Regulating purely resistive loads - potential effects on a MultiPlus

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jbakuwel asked Apr 26, '20

Regulating purely resistive loads - potential effects on a MultiPlus

Hi,

I'm designing a power system for an off-grid house that has a number of significant power consumers: two hot water cylinders (1+1+2KW), an electric car, an electric oven (3-4KW) and various kitchen appliances (kettle: 2KW). The power system consists of AC coupled PV inverters and a MultiPlus-II.

A control system decides where available power goes at which time, for example if someone requests power for the kettle by pressing a button and the hot water cylinder is being heated, that will be turned off or, ideally, regulated down before the kettle is switched on. Once the kettle is done, the system can decide to switch the hot water cylinder back on.

We're now going into our Southern Hemisphere winter and sunshine is getting a lot more patchy.

The electric car's charging is controlled by an OpenEVSE controller which is following the excess solar power. I would like to do the same for resistive loads, for example with something like this.

It would be great if the HWC could be regulated down & up closely following solar power generation.

According to the documentation of this dimmer, dimming can be achieved by Pulse Skip Modulation:

Method 1 — One or more cycles (sine wave signal) are transferred to the load, while following one or several cycles are blocked.
Method 2 — Partial transferrence of each sine wave to the load.

There is a concern that the resulting AC current wave is "dirty" and possibly not "really" legal on the grid. Would anyone be able to comment if this dimming would have any adverse affects on the MultiPlus? There can be multiple consumers all being "dimmed" at anyone time.

regards,
Jan

Multiplus-II

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Kevin Windrem commented · Apr 26, 2020 at 05:02 AM

I have an espresso maker that pulses power to regulate temperatures. I think that's more or less your method 1. It gives my Multi Compact fits but it handles it for the brief brewing cycle.

Method 2 is the classic SCR/TRIAC light dimmer which does dirty the sine wave a bit. If you are switching 2KW, that's a pretty nasty hit part way through the cycle. Expect some buzz from the sharp current edge.

If your goal is to reduce average power, and have sufficient peak power, I'd recommend cycling the heaters slowly, maybe interleaving them.

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Modifications Space

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Answer 1 · 2020-04-26T06:15:45Z

Hi Kevin,

Thanks for your reply. I have a few goals:

balancing/regulating power consumption so a 5KW MP can happily look after the power needs.
reduce battery cycling, especially in the winter with the sun disappearing and reappearing.
prevent the PV inverters from switching off and then regulating themselves back to power demand levels when they detect a substantial decrease in load. This happens in particular when the batteries are nearing being fully charged.
increase the longevity of the system by preventing hits (big loads suddenly turning on or turning off).

If I would implement method 2, I'd look at using trailing edge dimming instead of leading edge. I don't have enough experience with this kind of electronics to properly assess what "hits" I'm introducing with the various methods.

There are also voltage regulators, such as this one. It could be that the name is misleading that it is also a TRIAC based dimmer but the text suggests otherwise.

I have thought about some form of interleaving, for example the first x cycles go to HWC#1, the next y cycles go to HWC#2. Then when power is requested for the kettle, x and y are dropped immediately to fit z cycles in for the kettle and once the kettle is finished, x and y are increased again.

There will be parts of the day when there's only one significant load though and then interleaving won't work. It would also be nice to have a "slow start" for all those loads as well: there's no need for the full 2KW of a kettle to kick in if that could be slowly built up over a number of seconds from 0 to 2KW.

cheers,
Jan

Answer 2 · 2020-04-26T16:06:04Z

Kevin Windrem answered · Apr 26, 2020 at 04:06 PM

The best way to regulate AC power is to pulse-width modulate the AC sine wave at 20KHz or more, then filter that. The result is a pure sine wave at reduced amplitude. That is what happens inside the Multi. It takes more circuitry but provides the best control including ramping up control.

I have not seen any issues with the Multi responding to an instant demand for power so I wouldn't be too concerned about simply switching on your loads. As I said, switching the load on and off a few times a second does challenge the Multi. A faster cycle MIGHT be less obtrusive but you also have the chance to do it more slowly. Victron designers might be able to provide more quantitive advise to you.

I have a fairly simple load control system I built to manage emergency generator power. It manages 4 loads and gets a "demand" signal from them allowing other loads to be shed BEFORE adding a new one if an overload would be created. It does require digging into the managed loads to get that "I need power" signal then interrupting the actual activation of the load. But it works very well.

Good luck with your project.

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Answer 3 · 2022-09-12T23:02:13Z

Lawrence answered · Sep 12, 2022 at 11:02 PM

I know its probably a bit late but MYPV have a product called the ACTHOR now that can proportionally drive the elements matched to the excess AC coupled solar. It uses frequency shift, so as the frequency rises to shut down the AC solar, the element power is ramped up. Its fully supported by Victron.

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