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?

 

[rbulsara]

bimr:

Am I pinch hitting for someone? I did not even know he existed. Why are you keeping him? Find another one!

 

[davidbeach]

I don't know what recourse you have. But you will find that nobody familiar with how utilities size service transformers will say that the utility transformer is undersized. If at full load of your system, not transient conditions associated with motor starting, your 480V bus voltage is above 456V the utility has met their service obligations. My guess is that you will find it much higher that 456V, and that you will find it closer to 500V than 480V when unloaded. Most utilities have rules about how large motors are to be started, your 450HP is a bit much to be dealing with for a near full voltage start.

The utility doesn't give a hoot about your step-up transformer, they're happy to sell you the transformer losses. Your motor on the other hand has a real problem with the step-up transformer. If your motor won't start below 80%, your step-up transformer may leave you close to 80% during a "full voltage" start, throw in the RVAT starter and you will be well below 80%, that's why your step-up transformer is a serious mistake. The utility's planning is probably for a motor that can start at 50% voltage, without the intervening step-up transformer, don't expect them to counteract bad design or unusual conditions on the customer side of the meter.

Probably your best option is to get a second, 2400V, service from the utility. You'll be happier, your motor will be happier. Probably can do that for well less than $175k.

The major point is that the utility service transformer is properly sized and the problem is elsewhere, even if that means the solution is more difficult.

 

[rbulsara]

Bimr:

Firstly, your aim should be to resolve the issue (it is not even established that it even exists), and not sue anyone.

If you are looknig for who to sue, consult a lawyer. You have come to a wrong forum!

 

[davidbeach]

Wasn't this already beaten to death in thread238-228405?

 

[rbulsara]

Aha!

May be doing the same thing (asking the same question to same people) again and again would yield a different result?

 

[racobb]

BOTTOM LINE...This is NOT a utility problem! Time to stop pointing in that direction. It has become yours and your problem is the result of a poor design! The utility appears to have provided the "industry standard" level of service. Abuse of that service by poor design does not constitute a utility issue.

Does the step up trans. remain energized during periods of inactivity with the pump? If so, get it turned off. If energized 24/7 your customer/or you are paying for the no load trans. losses for 8760 hours per year.

One other issue we have not discussed is the run time on the pump. You also have the trans. load losses when the pumps runs, and that in addititon can be significant if the pump has a high run time.

David is correct, you probably have 490 to 500 volts at your service.

Your comment about the step up transformer boosting voltage is correct, but it comes with a cost. You say that your service voltage drops to 440 on pump start. Probably so but it should still start if not for the additional voltage drop of 4 x %Z of the step up transformer.

Possible Solutions:

Just try setting the RVAT to a lower level and see if it works, forget the pump mfg. and do the test! The pump should start unloaded.

Get a price from the utility for a second service at 2400 volts and compare that to the VFD insatlled cost.

The site obviously needs the pump and it is time to get it working.....as if you have not been trying right!

Alan

 

[bimr]

davidbeach (Electrical) 26 Feb 09 20:16
Wasn't this already beaten to death in thread238-228405: RVAT starting question for large well motor?

Not really. Since then the system has been tested by an electrical testing firm. The system has also been in operation for several months.

Racob above has added some additional comments that were not part of the discussion then

 

[bimr]

racobb (Electrical)

You made many good comments.

“Does the step up trans. remain energized during periods of inactivity with the pump? If so, get it turned off. If energized 24/7 your customer/or you are paying for the no load trans. losses for 8760 hours per year.”

How do I calculate the cost penalty for this? Is this a significant cost?

“One other issue we have not discussed is the run time on the pump. You also have the trans. load losses when the pumps runs, and that in addition can be significant if the pump has a high run time.”

The pump typically runs about 8 hours per day.

“Just try setting the RVAT to a lower level and see if it works, forget the pump mfg. and do the test! The pump should start unloaded.”

The RVAT is more or less out of the equation because of the way that it is programmed. The well pump has been starting every day for 3 months without problem. 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.

The utility has also recently offered a larger transformer for $10,000 plus $100 per month.

“Your comment about the step up transformer boosting voltage is correct, but it comes with a cost. You say that your service voltage drops to 440 on pump start. Probably so but it should still start if not for the additional voltage drop of 4 x %Z of the step up transformer.”

Another good comment. I assume that this is really what the problem is. Is there a way to calculate the voltage drop through the step up transformer?

 

[dpc]

Yes, the % impedance stamped on the transformer nameplate is used to compute the voltage drop through the transformer.




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

 

[stevenal]

"The system has also been in operation for several months."

Work load a little light?

"I do have a motor with less than the usual starting current demand."

Just how is this a problem?

 

[bimr]

"I do have a motor with less than the usual starting current demand." What I meant is that the starting current for the motor is 4 times instead of the typical 6 times the operating current.

"Just how is this a problem?"

Last weekend the voltage coming in from the utility was 4% less and the well motor failed to start.

 

[stevenal]

And the failure mode?

 

[rbulsara]

bimr:
4% utility voltage will not fail a motor to start. Utility supplies are entitled for 10% voltage variations. What and how exactly it "failed"?

There may be some other issue. If motor draws less starting current means that the "load" does not require a lot of starting torque.

You are trying to solve a problem which is not there, as far as I can read from your postings.

 

[waross]

With two 500 KVA transformers in series with a 450 Hp pump, you almost have a system that inherently provides reduced current starting.
Have you considered an auto transformer to boost the input to the 480:2400V transformer when the incoming voltage is low? That may be your cheapest solution. You would want a closed transition or a very fast open transition to take the auto transformer out of service once the motor starts. A slow transition may result in mechanical damage to the pump or pump motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bimr]

"And the failure mode?"

Well Motor will not start. If the well motor does not start in 5-10 seconds after the well motor is energized, the motor protection ciruit operates and ends the well start

At that point, the plant operator turned the generator on to run the well motor.

 

[bimr]

rbulsara (Electrical)

“4% utility voltage will not fail a motor to start. Utility supplies are entitled for 10% voltage variations. What and how exactly it "failed"?”

I am telling you exactly what happened. Voltage normally has been 500V. For some unexplained reason, the utility voltage dropped to 480V which did not provide enough power to start the motor. I know the voltage reading is correct because there is a power monitor.

The utility then adjusted the transformer tap and that provided enough power to start the motor. The tap raised the voltage to 505V.

 

[racobb]

"Racobb above has added some additional comments that were not part of the discussion then."

Don't know that I have added much different than has already been said here, just beating the same old dead horse with a new hammer!


Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[rbulsara]

bimr:

As you write more it makes less and less sense. You could have adjusted the taps on your own transformer, IF that was the issue.

Can you elaborate what exactly happened, motor stalled? A fuse blew? Breaker opened? Breaker tripped before picking up speed?

 

[bimr]

Starting sequence – when well pump fails to start
1. Control system calls for well motor to start. Incoming utility voltage at 480V
2. RVAT energizes well motor at 80% voltage. Nameplate voltage of motor is 2300V. Voltage to motor unknown. Design calls for 2400V.
3. ~ 2 seconds later, the RVAT transitions on timer signal to from 80% voltage to line voltage.
4. Incoming utility voltage drops to 420V
5. ~ 5 to10 seconds later, the motor protection device shuts off the well motor start because the motor is not .,running.


Starting sequence – when well pump is able to start
1. Control system calls for well motor to start. Incoming utility voltage is at 500V
2. RVAT energizes well motor at 80% voltage. Nameplate voltage of motor is 2300V. Voltage to motor unknown. Design calls for 2400V.
3. ~ 2 seconds later, the RVAT transitions on timer signal from 80% voltage to line voltage.
4. Incoming utility voltage drops to 440V
5. ~ .5 seconds later, the motor operating at normal speed

 

[stevenal]

505 is above the ANSI range A utilization and service limits. If you keep it there, you may have problems with other equipment.
Sounds like the problem is with the protection that trips too quickly.

http://www.selinc.com/techpprs/6338_AdvancedThermalMotor_EL-MZ_20081006.pdf

http://www.selinc.com/techpprs/6266_FieldEvaluation_EL_20070521.pdf

http://www.selinc.com/generation.htm