Using my Solar when the grid is down - will this work ?

[curto]

Guys,

I have 23Kw of Solar on 2 x 3 phase inverters on the house. About to get a single phase A/C coupled battery solution.

In the event the grid goes down - obviously my inverters will not work.

I had a thought to do the following

1) Isolate from the grid
2) Couple from the backup feed of my battery system to a VFD 1phase - to 3phase converter
3) Create a micro grid to enable the inverters to startup and feed house loads etc and charge the battery as required through the AC coupled inverter.

Would this work or would the inverters try to backfeed the VFD device and cause some havoc ?

And to add to this my inverters both support Zero Export control - which can be set either through an energy meter or through setting a modbus register which is how i would control it.

In this instance if we were without grid power for any lengthy period i could also isolate a number of my solar strings through DC isolators that are mounted next to each inverter (thus varying the total amount of PV being harvested at any point.



Craig

 

[Boondock Saint]

Hi Craig,

What are the inverters that you already have in your grid tie?

How much battery are you thinking to go with?

 

[Warpspeed]

Almost certainly will not work.

VFDs are made to specifically run ONLY three phase motors.
They use some very clever software that detects if there is any problem with the motor and shut the VFD down instantly if any abnormal operating condition exists. Having no load, or even slightly unbalanced loads will trip a fault condition.

How are you going to run a VFD with mixed domestic loads if any unbalance between phases shuts down the VFD ?
You will be trying to fool an intelligent well protected device, and it will win every time.

 

[curto]

Hi Craig,

What are the inverters that you already have in your grid tie?

How much battery are you thinking to go with?

Currently we have 2 Solis 3 phase inverters - one x 25KW and a 2nd 5Kw. We are looking at getting 3 x GoodWe SBP5000 AC coupled inverters with Alpha ESS 13.3 KWh battery packs.

The backup feature is a secondary - our main thing is to sell back to the grid when prices are high - hence why we want to push out as much power as quickly as possible - each of the GoodWe is rated for 5KW sustained - and we have a 10KW per phase limit for sending to the grid.

Craig

 

[curto]

Almost certainly will not work.

VFDs are made to specifically run ONLY three phase motors.
They use some very clever software that detects if there is any problem with the motor and shut the VFD down instantly if any abnormal operating condition exists. Having no load, or even slightly unbalanced loads will trip a fault condition.

How are you going to run a VFD with mixed domestic loads if any unbalance between phases shuts down the VFD ?
You will be trying to fool an intelligent well protected device, and it will win every time.

OK thanks for the explanation. The idea was not to run any loads from this - it was simply to "excite"the 3 phase inverters so they thought the grid was there and could then charge our AC coupled batteries in the event of a grid outage

 

[Warpspeed]

VFD's go through a startup routine that involves a lot of self testing, so you cannot simply disable a few sensors and defeat the protective systems.

See if you can borrow a VFD from somewhere, and try to run it first with no load at all, then with just one or two light bulbs. It will not start up.
Try it with three EQUAL light bulbs and it will run fine. Add a fourth extra bulb across one phase only, and it wil stop, and bring up another error message.

There is zero chance of back feeding through a VFD, simply because it has a rectifier at the input that expects to see an incoming ac voltage. Bypassing the rectifier will not work either, because it expects to see an ac voltage on all three incoming phases. If you lose one incoming phase, you get yet another error message and another shut down.

Only way this is going to work is if you have a complete alternative standby off grid system that you can use if the grid ever takes a dump.

 

[labeeman]

VFD's go through a startup routine that involves a lot of self testing, so you cannot simply disable a few sensors and defeat the protective systems.

See if you can borrow a VFD from somewhere, and try to run it first with no load at all, then with just one or two light bulbs. It will not start up.
Try it with three EQUAL light bulbs and it will run fine. Add a fourth extra bulb across one phase only, and it wil stop, and bring up another error message.

There is zero chance of back feeding through a VFD, simply because it has a rectifier at the input that expects to see an incoming ac voltage. Bypassing the rectifier will not work either, because it expects to see an ac voltage on all three incoming phases. If you lose one incoming phase, you get yet another error message and another shut down.

Only way this is going to work is if you have a complete alternative standby off grid system that you can use if the grid ever takes a dump.

Unless it is a single-phase input VFD but the dc voltage it expects to see is 300+vdc

 

[tanoshimini]

Scrap the ac-battery, and setup three Victron multiplus-ii's (or quattros if you want more inverter/charger capacity), which can work together to produce three-phase power, are designed to be grid-forming, and can easily be wired / configured for proper islanding in grid down. Then, hook up as large a batteries as you like. The 48v models will happily (and are rated to) run at 60v. You get seamless cut-over as a bonus.

I run a single 15kva quattro (48/240v/1ph) with 100kwh of battery @60v nominal. I have ~20kw of solar and use Enphase micro-inverters, ac-coupled. You'd use more (smaller) inverters to run 3ph, but it'd be the same basic idea.

 

[Warpspeed]

Unless it is a single-phase input VFD but the dc voltage it expects to see is 300+vdc

Yes, but you still cannot back feed 25Kw through a single phase VFD.

 

[pollenface]

The problem with tricking grid tie inverters to work without grid is that the loads need to match the power being produced (if you have power limiting inverters it may be possible - I don't have experience with those).

I think you're better off having a dedicated off-grid inverter available to plug in to an essential household circuit when needed.

 

[nonoceb]

You can't because gridtie inverter need a dump load with precise frequency and voltage range to work!
If you like the "danger life" and your solar array open voltage is near from the DC bus voltage of 400Vdc or internal high voltage in AC offgrid inverter you can supply directly them from your solar pv.
More safe, just buy battery, a solar charger and off grid inverter and share PV to grid tie inverter and solar charger
A little battery with a small charger is not really an option with this offgrid inverter

 

[stienman]

Anything is possible, but this is just not worth the headache. In order to make the load on the inverter look exactly like a motor, you're going to essentially have to build another inverter, with magnetics, phase balancing, active monitoring and control, etc. The time and resources you'll spend making it work will be more than simply buying the right tool for the job.

 

[Warpspeed]

VFDs are a bit like grid tie inverters in some ways, they are very fussy about starting up.
Unless everything appears perfect they will just spit the dummy and shut down.
I cannot see any way to make this idea work.

 

[efficientPV]

The basic principle of grid tie is that it keeps raising voltage till it draws enough current from the panels to drop the panel voltage down to power point. The grid can sink a lot of power, your house can not. It is quite easy to connect a sine wave inverter to your house when the grid is down and fake it into working. The problem is the line voltage will keep rising if the load is less than the panels can put out. That meas voltage will feed back into the inverter. Internally thru diodes this will pass right to the HV capacitors and cause them to fail from over voltage. Some inverters can feed that power back into batteries. But, all day? Dumping power isn't as easy as it looks. A shunt regulator drawing off over voltage could be the solution. Not something you will find off the shelf.

 

[tanoshimini]

The Victron units can be configured to do power limiting by frequency shifting, when offgrid. I use this feature on mine.