High Voltage Cables

if i understand the question? forgive me if i aim too low.

high voltage cables above ground are not insulated and therefore rely on an air gap for insulation between phases, and ceramic for insulation from earth.

when they go underground in a multicore cable, they are individually insulated by a plastic coating.

It's true that the air provides a means of insulation if the distance between conductors is sufficient. Add debris, birds, and squirrels, and wind-then the span must be sufficient to the end that no part of their body can touch two (2) cables at the same time, and the wind should not blow them together.

When the cables are brought into a peckerhead and directed down the pole, then typically from the initial point of splice the cables have protective jackets to protect them from wild animals and debris.

Ing. Robert Forbus

This (above) is the answer you seek: Birds don't fly or perch underground. Nothing more complicated than that.

how much high voltage do you mean? 10kv,30kv,50kv or higher kv?
Of cause if you have enough money to spend, I can offer you such high voltage cable of which distance is less than 1 inch.

Everything the others have posted here is true but with underground transmission lines the interaction between the cables is more accentuated. An approximation of a two conductor transmission line is the circuit below

What you have is a series of Inductive Resistive Capacitive Conductive units all strung together, the more the better the approximation. You can then use this approximation of the transmission line to then predict what will happen under pre determined loads and inputs.

The example I have given is for a two conductor transmission line and you would add a third series of components connected to the existing two for a three phase line. You can actually get boxes made up like this with variable value components that you can use to test out your analysis.

I wont go into the details of the analysis here primarily because there isn't the time or space and secondly because I would need to go back to my notes for 30 year ago to refresh my memory. From the circuit however you can see that there can be some very interesting results under certain conditions. If you want to learn more read up on TRANSMISSION LINE THEORY but be prepared it's a fairly big subject.

View the individual activated conductors strands inside each stranded cable with an IR camera and be further puzzled to find that one "hot" strand may be carrying the entire load rather than sharing equally. Think of it like watching a highway crew on the job. Many stand guard watching one work. Some stand closely. Others view from afar. But nare dare but one do the work.

about the High voltage, lets take for instance 11kV.

since its underground what i thinks you should be concerened about is the risk of exposure or cut.

because if its was not properly handled during installation, a minor cut can lead to rupture.

damge is likely to occur via that than thru touch.

Cable cost, attenuation, safety and coupling are also considered. Current travels mostly on the outside of the conductor, multiple strands implies more surface area and flexibility ie bend radius. Single strands cost less to make, even less when they are not sheilded. Spacing isolates them from touching and or grounding out. When they go in the ground, the conduit is usually a narrow trench, cheaper to dig. Conduit has curves and bends, so pulling a rigid single strand is harder to pull and resists bends and turns. The sheilding compensates as insulator for signal coupling in close proximity of adjacent cables. I do not know the economics of over head verse under ground costs, but I would guess over head cost less and is easier to manage. However it does look ugly and can be knocked over in high wind, or heavy ice. In canada I think most of their high power lines are under ground, to lower maintenance costs and the landscape looks prettier.

For a proper design, consult a Professional Engineer, in transmition line. All of the above are true but economics usually drives design and risk is managed. The HV lines do not carry as much current to save on line voltage drop, as the main reason. They also transmit in 3 wire and then need to be converted to 4 wire for use in most applications. This also saves money, fewer wires to hang. They are usually 120 degrees 3 phase delta lines and the load on each phase must be balanced, that is they all should draw the same amount of current as the adjacent phases. When designing a load like a factory or campus, equally distribute the load so that they all have the same power factor, same average load and pay particular attention to peak draw, when inductive based motors turn on, as these tend to pull more current, throw the lines out of phase and can trip breakers up stream. Installing capacitors on site at the load is a way to compensate. Designing the right capacitors requires analysis by an Engineer. We use actual power in watts, and we pay for power in KVA, the imaginary part, based on the loads inductance, is where you can save money. The power company will charge more for the power, you actually get to use less of it. To lower your peak usage requirements, turn on motors in a sequence that allows the surges to settle before the next motor turns on. Try to use 3 phase motors as they equally draw current from all three lines at the same load. Also use a VFD to slowly ramp up to speed and lower your initial inductive draw. All this usually cost more one time, but over time, cost less.

If you need more help, let me know. This is not something you want to guess at.

Good example, Cornstoves! I may add that when the cable is contained in underground conduits, it is typically sized larger as it must shed heat and there is virtually no air flow possible; and yes, thermographic examination of conductor heat is virtually impossible. Underground is my first choice, as it is less vulnerable to weather, wild animals, and other on-site construction that typically comes up later and requires the use of a mobile crane. Just beware the guys who install subterranean data cable, as they will nail it every time.

Ing. Robert Forbus