Power Quality Audit / Harmonic Analysis

This term refers to a wide variety of electromagnetic phenomena that characterize the voltage and current at a given location of a power system. Any power problem manifested in voltages, current, or frequency deviations those results in failure or malfunctioning of customer equipment.

Power quality has become increasingly important for industrial and commercial electric power customers, particularly as todays control processes rely on computerized equipment which is sensitive to power system interruptions and disturbances.

Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate (termed the fundamental component, usually 50 Hz or 60 Hz). Harmonics combine with the fundamental voltage or current, and produce waveform distortion. Harmonic distortion exists due to the nonlinear characteristics of devices and loads on the power system.

Nonlinear devices can usually be modelled as current sources that inject harmonic currents into the power system. Voltage distortion results as these currents cause nonlinear voltage drops across the system impedance. Harmonic distortion is a growing concern for many customers and for the overall power system due to increasing application of power electronics equipment.

Harmonic distortion levels can be characterized by the complete harmonic spectrum with magnitudes and phase angles of each individual harmonic component. It is also common to use a single quantity, termed the Total Harmonic Distortion (THD) or the Distortion Factor, as a measure of the magnitude of harmonic distortion. The term expresses the distortion as a percentage of the fundamental (pure sine) of voltage and current waveforms.

The example below shows that the distorted curve consists both of the fundamental frequency (e.g. 50 Hz) and of superimposed 5th (250 Hz) and 7th (350 Hz) harmonic frequencies.

Harmonics are created from equipment’s containing electronics that control other apparatus, e.g. variable speed drives, soft starters, static compensators, rectifiers and arc furnaces, etc.

High currents in neutral conductors

It is described as a short duration voltage decrease (decrease in RMS voltage or current at the power frequency for durations of 0.5 cycle to 1minute). It may cause due to short circuits, large motor start-up, transformer energizing and sudden load variations. Due to voltage sag, equipment may trip and energy losses can occur.

It refers to a measured voltage having a value greater than the nominal voltage for a period of time greater than 1 minute.

Over voltages can be the result of load switching (e.g., switching off a large load), or variations in the reactive compensation on the system (e.g., switching on a capacitor bank). Poor system voltage regulation capabilities or controls result in over voltages. Incorrect tap settings on transformers can also result in system over voltages.

It refers to the varying luminance of light sources under conditions of varying supply voltage levels. Voltage fluctuation is the electromagnetic phenomenon and flicker is an undesirable result of that phenomenon. It may cause due to arc furnace operation, large motor start-up and cycling on/off of large loads, rapid variations in load current magnitude. Flickering may distract the human eyes and malfunctioning of sensitive equipment’s.

It may cause due to short circuits, switching operations, lightning etc., the effect of transients is over currents, equipment aging and breakdown.

As per IEEE, it is defined as the maximum deviation among the three phases from the average three-phase voltage divided by the average three phase voltage. It may cause due to single phase loads, faulty three phase loads, etc. The effect of unbalance is malfunction of three phase equipment’s.