Immersun output switching rapidly

Evening all. I wonder if anyone out there can help me. I have an Immersun box which diverts excess solar current to my immersion heater element. It sort of works however when it 1st kicks in it pulsates the power very rapidly (probably 10 times/second) and gradually slows over say 5 minutes until the supply to the element is constant. I have had to replace the element once already, possibly due to the high number of ons and offs it was seeing. Any ideas how I can stop the rapid Immersun output power switching? There does not seem to be an option to set a dead-band for example? Thanks. David.

David - what PV inverter are you using?

The reason I ask is that I had a similar problem with my Fronius Primo, (and not an Immersun but a Mk2 diverter from @calypso_rae) but it arose because of export and/or grid voltage limits. In my case, I am limited to 3 kW export (7 kW system) and my HWS sits on the grid side of the inverter smart meter (so it isn’t seen as a household load).

What I think was happening was that the diverter and inverter were counteracting each other such that turning on the water heater triggered excess “export” and the inverter then reduced output turning off the diverter. This also affected the line voltage triggering additional feedback (we are on a long rural line that often sits around 250 VAC). This happened very quickly (every few cycles) resulting in a very low average PV output.

My inverter is a SMA Sunny Boy SB 4000TL-21. I am very far from being a pro but what you are saying does seem to make sense. Do you think that could be what I am experiencing? Also, can you suggest a solution as I am nervous to connect my immersion to the Immersun box as fairly soon the immersion element will no doubt fail again. Thanks, David

I’ve no experience of Immersun, but it does indeed sound like a feedback problem as Anthony says. As for a cure - there might not be anything you can do about it, my feeling is it’s something that’s going to require a software change either in the Immersun to control the rate at which it ramps up (or ramps down) the power to the immersion element, or in your inverter to control the rate at which that responds.

I think your only hope of a resolution is to contact both Immersun and your inverter manufacturer and explain the problem - though of course it’s highly likely each will blame the other.

Now if it was Robin’s or MartinR’s diverter, we could do something about it.

I’m not sure if my “solution” would work, as I have no experience with any other systems. Our installation is not typical in that the hot water circuit is located in the meter box about 20 m from the house and PV inverter. There is a voltage drop of a few volts when the heater is on (it’s a 20 A heater), and an increase of a few volts when PV export is at maximum (3 kW limited). So a significant voltage swing is possible as heaters and export control systems operate.

The first thing I did was to temporarily turn off the export limit control to see if this affected the heater. I could do this through the service access to my inverter after speaking with the manufacturer and installer. It was very straightforward - basically uncheck a box in the software settings page.

This improved the issue a bit, but after speaking with a Fronius representative they also said it could be due to their stringent grid voltage control systems where output is reduced according to the state of the AC line voltage. They have a number of setpoints where output is throttled down starting at eg 250 VAC and suspended completely at 260 VAC (I think). They said a new version of the software would relax/fix this and other issues, but I don’t think it has been released yet.

In the end I was able to fine tune a couple of the voltage/time limits which eliminated the problem - and also operated the Mk2 diverter in “anti-flicker” mode which takes the average of a lot more cycles before turning the SSR on or off. The export limit has no effect on the diverter operation now, and is kept to 3 kW as required by my connection agreement. The Mk2 diverter works brilliantly.

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Great feedback, thanks. I have a MK2 Immersun but can’t find the “anti flicker” mode. Maybe I have old software. Could you let me know where it is in the options? Thanks. David

Tel: ‭‭+33 7 87 26 27 85‬‬

Sorry David - The Mk2 is not an Immersun - it’s Robin Emley’s unit as described here Mk2

And full details of the DIY diverter are on Robin’s own website: