Good day to everyone,

I've just joined but have been studying many posts on this forum for some months now. It has been very helpful, thank you all.

I've configured my proposed system as best I can from all the info gathered. I've not purchased anything yet (except the generator which I've had for a while now.)

I'm buying 3 phase power directly from ESKOM and it's really expensive. (R3.5k monthly for 1000kWh average use). I'm committed to go OFF-GRID completely.

I would appreciate very much if members can comment/make suggestion etc. on my proposed system.

Here follows a brief summary:

CURRENT AVERAGE POWER USAGE:

MONTHLY:         1000 kWh

DAILY: 33.3kWh

HOURLY:             1.38kW

PROPOSED OFF-GRID SYSTEM

PV ARRAY (JA Solar JAM72S01)

18 off 370 Watt 72 cell Mono Modules. Total Pmax =  6660 W

                Module Voc = 48.18V, Isc = 9.91A

                Connected as 3 groups of 6 modules (2S3P) 2220 Watt per inverter

                Voc = 48.18V x (2S) = 96.36Voc max, Isc = 9.91A x (3P) = 29.73A max

                20 Amp Series fuses per string

INVERTERS

3 off STECA Solarix PLI 5KVA (same as Axpert MKS 5KVA)

                Connected in 3 phase mode (1 inverter per phase)

BATTERIES (at this time I can not reveal the Brand)

3 off 48V-240AH LiFePo Packs with internal BMS (no communications)

                All 3 Packs connected in parallel to all 3 inverters simultaneously

                Total Battery capacity is 34.5kWh

 

• Cell Type                                            40152L (3.2V, 17Ah)

• Nominal Voltage                                 48V

• Nominal Capacity(0.5C,25℃)             240A

• Recommended Discharge Current        30A

• Max. Continuous Current                    60A

• Max. 10 sec. Pulse                              80A

• Cut-off Voltage                                   40VStandard Charge

• Charge Voltage                                    57.6V

• Recommended Charge Current              40A

• Max. Charge current                             60A

• Other

GENERATOR

15kVA 3 phase Synchronous Alternator driven by a 2 cylinder Deutz Diesel

                Equipped with a custom made Auto Start controller/manager

 

Thank you in advance,

Trober.

 

 

 

20 minutes ago, Trober said:

Total Pmax =  6660 W

So at 5 hours average peak sunshine, that's just enough to cover one day's consumption if we ignore losses and inefficiencies. Also no provision for rainy days, do you intend keeping the grid connection, or will you run the generator for the rest? If you intend getting rid of the grid, I would suggest dropping consumption or making the system even larger.

25 minutes ago, Trober said:

3 off STECA Solarix PLI 5KVA (same as Axpert MKS 5KVA)

Same old upsetting story... if you intend remaining connected to the grid, you may need to look at an inverter that is approved for such use. This one has... issues :-)

I hadn't seen you are going to install a pack of three batteries.

Edited October 31, 20186 yr by Javi Martínez

Before buying the inverters, could you test if generator suits well with inverters?

 

I don't like too much inverters without comunication with BMS.

What would happen if PV is charging low and there is a peak of discharge and the BMS breaks?

At least, BMS should be "Two Ports"

Hi Plonkster,

Because my fixed costs for ESKOM are high, I need to go off-grid completely. Grid-tie is not an option.

My consumption of 1000kWh per month cost breakdown as follows (approx):

R1000 for 1000 kWh, R2000 for connection/service fees, R 500 VAT = R3500 per month.

I agree - only enough for one day. The gen runs pretty efficiently at about 2 liters diesel per hour at medium load so it will have to make up the shortfall and re-charge the battery bank.

I have one more geyser to convert to solar. Hopefully that will drop total consumption by perhaps 20%.

When the budgetallows, I can possibly add more batteries?

During the winter I measured power available from a single test module on the roof. If I tilt it towards the East in the morning and track up to midday (where the hinged half-axis tracking stops), the power output is 20% more than if it remained flat. I can build this "half-axis" tracker quite easily (like a louvre window) that simply rises the western side of each module to the east.

There is a limit to how high it can tilt due to mutual shading, but the target roof area is large (200 msq) so they can be spaced to allow higher tilt angles. By 10AM I get Pmax up to 12:00.

It's helpful that the available PV yield is increased in the morning when the batteries will be at lowest SoC.

 

 

I would say you are a bit light on the solar panels. I am off grid, using about 10-12 kWh/day from a 3kW solar array. For your consumption I would be looking at 8-9kW of solar. Battery size looks good. Generator backup is essential. I'm runnng on genny now as I was pumping water to fill the swimming pool while cloudy today.  

Hi Javi,

Yes, 3 packs of 240Ah in parallel. (720Ah total)

Great idea to test first. I can borrow an inverter for such a generator match test.

 

Diesel generators work better as the engine is governed to a particular RPM equivalent to 50Hz on the generator. Cheap petrol generators are the problematic ones as they try and control the engine throttle using voltage.

On your system size, are you sure you cannot bring your usage down at all? It is much cheaper to bring down your usage, rather than try and replace with solar. Also you will likely generate excess power during the day, so any high load applications can be put on timers to run during the day, rather than relying on batteries. You can likely shut off just about everything apart from absolute essentials from 12pm to 5am. Get a power monitor and hunt down all the useless loads that are running 24/7 for no reason. All the appliances on standby, chargers, computers, etc etc add up very quickly to a good few kWh per day. 

Edited October 31, 20186 yr by DeepBass9

I don't know the nature of your load, but I think you have hard think about what is making you go for a three phase system.

If it's just because that's the generator you have, it might be better to recoup some costs selling it and starting out with single phase system. Or even consider rectifying the output to DC.

Single phase system can grow in capacity in steps, and I think you should consider what a single inverter failure would mean on a 3phase system as opposed to a single phase set up with three inverters.

You can grow in capacity 1 inverter at a time, not 3 at a time, that costs a lot.

 

 

 

 

 

Edited October 31, 20186 yr by phil.g00

True, I had 3 phase Escom, but just bridged it all to 1 phase when I went off grid. Unless you are using equipment that specifically requires 3 phase power, there is no need for it otherwise.

30 minutes ago, DeepBass9 said:

I had 3 phase Escom, but just bridged it all to 1 phase when I went off grid.

Hi Deep.. I had to read this about four times before I could understand what you were trying to say. Sat here imagining the sparks flying all over the place while attempting to get that bridge in place.. 😅

Er, not so complicated, the live of each phase is bridged, and then , voila! Three phase is one phase. I paid an electrician good money to install the 2 little wires. 

15 hours ago, Trober said:

R2000 for connection/service fees

That says it all. Understood, I agree with you :-)

12 hours ago, DeepBass9 said:

Unless you are using equipment that specifically requires 3 phase power, there is no need for it otherwise.

Over 15kva it is usually adviseable to go three phase. You're already pushing around 65A RMS at this point which is a lot for a single pair of cables. @Troberis starting with 15kva, so I suppose single phase is in order if there is no intent to grow the system. Any further.

13 hours ago, phil.g00 said:

You can grow in capacity 1 inverter at a time, not 3 at a time, that costs a lot.

You may also not be able to find a compatible model later. Additionally, though the Axpert is robust and dependable as Taiwanese/Chinese inverters go, it is much more common for them to fail compared to the European ones... and losing one phase in a 3-phase system is a deal breaker, but losing some power on a single phase is not a complete disaster. Call me biased and point out my allegiances if you will, but I would not sleep easy with this setup :-)

In other words, please don't build a three-phase setup with that... unless you buy spare inverters from the start.

Edited November 1, 20186 yr by plonkster

Thanks for all the good advice. Much appreciated. It seems I have a lot more to think about.

SOME MORE BACKGROUND

I live on a small holding east of Pretoria. There are 2 residences and both me and my tenant are self-employed with our workshops adjacent. We live and work here 24/7.I have no municipal services so I also need to pump and manage my own water supply, security, refuse et al. These are reasons for higher than average household usage. Water pumps, compressors, workshop tools are 3 phase (all motors have 3 phase dropout protection). The quite extensive power network is all wired 3 phase.

All lighting is LED and 2 of 3 geysers are equipped with Evacuated Tube collectors (very seldom requiring electricity) . All security lighting is on day/nite switches set to minimum "on" time. Gate motors, electric fences etc. are already solar powered by their own local modules (simple DC battery charging - no inverters). The only item that can make a useful power reduction will be the 3rd geyser soon to get its own solar collector. Two years ago when I became "energy" conscious, I was using 2000kWh pm, so at this stage I've effectively halved my usage. It's the "diminishing returns" problem making it ever more difficult to reduce even further.

The plan is to allow (for example) water pumping and compressor charging only during times with excess power from the PV array (after batteries charged) and/or when the generator has to run to make up shortfalls. This may be necessary daily. If so, the projected diesel cost will be around R700 pm. (not ideal but acceptable). Fortunaly the Deutz diesel seems in good nick and will hopefully last a few years during which time I can either extend the system or reduce energy usage.

 

 

 

1 hour ago, Trober said:

Water pumps, compressors, workshop tools are 3 phase

Aaah, so single-phase is out then :-)

Well I tell you what... those Axperts are good starter inverters. Once you want to upgrade, come talk to me :-P

(I don't like the Axpert, I will just be honest and say it. And I work for a competitor. Full disclosure. But I disliked them before that came about :-) ).

I would do a cost benefit analysis as to whether converting your tools to single phase is cheaper than putting in a 3 phase solar system. I had  3 phase borehole pump that I sold and replaced with a smaller single phase, which produces the same amount of water, just takes longer to do it. The 2.4kW load of the 3 phase pump was also problematic, so the 800W pump I am using now is better suited for solar.

Edited November 1, 20186 yr by DeepBass9

Plonkster,

I can get the Steca (axpert) for R10800 ExVAT. So that's R32k for 3 giving 12kW total available power. There's also some redundancy here since I can (temporarily) run single phase while doing repairs. The 3 phase gen can do the important 3 phase stuff - pump water etc.  (BTW, I'm planning later to get a VFD for borehole pump with it' own PV's)

I would also prefer a European type over Axpert. What inverter(s) would you suggest?

DeepBass9 has a good point. I will re-look at that option. Some smaller motors can easily be converted to 1-phase.

But I like the idea of having 3 inverters. Unlikely that 2 will fail simultaneously so I can get by until repairs done. With a single large inverter its total downtime if it fails.

I'd have to side with DeepBass9 here.

Ideally your generator will hopefully be phased out, so it should have a diminishing impact on your decision.

But there is still the option of a transformer/rectifier set up for extra power using the same generator.

Consider what 3 x single phase offers over 1 x 3 phase:

(I am assuming the same 3 5kVA inverters in different configurations)

1. You get to grow your set up say 5 KVA at a time, instead of in 15KVA jumps. What happen when you want a 20kVA ? a single phase system you can add 5kVA but 3 Phase you have to add 15KVA.

2. 3 Phase also restricts to single phase loads  combined to 5 kVA,  whereas for single phase you have all that load combination wriggle room up to 15kVA.

It will be a mean trick to balance your single phase loads and you 3 phase loads, in practical terms you are building in regular generator usage into the system.

3. With 3phase if one inverter dies, you lose 15kVA, with single phase inverters you can still limp along with 10kVA.

Maybe a phased approach might be worth considering. Go solar initially and power all your single-phase loads.  Borehole pump motors don't last forever, but replacing it won't break the bank.

Your system dictates that you'd probably have to start the genny for the heavier 3 phase motors anyway, so maintain that in the meantime and see what single phase options come along in time.  That way your tennant will have plenty of notice to adapt as well.

Edited November 1, 20186 yr by phil.g00

23 minutes ago, Trober said:

I would also prefer a European type over Axpert. What inverter(s) would you suggest?

My blood is blue. Not the rugby team though. I also don't drive Ford. But in other respects :-)

Thanks phil.g00,

Perhaps I've got it wrong but here's my thinking regarding the (3 x 1-phase inveters) vs  (single 3-phase inverter):

1) The only 3-phase(ness) about the 3 inverters is that they are synced to produce 3 off single phases in 120 degree phase angles to each other.

2) Each inverter (I assume) should be able to supply different loads (up to 4kW each) on its own phase without concerning itself with the other 2 phases.

3) A 3-phase motor will add equal load to each inverter and function normally (provided all 3 phases in correct phase orders are present).

4) In essence therefore, these are 3 separate load circuits, sometimes with equal loads, sometimes not.

5) Each phase can later be beefed by adding (in parallel) another inverter. This need not be done on all phases simultaneously.

 

My assumption 2) is where my concern lies. Can anyone confirm that the Axperts in 3-phase sync mode will allow this?

 

 

Off grid you will always have use for the generator. 2 or 3 days of cloudy weather are more common than you think. 

1) Correct

2) Correct, with the caveat that the 3 phase configuration probably can only be deployed if there is 3 inverters. I think you maybe at the mercy of manufacturers fine print if you tried to get two phases of a 3 phase system with two inverters. So in that sense they are concerned about the presence of an inverter on the other phases. I don't know, but I think if you were just to solely and constantly supply a single phase load with a generator, this would reflect back as unbalanced mechanical stress on the generator and shorten its lifespan.

3) Correct, so a 3kW motor would load each phase by 1kW.  However, in a 3ph system that includes 1ph loads, you will have to deal with a 4kW ceiling /phase. Now I don't know what happen's when 1 inverter shuts down, but again I'd guess that the two other phases are commanded to shut  down as well. (See 2)  

4) Correct, each with a 4kW ceiling.

5) This one I don't think you can make this assumption. Again I think you are in a master-slave relationship that has a prerequisite of equal capability per phase.

So I think you'll be restricted to not only adding identical inverters, but three at a time.

 

Edited November 1, 20186 yr by phil.g00

2 hours ago, Trober said:

2) Each inverter (I assume) should be able to supply different loads (up to 4kW each) on its own phase without concerning itself with the other 2 phases.

Not necessarily. In a perfectly balanced 3-phase system there is no current on the neutral wire, which means all the current arriving from inverter 1 actually goes through inverter 2 and then eventually through inverter 3. For delta-wired motors this is of course implicitly true as well. That means there is some level of "concerning itself with the other 2 phases". It would mean that in terms of size, it is much simpler if they are all the same size. No possibility of the stronger one making more current than the other two can handle. Or that is my take on it.