Electronic Circuits and Projects Forum

What does your GTI pull the 235w panels voltage down to when its running?

If it was me I would just use a zener and a fair sized NPN transistor or similar setup to make a simple voltage limiting shunt of sorts.

I would suspect that to pull the panels OC voltage down from 34 to 28 may not take all that much actual current.

I connected a 235W and 80W panel in parallel and it seemed to work

As the panels have different ouput voltages (34V v <28V) presumably you have some way of preventing reverse current flow through the lower voltage panel?

I think the voltage comes down to 17 - 18V, when I was measuring current (like 5Amps in bright sunshine) it didn't seem very high for such a large panel but my DMM had a flat a battery which I later found affected it's accuracy. I have some high power resistors I was going to try loading the panel with tomorrow to see how much it takes to bring the voltage down. Obviously that isn't the ideal long term solution, just to see how big the transistor needs to be. Typically the panels came from left over from a 340kW solar array and so not exactly made for home use. Ideally I want to add panels as I go along, I'm guessing there isn't an issue with connecting different power rated panels in parallel as each has a diode in series with them. The only thing I can think of is the MPP for diff size panels is different.

Another question I wanted to ask was i know I can connect pretty much as many GTIs onto the same mains supply but is there any issues with connecting them on the DC side too, 235+235+80+40W panels connected in parallel to 350+350W GTIs with DC inputs and AC outputs joined together?

Alec_t, all the panels have blocking diodes in series so no issues with reverse current etc.

Thanks,

Jules.

At 34 volts OC I would take a guess that they may be rated to produce around 9 - 10 amps at 24 VDC. If your GTI needs to see 28 volts or less the panel may only need a amp or two of load to keep it at 28 VDC so from there you may only need to dissipate around 60 watts of power which is not much for a common TO-220 or larger sized transistor to handle.

The thing is if your GTI's are pulling the panels down below 28 volts when running the voltage limiter wont be getting used at all.
Also if the system is hooked up all the time I would assume that every morning as the sun comes up the GTI would be coming on automatically while the panel voltage was well below 28 volts keeping the voltage down anyway.

Thats my take on this so far.

How about som pics for us?

I agree, the voltage limiter would hardly ever be used. I can imagine if the inverter restarts or the mains goes down it would release the DC to rise and get stuck again. I did some more experimenting today, while it wasn't raining and found that with just the 80W panel connected it drew 5-6Amps. With the 235W connected after, the current only came up to 7-8Amps. I think that the 80W panel isn't really working very hard once the 235W is connected. I haven't got enough DMMs yet to measure every parameter and annoyingly my meters can only measure up to 10Amps. I haven't found the box of high power resistors yet so made a current shunt with some copper wire to be able to measure higher currents than 10A. Seems to work, see pics. I'm going to build a triangle mounting to make the panels face into the sun better. And make up a board with all the GTIs, switches, elec meter etc on.

It might be better to have the larger panels separate from the smaller ones.

I've got a pretty new electric meter (Landis Gyr E110 Type 5235A) which appears to be the basic model which isn't capable of measuring power flow in both directions. It has a pulsing red LED which blinks for every watt-hour I use and goes to solid red when I try to feed back onto the grid - I think that means it won't run backwards when I produce more power than I use. Annoyingly it doesn't have tenths of kWh so I can't easily see it running backwards. I've bought an electric meter to put on the same board as the GTIs. I think if I switch everything off and measure 1kWh from the solar panels with my bought elec meter and compare it with if the E110 meter registers a decrease. I also plan to have a PIC circuit to log DC power / AC power generated from the array.

Jules.