Formal Homework 3: Generation and Transmission of Electricity

Q1.A thermal power station burns Fossil Fuels and has an efficiency of 40%. A combined heat and power station burns biomass and is more efficient; it uses heat to produce hot water for homes as well as generating electrical energy. The energy output for each power station is shown in the diagram below:

Thermal power station

Combined heat and power station

(a)(i) State one fossil fuel that might be burned in the thermal power station. 1

(ii) State one source of biomass that might be burned in the combined heat and power station. 2

(b)(i) Calculate the percentage of waste heat for the thermal power station.3

(ii) Calculate the total percentage useful energy output of the combined heat and power station. 2

(8)

Q2.Electrical power is distributed by the National Grid system. Transformers are used in this grid system.

(a) State why transformers are used in this grid1

(b) The voltages required at various stages of the grid system are shown:

Stage / Voltage (volts)
Super Grid / 400 000
National Grid / 132 000
Heavy industry / 33 000
Light industry / 11 000
Homes / 230

A transformer transfers power from the National Grid to heavy industry. The primary coil of the transformer has 6000 turns.

(i)State the voltage at the primary coil.1

(ii)State the voltage output at the secondary coil1

(iii)Calculate the number of turns in the secondary coil3

(6)

Q3.Model power transmission lines are set up to demonstrate howelectricity is distributed from a power station to consumers.

(a) Why must a.c. be used in this circuit?1

(b)State the purpose of transformer X.1

(c) For safety reasons, in this model, the voltage in the transmission lines must not exceed 25 V.

Show that the transmission voltage is within this limit.3

(d) The current in the transmission lines is 200 mA.

The transmission lines have a total resistance of 20 Ω.

Calculate the total power loss in these transmission lines.3

(8)

Q4.Electricity is often transported long distances from power stations, where it is generated, to towns and cities, where it is needed. Typically, electricity is carried in overhead power lines. However, in recent years, there have been increasing calls for electricity cables to be buried under the ground.

(a) State two areas where electricity transmission cables might be buried underground. 1

(b) Give two advantages of overhead power lines compared to underground cables. 2

(c)Give two advantages of underground cables compared to overhead power

lines.2

(5)

Total (27)