What are harmonics in electrical networks

In the ideal case, the electric network should have an alternating voltage, which changes according to a sinusoidal law with a frequency of 50 Hz (50 times per second), if we are talking about domestic networks. In practice, the situation is different - the voltage is far from a sinusoidal shape, it is distorted, not only along the fronts, but along the entire length it is filled with various bursts and noise. This phenomenon is called harmonics in electrical networks. In this article we will take a closer look at what it is and why harmonicas are dangerous for equipment connected to the network.

Content:

Harmonic Detection
Sources of interference
Consequences of harmonic interference

Harmonic Detection

The graph of the signal, which varies according to the sinusoidal law, has the form:

But this is significantly different from the real form of voltage in the electrical network:

These jag and bursts are also caused by harmonics. We will try to talk about this phenomenon in simple words. The graph above can be represented as the sum of signals of different frequencies and magnitudes. If all this is added up, then the result will be just such a signal. An example and result of the addition of signals is shown in the graph below:

Harmonics are distinguished by numbers, where the first harmonic is the component with the largest value. However, such a description is too brief. Therefore, let's give a formula for determining the harmonic value. This is possible with harmonic analysis and Fourier expansion:

From this formula, one can also distinguish the frequencies and phases of the harmonic components of the electric network and any other sinusoidal signal.

Sources of interference

A number of equipment can be attributed to interference sources, from household appliances to powerful industrial electric machines. To begin, let's briefly consider the causes of their occurrence.

Harmonics in an electric AC network arise due to the characteristics of electrical equipment, for example, due to the nonlinearity of their characteristics, or the nature of current consumption.

For example, in three-phase networks in the magnetic circuits of transformers, the lengths of the magnetic paths of the middle and extreme phases differ almost 2 times, and therefore their magnetization currents differ up to one and a half times. From here arise harmonics in three-phase networks.

Another source interference in electrical engineering, these are electric motors, both three-phase synchronous and asynchronous, and single-phase, including universal collector motors. The latter type of engine is used in most household appliances, for example:

washing machines;
Food processors;
drills, grinders, rotary hammers, etc.

As a result of the operation of switching power supplies, high-frequency harmonics (interference) occur in the electrical network. To understand how they are formed, you need to have information about their internal structure. This is due to the fact that the primary current of the UPS is different from continuous, it flows only when the power semiconductor switch is open.And the latter opens and closes with a frequency above 20 kHz.

Interesting: The operating frequency of some modern switching power supplies reaches 150 kHz.

To reduce these harmonics, electromagnetic interference filters, such as common-mode chokes and capacitors, are used. To improve the graph of current consumption relative to the supplying single-phase voltage, active power factor correctors are used (Russian KKM, English PFC).

Such power supplies are installed in:

LED lamps;
Electronic ballasts for fluorescent lamps;
computer power supplies;
modern chargers for mobile phones;
TVs and other equipment.

Also, these power supplies include frequency converters.

To measure harmonics in the network, you can use multifunctional measuring instruments from EKF. In addition to harmonics, these instruments can measure network parameters such as current, frequency, voltage, active, reactive and apparent power, as well as power and frequency factors. In addition, multifunctional meters make it possible to control, analyze and optimize the operation of power equipment, systems and industrial circuits. They are quite easy to install and can be configured for any current transformer.

Consequences of harmonic interference

The presence of harmonics in an electrical AC network causes certain problems. Among them - increased heating of electric motors and power wires. The effects of harmonics are engine vibration. Further consequences may be different - from accelerated wear of the bearings of the rotor of the motor, ending with a breakdown on the body of the windings from increased heat.

In electrics, false alarms of switching and protective equipment are found - circuit breakers, contactors and magnetic starters. In sound equipment and communication technology, interference occurs due to harmonics. They fight with them in the same way - by installing electromagnetic interference filters.

The video below describes what harmonics and interharmonics are in the mains:

In conclusion, I would like to note that harmonics in electrical networks, in principle, are of no benefit. They only cause malfunctions, false alarms of switching equipment and other manifestations of instability in work. This can cause not only inconvenience in operation, but also economic problems, losses and emergency situations that can be life threatening.

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