Running a grid tied inverter without the grid...

[Mike90250]

The sky is falling. Run from the deadly solar panels, slashing windmill blades, and strangling coils of copper wire.

[Mr. Mik]

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.

Thanks for the explanations. I have a backup generator and transfer switch already, but when I install a solar system, I want to be able to use it instead of the generator when possible.

I disagree with your first statement: Micro-inverters do exactly what you say would be outrageously expensive, but with power gain instead of loss and at apparently only slightly higher initial cost!
And, although 600V is technically not considered "high voltage", it is most certainly very lethal and in many circumstances more dangerous than AC.

600V is definitively not "perfectly safe".

[russ]

As you seem to be such an expert at this maybe you should start a blog.

You can not do what you want on a cost effective basis.

No one does and many people are in situations similar to yours - say all of southern California for example.

[Mr. Mik]

Would it not be quite easy to design a grid tie inverter with added voltage control?

All it needs to do is stop pumping current into the "grid" when the target voltage is being exceeded. Do GTI's not already do this, anyway? I have read it a few times that the GTI stops as soon as the grid voltage (or frequency?) is "out of specification". I assume that would include a voltage spike in the "grid".

If a GTI stops generating fast enough if the "grid" voltage rises, then it could be used as a stand-alone power system, if an additional inverter (battery powered or generator/inverter) is present to generate the reference voltage and wave-form (and maybe provides power for peak demands and surges).

[Mr. Mik]

As you seem to be such an expert at this maybe you should start a blog.

You can not do what you want on a cost effective basis.

No one does and many people are in situations similar to yours - say all of southern California for example.

If it is so patently impossible, then how come no-one clearly states the law of physics that it would break?

I don't understand why I seem to be an expert too you.

Are you an expert? If so, then you should be able to explain it simply. I'll get it, go ahead!

[russ]

I do not need to explain it - it is not done anywhere that anyone here has heard of.

If you make your own inverter or pay to have it made the utility won't allow you to connect it to the grid anyway. They have zero interest in such jury rig connections and rightfully so.

You are playing in the public domain and therefore have to follow the rules established for the public domain.

The only other way is to spend millions developing something like you want and try to get it approved by the certification authorities. After that the battle with the utility could begin.

The utility owns the football and consequently pretty much make the rules - in conjunction with the state.

You are one of very few people worried about the voltage on the roof and most of your points do not hold water. Fire departments have been through this in depth as they are usually the ones that would have to go on the roof in an emergency. They have no real problem with it.

A bottle of propane in the garage is far more hazardous - to the fire fighters and to yourself. 600 volts DC is no big thing - won't kill you faster than lower voltages if precautions are not followed. Chemicals stored in a garage can be far, far more dangerous in the event of a fire.

Russ

[Mike90250]

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

Do you drive a car? walk on the street ? Drink from fragile glass containers? You need some honest risk assessment, and not focus on overblown reports about lethal solar. And move away from that dangerous fire zone, or put a sprinkler system on the house, use de-ionized water, it's non-conductive.

Putting metal staples through electrical cable is not an accepted practice. Using that as an example of dangerous voltage is silly and overblown.
Stringent install procedures prevent a lot of problems.

[Sunking]

I disagree with your first statement: Micro-inverters do exactly what you say would be outrageously expen

I concur I overlooked microinverters as I do not use them, but it does not change anything. Microinverters are exactly like the string cousins just a smaller scale with an added communication link so they can communicated between themselves. What all GTI inverters have in common is they are current supplies, not voltage supplies. This is what you cannot get around unless you have a hybrid inverter.

A hybrid inverter is actually two inverters in one package. One inverter is a GTI current supply, and the other is a battery inverter voltage supply. The problem with any Hybrid system is they use batteries, and they require a generator. Batteries and the hybrid inverter are an extra expense that are rarely ever used.

[Mr. Mik]

Summarising what I think I have learned:

1) Grid-tie inverters do not react fast enough to situations when the "simulated Grid" cannot absorb all the current produced by the GTI. (I'm not 100% convinced of this, but it seems to be the main point of those who say it cannot be done). But, I believe that well designed GTI's must have a safety system that prevents them from dumping power into the real grid during voltage spikes - or not? If they do not have such an over-voltage safety feature, then voltage fluctuations in the grid would be made worse by the GTI's.

2) Power companies / legislators hold the cards and will not allow the connection of untested devices to "their" grid. Testing and approval costs lots of money, millions or thereabouts. Whatever gets connected to the grid must be compliant with these rules.

3) GTI's would work if they could be "convinced" that they are connected to "the grid". But they will continue to dump current into this simulated grid even when the power is not being used, potentially frying everything that is connected to the simulated grid in the resulting voltage spike. (Same doubts as per 1) apply - is there really no maximum voltage at which a GTI stops providing current?)

4) If the house is disconnected completely from the grid (manually, during prolonged grid outage), the rules for grid connection do not apply. A well designed system could possibly be installed/certified for relatively low cost if it cannot interfere with the grid.
That might be wrong in some legislations and right in others.

5) It is possible to build AC voltage sensing circuitry that responds fast to voltage spikes. Surge arrestors are an example. It must also be possible to build something that takes over from a surge arrestor within a fraction of a second, to then turn on a load that reduces the voltage to within safe levels. (Ideally that load would be a battery capable of accepting high charge currents at short notice.) But, as a crude solution, one could also use a halogen lamp heater element to simply burn off the excess energy.

This system would just bleed off energy whenever the AC voltage rises above 255V (or some slightly different more appropriate value). I think even I could build a device that reliably turns on a relay whenever the AC voltage rises above 255V. The relay could switch on a load that is equal to the entire maximum AC output of the connected solar system. This, combined with the already present load, would drop the "simulated grid" voltage so that the safety cutoff engages. The GTI turns itself off. A very crude solution, but....
Someone with much better electronic capabilities than myself could design something that drains only as much power as required to keep the voltage in check, despite of the unregulated output of the GTI into the house network (while it's completely disconnected from the grid).

This device would be connected on the off-grid side of the transfer switch, making it impossible to ever interfere with the real grid.

[Sunking]

1. GTI are current sources, not voltage sources like a battery of the utility. They are made to operate into a extremely low impedance of the utility grid. Therefor when the gird is disconnected the grid is disconnected or even interupted for a moment and the impedance rises, the GTI will disconnect and wait at least 5 minutes after power is restored and normalized.

3. What you can use a hybrid GTI which is actually two inverters in the same box. One is the GTI which is a current source. The other is a battery inverter which is a voltage source made to operate when the grid fails. It is not a legal issue, it is a life safety and property damage issue.

[ckchok]

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.Mike,

Can you guide me which website i can have more schematic or wiring diagram on how to connect my grid tie inverter in my home? I would like to integrate with my solar panel, charge controller and battery!

Thank you!

[russ]

You are talking about a hybrid system with the specific type of inverter for that - same as Sunking mentioned in the previous post.

There won't be a schematic such as you are asking for.

Russ