Power supply issue testing solar generator - Page 1
Forum section: electronics / general

author: artv
author_rank: Contributor
timestamp: May 11, 2022, 08:11:11 pm
content: HiI have a solar generator with 35-150V input, max 12A / 700W. I'm try to test this input towards its max - since I don't have enough panels or solar to get the full 700W.When I plug in my bench supply - and set to its max 60V / 5A it pulses between full 60v 5A and zero - constantly switching between constant voltage and constant current.If solar panels are connected it can receive well under 5A no problem - but it appears the bench supply is max'ing out current constantly.I've got 4 bench supplies and none of them work how I was hoping. The best result I get is two 10a 30V supplies in series - that can deliver 7.5A at 60V and I can drop current drop off ok. But if I drop voltage I get no current? Which doesn't make sense to me since the solar generator can support from 35V?I'm no doubt being an idiot but am I make incorrect assumptions or is it an issue with my power supplies - they're pretty cheap? Ideally I'd have a 100 or 150V 15A supply but can't afford it and not even sure it'd help?Any help much appreciated!Thanks.

author: Faringdon
author_rank: Super Contributor
timestamp: May 11, 2022, 10:57:18 pm
content: So you are putting your bench supply into the solar generator instead of the solar panel in order to test the solar generator?Well, the solar generator, would be doing MPPT on its input...because it would expect a solar panel to be connected...so i am guessing that this is happening with your bench supply....the bench supply is just too low output impedancefor your solar generator.

author: trobbins
author_rank: Frequent Contributor
timestamp: May 12, 2022, 12:13:00 am
content: Your bench power supply needs to be modified to include some series resistance (to provide voltage droop in the CV region of a PV module), and some shunt resistance (to provide current droop in the CC region of a PV module). The MPPT solar interface may then be able to move to a stable MPP. Say the MPPT wants to see voltage droop along a load line from 60V at 0A to 55V at 2.5A, then add a 2 ohm series resistance. Say the power supply has a 5A CC output, and you want the interface to see 5A droop to 2.5A as voltage increases from 0V to 55V, then add a shunt 22 ohm resistor across the power supply. The MPPT may then hopefully dither around a 50-55V 2-2.5A MPP.

author: artv
author_rank: Contributor
timestamp: May 12, 2022, 06:31:07 am
content: Many thanks. I can't say I really understand your reply but it sounds like the issue won't be solved by just buying a bigger better power supply.In simple terms - if I want to see say 2.5A at 40V going into the solar generator what would I have to do?I understand that the power station has an MPPT controller - but at 60V 7.5A it does exactly what I'd expect?Thanks again for your help!

author: Marco
author_rank: Super Contributor
timestamp: May 12, 2022, 06:43:56 am
content: Try giving it 45V with a 20W 2 Ohm resistor.

author: artv
author_rank: Contributor
timestamp: May 12, 2022, 08:15:07 am
content: Many thanks! That does sort of work - but the voltage and current do jump around still - with the power supply switching between CV and CC mode.How did you come up with the 2 Ohm 20W for 45V figure so I can play around with this myself? But at least I can confirm the minimum voltage required to charge the generator.Thanks again. By the way if I got a 60V 15 or 20A power supply would that help?

author: Marco
author_rank: Super Contributor
timestamp: May 12, 2022, 10:36:24 am
content: 2 Ohm*2.5A+40 and a little prayer. Simulating the nonlinear response accurately is a lot more effort than prayer.

author: Marco
author_rank: Super Contributor
timestamp: May 12, 2022, 03:47:40 pm
content: If you have a constant current load you can modify, it would be relatively simple to create a solar panel simulator with it. You can modify your power supply too, but I imagine that one is a little more expensive. Cheap electronic load kits abound.Say you want to simulate a panel with maximum power point at 40V/2.5A, set the power supply to ~50/2.5A. Load the power supply with a couple Watt 1K resistor. Drop the voltage from the power supply output with say 40V worth of zeners to a resistor, use that dropped voltage to drive the control opamp of the constant current load (to get an adjustable voltage drop you could have say a 100k resistor with an adjustable BJT current sink to drop it most of the way, then do the last say 10V with a zener, this way you don't need a negative rail, or with a negative rail you could just have the resistor and current sink).So first as voltage starts climbing the 1K load resistor will give a small downward slope in output current, then when the zeners start conducting the constant current load will gradually take all the current preventing voltage from rising much further (not as brick wall as the CC/CV transition of the power supply, so the MPPT can react). Very similar to a solar panel.PS. you can put a resistor divider at the bottom of the zeners to make the transition go a little slower.

author: trobbins
author_rank: Frequent Contributor
timestamp: May 12, 2022, 11:52:57 pm
content: Quote from: artv on May 12, 2022, 08:15:07 amAs well as the series 2 ohm, just add a shunt 22 ohm (60V 250W) across the power supply, to shunt a portion of current so that when the power supply is in CC mode then the MPPT interface sees a drooped level of current depending on the voltage it experiences. That means the MPPT sees a power supply with both CV droop and CC droop - much like a PV module generates.

author: Marco
author_rank: Super Contributor
timestamp: May 13, 2022, 03:22:23 am
content: The maximum power point will still be at the CV/CC switch point of the power supply, still unstable.Can't make the maximum power curve C1 continuous and purely in the CV or CC region of the power supply with just resistors.PS. well I guess you could just double the voltage of the power supply and use a very high value very high power output resistor (resistance wire in a bucket of water levels). Then the curve would be C1 continuous and in the CV region. But the curve wouldn't really look like the one from a solar panel.

author: trobbins
author_rank: Frequent Contributor
timestamp: May 13, 2022, 03:39:35 am
content: It depends on the MPPT algorithm (as to how it dithers or locates itself at/near the MPP), and how benignly the power supply transitions from CC to CV at what will be the MPP. Imho, the resistors are a simple way to test the total system response with what the OP has available. I basically used that technique in a 600V 20A SCR regulated power supply for testing grid-tie PV inverters.

author: Marco
author_rank: Super Contributor
timestamp: May 13, 2022, 07:33:57 am
content: It's a hill climber and it will always end up at the CV/CC switch point and oscillate around it. It works to a point, but it's not an accurate simulation of a solar panel. No amount of dithering will tell you where the intersection is between two (almost) straight lines looking at the points on only one of the lines.The series resistor does pretty much all the work to form the angle at the peak of the hill, the parallel resistor makes a wildly unrealistic simulation a tiny bit more accurate. If the MPPT can deal with the C1 discontinuous power "curve" it can probably deal with power increasing exactly as V*Ilim on the way up instead of 0.99*V*Ilim.

author: trobbins
author_rank: Frequent Contributor
timestamp: May 13, 2022, 12:25:18 pm
content: The OP wants to test that his MPPT interface can operate near the maximum power level that he expects (assuming he has interpreted the MPPT capabilities correctly). If the MPPT can stay at, or close to the MPP, then the transferred power could well be fairly constant, and hence confirm his power transfer expectations. I just think that is the simplest path to test that expectation.Whether the two simplistic slopes on either side of the MPP allow for a fairly constant power transfer through the MPPT, or whether more aggressive slopes are needed, is certainly not a given. But at the moment I think there are two many operating unknowns to say this type of test should be written off from the start.I just looked up the custom 600V 20A supply we made 11 years ago - as well as 1 ohm and 900 ohm droop resistors, we used a Picaxe to control the regulated power supply V-I output, so not that simple, but the intent was to closely represent a PV array.artv, have you checked if the MPPT interface needs to operate as a floating supply? Most grid-connect inverters specifically test for PV isolation from earth before they are allowed to start.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 13, 2022, 12:49:14 pm
content: Try connecting a MOSFET based current sink (constant current load) in series with the supply.