[RE-wrenches] utility line voltage issues

Sun Aug 16 13:47:50 PDT 2009

Correction: 4 KV

----- Original Message ----- 
From: "Joel Davidson" <joel.davidson at sbcglobal.net>
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
Sent: Sunday, August 16, 2009 12:08 PM
Subject: Re: [RE-wrenches] utility line voltage issues


> 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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