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Corner delta 3 phase transformers

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.

RE: Corner delta 3 phase transformers

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?

RE: Corner delta 3 phase transformers

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

RE: Corner delta 3 phase transformers

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.  

RE: Corner delta 3 phase transformers

(OP)
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?

RE: Corner delta 3 phase transformers

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

RE: Corner delta 3 phase transformers

(OP)
7200 primarys are Y and secondary is corner ground delta

RE: Corner delta 3 phase transformers

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

RE: Corner delta 3 phase transformers

(OP)
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

RE: Corner delta 3 phase transformers

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.   



RE: Corner delta 3 phase transformers

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

RE: Corner delta 3 phase transformers

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.  

RE: Corner delta 3 phase transformers

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

RE: Corner delta 3 phase transformers

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.

RE: Corner delta 3 phase transformers

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.

RE: Corner delta 3 phase transformers

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

RE: Corner delta 3 phase transformers

(OP)
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.

RE: Corner delta 3 phase transformers

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

RE: Corner delta 3 phase transformers

(OP)
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 ?

RE: Corner delta 3 phase transformers

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

RE: Corner delta 3 phase transformers

(OP)
What does it mean when connecting 3 phase transformers, additive or subtractive.
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

In addition to what was already asked, how much single-phase load do you have?

RE: Corner delta 3 phase transformers

Check the connections on your transformer bank. If the primary of a wye delta bank is connected to the primary neutral, it will cause several issues.
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

(OP)
I have three banks of transformers in the mill. I get unbalanced current on every 3 phase motor on these banks. The primary 7200 is a Y. The 3 primarys are tied together but not grounded. I only have three lighting transformers in the mill. 2  100 kva and 1 60 kva  480 to 110. None of these are loaded. One sub runs a solid state hi freq 400KW welder and a 500KW seam annealer. Our other plant which feeds from the same 7200 supply is one mile down the road and the transformers there are YY. We have the same equiptment and problem there also. 99.9 % is 480 3 phase equiptment.

RE: Corner delta 3 phase transformers

It sounds like the utility is supplying unbalanced voltage on the primaries.
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

(OP)
We have two pipe welding mills that use 95% 480 3 phase equiptment.. At both of these facilities when we read the current on a 3 phase motor it is way unbalanced. For instance a 20 HP motor reads 15 19 22 amps.
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 can't get or trust a good indication of primary voltage from the utility, there is a second option.  Shut down all of your loads and measure your incoming voltage at the transformer secondary.

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

Let's look at the information that we have.
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

(OP)
We have two plants. One has three banks of tranformers that are corner grounded delta. The other plant has two banks of Y secondaries. Both plants exibit this unbalance. When I mention to the electric company about checking their incoming lines to their sub it gets real quite. We use a REA utility co. which buys power from SWEPCO.  Everyone tells me that SWEPCO would know if they had a problem.

RE: Corner delta 3 phase transformers

It seems to me that your REA already knows that it has a substation problem. (one transformer/regulator consistantly hotter than the others?)
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.   

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