[RE-wrenches] AC Coupling

Marv Dargatz Marv.Dargatz at solaredge.com
Fri Sep 2 17:51:58 PDT 2011


This all works fine, until you fire up a generator to charge batteries.  If the generator remains within voltage and frequency limits, you stand a good chance of overvoltaging the batteries.  In this situation it is best to disable the utility-interactive PV system while charging batteries from the generator.

The SMA system gets around this limitation by using RS485 communication between the SI and the SunnyBoys.  When set to off-grid mode, the Sunnyboys will limit their output power limit from 100% to 0% based on commands from the SI, so battery voltage can be limited, even when charging from the generator.


See ya!

Marv
Director of Technology and Support, North America
SolarEdge Technologies, Inc.
Tech Support North America:  +877.360.5292
Mobile: +530.392.0356

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Brian Faley
Sent: Friday, September 02, 2011 5:21 PM
To: Allan at positiveenergysolar.com; RE-wrenches
Subject: Re: [RE-wrenches] AC Coupling

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

bfaley at magnumenergy.com<mailto:bfaley at magnumenergy.com>

________________________________
From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at 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
Allan at positiveenergysolar.com<mailto:Allan at 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

bfaley at magnumenergy.com<mailto:bfaley at magnumenergy.com>




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