autotransformer

[tulum]

We have a situation where we have a long run of cable feeding some equipment.... the cable drop has reduced the voltage to approximately 510V 3p from 600V (fed from 13800VD - 600Y/347V 1000kVA dry type transformer with a 15A NGR). The equipment is having trouble 250HP)due to the drop and we would like to place a autotransformer at the end of the line so we could raise the voltage to the load. (1) What kVA autotransformer would be required?, and (2) would it affect the grounding system? (3) Besides replacing the cable is there a better solution?

 

[waross]

The problem of using auto-transformers to mitigate voltage drop is high load voltage under light load conditions.
Usually larger or parallel cables are the solution to voltage drop issues.
With very long runs, It may be economical to use transformers to raise the voltage by a factor of 5 or more and then drop it down with a second transformer at the receiving end. There may be regulatory issues with introducing a higher voltage class into some plants.
You can probably use an auto-transformer to boost the voltage up to 4160 Volts, but some codes prohibit using an auto transformer for step-down service under some conditions.
If the low voltage winding or low voltage connection becomes open circuited the high voltage winding will be in series with the load and unacceptably high voltages will be seen at the load under light load or open circuit conditions.

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

 

[MatthewDB]

Is the load strictly a 3 wire load or is there also neutral connected loads? I would presume no, because in North America neutral loads are forbidden with impedance grounded systems.

You have to make sure the auto-transformer has a zero sequence path to properly operate the ground fault detection. An open delta auto-transformer made with two single phase units will not interrupt zero sequence and hence not bother the ground fault detection. If you do have neutral loads, three single phase transformers connected in Y have to have the neutrals connected to the system neutral or they will block ground fault currents and prevent the fault detection from working.

As for sizing: Take the motor service factor current and multiply by 1.25. This will be the current in the series winding. I would suggest you use 600V:120V transformers. At 510V in they will give you 612V out. If that's what you choose, use the service factor times 1.25 times another .12 to get the kVA rating needed.

You will have to address warros's concern about open circuit voltage unloaded. Is the motor controller at the load or at the feed end? If the controller is at the feed end, you don't have too much of a concern unless the motor can run with zero load. If the controller is at the motor end, you will have concerns about the open circuit voltage burning out the controls. You'll have 720V when the motor is off. I would start with putting the transformer after the contactor and seeing if the controls have enough voltage to operate.