Isolating generator from solar system

I have a customer who just had a whole house generator installed at their home where I had put solar panels a couple of years ago. The solar system backfeeds the main panel through a 40-amp breaker. The power to the house comes underground for a pedestal about 150' from the house. The electrician put the transfer switch by the pedestal and ran power down from the generator which is off in a different direction. Normally I would either tie the solar into a line-side tap on the utility side of the transfer switch or connect the solar to the main panel by way of a normally open contactor that is tied to the dry contacts in the transfer switch (when utility power fails, the contactor opens). Currently there is no way that I know of to tell where the power in the house is coming from (utility or generator) and no way to connect the solar to the transfer switch way down at the pedestal. (Apparently the conduit to the house is thought to be damaged, so pulling the main feed to the house and adding a signal wire is not possible).
Anybody got any ideas?
 

mopowr steve

Senior Member
Does the solar have a negative impact on generator performance? Since the solar only adds capacity but still needs the 60 cycles as a reference for the inverter to produce output.
 
Does the solar have a negative impact on generator performance? Since the solar only adds capacity but still needs the 60 cycles as a reference for the inverter to produce output.
If the solar comes on when the generator is running, it has the potential to backfeed the generator causing significant harm.
 

synchro

Senior Member
Would using a low voltage 2-wire cable rated for direct burial that's placed at least 6 inches below the surface work for a low voltage signal line at 30V or less? This would drive the contactor you mentioned, and so worst case if the wire gets cut the solar won't operate. It could be fused to offer additional protection in the low voltage circuit if appropriate.
 

winnie

Senior Member
I'm sure this situation is going to come up more and more frequently.

There is no physics reason that says you _can't_ have the solar power system running with the generator. It is all about the controls.

When the solar panels/inverter are supplying more energy than the home is consuming, and the system is not connected to the grid, then _something_ has to give, preferably is a well defined fashion that permits continued operation, but the _something_ could be the magic smoke that keeps everything working.

Do there exist solar inverter and generator combinations which are smart enough that when operated without the grid connection, if solar production exceeds house consumption, that the system will simply do the right thing and reduce solar production?

-Jon
 

GoldDigger

Moderator
Staff member
I'm sure this situation is going to come up more and more frequently.

There is no physics reason that says you _can't_ have the solar power system running with the generator. It is all about the controls.

When the solar panels/inverter are supplying more energy than the home is consuming, and the system is not connected to the grid, then _something_ has to give, preferably is a well defined fashion that permits continued operation, but the _something_ could be the magic smoke that keeps everything working.

Do there exist solar inverter and generator combinations which are smart enough that when operated without the grid connection, if solar production exceeds house consumption, that the system will simply do the right thing and reduce solar production?

-Jon
There are such systems, but they most often include a battery bank and an inverter capable of off-grid operation from the battery bank as well as grid interactive operation. Pure grid interactive systems with throttling are rarer but are marketed in countries that prohibit feed back into the grid completely.
 

ggunn

PE (Electrical), NABCEP certified
There are such systems, but they most often include a battery bank and an inverter capable of off-grid operation from the battery bank as well as grid interactive operation. Pure grid interactive systems with throttling are rarer but are marketed in countries that prohibit feed back into the grid completely.
You might could make this happen with a SolarEdge inverter and their Electricity Meter.
 

pv_n00b

Senior Member
I would go the trench route to the transfer switch. I remember having to do a similar setup. The generator and service were near the house and the PV interconnection was in a building pretty far from the house, back-feeding a subpanel.

As an alternative, you can set up the PV system with a dropout relay to trip off on loss of AC and require a manual reset to turn it back on. There will be a loss of AC between when the grid goes down and the generator picks up the load, even with an automatic transfer switch. The homeowner needs to be involved and they need to do things right. They can't reset the PV system when on generator power. They can't manually switch from the energized grid to the running generator with the transfer switch without manually shutting the PV system down, depending on the reaction speed of the dropout relay.
 

synchro

Senior Member
Power is most often expressed in dBm or milliwatts; less often, peak power in volts is indicated. However, it is most convenient to understand using dBm. Since, using them, it is easier to go to the attenuation and length of the cable, which can be called with the help of such a generator.
Yes, if you're talking about RF signal generators with a source impedance of typically 50 ohms or sometimes 75 ohms. And usually with coaxial cables with the same characteristic impedance.

However, this forum deals primarily with AC power at 60 Hz or 50 Hz, and usually dealing with watts, kilowatts, megawatts, or more. And matching the load impedance to the source impedance (which is typical when using RF transmission lines) is usually a very bad thing in AC power distribution and usage :happysad:
 

texie

Senior Member
If at all practical I would and hard wire a control circuit. I would be a little concerned of AC driving a small relay at that distance though due to capacitance. DC would be a better choice.
 

tortuga

Senior Member
I have a customer who just had a whole house generator installed at their home where I had put solar panels a couple of years ago. The solar system backfeeds the main panel through a 40-amp breaker. The power to the house comes underground for a pedestal about 150' from the house.
Anybody got any ideas?
Just out of curiosity whats the make, model and kW rating of the generator?
And the Make, model kW rating of the inverter?
Its becoming quite common to synchronize several generators instead of installing one, I doubt it would be a problem for them to synchronize.
https://selinc.com/solutions/Generator-and-Microgrid-Synchronizing-Systems/
However it would be expensive.


If the solar comes on when the generator is running, it has the potential to backfeed the generator causing significant harm.
Can you elaborate on this ?
 

ggunn

PE (Electrical), NABCEP certified
RE: "If the solar comes on when the generator is running, it has the potential to backfeed the generator causing significant harm."


Can you elaborate on this ?
I can. If the PV system is connected on the generator side of the transfer switch when the generator is running, and if the PV system recognizes the output waveform from the generator as the grid, and if the demand from the loads falls below what the PV system is producing, the PV system will backfeed the generator, which would be bad news for most generators.
 

pv_n00b

Senior Member
Originally posted by smithgordonm View Post
If the solar comes on when the generator is running, it has the potential to backfeed the generator causing significant harm.



Can you elaborate on this ?
Unlike generators, grid-tied PV inverters are not designed to adjust their output to match a load. They are designed to converter all the power they can from the PV array to AC power and send it out into an infinite grid. So in an off-grid situation, if the load in the system is less than the output of the PV inverter there will be nowhere for the inverter output power to go and the inverter has no way to reduce its output power, this will damage something probably the inverter or the generator control circuits.
 

tortuga

Senior Member
Thanks. So in our off grid or no grid PV & generator scenario the generator would just sense the load as as Real Load - Solar = Load and adjust accordingly. However if the sum is less than 0 then were back feeding the generator and the generator should drop out.
 

winnie

Senior Member
I thought most inverters anti-islanded....?
I am not an expert on this topic, but my understanding is that the anti-islanding works by sensing the voltage from the grid and then adjusting the inverter output to 'match' (where 'match' means be just the right amount above that power flows from the PV system to the rest of the system).

It seems that a small backup generator could provide enough 'excitation' to defeat the anti-islanding so that the PV system supplies power to the rest of the system. But now the rest of the system is the house/connected load and the generator, but not the grid.

Now the question is 'what happens' when the inverter regulation logic tries to play with the generator regulation logic, in the circumstance of the PV system producing more power than the connected loads.

If the system is well designed but not intended for this sort of use, then everything simply shuts down when the voltage goes out of tolerance. If the system is sort of clever then the voltage climbs to the top of the tolerance range and then the inverter simply reduces its output. If the system is really clever then the generator reduces its output to some minimum and then the inverter starts dialing back. And if you are unlucky then the magic smoke exits the important bits.

-Jon
 

ggunn

PE (Electrical), NABCEP certified
I am not an expert on this topic, but my understanding is that the anti-islanding works by sensing the voltage from the grid and then adjusting the inverter output to 'match' (where 'match' means be just the right amount above that power flows from the PV system to the rest of the system).

It seems that a small backup generator could provide enough 'excitation' to defeat the anti-islanding so that the PV system supplies power to the rest of the system. But now the rest of the system is the house/connected load and the generator, but not the grid.

Now the question is 'what happens' when the inverter regulation logic tries to play with the generator regulation logic, in the circumstance of the PV system producing more power than the connected loads.

If the system is well designed but not intended for this sort of use, then everything simply shuts down when the voltage goes out of tolerance. If the system is sort of clever then the voltage climbs to the top of the tolerance range and then the inverter simply reduces its output. If the system is really clever then the generator reduces its output to some minimum and then the inverter starts dialing back. And if you are unlucky then the magic smoke exits the important bits.

-Jon
Such systems are possible to design but are far more expensive than straight grid tied systems and are strategically very different. Grid tied systems are purposed to link with the grid and offset the electrical utility bill. A system built the way you describe is designed to allow the system owner to operate off grid sustainably. One needs to assess the needs of the customer and manage their expectations accordingly.

For example, we get questions all the time from homeowners who have a generator for power outage backup and want to add a PV system. They frequently complain when we tell them that their PV system will go off line when the grid goes down. What they need to look at is what contribution to their bottom line the PV system would make if it ran during outages. If outages are generally short and infrequent, it makes no sense for them to spend the money it would take to build in the capability for the genny and the PV system to play well together.
 
Top