[RE-wrenches] Buck-Boost transformer to mitigate high voltage drop?

Will White will at solarenergy.org
Thu Aug 18 08:49:58 PDT 2016


I've had the exact same problem except we also had high voltage at the
transformer due to a utility substation being rebuilt.  We had to open the
voltage window of the inverter and that solved the issues.



On Wed, Aug 17, 2016 at 2:12 AM, <don at energysolarnow.com> wrote:

> Howard-
> Any voltage drop at the house will be a function of current drawn by the
> house load. Shut off most loads and there will be minimal voltage drop.  If
> there was a low voltage issue now before solar is installed, there would be
> symptoms like florescent lights not working well. That's why they flicker
> when a big motor kicks on nearby.
> A transformer could be used to boost the voltage to the house, but that
> assumes more or less constant load. When the house load is low there is
> minimal voltage drop and the boosted voltage may become too high. The
> utility service transformer may have different wire taps to adjust its
> output voltage as well. But again, this assumes more or less constant load.
> When there are long wire runs it is more common to use a pair of
> transformers, one at the utility drop and one at the house, to boost (or
> step up) the voltage at the service drop end to, say, 480 with a 2:1
> transformer. That is, the ratio of number of primary to secondary windings
> is 2:1. Then at the house another matching transformer bucks (steps down)
> the voltage back to 240.
> The current in the long wire run is cut in half, and so is voltage drop.
> This boosted voltage can be higher-- thousands of volts are not uncommon
> on ranch properties with miles-long wire runs.
>
> A solar inverter senses the voltage at it's connection point, and raises
> it's voltage a couple volts higher to push solar current into the wire. If
> there is voltage drop in the wire, the solar inverter must raise it's
> voltage even higher to overcome voltage drop and still be a couple volts
> above the grid. If voltage drop is severe, the inverter will sense that
> it's voltage is getting too high out of range and fault with a grid voltage
> error. Grid voltage is not changing, inverter voltage is.
> For example, we had an inverter sited at a 10 KW array connected to a
> garage with 200' of existing 6 AWG AL buried wire. The attendant voltage
> drop from nearly 40 amps solar current caused the inverter to raise it's
> own voltage so much to overcome voltage drop that it faulted. It was
> necessary to tweak the software settings to allow the voltage range to be
> extended.
>
> When your client's 20 KW solar inverter is sited at the house, any solar
> current will first be consumed by the house loads. This reduces current
> through the long wire run and so reduces voltage drop. When the solar
> current matches the house loads, no current flows through the long wire run
> and there is no voltage drop. Everyone is happy, except the utility company
> who now doesn't get to sell energy.
> Excess solar current not consumed by house loads will run back through the
> long wire to the utility grid, causing some voltage drop. This must be
> overcome by the inverter raising it's voltage to still be a couple volts
> over grid-voltage-plus-voltage-drop.
> Without running the numbers I suspect that the less-than-40 solar amps you
> may run into 4/0 AL wire will not be a problem.
>
> You will want to measure the voltage at the house under low and maximum
> load conditions.
> If the load is at max when the sun shines, a solar inverter will tend to
> help alleviate any low voltage issue at the house, as Ray W. wrote.
>
> Don Barch
> Energy Solar
>
>
> Date: Tue, 16 Aug 2016 14:23:27 -0600
> From: Ray Walters <ray at solarray.com>
> To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
> Subject: Re: [RE-wrenches] Buck-Boost transformer to mitigate high
> voltage drop?
> Message-ID: <c0339c2a-4d02-f64c-2512-fb1a7bbc0c8a at solarray.com>
> Content-Type: text/plain; charset="windows-1252"; Format="flowed"
>
> My guess is that in your case the GTPV will just help alleviate the volt
> drop situation. I wouldn't start adding transformers, etc until you are
> having a problem. Put in the contract that this extra cost may be
> necessary for proper function, but I would keep it as a backup. Based
> on the numbers you just mentioned, you would only be selling a surplus
> of 30 amps, and the existing wire should be adequate for that.
>
> R.Ray Walters
> CTO, Solarray, Inc
> Nabcep Certified PV Installer,
> Licensed Master Electrician
> Solar Design Engineer
> 303 505-8760
>
> On 8/16/2016 1:16 PM, Howard Arey wrote:
> >
> > Answers to all:
> >
> > No ? I did not measure voltage at the house. Right now, only an
> > assumption of voltage drop based on my perception of what ?typical?
> > loads might be on this 150-amp service. I wasn?t expecting to see such
> > small wires for such a long run from transformer to house when I
> > showed up there without a voltmeter. He is a heavy user, using 6,000
> > kwh in some months
> >
> > We?re looking at a 20KW system (he has some other expansion
> > requirements, he has the roof space, and the need.) I roughly estimate
> > that he?ll use onsite 40-50 amps of the 82 amps at peak inverter power
> > output.
> >
> > Interesting to hear of one transformer response and hope to learn more
> > on that, too.
> >
> > *Howard ?Scot? Arey*
> >
> > Owner, Solar CenTex
> >
> > 254-300-1228
> >
> > scot.arey at solarcentex.com
> >
> > www.solarcentex.com <http://www.solarcentex.com/>
> >
> > https://www.facebook.com/SolarCentex
> >
> > Rated #1 Solar Installer in Texas by Solar Reviews
> > <http://www.solarreviews.com/solar-companies/top-100-
> residential-solar-power-contractors-2016/texas/>__
> >
> > Solar Centex Logo - No Back
> >
>
>
> On Tue, Aug 16, 2016 at 12:16 PM, Howard Arey <scot.arey at solarcentex.com>
> wrote:
>
> Good day,
>
>
>
> I have a potential client that has an existing 1,000 foot run from the
> meter and transformer at the street to the home. This is 4/0 Al and quick
> calcs show the voltage drop at over 12%. He acknowledges that the wire is
> undersized to keep voltage at/about 240 volts (I have not measured directly
> at the street / transformer yet).
>
>
>
> I know the wire has the necessary ampacity to carry his current service
> needs and to carry amps from the to-be-suggested solar system.
>
>
>
> My concern is that the voltage drop will change the 240 volts at the
> street/transformer/meter to 211ish (or will I see a higher 268?sh voltage
> at the far house end?) at the house. The inverter will try to synch to this
> voltage but of course this is outside of typical inverter parameters.
>
>
>
> So, instead of re-running much large wire, does anybody have experience and
> comments about potentially using a buck-boots transformer at the home to
> get us back to 240 volts? This would need to be service entrance sized.
>
>
>
> *Howard ?Scot? Arey*
>
> Owner, Solar CenTex
>
> 254-300-1228
>
> scot.arey at solarcentex.com
>
>
>
>
>
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-- 
*Will White*
Curriculum Developer

e: will at solarenergy.org
w: www.solarenergy.org
p: 802-272-3092

PV Installation Professional
# 093006-34
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