One thing I have read during this thread, is the voltage rise at the
inverter output required to "sell to the grid". I have heard all sorts of
numbers from "less than a volt RMS" to "2-3 volts". As an engineer, I would
expect the number to depend on the AC current being exported and the
impedance of the grid seen from the distribution panel.
Let's say we want to export 2,400 W-ac at 240 V-ac, or 10 A-ac, and the
dynamic impedance of the grid is 0.1 ohm. The voltage rise needed to push
that 10 A onto the grid is 1 V-ac (i.e. 241 V-ac).
I am going to do a test on our system today, monitoring the L1-L2 voltage at
the inverter breaker and cycle the PV system on and off. Should be
interesting!
One other point: since IEEE929 and IEEE1547 require the inverter to shut
down within 16 ms (one full period of 60 Hz) -- correct? -- what is the
typical averaging time (Tavg) for a typical RMS voltmeter (the "M=mean" in
RMS)? If the voltage excursion occurs between 0.16mS and a fraction of Tavg,
then the RMS will miss the event. Seems to me, we have to have a peak
reading voltmeter or something equivalent.
- Peter
Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parr...@calsolareng.com
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
-----Original Message-----
From: re-wrenches-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Bill Brooks
Sent: Monday, August 17, 2009 9:05 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] utility line voltage issues
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: b...@brooksolar.com
www.BrookSolar.com
-----Original Message-----
From: re-wrenches-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@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.david...@sbcglobal.net>
To: "RE-wrenches" <re-wrenches@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" <wkort...@eesolar.com>
To: <re-wrenches@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.
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