Open LV Trial - Making local substation data available to community groups

I’ll latch onto any chance to fill gaps in my own knowledge(*), to offer/ask @Transparent about a geometric way of thinking of things (this is a question not an answer)…

I picture the three phases as three arrows, ideally the same length, and at 120 degrees to one another, rotating around (at 50 Hz, in the UK).

I recall that phases being balanced is a matter of the resultant (vector addition) being zero.

Do the losses correspond to the length of the resultant (which is in practice never exactly zero?

Or the squared length?

[This is of interest because as something goes to zero, its square goes to zero a lot more quickly.]

If the latter (which would be my gut expectation, half-remembering that in the electrical engineer's use of complex variables, power is something like the product of one complex voltage and a complex conjugate current, says I, bluffing furiously) then so long as the phases are moderately well balanced, then the losses are probably small enough not to worry. @Transparent's comment that the phases weren't very well balanced in the first place may count for a lot here…

(*) I was a little embarrassed to have to edit “fill gaps in my ignorance” to the intended “fill gaps in my knowledge” :-)

Oh that my ignorance could be complete and with no gaps at all...

@Simon1D  wrote:

My knowledge of substation transformers is close to zero, but they are evidently massive (tonnes) and benefit from active cooling.

Not the sort we’re talking about here.

The sub-1000kVA transformers used in substations with an 11kV input are passive cooling.

The core and windings are held in a bath of oil which circulates upwards by convection, and then out to finned cooling pipes on the outside. The exact arrangement varies, but here’s a photo on the left and then a couple of views of my simplified 3D model:

On a lightly-loaded ground-mounted transformer you would typically see 30-40°C at the point where the oil leaves the main casing.

There’s a transformer about 5 miles from me which runs too hot to touch. This can lead to premature aging of the internal insulation.

@Jeffus  wrote:

Do all 3 phases ideally need to be in perfect balance so there is no loss?

We’ll never reach that ideal situation on a substation supplying domestic customers because 99.999% will be using single-phase devices!

I don’t know what tolerance levels are regarded as ‘satisfactory’ by the DNOs. They’ve just got used to running them ever more out-of-balance over the last 20-years.

The break point in time was when the Government started offering FIT payments to install PV Solar Panels. Since then the losses on the Distribution Grid have doubled.

Now that Heat pumps are being promoted we’ll also be seeing increased losses due to harmonics. The problem there is that harmonic losses get propagated back through the Grid to the much larger 33kV and 132kV transformers.

That annoying hum you can hear when nearby is evidence of harmonic loss. If it was at 50Hz then it would be below the audible threshold.

I’ve written more extensively on this subject in the Topic Balancing the Grid.

There’s an informative 4.20min video on YouTube which takes you around a Grid Supply Point transformer (330kV input; 132kV output). The transformer gets energized at 3m45s if you’re just interested in hearing the harmonics!

The “tertiary winding” referred to in the video clip is to produce 11kV. This then gets transformed down to 440v 3-phase to operate the pumps and cooling fans.

The way in which these auxiliary devices get powered is extremely important. In the event of a nationwide National Grid collapse, there must be a way to bring the complete system back online again. So you can’t ‘borrow’ 11kV from an adjacent transformer to run the cooling system. The entire transformer mechanism must be capable of self-start when the breakers are closed.

I’ve enjoyed reading up on this thread today. But also I’ve realised how little I understand this infrastructure. Simon’s analogy of 3 arrows at 120 degrees confused me further. I’ll look to do some reading on this subject over the weekend, and I notice our forum site doesn’t have much to help me with this… yet.

Oh wow, yes at 3m45s it kicks off. A perfect musical 5th interval in fact, a tonic and a 5th above it. I’ve heard ambient musical performances just like this transformer! 

Fascinating! Many thanks, @Transparent!

I had missed answering this point from @Jeffus 

Is it difficult to switch individual homes or groups of homes from one phase to another, appreciating this isn't a long term fix?

In most cases it’s time-consuming and labour-intensive to change the phase supplying a house.

The majority of our 3-phase 440v feeds are underground. It means digging up the road to access the connection point.

Moreover if the cable run to the house itself is ever renewed or replaced it’s common practice to connect it to whichever conductor was at the top of the bundle when you locate it in the trench.

I live in an area with lots of above-ground feeds. These are now being changed to Aerial Bundled Cabling (ABC). This is 3 separate cables (or 4 if Neutral required) twisted together with connections made in pole-top boxes.

Connections to a house can be made using Insulation Displacement Connectors which are tightened onto the ABC cable with a spanner. This automatically provides a water-tight seal without having to cut the feed-cable or switch off the power.

There is no map which shows the phase supplying each house and it would be difficult to produce one. The three phases do not have a ‘master set’ anywhere in the country which can be used as a reference. Each transformer through which the electricity supply passes will rotate the phases +30° (Dyn-1) or -30° (Dyn-11).

In effect, houses have been randomly assigned to a phase. This was fine when each property had roughly equal demand, but that’s no longer the case.

You’ll be proud to hear I’ve been bringing up phase-imbalances to my friends over the weekend, @Transparent.

My engineering friend explained the difference between a single phase and three phase supply using a rope-wave analogy, and I thiiiiink I can visualise things now.

 

There is no map which shows the phase supplying each house and it would be difficult to produce one. The three phases do not have a ‘master set’ anywhere in the country which can be used as a reference. Each transformer through which the electricity supply passes will rotate the phases +30° (Dyn-1) or -30° (Dyn-11).

In effect, houses have been randomly assigned to a phase. This was fine when each property had roughly equal demand, but that’s no longer the case.

This is where my understanding gets a bit ropey (excuse the pun), why do the phases need to be rotated or is this a by-product of them being ‘transformed’?

Excellent question @Jess_OVO - have a gold star

The reason for rotating the phases in a transformer is to make best use of the magnetic flux in the core. You don’t want the flux to decrease to zero and then increase again as the next phase of the incomer rises in voltage.

Think of the magnetic flux as a temporary energy store.

A Local Substation transformer is usually wired in a Delta-Wye format. The primary 11kV windings are the Delta, and the secondary Wye windings have a central connection point which provides the neutral.

Higher voltage 3-phase transformers on the Distribution Grid are more likely to be Star/Delta winding format.

Historically the transformer manufacturers have sought to maximise the efficiency in the forwards direction – from primary to secondary.

The consequence of this is that efficiency is lower in the reverse-power direction – when homes are feeding back to the Grid.

The Standard Technique procedures (the “rule book”) state that a transformer may only accept 50% of its rated capacity in reverse-power mode unless it has been manufactured and specified for greater efficiency in the reverse direction.

The most critical parameter is when reverse-power is being applied with the phases imbalanced. Energy losses will be roughly double those associated with the same amount of imbalanced power in the forwards direction.

This characteristic helps to explain why a DNO will normally accept 32A import for Low Carbon Technology single-phase loads such as Heat pumps and EV charge-points, but only 16A (3.68kW) for export from Solar Panels.

Wow - things did get very technical very quickly there, @Transparent !

So from my understanding on things the current transformer set-up is more designed for energy to be imported from the grid rather than exported to the grid from individual homes. Obviously this will become more and more relevant as more homes install devices capable of exporting energy.

So in the current situation, how are transformers (or substations) adapted to cater to homes exporting larger amounts of energy, or would it be the case that the DNO would limit the number of applications it grants to exceed the usual export limit depending on the devices already installed - ie. you may be denied an application to export from a larger amount of solar panels if you neighbor or someone also linked to the same substation is already exporting over a certain amount of energy?

Not sure if that fully makes sense but trying to keep up! 

Yes it’s the case that a DNO can reject an application for export from your house due to the existing levels of export from your neighbours. That’s a safety issue and trumps all other points of negotiation.

The issue depends on whether you and the neighbours are on the same phase of course. If the Substation transformer is still well within its overall Reverse-Power capacity, then moving a couple of houses to different phases may actually make things better and reduce losses.

You can begin to see why Western Power decided to keep the original OpenLV Project alive for just the Community Groups. They have the ability to coordinate demand and supply between neighbours in ways that Energy Suppliers and DNOs cannot!

We must understand that there is difference between

a: trying to ‘balance the grid’ solely on the matter of supply & demand

b: taking a holistic view of the Distribution Grid, such that its enhancement also permits:

• reduction of losses through phase-imbalance, harmonics etc
• increasing the proportion of renewables exported to the grid

Option (a) is short-sighted and can make (b) even more difficult to implement.