Causes of Harmonic Distortion

Harmonic distortion is the presence of unwanted frequency components in a power system. These unwanted components are integer multiples of the fundamental frequency (usually 50 or 60 Hz) and can significantly impact the performance and reliability of the power system. There are several causes of harmonic distortion in power systems, which can be broadly categorized as follows:

Non-linear Loads: The primary cause of harmonic distortion is the presence of non-linear loads. These loads draw current in a non-sinusoidal manner, causing the current waveform to be distorted. Examples of non-linear loads include power electronic devices such as rectifiers, inverters, adjustable speed drives, and electronic equipment like computers, fluorescent lights, and LED lamps. The switching operations in these devices generate harmonics, which can then propagate through the power system.

Power Electronic Devices: As mentioned above, power electronic devices like inverters and converters are significant sources of harmonics. The rapid switching operations in these devices can generate high-frequency harmonics that can propagate throughout the system. In addition, the control algorithms used in these devices can introduce interharmonics, which are non-integer multiples of the fundamental frequency.

Magnetic Saturation: Magnetic saturation in transformers and other inductive devices can also lead to harmonic distortion. When a transformer becomes saturated, its magnetizing current becomes non-linear, leading to the generation of harmonics. This phenomenon is more likely to occur during high-load conditions or when the transformer is subjected to an overvoltage.

Resonance: Resonance can occur in power systems when the system's natural frequency aligns with one or more harmonic frequencies. This can cause harmonic amplification, significantly increasing the harmonic distortion levels. Resonance can occur due to the interaction between the system's inductive and capacitive components, such as transformers, capacitor banks, and transmission lines.

Faults and Asymmetrical Conditions: Faults, such as short-circuits, or asymmetrical conditions, such as unbalanced loads, can also cause harmonic distortion. These situations can result in non-sinusoidal currents and voltages, generating harmonics.

Types of Harmonic Distortion

Harmonic distortion can manifest in various ways within a power system. The different types of harmonic distortion, namely voltage and current harmonics, each have unique impacts on power quality and the operation of electrical equipment.

Voltage Harmonics: Voltage harmonics are distortions on the voltage waveform that occur at multiples of the fundamental frequency. These distortions are generally caused by non-linear loads drawing non-sinusoidal current from a sinusoidal voltage source. Voltage harmonics can lead to several problems, including additional losses and heating in electrical machines, misoperation of protective devices, and malfunctions in sensitive electronic equipment.

Current Harmonics: Current harmonics are distortions on the current waveform, often generated by non-linear loads. These loads draw current in pulses rather than in a smooth sinusoidal manner, creating harmonic currents. These harmonic currents can cause excessive heating in neutral conductors, transformers, and motors, leading to premature equipment failure.

Harmonic Indices and Measurements

The presence and impact of harmonic distortion are measured using various harmonic indices. The most common measurements include Total Harmonic Distortion (THD) and Individual Harmonic Distortion (IHD).

Total Harmonic Distortion (THD): THD measures the distortion of the voltage or current waveform compared with a pure sinusoid. It is a global index that considers all harmonics present and is expressed as a percentage of the fundamental. THD can be calculated for both voltage and current waveforms. High THD values indicate a high degree of distortion, which can negatively impact the operation of electrical devices and equipment.

Individual Harmonic Distortion (IHD): While THD provides an overall measure of distortion, it does not provide information about the contribution of individual harmonics. IHD is used to determine the magnitude of each harmonic as a percentage of the fundamental frequency. This information is crucial when identifying and targeting specific harmonic sources for mitigation.