on and off grid inverters

System · August 2010

In an experiment I would like to use an off-grid inverter as a starter to "my grid power". I would like to use on grid inverters to sync and increase wattage along my grid's lines. Is this possible or would it blow up my off-grid inverter? I'm assuming an on-grid would not have problems (I don't see why it would).

I know someone will say "it's stupid and why not buy (blank)?" My only answer is that it would be a cheaper way to get emergency power if the grid was down. Throw the main breaker on the house and hook up a small off-grid inverter. The on-grid inverters would allow me to use my full array of solar panels (even recharging the off-grid's battery bank).

My second question: could a blocking diode be installed to prevent the additional wattage from the on-grid inverters from being back fed into the off-grid inverter?

BB. · August 2010

Re: on and off grid inverters

No, it is not possible to connect a plain battery powered Off-Grid Inverter and feed/share/increase power/reduce billing costs by connecting the AC Off-Grid inverter directly to the Grid. At best, you will let out the "magic smoke" from the inverter (yes--a joke)--And at worse, you can start a fire and/or hurt somebody.

This is a variation of people miss-wiring a Backup Emergency AC Generator system to their home. In general, this will damage the genset and blow circuit breakers. If done during a power outage--it is possible to electrocute people/utility works during a power outage/lines down situation. Electric Utilities have the right to disconnect your utility power (pull your meter) if they discover "illegally" installed equipment. And it is possible your homeowners insurance will not cover and damage/losses/legal actions.

There are Grid-Tied inverters (no batteries, no transfer switches) that are used all the time for solar power. They are designed to inject/push AC power into the grid and power your local appliances, and can even run your utility meter backwards. It is the most popular and cost effective form of solar PV power out there today. I have a system on my home--and it just works (and my electric bill is a minimum charge of $5.50 per month). However, a pure Grid Tied Inverter system cannot provide any power during an emergency.

The normal method to provide battery backed AC inverter power is to use a Transfer Switch between the load and a) grid power or b) off-grid inverter. Basically, the transfer switch will connect the grid/utility to the load (when AC power is available) or switch the load to the off-grid inverter instead which will provide power to your load.

As with anything--the above is a vast simplification of the basic options available. For example, there is a very nice Hybrid Inverter that can operate as both a Grid Tied inverter (when utility power is up and running) or like an off-grid inverter to supply emergency/solar power to your loads. And there are even other, more sophisticated options out there too. But, you should not go "blindly" and start purchasing equipment and setting it up.

For the most part, you should define your requirements/needs first (grid, off-grid, backup emergency power, solar power, reduce power costs, or whatever). Then define how much power you will need to produce (Watt*Hours, Watts, etc.).

Next, you can look at the available hardware and figure out the best system for your needs and cost point...

In general, it is very difficult to start with a "small system" and scale it up in a cost effective manner. For the most part, the small system components need to be replaced with larger (and more expensive) components/wiring/building permits/possibly utility approvals/etc. as the system is changed/improved/enlarged over time.

And, if your intention is to save money--Look at conservation first... It is almost always less expensive (and a better investment) to reduce your current energy needs first (energy star appliances, switching to lower power devices like laptop computers, turning off unused appliances, insulation, etc.).

I did a lot of hand waving here--because I am not really sure where you want to go from here. Please feel free to ask more questions and tell us the details of what you are looking for.

-Bill

System2 · August 2010

Re: on and off grid inverters

Thanks BB for the long reply that had nothing to do with what I was asking. I understand what could happen by feeding power back into the utitity company's grid when the power is out. Let us start from the begining.

Let's throw out real world situations and say I am powering my tree house. There is NO utility company involved and no insurance company. I have lots of solar panels a small off-grid inverter, batteries and a bunch of on-grid inverters. Once again there is NO utility company involved, this renders the on-grid inverters useless. So I hook up my small off-grid inverter and my batteries. Suddenly I have power and a small grid that the UTILITY COMPANY HAS NO CONTROL OVER. Can I now hook up on-grid inverters to increase wattage along the off-grid inverters line? (Potential problems: 1: Wattage from the on-grid is NOT blocked from entering the off-grid. This then blows up the off-grid inverter shutting down my tree house's power.(magic smoke) (Potential Solution: Add a blocking diode to prevent power from entering the off-grid backwards.) 2: The on-grid inverter won't sync to pure sign off-grid inverter) [this is very unlikely].

As far as breakers let's assume the circuit has large enough breakers to handle the load.

stephendv · August 2010

Re: on and off grid inverters

What you're describing is referred to as "backfeeding" or "AC coupling" or sometimes something else

I'm most familiar with the SMA equipment which was designed to be used this way in the first place, but you can achieve a similar result with other equipment as long as there's some way to prevent overcharging the batteries, so firstly a description of the SMA gear:

Their battery inverter is called the Sunny Island (SI) and was designed to be AC coupled with their on-grid Sunny Boy (SB) inverter by connecting the output of both inverters to the AC bus (or mini-grid). The SI keeps track of the battery state of charge, and when it reaches 100%, the SI increases the grid frequency above the nominal value (60Hz if you're in the US). The SB detect this increased frequency and throttle their output accordingly. The nice bit is that this throttling is variable based on the frequency. So at 60Hz the SB produces 100% power, and 62Hz it produces no power (and all values in between). So the SI throttles the SB by using the frequency so that the output of the SB exactly matches the loads on the AC Bus.

All on-grid inverters will have an acceptable frequency and voltage range that they require before connecting, and as you point out, most off-grid inverters will be able to produce the required sine wave without a problem. In Europe some countries require an additional check by the inverter which is the network impedance. The impedance of the grid is higher than the mini-grid in an off-grid setup, so just connecting a SunnyBoy won't work. The SB requires a configuration setting to prevent it from checking grid impedance as well, once this is disabled, it'll work fine in an off-grid setup. Depending on what the US requirements are, you might not need to worry about this - the on-grid inverters might just check voltage and frequency, in which case, you're already good to go.

Turns out most other off-grid inverter/chargers will also charge a battery bank when there's surplus AC on the output.
Apparently with the latest firmware for the Xantrex XW it also increases the frequency above nominal values to knock on-grid inverters off-line when the batteries are fully charged. The difference with this and the SMA system, is that it's all or nothing: either the on-grid inverters are connected, or they're not, there's no ability to vary the output. (This based on hearsay, I'm not actually familiar with their system)

Other off-grid inverter chargers might also work (the Victron Multiplus and Outback's do), but then you'll have to build or buy your own solution to prevent battery overcharging. An optional dump load could be one way, disconnecting the PV output with a relay could be another...

BB. · August 2010

Re: on and off grid inverters

Two things to add to what Stephendv said...

The off-grid inverter should be as large or larger than the GT inverter so that it is able to accept all power supplied by the GT inverter.

The second is a question adding a diode (I presume a DC blocking diode to the DC input of the off-grid inverter)... It may prevent the off-grid inverter from feeding any power from the inverter into the battery bank--but then you you have no buffer for GT power (when GT inverter exceeds local AC loads), resulting in no charging of the battery bank from the AC power (charging is a good thing--if you setup some sort of regulation circuit for the battery/GT/OG system), and it is possible that you may over voltage the DC input of the off grid inverter (some off grid inverters--many?, all?-- will shut down if the battery DC input voltage is exceeded).

It seems like it would be difficult to increase the off-grid inverter's output by adding Grid Tied inverters to its output... The Sunny Boy (and perhaps the upgraded XW Hybrid) inverter may have the ability to throttle back a GT inverter by raising the line frequency and how much power/current is back feeding the off-grid inverter--but I don't think any others do.

So, you are left with one off-grid inverter and 0-100% output available for your loads. And one GT inverter that has variable and unpredictable (if partially cloudy weather) with 0-100% of its output.

The Sunny Boy GT inverters, I think, can have variable output based on line frequency. The others will either be on or off (with 5 minute delay) based on frequency--So, it would seem that being able to add significantly to your total available power (GT PV system +OG inverter capacity) is possible, but not really predictable enough to be of much use.

At this point, the Sunny Island is the only off-grid hybrid system (off grid + grid tied) system that I know of which makes an integrated system like you ask about. Even then, I am not sure that the response time of the GT inverter would be fast enough where you could have the GT inverters give you more useful grid power during sunlight to power loads that are significantly larger than the off-grid/master inverters.

I am sorry about the first reply... You had "my grid power" and a couple of "grid" in your question that had me missing that you wanted to do this all off-grid and/or behind a transfer switch.

I try to keep my answers towards safety until I better understand the application.

-Bill

Cariboocoot · August 2010

Re: on and off grid inverters

Even if you could "back feed" an off-grid inverter from one or more grid-tie types without it frying, there's something else to consider: you have no storage capacity for the power produced by the GT's. So when the sun is shining and all the panels are putting out, where does it go? If there aren't sufficient loads attached, it goes nowhere. That means you've wasted a lot of money on the ability to produce power only when the sun shines, which is not necessarily when you need to use it.

Just my two cents, Canadian so it's not worth much.:roll:

System2 · August 2010

Re: on and off grid inverters

I understand your position Cariboocoot, for the most part you're right. Where I think your mistaken is that when a 30 thousand dollar system becomes useless when the power is out. It doesn't matter how bright the sun is shining those days either.
Last year my neighbors a block away lost their power for over a week. I live in a decently populated area and it was the middle of summer. It would make sense for me to be able to turn on my on-grid inverters if such an emergency happened (and not kill utility workers). TURN OFF the main breaker (protecting the utility company), hook up an off-grid, and the on-grid inverters kick on. Hopefully it will keep the refridgerator or furnace running to preserve food or keep us alive.

This would be cheaper and easier than running a generator for a week straight. It also allows you to use your solar panels when you need them most (when you don't have power). It also saves you money on permits and install prices of getting a large hybrid inverter. Some places even require batteries to be stored outside in ventilated areas to prevent explosions from hydrogen gases (another problem with hybrid inverter). This also allows me to use my vehicle as a generator and most inverters even turn off to protect my battery.

Your right it isn't worth much but neither are solar panels if you truly break down the costs!

on and off grid inverters

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