Triggering the Grid-tie inverter with generator without Utility power?

tipu_sultan · July 2013

Would it is possible to Connect high power on-grid inverter with low power generator with out Utility power.(In case when Generator will never stop)

Load=15KVA On-Grid Inverter capacity= 20KVA Generator Capacity = 1KVA Utility Power = NIL (Not available)

Would the above system works? If there is no Utility Power and only power supplied by the Generator (In presence of Solar Energy only)?
Would it be the safe way to install above system?

BB. · July 2013

Re: Triggering the Grid-tie inverter with generator without Utility power?

Welcome to the forum Tipu.

In general, you have two major issues to address...

The first is that many small generator sets do not have "accurate" enough frequency (50/60 Hz +/- 1%)--The GT inverter needs to see frequency (and voltage) within range to turn on. If the frequency goes outside of range, the GT inverter will shut down for ~5 minutes before attempting to reconnect.

Second issue is that a GT inverter will pump out power if frequency (and line voltage) is within regulation (and you have a good Sine Wave source).

There is now upwards of 15 kWatts being pumped into your "local grid"--Where will that power go? If the loads (heaters, motors, electronics, etc.) is greater than the output of the GT solar inverter+array+full sun, then it can work (assuming voltage+frequency are in spec.).

However, what happens when the loads are less than the GT inverter's output power... That power has to go somewhere. It either raises the voltage too high and shuts down the GT inverter (a "safe" and reliable action) and/or it feeds power backwards into the generator set (really an alternator set). What happens there? I don't know... The diesel/gasoline engine cannot turn faster (frequency of GT inverter is locked to RPM of genset). So--only thing left is to drive current backwards through alternator windings and controller circuitry. Voltage and over heating problems? Don't know...

Anyway--Would suggest not even trying this unless you are willing to risk the genset and/or you have fixed loads (resistance heating, water pumping, etc.) that exceed the GT inverter's output at all times.

And I can think of a problem with motor loads--Most pumping systems (for example) have PF in the 0.6-0.8 range. The GT inverters typically output PF=1.0 power, and the genset will have to supply all "out of phase" current... Not sure if genset/regulation circuits will be reliable (PF seen by genset will, I would guess, be well below 0.50 with large un-power factor corrected loads). Generic gensets probably are not "happy" with very poor power factor loads.

-Bill

PS: I am not a power engineer--But these are my basic concerns with your suggested configuration.

If you wish to "mix" solar with generator power, then an alternative would be to use a "Hybrid" AC Inverter with "Generator Support" function. Such an inverter system can successfully "mix" generator+solar+battery together reliably.

Hybrid inverters are not that much more expensive (inverters capable of GT and Off Grid battery operation)--But what does cost money is the battery bank and associated wiring/maintenance. Not cheap.

Demonstration of Generator Support

ggunn · July 2013

Re: Triggering the Grid-tie inverter with generator without Utility power?

tipu_sultan wrote: »

Would it is possible to Connect high power on-grid inverter with low power generator with out Utility power.(In case when Generator will never stop)

Load=15KVA On-Grid Inverter capacity= 20KVA Generator Capacity = 1KVA Utility Power = NIL (Not available)

Would the above system works? If there is no Utility Power and only power supplied by the Generator (In presence of Solar Energy only)?
Would it be the safe way to install above system?

Generally speaking, no. Grid Tied PV inverters cannot deliver power to loads on demand*; they deliver all they can no matter what or they turn off. You cannot "fool" a GT inverter into thinking that the grid is present with a tiny generator unless its waveform is very clean and your loads are always more than the inverter output and less than the inverter output plus the generator's maximum output.

* with the exception of the new line of SMA inverters that can power a single outlet when the grid is down as long as the array is getting sunlight, but you don't need a generator to get them to do that.

hughng · July 2013

Re: Triggering the Grid-tie inverter with generator without Utility power?

tipu_sultan wrote: »

Would it is possible to Connect high power on-grid inverter with low power generator with out Utility power.(In case when Generator will never stop)

Load=15KVA On-Grid Inverter capacity= 20KVA Generator Capacity = 1KVA Utility Power = NIL (Not available)

Would the above system works? If there is no Utility Power and only power supplied by the Generator (In presence of Solar Energy only)?
Would it be the safe way to install above system?

I have the same thought but look at a little more reliable power source instead of the generator. Take a look at this hybrid inverter http://www.magnumenergy.com/Products/MSH-REseries.htm. I asked the manufacture if it would work and they said it would.
I haven't able to try it out yet because I just spent all my fund on the system and I don't have a way to disconnect the grid source currently since my system is hooked up with line side tap instead of to the main panel. I will have to get an electrician to put a breaker between the meter and tap box so I can safely shutdown the power prior hookup to this inverter.

Anyway, take a look at the inverter and see if that would work for you.

Peter_V · July 2013

Re: Triggering the Grid-tie inverter with generator without Utility power?

It depends...

Depends on the generator and depends on the inverter(s)

First you need an inverter that can scale back power production if the voltage goes to high (or using some other mechanism) AND you need a generator that can supply a stable enough frequency for the inverters AND can handle a voltage higher than the one where the inverters start scaling back output.

While it's not officially supported, the Enphase micro-inverters will work this way. This is because each microinverter has a slightly different shutdown high voltage, plus the variable wiring length (from each inverter) adds a little voltage loss. Net result is that the inverters start shutting down one at a time as the voltage goes higher.
Since each inverter is putting out 150-250 watts, this reduces your power production in ~200 watt steps. As long as all the inverters shut down before you hit the max voltage limit on the generator, no problem.

Note: Your generator also has to be large enough to handle the ENTIRE load if a cloud covers the sun, etc. And of course you get nothing from the inverters at night, etc.
Note2: Even though this works (I've seen it tested) Enphase discourages using their inverters this way and if something blows it might void their warranty.
Note3: I seem to recall at least one GT inverter manufacturer that offered inverters specifically designed to work with generators, but I can't recall which one.

deantalbott · September 2020

I agree with the forum on combining small generator and solar loads, Arizona Solar has helped me set up similar scenarios successfully..
Consider splitting your loads. With SMA secure power you can power lighting, communication and food storage loads in my house. We have 2 Refrigerators and a small freezer. Secure Power on the 6.0 will handle that during the day. Then operate the 3 ton AC unit with a small generator. When the sun sets, turn off the AC unit and switch the generator over to the small loads. This is clearly a cheap Kludge, but can be wired safe and code compliant. It just takes a trained individual to operate during power outages.

Triggering the Grid-tie inverter with generator without Utility power?

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