What is a harmonic?
What is a harmonic? Harmonics are voltages and electric currents that occur in a power grid due to some type of electrical charge.
The cause of harmonic creation
Advances in technology in the manufacture of semiconductor components (such as diodes, thyristors or SCRs, IGBT, GTO, etc.) have led to the use of semiconductor controllers instead of contactors and relays in automated processes. These components can be seen in abundance in permanent power supplies used in hospitals, banks, protection centers, etc. The main feature of these components is the deformation and shape of the current and its transformation into waves that do not have a sinusoidal shape but have periodicity.
In the normal and alternating current of power networks, the sine voltage at a certain frequency is usually 50 or 60 Hz when a linear bioelectric is connected to the system, in which case the sine current is formed with the same voltage frequency. When a non-linear load, such as rectifiers, is connected to the power system, it creates a current that is not exactly sinusoidal.
The following figure shows two examples of currents. The first stream has no distortion and the second stream has distortion. If it is observed that if the power consumption is equal in both loads, the effective current in the distorted load is significantly more than the smooth load without distortion; And that means more overload and more losses.
What is a harmonic? Mathematical calculations (Fourier series) have proved that each alternating wave is composed of a set of sine waves with the main wave frequency and its multiples. The wave with the main frequency is called the main wave and the other waves are called harmonic according to their frequency. For example, the third, fifth, seventh harmonic, etc. In other words, in a network with a frequency of 50 Hz, the third harmonic will have a frequency of 150 and the fifth harmonic will have a frequency of 250 Hz.
Distorted waveforms alone do not provide information about harmonics. What helps the designer with harmonic analysis is the harmonic spectrum diagram.
Conventional measuring devices are not able to measure harmonics. Measurements in harmonic systems can be performed by True RMS devices. Frequency spectra are also measured by experts using power analyzers.
Total THD harmonic distortion
The most important parameter for measuring harmonics in a power grid is the THD (Total Harmonic Distortion). In fact, THD represents the ratio of the sum of harmonics to the value of the parameter at the main frequency and is expressed as follows:
In the above relation An effective value or RMS harmonic n. = 1 is the main frequency.
The most common harmonic generators, which are the same as nonlinear loads, are:
2- AC motor speed controller
3- DC motor controller
4- Frequency converter
5- Lighting dimmer
7- Charging the battery
8- Arc furnace
9- Arc welding machine
- Electronic ballast
11- Induction furnace
- Switching power supply
Problems caused by harmonics
Depending on the type, number, and amount of harmonics produced by the load, the general problems created in the network are:
1- Overheating of conductors 2- Strong and weak light of fluorescent lamps
3- Capacitor failure
4- Excessive heating and failure of transformers and motors
5- Blinking gas lamps
6- Operating automatic fuses
7- Burning fuse for no apparent reason
8- Interference in telecommunications
9- Interference of telephone lines
- Malfunction of measuring devices
11- Overheating of the keys
- Locking computers
Today, the biggest concern of the electrical industry is to deal with harmonics and their consequences.