Running a grid tied inverter without the grid...

[Mr. Mik]

What are the options for keeping a grid tie inverter running if the grid is down for prolonged periods?

I'm not talking about defeating the safety features of these inverters while they are still connected to the grid. I am instead looking for alternative ways to make the grid-tie inverter "believe" that it is connected to the grid, when it has in fact been completely disconnected from the grid.

Could a small generator or battery powered inverter supply 220-240V AC so as to simulate the grid - and get the grid tie inverter to work?

[russ]

Hi Mik - Welcome to Solar Panel Talk!

To do what you are talking about you need what is called a hybrid system - that includes a 'hybrid' type of inverter and batteries - gets expensive.

Russ

[Mike90250]

it could, but then you have your large grid tie inverter, attempting to dump 3,000w into the "grid" Without a perfectly matched load, the voltage will quickly sag or rise past the safety thresholds, and the inverter will shutdown, and reset for 5 minutes.

Grid tie inverters do not have voltage regulation, they try to dump every available watt from the variable sun (clouds) into the infinite grid, and rely on the grid to keep the voltage constant.

What you want to try, would work for about a half second, and likely fry your "reference" inverter

[Mr. Mik]

Thank you for the explanations!

I thought there might be some problem when the energy has no-where to go.

How about this idea then:

Instead of having many more nonsensical ideas, I ask how to achieve what I want....

I live in Australia and would like to install a 1.5kW (or larger) grid-tie system while government incentives last. But, I also want to be able to use the power from the solar cells if and when the grid is down.

I already have a sine wave generator (6.5kW) and professionally installed grid-disconnect switch for the entire house (Grid / OFF / Generator).

I do not want to store much solar energy to use at night, I'd be happy to only run washing machine and other power-hungry applications when the sun shines. If I really need to, I can just turn on the generator.

What are the options to build such a hybrid system?

[Naptown]

Sunny Island
batteries, charge controller transfer switch come to mind immediatly however I tend to be a grid tied and thermal guy.

[Mr. Mik]

Thanks, Rich!

Are there any inverters that can work as grid tie inverters, but can also work as stand-alone inverters when disconnected from the grid?

This could be automatic or manual, but I'd prefer manual, because my generator does not have auto-start. Someone needs to jank the cable.

I have many NiMH batteries salvaged from first generation NHW10 Prius cars. I could build a small capacity battery for buffering during brief cloudy periods, not for night use or use on cloudy days. Some of these batteries have increased self-discharge rates, but can still produce 100A peak and 32A continuous, but they have only 6.5Ah capacity.

Am I right when I assume most modern solar grid tie systems operate on around 60V DC on the solar input side?

[Naptown]

depends on the inverter
Most require around 300 volts to start with a max of 600 volts.
Enphase micro inverters will operate on about 35 volts but will drop out when the grid drops out These are grid tied only
Look into the Sunny Island made by SMA they have a good website with all the tech specs

[Mr. Mik]

Low(er) DC voltage systems...

Is it essential to have such a high DC voltage system on the roof?

How much efficiency drop (or increased cost for really large DC cables) would it cause to have a system with a safer max DC voltage of less than 60V?

I am very concerned about the lethality of such a system.

Imagine trying to fix your roof on a sunny day after an earthquake, or a cyclone, or when a tree fell through it.

Even the neighbours shed, going walkabout in a cat 3 cyclone and landing on your roof, will make your roof a lethal "no-go-zone" during daylight hours if you have a >60V DC system installed. In fact, a single piece of scrap metal landing on your roof can turn it into a lethal "no-go-zone".

My roof is made out of metal. How stupid would it be to install a device that has the potential to make contact with the roof lethal whenever there is some sunshine?

The Australian Gvt. is currently having some problems with fixing the ceiling insulation debacle which killed several people by electrocution, caused by adding a metal insulation film and stapling it to electric cables.

[russ]

Hi Mik - You missed a couple:

1) an asteroid hitting your roof
2) an airplane crashing on your roof.

One has to think of the relative possibilities. In the Concepcion, Chile area you do have to be ready for an earthquake at any time. Most of the rest of the world has less chance, ranging on down to zero.

I live in an area where earthquakes do happen abd my preparation is only to have a bit of extra food on hand - especially coffee1

The foil insulation thing was caused by installers either 1) not knowing what they were doing or 2) not caring. Kind of hard to prevent stupid or uncaring.

The 'lethal no go zone' is a bit of an overboard description.

[Mr. Mik]

I don't think so!

The most likely and risky event in my location is fire. Loads of fuel load is located close by in the forest, certainly enough to melt the insulation of a few cables and burn the paint of the corrugated iron roof.

Houses typically burn down in bush fires because of leaf debris caught in the gutters - but how much will be stuck in the solar panel system after a couple of days of 50km/h dry westerly winds at near-zero humidity? Certainly enough to catch on fire when the embers start raining down during a bushfire, and most likely enough to fuse the solar system to the metal roof somewhere. In addition to the smoke and heat and wind, you would have to battle likely electrocution while fighting to protect your house!

Cyclones and thunderstorms with powerful wind gusts are also a very distinct possibility where I live. The sort of wind that easily manages to throw your neighbours shed, garage door or trampoline onto your roof. Of course it will probably keep on flying further than my roof, but would likely penetrate and mangle the solar system and the roof, connecting them electrically.

It's not far fetched at all and I'm simply thinking ahead.

So back to my question: How much efficiency drop (or extra material cost for heavy gauge DC wires) would it cost to use a relatively safe voltage of 60V DC or less?

[Naptown]

It' not the loss of efficiency it's finding a string inverter that will even wake up at those voltages.
A micro inverter would wake up but would need a stable grid to do so ending the off grid scenario.

Even with a micro inverter you would still have 208 or 240V coming off the roof. it would be AC however and by cutting grid power you would hut them down.

[Sunking]

So back to my question: How much efficiency drop (or extra material cost for heavy gauge DC wires) would it cost to use a relatively safe voltage of 60V DC or less?

Why would yu want to run a grid tied system at 60 volts DC? No grid tied systems work at that low of a voltage, nor would you want it too, the power losses and expense to overcome it are outrageous. 600 volts is not considered high voltage and perfectly safe.

But to answer your GTI with battery backup can be done with hybrid inverters. They are very expensive and require your whole house to be rewired to accommodate an transfer switch, plus the added expense of batteries. A far better solution is a grid tied system with emergency back up generator.

[Naptown]

A grid tie could be run at <60vdc. (Think Enphase) however there would no chance of running it off grid without spending a fortune on a setup that would simulate the grid. Even then there would be severe limitations on what could be done or how much power could be produced. Given enough money (read more than you could imagine) it may be able to be done. It's just not practical to do so.

[Sunking]

A grid tie could be run at <60vdc.

Yeah I forget about Enphase or other micro inverters, never use them.

[Naptown]

I would imagine then that in your neck of the woods the architecture allows for string inverters and there are few trees. Around here it is hard to find enough roof facing in the same direction to allow for a single inverter and too many darn trees. We use micro's almost exclusively. Out of 30+ installs last year only 4 used string inverters.