Electricity and the Grid

SPH4C

Power

Recall that the power delivered to a circuit or consumed by a load is:

Power

Recall that the power delivered to a circuit or consumed by a load is:

P V I

or P V I IR I I R

or P V I VV

R

V

R

( ) 2

2From Ohm’s Law

The Power Grid

Electrical power travels from a power plant to your house via the power distribution grid.

The Power Plant

The power plant contains a spinning electrical generator. What spins the generator could be hydro power. . .

The Power Plant

. . . but is more commonly a steam turbine, powered by steam from the burning of fossil fuels, or a nuclear reactor (or concentrated solar).

Alternating Current

Remember that generators produce AC power:

3-Phase Alternating Current

Power plants actually produce 3 different phases (offset by 120o) of AC power simultaneously, so that at any given moment, one the three phases is near its peak, producing relatively even power for high-powered (e.g. welding) equipment.

4 Wires

A power plant therefore has 4 wires coming out of it, the 3 phases and a neutral zero-voltage reference ground return path for electrons common to all three (in some circuits the Earth itself may be used as ground).

The Substation

The wires leave the generator and enter the electrical substation where the generated voltages are converted to higher voltages (by

The Substation

The wires leave the generator and enter the electrical substation where the generated voltages are converted to higher voltages (by transformers) for long-distance transmission.

Transmission

The high-voltage transmission lines are supported by large steel towers.

(The wires at the top are ground and are placed there primarily to

Transmission

The high-voltage transmission lines are supported by large steel towers.

(The wires at the top are ground and are placed there primarily to attract lightning.)

Why high voltage?

The long-distance transmission lines often carry 150 – 750 kV. Why?

Why high voltage?

The long-distance transmission lines often carry 150 – 750 kV. Why?

Recall P = VI. So the current drawn by the substation is I = P/V.

Also, P = I2R, so the loss of power along the line (the line loss) is:

Ploss = I2R = (P/V)2R = P2R/V2.

Why high voltage?

Ploss = P2R/V2

Note that line loss is least for high voltages.

Why high voltage?

Ploss = P2R/V2

Note that line loss is least for high voltages.

(And that line loss is increased when the power is increased, i.e. you have your lowest efficiency at times of peak demand.)

Stepping Down

For power to be useful in the home, the voltage has to be stepped down again (to less than 10 kV) at local substations and split for distribution.

Regulator Banks

You may also see local regulator banks, regulating the voltage along the lines.

Taps

Taps split the phases (your house only needs one).

The Last Step

And local transformers step the voltage down to what is required to run your household appliances.

The Last Step

And local transformers step the voltage down to what is required to run your household appliances (in North America, 120V).

AC adapters

Note that some household devices do require direct current.

Alternating current can be converted to direct current using adapters.

Summary: so why AC?

Summary: so why AC?

Large electrical generators happen to generate AC naturally, so conversion to DC would involve an extra step.

Summary: so why AC?

Large electrical generators happen to generate AC naturally, so conversion to DC would involve an extra step.

Transformers must have alternating current to operate, and the power distribution grid depends on transformers.