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Power System Harmonics

Harmonics are a distortion of the normal electrical current waveform, generally transmitted by non-linear loads. Switch-mode power supplies (SMPS), variable speed motors and drives, photocopiers, personal computers, laser printers, battery chargers and UPSs are examples of non-linear loads. Single-phase non- linear loads are prevalent in modern office buildings, while three-phase, non- linear loads are widespread in factories and industrial plants. A large portion of the non-linear electrical load on most electrical distribution systems comes from SMPS and variable speed drive equipment.  These non- linear loads draw current in high-amplitude short pulses that create significant distortion in the electrical current and voltage wave shape. Total harmonic distortion (THD), is a measure of the total harmonic currents flowing and is given as a percentage of the fundamental current (THD%). The distortion travels back into the power source and can affect other equipment connected to the same source. Most power systems can accommodate a certain level of harmonic currents but will experience problems when harmonics become a significant component of the overall load. As these higher frequency harmonic currents flow through the power system, they can cause communication errors, overheating and hardware damage, such as: - Overheating electrical distribution equipment, cables, transformers & generators - High voltages and circulating currents caused by harmonic resonance - Equipment malfunctions due to excessive voltage distortion - Increased internal energy losses in connected equipment, causing component failure and shortened life span - False tripping of circuit breakers and protective devices - Metering errors - Fires in wiring and distribution systems, especially in the neutral conductor due to the presence of 3rd harmonic (zero sequence current) - Generator failures - Crest factors and related problems - Lower system power factor, resulting in penalties on monthly utility bills There are a number of approaches that can be applied to reduce harmonics on a system or to manage their effects including passive filtering for targeting specific harmonics through to full spectrum generation of negative harmonic currents.