back feed xformer possible? 2

[maxPE]

As an ME Im a little out of my field but I have a 3000 foot overhead run to make from where Edison wants to put the meter in (4 wire/277/480) to my well. I was thinking of using a std residential type xformer to step up the 277/480 to 4800 or another voltage for the run and then step it down at the terminus for the well pump (240vac, 30 hp). Is this a 'bad' idea? Reversing the xformer feed direction incures losses or danger or? Does anyone have a better idea or is this reasonable. I was hoping to use surplus or used xformers. BTY, this is on my own farm. Thanks!

Max Hovaten P.E.

 

[busbar]

thread238-44184

 

[digitrex]

Hello MaxPE,

I am not residing in US but hope I can give some opinion.

Your 4800/480V transformer's primary/secondary windings should be connected in Delta/Star configuration. I don't see any problem in reversing the power flow. However, if you operate a 4800V circuit (the overhead line in your farm) without any neutral grounding, you will have (very) high risk of the following problems:

a. overvoltage on your 4800V line, when there is an earth fault in 480V system or somewhere else in public grid. This overvoltage will damage your 4800V line insulation or supports.

b. No suitable protection for the 4800V line against earth fault(eg trees touching the lines). A tree can grow and touch the 4800V line with little leakage current to earth and then you or someone in the farm will fill electric shock when touching the tree.

I am sure the cost of installing a 4800V line is more expensive than the 480v line.

Why not accept the expected losses through a long 480V lines and just connect the power through a 480V line(probably oversize it a bit) into your farm?
I also suggest that your motor(35kW) in the farm to be fitted with soft starter or other means to reduce the effect of voltage drop during starting.

 

[jbartos]

Suggestion: 480V is appropriate voltage to power 30hp motor over the 3000ft distance. 5kV level is used for motors ~250HP up. 480V line should be noticeably less expensive in comparison to step-up and step-down 5kV level transmission line.

 

[alehman]

I assume this is a 3-phase motor? If your service is 480V, you'll need a transformer for your 240V motor.

30HP at 480V 3-phase would be 40 amps. To keep voltage drop <3% (NEC recommendation) at 480V, 3-phase, you would need to use 250kCMIL conductor. If you want to allow say 5% voltage drop you could use 3/0 Cu or 4/0 Al.

At 4160V, you would have 4.62 amps. #6 Cu would be fine for that.

I'm not sure how much a soft starter would help.

 

[jbartos]

Comment: 5kV level transmission, overhead or underground, poses increased insulation and safety concerns.

 

[maxPE]

Thanks all for your helpful comments. My contractor has a good supply of used 4800V primary transformers. Additonally the Edison planner suggested that the disconnect box would be very expensive if I did high voltage metering. The cheap xformers imply doing the 3000ft run at 4800 but the high cost of a disconnect implies going with 480V transmission and this implies I need a 480 to 240 3 phase transformer bank. These dont sound too common to me. If I go with 480V transmission the cost of cable will be high...hmmmmm. No easy answer especially since both the Edison man and my contractor havent given me real cost data!

 

[jbartos]

Suggestion: Visit

http://www.thomasregister.com

and type Cables: Electric under Product or Service, which will return 420 companies to approach to for an affordable underground low voltage power cable price

 

[jghrist]

Why can't the metering be done at 480V before the step-up?

How much would Edison charge you for a separate 240V 3Ø service at the well? Standard voltage is probably 120/208V; the motor manufacturer might need to be consulted to see if 208V is acceptable.

 

[busbar]

Some considerations—
Motor terminal voltage during starting and running
Short-circuit and ground fault protection of transformer sets
Disconnecting means for transformers
Line clearances, spacing, grounding, hardware
NEC or NESC, or both?
Local/state ordinances for line construction/operation
Inspection/permitting for nonutility poleline

 

[busbar]

..and MV surge protection

"Weeding out" method for used gear—particularly oil-insulated transfomers

 

[GerH]

I am not familiar with US standard voltages. I assume that 480V is line to line. Now if you step up to 480 Line to Neutral you can generally use the same rating switcgear and insulation as most equipment is rated at least 600V Line to Neutral.

This would reduce the current to about 60% and still retain Low Voltage rating. In Europe the equivalent is a 660V Line to line. You might be able to change the winding connections of a 2nd hand traffo to create the 480V L-N voltage.

Reduced voltage ( soft starter ) is not used to help the motor start but to avoid disturbance to other consumers. I don't see any problem with starting as the cable will act as a reduced voltage starter so limiting the voltage dip experienced by other consumers.

What you need to consider is the effect of sustained running with reduced voltage e.g. 15% drop due to losses along the line. This could cause the motor to burn out on overheat. An option is to step up the voltage at the motor end by say 15% to 20%.