Electricity is the main source of power in our daily lives. Convenience and cleanliness are among the most important advantages of using electricity. It is simple to access this virtually unlimited supply of energy at any home in Hong Kong. You simply plug an appliance into the mains supply, turn on the switch and run the appliance as long as you wish. Unlike burning fuels, electricity would not produce any undesirable waste or emission that pollutes the area of use. Without the danger of gas leakage or the storing of inflammable fuels, electricity is also one of the safest power sources.
With the extensive use of electricity, and the wide geographical distribution of users, an effective transmission and distribution system is essential. It is this extensive network that enables electricity to reach almost every family in Hong Kong from the city centre to remote areas. The history of electricity transmission can be dated back to 1883, when Thomas Edison first introduced an economically viable model for generating and distributing electric power. Edison's greatest achievement was perhaps not the invention of the light bulb or any other single application, but the universally applicable electricity transmission system which has lit up the whole world. Modern electrical transmission and distribution systems are the result of conscientious efforts and design skills of engineers to ensure high energy efficiency and safety. High energy efficiency means the loss of power through transmission is minimized. In this module, the theories and the design of transmission and distribution systems will be discussed in detail.
Electricity is transmitted mainly through overhead lines or underground cables. Due to the resistance of the transmission wires, there is always some loss of power through the heating effect of current. The electricity transmission systems must be designed in ways which reduce this loss as much as possible.
Electricity generated in power stations is raised to a very high voltage for transmission. This is an essential way to reduce heat loss. Do you know why? Consider electrical power transmitted at voltage V and current I . The power transmitted P is given by
For a fixed amount of electrical power transmitted, a higher voltage would give a smaller current. To see this relationship more clearly, one can write
Thus for a fixed power transmitted P, the current I is inversely proportional to the voltage V. When the current flows, the power loss by heat Ploss on a segment of wire of resistance R is given by
The power loss is proportional to the square of the current, thus a small current greatly reduces heat loss. As seen from equation (2), a small current can be achieved by using a high voltage. For example, if we double (×2) the transmission voltage, the current would be halved (×1/2), and the power loss would be reduced to a quarter, (1/2)2 = 1/4, i.e. 25% of the original value. In the following activity, you can systematically measure the power loss in a circuit for various transmission voltages, verifying that the loss is smaller for higher transmission voltages.
We see from equation (3) that the power loss in the transmission wire Ploss is directly proportional to the resistance R of the wire. The lower the resistance, the lower will be the power loss. How do engineers design a transmission wire that has the lowest possible resistance and yet is economical? The first consideration is the choice of material. Metals are good conductors with low resistance. Copper and aluminium are the most commonly used metals in transmission wires. They are very good conductors, cheap, resistant to corrosion, and strong. The resistance of the transmission wire is lowered by making the wire thicker. Thicker wires have larger cross-sectional areas and therefore lower resistance. The low resistance property of the overhead lines can actually be "seen". You can see birds standing on high voltage overhead wires without getting hurt. Theoretically the bird standing on the wire with both legs would cause a current to flow via its legs into its body. So why doesn't the bird get hurt?
To answer this interesting question, let us think of the situation as two resistors connected in parallel: one resistor is the body of the bird itself (the resistance between the two legs), and the other resistor is the small segment of wire on which the bird stands (Fig. 5). Since the wire segment is very short, thick and made of good conducting material, its resistance must be very, very small. On the other hand, the bird's body is almost an insulator having a very high resistance. When a potential difference V is applied to a resistance R, the current I is given by
Since both the bird and this wire segment are subject to the same potential difference (resistors in parallel), the current that passes through the bird (very large resistance R ) must be much smaller than the current that passes through the wire (very small resistance R ). In other words, the bird doesn't get hurt because the current flowing through its body is extremely small!
Electrical transmission by overhead wires
Overhead lines are held high above the ground by metal towers called pylons. Since a metal tower conducts electricity very well, how can engineers prevent electricity from leaking to the ground (i.e. the earth) through the tower? If you look at a pylon carefully, you will see that the overhead lines are held by a stack of discs hanging from the pylon. This stack of discs is a series of suspended insulators which prevents the line from being electrically connected to the pylon. With no connection to the line, the pylon is not earthed. The structure of each insulating disc is shown in Fig. 8. The insulating part of the disc is made of porcelain, which is a very good electrical insulator. It is resistant to weathering by wind and rain, and is also mechanically strong enough for holding the heavy lines. Metallic components are fixed to the top and the bottom of each porcelain disc to link the discs together to form a hanging chain (see Fig. 7).
The number of discs used to hold the wires is carefully chosen to provide sufficient insulation. Engineers must carefully consider the possibility of current leakage through the stack itself, and through a discharge between the pylon and the wire in the air. More discs will give better insulation and provide a thicker layer of air to prevent conduction through these two pathways. The umbrella shape of the insulating disc also has a special purpose: it prevents water from forming a conductive path along the stack during rainy days. Besides low resistance, overhead lines should also be strong enough to withstand wind and their own weight (1 km length of metal wire can be very heavy!) and they must be resistant to corrosion by rain. Cored wire has a strong core such as steel inside for added strength.
Technically, maintaining a high voltage transmission is not a problem. However, high voltage means greater safety measures. Direct contact with the wire is of course extremely dangerous. Even getting close to the wire may trigger an electrical discharge from the wire to the body through the air, just like lightning. Safety must be carefully considered when setting up high voltage transmission. The power companies in Hong Kong (HEC and CLP Power) have safety guidelines for on site workers needing to work near the overhead wires. Safety Zones are determined so as to ensure a minimum distance between the working person/equipment and the transmission wires. No work is allowed in bad weather such as lightning storms and typhoons. Specific considerations are given to nearby trees that may be cut or blown down.
In well-developed urban areas where no land space can be spared for building pylons, underground cables are used to transmit electricity. As their name suggests, these cables are buried underground, avoiding any danger of contact during operation, and the effects of bad weather conditions like a thunderstorm. Although underground cables have many advantages, designing, building and installing underground cables require more advanced technology. This explains why they are as much as 10 times more expensive than overhead wires. Unlike overhead wires, underground cables must have a very good electrical insulation because they are in direct contact with the soil. Their direct contact with the soil may lead to mechanical damage and problems with cooling. The sophisticated design of modern underground cables includes a metallic cable sheath to protect the cable from any mechanical damage, sufficient insulation to prevent current leakage, and strengthening materials to allow the cable to withstand the high stress due to the heat released during high voltage transmission. Typically, for high voltage transmission (e.g. 132 kV or 275 kV), the system predominantly comprises fluid-filled or cross-linked polyethylene insulated cables.
Traditional installation of underground cables involves trenching (digging up the ground). This requires detailed planning to minimize the impact on traffic and inconvenience to the public. For an urban territory like Hong Kong Island, the dominance of narrow roads and existing underground services make this kind of installation difficult. The use of cable tunnel is one of the feasible solutions for overcoming these constraints. In all cases, dedicated supervision and protective measures are required during the installation of cables. Some comparisons between overhead lines and underground cables are listed in the table below.
As mentioned, a very high voltage transmission can minimize the power loss of wires through the heating effect. Yet electricity generated from power stations is not at such a high voltage as that running through the transmission cables. Note also that electricity is supplied to end users at a low voltage, making changes in the voltage necessary. In the power stations, the voltage has to be raised for transmission. Raising the voltage is called step up. Before reaching end users, the voltage has to be lowered. Lowering the voltage is called step down. The step up and step down of voltage is done by transformers. These work on the principle of electromagnetic induction.
A transformer basically has two coils from two separate circuits, namely, the primary coil and the secondary coil. The primary coil is the input coil and the secondary coil is the output coil. The coils are wound on the two sides of an annular iron core. When an alternating current (AC) is fed to the primary coil, it generates a magnetic field which changes at the same frequency as the current. The iron core has the function of guiding almost all the magnetic field lines to pass through the secondary coil. The changing magnetic field then produces a current in the secondary coil by electromagnetic induction. Considering the relationship between the input voltage (primary voltage Vp ), the output voltage (secondary voltage Vs ), the number of turns Np in the primary coils and the number of turns Ns in the secondary coils, it can be shown that
From equation (5), we see that if Ns > Np , then Vs > Vp , i.e. the voltage is raised and the transformer is said to be a step-up transformer. If Ns < Np , then Vs < Vp , i.e. the voltage is lowered and the transformer is said to be a step-down transformer. In summary, stepping up requires a transformer whose primary coil contains fewer turns than the secondary coil, and stepping down requires a transformer whose primary coil contains more turns than the secondary coil. An ideal transformer causes no energy loss, i.e. all the input power is converted to output power. Applying P = IV, we have
where Ip is the primary current and Is is the secondary current. Using (5) and (6), we have
Equation (7) relates the input and output currents for an ideal transformer.
In reality, transformers are never ideal, there is always some energy loss. This occurs in three main ways. First, the coils themselves have resistance. As currents flow, some energy is lost as heat. Second, some energy is dissipated as heat when the iron core undergoes continuous magnetization and demagnetization. Third, the changing magnetic field induces currents called eddy currents in the iron core. This produces heat in the core. However, modern transformers have very high efficiencies of up to 90% - 99%. Energy loss is greatly minimized by using wires of low resistance, "softer" iron, and laminated iron core to reduce eddy currents.
After electricity is generated from the power stations, it is stepped up to a high voltage and transmitted through overhead wires, underground cables and/or submarine cables to different regions of Hong Kong. There the electric power is then stepped down to a lower voltage by large capacity transformers. The power is then distributed to customer substations, which are located much closer to the users. The substations further step down the voltage and finally distribute the power to the customers for use. The following table shows the generation, transmission and distribution voltages of the two power companies in Hong Kong:
To see an overview of electricity transmission and distribution in Hong Kong, take a look at this animation.
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