Three-phase water heater & PV diverter

Getting Started
1

Hi all,

I’m writing this post on the recommendation of Robin Emley. I’d like to buy his PVRouter and use it, because we’ve some technical question.
I live in France and my house has a 3-phase power supply from the grid because of 2 devices:

  • the heat pump (Stiebel Eltron WPF16) can only work in 3-phase because of its power
  • the water heater, connected in 3-phase Y. Its power is 3kW distributed equally on all 3 phases.

Since a couple of weeks, I’ve a PV installation (6,175 kWp) which is 3-phases too.

In France, the energy meter calculates the power taken on each phase, sums it, and you’ll get the resulting power used. If the sum if positive, the electricity is taken from the grid, if it’s negative, it’s injected into the grid.

So now, I’d like to control my water heater through a PV-Router so it’ll take only at most the surplus of energy produce by the PV-Panel and not used yet by other devices in the house.

Our idea (from Robin and me) was to connect the water heater to all 3 outputs of his router. Would it work properly? what must be changed in his router (hardware changes and/or software changes)? As far as I know the load must stay fully balanced over all 3-phases.

thx in advance for your help
Fred

2

Does that mean that you have a neutral connection, or not? If the three heating elements are connected in star, it’s not necessary to have a neutral connection, but you could have one. If there are three wires (brown, grey, black) plus the protective earth connection (green/yellow), you don’t have the star point connected to the neutral. If you have a blue wire as well, you do have a neutral connection.

What you might need to change will depend on the answer to that.

3

Exactly, there’s no neutral connection for the water heater.

But I think, I could connect a neutral cable in the middle of the Y. As long as the device stays fully balanced, no electricity will flow in the neutral connection.

4

With no neutral connection, you will only need to switch two of the phases, you can leave the third phase connected (only switched by an isolating switch and circuit breaker for protection). If you switch one other phase on, you will have a reduced load of 1.5 kW spread equally over the permanently connected phase and the one that is switched on. This means that you can have three stages of loading: zero, 1.5 kW or 3 kW. It is not necessary to load the three phases equally: when you are using other things, I can almost guarantee that the three phases will not be loaded equally. But your power company would like it if you did - especially if that was your total house use and not just the immersion heater.

If you and Robin between you decide to switch all three phases and there is no neutral connection, then you have a choice of: zero, 1.5 kW spread equally over your choice of any two phases, or 3 kW spread equally over all three phases.

If you have the neutral connection, then you must switch all three phases. Then you have a choice of: zero, 1 kW on one chosen phase, 2×1 kW on your choice of 2 phases, or 3 kW spread equally over all three phases.

If you have a neutral connection and you can switch it but leave one phase permanently connected (I don’t know if that’s possible with Robin’s PCB design), then you have a choice of: zero, 1 kW on the permanently connected phase, 1.5 kW spread equally over your choice of that permanently connected phase and one other, 2×1 kW on your choice of that permanently connected phase and one other, or 3 kW spread equally over all three phases.

Each of these might allow you to achieve a better overall balance for the three phases for part of the time; also you might be able to reduce flicker, because instead of switching 4.3 A on each phase simultaneously, you would be switching between zero and 3.75 A, or between 3.75 A and 4.3 A, or between zero and 4.3 A on one or two of the three phases.

That is indeed correct, but does it matter? I would say it is good when that happens, but it is not essential with loads of only 4.3 A per phase.

If you connect two heating elements in series (with no connection to neutral), then you will be putting a 860 VA reactive load on the two phases, inductive on one and capacitive on the other. The inductive load will cause the voltage to drop slightly, the capacitive load might cause the voltage to rise slightly. But that should not cause a serious problem.

5

Thx a lot for all the details. But unfortunately that’s not really what I’d like to do. I don’t want such a discret variation, I’d like it in continuous, so even I’ve a surplus of 500W, I’d like to “inject” these 500W to the heater.
It seems the czech Watt… Mx can do that (3f mode).

6

By turning on the load for one sixth of the time ( = 500 / 3000), you will average 500 W to the heater. Having only today looked at details of the WATTrouterMx - that too appears to work on exactly the same principle.

When people buy a commercially produced “closed” design, they usually find that it works well provided that it will do exactly as they want. If it will not, there is no room to manoeuvre. With a design where all the details are published and it is possible, with sufficient knowledge, to adapt the algorithms to suit your needs, that difficulty is surmountable.

7

Yes you’re right. Since I don’t need all the functionality of the WattrouterMx (and to minimize the costs), that’s why I’d rather buy the router or Robin.
In fact the point is how did this 3f mode work. Are all 3 phases turned off/on simultaneously? Is it burst, phase control?

8

Can you clarify what you mean by “3f” mode? And which device - the Mk2PVRouter or the WattrouterMx?

I cannot answer in detail for the WattrouterMx, but Robin can adjust his software so that it works the way you want it to. His units normally use burst mode, but he does have phase control software that can be used with commercially produced power switches that incorporate the necessary filters.

What I suggested in post no.4 would not make the phase balance worse, and would possibly improve it by allowing the immersion heater load to correct an imbalance created by other appliances in the house, and at the same time dumping surplus PV production into heating the water. Burst mode will always have the potential to create flicker, by arranging the loads so that, when PV production was low, a lower current was taken, the rate of flicker could be reduced and so would the disturbance that flicker creates.

It would all depend on how clever the control algorithm can be made. Robin’s unit always monitors the grid connection, so it always knows the energy flow in each phase. If it has (say) phase currents of 3 A, 2.5 A and 1 A exporting, it should be possible to connect two heaters in series between phases 1 & 2 to draw 3.75 A, and the currents when the dump load is on will be 0.75 A, 1.25 A importing & 1 A exporting. The dump load will be turned on and off so that the nett energy flow to the grid is zero - if my maths is correct, that will be with an on-off ratio of 6.5 - 1. Alternatively, if the dump load is treated as a single balanced load, the currents when the dump load is on would be 1.3 A, 1.8 A & 3.3 A importing.

9

I mean the 3f mode of the WattrouterMx… It seems it’s intended for 3-phase water heaters connected in Y. The doc/FAQ says too, a neutral cable must be added.
I don’t think I could encounter any flickering effect. When my heat pump starts, even with much more load than the water heater, I didn’t notice anything till now.

10

That sounds good. You should expect the flicker from a dump load to be more noticeable than starting the heat pump, simply because the heat pump starts occasionally, while the dump load will operate continuously - but only during daylight hours of course, when the need for indoor lighting is less. So we can hope that flicker will not be a problem.

11

That’s not so easy to choose which router to select. Beside this heater “problem” I’ve already in my house an energy manager (SMA Sunny Home Manager) to control my dishwasher, my washer and my dryer through EE-BUS.
I can control my heater too (via a 3-phase relay controlled by a smart outlet controlled by the energy manager). Unfortunately it’s a on/off control.
My dream is too optimize the whole stuff :stuck_out_tongue_winking_eye:

12

If you have another unit trying to do the same thing - you could be heading towards big trouble. Another member here had exactly that problem not very long ago - each unit was trying to optimise the other one! I solved the problem for him by suggesting he changed the way the two systems were wired, so that one (I think it might have been an EV charger) was seen as part of the normal house load by the PV diverter. Unfortunately, I can’t remember the title of that topic - nor any other details.

So provided that the Sunny Home Manager cannot see the PV dump load, and Robin’s (or anything else) sees the Sunny Home Manager and the dishwasher etc as part of the normal house load, the two systems should work together and not against each other.

13

I’ll try to take a look at the EE-BUS protocol and see if I can add it to Robin’s router. That would be probably the best solution…
Through this protocol, depending which program I selected on each of my appliance, the Sunny Home Manager gets information about the electricity needed for the active program. Then depending on weather forecast/priority/price/… The Home manager will remotely start the appliance.
The Home Manager has an internal meter to measure the PV energy produced on each phase (+ sum them all) and the SMA energy meter provides the home manager the energy taken from the grid or injected into the grid.

14

I do not think this will be necessary. Provided that the PV dump load can be connected on the supply side of the point where the other device measures its grid current, and the Mk2PVRouter can measure the total grid current, the two systems can exist quite independently. The Mk2PVRouter will adjust the dump load automatically so that the other appliances have priority, but only when the Home Manager has made them to be active. And remember, the Mk2PVRouter will react to a change within 40 ms, and adjust the energy dumped accordingly.

As I understand it from what you have written, if you feed the Home Manager with information about the state of the dump load, you might find that your other appliances are being switched on and off every few seconds.

15

No, the Sunny Home Manager knows about what kind of load it is. For exemple if I define the load as a heater, then I’ve to enter the power rate, the minimum working time when it switches on, then the minimum time between a off and the next on. For the appliances, the Sunny HM switches them on (well not exactly, it sends a “switch on” command to the appliance through EE-BUS as I could do it with the Homeconnect app) and the appliance informs the Sunny HM when it’s finished. My appliances are always on so they can be remote controlled any time.

16

What I am trying to say is, the Mk2PVRouter will adjust the average power taken by the dump load so that, when possible (that is, there is a nett export of power that is less than the power that the dump load can absorb), the average power flow to and from the grid is zero. When the nett export of power is more than the power that the dump load can absorb, then energy will be exported to the grid, because you can do nothing with it. If under those conditions you or your Home Manager decides to start an appliance, the Mk2PVRouter will still, even though it does not know anything about the other system, try to adjust the dump load to give maximum heating while maintaining an average zero power to or from the grid.

17

just out of curiosity- since I do not have ready access to 3 phase. how are they pulsing the diversion. just pulse one phase or pulsing sequentially after each cycle or several cycles – to limit wear on the element I would probably pulse sequentially - something like wear leveling on a SD card…

18

That’s up for debate. Fred would like all 3 phases switched together, to apply the full 3 kW load to all 3 phases equally. In that case, the wear on the elements will also be equal. But as I pointed out earlier, if less than 3 kW of PV is available, it’s possible, by connecting just two phases, to have two heating loads in series to give 1.5 kW shared over two phases - and if those phases can be chosen to be the most lightly loaded pair, then it will likely improve the balance between phases.

I would guess that the 3-phase heater is actually 3 separate 230 V, 1 kW units, just wired in star, so replacing one should not be a problem.

Neither do I, nor most domestic consumers in the UK! Here, only very few houses are not on a single phase, 240 V supply. A 3 kW immersion heater is common here, drawing 12.5 A. So I don’t really understand the concerns with an out-of balance current of just 4.3 A.

[Ignore the "Test response … " link, something funny happened between Discourse and my browser.]

19

Just to understand, the 3kW isn’t a problem at its own. I’ve 3-phase at home because of the heat pump, it can only work like this. Then the heater is 3-phase to be able to have a cheaper contract. If the heater would be single phase then I’d need 18kVA, so 3x6kVA, limited at 30A for each phase. If the heater is 3-phase, then I can go down to a 15kVA (limit of 25A for each phase) contract which is like 20% cheaper!
I also think about putting my induction plate to 2-phase.

20

I always understood why the the pump motor needs a 3-phase supply, and why you can get a cheaper tariff. Everything I’ve written supports that, nothing I have suggested would make it worse.

I am not - was not - suggesting putting all three phases of the immersion heater onto one phase, but I do think that using one, or two, or three phases selectively when there is less than 3 kW of surplus power will be an improvement, if it is possible to design and implement the algorithm to control it.