Hi Allen,
I'll try to describe the typical AC coupled system as well as I can. In such a system, a NON-grid tied battery based inverter (i.e. Magnum MS PAE) is connected as a backup inverter, only supplying AC power to consumer's loads when the grid is down. The micro inverter(s), or string inverters are connected on the load side of the inverters transfer switch, in parallel with the inverter output so they see AC grid power (when available) or inverter power when the grid goes down. The micro inverters / string inverters in such a system are called AC coupled inverters because the point of coupling is the AC system rather than the DC system. This allows the battery based inverter to provide the grid reference to the AC coupled grid-tied inverters when the utility power is not present - Backup mode. In that mode the inverter will form the reference for the grid tied inverters and any surplus power supplied by the grid-tied inverters that is not being consumed by AC loads will charge the battery of the inverter. In this inverter mode, once the battery is full, the inverter senses it and shifts the inverter frequency by .5hz to force the grid tied inverters off, keeping them from exporting power, thereby stopping the battery from overcharging. On a large system, a DC diversion load is usually added in parallel with the battery to reduce the on/off cycling of the grid tied inverters, which once the frequency shift happens, will try to reconnect every five minutes without it. Using the frequency shift method only is rather crude, because it essentially is a bang/bang controller - off or on. Its not a matter of our lack of confidence in the approach, the question is do you want the battery voltage swinging around by several volts. In a DC diversion mode system, the surplus energy from the grid tied inverters can be put to work as a useful dump load rather than just off lining several kw of PV. We had a white paper describing the AC couple mode. I'll see what we can do to make it more widely available, since the interest in such systems seems to be increasing. Brian Faley Chief Engineer Magnum Energy 2111 W Casino Rd Everett, WA 98204 425-353-8833 bfa...@magnumenergy.com ________________________________ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Allan Sindelar Sent: Friday, September 02, 2011 4:45 PM To: RE-wrenches Subject: Re: [RE-wrenches] AC Coupling Brian, Could you speak a bit more to this, please? My understanding is that the need with a string (or, I suppose, micro-) inverter and a battery-based inverter is for a way to disconnect full batteries from being overcharged when the grid is down and sell is disabled. The SMA approach is to shift frequency to reduce string inverter output. I thought that the battery-based (Outback, Magnum) approach is to use a contactor to open string inverter AC based on voltage (crude, but effective). So what I understand is that the PAE inverters shift frequency: do you mean it simulates the shift that a Sunny Island would do? And if so, why is a diversion load recommended, and how is it configured into the system? And where can we read more about how this is designed to work? Thank you, Allan Allan Sindelar al...@positiveenergysolar.com <mailto:al...@positiveenergysolar.com> NABCEP Certified Photovoltaic Installer NABCEP Certified Technical Sales Professional New Mexico EE98J Journeyman Electrician Positive Energy, Inc. 3201 Calle Marie Santa Fe, New Mexico 87507 505 424-1112 www.positiveenergysolar.com <http://www.positiveenergysolar.com/> On 9/2/2011 4:39 PM, Brian Faley wrote: Hi Larry, Magnum MS PAE inverters do indeed shift the inverter output frequency when the battery is full in order to disconnect grid-tied inverters used in AC coupled systems. It was designed to work in conjunction with a DC diversion load - which is recommended if the system is larger than a couple panels. Regards, Brian Faley Chief Engineer Magnum Energy 2111 W Casino Rd Everett, WA 98204 425-353-8833 bfa...@magnumenergy.com
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