[RE-wrenches] utility line voltage issues

Joel Davidson joel.davidson at sbcglobal.net
Sun Aug 16 12:08:36 PDT 2009


Hello William,

SCE has some 4kVA long, skinny feeders that get voltage sag during high 
usage periods (August afternoons air conditioning and December nights xmas 
lights and heating). High grid voltage can occur in some areas when SCE 
increases the voltage to compensate for voltage sag. High grid voltage can 
also occur when utility and/or customer wires are undersized.

The CPUC, not lawyers, tell electric utilities to fix grid voltage problems. 
You need to file a complaint with specific information to the CPUC. With no 
PV system on and using an rms meter, measure the voltages. Record the times 
and in what neighborhoods are you measure high or low voltage. Then file a 
written complaint to the CPUC (contact info on the back of an electric 
bill). The CPUC accept the complaint, investigate, and tell SCE to correct 
the problem(s).

If the grid voltage is within acceptable range and the grid-tie inverter is 
still shutting down, then leave the inverter off and measure grid voltage at 
the inverter AC in. If voltage is high, pull the grid-tie breaker and 
measure grid voltage. It should be within range. If not, then the wiring 
from the grid-tie breaker may be undersized or the inverter input voltage 
setting may be out of range.

Joel Davidson

----- Original Message ----- 
From: "William Korthof" <wkorthof at eesolar.com>
To: <re-wrenches at lists.re-wrenches.org>
Sent: Sunday, August 16, 2009 10:06 AM
Subject: [RE-wrenches] utility line voltage issues


> I'm beginning to wonder if the allowed voltage range for grid-tie 
> inverters (+/-10%) is too
> sensitive in some networks and contributes more harm than benefit.  This 
> is close to home.
>
> We actually have a significant number of customers who've had trouble 
> with grid voltage
> causing their systems to go offline at various times. I think most or  all 
> are SCE customers.
>
> So I've been having problems with the inverter at my own house going 
> offline due to high
> utility line voltage. We actually have 4 other systems in the 
> neighborhood, with two more
> going in in the next couple months. All of them are about equally 
> affected, even though
> each house is feed by a different transformer. I did a bit of research 
> and found that some
> of the grid hardware in the neighborhood is very old---some of the  oldest 
> in existence---
> dating back as far as 1892. You can read about the San Antonio Power 
> Plant (hydro)
> and the Pomona substation online.
>
> Going back to the 1950's, most of the local neighborhood sub- transmission 
> in the US is
> at a voltage around 12 kV or more, with transformers for the  120/208/240 
> or 277/480.
> But some older neighborhoods that haven't been upgraded (and some 
> campuses) use
> an intermediate system, usually 2400/4160 volts. My neighborhood is  still 
> mostly fed by
> the old 2400/4160v network.
>
> Generally, the utility voltage at my house is in the mid 120's---  around 
> 125 vac per phase.
> But at times, the voltage goes up higher---two weeks ago I saw 129 to 
> 130V per phase.
> That voltage was high enough to put all of the inverters that I  checked 
> offline for much
> of the day.
>
> So I've had to call in "voltage trouble" complaints to the utility at 
> least a dozen times
> over the past 5 years in response to seeing inverters offline and line 
> voltage about
> 8% above nominal.
>
> This periodic voltage problem has been going on for years, typically 
> worst in summer.
> I upgraded with new service panels and heavier feeders at each of the 
> houses to do
> my part to help. The circuits are now sized so that voltage drop in  every 
> case from the
> inverter terminals to the utility entrances are all under 1% at full 
> solar output.
> Six months ago, utility crews replaced some transformers around the 
> neighborhood
> (some appeared to be original), yet they didn't move any customers  from 
> the old 4kV
> supply to the newer 12 kV supply.
>
> I've read that the utility's operating guidelines call for keeping the 
> supply voltage
> within +/- 5% of nominal voltage at the customer's service terminals.
>
> If the utility actually stays within 5%, and if solar inverters allow  for 
> voltages to vary
> up to +/- 10% from nominal, then things _should_ be okay. However, in 
> practice, I see
> the inverters going offline right at 129 to 130V per phase, not 132V 
> (+10%).
>
> In response to the last trouble call, I talked with the crew that came 
> out. I think their
> response helps explain the problem... my issue seems to go back to the 
> substation.
> Apparently other customers on another feeder from the same substation 
> complain
> about low voltage---around 108-110V per phase. So they raise the voltage
> regulator at the substation and everyone goes up 5%. It's easy to see  how 
> the drop
> on a 4kV feeder could be 1% on one feeder and 10% on another----  example:
> a load of 865 kW (~400 homes with A/C) or 120 amps at 4kV 3 ph, in 2 
> miles of #2
> copper is a voltage drop of 10%... yet 300 kW going 3000 ft gives a 
> voltage drop
> under 1%. If voltage drop is 10% on one feeder, but only 1% on a 
> different feeder...
> it's tricky for the operator to keep everyone reliably within 5% of 
> nominal.
> In my neighborhood, SCE technicians apparently view it as normal 
> operating
> procedure to supply 126 or 129 volts per phase.
>
> So it seems there are several possible remedies:
> 1) ignore the problem and hope I don't lose too many kWh's due to grid 
> overvoltage
>
> 2) I could install transformers to lower the voltage to the inverters  by 
> ~5%----so far,
> grid voltage in my neighborhood has *always* measured at or above 
> nominal.
>
> 3) if it weren't for the UL and IEEE standards, solar inverters could  be 
> set with values
> that made them compatible with prevailing utility operating  conditions, 
> perhaps
> +/- 15% of nominal voltage.
>
> 4) at a cost of many millions of $$$, SCE could completely replace the 
> local
> distribution system with new 12kV facilities, even though annual load 
> growth
> on the old wires is trivial or possibly negative...
>
> 5) at a more modest cost, SCE could make strategic upgrades to lower  the 
> load
> and voltage drop on the most stressed parts of the old network
>
>
> How have other installers dealt with utility voltage problems?
>
> /wk
>
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