Harmonics

Harmonic Analysis of Power Circuits by PowerSight Meters

Harmonics analysis is included as standard in the PowerSight models PS4500 and PS3500 which measure to the 63rd harmonic.  It is an option for the PS2500.

At a utility power company's generators the shapes of the current or voltage waveforms are very close to a perfect sine-wave of one frequency e.g. 60Hz. As consumers use power the waveform becomes distorted.  The distortion is caused by the nature of how the various loads consume power.   The extent of the distortion depends on the load types.  Electronic loads are well-known for creating distortion of power waveforms.  Here’s an example of a voltage and current waveform from an electronic load:

VI Waveform Harmonics

The voltage waveform is very sinusoidal whereas the current waveform looks more like a series of pulses. Thus the current waveform has a high harmonic content.

The extent of the change in shape (distortion) is quantified by measuring the harmonic content of the waveform. Waveforms of any shape can be analyzed mathematically;  any distorted waveform can be broken down into a series of waveforms of single frequencies, at different amplitudes and phase positions. The different frequencies are the harmonics. (A harmonic is an integer multiple of the base or fundamental frequency, so the 3rd harmonic of 60Hz would be 3 x 60 z = 180 Hz.) All PowerSight models incorporate harmonics measurement to the 50th or 63rd harmonic.

The harmonic analysis results of the above waveform are shown in the following PowerSight graphic reports of harmonic spectrum and table:

Current Harmonic Spectrum

The table shows the odd harmonics and even harmonics with the detail of the individual harmonic magnitudes in volts and their phase angles, here is the table data exported so you can see the values better:

Harmonic Analysis Table

Effects of Harmonics

Power distribution systems and equipment are designed to work at 50 or 60Hz. Harmonics at extreme levels can create excessive heating in electrical conductors, motors, transformers, and other components that will either overheat devices and conductors to cause mis-operation or failure.  At lower levels of severity continuous heat stress will reduce the operating life of electrical equipment leading to premature failure. 

In addition to waveform snap-shots and their harmonic content analysis, PowerSight meters also log or trend THD (Total Harmonic Distortion) over time as in this example:

Total Harmonic Distortion Graph

The IEEE standard 519 recommends that limits for voltage THD be kept to less than 5%, plus it further specifies recommended maxima for each harmonic order depending where on the power distribution system harmonic measurements are taken. More information on harmonic analysis of power to this standard can be found here: IEEE-519