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􀂃 Give examples of four methods used to reduce or eliminate harmonics from a

distribution network.

Time

You will need 3 hours to complete this section

Resources

Access to the Internet, access to spreadsheet package, calculator, pen and scrap paper

S7: Supply Harmonics

109 EEE8044: Fundamentals

7.1 Effects of Harmonics

The presence of harmonics on the supply network is often referred to as a form of

“pollution”. Harmonics reduce the power factor of the load and lead to problems of

increased electrical losses, unnecessary oversizing of cables and transformers and possible

failure of power factor correction capacitors. The presence of harmonic components in the

current and voltage waveform also causes problems with metering and the satisfactory

operation of communication systems, electronic equipment and relay protection devices.

Details of harmful harmonic effects [1] on some common types of electrical equipment are

discussed the following sections.

7.1.1 Transformers

Compared to operation from a purely sinusoidal supply, the main effect of harmonics is to

increase transformer losses. Voltage harmonics give rise to an increase in iron losses, and

current harmonics cause an increase in copper losses and stray eddy current losses.

7.1.2 Cables

The flow of harmonic currents in a power cable will lead to higher losses and possible

derating when compared to operation with a sinusoidal load. Furthermore, the flow of high

frequency current harmonics will cause extra heating over and above what we might

expect from considering the rms value of the non-sinusoidal current waveform.

Third and other triple harmonic frequencies produce zero-sequence currents that flow in a

4-wire system and can cause overheating of the neutral conductor.

7.1.3 Capacitors

Power factor correction capacitors used in the presence of non-linear loads provide a low

impedance path to the harmonic currents leading to capacitor overloads increasing

capacitor heating and dielectric stresses.

Another major concern when using power factor correction capacitors is the possibility of

introducing resonance into the system. Resonant circuit conditions will occur when the

capacitive and inductive impedances of a circuit are equal, leading to unacceptable levels

of harmonic distortion much higher than would be the case in the absence of resonance.

S7: Supply Harmonics

110 EEE8044: Fundamentals

7.1.4 Switchgear

Electrical losses and heating in switchgear and fuses are increased by the presence of

harmonic currents resulting in a reduction of the steady-state current rating of the

equipment.

7.1.5 Metering and instrumentation

Metering and other instrumentation devices are usually designed for fundamental current

conditions and may give inaccurate readings in the presence of harmonics. However, the

levels of distortion must be pretty high, as can happen in the presence of resonant

conditions, before any significant errors are reported.

7.1.6 Protective relays

Because of the variety of relay designs and nature of distortion that can occur, it is almost<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。