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Zero feed-in with ESS and heating resistance for leftover power - best way to control the resistance (PWM, leading/trailing edge, ...)


We've built a system with Multiplus II / Cerbo GX, Pylontech batteries and a Fronius inverter - and as the goal is zero feed-in, we added some heating resistances to heat up rooms and water for showers etc. with leftover power.

A RaspberryMatic instance is reading the live values from the Cerbo GX via Modbus/TCP (really great how easy it is to read/set values btw!) and controlling three 1 kW heating resistances via an ESP32 (Arduino-like, providing web-controllable relays). Currently, this results in possible steps of 333W, 500W, 1000W, 1500W, 2000W, 3000W.

This gives the possibility to realize almost whatever you want between the live values (pv power, ac consumption, ESS SoC, daytime, ...) and the ESP32 outputs by creating RaspberryMatic applications or scripts.

The first results are great, but - yeah the regulation steps mentioned above are quite big. It would be better to be able to regulate between 0W and 3000W without any steps :-)

So the question is: What suits best for the MultiPlus, especially with regards to micro-grid capability? I suppose that leading/trailing edge regulation isn't really good as it will have a big impact on the sine wave. Full wave control also seems like a bad idea, as it will stress the MultiPlus' regulation and/or the batteries.

The softest approach (also trying not to stress the inverters semiconductors too much) seems to be PWM with MOSFETs, which the ESP32 could also control easily at higher frequencies. But it will probably require filters to avoid EMI/disturbances in the grid, even with the resistive loads here..

Any thoughts on this are welcome!

ESSRaspberry PiModbus TCPFronius
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1 Answer
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jamierobinson answered ·

You could try getting in contact with Hugh Piggot from Scoraig Wind Electric. He has developed a system which controls AC heating loads from the PWM signals from a Tristar controller. He calls it a Tristar follower. I have used several of these in the past. The advantage is that you only need a single heating element.

I have also used a Siemens LOGO multifunction relay for load diversion to great effect. It will switch a series of small loads on or off progressively according to excess power generation. Very user friendly but you do need small individual loads.

Afraid I know nothing about RaspberryMatics. I am much better with a screwdriver than a computer!!

Hope this helps

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