Grid Overvoltage on Sunny Boys [RE-wrenches]
Bill Brooks
billb at endecon.com
Fri Sep 20 08:42:00 PDT 2002
Jeff,
The utility is supposed to stay within 5% of nominal voltage at all times.
This allows for slight variations outside that window
It is the responsibility of the installer to measure the site voltage and to
notify the utility, with the help of the customer, that the site voltage is
too high. We are typically the only ones that actually notice high voltage.
Occasionally, a customer might suspect it because they are blowing
incandescent bulbs, but other than that, high voltage is hard to detect
without a meter.
If the utility voltage is routinely above 252 V, the customer has a reason
for complaint and the utility is obligated to deal with the situation. It is
often a hastle to get the utility to move on the issue, so it is usually the
installer's responsibility to work the issue. They will usually bitch and
moan and say that if they lower the voltage the sky will fall, but just
stick to your guns and make them do it. Have fun.
Bill.
-----Original Message-----
From: Jeff Clearwater, Ecovillage Design [mailto:clrwater at earthlink.net]
Sent: Wednesday, September 18, 2002 4:46 PM
To: RE-wrenches at topica.com
Subject: SPAM: RE: Grid Overvoltage on Sunny Boys [RE-wrenches]
John and all,
Thanks so to John B. for his usual thorough and quick responsiveness on
this.
I'm glad SMA is taken this seriously and pursuing some fixes or at
least negotiations with UL and the Utilities. In the customer's
eyes, we've sold them, for almost a year now, on the superiority of
SBs only to turn on the system and have it down 1/5 of the time on
hot summer days! Though it is no real fault of SMA, it doesn't bode
well for estimated KWhrs/mo or payback periods - especially when the
systems are commissioned during those hot summer days.
As to the 4 factors John outlines below:
1) In our case my Fluke 87 III is measuring 258 VAC at the AC
distribution panel when it's going into Grid Failure Mode and the
Sunny Boy Control (through a RS485 interface) is showing 260 VAC to
be the point at which they drop out.
We do have perhaps some slightly higher voltages than some customers
might have due to the fact that this is a Hi Leg Delta 3 Phase
installation so that one leg isn't drawn down by 120 VAC loads at the
Lodge and Residences as much as the other two. But this is minimal
(maybe 1 volt) And all 4 SBs show the overvoltage within minutes of
each other - the two inverters that share the Hi Leg do drop out a
little bit quicker.
2) Grid Impedance in our case shouldn't be a great factor. We are
running the 4 SBs into a 400 Amp 3 phase service.
3) Voltage rise due to wire loss is definitely not the issue as we
are below 1% with our wire loss. (since we are running on wire
planned for expansion to 10 more KWs)
4) The fourth I guess we don't have any control over unless SMA can
work something out with the appropriate agencies.
So our primary problem is utility grid voltage being ramped up on hot
summer mornings and there is little I can change at the installation
side except getting a waiver on the VAC max set points if given
permission to do so.
Only 2 volts would do it from the measurements I've seen. Most of
the time the SBs are on the edge - oscillating between the 1 long, 2
short blink yellow failure mode (grid failure) and blinking green
(starting up) Before the blinking green can go to full green it
falls back to grid failure. When reviewing the 15 minute interval
data points on VAC from the Sunny Boy Control, you can see that the
voltage hovers in the 260-261 Vac range and as soon as they fall
below 260 it goes into the start-up, blinking green mode. It'll go
back and forth all morning indicating that we are not far from a set
point that would work.
It seems to be that if SMA has gone to such lengths to meet UL and
IEEE requirments to stay within the legal limits, that if the grid
can't operate within those limits itself that they should cut us some
slack on set points. I'm hoping something can happen on that front!
Thanks again John and all at SMA.
Jeff Clearwater
Ecovillage Design
>Jeff,
>
>Thanks for bringing this up. I will try to explain what we are seeing
>out there and what I think is going on. Hopefully this will help your
>situation and others out there who are seeing the same thing.
>
>We are seeing high voltage issues in a few installs out there.
>Everything seemed to be fine until the weather got hot and the utilities
>started bumping up the voltage to deal with big afternoon air
>conditioning load. The problem is that the voltage is really high in
>the morning when the AC is not running. Typically we see this issue in
>multiple inverter residential installations. I think there are four
>factors which combine to give us this problem.
>
>The first issue is high grid voltage. Technically the utilities are
>supposed to provide service voltage within "ANSI Range A" (112 to 126 @
>120 Vac and 224 to 252 @ 240 Vac). They are allowed to go into ANSI
>range B "occasionally". IMHO, Every morning all summer long does not
>qualify as occasionally.
>
>We first saw this on a 10kW residential system in Vacaville, CA. PG&E
>came out, measured, and ultimately fixed it because it was above 252 (in
>accordance with the ANSI standard). The system owner had to make a fair
>amount of noise before they did it but they did do it and everything is
>working fine now. PG&E is pretty good and other utilities may have
>different limits and/or policy's.
>
>This second issue is grid impedance. A brief aside: In Germany they
>monitor grid impedance as part of the anti-islanding scheme. When we
>came to the US I started asking people about grid impedance and guess
>what, nobody had any idea what it was. I could not even find a US meter
>to measure it and had to order one from Germany.
>
>As we drive current back into the grid the voltage has to increase in
>accordance with Ohms law (V=IR). At 10 kW the peak current would be
>about 42 Amps. After taking all kinds of impedance measurements in both
>residential and commercial installs, in several States I found the grid
>impedance (and corresponding voltage rise at 10 kW) is roughly as
>follows:
>
>100 Amp service: 400 mOhms >> 16.8 Volt rise
>200 Amp Service: 200 mOhms >> 8.4 Volt rise
>400 Amp Service: 100 mOhms >> 4.2 Volt rise
>
>The third issue is wiring losses which in our case shows up as more
>voltage rise. Normally I recommend you try to design for 1 to 1.5%
>losses on the ac side (2.4 to 3.6 Vac rise). If you design for the NEC
>max of 3% you would be at a rise of 7.2 Vac. Remember, this has to
>include all the TOTAL losses between the inverters and the meter
>location - wire, connections, breakers, sub panels, switches, etc.
>
>Lastly, the UL / IEEE anti-islanding requirements make the inverters
>very sensitive to high grid voltages. The UL / IEEE limit where we have
>to be off line is 264 Vac RMS. This is under any condition regardless
>of power level if noise and/or spikes on the grid. In actuality we trip
>at about 262 to 263 so that we can be sure we are gone at 264.
>
>We are seeing tripping on high grids with the threshold being about 250
>to 252 Vac (at rest) or so. We think this is due the above factors plus
>some noise or spikes on the grid. We are working the issue with SMA DE
>and UL to see if we can't make the inverter a little less sensitive to
>spikes and noise and still meet UL / IEEE requirements.
>
>When you start at a grid voltage of 250, add in another 8.4 Volts for
>grid impedance (200 Amp service), 3.6 Volts for wiring, and a little bit
>more for noise you can see how it is pretty hard to stay below the UL
>Limit of 264 Vac.
>
>As we (the industry) start putting inverters out there we are seeing the
>grid is not always within specification. There are reasons they pump up
>the voltage in the summer time and so they are reluctant to drop it back
>down. If you are above 252 Vac (at rest) then you may be able to get
>them to install a bucking transformer to bring the voltage back down for
>your house while the voltage on the feeder remains high. Ideally have
>them come out and measure when the inverters are running so the voltage
>they see will be highest.
>
>Another approach is to get a written waiver from the Utility to allow us
>to bump up the set points beyond the UL limit for that specific
>installation. In my experience they don't want to do this either. Even
>if you do get the waiver this means the inverter has to come home to SMA
>for a short vacation because UL says we can not do it in the field.
>
>As installers, the only thing we really have control over is the wiring
>losses. The cost to increase the wire sizes to get down below the 1%
>range may be money well spent if you are doing larger residential
>systems.
>
>I hope this helps explain the problem even if it does not give you any
>really good solutions.
>
>If you have any questions, or if I can be of any further assistance,
>please do not hesitate to contact me.
>
>Best Regards,
>
>John Berdner
>
>SMA America, Inc.
>20830 Red Dog Road
>Grass Valley, CA 95945
>530.273.4595 (voice)
>530.274.7271 (fax)
>
>
>-----Original Message-----
>From: Jeff Clearwater, Ecovillage Design [mailto:clrwater at earthlink.net]
>
>Sent: Wednesday, September 18, 2002 12:23 AM
>To: RE-wrenches at topica.com
>Subject: Grid Overvoltage on Sunny Boys [RE-wrenches]
>
>Hey Folks,
>
>The 10 KW (4 Sunny Boys) I just installed is losing a couple of hours
>each morning due to late startups due to the fact that the Grid
>Voltage is hovering above 260 VAC so the SBs see that as a grid
>voltage failure. I suspect that Southern Cal Edison is ramping up
>thier voltage in anticipation of air conditioner loads on hot days
>and it's not until those air conditioners start coming on that the
>grid voltage falls below 260 and the SBs start up.
>
>Anyone else experiencing this?
>
>Jeff
>--
>~~~~~~~~~~~~~~~~~~~
>Jeff Clearwater
>Ecovillage Design Associates
>Home, Community and Village Scale Renewable Energy Systems
>
>2525 Arapahoe Ave, Suite E4, #280
>Boulder, CO 80302
>1-800-440-2523 PIN7071
>Fax: 720-528-7813
>clrwater at earthlink.net
>jeffc at ic.org
>
>Council Member - Ecovillage Network of the Americas -
>http://www.ecovillage.org
>Founder: Ecovillage Research, Development, and Demonstration Program:
>http://www.home.earthlink.net/~clrwater/RDD/rdd.html
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
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--
~~~~~~~~~~~~~~~~~~~
Jeff Clearwater
Ecovillage Design Associates
Home, Community and Village Scale Renewable Energy Systems
2525 Arapahoe Ave, Suite E4, #280
Boulder, CO 80302
1-800-440-2523 PIN7071
Fax: 720-528-7813
clrwater at earthlink.net
jeffc at ic.org
Council Member - Ecovillage Network of the Americas -
http://www.ecovillage.org
Founder: Ecovillage Research, Development, and Demonstration Program:
http://www.home.earthlink.net/~clrwater/RDD/rdd.html
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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