Harmonic measurement in electrical networks

Devices used to measure the indicators

Device selection

The traditional observation and measurement methods include:

• Observations using an oscilloscope

An initial indication on the distortion affecting a signal can be obtained by viewing the current or the voltage on an oscilloscope.
The waveform, when it diverges from a sinusoidal, clearly indicates the presence of harmonics. Current and voltage peaks can be viewed.
Note, however, that this method does not offer precise quantification of the harmonic components

• Analogue spectral analysers

They are made up of passband filters coupled with an rms voltmeter. They offer mediocre performance and do not provide information on phase displacement.
Only the recent digital analysers can determine sufficiently precisely the values of all the mentioned indicators.

Functions of digital analysers

The microprocessors in digital analysers:

• Calculate the values of the harmonic indicators (power factor, crest factor, distortion power, THD)
• Carry out various complementary functions (corrections, statistical detection, measurement management, display, communication, etc.)
• In multi-channel analysers, supply virtually in real time the simultaneous spectral decomposition of the currents and voltages
  

Analyser operation and data processing

The analogue signals are converted into a series of numerical values.
Using this data, an algorithm implementing the Fast Fourier Transform (FFT) calculates the amplitudes and the phases of the harmonics over a large number of time windows.
Most digital analysers measure harmonics up to order 20 or 25 when calculating the THD.
Processing of the successive values calculated using the FFT (smoothing, classification, statistics) can be carried out by the measurement device or by external software.

Procedures for harmonic analysis of distribution networks

Measurements are carried out on industrial or commercial site:

• Preventively, to obtain an overall idea on distribution-network status (network map)
• In view of corrective action:

  - To determine the origin of a disturbance and determine the solutions required to eliminate it
  - To check the validity of a solution (followed by modifications in the distribution network to check the reduction in harmonics)