SET Chapter 11 PDF - Electrical Supply Systems

Learning Outcomes Explain how electricity is transmitted from a power station to consumers Explain how electricity is distributed in residential, commercial and industrial premises Explain the function of transformers in the transmission and distribution of electricity State the voltage...

Learning Outcomes Explain how electricity is transmitted from a power station to consumers Explain how electricity is distributed in residential, commercial and industrial premises Explain the function of transformers in the transmission and distribution of electricity State the voltages for the generation, transmission and distribution of electricity in Singapore 1.11.1 Introduction Electricity is generated in a power station. The type of fuel consumed by the power station depends extensively on the natural resources available, which include coal, oil, natural gas, nuclear power, water, diesel fuel and biomass (or waste). There are also more environmentally friendly ways of generating electricity on a smaller scale, such as by using wind power, solar power and tidal barrages. Some common types of power stations are: thermal power stations hydro-electric power stations nuclear power stations The generation of electricity is a process of energy conversion. The stages of energy conversion in a thermal power station are shown in Fig. 1.11-1. Oil Coal Heat Mechanical Electrical Natural gas Fig. 1.11-1: Stages of energy conversion Unit 1.11 | Electrical Supply Systems 76  1.11.2 Components of a Typical Thermal Power Station Fig. 1.11-2 shows the main components of a typical thermal power station. Water Fig. 1.11-2: Typical thermal power station 1.11.3 Generation of Electricity Electricity is generated in a thermal power station by the following processes (Fig. 1.11-3): Heat from combusting the fuel is used to turn the water in the boiler to steam. The powerful jets of steam are directed to drive the blades of the turbine. The turbine, which is coupled to the rotor of the AC generator (or alternator), in turn drives the rotor of the generator. The rotation of the rotor eventually results in electricity being produced by the alternator. Fig. 1.11-3: Process for Generation of Electricity The alternator produces AC electricity. The combination of the turbine and alternator is known as the turbo-alternator. The alternator in the power station produces a three-phase supply. Unit 1.11 | Electrical Supply Systems 77  1.11.4 Transmission of Electricity The electricity generated at the power station must be increased to a very high voltage, by step up transformers, before it can be transmitted over a considerable distance through a network of cables to the main substations, which in turn feed secondary or smaller substations. A step-up transformer is used to increase the voltage. The cables used for the transmission of electricity are known as transmission lines. Stepping up the voltage before transmission reduces the amount of current flowing in the cables, which results in: smaller cable sizes and switchgear capacities; a reduction in voltage drop in the transmission lines; and lower power losses in the transmission lines. Unit 1.11 | Electrical Supply Systems 78  1.11.5 Distribution of Electricity After transmission, the high-voltage (HV) electricity is fed to smaller substations, where its voltage is reduced, by step-down transformers, before it is distributed to consumers. A step- down transformer is used to decrease the voltage. The cables used for the distribution of electricity are known as distribution lines. Distribution lines include: feeders, which are conductors that connect one substation to another; distributors, which are conductors that are tapped to supply consumers with electricity; and service cables, which are conductors that connect the distributors to the consumers’ premises. Figs. 1.11-4 and 1.11-5 show how electricity is transmitted and distributed to consumers, which can be done either by overhead lines or underground cables. Power station Step-up Transformers Step-down Transformer Fig. 1.11-4: Transmission and distribution of generated electricity Overground box (for Power distribution station of electricity) Fig. 1.11-5: Typical voltages used in the transmission and distribution network Unit 1.11 | Electrical Supply Systems 79  1.11.6 Distribution System for Large Consumers Fig. 1.11-6 shows a typical distribution system for large consumers of electricity: For some large consumers like refineries, the supply is taken directly from the transmission lines at 66 kV. There are other large consumers (in the industrial and commercial sectors) where the supply is taken from the distribution lines at 22 kV or 6.6 kV. Examples are factories, and multi-storey shopping complexes. A local substation is required at the premises to enable the consumers to reduce the voltage for their use, through step-down transformers. Refineries Hotels, Factories Shopping complexes Fig. 1.11-6: Supplying electricity to large consumers Unit 1.11 | Electrical Supply Systems 80  1.11.7 Distribution System for Small Consumers Low-voltage (LV) consumers are provided with: single-phase, 230 V, 50 Hz and three-phase, 400 V, 50 Hz supply; or single-phase, 230 V, 50 Hz supply. The system used for distribution is the three-phase, four-wire system shown in Fig. 1.11-7. Fig. 1.11-7: Distribution system for small consumers The star point on the secondary side of the delta-star step-down transformer is connected to the earth; it also serves as the neutral terminal for single-phase supply. On the secondary side, the voltage between any two phases is 400 V, 50 Hz, while the voltage between any phase and the neutral is 230 V, 50 Hz. Unit 1.11 | Electrical Supply Systems 81