Electricity Generation and Transmission in Hong Kong

[Print]

We cannot live a day in comfort without electricity. Our lighting, home appliances, computers, the mass transport systems (e.g. MTR and KCR trains), and numerous other machines are all powered by electricity. In Hong Kong, we are consuming over 3.8 × 10¹⁴J of electrical energy every day! [1] In this module, you will learn how such a large amount of electrical energy is generated and transmitted.

In Hong Kong, electricity consumed is generated using coal, natural gas and nuclear energy as the sources.

Fig. 1   The Lamma Power Station of HEC (photo courtesy of HEC)	Fig. 2   The Castle Peak Power Station of CLP Power (photo courtesy of CLP Power)

Electricity generation in power plants

Coal-fired power plants

The power plants in Hong Kong use mainly coal and natural gas as fuel to generate electricity. The fuel releases chemical energy upon combustion; hot steam or gas is generated and used to drive turbines, thus converting part of the internal energy of the hot steam or gas into useful kinetic energy. Turning turbines then operate a generator to produce electricity. Examples of coal-fired power plants can be found in the Lamma Power Station of The Hongkong Electric Co., Ltd. (HEC) and Castle Peak Power Station of CLP Power Hong Kong Limited (CLP Power).

Do you know the different components of a coal-fired power plant, and how these components function to generate the electric power we need? Test yourself with the game below:

Click on the game below to learn how a coal-fired power plant works:

Fig. 3   Generator and turbine of Black Point Power Station, CLP Power

In Hong Kong, power plant generators are turned at a precise rate of 3000 rpm during operation to generate an AC of frequency 50 Hz. In order to withstand the tremendous force in rapid rotation and to optimize the output during operation, the design and construction of the generator is detailed and complex.

Although the actual construction of a power plant generator is complicated, it still relies on the basic principle of electromagnetic induction to generate electricity. Have you learnt this principle in physics lessons? When a coil is moved through a magnetic field, an induced current flows in the coil. You can also get an AC current with a simple AC generator found in a school laboratory. However, an AC generator that serves us every day in a power plant looks very different. See the activity below:

Natural gas power plants

Natural gas is gaining importance as the fuel for electricity generation in Hong Kong as well as worldwide. Natural gas is cleaner when burning and can burn at a higher temperature. A higher temperature is more favorable in achieving higher efficiency for electricity generation. But a high temperature also places higher demand on the material of the turbine. The recent advancements in metallurgy, ceramics and material technologies allow natural gas to be used efficiently in electricity generation.

Fig. 4   Steam turbine of a power plant generator. Photo courtesy: CLP Power

Gas turbines can be used in power plants that use natural gas or some forms of fuel oil like industrial gas oil or diesel oil as the source of energy. The technologies applied to gas turbines for electricity generation are borrowed from those developed for aircraft jet turbines. Gas turbine power plants designed for combined cycle operation have a higher efficiency. Once the hot gas has pushed the gas turbine, this gas has a lower temperature than before, but is still very hot and can be used to heat steam. The hot steam in turn pushes a different set of turbines to generate extra electricity. Thus the overall efficiency is increased when compared to that of a single cycle operation. The Black Point Power Station in Tuen Mun is an example of a combined cycle natural gas power plant and has a maximum efficiency of 52.9% [2], compared to the 34% to 40% efficiency of coal-fired power plants [3]. The Lamma Power Station also has a combined cycle block of two gas turbines with steam generators to increase efficiency and generation capacity.

The following Flash animation will help you get a clearer picture of how a combined cycle operation can enhance the energy efficiency of power generation.

Nuclear power plants

Fig. 5    The Guangdong Daya Bay Nuclear Power Station (photo courtesy of CLP Power)

CLP Power also obtains power from the Guangdong Daya Bay Nuclear Power Station. Nuclear power stations use uranium as the source of energy for electricity generation. Uranium is a kind of heavy metal element. When it undergoes nuclear fission reaction, a large amount of energy is released and this is used as the source of energy for electricity production. While conventional power plants use chemical energy, nuclear power plants use nuclear energy for electricity generation.

Transmission and distribution system in Hong Kong

Fig. 6    Overhead lines and substations of CLP Power (photo courtesy of CLP Power)

Power plants are usually far away from populated areas. Underground cables and/or overhead lines are required to transmit and distribute the electricity from the power plants to the users living in populated areas. A system of underground cables and overhead lines forming a transmission network carries the electricity over long distances to the substations close to populated areas. In the substations the voltage is stepped down. Through another system of cables forming the distribution network, the electricity is distributed over shorter distances to the users.

In Hong Kong, transmission is done at the voltage of 275 kV and 132 kV for HEC and 400 kV and 132 kV for CLP Power [4].

Fig. 7    A schematic diagram showing the transmission of electricity

The use of high voltage for transmission over long distances is energy efficient. Although the resistance per unit length of the thick transmission cables is very low, over long distances, the total resistance is significant. Transmitting at high voltage reduces the current passing through the cables, and hence the power lost as heat along the cables, which is equal to I ²R, is also reduced.

The following diagram shows the transmission system of CLP Power.

Fig. 8    The transmission system of CLP Power (click here to view an enlarged version) (image courtesy of CLP Power)

When electricity is generated at the power plant, the output is always in the form of an alternating current (AC) of 50 Hz to easily facilitate voltage step up or step down. The frequency of the alternating current is fixed at 50 Hz throughout the whole generation, transmission and distribution processes. Depending on the design of the generator, the initial voltage output from the generator may be different. The output voltage is about 22 kV. So the output voltage has to be stepped up to the transmission voltage through the use of transformers. The transformers in the electricity transmission and distribution systems are very large, close to the size of a house and have to handle a lot of power. They are also designed to be around 99% efficient [5].

The transmission voltage is very high. For safety and practical reasons, when the cables get close to populated areas, the voltage is stepped down and distributed by the distribution system. In Hong Kong, the distribution voltages are 11 kV, 22 kV or 380 V (three-phase) for HEC and 11 kV, 33 kV or 380 V (three-phase) for CLP Power. The stepping down of the voltage is done by the transformers located in substations. The distribution system distributes electricity throughout populated areas. After further stepping down, electricity is supplied to the users at 50 Hz AC, and at the voltage of 220 V (single-phase) for the mains supply and 380 V (three-phase) or industrial use.

References