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Forum Index : Other Stuff : Current Limiting.
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| Davo99 Guru  Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
I have a 2 Kw GT solar inverter. I want to connect it to a load and limit it's output to around 1200W give or take. I was thinking of using some Halogen 500W tubes as resistors/ current limiters. 1st question, will a 500W lamp used as a resistor pass 500W @220/250V or is there another equation as I'd have to use these in parallel, perhaps with a 150W lamp also in parallel to get to the required wattage? I know it may be perfectly simple in that it does pass 500W but I also know the resistance will change as the lamp heats up so measuring cold is no good and paralleling may effect the current equation. 2nd question.... As I want to connect the lamp limiters to a solar Inverter, it will need to be connected to a current source, which is fine. Being that there will be power already to this circuit which is what I am trying to feed and eliminate mains draw as much as possible, will the inverter still try to push the full 500W and ramp up to do that or again will there be some electrical trickery at play and I'll get a current drop? 3rd question, if this is viable, and I'm not the least bit worried about wasted energy as I can use that heat anyway, how many 500W tubes do I need with a 150W lamp to get it close? Sorry, 4th question, Would I be better off with say a 1000-1200W heating element from a stove? Would a Fan heater work if wired in series with the load? Pretty sure I have one which I measured a while back at around 1200W on the low setting. Would be handy as it's all pre-packaged, easy to adapt and relatively safe. Thanks for any enlightenment! |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Any ideas?? |
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| mab1 Senior Member  Joined: 10/02/2015 Location: United KingdomPosts: 209 |
not sure i understand what you're trying to do: gti connected to an a.c. circuit and you want ~1200w out to minimise draw from the grid? I assume you're putting the lamp(s) between the current source and the gti? no, it'll pass about 2amps @ 230v drop across the lamp (amps=watts/volts). As to how many watts are going into (or out of) the GTI at that point? well that depends on the voltage across the GTI terminals (watts=amps*volts): at 100v across the GTI, that'll be ~200W. |
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| mab1 Senior Member Joined: 10/02/2015 Location: United KingdomPosts: 209 |
again, i'm not sure i understand the question: the inverter connected to current source+limiting resistor should track up & down within it's input voltage limits and find the point where it's getting maximum power. don't know; I would hazard a wild guess that 1 500w lamp (max 2amps) would give you a max power point at around 1.5amps. You may just have to suck it and see. of course, it you current source is PV panels then connecting 1200w of panels directly would limit the output to ~1200w - as i say: i'm not sure what you're trying to do. Edited 2021-01-20 04:20 by mab1 |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
I am not at all sure I understand what Dave is trying to do either. But one way to reduce solar power going into a grid tie inverter might be to simply disconnect enough solar panels "as required". If there is a high voltage series string, just tapping down lower in voltage, by bypassing one or two panels might work. The mppt will just find a new operating point at a lower voltage and power level. Disconnecting whole strings could presumably provide much larger changes in power. It would need to be well thought out so that continuous arcing of contacts did not result. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Sorry if I put it badly. Tried to make it concise, doesn't work for me! :0( I want to connect The GTI to feed the 3rd leg of my 3 phase AC. I want to limit the current because the meter it is on records any excess backfeed power as consumption. Therefor getting as close as possible to what that leg of the AC consumption which is about 1200W is the goal. It will still have to be wired to the control board of the AC unit but if the power is coming from the GTI is will save the draw on the grid power as normal as long as the GTI will push through the lamps used as the current limiting. I set up a GTI like this before on a relay and it worked fine just I moved the sequence of the 3 phase which the AC was fine wih as long as they were in order so the indoor unit was on a phase I'm feeding back. Now the phase connected to the meter I can't feedback pulls much less current than the 2 KW GTI wants to provide so I'd not be gaining much hooking it direct as before and I need to " Tune" the output somewhat. My other far more expensive and somewhat inconvenient option setup wise would be to buy a 2 phase to 3 phase converter. If I can get the same result limiting the output other ways, that would be preferable of course. Its more desirable to limit the AC output of the GTI to make that a constant. Limiting the panels would mean the feedback was all over the place with the weather where as If I put say 2KW of panels on the GTI and limit it's output, I have a lot wider ability to supply the 1200W the Compressor whether the GTI has enough input to supply 2.5 KW or even 1000w which would still be a good outcome. I want more power AVAILABLE to the GTI than what I will use most of the time to account for fading light at the end of the day when the solar will be fading off. That way I'll be drawing less from the grid later in the day. It would also be beneficial in winter when the radiation is lower and we use the AC for all our heating. The AC or this 3rd leg thereof is the only thing on that annoying meter than I can't feed back and it irks me no end that I'm shutting down inverters on the other phases because I have too much power but then having to take it from the grid on this one. I have looked for small GTI's but they seem hard to come by and the few I have found of course the people selling them seem to have priced them as if they were solid Gold. If the 500W Lamps will flow 500W when used in series with the AC, that's great. I take it that if I splice a fan heater also in one line from the GTI, that should work the same? I wasn't sure if putting the thing in series would give a different power output and what I looked up on the net was more confusing than clear. The weather is predicted to be in the 40s by the weekend so i'm keen to get this going! :0) |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Still none the wiser I am afraid Dave. I have three phase power here, and a "smart arse" meter. It just records an overall result of all three phases as a single Kwh consumption figure. If I were to feed anything back on any or all phases, it would just record that also as a single combined figure as well. Before the smart arsed meter was installed, I had a three phase spinning disc meter that also recorded a single Kwh figure. Cheers, Tony. |
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Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1022 |
Hi Davo, I got an idea of what you are trying to do...I think. Loading up that phase with the lights to use up the extra power the GTI is putting out, could work but not so easy to regulate, probably a switch board with many different size loads , and switch in/out what ever you need. A smaller GTI would do the job better, but you know all ready they want $$$ for things they shouldn't. One option is if the GTI has a built in current limit that could be changed to suit, most likely that this setting would be locked in and password protected, or it may not have this option at all. Sorry, not much help to you, just sort of confirmed what you already know. Cheers Aaron Off The Grid |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Thanks for the input anyway gents. I think I should be up to going out tomorrow and setting up a test for my theroy. I wasn't thinking of using the lamps to burn off extra power but rather use them as current limiting resistors. I would wire one output from the GTI straight to the load and the other so the power had to travel though the lamp rather than across both sides. The idea of using the lamps to burn off excess power isn't bad though. If the GTI is over clocked say with 3 KW of panels to keep it at max output and the lamps are sized so at to burn off 800W of that leaving me with the 1200w I want, Should work exactly the same. The GTI output and the consumption of the lamps would be constant and that's what I want. I ordered some current activated switches the other day. These too could help if I can set them well enough. Might be able to switch off the lamps as the power from the GTI Fell. I was also wondering about a PWM controller this afternoon. If I put one of them behind the GTI going to the compressor motor, I wonder if that would work? Not sure how it would go with the electronics of the GTI or if the compressor motor would not be happy? As it's 3 phase and a motor I might get away with it. Wish I had something to test it on first though. Thanks again. |
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| noneyabussiness Guru Joined: 31/07/2017 Location: AustraliaPosts: 512 |
Maybe poida could chip in with a arduino, mosfet driver and some code to make an adjustable pwm controller for GTI's. Its definitely doable, i have 4 on my mini grid all controlled by same method for battery charging.. very easy... nothing has been bothered by it on the output of them as yet.. A simple way is of course a resistor, bulbs will work burning off excess power, you could also try defrost resistors in fridges, usually built for these voltages and between 100w to 300w depending on the fridge. Easy if you are friends with the dump bloke, heaps there... I think it works !! |
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| mab1 Senior Member Joined: 10/02/2015 Location: United KingdomPosts: 209 |
I doubt that using the lamps as series resistance on the a.c. side of the GTI will work TBH; the GTI's have an a.c. voltage limit set in firmware (depends on country- e.g. here in UK it's 264v; in some eu countries it's 253v) and they stop exporting altogether if the voltage at their terminals exceeds that setpoint. Having the lamps between the pv and the d.c. input of the gti may well work ok, but will require some experimenting to get the output limiting in the right ball park. getting into the firmware settings and setting the export limit would be best - if you can. the current activated switches sounds workable. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Yes, that had occurred to me but hadn't quite got that far in the idea. I could not estimate if the draw of the compressor would keep the voltage down enough or the GTI would just keep trying to ramp the voltage to push all the power till it tripped out. I have some adjustable little PWM boards I got from Fleabay. I regularly use one to run a 3.6 Kw water heater element from a power point. I turn it down to about 1500-2000W and it is perfectly happy. Not sure if a 3 Phase compressor would be though. The thing with using the Pwm is I think it will be unfortunately Linear in it's output. If the GTI Drops from say 2000W output to 1500w, even though there will be enough for the 1200W I want, it will fall off on the output. I imagine the resistors will be a bit more like a water pipe. As long as there was enough water to fill the pipe, it would flow at full output regardless of how much water was behind it. Perhaps the question I should be asking is " How Do I limit a 2Kw GTI to always give 1200W output or whatever below that is available?" Anyone know of any ( preferably older) GTI's that can be turned down in the software? |
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| noneyabussiness Guru Joined: 31/07/2017 Location: AustraliaPosts: 512 |
I think you are referring to " surge " with the linear comment, which can be true, however there are capacitors inside your GTI that will cover a small surge ( the grid should cover it anyways) ... PWM can be implemented to mimick a " resistor " in series with incredible accuracy. My aurora inverters are " adjustable" via software, not sure about specifically 1200w but works off percentage of total output, so theoretically should be fine. Have you thought about just putting a specific load on the output ?? So say a 1800w inverter going flat out, with a 600w fixed load on output.. you may use some power when sun goes behind a cloud etc. but really easy to implement and solid. I think it works !! |
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| noneyabussiness Guru Joined: 31/07/2017 Location: AustraliaPosts: 512 |
aurora inverter communication A write up I did earlier... I think it works !! |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
I think you put me onto one of those RS plugs a while back and may have sent me some software as well. I got the plug but never got round to doing anything with it as I cured the problem I was having at the time with voltage rise by putting in a 3 phase 6mm Circuit. I have bought a bunch of new inverters dirt cheap that have the same RS port and the disk for the software but they have been working fine as plug and play so I haven't bothered messing with them. Maybe it's time to get out the lappy and load it up and connect the plug and see if I'm sitting on the solution all along. They are pretty basic inverters, JFI suntwins, and not sure they would do a lot but.... Guess I'll have to look and see. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Seems the Inverters I have are only " Tuneable" for safety parameters like line cut off voltage, frequency etc. Not surprised. Also found Vids and mention of a Current limiter that appears to interface with Inverters to Current limit but can't find them for sale on the net anywhere. Can't believe how damn difficult a simple thing like this is. I did find these Micro inverters that seem a reasonable price for the job and hopefully would last long enough to pay themselves off. Micro inverter I was looking for a quality brand but from what I can find, micros are now old hat and the name brands have all gone to power optimisers which have to work with a regular inverter. That blows the cost out straight off. These micros state an input current limit but I can't see any problem doubling the panels in parallel. Thing is going to wok like any other inverter and only pull what it needs anyway. Given the AC gets used more in winter than summer, I'd want to make sure there was double the panels to have a hope of getting the power I want out of the thing for decent time. Spose I can always put a change over switch or a relay on the output so when the AC isn't running, the power from the panels attached will be contributing to another phase and I won't have 2.4 Kw of panels sitting there going to waste. Couldn't care less in Summer but in winter, I need all I can get. Question then would be, do I run the new 360W panels with this or use the pile of 270s I have? Might be able to squeeze 8 of them on the verandah. Shades earlier than the rest of the roof thanks to the tree I hate but Mrs won't let me cut down but it might just have to get a severe haircut.... :0) Might also come down to load management. Run the AC a bit warm to heat soak the place while there is good sun so less is needed at night although the place does tend to cool off quick in winter. |
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