Ensuring fast switching between solar and grid

[ABarbarian]

I am planning a solar system to charge 10-20 computers, ~4-5000W inverter, I strongly want uninterupted power to these computers during power cuts, but also when switching to grid when solar/battery charge is low.

I am reasonably convinced I will need to run my load off batteries all the time (online UPS) to avoid interuption during a power cut where the inverter switches to batteries.

Any advice on the best method to reduce interuption when the batteries are low and need to switch to grid?

I understand hybrid inverters have an ATS that will do the switch and I guess this would be the most reliable switching mechanism. I have seen separate ATS that would perform the switch fast enough that a computer does not power off, but am not convinced it would be good enough for the large number of computers I am running.

I want to avoid separate (online) UPS's, I have not had good luck with them and would prefer a more unified setup, also want to keep prices down.

Interestingly I have been running a solar system that does the above, maybe not correctly wired although it has been in use for ~9 years, I have an inverter (STECA EXTENDER XTM 4000-48) that runs off batteries constantly and alarms when batteries are low, at which time we open a large switch that connects the inverter to the grid, once we pull that switch the inverter pulls its charge from the grid, and I think also charges batteries from the grid. Once we feel the batteries have enough charge we turn off the switch. It is an old system and was programmed by the installers so not sure it's configuration, but when doing this we never have any computers power drop.

 

[OffGridInTheCity]

In my off-grid solar system for my home, I have individual components (instead of all-in-ones) and I use ATSs / APC UPSs for sensitive circuits as the system switches between solar/inverter power and grid- twice daily 2/3 of the year..

Some of the all-in-one units aren't UPS so much as they are 'run-off-the-battery-all-the-time' to invert. AC comes in but is converted to DC for charging / pass-thru. The AC coming out is from the DC(Battery) -> inverter. With this arrangement there isn't any actual switching at the AC output side so sensitive circuits are OK.

I believe MPP Solar and GroWatt operate this way - but I'll leave it to other to confirm. For MPP Solar there may be nuances between off-grid and grid-tie models - but the concept of AC -> DC -> AC is definitely available in today's equipment.

 

[TomC4306]

I have a pair of MPP Solar PIP3048LV MK inverters (all in one) that perform this function flawlessly.
Solar/utility/battery mode blends the three inputs, preferring solar and utility backed by the batteries.
Solar/battery/utility blends the three inputs preferring solar and battery, backed by utility.

 

[Jim Burrow]

Because my MPP 2424lv controller cannot handle more that 22 discharge amps without overcharge and turning off, for 60 seconds, what I did what to go to Walmart and Home Depot and bought 3 Battery Backup units for around $45 each. Now when my power to the house goes out, all the computers, TV, Electronic for the internet stays on. I have one light that is not on one of the batteries back up units. That is how I now the converter overloaded and shut down.

I also changed my low voltage setting to 24.5v to keep the battery current from going over 20 amps for the wattage my house draws on the average when the solar panels are not getting any more sun light. But once in a while it still goes out, but the battery backup has never failed me.

If you are mining crypto with 20 computers, you definitely need battery backup units. - Good luck!

 

[Bossrox]

I use an 80 amp relay I got off amazon. My pc never shuts down but compressor appliances can be a problem only for the reason the sine wave might be 180 degrees out of phase during a switch which stalls compressors.

 

[TorC]

If you're putting in a new inverter, better hybrid inverters often even have a grid support mode, where if there's good enough power they'll add to a weak grid/generator input, or charge with while inverting if it isn't good enough to pass through and support. The Outback Radian series can do this, among others. Some modes can consume a bit of power to remain synchronized with the grid, while others, AFAICT, operate independently and synchronize and transfer only if the battery gets low.

Even an inverter that has a passthrough only in UPS mode will generally start up inside half a cycle of a power failure - ~16ms. The reverse, inverter to grid, is usually done by synchronizing to grid, then transferring at zero crossing so there is no interruption to power.

 

[Lt.Dan]

Some of the all-in-one units aren't UPS so much as they are 'run-off-the-battery-all-the-time' to invert. AC comes in but is converted to DC for charging / pass-thru. The AC coming out is from the DC(Battery) -> inverter. With this arrangement there isn't any actual switching at the AC output side so sensitive circuits are OK.

I believe MPP Solar and GroWatt operate this way - but I'll leave it to other to confirm. For MPP Solar there

The MPP and Growatts are NOT inverting all the time. I do not know of any that do, so let me know, because I'd like to buy a couple. They are physically bypassing grid current when in bypass mode, and the inverter turns off.

 

[Hedges]

Sunny Island keeps the phase lined up.
When reconnecting, it synchronizes before closing relay, so glitch-free.
When disconnecting because grid failed, there will be dead time. I think 30 milliseconds, but not sure. And it may vary with type of brownout/blackout.

 

[TorC]

I just looked a bit more at the Radian manual. AFAICT, several of its modes will maintain the inverter active and synchronous with the grid, so the worst that will happen is a slight and brief brownout as it takes the load if it's relying heavily on the grid. Even that is little different from starting a large motor or other big surge item. In particular, the UPS mode which looks like it passes through the grid, but has a 42W idle to keep the inverter running in case of grid failure so the inverter can seamlessly take over. See the table on page 19: https://outbackpower.com/downloads/...adian_8048a_4048a/gs_8048a_4048a_operator.pdf

 

[RedTechNeck]

I use an 80 amp relay I got off amazon. My pc never shuts down but compressor appliances can be a problem only for the reason the sine wave might be 180 degrees out of phase during a switch which stalls compressors.

There is an old fashioned trick the old timers used to synchronize generators. Simply put an incandescent light bulb in between the two "hot" lines you want to synchronize. When 180 degrees out of phase, the light will glow bright, when synchronized it goes dark. Slight RPM differences between generators would cause a "beat frequency" between the lines so the light would go on and off at a low cyclic frequency. The trick was to throw the switch connecting the two generators just when the light went dark. Once connected they would lock in and never go out of sync unless badly overloaded. Both generators need to have high slope regulation so that voltage falls off more rapidly with load than as a single generator. That way the gen's share the load better.

I suppose a redneck method could be devised where a light bulb is used and a photo transistor, photo cell, or PV sensor could be used along with an appropriate drive circuit to kick in a relay when the bulb went dark to connect the circuits when they are in sync. Just make sure the bulb is an old fashioned type. It wont draw any power when the lines are in sync so no energy will be wasted after the connection has happened.

I won't do the grid tie BS. A generator auto-start accomplishes the same thing when solar is not available if batteries get exhausted. The trick is to have a large enough battery bank where that doesn't happen very often. The grid people really rip off folks with grid tie because they don't pay anywhere near what they charge for the power you give them. So when you need power, your getting the screws even if you gave them a lot more. This "use the grid for a battery" mentality is seriously flawed in that regard. I think the power companies have some serious "back door" investments in the grid tie solar industry because they can get lots of power a lot cheaper from niave solar owners than they otherwise would get it. I refuse to be their milk cow. The other argument that will get made is that it costs money to maintain the grid, although most grids are very shoddy and very badly maintained. That's why they go down so easily during storms. Why invest that capital money in their system when you can invest it in your own system? Properly done and with enough redundancy, your system will be more rubust than theirs too, with fewer outages during storms and other events.

 

[ABarbarian]

Some of the all-in-one units aren't UPS so much as they are 'run-off-the-battery-all-the-time' to invert. AC comes in but is converted to DC for charging / pass-thru. The AC coming out is from the DC(Battery) -> inverter. With this arrangement there isn't any actual switching at the AC output side so sensitive circuits are OK.

I think you can call 'run-off-the-battery-all-the-time' UPS functionality, it seems functionally similar to an "online" UPS which does not need to switch to the inverter/battery supply when power cuts..

 

[Bud Martin]

MPP Solar MK series: Zero transfer time between line mode and inverter mode

MPP Solar Inc » PIP-MK Series

www.mppsolar.com

 

[ABarbarian]

I won't do the grid tie BS

Thanks for replies, the MPP zero transfer time inverter looks interesting, although RedTechNeck's comment about not using grid has got me thinking too.

If I went zero transfer I could reduce batteries and panels to have only emergency capacity, rather than enough to power the entire load each day.

The generator idea is good too, although I have seen people talking about this and that the prices are not too high I have not looked into it because I have no experience with generators, however we have been caught a few times with our old system where we have both a grid power cut and batteries running low at night time, so needed to shut down until grid came back or the next day, generator would solve that as all we need is enough petrol to keep things running. Also we will not be feeding into the grid because that is not our priority and there are regulations/risks doing that in our location that I want to avoid, if I completely disconnected from grid no danger (also no danger of lightning strikes on the grid cables burning out our system which some people in my location had happen).

 

[timselectric]

The MPP and Growatts are NOT inverting all the time. I do not know of any that do, so let me know, because I'd like to buy a couple. They are physically bypassing grid current when in bypass mode, and the inverter turns off.

Growatt and probably mpp can operate in both of these modes.

 

[Lt.Dan]

Growatt and probably mpp can operate in both of these modes.

Can you show me how to make my LV6548 charge the battery while simultaneously inverting power? MPP cannot figure it out.

The LV-MK Series is an exception, and I forgot about that model because it is such a low output that a lot of people cant use them without paralleling several.

EDIT: I just went through 3x different models of Growatts manuals and they all say the same thing at the bottom, "If this off grid solar inverter is working in Battery mode or Power saving mode, only solar energy can charge battery. Solar energy will charge battery if it's available and sufficient."

 

[timselectric]

I'm not familiar with all of the settings on that unit.
But, the inverter needs to be set to SUB (solar/utility, battery). This uses solar to power the loads. And let's grid help, if needed. The battery is only used, if solar and grid are not available. Then, you can set charging to SNU.

 

[Lt.Dan]

I'm not familiar with all of the settings on that unit.
But, the inverter needs to be set to SUB (solar/utility, battery). This uses solar to power the loads. And let's grid help, if needed. The battery is only used, if solar and grid are not available. Then, you can set charging to SNU.

So this is using GRID power to power the loads and the battery is non existent in the equation. It does not charge the battery, and reinvert the DC power to power loads.

This is exactly like MPP units as well.

 

[timselectric]

The battery will be kept at full charge, either by solar or grid. And only be used if solar and grid are not available.

 

[timselectric]

Solar is first priority for the loads, with grid assisting if necessary.