Grid Overvoltage on Sunny Boys [RE-wrenches]

Wed Sep 18 12:41:25 PDT 2002

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
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

- - - -
To send a message: RE-wrenches at topica.com

Archive of previous messages: http://www.topica.com/lists/RE-wrenches/

List rules & etiquette: http://www.mrsharkey.com/wrenches/etiquete.htm

Check out participant bios: www.mrsharkey.com/wrenches/index.html

Hosted by Home Power magazine

Moderator: michael.welch at homepower.com

- - - -
To send a message: RE-wrenches at topica.com

Archive of previous messages: http://www.topica.com/lists/RE-wrenches/

List rules & etiquette: http://www.mrsharkey.com/wrenches/etiquete.htm

Check out participant bios: www.mrsharkey.com/wrenches/index.html

Hosted by Home Power magazine

Moderator: michael.welch at homepower.com

==^================================================================
This email was sent to: michael.welch at homepower.com

EASY UNSUBSCRIBE click here: http://topica.com/u/?bz8Qcs.bz9JC9
Or send an email to: RE-wrenches-unsubscribe at topica.com

T O P I C A -- Register now to manage your mail!
http://www.topica.com/partner/tag02/register
==^================================================================