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?

 

[stevenal]

I'm assuming "not running" means motor is still accelerating at less than normal speed, not that the motor remains locked.

 

[rbulsara]

You just need(ed) to adjust votlage and timing.

You could wait for say 4 sec before applying full voltage. The whole purpose of the reduced voltat start is to start the motor at reduced voltage and revert to full voltage when the motor has picked up 70 to 80 % of the rated speed.

Also as stevenal suggest, you adjusted the tap of the wrong tranformer and exposing other "480V" loads to overvoltage conditions. You need to adjust the taps on the your tranfomer only feeding the motor.

After all this efforts, how do you not know the rated voltage of the motor?? That would be the first thing needed to know. Adjust tap close to its rated voltage.

Using 80% tap on RVAT is waste of RVAT too.

 

[bimr]

The motor starts in 0.6 seconds because of the way the pump is constructed. The pump has a long shaft with a narrow rotating element. The well pump is either going to start or it will not. Lengthening the time for the motor to start will not make any difference.

The RVAT is typically required by the utility. This utility requires that the RVAT be set at 80%

If you have a typical motor with 6 times the starting current, the RVAT would make more of a difference.

In this case with this well pump, since the starting current to operating current ratio is only 4 times, the RVAT is probably not necessary..

Part of the problem has been the motor protection device. The device requires a narrow window for operating voltage in order to operate. Too high a voltage will not allow the motor to start as well. The device also takes experienced staff to reprogram.

The tap was adjusted on the step up transformer at one point. Unfortunately because it is medium voltage equipment, you have to hire qualified personnel to change the taps.

 

[davidbeach]

The device also takes experienced staff to reprogram.

because it is medium voltage equipment, you have to hire qualified personnel to change the taps

Both very good things. When you are playing around with equipment this expensive it is best to have people who know what they are doing.

 

[rbulsara]

bimr:

The motor starts in 0.6 seconds because of the way the pump is constructed. The pump has a long shaft with a narrow rotating element. The well pump is either going to start or it will not. Lengthening the time for the motor to start will not make any difference.

The RVAT is typically required by the utility. This utility requires that the RVAT be set at 80%

None of your statement makes any technical sense, in fact they are not even believable. I am now convinced that you are not interested in technical issue, you some how after someone for reason only known to you.

 

[waross]

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

 

[wiretwister]

Your problem could possibly be that the step up transformer is of a cheep construction and is probally "primary turn compensated".
this would cause the voltage to drop when a 150% load is applied.
This transformer 2400/480 Y:Y was manafactured for stepdown applications.
See if I am correct.

wiretwister

 

[bimr]

rbulsara (Electrical)

What part of this don't you understand?

"The motor starts in 0.6 seconds because of the way the pump is constructed. The pump has a long shaft with a narrow rotating element." (This information is taken directly from Byron Jackson's product literature - the pump manufacturer)

http://www.flowserve.com/vgnfiles/Files/Literature/ProductLiterature/Pumps/ps-50-3-e.pdf

If the pump had a large diameter rotating element, the pump would take longer to get to speed. Simple physics, that an experienced engineer would understand.

When the motor is energized, it is either going to start or not start. Byron Jackson advice is to not keep the motor energized longer than 10 seconds, if the motor has not rotated.

"The RVAT is typically required by the utility. This utility requires that the RVAT be set at 80% "(This 80% requirement is written into the utility customer service agreement by the utility).

 

[bimr]

wiretwister (Electrical)

One of the electrical engineers also suggested this. The step up transformer has a 5% impedance and he was concerned about the impedance. Square D, the manufacturer of the transformer said that this is a standard impedance value for this type of transformer.

The other electrical engineer suspected that the utility was using a low quality transformer.

 

[racobb]

#1)From one of my previous posts:
"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."

I was attempting to show you that voltage drop in the step up is also a huge problem....with Z=5% you have an additional 15 to 20 % voltage drop (3 to 4 times the %Z) on starting, just through the step up trans alone. It needs to go!

#2)"The RVAT is typically required by the utility. This utility requires that the RVAT be set at 80% (This 80% requirement is written into the utility customer service agreement by the utility)."

I think what you mean is that the RVAT may be set at a maximum of 80%....has nothing to do with settings less than 80%....utility is concerned about max, not min setting.

#3)"The other electrical engineer suspected that the utility was using a low quality transformer."

What a Crock! I suspect he does not know the difference!

We can beat this up forever, but until you lower the tap on the RVAT and see if it will work, or until you replace some equipment, you will always be operating at the edge of the motor starting envelope and will continue to have problems.

If no one at the facility is willing to review these posts and make a decision, then they need to hire a consultant, pay him 10 to 20 grand, so he can tell them the same thing!

Alan

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

 

[dpc]

Too much conflicting advice in this thread....

Attached is power flow solution showing a 2300 V 450 hp motor starting across the line with two 500 kVA, 5 % transformers in series.

I assumed 6x Full load current at 0.2 pf at locked rotor.

Voltage at the motor terminals at locked rotor is only 63%, ignoring voltage drop in the cable the motor. A reduced-voltage starter is not a solution and would be part of the problem. The utility can make all the rules on reduced voltage starting that they want, but they are still subject to the laws of physics like the rest of the universe.

Voltage drop on the utility side of the service transformer is negligible - admittedly with a lot of assumptions, since there is no data.

If the motor can accelerate the pump with 40% of normal motor starting torque, then adjust the motor protection accordingly and let it start. The fact that a relay tripped does not necessarily mean the motor cannot start safely. If not, something will need to change. Before you start spending money on new transformers, etc, you should get a EE to do an evaluation of the impact of the changes and probably a transient motor starting analysis.

In my opinion, based on the limited information provided, the problems are on the customer's side of the service.

Good luck.

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

 

[edison123]

May be, you can think of having a run-of-the-mill 480 V VFD on the primary of the 2300 V trafo ? That shouldn't cost much.