Ideally, there should be an alternating voltage in the electrical network, 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 form, it is distorted, not only along the fronts, but also along its entire length, 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 harmonics are dangerous for equipment connected to the network.
Content:
- Determination of harmonics
- Sources of interference
- Consequences of harmonic interference
Determination of harmonics
The signal graph, which changes according to a sinusoidal law, looks like:
But this is significantly different from the real form of voltage in the electrical network:
These jags and spikes are caused by harmonics. We will try to describe this phenomenon in simple terms. The graph shown above can be thought of as the sum of signals of varying frequency and magnitude. If you add all this, then the result is just such a signal. An example and result of the addition of signals is shown in the chart below:
Harmonics are distinguished by numbers, where the first harmonic is the component with the largest value. However, this description is too short. Therefore, let's give a formula for determining the magnitude of the harmonic. This is possible with harmonic analysis and Fourier series expansion:
From this formula, it is possible to single out the values of the frequencies and phases of the harmonic components of the electrical network and any other sinusoidal signal.
Sources of interference
Interference sources include a variety of equipment, ranging from household appliances to powerful industrial electrical machines. First, let's take a quick look at the reasons for their occurrence.
Harmonics in an AC electrical network arise from 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 by almost 2 times, therefore, their magnetizing currents differ up to one and a half times. From here, harmonics arise 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, hammer drills, etc.
As a result of the operation of switching power supplies, high-frequency harmonics (interference) arise 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 current of the primary winding of the UPS differs from the continuous one, 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.
EMI filters such as common mode chokes and capacitors are used to reduce these harmonics. To improve the graph of current consumption relative to the supply single-phase voltage, active power factor correctors are used (rus. KKM, eng. PFC).
These 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, frequency converters can be attributed to these power sources.
Consequences of harmonic interference
The presence of harmonics in an AC power grid causes certain problems. Among them - increased heating of electric motors and supply wires. The effects of harmonics are motor vibration. Further consequences can be different - from accelerated wear of the motor rotor bearings, ending with a breakdown on the winding housing from increased heating.
In electrics, there are false alarms of switching and protective equipment - circuit breakers, contactors and magnetic starters. Interference occurs in audio and communication equipment due to harmonics. They are fought with in the same way - by installing electromagnetic interference filters.
The video below describes what harmonics and interharmonics are in the power grid:
In conclusion, I would like to note that harmonics in electrical networks, in principle, do not have any benefit. They only cause malfunctions, false alarms of switching equipment and other manifestations of instability in operation. This can cause not only inconvenience in operation, but also economic problems, losses and accidents that can be life-threatening.
Related materials:
- Why does the circuit breaker work?
- Voltage drops in the network
- Reasons for burning out LED lamps
