The old driver popularizes science for you What is a harmonica? How are power system harmonics generated?

01 Harmonic Value

In vibration theory, it is considered that sine wave with a certain frequency and largest amplitude in wave generated by vibration is called fundamental wave. Other wavelets with frequencies above fundamental frequency are called harmonics.

Definition of harmonics in a power system: Fourier expansion of periodic non-sinusoidal electricity (voltage or current), in addition to obtaining same component as main frequency of power system, a number of components exceeding fundamental frequency. power grids can also be obtained, this part of electricity is called harmonics.

(main wave and its expression)

(fundamental wave and 2.3.4th harmonic)

(Composite wave after adding harmonics 2.3.4)

02 Harmonic generation

The power system is a single whole, consisting of four links: electricity generation, electricity conversion, electricity transmission and distribution, and electricity consumption. Each link can generate harmonics.

Link to power generation: skip...

Link transformation: a bit...

Power transmission and distribution line: an omission...

Communication with electricity: harmonics in an electrical system are mainly caused by non-linear loads. Since a sinusoidal voltage is applied to a non-linear load, fundamental current is distorted, creating harmonics. The main non-linear loads are UPS, switching power supply, rectifier, frequency converter, inverter, etc.

Third harmonic formula:

Complex formula:

03 Meaning of non-linear equipment

In short, waveform of current generated by equipment is intermittent or abrupt. It is not a continuous function that can be governed by a linear law. It is usually described by a Fourier series.

Example:

The figure above shows a three-phase rectifier circuit. The rectified current waveform is a stepped square wave. The figure on right shows frequency spectrum expanded according to Fourier series. It can be seen that proportion of fifth and seventh harmonics is very high.

Another example:

The figure above shows a single-phase rectifier circuit. The waveform of rectified current is intermittent. The figure on right shows spectrum extended according to Fourier series, in which third harmonic fraction is very high.

04 Features of third current harmonic

Third harmonic current is mainly generated by single-phase non-linear loads (such as fluorescent lamps, energy-saving lamps and ballasts, etc.).

Due to particular frequency, consequences of third harmonic on neutral line of mains are especially serious. In a three-phase electrical network, phase difference of each phase of fundamental wave is 120°, and phase difference of third harmonic is 360°.

As you can see from figure below, when third harmonic of current in each phase line is collected on neutral line, its instantaneous value is directly added across phase, so third harmonic of current on neutral line is usually about 3 times larger. third harmonic of current in phase line is even greater than fundamental current in phase line.