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Controllable grid tie inverter?
#1
Hello everyone, 

I am working on a battery, that should cover our nightly energy consumption. We have ~6.5kwp of photovoltaic panels. They produce about double the electricity that we need and I would like to charge my battery at daytime and drain it at nighttime. 

Therefore I would need a way to control the current that goes into the grid at night. Idealy I would like to use an inverter, that just sends as much power into the grid, as our house needs. I think our solar inverter is not ideal for use with a 14s battery (which I plan to build) and also is not controllable (it just pumps the whole power from the solar panels into the grid). 

I would be glad to get some inputs about possible inverter setups. The inverter should be compatible with 3 phases (ideally individually cover the phases) and 230v/50hz and should deliver a minimum of ~2kwp (I think it would be ok to get higher peaks from the grid at the moment... I rather would like to cover the permanent load).

Thank you for your inputs in advance!

Cheers 
elkooo



PS: right now I have ~800 18650cells lying around and growing, ~200 are tested and are <80mΩ >1700mAh... perhaps I'll even drop out the cells below 1900mAh... suggestions?
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#2
There is no requirement to alter your existing installation, just add a bidirectional inverter, and associated equipment to it, such as .....



https://www.victronenergy.com/live/ess:d...ion-manual
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#3
"Warning : Parallel and Multiphase systems are complex. We do not support or recommend that untrained and/or inexperienced installers work on these size systems."
https://www.victronenergy.com/live/ve.bu...se_systems


Depends on what approach you want to go down.

Is your phase to phase 240V or higher around 415V ? 240V phase to neutral or 140V ?

2kW for a 3 phase balanced load or 2kW per phase capability and resulting 3.5kW 3 phase balanced load capability ?

If you have a 3 phase star type incomming supply (common neutral) then 3 x separate 1kW inverters may work out better and provide 1.7kW 3 phase balanced load capacity. The inverters would need to be fully isolated types if separate because your battery is the common input point and be able to cope with the current profile an imbalanced 3 phase supply may end up with.

The selection of compatible inverters then depends on your level of grid compliance....

These are to give you an idea of one approach, not necessarily to use...
https://secondlifestorage.com/t-Limiter-...ad-setting

These units are relatively cheap, however I would not expose them to 415V phase to phase differentials as they may burn your house down and the sine wave output may not work too well with the resulting current demand from a 3 phase setup (phase to phase loading impact on current timing).

They are also lacking "official certification" in I think every country in the world that has any good grid standards... but they work and would "probably" pass grid tests.

There are other inverters and configurations you need to have a look at and read up on for 3 phase installs, more so if you have a lot of imbalanced loading..

Sometimes, covering the relatively small base that is on 24x7 can return far more than you think... at a lot lower cost.. rather than chasing the brief peaks.
elkooo likes this post
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#4
(08-09-2019, 11:08 PM)completelycharged Wrote: "Warning : Parallel and Multiphase systems are complex. We do not support or recommend that untrained and/or inexperienced installers work on these size systems."
https://www.victronenergy.com/live/ve.bu...se_systems


Depends on what approach you want to go down.

Is your phase to phase 240V or higher around 415V ? 240V phase to neutral or 140V ?

2kW for a 3 phase balanced load or 2kW per phase capability and resulting 3.5kW 3 phase balanced load capability ?

Hi completelycharged
Thank you for your thoughts & ideas! Very helpful information!

Our power system is 3phase so called 400v (probbably the mentioned ~415v) --> 240v per phase 120° Y type arrangement with common neutral. I think in Switzerland most houses are wired like this (except they are really old or at really remote locations). The load is spread over the house, so that all single phase wall outlets and devices are more or less equally assigned to one of these three phases in order to have similar loads on every phase (and we also have many devices that are hooked up to 3phase like cooking stoves, washing machines and in my case the workshop has 3phase outlets for my 3phase tools). 

I would like to use as much energy off of my solar system as possible. Therefore (and because the grid power provider charges me for every phase individually) it would make sense to provide power to every phase individually. I didn't think of the common battery-ground being a possible problem for the inverters as I must admit (seems to be a similar problem as measuring individual cell voltages when the measuring device has common ground with the battery pack...

Just had a look at your inverter... now, how do I find out if a certain inverter is «completely isolated»? Is this true for the mentioned one?  Huh
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#5
The Y arrangement is star where each phase is 120 degrees out of sync with the other and for 400V phase to phase in a residential property this is normally not done because a lot of appliances are only insulation rated tested for the nominal phase supply they will see plus a margin, i.e. for 240V they may be tested with 500V DC and 400V AC gives a waveform peak of 400V * SQRT(2) or just over 560V. The phase to phase higher voltage risks are what makes normal wiring in a lot of countries single phase or split phase as a norm, anyhow, that's just background...


The problems arrise with the three phase loads, all your single phase to neutral loads are easy to deal with - just use 3 standard small inverters as long as there is no phase to phase load connection.. Each phase is then just a separate supply and view it as if you just have 3 supplies to the house.

With phase-phase loads it get's a bit more interesting...


The issues with single phase inverters on a 3 phase setup is that due to the phase shift between each unit which results in a current flow per phase for a single phase-phase load having a current flow at times reverse to the voltage (i.e. the current flow reverses while the voltage is still positive on one phase, which requires the iverter go into a sort of charge mode)



 

The area in the circle shows the current reversing when the red phase for example is still positive. This is were ALL high frequency inverters that are not specifically designed to operate in reverse power flow will likely go up in smoke or cause the output to stop.

For an inverter to work properly it then needs three critical elements:
1. Waveform feedback tracking throughout the voltage profile to cope with the out of sync current flow pattern and quite a few inverters only spot sample the voltage for feedback, making them more susceptable to output harmonic distortion with some loads.
2. The ability to back-flow power through the inverter into the battery / DC bus line.
3. Full isolation (rated for above phase-phase voltage, i.e. 600V as per cable ratings) between the battery/DC line and the AC output, which is typically achieved via a regular transformer.

For balancing only 2kW you may be better off splitting out all the single phase loads to separate boards and then just use single inverters on each phase as if they were isolated supplies.

For 3 phase, from what you have said, you need to do a lot more reading and research before you go down this route so that you understand the issues, risks and options.

Hope this helps.

Read this to understand what metering setup you have and how your kWh value is then determined as this will also influence your decisions.

https://www.resinc.com.au/blog/how-do-si...e-property
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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