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Glad to have stumbled across this forum, looks like there is a fair bit of knowledge floating around in here.
I've recently had a 3kW (Xantrex GT2.8 and 3.04kW of panels) grid tie solar system installed on my home, it's been working great.
When this system was installed I got the sparky to wire in a change over switch that allows the manual connection of my Honda EU20i generator via heavy duty extension lead to the essential circuits within my home.
I had a chat with the solar guy and he was sure the Xantrex wouldn't sync to the generator but after some testing with him, he was proven incorrect. The Xantrex syncs to the generator output and produces power just as if it was connected to the grid.
This was a test that got me thinking about how to make the best use of the solar power during a grid failure and how to overcome the limitations of the current system, you see if the load doesn't exceed the inverters output at all times the inverter will attempt to drive the "mini-grid" voltage up to the point the inverter trips out on high voltage (265V), waiting 60 seconds once the "mini-grid" has returned to normal and then reconnects, possibly cycling in a never ending loop. I've previously gotten around this by either switching off 1/2 of the panels connected to the inverter to reduce power production or switching on more loads within the home, both of these methods aren't very efficient in terms of utilising power produced from the sun and minimising power generator from the Honda generator.
What is needed is essentially a 240V dump load which can automatically vary dissipated power based on the "mini-grids" instantaneous voltage level, quite similar in concept to that of a dump load used in wind mills I believe.
I did some research and think some large say 3kW resistive load (water heater or resistor bank) could be coupled together with some thyristors to form the variable load, add to this a circuit that can control the firing of the thyristors based on voltage feedback and we should be cooking!
So now I hope I've been able to explain exactly what I am trying to do and what I think the circuit should basically consist of, can you guys provide recommendations for particular devices, control circuit schematics or possible even off the shelf control devices?
I'm not really sure where to start in terms of getting my ideas into a working appliance.
Design specs I've considered are 3000W resistive load (should really only need to dump inverter max power output which is 2850W), ability to control say 15A which allows for a bit of a safety margin on that side of things. No idea on control circuit at this stage.
Cheers izy
VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539
Posted: 01:50pm 06 Nov 2011
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Hi Izy
I would try a 1/2 HP electric motor with a flywheel as a load stabilizer before going into a complicated electronic circuit. I have used this method on small generators with bad regulation running sensitive equipment, I haven't tried it on a GTI but it should help.
Yeah I did think about that idea briefly, but the problem is its really only good for a few seconds to maybe a number of minutes worth of "correction", I could possibly need the entire inverter's output (2850W) dumped for hours at a time, if there is not enough load in the house or none at all in the worst case scenario.
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 08:34am 08 Nov 2011
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Your inverter may not be too happy with a phase controlled load, zero load over the first part of each cycle then a heavy load over the last part of the cycle.
It may be perfectly o/k, or it may not be.
If your inverter has some kind of voltage feedback, it might freak out....
How about a motor driven 10 amp variac to use with your resistive dump ?
Full load control right down to zero, and resistive all the way.Cheers, Tony.
VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539
Posted: 02:23pm 08 Nov 2011
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Hi Izy
A HWS with a high set thermostat would be the easiest dump load at 240 volt level as the worst case is it would boil of a bit of water if you are not using any hot water.
All the best
BobFoolin Around
yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166
Posted: 12:41pm 09 Nov 2011
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Izy,
I can see where you are going with this, in small islanded grids they are called dynamic resistors. I have heard that the effect can be recreated with a high frequency pulse width modulation control on a heater or whatever.
In these situations the generator armature slows its rotation with increased load and this can be used as an input to control the dummy load, it is very effective with a diesel generator and wind turbines running together, as the turbines can spin loaded rather than shutting some of them down when the user load drops.
Your inverter generator puts out a stable 50 Hz, so that is out as an option, I cant think of anything that has a similar linear and direct correlation to your total system load.If the voltage difference between the generator and grid inverter is not kept under control the GTI output may start to hunt wildly.
I wonder if the grid inverter would let a small inverter-generator clamp it open and active on the tighter European voltage settings? probably not!
You must have read the document on King Island down Tassy way, your 100% correct my idea is exactly the same in concept but mine will vary the load based on voltage where as the king island one works on frequency.
PWM on a heater coil is basically the same as thrystor control but you bolt in a AC-DC converter first, it could potentially respond differently in control sense as well as you could operate it at many kHz.
I guess I should probably model the proposed circuit and get some kind of response from it, see if it's possible to build a stable system first.
yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166
Posted: 11:42pm 09 Nov 2011
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One of the wind farms near me is built by roaring 40's they are connected with Hydro Tasmania so their designers are all over the King Island thing. It is a big issue here, there has been at least 7 wind farms shelved in my area because the transmission lines have not got the capacity between here and a major city.
I found this on the selectronics accessories page recently.
AC Coupling ON Grid
A standard Kaco inverter can now be used as an AC MPPT in Grid Backup systems.
The AC Coupling method can significantly reduce installation costs by allowing the PV to be configured to 600V O/C. This also allows Backup to be added to an existing Kaco Powador 2002, 3002, 4202, 5002 or 6002.
Presently not suitable with other brands of inverter.
I assumed it involves an additional inverter/battery charger just proves the old saying "more ways to kill a cat than choking it with butter"
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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just proves the old saying "more ways to kill a cat than choking it with butter"