PV assistant - mapping between voltage and frequency

DIY
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1

Hi folks,

I’ve got an off grid system with two Quattro 48/10000/140-2x100 connected to BYD Premium LVL batteries and three 4kW SMA Sunny Boy inverters. Per https://www.victronenergy.com/live/battery_compatibility:byd_b-box#veconfigure_settings the float voltage is set to 55V and absorbtion to 56.5V. The PV assistant is set to reduce generation at 51Hz, minimum at 51.5Hz and disconnect at 52Hz. (This seems a bit unusual to me, should it not be reduce from 51-52 and off at 53?)

I’d like to use some frequency controlled dump loads which activate before the PV inverters reduce their output, and I have some dump loads connected to AC out 2 which I’d like to activate afterwards. I need to better understand the mapping between frequency and voltage so I can configure everything correctly. I’d prefer not to activate dump loads based on state of charge because I would like to allow the batteries to reach 100% SOC in order to give the BMS the chance to balance the cells correctly.

  1. Can I just change the “reduce generation” frequency to 50.5Hz and set my dump loads to come on at, eg, 50.5, 50.6, 50.7 Hz? If I leave the PV inverters set to reduce at 51Hz, this should start dumping load before reducing generation, right?
  2. I’d like to use the programmable relay to turn my AC Out 2 dump loads on and off, but this means I need to know what the voltage for e.g. 50.8Hz corresponds to. Is it a straight line relationship between the float voltage (55V at 50.5Hz) and absorbtion voltage (56.5V at 51.5Hz) so I should turn my AC Out 2 dump load on at 55.45V?

Thanks,
Rob

P.S. I see from PV Inverter Support Assistant [Victron Energy] some voltage levels used for limiting charge output, but this seems separate to the frequency control which mentions the absorbtion voltage. And perhaps the BMS will override the charge current anyway.

2

Hi, i use soc for my dump load control, and i can still reach almost 100% soc. My dump load have 2 stage low power and high power, low power is active always until soc is 86% then high power is active.
Script have SOC hysteresis of 5 % then SOC goes up, low threshold is SOC-5%, so if the system dont produce power, dump load shutoff after -5%. Script reset soc if it is below 46%.
The system is not perfect, but it works.
PS. It’s all a matter of priorities, my goal is to charge the battery and heat the water

3

Thanks! I set up something like this with the frequencies and it seems to work, but it has a serious downside that the dump loads flip on and off quite often because applying the load brings the voltage - and hence frequency - down. Eventually they do kind of duty cycle themselves about right and match the consumption to the surplus generation, but with many on off cycles and lots of small charge and discharge cycles on the battery. Voltage triggering the AC Out 2 relay just flat out didn’t work, need to try and fault find that when I’m there soon.

So I think you’re right - I need to respond to SOC and then I can set my activation and hysteresis in terms of the battery filling and emptying, and it will be a lot more stable as the loads will turn on and off in a predictable sequence. Working on a script to connect the Shelly relays to the Cerbo via MQTT to monitor SOC (and keep the dump loads shut off when on generator power!).