Transformers in Series, How does it effect the power? 6

[bimr]

I have a step down transformer from the utility that supplies 480V to my facility. The utility say that it is rated for 500 KVA. I don’t have any more information on the utility transformer.

Inside my facility, I have a step up transformer (to 2300V) that is powering (only) a well pump rated for 450HP, 2300V

The inside transformer is a dry type 500 KVA unit with 5% impedance.

The utility transformer is operating at close to maximum rating when the pump is operating at capacity.

The problem is that when the well is started, the power is marginal to start the well pump motor. If the utility voltage drops slightly, the well motor may not start.

What size of transformer should the utility have supplied?

Does the impedance (of the transformers in series) have an effect on how much power comes through the transformers?

The utility was supplied with a load letter listing all equipment in the project. Now the utility rep is saying that there is no problem on their end, it is my fault. The utility is the one that sized the utility transformer.

Any comments?

 

[racobb]

Sounds like the utility transformer is probably sized pretty well for your load since you say that it is "near" capacity when the well pump is running. Are you starting this pump across the line, or do you have compenstaion to limit the inrush from the start?

Alan

 

[bimr]

The well motor is starting across the line, with about 4 times current during the start.

 

[racobb]

I am surprised that the utility will allow you to start the motor across the line, must not be affecting anyone else on the system. I would expect your lights go out or almost on starting. You really need to use starting compensation for a motor that large.

Alan

 

[bimr]

Actually, I have an RVAT installed. However, the RVAT is programmed so that it is more or less an across the line start.

If the RVAT was set at the typical 80% voltage to start, it would drop the voltage further and make the motor starting problem worse.

As it is, the well pump motor takes 4 times the running current to start instead of 6 times for the typical motor. In addition, the well motor starts in 0.6 seconds as well. So the RVAT may not be necessary for this application.

I am more interested in answers to the questions in my first post.

What size of transformer should the utility have supplied?

Does the impedance (of the transformers in series) have an effect on how much power comes through the transformers?

The utility was supplied with a load letter listing all equipment in the project. Now the utility rep is saying that there is no problem on their end, it is my fault. The utility is the one that sized the utility transformer.

Any comments?

 

[davidbeach]

What does the project electrical engineer have to say about this? If the utility transformer does not see 120% of nameplate for extended periods of time and you did not specifically ask for and pay for excess capacity then the utility transformer is not undersized. The design on the customer side of the transformer has some serious problems though, should have been a 2400V service rather than a 480V service.

 

[racobb]

Every series device you put in an electrical circuit has impedance and will cause voltage drop across it when current is passed through it....especially at 4 times rated. Both of your transformers are no doubt experiencing severe drop when you start the motor.

But David and the utility are correct in that you are causing the problem and it will be left for you to fix, or pay the utility to fix it for you. They could install a larger trans. but your load does not indicate this is needed. Larger utility trans. would help some, but possibly not enough. The second transformer will continue to be a problem when starting across the line. Should not be there!

Alan

 

[racobb]

You could also ask the utility for a second 2400 v service, but at this point they may charge you for the entire cost.

Alan

 

[BSEE1980]

The xmfr z's will sum, making the voltage regulation even worse. I can see this will be a problem.

Check your power system + xmfr + cable voltage drop calculations at 100% and 400% of motor current. You may end up with such poor regulation you will burn up your pump motor.

Any way to start the pump with no/limited load?

@ 746 w/HP, your watts @ full load will be 335,700. Assume a .8PF, and that will be 419,625 VA. Is the 500,000 va transformer big enough depends many other factors including any other loads. If this is the only load, technically yes, it should be OK. But there are LOTS of other factors, like system z%, that we have not even discussed here yet.

These guys have given you some valuable insights and suggestions or maybe work arounds. If the only question you have is regarding the xmfr, technically it is probably OK.

You might need a power system study to fully answer everything-you have many variables here.

good luck
Steve

 

[bimr]

I am not defending the designer, this is just the way that it is.

Actually, the voltage drop was measured. The incoming voltage drops to 440V on starting. The step up transformer voltage drop was not measured, but I don't think there is a problem there.

I also have an emergency generator and transfer switch 480V rated. The designer stated that he went with the lower voltage equipment because of the cost and servicablity factors. It is hard to get medium voltage service people.

All that is water over the dam now. I am just trying to resolve this the most cost effective way.

I also don't understand why the utility should not be providing enough power to start the motor. At some point, every motor has to start.

 

[davidbeach]

Utilities generally have policies banning DOL starting of motors more than some number of horsepower; typically in the 50Hp-200Hp range when served at 480V. You need a 2400V VFD, or some other costly modification. It isn't the utility's problem, though I'm sure they would be willing to let you pay for a larger transformer, and an additional monthly charge for excess capacity. Better luck next time on the design.

 

[racobb]

If you cannot get your RVAT adjusted so that it will start the motor, and I would think you could, you might consider a VFD if it could work better. Does anyone here think a VFD would be better? The first thing I would do is find out why the RVAT does not do the job. Does it have a problem?

Unfortunatley the utility will supply capacity for continuous loads, not inrush unless you can somehow persuade them (good luck) or pay them to do it. They would probably add on a perpetual facility charge for the extra equipment. This can be 2% of the extra installed cost every month (24%/year!) for the life of the facility. Generally it is most cost effective for you to do it yourself.

When the utility sees your step up transformer, they will absolutely tell you that the second transformer is a major problem with the full voltage starts.

Alan

 

[bimr]

Again, the RVAT was probably required by the utility to staisfy the across the line start issue. So, technically, I do not have an across the line start.

Practically, I do have an across the line start because of the way the RVAT is programmed.

Regardless, I do have a motor with less than the usual starting current demand. The motor also does not start at full load since the well drains back.

 

[dpc]

I don't see that the utility has done anything wrong here, unless there was some specific requirement regarding starting of this pump. The real problem is the step-up transformer not the utility transformer. Probably a severe lack of communications, but I don't think the utility is on the hook for a larger transformer. But they might be willing to provide a larger one, for a cost, of course.

When you say the pump fails to start - what exactly is happening. If this is a centrifugal pump, the motor should be able to start it at a fairly low voltage.

With the captive transformer in series with the service transformer, there is a significant voltage drop across the two transformers during starting so I agree that a reduced voltage starter is just going to make things worse. The captive transformer is already acting like a reduced voltage starter.

Have you discussed the possibility of adjusting the primary tap on the utility transformer (and/or the captive transformer)? Normally the utility transformer will have primary taps that allow +/- 5% adjustment of the secondary voltage. You could try lowering the primary tap to boost the voltage on the low side up 2.5% or 5%, if that hasn't already been done. You could also look to see if the captive transformer has taps.

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[rbulsara]

bimr:
The short answer: A well pump motor with RVAT should not have any issues starting with the set up you are describing.

Long answer:

Firstly, it is not clear from your first post, if the problem really exists or you are antcipating an issue.

Your statement:

If the RVAT was set at the typical 80% voltage to start, it would drop the voltage further and make the motor starting problem worse.

is not true. RVAT will reduce the starting voltage and so will the startikng torque and the current. The very reason RVAT exists. Well pumps typically have low starting torque requirements so even at 50% tap it should start OK. I would start out at 65% tap and go from there.

 

[bimr]

The well pump motor manufacturer states that the well motor will not be able to start unless the voltage is at least 80% of nameplate. Therefore, the RVAT should be programmed only at the 80% tap.

The well motor manufacturer generally recommends no RVAT since the motor starting current is only 4 times the operating current. However, some utilities specify that you have to use an RVAT.

Typically an RVAT would be programmed to transition from 80% to across the line once the motor has started. We have programmed this RVAT installation to automatically transition from 80% to line by the internal timer in the RVAT.

The problem that I have with the utility is that the utility transformer is operating at 90% of rated capacity with little reserve capacity to start the motor. All this information was listed on the load letter and the utility rep sized the transformer too small. Now, the rep is trying to blame someone else.

Last weekend the voltage coming in from the utility was 4% less and the motor failed to start. The tap was changed on the transformer and the motor was then able to be started

 

[bimr]

If you cannot get your RVAT adjusted so that it will start the motor, and I would think you could, you might consider a VFD if it could work better.

A VFD would work, but a VFD would cost about $175,000.


When the utility sees your step up transformer, they will absolutely tell you that the second transformer is a major problem with the full voltage starts.

Why do you say that the step up transformer is the problem? The problem seems to be that not enough power is coming in from the utility transformer. The step up transformer is boosting the voltage.

 

[davidbeach]

the utility transformer is operating at 90% of rated capacity

Then why do you keep implying that it is undersized? It isn't even getting close to 120%. Utilities size their transformers differently than customers. For a service transformer that is not undersized and you won't get anywhere with the utility trying to tell them that it is undersized. Your step-up transformer is the problem, this should have been a 2400V service. It isn't, but that isn't the utility's problem, it is your problem. You can fix your problem on your side of the meter, or you can pay the utility lots and lots of money to make your problem less of an issue, but you can't make it the utility's problem.

 

[rbulsara]

bimr:

I think you got the appropriate opinions here. You may not have read what you wanted to hear, but that is what it is.

For anything else hire another consulting engineers and take their advice.

 

[bimr]

rbulsara (Electrical)

I have already hired a 2nd engineer. The problem is not what the fellow is saying. The problem is that he does not respond in a timely manner. ie he does not return calls for 2 days. I left him a message yesterday morning and he has not called back. Good way to lose a client. So you are pinch hitting for someone that does not return calls.


davidbeach (Electrical)

Granted I am not an electrical engineer. Perhaps you are reading something into my posts. My sole purpose is attempting to understand the situation with the utility and who is responsible.

Should I go back and sue the electrical design engineer? What recourse do I have? Is he just going to say that it is the utility's problem?

Why does one bother to submit a load letter to the utility when they will just provide you with a standard commercial electrical setup. At some point, shouldn't the utility supply you with something that meets your needs? It is not like you are asking them to put it a major substation.

I have also seem similar problems on other projects where the utility does not furnish adequate equipment. Granted there may be design issues there as well. For instance, on one building the elevator motor caused the lights to flicker because the transformer was overloaded.

I don’t see why the utility should be unhappy with the step up transformer since in the end, it uses electricity and they sell it. I can see why the utility does not want the step down transformer since it has a power loss associated with it and it is on the utility’s’ side of the meter.