Why are consumers located near high-voltage lines typically not connected to those lines but instead are connected through a lower high-voltage line?

Question

A typical grid uses 110..500 kilovolts lines to deliver electricity to substations which lower that to 6..20 kilovolts and then lines with that lower voltage get to consumers where yet other substations are located which finally lower those 6-20 kilovolts to consumer voltage (100 or 230 volts or whatever the local standard is).

Those 110..500 kilovolt lines often pass through areas where those consumers are located. Consumers could be connected to those lines via transformers accepting say 110 kilovolts and outputting consumer voltage. Instead those lines run to faraway somewhere and then another powerline runs back with some lower voltage and a consumer is hooked to the latter. That's a lot of extra wiring.

What's the reason for this design? Why not hook consumers to the closest powerline?

Answer 1

HV (66kV - 500kV) is... difficult to deal with.

I will rattle off reasons I can think of from the top of my head.

All figures that follow (weights, dollars) are order-of-magnitude guesstimates.

Clearances

Let's use 220kV as an example. The Australian HV substation standard AS 2067 nominates the following clearances required for 220kV equipment:

• Phase to earth - 2100mm. That is, no 220kV conductor may be within 2 metres of any earthed conductor (say, a transformer tank, or a steel pole.) Edit: Actually, I should have quoted the Non Flashover Distance (N) here.
• Phase to phase clearance - 2,415mm. That is, the 220kV aerial conductors must be at least 2.4m apart at all times.
• Horizontal safety clearance - 4,125mm. All live parts must be at least 4,125mm above any surface a person can stand on.
• Vertical safety clearance - 3,565 mm.

Which is to say there is no such thing as a 'compact' 220kV substation. (Well, there is; substations based on gas-insulated switchgear can be very compact, but you don't want to know how much they cost.)

The minimum size for a 220kV substation, containing the required equipment and maintaining all these clearances, is at least a 20m × 20m square, i.e. the size of a suburban block of land.

It would also have to have structures at least 4 metres high, which is hard to blend into the suburban landscape.

In addition to the above clearances required to prevent people getting directly electrocuted, you also have to contend with -

• Fire safety radius in case a transformer drops 10,000 litres of insulating oil and catches fire. From memory, at least 10 metres.
• Radius in case of electrical explosion. Typical threshold radius for receiving 'survivable' second-degree burns can exceed 10 metres for some energetic kinds of faults. Definitely no civilian housing allowed inside this radius.

Protection

A fault on the 220kV network must be cleared rapidly, or it will drive the whole grid into an unstable state (i.e. blackout.) The 'critical fault clearing time' to avoid a blackout is usually much less than 1 second.

Very expensive protection schemes (line differential with optic fibre pilots, distance protection) are used to ensure this high speed of protection. These protection schemes must be installed at every terminal of the 220kV line.

Once we account for the cost of -

• 220kV circuit breakers - about $200,000 each, minimum three required per substation - two for the incoming/outgoing circuit continuing past the substation, and one for the T-off = $600,000
• two sets of three-phase protection current transformers rated 220kV, and "enough" continuous amps - about $50,000 a set (ballpark) = $100,000
• two sets of protection relays - each with a redundant duplicate - about $20,000 each = $80,000. (Note: duplicate "X" and "Y" protection is standard for HV substations.)

... we are up to about $780,000, just in protection equipment, per substation. And we haven't even started buying transmission line termination hardware, surge diverters, busbar, support structures, earthworks, fencing, concrete, control PLC's, control hut...

(Compare 22kV distribution transformer protection, which is usually just a set of three-phase expulsion dropout fuses, total cost maybe $2,000.)

Transformers

220kV transformers are large, by dint of all the insulation required inside them to prevent flashover. There is no such thing as a "small" 220kV transformer - the smallest one I have seen is rated 60 MVA and weighs about 10 tons.

Contrast typical pole-top transformers 22/0.415kV which are rated 500kVA or less. The weight is important because there is a maximum limit to what you can have on top of a wooden pole. I am no structural engineer, but you certainly wouldn't want to pole-mount anything more than a ton.

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Is that enough reasons?

Answer 2

A major reason is that these lines are for long distance transmission and interconnecting large grids.

Imagine a highway. Mostly they have exits every few miles in built up areas, and sometimes more frequently than a mile in particularly strange cases, but for the most point they are intended to allow fast, efficient travel from long distances away. While there are clearly houses and businesses near highways, if each one had its own onramp and offramp, not only would the infrastructure resources be significant, but every time you have a problem with an onramp or offramp that ends up closing a section or a lane of the freeway for a period of time you impact many, many more people.

If you start building out more substations you increase the risk of down-time for the transmission line due to substation issues.

Further, smaller grids are actually connected to several larger grids with switches, which are then sometimes connected to more than one transmission line with switches. This allows a problem on any given line or grid to be routed around, and results in power loss that is localized to the problem. Transmission lines are harder and more costly to work on and repair, and are critical backbone infrastructure for national electrical grids. When power plants go offline for any reason, power plants much further away can take up the slack due to these lines.

Lastly, electrically they are phase balanced for the most efficient transmission of electricity. Individual substations and grids are designed so that the power factor is as close to 1 as possible. Lower power factors result in energy loss in the lines and transformers, which requires more substantial conductors. These lines are not meant for poorly matched AC loads. Industrial customers who connect to the higher voltage lines often have to add power factor correction if their plants are not properly balanced. Connecting a home or neighborhood more directly to a transmission line would require an even greater investment in the substation needed to service them so the transmission lines are unaffected. Other high voltage lines merge a lot of customers with poor power factor, but by mixing small industrial users (lots of motors) with home users (lots of switching power supplies) the substations can balance the power factor for a much lower cost and smaller facilities. Placing unbalanced loads directly on transmission lines would lead to more significant headaches than the power transmission coordinators already face.

They really are not designed for small consumers.

Answer 3

Imagine if we actually did this, and we had power lines run through a neighbourhood or alongside, and every house connected directly to these power lines rather than a substation, it'd be pretty silly

I drew a picture to demonstrate how silly it might be:

Fortunately, the Swedes built things far better than my drawing skills:

Those are telephone wires by the way, they can get somewhat close without terrible terrible things happening to the wires (and people nearby).

Now imagine those cables are heavy duty power line cables. Imagine you couldn't pack them so densely and had to give each line individual clearance. Imagine the additional supports for when tower blocks and apartment buildings block direct line of sight, additional structures along the way to support all the additional cabling and the weight and tension needed to hold it in place.

Imagine the impact all these heavy duty high voltage cables have on reception and radio transmissions, and the numerous micro substations for every house.

I drew another picture, it's a tiny village with power lines adjacent:

We could bury the power lines most of the way, but that's a lot of digging to lay pretty dangerous power lines, it's all going to get very expensive (which it already is).

A simple solution would be for several adjacent houses to share a cable and substation. Stations of sufficient size would be cheap enough to support entire neighbourhoods while saving on construction costs and reducing the number of cables. This is all starting to sound familiar...

Answer 4

I am thinking more that it's due to system protection. If you tap-off the transmission and successfully step it down to your house-hold voltage, it would be very expensive to the utility if a fault happens at your location.

Also it's cost effective to have a central system protecting the central transformer, and the main transmission line. Furthermore the cost of the transformer to step down the transmission line voltage from around 69Kv, 138Kv, and so on, to 120V would be crazy expensive to pursue.

So it has both technical and economical benefits to have the layout as it's today.

Answer 5

I think it's because the main goal of a high voltage line is transmission. This is because at high voltages the power lost caused by I2R is lower than using lower voltage (for the same Power [W], higher voltage => lower current)

Besides that, you can connect to high voltage line by using a transformers, maybe 500/0.4 kV, that would be unacceptably expnsive.