Voltage Drop Solution - Intentional VD over 5%

[Mindrunnder88]

Have a curious challenge with an idea for an interesting solution...

Have a site that is supported by a primary power source at the site (480Y/277V 3PH, 4w). We are intending to install another supply from a secondary power source that is about 4000' away. The typical load on the secondary will be well under 100A. However, certain instances (maybe for an hour or two a week) will result in this load jumping to a max of 200A. The cost of the secondary is our primary enemy, namely due to VD. Going to 600V using xfrms on each end, at 3% we would be looking at 3 sets of 500 for the run (over $350K for just the wire here).

Here's the idea that needs vetting. As the supported loads are primarily motors (and other inductive loads), we size to 7-10% VD (to stay within the voltage range of the motors), which would allow us to use one set of 500 instead of three. The motors will draw more current, but with the oversized wire, we won't even come close to ampacity limitations. The increased current will cause more VD, but it tapers off pretty quick. So all seems to check out.

Obviously, medium voltage would seem like a smart option for this situation, but there are specific reasons we are staying away from that.

Anybody see any issues with this (other than making sure we are seeing voltage at the equipment within range)?

 

[charlie b]

It won't be an NEC issue. The 5% VD "limit" is only a "suggestion." Is the site built already (i.e., the secondary motors in place)? If not, do you have the ability to work with motor manufacturers to buy motors with a greater tolerance for low voltage? You can also look into placing a buck-boost transformer at the load end.

 

[Mindrunnder88]

It won't be an NEC issue. The 5% VD "limit" is only a "suggestion." Is the site built already (i.e., the secondary motors in place)? If not, do you have the ability to work with motor manufacturers to buy motors with a greater tolerance for low voltage? You can also look into placing a buck-boost transformer at the load end.

Good insights! Finding buck-boost designed to a 10% Voltage difference seems to be the issue... as most seem to be 5%. I suppose this would still get us back to the 5% level though. Any suggestions on a MFR that makes an xfrm that would boost 10%?

 

[petersonra]

Good insights! Finding buck-boost designed to a 10% Voltage difference seems to be the issue... as most seem to be 5%. I suppose this would still get us back to the 5% level though. Any suggestions on a MFR that makes an xfrm that would boost 10%?

i would just call acme and ask their tech support people.

I have had really good luck with them pulling something out of a hat.

 

[drcampbell]

If you've done the math for the maximum loading/maximum voltage drop case and verified that everything will be within limits, there shouldn't be a problem.

Have you considered what will happen when motors turn on and the voltage drops from, say, 470 to 440 volts, then comes back up? Will this burble be acceptable? If not, boost transformers are not the solution; they'll raise the voltages without affecting the burble.

Boost transformers may increase the voltage to unacceptable levels under a low-load condition. Be sure to calculate both extremes.

Have you considered installing power-factor-correction capacitors to minimize current and voltage drop?

Have you considered changing taps at your primary power source so that it starts out 5% high? Making room for more voltage drop might enable you to reduce the wire size.

 

[oldsparky52]

I too would look at using the taps on the transformers to help the voltage drop instead of a buck-boost.

Either way, the concern about too high a voltage when there is low loading and the POCO voltage is at the high end of it's range is a factor no matter how you attempt to raise the voltage.

 

[Mindrunnder88]

If you've done the math for the maximum loading/maximum voltage drop case and verified that everything will be within limits, there shouldn't be a problem.

Have you considered what will happen when motors turn on and the voltage drops from, say, 470 to 440 volts, then comes back up? Will this burble be acceptable? If not, boost transformers are not the solution; they'll raise the voltages without affecting the burble.

Boost transformers may increase the voltage to unacceptable levels under a low-load condition. Be sure to calculate both extremes.

Have you considered installing power-factor-correction capacitors to minimize current and voltage drop?

Have you considered changing taps at your primary power source so that it starts out 5% high? Making room for more voltage drop might enable you to reduce the wire size.

Great insights, thank you (DrSparky as well)!

The challenge we have is that the power is bi-directional, as the remote site will have solar and will feed excess generation back to the primary site when load at the remote site is less than generation. As such, anything we do at the XFRM's going one way is reversed the other direction... Failed to mention that in my original post :ashamed1: (apologies). The burble would seem to be a problem. We will have energy storage at both sites, but was not intending to design for voltage regulation.

It would seem power factor correction capacitors and/or start capacitors would help?

Regarding POCO voltage, the remote site has no utility feed. The long run from the primary site is intended to provide secondary power from an (E) POCO service at the primary site.