[RE-wrenches] utility line voltage issues

[Bill Brooks]

William and others,

The standard requirement in IEEE929 and IEEE1547 that is tested in UL1741 is
+10%/-12%. Admittedly, some utilities, particularly in more rural areas, may
get close to these limits at times. Just for a little history lesson that
very few people know about, this happens to be a very hard fought and won
battle. Initially when IEEE929 (the original PV interconnection document)
was being written, utilities wanted inverters to trip at +/- 5%. Realizing
that this would be a death blow to the PV industry, several of us worked on
the justification for why that did not make sense. We finally won when a key
utility engineer got on our side and the proper limits went into place. The
standard also recognizes that special locations, like island grids (Hawaii)
and other remote grid areas may need even wider ranges to handle typical
fluctuations. Widening voltage windows of the inverter requires utility
permission. 

The voltage limits we have now are good, but most people don't understand
the rest of the story. As was mentioned in the post, most PV inverters do
not trip at their actually limits, they typically trip a few percent inside
those limits (+8%/-10%). This is due to the difficulty of measuring ac
voltage accurately, and the penalty within UL1741, the test standard, if you
fail one of these limit tests. Since there is no penalty in the standard for
tripping early, but a large penalty for tripping late, inverter
manufacturers constrain their limits so that they trip within the limits
every time, even if their transducers are at the maximum offset allowed.

That is why it is so important for installers to try to stay within 1%
voltage drop on the ac side. Since the utilities are allowed to go to the
limits of +/-5% routinely (ANSI Range B), and outside that range for "short"
periods of time (a few hours is short compared to 8760 hours), a 3% voltage
drop will cause inverter tripping that is not the utility's fault--it is the
installers fault.

I'm not defending the utilities, but having responded to dozens of utility
voltage complaints, I can say that 80-90% of the complaints were the
installer's fault in having too high a voltage drop in their inverter output
circuits. The legitimate issue is that PV inverters are one of the only
things on the residential utility system that reacts to bad voltage. We are
now placing tens of thousands of voltage sensors on the utility grid, and
they will find bad places. The key is to be part of the solution, not part
of the problem--keep your ac voltage drop low. For every voltage drop dollar
you spend on the dc side of your design, you should put $2 toward the ac
side voltage drop--it is at least twice as important because it may
determine whether or not the system runs.

Bill.

Bill Brooks, PE
Brooks Engineering
873 Kells Circle
Vacaville, CA 95688
Office and Mobile: 707-332-0761
Office Fax: 707-451-7739
email: Bill at BrookSolar.com
www.BrookSolar.com




-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Joel
Davidson
Sent: Sunday, August 16, 2009 1:48 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] utility line voltage issues

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.
>>
>> 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.