Grid-tied system, no batteries, no grid!

aj164

I'm sure this question has been answered here before, but I haven't found it yet.

A lot of customers are asking how they could use their grid-tied systems during an extended power outage. Recently, some people were without power for nearly two weeks.

I'm thinking if you switch off the main breaker (isolating yourself from the grid) and then run a generator feed into the home, the grid-tied solar inverter will supplement.

The question is what happens when you're producing more solar than you use? Will the inverter (e.g. Xantrex GT 3.3) throttle its output, or will it push the AC voltage to the upper limit, shutdown, and retry (over and over)?

Clearly, the inverter is quite capable of throttling its output in other situations, such as over-temperature and the typical ramp-up after a cold start.

-AJB

mike95490

Re: Grid-tied system, no batteries, no grid!

aj164 wrote: »

push the AC voltage to the upper limit, shutdown, and retry (over and over)?

That's the way I've understood it would work.

I shopped around on flea-bay for some 400 - 48 DC-DC converters, and had a scheme where I could use 4 car batteries as "surge tank" daytime to run an inverter for the fridge, and a light or 2 at night. But the cars dont work till the batteries go back into them.

And that's why a hybrid system costs more !

Someone was scheming with a inverter and battery charger, so a simple GT system could charge and run a battery bank, but there is now a large equipment cost too, full hybrid system again.

BB.

Re: Grid-tied system, no batteries, no grid!

Read this thread... Both Solar Guppy and Jim/Crewzer have posted documents/links to a description of how a Grid Tied solar array can be used to back drive (or AC Couple) a "standard" off grid inverter (I would suggest that a True Sine Wave inverter be used for this project--as that is what appears to be documented... A MSW inverter may work... But I do not know enough about the issues to say how well or what the limitations may be.

Remember that if your Solar Panels+GT inverter outputs more than your AC Loads... The "excess power" must go somewhere... With an "AC Coupled" inverter, that power goes "backwards" through the inverter and charges the battery bank (in an "uncontrolled" manner, some sort of dump controller and/or/both feedback to the GT inverter is needed to prevent overcharging unless you are 100% watching the system).

The other issue is back feeding a generator... I am not sure that any
AC genset (standard alternator or Inverter Generator--AK Honda/Yamaha) can absorb any energy at a useful amount and/or at all when connected with a GT inverter... For the AC genset to become a load, I would assume that the generator would have to be spun faster than 60Hz (1,800 or 3,600 RPM)--and with a GT Inverter, their AC frequency settings are very close to 60Hz--so more than likely, the voltage/frequency will go out of regulation (if load is less than GT inverter power output)--the GT inverter will fault (turn off for 5 minutes) and the generator will supply 100% of the load until the GT inverter attempts to reconnect. It is also possible, with a standard AC Alternator Genset, the the frequency stability will not be tight enough for the GT inverter to ever connect in the first place (plus other issues, GT inverters are typically 240 VAC, and small gensets are typically 120 VAC--A larger genset may work a bit better).

Don't know how a Inverter type genset will behave. But, as long as the load is greater than the GT Inverter output (and you have the 120/240 VAC connections addressed--then it should work).

In the end, using a GT inverter with an off grid inverter (and properly wired AC Transfer Switch to prevent injuring/killing a lineman)--and setting up a couple of independent charge controller systems (one dump load, one feedback to the GT inverter to turn off--both when the batteries are "full")--seems to be the most do-able method out there to allow the use of a GT Inverter when the grid is off-line.

Just opening the mains breaker and hooking up a generator is illegal in most/all places in the US when connected to the Utility.

SouthWest Wind Power is suggesting/possibly selling this as a method for using their Grid Tied Wind Turbine in off-grid installations (they have enough other issues with the GT Turbine itself (not the AC Coupled DC Inverter part) that would suggest that any new customers look at existing SWP GT installations/feedback before investing in their solution).

-Bill

PS: The GT type inverter is designed to put 100% of the available solar energy back into the utility feed. There is no "throttling" mechanism available to us humble consumers that would allow us to automatically "dial back" the power when less is needed. The only feedback control known right now is to A) let the inverter fault--apparently some DC Inverters will allow the frequency to drift up when the battery is full(???) or Feedback from a battery controller that turns off the GT inverter (either through a relay that opens the GT inverter AC feed, or some GT inverters have an "on/off" control line available that you can wire into).

In the end, a combination of something like "B)" (turn off GT inverter) and using a "dump controller" on the battery bank itself (with some additional logic--to prevent the dump controller from being turned on when AC utility power is present and the batteries are being "equalized").