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.

 

[ttind]

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.

 

[magoo2]

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

 

[waross]

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

 

[ttind]

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.

 

[waross]

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

 

[ttind]

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 ?

 

[JBinCA]

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

 

[waross]

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

 

[ttind]

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.

 

[subtech]

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.