Guru
Joined: 23/10/2015
Location: FrancePosts: 437
| Posted: 09:12am 15 Feb 2021 |
rogerdw,
I have not built a 'Warpspeed Inverter' yet. I am watching you with great interest. I just do the OzInverters.
AC COUPLING , or Back charging the batteries
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.
I have always seen AC coupling as the way forward, rather than just DC and many expensive charge controllers.
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 constantly only do 4.2kW, 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 there crap toy Inverter.
'Oztules' stepped in, and like me he wanted a true and real Inverter that would do the AC coupling using a H bridge design without all the fuss. Our Mantra .... KEEP IT SIMPLE, MAKE IT ROBUST, and importantly MAKE IT COST EFFECTIVE.
A very cost effective solution for handling that 19kW of power from the PV array, 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 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 there 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.
The 48vdc OzInverter creates a very stable 240vac or 120vac at a stable 50H or 60HZ and therefore allows 'AC Coupling' the GTI's back charge through the OzInverter to the battery bank.
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.
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.
I trust this information helps.
Edited 2021-02-15 19:25 by Clockmanfr