Inverter output in standby load panel

[Energy-Miser]

I was asked what do you think will happen to a standby generator when the utility power goes down, if the output of a solar inverter is fed into the load center that is connected to the automatic transfer switch of this generator. I thought in that case, the inverter will continue to feed the panel, while the generator is also runing, and if the load is less than the total generation, then the excess generation from the solar inverter will backfeed into the genrator, damaging it. Is this a correct analysis of this situation? Thanks,

 

[techntrek]

The inverter should sync to the generator, if the generator's output is stable enough for the inverter. Grid-tie inverters tend to be picky on voltage and frequency fluctuations and most smaller gensets are too dirty (I assume this is for a homeowner). Get up to commercial grade and there's a good chance. But then the inverter often has a delay after utility dropout before it comes online again so during shorter outages only the genset will be online.

As for backfeed damage, that's not an issue - the inverter isn't going to push, loads pull. Its like worrying that using a 100 amp DC transformer for a doorbell will burn it out - the doorbell is still only going to use the amp or three it needs, no damage done.

 

[SeanD]

I saw this post last night and I was hoping to see some other reply before throwing in my 2 cents but oh well. Working in the solar industry this question comes up from time to time. Whenever I mention it to my boss I get the same responses EM got.

A Grid-tie inverter will not work with a generator because 1) of potential sync issues 2)and Potential back-feed from solar system to generator.

I have been told the only way to get this to work is for the solar system to be installed with a battery back up system. The cost of this set up is much more than any customer has been willing to spend so we have never installed something like this.

This is something that I am not all that familiar with so I would love to get some more info.

If I follow, techntrek's logic and assuming we can get the grid-tie inverter to sync to the generator, How is this going to work out?

Going along with the push/pull logic.

Scenario #1) Load is higher than max solar output. Grid goes down. Generator kicks on. Inverter syncs to generator. Load pulls from solar system. Generator adjusts output to match remaining load.

Scenario #2) Load is less than the output of a solar system. Grid goes down. Generator kicks on. Inverter syncs to generator. Load pulls from solar system. Generator adjusts output to 0??. Assuming it is possible for the generator output to drop to zero and inverter to still be synced, what happens with the excess production from the solar system?

Like I said, this is not something I am very familiar with but scenario #2 doesn't sound like it will work out. Can anyone provide clarification?

 

[ty]

This would be a Hybrid system, and would be taken into account when the site survey is done.

Edit:
Many inverters are specifically designed to integrate and control multiple energy sources.

 

[techntrek]

Many inverters are specifically designed to integrate and control multiple energy sources.

The Xantrex XW series does this. They can be configured to start a genset to assist with heavy loads, then the inverter syncs up and they work together. But I still think that for most off-the-shelf grid-tie inverters the generator output will fluctuate too much to allow for sync.

 

[gar]

100119-1603 EST

I can not answer your question relative to any particular real equipment. However, I will provide you with some food for thought.

Consider a solar panel with no connected load and sun shining on the panel. What happens? Does it explode? Does it reflect more light than if it had a load? Does it get hotter than if it had a load? Does it produce so much voltage that it arcs over? Does it scream I need a load?

Do an Internet search and find out what the volt-current characteristic curve is for a solar panel with different levels of solar input energy.

Next consider two adjustable DC voltage sources. Each with an adjustable internal resistance. And an adjustable load. Connect the two DC sources in parallel with both negatives connected together and to one end of the load. Connect together the two positive outputs and this point to the other end of the load. The internal resistances are between the positive terminal of the ideal voltage sources and the common plus point of the load.

Adjust the voltages, internal resistances, and the load to see how the voltage and current varies in different parts of the circuit.

This will give you basic theoretical answers to your question, but may not tell you about some real world problems with specific equipment.

.

 

[Energy-Miser]

Thanks. Everyone brings up very good points regarding this configuration. I know generators and solar generation can get along well, if you keep them totally seperate, with generator feeding a subpanel with selected loads in it, and solar back feeding into the main panel. In this scenario, the generator and the solar work in a mutually exclusive manner. The thing is that we have a customer who is interested in the gen set and the solar cooperating during an outage, and I just don't see a clean design solution for this (short of involving batter storage). I will look into the Xantrex model though that was mentioned in one of the posts, perhaps that's the way to go. Please continue with your suggestions, it will be much appreciated. Best, E/M.