[RE-wrenches] AC Coupled Re-visited

[Phil Undercuffler]

Jeff,

How would the battery based inverter control the charge?  Most modern
control algorithms are based around some version of pulse width modulation
-- PWM.  You open the circuit for a tiny fraction of a second, and as the
battery voltage rises the open (off) pulses get wider, and the closed (on)
times get shorter.  To those of us living in meat time, it looks like the
current is magically tapering.

However, the GT inverter isn't a generator that can have loads unplugged and
plugged back in -- it's a current source.  *If *you could open the circuit
for a millisecond without tripping the anti-islanding protection, the
voltage on its output terminals would begin to rise as the BB inverter
"tapered" the current.  You may have seen a similar reaction if you've ever
measured the voltage on the PV side of an old-fashioned PWM charge
controller when the batteries near full and the charge controller begins to
regulate -- the voltage on the PV side begins to drift towards module open
circuit voltage.  Once the voltage on the GT inverter's output terminals
rose to the UL1741/IEEE high limit, the inverter would trip offline.

Therefore, the only practical way to regulate the charge is to either tell
the GT inverter to stop making power (ie, shut it down by forcing a
blackout), tell the GT to throttle back power output via communication (ala
Sunnyboy/Sunny Island RS485 and frequency shifts), or to divert that power
and absorb it doing some other work.  Per my other post, I now think this
latter solution is the better idea if you're going to mix brands.

Allan asked what would I recommend to accomplish this.  There's a couple
ways.

1.  DC diversion, using standard PWM controller and DC resistance load.
Advantages:  temperature compensated PWM charge regulation.  Disadvantages:
difficult to source and size diversion load, and you need to ensure the
regulation voltages don't interfere with normal charging and sell back
voltages.  I can share a good technical bulletin written by Morningstar on
sizing DC diversion loads, if you contact me off-list.  No magic bullet on
keeping regulation voltage out of the way of charging voltages -- it's
probably the one aspect that you will spend the most time getting right.

2. AC diversion, using relays driven by Aux Outputs and AC resistance
heaters.  I believe this is the most practical solution available today for
grid tie applications.  It's not PWM, but it's far more stable than 5 minute
off cycles.  Depending upon the brains driving your aux output, it's
probably temperature compensated.  AC heaters are commonly available in a
range of wattages and voltages, and they're dirt cheap.  Besides, it's AC
power that you're trying to absorb, so why make your BB inverter go through
the stress of having to convert that AC power to DC just to send it off to a
heating element?  If you want to get fancy and have multiple inverters
(therefore multiple aux outputs) in the system, you can do staged diversion
(1,000 watts of load come on at one voltage, an additional 2,000 watts comes
on .2v higher, etc).  Same complication of ensuring regulation voltage
doesn't interfere with normal charging and sell back voltages apply,
however.

3. Christmas Wish-list solution:  I've been trying to talk the guys at
Outback into creating a Diversion controller that can talk to the rest of
the system, provide temperature compensated PWM control of energy flow to a
diversion load, but most importantly know when grid power is present and
then stay out of the way.  That would ensure that there is no time where
you're buying AC power from the grid and dumping it to the heater, and it
eliminates all the gyrations of staggering voltage setpoints and hoping that
the multiple temperature sensors and devices will all play nicely under all
conditions.  Bonus points if it can work with AC or DC.  Extra bonus points
if it can work with any brand of equipment (wind or hydro, anyone?).
However, I'm just one voice in the wilderness -- if you would like to see a
solution such as this come to market, send Outback an email.  Let them know
Phil sent you ;-)

I think there is a lot of opportunity to add power reliability and stability
to traditional grid tie systems -- what we need is a way to do it easily and
effectively.  Here's to looking for a way!

Phil Undercuffler
Conergy



> ---------- Forwarded message ----------
> From: "Jeff Yago" <jryago at netscape.com>
> To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
> Date: Wed, 21 Oct 2009 19:53:11 -0700
> Subject: Re: [RE-wrenches] AC Coupled Re-visited
> There have been some good advice related to battery bank sizing and a
> better description of how Sunny Islands work with SunnyBoys, but I am afraid
> we are getting away from my original post that started this and that was:
>
> Since a battery based inverter has a battery charging section and all kinds
> of software control over the charging process when connected to the grid,
> why does the battery charging process "go wild" when the grid is down and
> the AC power is being backfed from a separate AC coupled inverter.  Also,
> why do we keep having to field wire and program an added power relay or a
> separate diversion load relay to prevent over-charging and destroying an AGM
> battery bank because the battery charging process suddenly has no clue how
> to charge the battery bank.
>
> Yes, the Sunny Island varies frequency which will cause an AC coupled
> SunnyBoy to be faked out and drop off line to avoid overcharging the battery
> bank, but it seems to me this is all just software programming, so why can't
> any inverter manufacturer simply control battery charging no matter where
> the AC power is coming from?  Why can't grid tie inverters used in an AC
> coupled application have the software to recognize a full battery condition
> and stop charging without having to drop off line on an out of limit grid
> condition and then cycle through the 5 minute wait?
>
> I am not an electronics engineer, but I don't see where any hardware
> changes are required, just some added software to select during initial
> setup.  Many of you may not deal with battery based systems, but half of our
> systems are and AC coupled is a great way to avoid long low voltage DC wire
> run losses.   We select generator type from a setup menu, how about "Press 1
> for standard inverter setup, press 2 for AC coupling".
>
> Jeff Yago
>
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