Transmission of Electricity

Khoo Yihan | Chua Cong Yang | Park Seong Jin

process

Process

Electricity is generated in power stations at 11000V to 33000V and then stepped up to 400000V by transformers.

400000V fed into the grid why so high voltage?

Power from the grid is stepped down in successive stages at substations and distributed to different consumers.

Process

factors

Factors

Electrical power is generally transmitted: Using alternating current Very high voltage

Factor: Alternating Current Used because the voltage can be

stepped up and down easily, cheaply and efficiently using a transformer

Recall how a transformer works Stepping up/down of current relies on the

change in magnetic flux Induced current will stop if there is no change

(in the case of a DC circuit) AC will continually ‘open and close’ the circuit

by continually changing the magnetic field

Factor: High Voltage

Problem faced: Electrical power from power stations lose power due to the heating effect of current in the grid cables

Heat lost is given by P = VI = I2R where I is the transmission current and R is the resistance of the cables.

With high voltage, power lost is minimum.

Factor: High Voltage

To reduce resistance, thick cables can be used (cross-sectional area increases), but this is very uneconomical and heavy to use.

Step up the current in transformers instead

sample questions

Example 1

Question:Find the power wasted per km as internal energy in the cable when 10/ MW is transmitted through a cable of resistance 1 Ω per km (a) at 10kV(b) a 200kV

Example – Solution (a)

(a) P = VI Transmission current I = P/V = 107W / 104V = 1000 APower loss per km Ω = I2R = (1000) 2

x 1 = 106WThis is equivalent to almost 10% of the power generated at the power station

Example- Solution (b)

(b) Transmission current I = P/V =

107W/ 2x105V = 50

APower loss per km = I2R = (50) 2 x 1 = 2500 WThe power loss would only be about

0.025% of the power generated at the power station