Solar & Generator

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shortcircuit2

Senior Member
Location
South of Bawstin
We always discuss that a Solar PV System should not be Loadside connected to an electrical distribution system that also has a Generator backup connected to it because you don't want the Solar PV System to back-feed into the generator.

We come to the conclusion here that the PV should be connected Lineside so there will be no problem.

Has anyone connected a PV System loadside to an electrical distribution system with generator so that the PV System can't operate when the system is on generator power?

If so how did you design this? With a relay or shunt trip breaker setup maybe?
 

iwire

Moderator
Staff member
Location
Massachusetts
The inverter won't run, it will not see the generator power as a utility supply.

I have tried this at a couple of places and the inverter will not go online.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
The inverter won't run, it will not see the generator power as a utility supply.

I have tried this at a couple of places and the inverter will not go online.

How does the inverter "know" that the source voltage isn't from the grid? :?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
The inverter won't run, it will not see the generator power as a utility supply.

I have tried this at a couple of places and the inverter will not go online.

That is not universally true. I have connected a Sunny Boy, a Sunny Island, and a Generac generator offgrid, and the Sunny Boy ran just fine when the generator was driving the AC bus. You cannot depend on the inverter not recognizing the generator as grid power to protect the generator from the inverter. In the setup I described the Sunny Island protected the generator.
 

iwire

Moderator
Staff member
Location
Massachusetts
How does the inverter "know" that the source voltage isn't from the grid? :?

I do not know.

In both cases the generator was 600KW or more, other building load on line and the cycles displaying 60 hz at the generator.

That is not universally true. I have connected a Sunny Boy, a Sunny Island, and a Generac generator offgrid, and the Sunny Boy ran just fine when the generator was driving the AC bus. You cannot depend on the inverter not recognizing the generator as grid power to protect the generator from the inverter. In the setup I described the Sunny Island protected the generator.

I wonder if that is a feature or a problem?
 

Carultch

Senior Member
Location
Massachusetts
I do not know.

In both cases the generator was 600KW or more, other building load on line and the cycles displaying 60 hz at the generator.



I wonder if that is a feature or a problem?

If the inverter is not set up to be intentionally operated in parallel with the off-grid generator, you should be connecting it on the utility side of the transfer switch, so that it never operates in parallel.

The most that the inverter would have to "know" that the ATS is in generator mode, without intentionally designing it to "know", is that the generator grid parameters might be a lot less rigidly maintained than the utility, and the inverter relay would trip, or not initiate the inverter after 5 minutes of stable grid power.

One way you could design the inverter to "know" which mode the ATS is in, is to measure the auxiliary contact of the ATS that will indicate its status as either an open circuit or connected circuit.

A generator should not operate below its minimum power ability, otherwise it will wet-stack the exhaust. Having an inverter operate in parallel, without inverter power curtailment, increases this risk.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
How does the inverter "know" that the source voltage isn't from the grid? :?

Many generators can't generate a 'clean' enough waveform to satisfy an inverter's anti-islanding checks. However, as ggunn mentioned, this is not to be counted upon as a design principle.
 

iwire

Moderator
Staff member
Location
Massachusetts
If the inverter is not set up to be intentionally operated in parallel with the off-grid generator, you should be connecting it on the utility side of the transfer switch, so that it never operates in parallel.

I did not, and it was not, there was no transfer switch. In both cases we were feeding the switchgear directly from genset due to utility outages.

I was hoping to reduce the large gensets fuel consumption by utilizing the 100 KW plus PV arrays these locations have.

The most that the inverter would have to "know" that the ATS is in generator mode, without intentionally designing it to "know", is that the generator grid parameters might be a lot less rigidly maintained than the utility, and the inverter relay would trip, or not initiate the inverter after 5 minutes of stable grid power.

One way you could design the inverter to "know" which mode the ATS is in, is to measure the auxiliary contact of the ATS that will indicate its status as either an open circuit or connected circuit.

A generator should not operate below its minimum power ability, otherwise it will wet-stack the exhaust. Having an inverter operate in parallel, without inverter power curtailment, increases this risk.

Let's not go off topic that far, I am well aware of wet stacking.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
There are two concerns about running a generator in parallel with a grid interactive inverter (GTI):
1. The generator output may not be stable enough in terms of voltage or frequency to meet the five minute qualifying time period for the GTI. On many inverters these limits can be loosened using factory access codes. The initial settings are part of the anti islanding protocols, so they may be unwilling to do.
2. If the standard GTI is getting full sun it has no choice but to put out full power. If that power is ever greater than the instantaneous local load the GTI will try to back feed the generator. This could damage the generator, the GTI, or connected equipment, and at best will cause the GTI to shut down completely. That will suddenly throw the whole local load onto the generator.
Some recent GTI models have the ability to use a current transformer to detect a potential backfeed condition and throttle back the GTI output as needed. Some of those same models allow controlled output at a power factor less than 1, keeping the entire reactive load from being seen by the generator.
Both capabilities have been added to meet the requirements of the state of Hawaii, where PV is such a high fraction of the generation capacity that network instability became a problem.

P.S. : Note that this zero export feature cannot be done simply from the terminals of the GTI itself since it does not know what the local load is. That is why a CT or other sensor has to be placed on the generator feeder to allow the feature to work.
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I would think that a 600 KW Catapilla genset has clean, steady power especially when driving a load.

Sure, I used 'clean' a little capriciously. The point is that inverters anti-islanding protocols might be tighter than what even such a generator provides. If I had to hazard a guess, it would be that your generator's frequency was few tenths of a Hz off.

Do you have more info on that?
GD explained the issues. I just meant that unless you have enough knowledge or control regarding the generator and inverter outputs, as well as typical minimum load, you can't make any assumptions about how the inverter will see the generator and what the consequences will be. Best to keep them from seeing each other unless you know exactly what will happen if they do.
 

electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
I have used relays before. DPDT 40 amp relay with 120 or 240V coil. You have to operate the coil from the grid voltage. I have picked up the grid voltage before through the transfer switch. The highest rated general purpose relay I have found is 40A. I have tried reconfiguring ATS's. I was able to do it on 100A rated Generac ATS's but somehow it would not work on 200A rated Generac ATS's. Never figured that one out.

So install the relay between the AC output conductors of the inverter and the interconnection point.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
One other point I'd like to make:

We field this question quite frequently when we have a customer who has a generator and wants to add PV, and they want to have the PV keep running when they are off grid and getting power from the generator. Yes, there are ways to make this happen but they are expensive. Unless the grid is very undependable for them it is never worth the expense. GT solar's purpose is to reduce electric bills. For most folks the contribution to their bottom line they get from having the solar run while they are off grid is negligible compared to the cost of building a system that will safely and reliably do that.

For most people this is true even if they don't have a generator. At my place the grid has been down maybe a couple of hours, total, over the last ten years. For me, spending the additional money for a PV system that would run when the grid is down would be a terrible investment.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
At my place the grid has been down maybe a couple of hours, total, over the last ten years. For me, spending the additional money for a PV system that would run when the grid is down would be a terrible investment.

I just wanted to add that most (maybe all) of that couple of hours was at night.
 

shortcircuit2

Senior Member
Location
South of Bawstin
Thanks for the feedback. I like electro7's post of using a relay. I would like to figure out how to apply that to larger systems...some way to isolate the inverter from seeing the generator at all.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Thanks for the feedback. I like electro7's post of using a relay. I would like to figure out how to apply that to larger systems...some way to isolate the inverter from seeing the generator at all.
That's what an automatic transfer switch is designed for. Put the ATS between the MDP and the protected loads panel with the generator on the PLP side and the solar on the other.
 
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