contactor questions [RE-wrenches]

Jeffery Wolfe, Global Resource Options jeff at globalresourceoptions.com
Sat Dec 6 19:57:41 PST 2003


Marco,

Our calcs show more like a 8 to 10% daily loss. The transformer losses 2%,
constantly, of full load. (IF it's good, more if it's a low quality unit).
The transformer is also always a little bigger than it needs to be (unless
you buy a cu$tom unit.) Since you're only generating for 8 hours (and only 4
hrs equivalent in the northeast, you have it much better in HI), that 2%
multiplies. (Basically, it's something like 24hrs/day divided by full load
hrs, times transformer loss = % system loss from transformer.)

The way we get around this is by using SMA inverters, and connect into a
208V system. No transformer. We just had a nice project at Harvard Business
School written up in SolarAccess that did this. Numerous other advantages
too, but if I start that, this will sound like an SMA add, and I don't even
own stock...

Anyway, my cost-effective solution for the problem you have is to use SMA
inverters. No one-off design for a contactor circuit. No contactor expense,
added efficiency (even with the contactor design, the SMA are more efficient
overall on the array), simpler installation. May cost more money for the
inverter (depending upon your size and pricing), but may not cost more per
kWh produced at the end of the day.

Having said all that, if you are required to tie into a 480V system for
distribution reasons, then even the SMA needs a transformer. So one way to
deal with that is to try to still tie in at 208V, and share an existing 208V
transformer that's already on 24/7. No ADDED phantom load then. Otherwise,
you're into a contactor and control circuit.

I think the KISS (Keep It Simple Stupid) way to do this is to put a timer on
the unit. Especially at your latitude, how much does the day length vary? So
just turn the transformer on and off with a time clock. If you want, adjust
the time clock 2-4 times a year. I've used a standard photocell to turn
pumps on and off, and I can tell you there is not much fine sensitivity
adjustment or ruggedness in the ones I've used.

If you do go the contactor route, I would suggest installing at least a
rudimentary alarm (a red LED on the wall?) that comes on when the contactor
is energized, so that it's easy to see if it's working.

Let me know off-list if you want more details on large SMA arrays.

Jeff Wolfe
Global Resource Options, LLP

-----Original Message-----
From: Marco [mailto:marco at pvthawaii.com]
Sent: Friday, December 05, 2003 3:05 PM
To: RE-wrenches at topica.com
Cc: douglas at pvthawaii.com
Subject: contactor cuestions [RE-wrenches]



Commercial grid-connected systems using the Xantrex PV series 3-phase
inverters with their dedicated isolation transformers mean that there's
going to be a not insignificant parasitic power loss during the nighttime
hours.  According to my own measurements and conversations with others, this
loss can somewhere between 3-5 percent, or even more, of the previous day's
kWh production.  One way around these inverter and transformer core losses
would be to install a three-pole contactor/relay that would disconnect the
PV equipment in the evening and reconnect it early the following morning.
The issues as I see them are: 1) extra cost for the labor, contactor and
additional control electronics, and 2) additional means of system failure in
case the contactor stays open.  One could use a photosensor to control the
contactor or even a standard mechanical or digital timer.

Has anyone out there gone this route of using a contactor?  Any feedback on
the pros and cons?

thanks,
marco

Marco Mangelsdorf
President
ProVision Technologies, Inc.
69 Railroad Avenue, Suite A-7
Hilo, Hawaii 96720 USA
(808) 969-3281, fax 934-7462
www.provisiontechnologies.com



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