[RE-wrenches] 120 volt grid tie inverter

Tue Nov 16 13:06:11 PST 2010

Sure the resistor will limit the current to (100V/R) but it may not be
necessary or even desirable. 

One needs to know how the inverter in question works and what it would do
with a very low impedance source (essentially a battery can deliver several
hundred amps at 12 V; let’s say on the order of 50 milli-ohms). 

A string of PV modules may present 400V with a short circuit current of 8A
(a source impedance of 50 ohms – a factor of 1000X higher impedance).

This much I know: grid tied inverters start at very high input impedance say
50,000 ohms (drawing very little current) and iterate their way up the “I-V”
curve presenting lower impedances as they go (and checking the power input
at each step). When the most recent step actually decreases the input power,
the MPPT algorithm reverses the step and goes back and forth, constantly
maximizing the input power.

Faced with a source on the order of 50 milli-ohms, the inverter will never
be able to find a maximum power point by lowering its input impedance. So my
question is the following “Will another specification (like maximum out put
power) limit this process and cause the inverter input impedance to sit at
that point which causes a maximum output power?” I doubt that things are
that simple. But the question could be asked of the inverter manufacturer.

One other thing to remember is that inverters have a very large capacitor on
their input which gets charged by the PV string. The capacitor is very much
like a battery (very low impedance) so at least in the beginning of the MMPT
algorithm, an inverter should not have a problem with a battery vs a
capacitor.

Finally, there are relatively simple circuits that can perform “current
limiting” functions and could be used with a battery. Unlike a resistor,
current limiters offer no impedance (to speak of) until the current limit
has been reached and then they do a good job insuring the current set point
is not exceeded. However, they are an “active circuit” and may not be
compatible with a MPPT algorithm.

Sound like someone should spend some time with a “super-tech” at one of the
grid-tied inverter manufacturers and suss this out.

- Peter

Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parrish at calsolareng.com  
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885

________________________________________
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of boB Gudgel
Sent: Tuesday, November 16, 2010 12:00 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] 120 volt grid tie inverter

On 11/16/2010 11:20 AM, Richard Scott wrote: 
The KACO blueplanet 1502xi inverter has a minimum MPPT voltage of 125Vdc but
cannot be connected to a battery.

I would not recommend hooking a battery up to the input of just any grid tie
inverter unless it was designed for it.
The current from a battery is significantly higher than that of a PV array
and can damage the inverter or cause it to fail in a way which could be
unsafe.

Sincere regards,
Richard Scott
KACO New Energy

One thing that could help to make these kinds of connections work would be
to add a power resistor in series, in-between the inverter input and
battery.

This will limit the current and make the battery look "more like" a PV
array.

Of course you will lose some energy in that resistor, but the MPPT algorithm
would have a much better chance of getting it right and current limiting 
safely.
If it's better than going without, then that's better than nothing.

And, of course, one would have to pick a proper resistance (a few Ohms,
typically),
and a minimum power rating of the resistor.

boB

On 11/16/2010 10:34 AM, Peter Parrish wrote: 
Bob,

So he is grid-tied. But his household loads are all 100 Vdc? I wonder how he
charges his battery bank
If it is PV with a charge controller (or wind),
then I think he can export to the grid and get both a rebate from his
utility and a Federal Tax credit. I assume that he is not interested in
powering the 20 HP motor for his sawmill! If he is charging his batteries
some other way (small hydro) I don’t know about the incentives.

But it looks like he is staring at a fully charged battery bank with
generation to spare and is thinking “Why not sell this juice to my utility
company? Fair question. I have no direct experience with the SB700, but it
has been around for eons, and presumably is rock solid and can MPPT at 100 V
all day long. The Fronius inverters only work down to 150 V. The Exeltech
inverters that I am familiar with work down to 200 V. There must be lower
voltage, grid tied inverters out there but the “battery backup” versions
assume 24 Vdc or 48 Vdc battery banks. So, with the exception of the SB700,
there seems to be a gap between 48 V (maybe 60 Vdc) and 150 Vdc.

- Peter

Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parrish at calsolareng.com  
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885

________________________________________
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of bob ellison
Sent: Tuesday, November 16, 2010 5:23 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] 120 volt grid tie inverter

Peter,
The fellow is living with a 120 volt DC battery bank, using it to power his
place and a 20 HP motor on his sawmill. 
He wants to reduce the electric bill further than he already has.

So we are looking for an inverter that will take 120 volt DC nominal from
the panels / batteries and send it to the grid.

I am supposed to meet him tomorrow and hope to have more info afterwards.

Thanks,
Bob Ellison

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Peter
Parrish
Sent: Monday, November 15, 2010 8:38 PM
To: glenn.burt at glbcc.com; 'RE-wrenches'
Subject: Re: [RE-wrenches] 120 volt grid tie inverter

Bob,

Do you mean they have a nominal 120 Vdc input? Or do you mean that they want
a 120 Vac (RMS) output? If the former, what output voltage do you want? If
the latter, what is their nominal dc input voltage (better yet the range of
DC input voltages over a range of typical temperatures and irradiance
values)?

- Peter

Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parrish at calsolareng.com 
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Glenn Burt
Sent: Monday, November 15, 2010 5:29 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] 120 volt grid tie inverter

I think there is still an SMA 700W inverter available that uses 120VAC.
-Glenn

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Bob Ellison
Sent: Monday, November 15, 2010 11:14 AM
To: RE-wrenches
Subject: [RE-wrenches] 120 volt grid tie inverter

I have a customer who wants to grid tie a 120 volt dc system
Does anyone know of an inverter that can do this?

Thanks
Bob ellison

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