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wer is delivered to the customer in very small quantities at low voltage
levels. A typical customer will consume only about 0.001% or 0.0001% of the output
of a large generator.
An efficient power delivery system builds upon these fundamental truths and has evolvedinto a system of hierarchical voltage levels that contains three distinct functions,generation, transmission and distribution.Generation plant such as nuclear power stations with high capital cost and low running costare economical only if operated for most of the time at near maximum output. On the otherhand, gas turbine stations with relatively low capital cost but high operating costs are
operated for short periods to meet demand peaks. Fossil fuel burning steam power stationstogether with nuclear power stations provide the majority of electrical energy consumed inthe EU.
The transmission system then transfers power in bulk from the main generation areas tomajor load centres at high voltage. The distribution system carries the energy to the most
S1: The Electricity Supply System
4 EEE8044: Fundamentals
remote customer, utilising the most appropriate voltage level. As power is dispersedthrough the system, it gradually moves down to lower voltage levels where it is moved inever smaller quantities on lower capacity equipment until it reaches the end users.
Exercise 1.1
Why do we bother with generation and transmission at very high voltage levels such as
400 kV and 275 kV? Why don’t we simply generate and distribute at a sensible voltage
level like 11 kV and then drop to 415 for domestic distribution?
Turn to the end of the book for suggested answers to the exercise
1.3 Generation, Transmission & Distribution in the UK
The United Kingdom benefits from a highly developed electricity interconnected networkwith a 400 kV super-grid transmission system.
Most electricity in the UK is generated in large power stations running on coal, gas ornuclear energy. These are connected directly to the 400 kV transmission network. As wellas these big generators, numerous small generators (anything from 100 MW to less than 1
kW output) are also connected to the low voltage distribution network. These are referredto as distributed (or embedded) generators and are usually fuelled by landfill gas,combined heat and power plant, wins energy, solar energy and other types of renewable
energy sources.The transmission and distribution networks are made up of many interconnected layers,each made up of many network components (cables, overhead lines, etc.) operating at acertain nominal voltage. These layers are connected via transformers allowing the flow of
power down through the system from the higher voltage network to the lower voltagenetwork.
Definitions of what is transmission and what is distribution vary a great deal amongdifferent countries, companies and power systems. In the UK, voltage class generallymakes the distinction. 400 kV and 275 kV are transmission. The distribution network
includes anything at 132 kV and below, i.e. 132 kV, 66 kV, 33 kV, 11 kV 6.6 kV and
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5 EEE8044: Fundamentals
415 V. Other voltages are present on the network, often for historical or geographical
reasons. Figure 1.1 demonstrates the interrelationship of the various networks.
The term ‘nominal voltage’ is used to refer to a particular layer of the electricity network.
Hence engineers will talk about the