back feed xformer possible?
back feed xformer possible?
(OP)
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.
Max Hovaten P.E.






RE: back feed xformer possible?
Thread238-44184
RE: back feed xformer possible?
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.
RE: back feed xformer possible?
RE: back feed xformer possible?
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.
RE: back feed xformer possible?
RE: back feed xformer possible?
RE: back feed xformer possible?
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
RE: back feed xformer possible?
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.
RE: back feed xformer possible?
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
RE: back feed xformer possible?
..and MV surge protection
"Weeding out" method for used gear—particularly oil-insulated transfomers
RE: back feed xformer possible?
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%.