Why cant you use electrical energy directly form a high voltage line?

Answer 1

This is done in order to minimize power losses in the power distribution network due to the resistance of the transmitting cables. It should be noted that for a given cable resistance, voltage drop, and thus power dissipated in the cable and not available to use, is directly related to the current flow through the conductor.

According to the Power Law: P = I2 ×R, that is power (in this case, power lost) is equal to current squared times resistance. To deliver power, it takes amps and volts. If you raise the volts, you can reduce the amps and still get the same power. If you reduce the amps, you lower the losses. Did you notice the squared term in the formula? That means if you reduce the current to one-tenth of the original value, your losses go down to one one-hundredth of what they were.

This is a huge issue for the utilities. Every kW lost is one they cannot collect money for, yet they still have to pay for fuel to generate it, they have to size the generator bigger to supply it, and they have to size the transmission system to carry it. There are other good reasons too (see below), but minimizing line loss is the $main$ one. A few transmission systems have been designed at 1.2 million volts. The utilities would have billion-volt systems if they could figure out how to do it.

2nd Answer

A major reason is that, to carry the same amount of power, if the transmission voltage is made higher, then, even though a thinner cable has a higher resistance for a given length, the cables can be made thinner and lighter in weight.

Use of a higher transmission voltage saves a tremendous amount of money in many ways. For example for the expensive material used for the cables (often a steel multi-strand core wound with an outer skin of copper, aluminum, or similar good conducting wires) and for the weight and costs of construction and erection of the towers that carry the cables across the countryside.

To carry 400 kV (= 400 kilovolts = 400 thousand Volts) the steel towers have to be taller and the porcelain insulators have to be longer than they would have to be for cables carrying lower voltages but the cost of making the towers taller and the insulators longer is far less than the cost of the extra weight of the much thicker cables that would be needed to carry the same power at a lower voltage. *** (See Note below for more explanation)

There are many other costs which have to be reckoned when deciding what voltage to use for long-distance power distribution. For example the high cost of the massive power Transformers and big switching stations that have to be included in the power distribution network; the power that is lost from the cables - radiated to the surrounding air as heat - because of the electrical resistance of the materials from which the cables are made.

The above answer just gives a very simplified overview of the kinds of things a skilled power transmission engineer has to work with and calculate when designing a new power transmission network.

3rd Answer

Transmission and quantum of electricity can be considered analogous to hydraulics. Reckon voltage as pressure, the longer the distances, the higher the pressure required to pump. That is why for long distance transmission high pressure (voltage here) is required, failing which, the power will not reach the destined end. It will dissipate on the way. Reckon current as quantity which will be drawn from the pipeline (cables here) at the pressure/voltage required.

*** Note:

If we use the Electric Power Equation we can get an idea of what the descriptions given in the answers above really mean:

P = V × I or, in words:

P (power) = potential difference V (voltage) times current I (amperage)

So, using simple mathematics we can say that:

I = P / V or, in words:

Current I (amperage) = P (power) divided by potential difference V (voltage)

Now, as an example, if a small town needs to have a supply of power W of say 1 MW (= 1 megawatt = 1 million watts) to be delivered over cables from a power generation station:

Calculation A:

If the voltage V used for transmission through the cables is 1000 Volts then the current I in the cables would have to be:

P / V = 1,000,000 / 1,000 = 1,000 amps

which would require a very thick and heavy, and therefore very expensive, cable and associated support towers.

Calculation B:

If the voltage V used for transmission through the cables is 400 kV (= 400 kilovolts = 400,000 volts) then the current I in the cables would have to be:

P / V = 1,000,000 / 400,000 = 2.5 amps

which can be carried safely in a very much thinner, lighter and less expensive cable and support tower.

Line supports are mainly two types:

1. poles

Poles are classified as wood poles, concrete poles and steel/aluminum poles.

2. towers

Towers are classified as self supporting towers and stayed/guyed towers.

Self supporting towers are in two types: wide base and narrow base.

Stayed towers are classified as portal type and V-type.

Answer 2

Electricity is transmitted at high voltages in order to minimize the the loss of power during transmission. The power loss during transmission is proportional to the square of the current (P : I2), so there is a strong incentive to reduce the current by as much as possible. For a given amount of energy, the current is inversly proportional to the voltage (I : 1/V), so the higher the voltage, the lower the current. And the lower the current, the lower the power loss squared.

For example, if the transmission power loss was 9 watts at 100 volts, then at 200 volts (double), the current would halve and the power loss would quarter. Hence the transmission power loss would only be 2.25 watts (1/4).

In mathematical terms, power can be measured by Joule's Law, as

P = V * I

where P is electrical power, V is voltage, and I is current.

Using Ohm's law (V=IR), where R is electrical resistance of a given circuit, we can substitute IR for V, and arrive at the transmission power loss equation:

P= I2R.

Answer

The primary reason for transmitting energy (not 'power') is to reduce the load current so that conductors of practical cross-sectional area and weight can be used. At low voltages, conductors would be far too heavy and expensive to use.

Answer 3

Because distribution losses are lower at high voltages than at low voltages.

Answer

The answer provided above describes an advantage, rather than the reason, for transmitting energy at high voltages. The primary reason is that, by using high voltages, the resulting lower currents make it possible to use cables with cross-sectional areas that allow the conductors to be light enough to be suspended from towers or poles. In other words, the use of high voltages is what makes energy transmission practical. The advantage of a saving in energy loss, as described above, is simply a bonus!

Answer 4

Assume that only 120 volts is transmitted from the power stations throughout the whole distribution network. Then for 10 megawatt [as used by a VERY SMALL town ] the current would be have to be very high so the cables would have to be very thick and heavy - and therefore very expensive to buy and to install - because if they were thin their resistance would cause a huge amount of power to be wasted, just heating up the air, which means the energy lost as heat from the lines would be enormous. But, by using 500 kilovolts to send 10 megawatt to the same town, the current would be very small so the cables can be thin and much cheaper to buy and to install and the energy lost as heat from the lines would be much less. That is the reason for using high voltage transmission lines.

Answer 5

Because DC cannot be stepped up and down like AC can.

Answer 6

We can up to 250,000 volts. That is almost always high enough, although if we could go higher it would be better for simulating lightning.