am I designing a "soft line"

[maxPE]

This forum has been very helpful. For reference, Im planning on stepping up 480 3 ph to 4800 for 4500 ft and then back down to 240 for submersible 35 hp pumps. Additionally I want to pull house power at the terminus. Someone mentioned TRANSIENTS.
This got me worried that the house will be drawing 1 kw or less when the pumps kick in. If I remember my physics, theres a L*di/dt that affects the conductance and my gut says that a transformer L might be pretty big. I was speculating that it may be preferable to pull the house power off the 4800 volt line with a house dedicated second xformer rather than have the house come off the pump xformer bank.

Thanks,
Max


you may contact me at my domain email.
max@engsrv.com

 

[jbartos]

Please, what is the question? If the pump starts, the house tap may experience voltage dips or sags, depending on the transmision line voltage drop caused by the motor inrush currents.

 

[busbar]

1. “…for submersible 35 hp pumps” explains 240 instead of 480V motor load.

2. With rating depending on grounding for the 4800V circuit, it is routine for many to install MOV surge protection at each transfomer set.

3. The “house power” add-on part somewhat depends on voltage dip at motor starting. The added ‘third node’ complicates things a little.

 

[maxPE]

Re jbartos,

The question is, assuming that the house is drawing 1 kw through the step up transformer (480 to 4800) and then I start the well motor (100 amp@ 240) the initial in rush current may be as high as 300 amps or more. Will the 'step up' transformer be able to respond immediately or will the output voltage (4800) droop significantly?

max


you may contact me at my domain email.
max@engsrv.com

 

[dspDad]

Both the step up and the step down transformers have to be rated for the peak loads. The drop in the 4800 v line is probably not going to be significant. It would be less expensive to put a 1 kW battery backup at the house to maintain line voltage during the transient, or to ignore the momentary dimming of lights.

Your voltage drop can be calculated easily from the resistance of the entire system. Another issue that might be more important is to make sure the transformers don't saturate momentarily, as you will probably blow a circuit breaker and won't be able to start the pump at all.

My experience is that 20 volts of drop for half a second is not very objectionable if the only problem lights dimming and microwave ovens changing pitch. 40 volts begins to feel like an impending blackout.

If the transformers are adequate, there won't be any 'delay'

DspDad

 

[alehman]

Looking at order-of-magnitude approximations: As an example consider a bank of (3) 15kVA transformers with typical impedance of 3%. That would be about 15 ohms at 4800V. You will have three transformer banks in series (utility service, your step up and your step down). Using numbers from the previous thread, the line might be about 1.4 ohms.

To calculate this accurately you would need to run out the numbers using complex impedances and correct load power factor.

 

[jbartos]

Comment on maxPE (Nuclear) Apr 22, 2004 marked ///\\Re jbartos,
The question is, assuming that the house is drawing 1 kw through the step up transformer (480 to 4800) and then I start the well motor (100 amp@ 240) the initial in rush current may be as high as 300 amps or more. Will the 'step up' transformer be able to respond immediately or will the output voltage (4800) droop significantly?
///The weakness is in the voltage drop size, not the in the response speed. The response speed is given by the motor acceleration time and associated length of the inrush current. This current may cause light dimming, and motors stall. It would be good to simulate your project first by a suitable software.\\\

 

[maxPE]

reply to jbartos...

Do you have an idea or a "suitable' software? Possibly I could get a "tria;" version.



you may contact me at my domain email.
max@engsrv.com

 

[dspDad]

Max,
Your line will see almost no inductive element due to the transformers. There may be some inductive kick from the pump when you energize and de-energize it, but this will be reflected dumped back into the mains a mile away.

Transients (other than the momentary voltage drop during startup) will be fairly small. The inductive kick will be fairly small. Unless you bury the lines underground, you need to protect from lightning strikes, and building codes and/or a sense of self-preservation dictate getting grounding done right.

I assume you're getting commercial power transformers.
Two 25kW units at each end would probably work for a single 35 hp pump plus any miscellaneous.
If you have multiple pumps totaling 35 hp, (since you did use it plural), then stage the turn-on and off by a few seconds delay and you could cut the size of the transformers about in half.

Also, 240 is not a standard 3 phase voltage, and is fairly low for a 35 hp application. If you don't already own the parts, maybe you should consider a 480 volt installation, and get a step-down transformer that provides both 480 and 110/220.

Thanks,
DspDad