August 4, 20233 yr Hi All, Finally connected PV Wednesday late afternoon… Some background on my setup. Synapse 5.0K+ inverter and Synapse 4.8kWh battery with 10 x 420W panels in a 2S5P configuration. Power source/priority configured as SUB. According to the datasheet & label on side of inverter the MPPT specs are as follows: Min Solar Voltage 40Vdc Max Solar Voltage (Voc) 145Vdc MPPT Voltage Range 60-115Vdc Rated Current 80A Solar panel spec is as follows: 420W monocrystalline Maximum Power Voltage (Vmpp) 42.50Vdc Maximum Power Current (Impp) 9.89A Open Circuit Voltage (Voc) 50.50Vdc Short Circuit Current (Isc) 10.54A Installation is 10 x panels in a 2S5P configuration. This should give me a theoretical: Total max power of 4200W (slightly over the 4000W rating, but well within the 15% over-rating rule of thumb) Open Circuit Voltage of 101Vdc (well under the rated max of 145Vdc) Max Power Voltage of 85Vdc (within the 60 – 115 Vdc range) I’ve noted strange MPPT behavior (or at least not what I expected) where the PV power output seems to be very erratic/unstable when the output the PV can provide is higher than the load (example 09h00 – 09h05 and again 09h32 onwards). Example below: Early morning when the sun isn’t yet fully on the panels the load (idle load + pool pump) is higher than what the PV can produce. MPPT power output is relatively stable (doesn’t jump around as much). No power is drawn from the battery and shortfall is drawn from utility. At least the power blending (PV + Utility = Load) on the DC bus seems to work. At 09h00 the pool pump switched off for 5 minutes. This is where things start getting wonky and PV output becomes erratic. My theory is that this is because the potential PV output exceeds the required load. At 09h05 the pool pump switches back on, load is higher than PV production and PV output is stable again. This continues until 09h30 when the sun is directly on the panels and the possible PV production is higher than the load. PV output becomes erratic again. During these dips the PV supplies less than the load which then results in grid and battery power being used (cost and additional wear on the battery?). Has anyone seen this before? Any idea why this happens? A similar thing happened yesterday. The load coming on and off between 08h30 and 13h30 is pool pump (switches off for 5min every hour for the creepy to stop feeling sorry for itself in a corner somewhere). The higher load from 14h30 to 14h50 is a hair dryer and again a 2kW geyser from 15h00 to 15h30 (sun no longer directly on the panels). During these periods of higher load or perhaps when load is higher than what MPPT can provide the PV output seems to be stable. I assume the MPPT is then at max power point. As soon as the load drops below what the PV can provide the PV output becomes unstable.
August 4, 20233 yr Author Update. The plot thickens. As soon as utility supply falls away (loadshedding that started at 12h00) the PV supply smooths out even though load is very low (I have a contactor that drops out the non-essential loads during loadshedding). I still don't understand why this is happening, but is it possibly related to the SUB configuration?
August 4, 20233 yr Hi Handysmurf. This is normal behavior. The Solar is Following your load. Seeing that your setting is Solar/ Util/Batt the grid takes the load and is then replaced by the solar. It takes sometime for the mppt to react. When Eskom isn't available the batt will fill the gap until the solar can take over. I would like to know whey is your load so erratic?
August 4, 20233 yr Author Hi @Jacques Ester Thanks for the reply. Is the load really that erratic? Looking at the image below the load (blue) is fairly consistent at 1kW, but the PV (yellow) fluctuates between 706W and 1.51kW.
August 4, 20233 yr It looks better on this graph but the mppt is still chasing the load. I am running is SBU so my battery takes the load and then gets replaced by the solar
August 4, 20233 yr Author What is strange to me is that the PV output is fairy stable as long as the load is larger than the PV supply. In the image below you can see that the PV supply increases as the morning sun gets brighter. As this time the supply is fairly constant. As soon as the PV exceeds the load this behaviors starts. I expected the PV to chase the load and then once it's in a position where it matches the load is will stay there constantly. It's almost like it overshoots and then overcorrects.
August 4, 20233 yr 11 hours ago, HandySmurf said: It's almost like it overshoots and then overcorrects. I think that's exactly what happens. Is it easy for you to switch off one or two of the five strings? If so, you might try that during one of these erratic times and see if that settles it down. It might be that you have a little too much PV power for the MPPT to react correctly. The other problem is that the power needed from the PFC stage (the AC to DC converter that is unique to King models) has a demand that is constantly fluctuating, and can't go negative. At low power, at times the algorithm might call for negative PFC power, but it can't achieve that, so the solar charger has to react to fix the over-supply of power rather than have the PFC supply less. I wonder if fully patched firmware (coming soon!) might improve its stability. At low load, there may be a lot of "quantisation noise" from calculations in whole amps (in places) that are not using rounding with the various divisions in the calculation. We have a "divide with rounding" implemented, and use it sparingly so far. It might be possible to use it more widely if that helps with stability. But the problem is quite possibly in the solar charge controller, and we don't want to patch that firmware. Overall, I don't think it's anything to worry about, but it's an interesting effect and it would be nice if we can figure out a way to reduce it.
August 7, 20233 yr Author After a bit of playing around I found that this only seems to happen while in SUB. When in SBU the MPPT seems to behave itself. We had a nice sunny day on Saturday so could test without having to worry about cloud cover influencing results. Ran SBU the whole day except the period from 10h08 to 10h34 when it ran SUB. Can clearly see the strange MPPT behavior in this window.
August 7, 20233 yr Author On 2023/08/05 at 1:57 AM, Coulomb said: Is it easy for you to switch off one or two of the five strings? If so, you might try that during one of these erratic times and see if that settles it down. It might be that you have a little too much PV power for the MPPT to react correctly. @Coulomb Thanks for the reply. Easy enough to do (can just pull the fuses for one of the strings in the combiner box) to test. I would be surprised if this was the issue though as the PV power (power, voltage and current) at this time is well within what the MPPT is supposed to be able to handle. Unless the spike is too quick for the monitoring software to detect. I'll try this when I get a gap and let you know.
August 7, 20233 yr It's certainly something to do with SUB mode; the utility power seems to be too high or too low for the situation. What is your main (U1) firmware version?
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