Homebrewed Electricity > Controls
AC COUPLING, OFF GRID & MINI GRID & back charging batteries.
clockmanFRA:
A brief information document for those who have allways wondered.
RENEWABLE ENERGY.
‘OzInverter’ Mini Grid & 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 to function and back charge the batteries correctly. And thirdly It raised the 240vac HZ frequency from 50HZ to 60HZ and domestic equipment failed.
It went back, and the company put a gagging order on me not to talk about their crap toy Inverter.
'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 15 to 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........ . 150 to 300 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 ‘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 using DC Charge controllers for the 5kW as these can gently finish charging that precious 48vdc 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.
noneyabussiness:
There is also " method D "
Pwm the dc side of the inverter, effectively having a variable " cloud " on the OGI , controlled via a arduino..
Good write-up as always though... informative ..
clockmanFRA:
Hi noneyabussiness,
Yes a 'Method D' is a possibility, and i have seen them made and put into action.
The GTI input from the PV panels is controlled, depending of the state of charge of the battery.
However, i have not implemented the 'Method D' as i have no personal empirical evidence, and i have reservations on the 'Method D' when used with multiple GTI's.
Scruff:
Method C and D sound the same to me.
Method C would be my preferred
Method E: Frequency variation triggering.
Method F: "Let 'er buck" let the AC coupling inverters trip over-current/over-temp/over-voltage and self isolate taking down the GTI via anti-islanding...I don't recommend it as a method but it works...maybe not for very long if you're using cheap gear though.
Scruff:
Method G: AC diversion with GTI isolation at a secondary threshold.
ie. Heat water...if thermostat opens isolate GTIs
Navigation
[0] Message Index
[#] Next page
Go to full version