Solar power system technical discussion only

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#3117956 19-Aug-2023 12:53

SteveXNZ:
So what’s the consequence of this? Nothing practically, until the power bill arrives. “Net metering” says my 3-phase meter must independently record import and export – no problems with that. But what doesn’t happen is “net billing”. I’m on Electric Kiwi’s Movemaster plan which applies differential time of day tariffs. If I use zero net energy over say the 7am-9am peak I should pay $0. Not so fast! I’m actually paying 40c/unit for what I’m importing, and receiving a 14c/unit credit for what I’m exporting! This makes no sense at either the transformer/utilities level or EK’s spot market level. I have an on-going dispute with them and have asked them to justify their billing practice and make it clear on their website.

How does the current flow actually work in terms of what the power company sees? This is the same problem I've pointed out earlier, the ESS tries to push power out to the grid to balance the draw from the grid, but wouldn't the outgoing power from the ESS get drawn straight back in again as incoming power to the load before it hits the meter (I've forgotten pretty much all of the AC circuit theory I learned way too long ago, not sure how accurate that is).

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#3117959 19-Aug-2023 13:21

SomeoneSomewhere:
Not being familiar with these systems, what you'll tend to find is that the system aims to zero out power flow through a point - that point being where the measuring current transformer (CT) is clamped around a cable.

Yup, that's exactly it, it's trying to balance the flows [*], which means it's drawing power off the ESS (energy storage system) to compensate for the draw from devices explicitly not on the ESS because I don't want to run down the ESS with them. It's a lose/lose situation, I put stuff like the stove outside the ESS to avoid it running down the batteries but it's still running them down without the benefit of them being on the ESS.

Any ideas on how to fix this? It seems like all it needs is a software update, don't push power out in excess of what the backed-up circuits require, or is it more complex than that? Unlike vendors like SMA, Enphase provide virtually no info on the technical details of how things work, the installer described them as being like Apple, really easy to use but with no access to internals.

Currently sitting here with the batteries run down to the SOC limit because it compensated for the HWC which was explicitly not put on the ESS to avoid this issue, grrr.

[*] Presumably in order to prevent all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light from total protonic reversal.

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#3117960 19-Aug-2023 13:22

Three phase supply is, in this case, best modelled as just three separate supplies. For you to export on one phase and re-import on another, the power has to go out your meter to the street, through the pole fuse, and to another customer, displacing power coming in through A phase of the distribution TX. It then has to be balanced by extra power coming in on B Phase, through the transformer, through the pole fuse, and to you.

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#3117961 19-Aug-2023 13:27

neb:
SomeoneSomewhere:

Not being familiar with these systems, what you'll tend to find is that the system aims to zero out power flow through a point - that point being where the measuring current transformer (CT) is clamped around a cable.

Yup, that's exactly it, it's trying to balance the flows [*], which means it's drawing power off the ESS (energy storage system) to compensate for the draw from devices explicitly not on the ESS because I don't want to run down the ESS with them. It's a lose/lose situation, I put stuff like the stove outside the ESS to avoid it running down the batteries but it's still running them down without the benefit of them being on the ESS.

Any ideas on how to fix this? It seems like all it needs is a software update, don't push power out in excess of what the backed-up circuits require, or is it more complex than that? Unlike vendors like SMA, Enphase provide virtually no info on the technical details of how things work, the installer described them as being like Apple, really easy to use but with no access to internals.

Currently sitting here with the batteries run down to the SOC limit because it compensated for the HWC which was explicitly not put on the ESS to avoid this issue, grrr.

[*] Presumably in order to prevent all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light from total protonic reversal.

Depends. Can it actually directly measure the power flow to the Essential/ESS circuits? That would require the system being set up with two CTs - one to measure total house power, one to measure only backed up power.

If it doesn't have the ability to read two CTs, then you would need to have the CT moved to the essential services contactor, and would give up the ability to measure the whole house power.

It doesn't sound like you want it to offset grid usage at all. If so - why not just either a) disable it discharging the batteries to offset load at all, or b) set the state-of-charge limit for non-emergency operation to perhaps 75%.

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#3117968 19-Aug-2023 14:32

SomeoneSomewhere:
Depends. Can it actually directly measure the power flow to the Essential/ESS circuits? That would require the system being set up with two CTs - one to measure total house power, one to measure only backed up power.

AFAIK it can, or at least it knows how much power the inverters are putting into the backed-up circuits, as well as how much is coming in from the grid, and presumably how much is going to the non-backed-up circuits. The killer is that at some point things may end up on a shared bus where it can't tell which power is going where, and Enphase publish very little about the internals of the system and don't provide any access to knobs and levers, so there's nothing you can do yourself. It seems like a straightforward thing to do, but given that they don't support something that seems obvious it may be that it's not possible.

That's actually a pretty annoying misfeature if the only possible option is to run your batteries down to zero every day compensating for loads you don't even want to have powered by the ESS.

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#3117970 19-Aug-2023 14:55

Can you post the documentation and line diagrams you have for the system? Because I guarantee that is not the only option.

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#3117976 19-Aug-2023 16:01

SomeoneSomewhere:
Can you post the documentation and line diagrams you have for the system? Because I guarantee that is not the only option.

This is the system diagram:

They really don't tell you much. I'll dig up the installer manual for the controller and post the wiring diagram page from that, which is the best you get, but it's more or less the above with better technical drawing abilities.

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#3117982 19-Aug-2023 16:16

neb:
SteveXNZ:

So what’s the consequence of this? Nothing practically, until the power bill arrives. “Net metering” says my 3-phase meter must independently record import and export – no problems with that. But what doesn’t happen is “net billing”. I’m on Electric Kiwi’s Movemaster plan which applies differential time of day tariffs. If I use zero net energy over say the 7am-9am peak I should pay $0. Not so fast! I’m actually paying 40c/unit for what I’m importing, and receiving a 14c/unit credit for what I’m exporting! This makes no sense at either the transformer/utilities level or EK’s spot market level. I have an on-going dispute with them and have asked them to justify their billing practice and make it clear on their website.

How does the current flow actually work in terms of what the power company sees? This is the same problem I've pointed out earlier, the ESS tries to push power out to the grid to balance the draw from the grid, but wouldn't the outgoing power from the ESS get drawn straight back in again as incoming power to the load before it hits the meter (I've forgotten pretty much all of the AC circuit theory I learned way too long ago, not sure how accurate that is).

So here's my understanding, though I'm not an electrical engineer. With a 3-phase supply you are essentially getting three semi-independent power sources coming into your home, and you distribute your single-phase appliances across them so they're roughly balanced. Each phase is shifted from the other by 120 degrees, but this is of no consequence for single phase appliances. If you have heavy 3-phase electric motors they will use all three phases simultaneously, utilising the 120 degree shift for improved efficiency and torque characteristics.

Your electricity meter (in my case an EDMI Atlas Mk10D) records a wealth of information relating to each phase, including per-phase import and export. But that information is not available to you, despite the fact that you're hosting the meter on your property and it's measuring your energy consumption. Instead the metering company reads the meter, and relays a subset of information to the electricity retailer. This is where things get murky, as I have no idea exactly what ends up with the retailer. What EK tell me is that they bill on grid import on the UN (uncontrolled channel) and grid export on the EG (solar channel), with energy consumption recorded at 30 minute intervals. Ie the per-phase information is lost/amalgamated, and finer detail as to the power flows is lost. I've asked EK to supply the fine granularity meter detail from the metering company until I'm blue in the face, but they will not do so.

For those of us with solar installations we have recourse to another method. CT clamps across each of the mains phase wires feed into a separate meter (in my case a DTSU666), which is used by the inverter to monitor power flows and report them (in my case to AlphaCloud) where they can be accessed by web and mobile apps. But again I don't have immediate access to this data - if I ask my installer I can get a csv snapshot of instantaneous power readings across each phase with intervals a few minutes apart. Great - but it's not easy, and not real-time. AlphaESS are also developing an API for data access, but it's limited. Still pursuing this one.

So what's happening when my inverter balances the power flows when supplying the house from the battery? If the draw across all phases is under 3.3kW per phase, no problem. The battery supplies all loads. But let's say the 3.5kW element in the hot water cylinder switches on. Instantly the inverter draws from the grid on that phase, let's say 2kW. To compensate it feeds 1kW back into the grid on the other two phases. It does this by raising the voltage on those phases to drive current back into the grid. So net zero energy consumption, but import and export are taking place simultaneously.

So what's happening at the transformer on the pole outside my house? It's taking 3-phase power from the 11kV overhead wires and stepping it down to 240V for household distribution to multiple homes, some 3-phase, some single phase. Pushing a bit of power back into the transformer just means the draw on that 11kV phase is a little less. If multiple users do this there's a natural phase balance, so no negative consequences. So to my mind there's no justification for retailers to bill import and export at different tariffs within the same tariff period.

Happy for an electrical engineer to correct my understanding, but I don't think I'm too far wrong.

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#3117983 19-Aug-2023 16:17

OK, managed to find the ANZ IQ3 installer manual not the US one which I'm sure wasn't posted two weeks ago when I last looked, here's the diagram:

And that pretty much dashes any hope of fixing the problem, if that diagram really is representative of what's going on inside the controller then everything is on a common bus and the controller just acts to drive the reading at the consumption CT to zero.

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#3117984 19-Aug-2023 16:29

Ah, I see. Red, black, or green/yellow striped dragons? Green dragons were deprecated a few years ago due to concerns over possible confusion.. /s

There's some drawings in the system controller quick install guide, but it is very much not a full manual and does not explain what is happening behind the covers.

However, I don't think you really want what you say you want. It would still be possible to completely discharge the battery by using loads on the backup side during peak power.

I think your best option is to set a different state-of-charge limit for 'peak-shaving'/cost reduction, versus the limit for emergency operation. This should be possible, but I don't have access to any GUIs to check.

Also, you probably should make sure your hot water is on a time switch not to operate in peak periods, and the batteries are only setup to export during peak periods.

EDIT:

And that pretty much dashes any hope of fixing the problem, if that diagram really is representative of what's going on inside the controller then everything is on a common bus and the controller just acts to drive the reading at the consumption CT to zero.

Yup. You could have an electrician arrange for the non-backup loads to be connected between the main switch/meter and the Enphase box, at which point it would have no idea what's happening with those loads.

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#3117987 19-Aug-2023 16:48

Wish I'd had this earlier, would have saved a lot of guessing at what's going on:

So there's one honkin' big common bus, at least for L1, internally, and anything pushed onto that goes equally to the backed-up and non-backed-up loads. Presumably there's some technical reason why you can't put a CT on the backed-up load and only push out the power for that, because otherwise it seems like a no-brainer solution to the run-the-battery-down-to-zero problem.

I'd looked into things like using a Paladin diverter for this but it's an awful lot of extra faffing around, and bolting giant lumps of stuff onto a visible wall may not go down well.

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#3117989 19-Aug-2023 17:02

So there's one honkin' big common bus, at least for L1, internally, and anything pushed onto that goes equally to the backed-up and non-backed-up loads. Presumably there's some technical reason why you can't put a CT on the backed-up load and only push out the power for that, because otherwise it seems like a no-brainer solution to the run-the-battery-down-to-zero problem.

The whole grid is one big common bus - we literally refer to the infinite busbar as a theoretical basis for dealing with the rest of the grid. You can absolutely place as many CTs as you want, wherever you want.

The question is why you want to zero out power usage by backed up loads, but not by non-backed-up loads. You pay the same for power used for both. Both will empty your battery bank if you let them. If it's economical to use batteries rather than the grid to supply load, it doesn't matter what the load is. If you want to ensure you always have 50% power available for emergencies, or to improve battery longevity, set a limit for 50% SOC.

ETA: You could also consider adding a power limit for discharge-battery-into-grid of e.g. 2kVA. Slower discharges improve battery longevity. The question really is why should we treat a 2kW hairdrier differently based on whether it's plugged into an essential or non-essential outlet?

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#3118000 19-Aug-2023 17:23

SomeoneSomewhere:
The question is why you want to zero out power usage by backed up loads, but not by non-backed-up loads. You pay the same for power used for both. Both will empty your battery bank if you let them. If it's economical to use batteries rather than the grid to supply load, it doesn't matter what the load is. If you want to ensure you always have 50% power available for emergencies, or to improve battery longevity, set a limit for 50% SOC.

Because it's not cycling the batteries hard every day. The backed-up load is about 0.5kW, I can leave the SoC at the default 30% and know it'll typically never go below around 60-70% most of the time, and charging and discharging will be relatively slow. With the current situation it discharges the batteries at quite a high rate until it hits the SoC limit and then recharges them at an even higher rate at 9pm, cycling them hard every singe day.

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#3118002 19-Aug-2023 17:31

SomeoneSomewhere:
You could have an electrician arrange for the non-backup loads to be connected between the main switch/meter and the Enphase box, at which point it would have no idea what's happening with those loads.

That apparently doesn't work, since it's on a common bus the 1m of conductor from the Enphase controller to the non-backup load represents a much easier current path than the (say) 200m of conductor to the street transformer, so the draw will be from the Enphase not the grid.

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#3118004 19-Aug-2023 17:39

neb:

Because it's not cycling the batteries hard every day. The backed-up load is about 0.5kW, I can leave the SoC at the default 30% and know it'll typically never go below around 60-70% most of the time, and charging and discharging will be relatively slow. With the current situation it discharges the batteries at quite a high rate until it hits the SoC limit and then recharges them at an even higher rate at 9pm, cycling them hard every singe day.

That's why I suggest setting the SOC to ~50% and setting a limit on charge and discharge rates. Much more predictable.

neb:

That apparently doesn't work, since it's on a common bus the 1m of conductor from the Enphase controller to the non-backup load represents a much easier current path than the (say) 200m of conductor to the street transformer, so the draw will be from the Enphase not the grid.

Common bus has nothing to do with it. Your neighbour is also physically connected via a hunk of copper, and it's not compensating for their usage, or for other grid voltage variations.

It's simply whether or not the current goes through the CT on the mains site of the controller box.

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