英语论文网

Supply Harmonics

Abstract

Electricity in the UK, and everywhere else for that matter, is generated, transmitted and

distributed using a system of three-phase sinusoidal voltages. Most industrial and domestic

equipment and appliances are designed to operate with 50 Hz sinusoidal voltages and

currents and it is therefore important to maintain the quality of the supply, which must

remain as close as possible to that of an ideal sine wave.

On the 400/275 kV UK transmission system, there are no harmonic producing loads and

the voltage waveform is almost a pure sine wave. However, some customer loads

connected to the distribution network do not take sinusoidal currents when connected to a

sinusoidal voltage and harmonics will flow. These are commonly described as non-linear

loads. These non-system frequency currents flowing through the distribution network can

cause operating problems on both the customer and supplier sides.

Such nonlinear loads include devices such as static power converters, arc or induction

furnaces, saturable electromagnetic devices, fluorescent and discharge lamps and switched

mode power supplies (PCs, VDUs, TVs, etc.). All these generate harmonics in the system.

Of these, the main source of supply harmonics are static power conversion equipment used

in industry for a variety of purposes, such as motor soft starts, rectifier equipment, variable

speed drives and uninterruptable power supplies. Such loads change the sinusoidal nature

of supply current (and consequently the ac voltage drop across the supply transformer)

resulting in the flow of harmonic currents in the power distribution system and the

distortion of the supply voltage at the point of common coupling with other customers, i.e.

the point in the public supply network at which other customers’ loads are, or may be,

connected.

Harmonic frequencies have integer multiples of system frequency. In a 50 Hz system,

harmonics will occur at 100 Hz, 150 Hz, 200 Hz, 250 Hz, 300 Hz, 350 Hz, and so on.

These harmonics are referred to, respectively, as the second harmonic, the third harmonic,

the fourth, the fifth, sixth, seventh and so on. The system frequency is the fundamental or

first harmonic.

In this unit, we will examine the effects of these harmonics and some of the national and

international harmonic standards and recommendations commonly used for guidance in the

design of power distribution systems operating with non-linear loads. Waveform distortion

limits at the point of common coupling are established and methods of harmonic reduction

and elimination are discussed.

S7: Supply Harmonics

108 EEE8044: Fundamentals

Learning Outcomes

On completing this section you will be able to

􀂃 Identify the main sources of harmonic pollution in a power distribution network

􀂃 Describe the effects of current harmonics and harmonic voltage distortion on two

common types of electrical equipment

􀂃 Define the total harmonic distortion (THD) and calculate its value at the point of

common coupling for simple circuits

􀂃 Compare national and international standards relati本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。