clip on amp meters.. I never heard of anything so far out in left field. They even put in a portable substation that feeds us and the same results. I suggest he check the company that REA is buying power from. Your thoughts ?
RE: Corner delta 3 phase transformers
If you still have the imbalance, then you know you're receiving unbalanced power. If you don't, it's still possible that the utility's regulators are not working correctly as they respond to your plant's load.
From my perspective, the phase to phase voltage imbalance that you report is indicative of either seriously imbalanced loads in your plant or a imbalance in the utility supplied voltage.
If you're using a true RMS voltmeter, then the harmonic distortion concern is nullified. If mroe detailed data is required, rent a power analyzer for about $2k.
Subtractive or additive polarity of a transformer has to due with the relation of the primary and secondary windings and would not pertain to your problem.
Best of Luck.
JB
RE: Corner delta 3 phase transformers
All motors exhibit a current unbalance.
I understand that all the plants on the same circuit show the same problem.
There is a 2% voltage unbalance.
One substation transformer runs disproportionately hot.
There are voltage adjusting tap changers in the system.
Most likely cause; The tap changer on one transformer is one tap out of step with the others.
Second choice; The primary supply to the substation has an unbalanced voltage.
Third choice; Both of the above. The incoming voltage is unbalanced and a tap changer is trying to correct and causing or worsening a phase shift.
In regards to the ammeter readings. It is common for the reading on the center cable of three parallel cables to be influenced by the other two phases being in contact with the outside of the tong of the clip-on meter. Most experienced electricians and technitions are aware of this and recognize the effect and try different meter positions until they get a reasonably accurate reading.
You may want to try switching the ground to another corner of the deltas to rule out any weird one-in-a-million high resistance ground faults that may be causing problems.
Do you have one transformer in each bank that is much hotter or much cooler than the others?
Are you able to do any trouble shooting at the plant?
Specifically, during the weekend, or slow period, is it possible to kill the transformer bank long enough to change the tap setting on one of the transformers?
I would not normally suggest running banked transformers on different taps. In this instance it may be justified as a trouble shooting measure. If you can mitigate some of the problem with an off voltage setting of a transformer tap, you may make a strong suggestion to the utility that they have a voltage problem and should check their tap changer.
If you are able to improve the situation by changing a tap, it is up to your judgement whether to run that way or return the tap to the original position and wait for the utility to respond. If you choose to run one transformer on a different tap setting, please co-ordinate with the utility and restore the setting immediately the utility completes repairs or adjustments.
Running your transformer banks on different taps will be treating the symtom rather than the cause, and may well mask the problem at the substation. The utility may then want to think that the problem has healed itself and never check their equipment. Your call.
respectfully
RE: Corner delta 3 phase transformers
RE: Corner delta 3 phase transformers
Also, the fact that you have both Y and Delta connected banks in service derived from the same feeder and they both exhibit the unbalance problem brings me right back to the substation. Waross has you on the right path. Also,I think it would be very wise and easily justified to find an independent firm in your area that investigates power quality issues and hire them to look into this for you.
You may need this professional help and you'll for sure need documentation if you wind up seeking damages for equipment that has died prematurely.
Lastly, don't ever assume that power companies are chock full of "power experts" (no matter who your power company is). While I believe that it is true that most power companies have some pretty good people, some aren't so good. I would be very wary about taking your REA's explanations/opinions at face value. I'd get some reputable outside help.