Posted by: @bretix

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I've not seen my local substation data

And you're very unlikely to!

There are 230,000 ground-mounted local substation transformers and a further 320,000 pole-mounted.

Only a few hundred will have ever had 'permanent' monitoring devices connected to them, most of which will have been decommissioned when its certification period lapsed.
I have access to a handful of monitored sites, which provides me with insights that few local distribution-grid engineers enjoy.

The Low-Voltage grid (11kV and below) is managed by experience and common-sense, rather than data and statistics.

Whereabouts are you in the UK, and who is your DNO?

Those solar panels on the farm roof will be active 365-days/year.
They are providing functionality which DNOs are eager to have... reducing demand-load on the 11kV network.

Posted by: @bretix

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@transparent

Love these discussions and love learning new things btw

I've not seen my local substation data but I guess it would make an interesting study. Most buildings are on phase 3 as there are either defunct farmer/small holding sites or working agri farms/storage.

There's also a humongous farmers hangar close by that is covered in solar panels on the south. I'm  guessing these must be isolated and switched off during the summer as the load on the grid would be massively significant.

 

 

Other than applying certain limits like the 80A service fuse and limiting grid export to 3.68kW, the DNO's have no control over their consumers energy use. They cannot stipulate that high energy consumption loads are spread evenly across all 3 phases, or that these devices are all switched on for the same periods to balance the loading on each phase. Even if the DNO's could do so, it could create the problem of high overall demand when the grid is struggling to cope.

I feel it is therefore imperative that the DNO's utilise methods to balance the phases, methods that are under their control.

As Transparent has repeatedly pointed out phase imbalance causes a flow of current in the neutral conductor, whereas in an ideally balance system there should be no or little current flow in the neutral conductor. I suppose to most this will seem strange, since they are used to the single phase system where current flows from the live conductor, through the load to the neutral conductor, and the current flowing in both the live and neutral would be the same.

This is different in a 3 phase system, since the waveform of each phase differs by 120 degrees, so the instantaneous voltage of each phase is different, which means that the current actually flows from phase to phase, with no current flowing in the neutral conductor when the loading is balanced.

@derek-m again forgive my schoolboy physics but I assumed the sine wave algorithms meant 3 phase was more stable? Are the implications that it's a stable 'wave' to the metre or the DB?

Or does it mean that the stability should be from the DB to the drawers of the current and voltage? Am I using the wrong lexicon 🤔 ?

Posted by: @ianmk13

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You seem to be suggesting a future when we could experience more frequent supply failures.

Yes... and most of that will be down to ignorance.

When I see an outage pop up on the NGED Regional alert page, I tend to check where it actually is on the grid (because I also have the network map). If it's particularly 'interesting' then I ask the Area Manager what was the cause.

If it's not far away, I will go to the site, talk to the engineer in attendance and take some photos.
That builds up my knowledge-base of what's actually going on around me.

The cable above had only been in use 2 years, but was on a new estate still under construction.

The national developer had saved money by employing an Independent Distribution Network Operator (IDNO) to install a substation and lay the 440v cabling.
In turn, they'd saved money by allowing the groundworks sub-contractor to dig the (shallow) trenches, and then using cables which had less than half the current capacity which NGED would've installed.

So when a couple of houses on this feed had EV chargers installed, the inevitable outcome was the insulation breaking down at the weakest point.

'We' need to start looking after our Distribution Grid.

If we noticed someone attacking a distribution-pole supplying our house with an axe, I'm pretty sure we'd do something about it.
And yet there are 'threats' against the resilience of our local network which are just as dire, but we tend not to realise what's going on.

Here's a recent photo of a trench which I noticed being dug with the intention of running a 33kV cable through it.
That's a re-routing of the main 33kV supply for my entire town and surrounding countryside! 😮 

The shallow trench runs up a 1-in-4 slope which is to be over-planted to the right with trees.
To the left of the trench, the ground will be excavated to form a level platform, on which a 2-storey commercial building is to be constructed in several years time...
... and that will need a strong fence with steel uprights for security.

Common sense alone tells me that this shouldn't be acceptable.
I don't need to be 'adequately qualified' nor understand the relevant sections of the ENA Engineering Specs.

I don't see the above issues as very much different to the load imbalances at @bretix' house.

They all come under the heading of "Using common sense to look after our electricity supply".

If we don't do this, the inevitable outcome is that the DNOs will need to dramatically upgrade the grid to provide greater resilience.
Ofgem will have to agree because integrity of supply is a statutory obligation... and will therefore approve yet more increases in our bills.

The two issues are closely linked...

... but you didn't see that reported in the press when Ofgem announced the next round of price increases just 4 days ago.

Posted by: @bretix

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@derek-m again forgive my schoolboy physics but I assumed the sine wave algorithms meant 3 phase was more stable? Are the implications that it's a stable 'wave' to the metre or the DB?

Or does it mean that the stability should be from the DB to the drawers of the current and voltage? Am I using the wrong lexicon 🤔 ?

I'm not certain what you mean by 'stable'.

The frequency of the grid is set by the speed of the generators at the power stations, which are all running at the same speed because they are synchronised to each other, at a nominal 3000rpm.

The rotor within each generator contains a powerful electro-magnet, which induces voltage in each of the three stator windings, which are located 120 degrees apart.

Considering the 3 phases, L1, L2 and L3, the voltage on each rises from zero to a positive peak, falls back to zero and then continues to a negative peak and then back to zero again, where the cycle recommences. Because the phases are 120 degrees apart, when phase L1 is at zero volts, phase L2 is at a negative voltage and phase L3 is at a positive voltage, so in a 3 phase system current could flow through loads from L3 to L2, L3 to L1 and L1 to L2. If you Google '3 phase waveforms' it should help with my explanation.

If 3 houses are each connected to a different phase, and each is drawing a current of 50 Amps RMS (Root-Mean-Squared), then when the current flowing from L1 through the load in house A, and back to the supply via N1 (neutral 1), reaches a peak, the current flowing to house B and house C will be negative, so it is actually flowing via N2 to L2 and N3 to L3. This should result in zero current flow in the DNO's neutral cable at the supply transformer.

It is when the system is imbalanced that current starts flowing in the DNO's neutral cable, which could initiate a system trip if the imbalance gets too great.

Posted by: @derek-m

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current starts flowing in the DNO's neutral cable, which could initiate a system trip if the imbalance gets too great.

... which it doesn't, because the Neutral is rarely fused.

Instead, the Neutral wire gets hot, which eventually creates thermal breakdown of the insulation.
Historically, Neutral conductors could be made thinner than the three Phases, and required less insulation between the copper and ground.

But if the Neutral is now regularly conducting current, then we have a million miles of underground LV cables which are being compromised.

It might help @bretix if I provide a diagram of how the windings are arranged in a local substation transformer.

The incoming 11kV 3-phase goes through three coils in Delta form.

The secondary 440v coils are in Y- formation, which allows the centre point to be defined as the Neutral.

And I wish your electrician was reading this so he could chip in !

Considering I've had 3 different electricians (2 commercial) over time working on my system I'm amazed these issues are never mentioned or spoken about at the very least in layman's terminology. Maybe that's also due to my ignorance in these aspects. Anyway.

Having a large mainly single storey,  roof space with capability of going beyond 50 KW solar generation at peak (if I covered my home with tiles and/or panels in the future) much in the same vein as the farmers building just down the road it only makes sense to me to be 3 phase, not that I'm commercial but apart from my own storage I would surely be taking the strain off the grid ?

Here's the classic 3-phase diagram of voltage against time, which will make better sense of Derek's explanations.

I've just drawn this from scratch, so we can 'play' with it and avoid copyright issues.

@derek  please let me know if you wanted any more labels.

@transparent

Thank you Transparent, I knew that I could rely upon you. I think that your diagram is quite clear and does not require any further labels.

I wonder if this diagram provides an idea of what happens when the demands on a substation are unbalanced across the 3 phases:

I'm starting with the three coils which form the secondary windings of the transformer,
but laying them flat on the ground with L1 away from the viewer.

The current is proportional to the distance from the end of the coil to the centre-point, which is the Neutral.

If that transformer supplies three identical houses (ie they are taking the same current) then the voltage is also even across them.

It's as if the houses are joined to the Neutral by springs, each under the same tension.

Now we're going to add another three houses further along the street.

House No. 4 presents exactly the same demand as did No.1
It applies the same force to the red spring as before.

No.5 is now a pair of semi-detached houses.
Yes, they require more power, and you can see the blue-spring has become extended.
But the effect is reduced somewhat because 5b has some solar panels on the roof.

The major change is presented by No.6.
He's got an EV charger for the VW ID3 on the drive.

The yellow spring is significantly more stretched, as you'd expect.

But look at the effect on the centre Neutral point which joins those springs.
It too has been pulled, and is no longer at 0v
Current is therefore flowing in the Neutral.

And that has further consequences...
... the red spring is also now elongated, even tho' the force exerted by house N.4 is no different to that of No.1

The greater demand made by a house onto one phase actually affects the supplies to neighbouring properties - even those on different phases.

In an extreme case, several houses all on the same phase and all with EV chargers can drop the voltage seen by other houses below the 216v statutory minimum...
... in which case their G98-certified PV inverters will cease exporting to the grid.

Our homes are not resting on an immovable grid, as we might've supposed.

I have attempted to address the subject of phase-imbalance by using analogies rather than referencing some of the excellent academic papers on the subject which can be found online. Here's a extract from Power Systems Operation with 100% Renewable Energy Sources, 2024, by Ritesh Robin Stanley in case you wondered what you're missing:

I have deliberately stuck to voltage & current variations, and avoided harmonics and phase-angle distortions.

May I now return to answer the original question from @bretix about what action he could take to better address the installation at his house?!

There are two main strategies:

• shuffle the existing loads so as to better distribute then between the phases
• expand on the existing SunSynk inverter and battery system to take the house 'off-grid'

1: Changing the distribution of loads between phases is far from ideal.

We've already noted that the high-current appliances wired to the Kitchen consumer unit are going to be operating at a different time of day to the EV-charger.

Personally, I'd still place the two 10kW heat pumps on separate phases, and keep the EV charger separate to those two.
All three will be running overnight.

We don't know if there are any non-kitchen high-demand appliances, such as electric showers (typically 10kW).
I'll assume not since the heat pumps will presumably be supplying one or more hot-water cylinders.

Your DNO was absolutely right to upgrade the house to 3-phase at their expense.
That max-loading of 55kW is massive!

You will probably never draw that amount of course, because both heat-pumps and the EV charger would need to started simultaneously, along with all the other fixed household appliances!

But that's the figure used by DNOs to approve LCT applications.
They use max. total demand to evaluate the stress on a substation transformer when it is re-energised after an outage.

Yes, switching appliances between phases will require more electrician time and another DB or two.
But another new DB might only be a 4-module wide unit, like the one at the bottom of this photo:

2: Taking the house off-grid is somewhat more appealing.

It was @derek-m who first mentioned such a possibility, although he was probably not expecting me to actually propose doing it!

Posted by: @derek-m

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The only way that it could be done is to use the 3 phase supply to charge a battery, to which is connected a single phase inverter feeding the home, but it would need to be quite a large inverter to be able to supply the possible peak demand. It is not exactly efficient or cost effective.

The basic idea is to run the house from the batteries all day.
I'm assuming here that the 3-phase SunSynk inverters do not mind supplying unbalanced loads, whereas the grid does!

The diagram here shows three single-phase inverters, but gives the general idea.
I can modify this later, according to how the discussion here progresses.

The off-grid strategy requires slightly less rewiring because all existing single-phase appliances and power sockets remain 'as is'.

To increase the available power to the house, add more inverters in parallel.

To increase the amount of time between re-charging, add more batteries.

Install as many solar panels as you wish to avoid re-charging from the grid, especially during summer.

It’s becoming very complicated now but can I brainstorm with you.  I had to get three phase in, Scottish power I believe wanted it but only run single phase.  I need a 3 phase meter I’m told, Scottish power don’t do that it’s my supplier octopus. With what you mention above would you be running separate meters for each string or does it all go into the three phase meter then the strings created from there to individual consumer boxes. During the summer my house has survived mostly on batteries and on average 2-3 quid a day electricity charging the batteries overnight. More panels are the way forward but to be efficient also more batteries and they are expensive, and would I recover the costs. Winter is the demanding time and solar falls down here, where more batteries charging on the 7p rate  would perhaps keep the house running until 11.30pm when the 7p rate kicks in again.  More solar panels could provide more power but a surplus during summer i cant use and send back to the grid. It’s fine regarding diy to a degree but we need to remember that our insurers won’t take kindly to this if anything results in a claim. Some of this needs done and signed off to comply with regs I would imagine. I told my insurers what I had installed and they weren’t interested other than my panels are not covered but the roof is if they come off and damage the roof, however given insurance policy I have no doubt it might be an issue should that happen.  Most of what you explain here would definitely be over the heads of the level of so called engineers attending my home.  I tried to explain start up voltage to one because he had overloaded my pcb and samsung spec a relay. He had no idea and samsung had to step in but the board was toast.