Homebrewed Electricity > Controls
Tell me about ac coupling
bigrockcandymountain:
Ok, so i have read a few timea now the benefits of putting extra solar through an ac grid tied style inverter and backfeeding your off grid system. A few details need explaining to get through my thick skull.
I would like to add about 2kw of pv to the new shop, and tie in to the existing off grid system. I already have 240v from the inverter at a normal panel in the shop. I also pulled wires to run dc from the future panels back to the house and in to a mppt charge controller to charge batteries with.
You guys are suggesting i could just use a grid tie inverter and connect to the 240v ac right in the shop. Here is what i don't understand.
When we have large ac loads and the battery inverter (magnum ms4448pae) is powering them the theoretical grid tie inverter would just help and take some strain off the battery inverter.
When it doesn't make sense is when i want the theoretical ac inverter to backfeed and charge batteries. My inverter can only be a charger or inverter, not both. So as soon as it switches to charging mode, the ac output dies and the gti would shut down too. How are you guys that do this getting around that?
DamonHD:
I think that I don't have the system layout that you are interested in, nor is it anything like a beefy as you'd want.
But ... I have separate off-grid and grid-tied systems each with solar PV and storage. Batteries are fed from a solar controller in the first case and AC-coupled in the second. The (tiny) off-grid system can take over a small load from the grid when the off-grid system has plenty of energy to spare. The grid-tied battery shaves flows to/from the grid but would not protect from a grid outage. Schematics are available for both.
https://www.earth.org.uk/expanding-off-grid-PV-system.html
https://www.earth.org.uk/Enphase-AC-Battery-2.html
Rgds
Damon
clockmanFRA:
Hi,
Firstly your DC to AC 120v or 240v Mini Grid Inverter must be truly BI-DIRECTIONAL.
Unfortunately most main Inverters add further complications into the controls that allows all sorts of trickery but also make things real complicated.
Okay this is my standard reply to AC Coupling concept when creating your own Mini Grid.
And Mods, sorry about mentioning OzInverter stuff, but just trying to explain a AC Coupling system that has been fully operational from 2015.
RENEWABLE ENERGY.
‘OzInverter’ AC COUPLING, and Back charging the batteries.
For true total domestic renewable independents, or as it is referred to OFF GRID or MINI GRID.
I have always seen AC coupling as the way forward, rather than just DC and many expensive charge controllers just feeding the batteries and powering a large Inverter that creates 240vac PSW, (pure sign wave).
Some time ago I originally bought a very expensive, so called Rolls Royce of commercial Inverters, 48vdc to 240vac, firstly it couldn't do the specified 6kW and found that it could only do 4.2kW constantly. Secondly it needed a lot of expensive propriety ancillary control equipment integrated into the system to function and back charge the batteries correctly. And thirdly It raised the 240vac HZ frequency from 50HZ to 60HZ and domestic equipment failed. Basically it was trying to be a 'Jack of all trades and master of none'.
It went back, and the company put a gagging order on me not to talk about their crap toy Inverter.
2014-15, 'Oztules', on Flinders Island, Tasmania, stepped in, and like me he wanted a true and real Inverter that would do the AC coupling using a H bridge design, and without all the fuss. Our Mantra .... KEEP IT SIMPLE, MAKE IT ROBUST, and importantly MAKE IT COST EFFECTIVE.
And hence we designed and created the efficient and powerfull ‘OzInverter’, that you can make your self for about 800 euros. And yes, I can supply all the necessary PCB boards and a DIY Book. See …………. http://www.bryanhorology.com/ozinverter.php
A very cost-effective solution for handling that 19kW of power from the PV panels arrays, is to use what is called 'AC Coupling'.
Houses that you see around the World with PV on the roofs will be using GTI's, (Grid Tied Inverters) that take the PV DC output and change it to AC and feed it back at a few volts more into the Mains Utilities Grid for that Country.
These GTI's are slave machines and need to see the correct AC voltage and the correct HZ frequency, so to get their internal electronics/transformer to operate before they will feed into the Grid a few volts over the Mains Utilities supplied voltage.
There are many Good Quality GTI's available second hand, i use fleebay etc, and i prefer GTI's with a toroid transformer, see below photo of previously used SMA's........ . 80 Euros each.
Now our 48vdc ‘OzInverter’ creates a very stable 240vac or 120vac at a stable 50H or 60HZ, depends on your country domestic voltage, and therefore allows 'AC Coupling'. The GTI's back charge through the ‘OzInverter’ to the battery bank any surplus power not being used on the OFF GRID or we call our ‘OzInverter’ created MINI GRID.
Three methods of controlling the back charging from GTI's is possible for 'AC Coupling'.
A. Use the Internal codes/settings in the GTI's to sequential shut down when a specified ac output voltage is reached. This works reasonably well with my system as some installations GTI,s are up to 400 meters away from the ‘OzInverter’ and batteries, and the batteries do push back slightly, but this depends on the cable voltage sag.
B. Use dc voltage comparators circuit to shut down the GTI ac side with a relay when the DC battery voltage rises above a charging rate voltage. But you will need to run a data cable to all your installations, and again allow for voltage sag.
C. Use PWM dc controllers that are connected directly to the 48vdc battery bank. These Diversion controllers, (Morningstar Tristar PWM at about 200 Euros each), will regulate the charging and any excess power when the batteries are full and will dump/divert to other permanently connected sources, ie, Air Heaters 2kW each or Hot water heaters, underfloor electric heating, etc.
The above are a cost effective and very reliable system.
A. is used most, and C. is my guaranteed safety system. Where i do not have access to the GTI internals, then i use B.
For a 19kW Array I prefer using 2off 1.7kW & 4off 2.5kW GTI's for 14kW, and i only use DC Charge controllers for the 5kW as these can gently finish charging that precious 48vdc battery bank. I also have 3off Hugh Piggott design wind turbines, DC coupled direct to the 48v battery bank
NOTE, I have not used a HF, High Frequency GTI, because second hand old heavy toroidal types are easily obtainable and very cheap, plus you get to use the GTI internal MPPT . 'Oztules' and others have experimented with HF types and do not find any real difficulties.
NOTE. Its best to use GTI’s at a max output of 2.5kW as above this the GTI tends to surge on a long cable connection.
I trust this information helps. I can go into each mentioned process far deeper if you so wish.?
So is your Mini Grid Inverter truly Bi-Directional.?
bigrockcandymountain:
Perfect. Thanks clockman for a great write up as always.
No, my inverter is not really bidirectional at all. The AC input(for me generator power) is a separate set of terminals and when the inverter sees power on there, a transfer switch actuates, and the inverter becomes a charger and the generator power just flows through the inverter and out the other side to power loads. The inverter siphons off some power to charge the batteries. It works well, but it seems like coupling ac to the output side will not work with this inverter.
I'm still not there on the building an oz inverter but i think I'm getting closer every day. What is the learning curve like for someone with very basic electrical knowledge, zero code writing knowledge. Etc? Also, is 120/240v split phase north american power more difficult?
The inverter i have is rated at 4400w and about 8kw surge. It won't start a 3hp single phase induction motor though. Getting an ozinverter ready for when it gives up would be a good idea.
clockmanFRA:
No, you don't need to write any code, as we use the 8010 SMD chip which according to 'FRACKERS' has about 64k of code already written in it.
Its the soft start that with large toroid's needs to happen, because a toroid can pull huge surges.
You just add in a centre tap when you are winding the secondary on the bare toroid, so 130 turns then take a tap at 65 turns, this means that all the control board stays the same, but with the taps you get 120vac on each side of the tap and 240 on the main secondary winding of say 130 turns.
I explain and show lots of photos etc, in the book.
Sometimes its best if you have a reasonable mechanical engineering skills as this is real POWER electronics/electrics, and some just electronic guys find it a challenging doing the chunky stuff.
The 3 PCB's are all through hole components and all readily obtainable, and the only SMD 8010 processor is fitted on a daughter board. The PCB tracks are wide and large and the solder points are reasonable large to avoid PCB substrate separation. The PCB's were deliberately designed to be easily repaired and components replaced with out difficulties, ie we kept it simple and robust.
Anyways i point out the critical bits.
Trust this helps.
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