Navitron Renewable Energy and Sustainability Forum

Energy/Electricity Storage and Use/Grid Connection => Off-Grid, Batteries & Inverters => Topic started by: billi on March 18, 2015, 04:37:49 AM



Title: Is AC-coupling more stress ?
Post by: billi on March 18, 2015, 04:37:49 AM
Hi , just wondering , cause i am planning to  set-up a off grid system on the canary Islands  ....

The inverter/charger in the Off Grid inverter (Victron 5000 or 8000 watt ) will then have to handle the charge of a 5-6 kw PV and the loads ..... is that more stress to the Victron Off grid inverter ?
Or
As you know , i  try to keep it simple ,  :P ;D   ,    one  MPPT chargecontroller could handle that 5-6 kw PV
I will have approx 21 PV panels  3 in one string  so approx 120 VoC  max 30 m wire  lengths

Any legal idea , how to combine 7 strings  into one cable  to combine them into the  charge controller ?

Thanks


Title: Re: Is AC-coupling more stress ?
Post by: camillitech on March 18, 2015, 07:40:08 AM
It is actually less stress on the inverter when you are feeding the loads directly. In fact there are times when you can be feeding a 5kW load and your 'off grid' inverter is doing nothing as the GTI's do all the work. Of course if you are storing the power then it's the other way around so it really depends on how you manage your loads. If it is to run 'aircon' and run your heavy consumers during the day then 'AC coupling' wins 'hands down', though I'm sure you know this. There may be some issue with 'microcycling' but this (as far as I know is just conjecture). If you are storing the power for later use then 'straight to battery' is the winner, again, I'm sure you know this.
Me, I use a combination of both, 'AC coupling' for inputs miles from the bank or operating at other than battery voltage and 'DC coupling' for stuff near the battery bank.

Good luck, Paul


Title: Re: Is AC-coupling more stress ?
Post by: biff on March 18, 2015, 07:47:43 AM
Hi Billi,
          The last time my wife and I were there on holidays, there were pv arrays all over the place. We stayed in Puerto Rico and I hired a car to drive all round the Island. The commercial hub on the coast outside Las Palmas is studded with 50kw+ wind turbines beside each commercial factory or retail unit but as you move round the Island to Puerto Rico itself, it seems that the big turbines are not allowed.  However there are solar arrays of all kinds and descriptions.
      As you drive towards Mogan on the coast road and look down towards the sea, you will notice Spanish villa type bungalows with corrugated type PV panels on their roofs. This is the only place that I have ever seen those panels. Even the hotel in Puerto Rico where we stayed, down near the harbour had a massive pv installation on its roof. It would be a great idea to get chatting to an installer out there because those guys have massive experience installing pv arrays.
   The wind turbines outside Las Palmas were kind of weird. There was one commercial estate of perhaps a hundred or so large factory units. Each unit had an approx 50kw wind turbine whose tower base was something like 20 to 30ft from its own commercial unit.  All the market gardening power was supplied by the same type of turbine as well. It was very impressive. So which ever Island that you go to, you will more than likely find the most recent type of gear and methods.
 My greatest regret was that I did not have a word of Spanish. The people were really helpful and obliging.
    You must take your camera and take some videos, and of course, "Keep it simple"  ;D
                                                                                                    Biff


Title: Re: Is AC-coupling more stress ?
Post by: freddyuk on March 18, 2015, 07:55:41 AM
This is how I did it with 4kw @ 8 strings into Blue Power 150/70. Each string fused. Not AC coupled. If you are off grid why AC coupled??
http://i1145.photobucket.com/albums/o517/freddyuk1/2014-12-13%2010.46.52_zpsvyzdx7k9.jpg


Title: Re: Is AC-coupling more stress ?
Post by: billi on March 18, 2015, 08:10:39 AM
thanks Paul  and Biff and Freddy

True ,  the possitive side of AC coupling is that , the GTI takes over the/some  loads during the day , but i am more concerned about   the Victron Inverter charger that has to take all the charge  then ... probably nothing to worry about ....

Freddy , is that string combiner box from Fronius?  (have you a link )... It looks  exectly  to what i need if i go with a Mppt charger  , 4 mm2 cable ? at what voltage ?

Thanks


Title: Re: Is AC-coupling more stress ?
Post by: camillitech on March 18, 2015, 08:36:56 AM
Billi, at only 30m why not just go 'straight to battery' and use some PWM diversion or the Victron switch. I'm sure that would be Rogeriko's solution and this is 'his forte'. I got 30m of 4 core 50mm SWA off eBlag for 275 delivered from Bristol to Raasay. 100mm square would be less than 3% loss at 114 amps and you cannot get simpller than that.

Freddy, great advantage can be gained by 'AC coupling' when you are 'off grid'.

1 You can integrate many and varied inputs over greater distances by just 'hooking up' to your 'AC bus'.

2  Grid tied equipment is often far cheaper and more efficient. For instance I have purchased a Proven 2.5kW grid tied wind turbine for 800 and it is now producing over 3.2kW in my 'off grid' system.

https://lifeattheendoftheroad.wordpress.com/2012/05/15/commissioning-the-powerspout/

It's not 'everyone's cup of tea' but is worth considering if you come across cheap GTI's or a 48v/110v/230v turbine and run a 24v system say.

Enjoy the holiday Billi



Title: Re: Is AC-coupling more stress ?
Post by: billi on March 18, 2015, 09:52:29 AM
Cheers,

If i go DC.....  with a cost efficient  MPPT controller  ....  ( PWM controllers are not easy to find  cheaper  at 80 A or 5000 watt PV )

I  will have  7 Strings  with 750 watt each string at about 120 VoC  with max 30 m more likely only 20 meter and a combiner Box like Freddy .... i guess i get away with 4 mm2 solar cable ? whistle

Any one a Power loss calculator handy ?

Billi









Title: Re: Is AC-coupling more stress ?
Post by: camillitech on March 18, 2015, 10:02:02 AM
I have this on my desktop http://www.paul-pelletier.com/LDCalculator/LDCalculatorPage.htm Billi.


Title: Re: Is AC-coupling more stress ?
Post by: freddyuk on March 18, 2015, 11:33:39 AM
I had a look at the cost of ready made combiner boxes and ran a mile!! Easy to make it up yourself. I can send you details.

I have done an AC coupled but problem is you need somewhere to dump the excess power as the Victron just switches on/off a load which comes out of the batteries. I am looking at getting a dedicated proportional controller designed and built for this application that will take the full PV AC input. No idea if it is viable. In the UK you need to keep the meter running to earn the FIT on an Off grid "non stand alone" install. In Ireland this is not an issue.


Title: Re: Is AC-coupling more stress ?
Post by: camillitech on March 18, 2015, 11:48:41 AM
I had a look at the cost of ready made combiner boxes and ran a mile!! Easy to make it up yourself. I can send you details.

I have done an AC coupled but problem is you need somewhere to dump the excess power as the Victron just switches on/off a load which comes out of the batteries. I am looking at getting a dedicated proportional controller designed and built for this application that will take the full PV AC input. No idea if it is viable. In the UK you need to keep the meter running to earn the FIT on an Off grid "non stand alone" install. In Ireland this is not an issue.

Hi Freddy,

in an 'off grid' set up you just put the meter on the inverter output, turn off any 'frequency shifting function' on you inverter and divert via PWM or AC relays. ALL the energy produced by your PV/turbine is then metered. That's how I do it, whilst not claiming FIT I am obsessive about accurately metering production.

Good luck, Paul.

PS, I would love a diagram of your combiner box as I'm planning another array.


Title: Re: Is AC-coupling more stress ?
Post by: freddyuk on March 18, 2015, 12:14:42 PM
Everything is going via the meter from GT inverters to Inverter charger so all measured. I need a PWM controller to only dump the surplus from the production directly to heaters. Does such a device exist taking battery voltage and then dumping AC direct from the GT's?


Title: Re: Is AC-coupling more stress ?
Post by: billi on March 18, 2015, 12:24:54 PM
... Lovely

Just found this one for cable loss .............. http://photovoltaic-software.com/DC_AC_drop_voltage_energy_losses_calculator.php    so 4 mm2 is sufficient  with 120 Voc

I will suggest  a 8000 kvA  Victron Inverter (there will be a 48 Volt air to air heatpump and an electric car )  and one chargecontroller  that can handle the 5000 watt PV ..... Its a "East-West" PV ..... mounting like this (http://img.archiexpo.com/images_ae/photo-mg/flat-roof-mounting-system-pv-applications-70009-3930663.jpg)

Do you think , i get away with one MPPT tracker in Lanzarote   with this mounting   ?

And sure Freddy  sent me details about the combiner box

Thanks


Title: Re: Is AC-coupling more stress ?
Post by: camillitech on March 18, 2015, 01:01:53 PM
Everything is going via the meter from GT inverters to Inverter charger so all measured. I need a PWM controller to only dump the surplus from the production directly to heaters. Does such a device exist taking battery voltage and then dumping AC direct from the GT's?

Some people have done it with Immersun type devices but why not use DC, if it's a resistive element it doesn't care, though you will have to come up with some kind of thermostat if it's heating water in a tank. My plan is to combine both, voltage controlled relays switching AC immersion elements on and off, then PWM to DC air dumps when the water gets too hot.

In the UK you need to keep the meter running to earn the FIT on an Off grid "non stand alone" install.

Sorry Freddy, I mistook this to mean your meter wasn't running all the time.

Cheers, Paul


Title: Re: Is AC-coupling more stress ?
Post by: billi on March 18, 2015, 03:28:33 PM
.... my outback controller  can do MPPT  and diversion load control  via PVM


Title: Re: Is AC-coupling more stress ?
Post by: nowty on March 18, 2015, 08:50:51 PM
I use an Immersun Type device with my AC coupled Off Grid setup and it works quite well. I use a time delay relay to give the batteries 1 to 2 hrs of further absorption after achieving 90% SOC.

Todays production with the device. Once the time delay relay fires, its up, up, up and the FIT meter spins till the sun goes down.  exhappy:
(http://s29.postimg.org/4gspdo0ab/With_Immersun_Type_Device.jpg) (http://postimg.org/image/4gspdo0ab/)

A similar sunny day last month without the device. Once your in absorption its down down down. facepalm
(http://s9.postimg.org/l2n92h2sr/Without_Immersun_Type_Device.jpg) (http://postimg.org/image/l2n92h2sr/)


Title: Re: Is AC-coupling more stress ?
Post by: billi on March 19, 2015, 04:45:46 AM
This should work as well  similar to FreddyUK  string combiner , or ?

for about 140 Euro

(http://i.ebayimg.com/t/DC-Freischaltbox-IG40-60-DC-JUNCTION-BOX-DC-FREISCHALTER-530-VDC-20-A-4-Strings-/00/s/MTAzMlgxNjAw/z/8zUAAOSwD0lUcxk3/$_57.JPG)


http://www.fronius.com/cps/rde/xchg/SID-A22DC308-8DD27FB7/fronius_international/hs.xsl/83_17025_ENG_HTML.htm#.VQpSz-EeQpY

Ok .... just for 4 strings  but still ... tidy


Title: Re: Is AC-coupling more stress ?
Post by: freddyuk on March 19, 2015, 09:11:29 AM
You need to fuse positive and negative cables if the system exceeds the panel limit and requires string fuses.
http://www.homepower.com/articles/solar-electricity/equipment-products/pv-combiner-box-buyers-guide

http://www1.cooperbussmann.com/pdf/1b416a65-f5ac-4730-ab77-9e2faa147945.pdf

The cost of carriers and fuses was crazy but now they are a fraction of the price. All you need is a 16mm drill bit for the glands. Also maybe worth considering lightning protection arrestor which can also clip on the Din rail. It will not stop a direct hit trashing the system but may prevent damage from stray high voltages from a nearby strike around the frames which can damage controllers and inverters.

As regards switching AC immersion loads - I think what you are doing is taking the power from the batteries via the inverter into the tanks? Relay on = 3kw load. What I want is direct feed from GT inverter supply only drawing surplus PV power (ie. less the onsite loads) when battery voltage is (100%) so no drain on batteries but meter keeps running heating cylinders (3 x 3kw) until they are hot and then a final air dump or whatever. I am still learning all this...