Generator Install with Solar

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JLCELEC

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Generator Install with Solar



Hello all,

Curious , has anyone installed a residential whole house generator with solar system already installed. I believe there is a way to do this im just not sure how. I don't have much experience with solar. Any help would be greatly appreciated...
 
JLCELEC said:
Generator Install with Solar



Hello all,

Curious , has anyone installed a residential whole house generator with solar system already installed. I believe there is a way to do this im just not sure how. I don't have much experience with solar. Any help would be greatly appreciated...
Click to expand...

As I replied over on the Engineering/Electrical Calculations forum:

A PV inverter should be interconnected outside the generator transfer switch. This will result in the PV system shutting down in the event of a power outage, which is better than having the PV system think the grid is still up when the house is on the generator. The reason for this is that if the PV system and the generator are both operating on the AC bus and the demand from the household loads drops below the output of the PV, the PV will backfeed the generator, which is very bad news for most generators.

This, of course, can be problematic if the interconnection point is a backfed breaker in the main panel of the house and the owner wants a whole house generator backup. The logical place for the transfer switch is between the meter and the panel, but this connects the generator to the PV inverter outputs.

I realize that some owners are doing this sort of thing expressly because they want their PV to keep producing during power outages, but it's not a great idea. The contribution to their bottom line by their PV system while the grid is down is usually minuscule compared to the damage the PV can do to the generator.
 
JLCELEC said:
Generator Install with Solar



Hello all,

Curious , has anyone installed a residential whole house generator with solar system already installed. I believe there is a way to do this im just not sure how. I don't have much experience with solar. Any help would be greatly appreciated...
Click to expand...

Well, I've done it in the opposite order. I participated in an install where the solar was installed after the generator. It was a large residental property, with a 500A service IIRC. The solar was installed on the line side of the service disconnecting means and automatic transfer switch, precisely so that the generator and solar would never be connected to each other. We even required the assistance of the generator tech to shut down the generator so that we could perform part of the installation.

As ggunn said, the PV should be installed on the utility side of the transfer switch. You will probably have some work to do to relocate that connection.

There are alternatives that will allow solar to coexist with a generator, but that would require more inverter equipment and is really beyond the scope of your question. Hopefully your customer has already studied this carefully and has made their choice of getting a generator accordingly.
 
The operation of a generator with a solar PV system can be done with a concept called "AC Coupling". Requires a special inverter and a battery bank.
 
iblittljn said:
The operation of a generator with a solar PV system can be done with a concept called "AC Coupling". Requires a special inverter and a battery bank.
Click to expand...
With some generators, yes. Some other generators do not produce a clean enough sinewave for a PV inverter to lock onto.

One of the best AC coupled systems is the SMA Sunny Boy/Sunny Island system, which actually uses two inverters - one to produce AC from batteries and the other to make it from a PV array. It's pretty slick but it ain't cheap.
 
ggunn said:
The reason for this is that if the PV system and the generator are both operating on the AC bus and the demand from the household loads drops below the output of the PV, the PV will backfeed the generator, which is very bad news for most generators.
Click to expand...

How will the output of the PV system backfeed the generator? The generator is not a load...why/how would the GTI send excess power to the generator?

Is the Voltage backfeed the problem or the Power backfeed? It probably has something to do with the generator windings...

Just searching for a clearer understanding of the problem.
 
shortcircuit2 said:
How will the output of the PV system backfeed the generator? The generator is not a load...why/how would the GTI send excess power to the generator?

Is the Voltage backfeed the problem or the Power backfeed? It probably has something to do with the generator windings...

Just searching for a clearer understanding of the problem.
Click to expand...
The generator is a demand source, i.e., a voltage source, that keeps the AC bus at a constant voltage by delivering whatever current is necessary to do that. A GT inverter, on the other hand, cannot regulate its current output to maintain line voltage - it behaves as a current source. If you have a generator running on an AC bus alongside a GT inverter and the demand from the loads on the bus falls below the output of the GT inverter, the only way that the generator can regulate the line voltage is to sink rather than source current, which most (if not all - I don't know a whole lot about generator design) generators cannot do without sustaining damage.

There are ways around this but all of them I know about are a whole lot more expensive than what would be the contribution to the bottom line from enabling the GT inverter to remain operational during the occasional power outage.
 
Put simply, if the load does not exceed the GT inverter's output, then the GT inverter will try to spin the generator backwards.

I don't know hardly anything about generator engineering except a few bits I've picked up from the forum here. :D But my understanding is that most if not all generators have thrust bearings that won't spin backwards. So whatever else that leads to, it will probably lead to loss of the AC sine wave reference, and thus the GT shutting down. Probably the generator stalling too, but possibly damage to the thrust bearing if done repeatedly.

I recall reading at least one story of accidental backfeeding (on another forum) where the generator's control electronics interpreted it as a fault and shut the generator down. Whether that would happen in most cases I don't know; I suppose it depends partly on the generator's safety and control features.
 
The GTI will try to drive the generator as a motor. Depending on the design of the generator this might drive the voltage higher or increase the speed of the generator. If you are fortunate this will just cause the generator voltage or frequency to go out of range and shut the GTI down.
In the worst case the reverse current will interact with the speed and voltage regulation of the generator to cause damage.
A normal GTI's anti islanding circuitry will simply never see the generator as a low enough network impedance and will not run in the first place.

Tapatalk...
 
I have always installed an isolating contactor to make sure the PV system does not start-up while the generator is running.

Simply run the AC power to the PV inverter through a contactor, then feed the coil on the PV contactor from the "utility" side in the Transfer Switch.

When the transfer switch is feeding the panel from the utility source it is also keeping the PV contactor closed and operable.
When the Transfer switch switches and is feeds the generator, the contactor opens because it is no longer held closed by the utility power.
PV cant start-up until utility power is restored.
 
davedottcom said:
I have always installed an isolating contactor to make sure the PV system does not start-up while the generator is running.

Simply run the AC power to the PV inverter through a contactor, then feed the coil on the PV contactor from the "utility" side in the Transfer Switch.

When the transfer switch is feeding the panel from the utility source it is also keeping the PV contactor closed and operable.
When the Transfer switch switches and is feeds the generator, the contactor opens because it is no longer held closed by the utility power.
PV cant start-up until utility power is restored.
Click to expand...
Or just interconnect the GT inverter at some point on the utility side of the transfer switch.
 
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