Transmission and Distribution of Electricity in Hong Kong  Print page

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.

Fig. 1   Thomas Edison's greatest contribution was perhaps his economically viable model for generating and distributing electric power. Fig. 2   The fantastic night view of Hong Kong is made possible by an effective electrical transmission and distribution system. (Photo courtesy of HEC)

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.


Transmission of electricity

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.


High transmission voltage

Fig. 3   High voltage transmission lines on a transmission substation

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

P = IV (1)

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

I  =   P
V
(2)

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

Ploss = I 2R (3)

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.

Activity: Efficiency of transmission of electricity


Low resistance transmission wires

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?

Fig. 4   Why don't birds get hurt when standing on a transmission line? Fig. 5   The current passing through the bird is extremely small compared with that in the transmission wire.

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

I  =   V
R
(4)

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!


Overhead wire and underground cable system

Electrical transmission by overhead wires

Fig. 6   Pylon holding overhead lines

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).

Fig. 7   Stacks of insulating discs are commonly used to hold high voltage transmission lines. Fig. 8   Note the umbrella shape of an insulating disc

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.


Safety concerns with high voltage transmission

Fig. 9   You may have seen this warning sign on pylons when hiking

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.


Electrical transmission by underground cables

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.

Fig. 10   A cable tunnel of HEC (photo courtesy of HEC)

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.

  Underground lines Overhead cables
Configuration and cost
  • high-voltage cables buried underground
  • higher per-km cost
  • high-voltage electric wires hanging on pylons
  • lower per-km cost
Application and protection against inclement weather
  • generally used in urban areas where no space for building pylon is available
  • less affected by lightning and storms
  • power carrying capacity is lower than that of overhead cable with the same size due to severe insulation constraint
  • generally used in rural / suburban areas where more space is available for building pylons
  • require appropriate measures against lightning and storms e.g. ground wire, surge arrestors, auto-reclosing scheme
  • Able to transmit more power than the same-sized underground cables
Installation
  • complicated, involves trenching, detailed route planning or the use of cable tunnel
  • straightforward, setting up of pylons and wires is relatively simple
Design and Construction
  • Insulation of cables important as they are in direct contact with soil
  • Protection against mechanical damage and thermal stress required
  • Longer construction period
  • No insulation needed on wire surface as the wires are naturally insulated by air
  • Little or no protection required, natural cooling of wire in air
  • Shorter construction period


Step up and step down of voltages

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.

Fig. 11   A simple transformer

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

Vs  =  Ns
Vp Np
(5)

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

IsVs = IpVp (6)

where Ip is the primary current and Is is the secondary current. Using (5) and (6), we have

Is  =  Np
Ip Ns
(7)

Equation (7) relates the input and output currents for an ideal transformer.

Fig. 12   Transformer used in electricity transmission (photo courtesy of HEC)

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.


Fig. 13   Transformer in a substation of CLP Power

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:

Electric company generation voltage transmission voltage after first step down by
large capacity transformers
after second step down by
customer substations
HEC 12.5 kV to 22 kV, depends on generator design 275 kV, 132 kV 22 kV, 11 kV 380 V (or 220 V single phase)
CLP Power 11 kV, 18 kV, 23 kV and 23.5 kV, depends on generator design 400 kV, 132 kV 33 kV, 11 kV, 380 V 380 V (or 220 V single phase)

To see an overview of electricity transmission and distribution in Hong Kong, take a look at this animation.

Flash animation: Transmission and distribution of electricity in Hong Kong

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