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What Are Power System Harmonics?

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  • Written By: Lakshmi Sandhana
  • Edited By: PJP Schroeder
  • Last Modified Date: 29 August 2016
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Power system harmonics are the by-products of electronic devices that occur when certain types are connected to power systems. Devices like uninterruptible power supplies, personal computers, and variable frequency drives create power system harmonics when they draw power in short pulses instead of in a smooth manner. The effects of these harmonics result in distorted voltages and issues that negatively impact the performance of systems.

Electrical supply sources aim to deliver sinusoidal voltages consistently. When nonlinear loads or any device that uses solid state power switching supplies to transform alternating current to direct current are connected to a power source, it results in power system harmonics. A linear load has a continuous current relationship in which the power is proportional to the voltage with both waves having the same shape. Incandescent lamps, motors, and heaters are some examples of linear loads.

The harmonic current-producing loads are nonlinear, where the current and voltage waveforms differ from each other in shape. When such a load draws power, harmonic voltages are generated by the impedance within the system. Ferromagnetic devices, discharge lighting units, and arcing devices are some examples of nonlinear loads. The effects of power system harmonics can be quite severe depending on the magnitude of the voltage distortion.

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In many ways, its effect is comparable to how stress affects the human body adversely. High levels of harmonic distortion can result in sporadic equipment operation, power system inefficiencies, and even shutdowns. The entire utility distribution system and any devices connected can be affected at various levels in a number of ways. For instance, power system harmonics can reduce the life of capacitors or cause them to fail or rupture. Components may trip, blow up, or be damaged without any discernible cause.

Harmonic voltage distortions cause generators to operate in an unstable way and overheat windings in transformers. Devices like telephones and computers may fail intermittently or exhibit interference. Users may end up with excessive power bills because of the utility meters record power measurements incorrectly.

Technicians use many methods to reduce power system harmonics, ranging from more efficient designs to using isolation and delta-delta transformers. With better design, the harmonics can be brought down by limiting the nonlinear loads to a certain percentage of a transformer's capacity. Delta-delta transformers shift phase relationships using various techniques, which are also beneficial. Isolation transformers attempt to solve the issue by stepping up and stepping down the system voltage to match.

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