Using a load bank to load shed with Photo voltaics.

[CandCproblems]

Good Day,

I have several applications where residential users have between 5- 15KW Photo voltaic grid tie in systems installed.

Currently there are not any means of storing the energy on site.

During times of utility power outage the inverters do not sense the sine wave an thus do not push back to the grid - as is normal with these installations.

I am looking to use a small generator in conjunction with a dynamic load bank.

We would isolate from the utility, and then connect the generator to the system to provide a sine wave to sync to. The purpose of the load bank is to absorb any excess energy generated by the cells that is not consumed through the residence.

Is there a commercially available device that would enable this interconnection?

Any help is appreciated.

 

[VEBill]

I'm guessing you want the PV system to power the house during a power failure. But there's no battery bank. So what happens when someone turns on the stove just as a cloud passes by?

Rather than a little generator to stimulate the grid tie inverter (to backfeed the poor generator) and all the other bits and bobs to make it work, why not just buy another inverter that is not a grid tie type? Just switch to that and be done.

If you were buying a generator, why not make it a $700 5kw one?

Do the grid tie inverters offer a feature to foirce them to run open loop? Seems like a "lazy" designer that wouldn't include that feature.

 

[Compositepro]

I agree with VE1BLL. Your proposal is a crude mechanical approach to solving an electrical controls problem. And solar cells do not need a load bank to absorb unused solar energy. Will your solar panel blow-up in sunlight if it's not connected to a load?

 

[CandCproblems]

Thanks for this,

I was using the generator to supply the sine wave and a small amount of VAR's required to make it work.

I understand that it may be easier to purchase a larger generator to service the demand...and probably with a lot less head ache, but need to cancel out the viability of this first.

 

[waross]

I have done a few generator installations.
My first approach would be to ignore the inverter and properly size a generator for standby use. Include an automatic transfer switch. Then connect your inverter downstream of the generator. It will automatically support the generator and save fuel.
If there are heavy A/C loads, it may be well to arrange for the A/Cs to drop out on a power failure nd have to be manually restarted.
If you have enough generator capacity to run all the A/C units at once it is still well to arrange staggered starting.
Having all the A/C online and trying to start together when the generator cuts in is a good way to trash many transfer switchs in a few minutes.
A generator that is too small may result in a very unhappy customer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[CandCproblems]

Waross - thank you for your input.

My concern is that by having the photo voltaics a online and the generator that there will be an issue with the grid tie in inverter. Do I need to change the inverter type?

 

[davidbeach]

There are (at least were) inverters that normally functioned as grid tied, but would also work with on-site generation. Haven't looked for two or three years, but back then the Sunny Boy folks had one.

 

[waross]

I'll try to draw a word picture.
1> Grid.
2> Service entrance switch.
3> Automatic Transfer Switch.
4> Breaker panel.
5> Grid tie inverter connected to the panel.
The inverter must be able to handle frequency swings from about 51.5 Hz down to about 49 Hz or 62 Hz down to about 59 Hz.
If the inverter is sufficiently frequency agile it will sync to either the grid or to the generator, whichever is feeding the panel.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Sparweb]

CandC,
You haven't specified which inverter is in these residential systems, so we can't tell if you need to change the inverter or not.

To amplify the suggestion from DavidBeach: The list of grid-tie inverters that can also manage off-line power supplies (generators or batteries) is as long as my arm. Selecting the type and model, that includes the functions and power range your customers need, isn't the hard part. I have one, built in 1996, that's still ticking today.

Tricking the inverter to stay on with a "false" grid just invites other problems. There may be some grid-tie inverters that advertise limited off-line support, but that would have to supply an "emergency" panel with loads that cannot exceed the instantaneous output of the solar array. At night that would be...

Maybe the real problem is that your customers were promised something that their system won't deliver.

STF