[RE-wrenches] AC Coupling

Brian Faley BFaley at magnumenergy.com
Fri Sep 2 17:20:47 PDT 2011


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

 

________________________________

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
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