Can a grid tie inverter work along side a battery based one?

comcenter · August 2012

Currently I have a Xantrex DR3624 battery powered inverter installed in my house with 8 lead acid, 6-volt batteries on a 24-volt configuration to supply electricity during blackouts. Would it be safe to install a grid tie inverter in one of the outlets in the house? I know that under normal circumstances, the grid tie inverter I'm thinking of buying would send power coming from the solar panels into the circuit panel and off to the grid, it would turn off during a blackout as a safety feature when it doesn't detect any current. But with another inverter online, the transfer switch on the Xantrex would place the batteries online, what would happen with the extra power coming from the outlet with the grid tie inverter? Is this safe?

BB. · August 2012

Re: Can a grid tie inverter work along side a battery based one?

Welcome to the forum ComCente!

Yes, you can probably do it and it will work.

There are issues:

If this is a MSW inverter, the GT inverter may not sync correctly. It may only work when you have grid power (sine wave) or TSW inverter
Many TSW inverters can actually be feed from a GT inverter and recharge the battery bank. If the inverter cannot accept power, the local AC voltage will probably go "high" and both inverters shutdown for safety.
If you recharge through the TSW inverter, what happens when the battery bank is full? It may "equalize" excessively and use excess water and wear out your battery bank sooner. In some cases, you could boil the battery bank dry and cause a fire/explosion
I have not seen a "plug in" GT Inverter that is UL/NRTL listed--And many of these inverters do not appear to be built well (early life failures, over heating, possibility of fire).
Adding a plug in inverter to a wall outlet can allow excess current in your home wiring (15 amps from main panel breaker + XX amps from GT inverter, into an overloaded outlet will overheat wiring).

There are TSW inverters designed to alter their 60 Hz frequency to ~61/59 Hz which causes the GT inverter to shut down. They do this to prevent battery over charging (and SMA has a line that does this very nicely with variable frequency and power output from the GT inverter).

Anyway, more questions/discussion?

-Bill

Cariboocoot · August 2012

Re: Can a grid tie inverter work along side a battery based one?

Welcome to the forum.

Answer: no.

First of all none of the plug-in inverters are listed by any safety organization. You can not do any type of grid-tie system without proper equipment, planning, and permits.

Second what you're thinking of is called "AC coupling" where an off-grid inverter is used to provide a sync signal for a GT inverter, thus allowing the latter to supplement the available power during daylight hours. For the most part this is tricky business even with the right equipment. Your DR 3624 is definitely not the right equipment as it is an MSW type inverter and will not provide the right output waveform for a GT inverter to sync to.

Do not do this.

bill von novak · August 2012

Re: Can a grid tie inverter work along side a battery based one?

comcenter wrote: »

Would it be safe to install a grid tie inverter in one of the outlets in the house?

No, for several reasons.

1) The DR3624 is a mod sine (i.e. square wave) inverter which will not give the grid tie inverter the input it's expecting. Best case the grid tie doesn't start up at all; worst case it tries to start and damages itself when it tries to operate into a bizarre impedance.

2) The "plug in" grid tie inverters are typically pretty cheap and cruddy, and do not have the sort of anti-islanding protection that better inverters do. What this means to you is that if the GT inverter _does_ happen to come up, and you unplug it, the male tines of the plug may be energized for some (hopefully short) time after you unplug them.

The way this might work is with better GT inverters (like the Enphase inverters if you want to go small) and a better bidirectional inverter (like the GTFX series from Outback.) But I'm guessing that's too much money.

comcenter · August 2012

Re: Can a grid tie inverter work along side a battery based one?

Wow, these are really good answers.
I'm glad I joined this forum, thanks all.

Jmilner · August 2012

Re: Can a grid tie inverter work along side a battery based one?

Taking this idea a step further,
If one had a pure sine wave inverter, used an isolating transformer (to protect the inverter from current surge).
Then attached this output to the grid tie would this work. And work SAFELY?

BB. · August 2012

Re: Can a grid tie inverter work along side a battery based one?

Jmilner wrote: »

If one had a pure sine wave inverter, used an isolating transformer (to protect the inverter from current surge).

An Isolation Transformer will not do anything for function/safety/protecting against surge current (that I can see). And you may lose upwards of 5% of the transformer's rating as wasted energy (i.e., a transformer just powered up consumes some power).

Then attached this output to the grid tie would this work. And work SAFELY?

This has been done with Off Grid TSW (true sine wave) inverters in times past. Some (many/few?) off grid TSW inverters are bi-directional in power flow (they not only provide battery power to AC loads, they can take power from AC GT Inverters and recharge battery banks--Note, never connect a Off Grid inverter directly to utility/grid power--you will ruin the inverter and possibly start a fire/etc.). It was first done (that I saw) with Skytream's Grid Tie based wind turbine as a way to support off grid applications.

The problem is that GT to OG connected inverters did not have any way of regulating battery bank voltage/charging and could damage (or worse) the attached battery bank from over charging. It is possible to attached a dump load to the battery bank and/or monitor the battery bank voltage and put a relay in the output of the GT inverter to turn off the GT inverter when the battery bank is full. Requires a fair amount of engineering skills to build/configure your own system (and figure out which TSW off grid inverter to use with your GT inverter).

Today, there are several OG Inverters designed to take the 60 Hz to 61/59 Hz (or a bit more) when the battery is "full". This "faults" the GT inverter and shuts them down until the frequency return to 60 Hz. Xantrex/Schneider "XW Hybrid" inverters and (I think) Magnum support this function (there may be others).

And SMA's Sunny Island series does a very nice job of this (the vary the frequency to the GT inverter and can control the GT inverter from 0-100% output instead of "on/off" as done with the others).

-Bill

David and Laura · August 2012

Re: Can a grid tie inverter work along side a battery based one?

There are a couple of less-well known vendors (at least in North America) that are offering solutions based around the concept of using solar inverters as part of an off-grid system. Two that we've come across recently are:

http://www.victronenergy.com/markets/off-grid/

Take a look at pages 6 and 7 of their Off Grid PDF brochure.

Also,

http://www.stecasolar.com/index.php?Single-phase_and_three-phase_AC_hybrid_systems

Control signalling for this sort of solution is _very_ touchy. You're in essence creating a micro power grid where all the devices have to balance the different power flows by reacting to changes in voltage and frequency. While this works pretty well when you've got a bunch of rotating generators at MW scales (with lots of inertia to help prevent voltage collapse or pole slipping), with today's inverters, you're depending on a bunch of algorithms to keep everything stable. I'd hate to see all the waveforms when you turn on your planer (or any large inductive load), or when a dark cloud causes all your solar inverters to reduce their power levels significantly. And things only get worse when you move to three-phase power.

Personally, I'd rather have a system based around an out-of-band communication channel to control power flows - You really have to be able to react to load changes at the sub-cycle level, and you need some sort of mechanism to deal with sudden load changes (this would be a good use for a flywheel).

stephendv · August 2012

Re: Can a grid tie inverter work along side a battery based one?

David & Laura wrote: »

Control signalling for this sort of solution is _very_ touchy. You're in essence creating a micro power grid where all the devices have to balance the different power flows by reacting to changes in voltage and frequency. While this works pretty well when you've got a bunch of rotating generators at MW scales (with lots of inertia to help prevent voltage collapse or pole slipping), with today's inverters, you're depending on a bunch of algorithms to keep everything stable. I'd hate to see all the waveforms when you turn on your planer (or any large inductive load), or when a dark cloud causes all your solar inverters to reduce their power levels significantly. And things only get worse when you move to three-phase power.

If we ignore the AC coupling bit and just consider a normal DC installation, then don't inverters already take care of all these issues like sudden loads coming on and going off? And they seem to do a good enough job of it by only monitoring voltage and frequency - there's no out of band signalling telling them when loads will come on, or go off.

And with AC coupled systems using this on-off approach as explained in the links above, the battery effectively provides the flywheel inertia that you're after. Too much power available on the AC side, then it just gets dumped into the battery and the inverter/charger has time to monitor battery voltage and take the appropriate action. If the GTI suddenly switches off on the AC side, then the inverter/charger responds as it normally would if a heavy load was suddenly turned on.

I don't have any experience with these AC coupled systems, but the solution doesn't seem too touchy to me. Both those brands (and of course Xantrex) who offer this solution are pretty well respected, so I don't think they'd propose solutions that won't work or would cause damage to any appliances.

ggunn · August 2012

Re: Can a grid tie inverter work along side a battery based one?

stephendv wrote: »

If we ignore the AC coupling bit and just consider a normal DC installation, then don't inverters already take care of all these issues like sudden loads coming on and going off? And they seem to do a good enough job of it by only monitoring voltage and frequency - there's no out of band signalling telling them when loads will come on, or go off.

Yes, but they are pulling power from the battery bank, which is a voltage source and delivers current on demand. The PV is only there to keep the batteries charged.

David and Laura · August 2012

Re: Can a grid tie inverter work along side a battery based one?

stephendv wrote: »

If we ignore the AC coupling bit and just consider a normal DC installation, then don't inverters already take care of all these issues like sudden loads coming on and going off? And they seem to do a good enough job of it by only monitoring voltage and frequency - there's no out of band signalling telling them when loads will come on, or go off.

Assume a simple system where you have two battery inverters both connected to common DC and a common AC bus, with no out-of-band signalling. When a heavy load switches on, both inverters will detect the corresponding voltage sag and frequency shift independently, and will start routing more current from the DC side to the AC side. However, if both of the inverters aren't coordinated, they will tend to overshoot and will then have to scale back. With out-of-band signalling, the two inverters can at least coordinate their ramp-up to evenly share the change in load and have less of an overshoot.

Things get even more complex as you add more inverters, introduce transmission losses and impedance between them, and add distributed loads, as an inverter can get an increasingly skewed view of what is actually going on.

stephendv wrote: »

Both those brands (and of course Xantrex) who offer this solution are pretty well respected, so I don't think they'd propose solutions that won't work or would cause damage to any appliances.

None of these are going to damage appliances. All we are saying is that they aren't going to win any awards for power quality. I know we certainly wouldn't hook any of them up to our more sensitive loads without first going through a double-conversion UPS, and some UPSes we've worked with will complain blithely and temporarily disconnect over a 1 Hz deviation in frequency, droop, etc, that can result from the above approach.

Cariboocoot · August 2012

Re: Can a grid tie inverter work along side a battery based one?

David & Laura wrote: »

Assume a simple system where you have two battery inverters both connected to common DC and a common AC bus, with no out-of-band signalling. When a heavy load switches on, both inverters will detect the corresponding voltage sag and frequency shift independently, and will start routing more current from the DC side to the AC side. However, if both of the inverters aren't coordinated, they will tend to overshoot and will then have to scale back. With out-of-band signalling, the two inverters can at least coordinate their ramp-up to evenly share the change in load and have less of an overshoot.

Just in case anyone is "tuning in late" I have to point out:

You can not take just any two (or more) battery-based inverters and connect the AC outputs together. They have to be designed to work together: stacked. When this is done, one unit is programmed as "master" and the other(s) as "slave(s)". They will not automatically co-operate on their own.

There is a big difference between a load coming on and pulling one inverter down forcing it to adjust to meet the increased current demand and trying to accomplish the same thing with two power sources connected together.

David and Laura · August 2012

Re: Can a grid tie inverter work along side a battery based one?

Absolutely. This must be a feature of the inverters, as you need one inverter acting as a frequency standard (master).

Our comparison above was between such systems with and without out-of-band signalling.

ggunn · August 2012

Re: Can a grid tie inverter work along side a battery based one?

Cariboocoot wrote: »

Just in case anyone is "tuning in late" I have to point out:

You can not take just any two (or more) battery-based inverters and connect the AC outputs together.

Well, you can, but you won't like what happens...

Cariboocoot · August 2012

Re: Can a grid tie inverter work along side a battery based one?

ggunn wrote: »

Well, you can, but you won't like what happens...

Hey, there's about a 1 in 120 chance they will synch anyway. For a while. Maybe. :roll:

Can a grid tie inverter work along side a battery based one?

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