What is Harmonic?
Harmonic frequencies are multiples of the mains frequency. For example, for a fundamental frequency of 50 Hz, the third harmonic is 150 Hz and the fifth harmonic is 250 Hz. Harmonics manifest themselves as current harmonics which flow through sub-circuits to produce voltage harmonics. In calculations and measurements, those in the frequency range from 100 Hz (2nd harmonic) to 2.5 kHz (50th harmonic) are considered. The range from 2.5 kHz to 150 kHz is called the supraharmonic range. Harmonics cause deviations from ideal sinusoidal waveforms in both current and voltage, as shown below.
So, what are harmonics? Harmonic currents and voltages are created in the electrical grid when non-linear loads are connected. In simple terms, harmonics are distortions in the sinusoidal waveform, formed as multiples of the fundamental frequency. These distortions lead to energy loss, overheating, and reduced equipment lifespan in power systems. Non-linear loads, such as rectifiers, motors, transformers, and various electronic devices, typically generate these harmonics. The most harmful harmonics, with the highest amplitudes, are the third, fifth, and seventh harmonics. These cause the most significant distortion to the sinusoidal waveform, as their high amplitude results in the waveform straying further from the ideal sinusoidal form. Consequently, suppressing and reducing harmonics is crucial for maintaining power quality. These unwanted frequency components in electrical circuits can cause major issues in electrical installations, which we have summarized briefly in our article.
In recent years, subharmonics have appeared frequently in scientific studies, but the frequencies defined as harmonics are exact multiples of the grid frequency, ranging from 100 Hz (2nd harmonic) to 2.5 kHz (50th harmonic) in calculations and measurements. The range from 2.5 kHz to 150 kHz is referred to as supraharmonics.
What is the 5th Harmonic?
The concept of the “5th harmonic” is closely associated with the overall topic of harmonics. The 5th harmonic refers to a frequency that is five times the fundamental grid frequency of 50 Hz, resulting in a frequency of 250 Hz. Harmonics, in general, are integer multiples of the fundamental frequency, and individual harmonic components are frequently measured and analyzed in electrical installations. For instance, harmonics can be categorized as follows:
The 3rd harmonic: which is 3 times the fundamental frequency, is commonly observed in single-phase loads and can cause overloading and overheating of the neutral line.
The 5th harmonic: which is 5 times the fundamental frequency, is prevalent in three-phase systems and can lead to resonance issues.
The 7th harmonic: 7 times the fundamental frequency, is also common in three-phase systems and can contribute to resonance problems.
In summary, harmonic frequencies refer to integer multiples of the fundamental grid frequency. For instance, when the fundamental frequency is 50 Hz, the third harmonic corresponds to 150 Hz, and the fifth harmonic is 250 Hz. Harmonics manifest as current harmonics, and these harmonic currents propagate into sub-circuits, inducing voltage harmonics. The frequency range from 100 Hz to 2.5 kHz is typically considered in calculations and measurements, while the range from 2.5 kHz to 150 kHz is categorized as supraharmonics. As evident, harmonics cause the current and voltage waveforms to deviate from the ideal sinusoidal form.
How to Detect Harmonics, Which Can Cause Significant Problems?
Network harmonics can be identified through power quality measurements taken using a power quality analyzer. The power quality analyzer displays current harmonics as THDi and voltage harmonics as the THDu.
Non-linear loads that distort voltage and current waveforms from ideal:
- Uninterruptible power supplies ve IT loads
- Variable frequency drives
- Rectifier circuits
- Thyristor-controlled circuits
- Arc furnaces
- Welding machines
- Transformers operating at saturation
- Transformers loaded above their rated power
Linear Loads:
- Ohmic loads, resistors
- Asynchronous motor
- Capacitors
Six-pulse bridge rectifiers generate harmonics that are 6n±1, where one is more or less than a multiple of six.
Six Pulse Bridge
Harmonic Spectrum of a Square Wave
Damages of Harmonics
Harmonic currents and voltages produce issues on both the utility and load sides. The damages, incidents, and related concerns caused by harmonics in electrical systems can be summarized as follows.
- Resonance can occur and damage the power factor correction system.
- Supply switches may trip at unpredictable times due to resonance events, causing the system to shut down.
- Motors and their cables can suffer from increased losses and overheating.
- Insulation can weaken and shorten the service life of equipment components.
- The neutral cable can be overloaded, resulting in overheating.
- Capacitors and associated equipment may have significantly reduced service life.
- Measuring instruments may be susceptible to inaccurate readings.
- Control circuits operating at zero point can malfunction.
- Transformers and asynchronous motors may overheat.
- Losses may increase. S = √ (P2 + Q2 + D2) D: Harmonic distortion.
Any consumer connected to the grid produces harmonics with linear loads, and these currents flow into the grid in proportion to the source impedance. This results in harmonic voltage distortion at the point of connection. The higher the harmonic current generated by the consumers, the higher the proportion of harmonic voltage distortion at the PCC point. In essence, each consumer adds an additional harmonic contribution to the network.
How to Mitigate Harmonic?
To prevent pollution of the grid by harmonics, general harmonic standards have been published. Businesses that exceed these standard limit values must reduce their levels below those limits. Various power quality solutions are available for suppressing harmonics generated by different loads, depending on their source. These solutions can effectively eliminate events and problems caused by system harmonics after suppression.
Didem Ergun Sezer
Electrical Engineer
Ergun Elektrik A.Ş.
didem@ergunelektrik.com
