Designing Off-grid (line interactive) system combined with on-grid system

RRD

Hello.

This is my first post.

I live in Romania, Sibiu.

I have grid connection 7kW installed (230V/32A).

My roof has one face SE (145deg) oriented with 35deg incline.

The prices for energy escalates these months.

(I also have natural gas water central heating with instant DHW, gas energy is 1/3...1/4 price of electric energy)

I planned to install an On-grid system with battery backup in order to maximize the self consumption, feed to grid the solar energy in excess and have backup for the grid blackout.

In this idea I wanted to use the system as offgrid for the night-time (~1.5...2kwh energy).

After I studied the specifications of few such inverters, there are my findings:

- not any on-grid inverter is accepted by the grid operator. There are some standards and many of these Chinese clone inverters does not fulfill the requirements.

- this kind of inverters have a long switchover time from grid to off-grid(backup), something of 500-3000ms time. This is not acceptable as UPS-like backup where the switchover time is 10-20ms.(there are communication setups, surveillance systems, personal computers and home automation sensors where the restart takes some time and the sudden loss of power cause unavailability, data loss, etc.)

In this situation, my attention is focused to a grid-tied interactive system (the inverter does not use the grid to feed energy but rather use it to power the loads bypass or in parallel with the solar when not enough, charge the battery, etc.

Growatt SPF 5000ES.

Pylontech US3000 x2

PV 360W x 14 ...16

(+ accessories, mounting system I think)

~ 7000eur (no labor)

(For the record, I know the return of investment in the off-grid systems, when the grid is available, is quite long.

But when you are angry on the "smart boys from energy", you wish to depend as less as possible of their energy.

Anyway, an off-grid system is more profitable than a new car)

In extension, I'd like to connect an on-grid inverter from the same manufacturer to have the possibility feed something to the grid.

Growatt MIN 3000 TL-XE series (2 MPPT, each 12.5A. 80-500V)

(the price we get for the energy feed to grid is calculated by the previous year average production price, no other taxes implied, which is now just ~25-30% of price we pay for energy we use; there are rumors in the market about a law for compensation 1/1 for feed energy/consumed energy, we are not sure if the compensation is done monthly, trimester or annual)

This inverter (it complies with the grid operator requirements) will be placed as required between the grid and the off-grid inverter

My questions:

- Is there any variant in which I can share some PV array from the off-grid inverter with the on-grid inverter? (I know it was discussed previously because that is how I landed on this forum)

- how safe is it to switch an array from one inverter to another? (situation during the sunny day, battery full, use diverter to spend extra energy to make hot water, cool air)

Also I know there are energy diverters which can detect the enetgy flow to the grid (on the AC line) and connect loads/variable loads (hot water tansks, water irigation pumps, ventilation cooling, etc.) to make use of the energy instead of feeding to grid for a price too small.

Please come with recommendation/suggestion/improvements about the idea.

(I am engineer in mechatronics, I work in SW testing, I have qualification in HighVoltage AC and LowVoltage, I know programming for personal/hobby use automation stuff, I was dreaming for photovoltaic for more than 10 years, I was playing with buttons and wires since I was 7).

mike95490

As far as I know, each MPPT controller channel, needs it's own PV array, scaled for the proper voltage range to match the MPPT circuit.  MPPT's can't share  a PV array, unless you use expensive HV-DC switches to "move" the PV to another MPPT.

Better quality hybrid inverters have fast transfer times, 8ms for the Schneider 6848 inverter
which weighs 125 lb (57 kg).  Pretty substantial.

I believe it has several programmable relays that can be set to activate devices at certain battery states.  Generally, once the battery is full, surplus power is sent to the Grid and reverse your meter

clockmanfran

High RRD,

What you are trying to do is very possible, but using commercially obtainable equipment will be expensive and just not cost effective.

Here in France, the Government run EDF Utilities, limit all households to a maximum of 3kW feedback into the main Grid. However all equipment and regulatory procedures required by EDF to connect to their Grid become hugely excessive and expensive.

So here in France feeding excess renewable green power we generate back into the EDF main Grid is no longer encouraged.

So our excess energy we just control and dump load some.

To Get around these issues 'we have tipped the whole standard process on its head'.  We have a main Inverter, that is by-directional, 48vdc to 230vac connected to a 48vdc battery bank, and our 19kW solar PV, Trackers, Arrays and roof mounted installations all use second hand GTI's that feed into our own Inverter created Mini Grid, this system is called AC Coupling.

AC coupling  its far cheaper concept for regulating and harvesting your own energy, rather than loads and loads of very expensive DC charging and connection equipment.

I talk and show 'Off Gridders' the AC Coupling System, at First they think its complicated and are led to believe (by the commercial boys) that they need masses of equipment.   I just love it when after a couple of hours showing and doing, their 'light bulbs' suddenly come on and they get it.   yeah!

Our Philosophy for a green and sustainable renewable energy system ........ keep it........SIMPLE, ROBUST, and  COST EFECTIVE.

RRD

In Romania the FeedInGrid phenomena is the beginning. In 2017 was adopted the law which define the "prosumer" (consume and produce energy max 27kwp installation, now 100kwp)

The government started in 2019 a project in which on-grid systems on 3kw minimum get 3000eur subvention (for free) and you have to come with 10%. Well, in the beginning of 2021 the authorized contractors started to mount systems and since the 2021 summer they started to get money from the government. (so it took 2 years from application documents to get the money, probably if it weren't covid-19, it would have taken one year)

There are some conditions for the applicants:

- you cannot build the system in DIY style. Energy certified and agreed contractors are only accepted.You buy the equipment from them and when there are free money from government, the contractors increase the prices.

- you cannot use batteries to inject energy in grid

- you cannot modify the system 6 years

- you have to keep the system working for 6 years

- you are monitored for the same period of time (smart meter, remote)

- you are qualified to sell the excess energy at average previous year production price, no taxes

- after that, the system is yours for free, you can upgrade it, modify it.

AC coupling idea was recommended to me by some of PV enthusiasts on SoftpediaForum.

The problem is there are indeed devices which you can use for AC coupling but most comes from Victron and others brands which is way too expensive.

On top of that, the "commercial boys" from Romanian market sells equipment which the market wants (On-grid, hybrid, off-grid, grid interactive off-grid).

So, the bidirectional inverter which supports AC coupling is some sort of niche product.

But I am wilingly to adapt to this solution if the cost is about 7000-8000E as I calculated previously.

Another factor which limits the choices is the foreign market.

Some buyers avoid to buy expensive equipment from outside the country. It is difficult to request service or return product in case of warranty issues.

That's why I prefer to buy from a romanian seller which I can find/sue if there are warranty issues.

Schneider Xantrex are a choice but:

Connext XW6048 is ~4800Eur

(with 4800 eur you can buy 3 expensive GTI with backup, or 5-6 off-grid inverters on 5-6kw power)

add 2x Pylontech 3000US = 2760Eur (which is minimal I say)

and 16 x PV 375W = 2430Eur

and do not forget about the 5-6kw on-grid inverter which is ~1000Eur (medium priced)

put some money for the accessories (PV fixing system, ATS-S/backuo box, AC/DC protection, cables...) and there is 12-14k Eur no labor included.

Probably in France/Germany, 14k eur is not a big value because of 2000-2500 usual salary (monthly).

But in Romania, the average salary is 700Eur. Those which have well paid jobs can earn 1500Eur, and these are the people which wants other things in life than eat, work, sleep, go party. 

Think about this, previously I was considering use of chinese inverters MPSolar/Easun of other clones because they are cheap.

I already raised the budget for more reliable equipment (eg. Growatt).

If I raise it more, the project will die before starting (my wife has a word to say when big expenses are to be done)

RCinFLA

Newer battery based hybrid inverters allow AC coupling of GT Inverters.

Growatt 3kW is a high frequency inverter, not of best quality.

High frequency hybrid inverters are not bi-directional in the same way a low frequency hybrid inverter (heavy low freq transformer) is.  A low freq hybrid inverter can continuously move from power pull to power push with just a slight modification to PWM control drive.  Most HF hybrid inverters have to make a mode change which takes longer and is not as robust as low freq hybrid inverters.  A LF hybrid inverter with GT inverters AC coupled can rely on battery to absorb a momentary back surge.  A HF inverter only has HV filter cap to absorb a back surge.

You need to do all the system paths efficiency calculations to find out if you will save money running a generator on natural gas.  Also, DC coupling for hybrid inverter grid push puts ripple current stress on batteries so you also should factor in longer term battery replacement costs.

Dave Angelini

You got that right on the quality. Growatt currently under recall for electrical safety..  AC coupling is not best for batteries in a small residential app. Especially if you do not know the exact compatibility of all of the inverters. It is an easy way to kill a battery bank even if you are an electrical engineer. 

The Op sited XW 6048 which has not been in support for over 10 years. The current replacement with a 10 year warranty is XW pro 6848 which is a low frequency design. XW input relays switch at less than 8 ms.  XW always have since it was designed in early 2002 and sales in 2006.

There are 600vdc rated A or B switched devices from Schneider Electric / Square D that can switch an array to different inverters or 600v DC chargers. Schneider MPPT-100-600v can run 8.5KW arrays. You can tell I am a fan of the company. They will be there down the road.

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Contacting Growatt Problematic?

With regard to getting in touch with Growatt, we had a comment last week on an old blog post from an Australian owner who had attempted to contact the company about an unrelated issue over a period of 48 hours via the 1800 support phone number. He mentioned that provided 3 options, all of which were directed to a full voicemail box. And this was before the recall was announced.

“As the phone is never answered you can’t talk to a person nor can you leave a message,” said the commenter. “My email for assistance has gone unanswered. (not replied to).”

I tried calling the number early this morning. While it was outside of business hours and the recording stated that, it also said if I would prefer a call back, to press 1 to leave a message. Doing that just resulted in the same recording being repeated – there was no option to leave a message.

It’s not a good situation at the best of times, let alone when a recall is active.

Hundreds of 8kW Growatt solar inverters sold in Australia are subject to a recall due to an electrical safety standards issue.

On Friday, the Australian Competition and Consumer Commission (ACCC) published a recall notice on its Product Safety Australia website concerning Growatt 8.2kW 8000 MTL-S inverters manufactured between 18 January 2021 and 26 February 2021. The units were sold/installed by several companies in New South Wales, Queensland and Victoria.

RRD

Thank you @RCinFLA and @Dave for sharing your knowledge.

I've got an offer for a system DC coupled way too over my needs and budget. Probably they were thinking "we got a reach customer".

Conext XW+8548E 230V        - 3342Eur

Conext XW- MPPT60-150 x2? - 513Eur

PV 345W x15                           -188Eur

Exide battery 2V/850Ah x24     - 424Eur

optional control panel              -236 Eur

optional gateway                     -538Eur

Total ~18000Eur

This inverter is quite good for what I need, still expensive.

I started to look for similar inverters/chargers and consider AC coupling with a cheaper GTI.

I've got some recommendation. Still searching.

eg:

Victron Multiplus 48/5000 -  1800Eur

other GTI which knows comunication (Fronius) - 1300Eur

Is it enough 2 x Pylontech US3000 3.5kwh for the beginning or should I start with 3 or more modules?

(3 modules will provide 37A (recommended) x 48V x 95% x 3 = ~5kw)

I have another offer for PV CanadianSolar 375W - 151Eur (it is cheaper than the one with 345W)