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harmonic that may be permitted anywhere on the system. Table 7.5 shows IEEE standard

519-1992 distortion limits at the various system voltages.

Table 7.5 IEEE Standard 519-1992 recommended voltage distortion limits

System Voltage Individual

Harmonics voltage

distortion

Total Harmonic

Distortion (THD)

69 kV and below 3.0% 5.0%

115 kV – 161 kV 1.5% 2.5%

Above 161 kV 1.0% 1.5%

Finally, there is IEC standard 61000 which not only limits the harmonic distortion levels

produced by electrical equipment but also deals with EMC compatibility and specifies that

electrical equipment should be robust enough to operate from supplies with voltage

distortions within stated limits.

Exercise 7.1

In Equation 7.2 can you suggest why we stop our summation at the 50th harmonic?

Turn to the end of the booklet for suggested answers.

S7: Supply Harmonics

116 EEE8044: Fundamentals

7.4 Harmonic Measurements

Site measurements of voltage and current harmonics are regularly carried out to assess

compliance with relevant standards, monitor the level of existing harmonics, help in the

planning and connection of new non-linear loads and for trouble shooting purposes in

situations where there is a harmonics related problem with the performance of electrical

equipment.

A range of measuring equipment is available for the analysis of nonsinusoidal voltages and

currents ranging from spectrum analysers that display the power distribution of a

nonsinusoidal signal as a function of frequency to specialised harmonic analysers that

measure the amplitude of a periodic function and gives the frequency spectrum of the

signal and even distortion analysers that give a direct reading of the THD.

Exercise 7.2

You need to perform some site harmonic measurements. Use the www to search for

possible suppliers of the three types of analysers and compare their specifications.

List the web sites that you visited below.

S7: Supply Harmonics

117 EEE8044: Fundamentals

7. 5 Harmonic Calculations

For most systems, the calculations of system harmonic distortion levels require the use of

some computer simulation software to aid the system planner in his analysis. However, for

simple distribution systems and some industrial supply networks, it is possible to perform

the analysis using simple manual calculations. This requires knowledge of the

characteristics of the harmonic generating non-linear load. For most power electronic

devices, it is possible to determine the harmonic spectrum analytically. For other types of

non-linear loads, such as arc furnaces, it is often necessary to use typical characteristics in

the absence of any better load information.

For purposes of analysis, most harmonic sources may be represented as ideal current

sources (at each harmonic frequency) with power flowing from the harmonic sources

through the network from the customer up into the higher voltage distribution system and

into other loads.

Hand calculations can be performed when the system under consideration can be

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