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)

Operating mode

The current and voltage are studied:

• At the supply source
• On the busbars of the main distribution switchboard (or on the MV busbars)
• On each outgoing circuit in the main distribution switchboard (or on the MV busbars)

For the measurements, it is necessary to know the precise operating conditions of the installation and particularly the status of the capacitor banks (operating, not operating, the number of disconnected steps).

Analysis results

• Determine any necessary derating of equipment in the installation or
• Quantify any necessary harmonic protection and filtering systems to be installed in the distribution network
• Enable comparison between the measured values and the reference values of the utility (maximum harmonic values, acceptable values, reference values)

Use of measurement devices

Measurement devices serve to show both the instantaneous and long-term effects of harmonics. Analysis requires values spanning durations ranging from a few seconds to several minutes over observation periods of a number of days.
The required values include:

• The amplitudes of the harmonic currents and voltages
• The individual harmonic content of each harmonic order of the current and voltage
• The THD for the current and voltage
• Where applicable, the phase displacement between the harmonic voltage and current of the same harmonic order and the phase of the harmonics with respect to a common reference (e.g. the fundamental voltage)

Keeping a close eye on harmonics

The harmonic indicators can be measured:

• Either by devices permanently installed in the distribution network
• Or by an expert present at least a half day on the site (limited perception)

Permanent devices are preferable

For a number of reasons, the installation of permanent measurement devices in the distribution network is preferable.

• The presence of an expert is limited in time. Only a number of measurements at different points in the installation and over a sufficiently long period (one week to a month) provide an overall view of operation and take into account all the situations that can occur following:

  - Fluctuations in the supply source
  - Variations in the operation of the installation
  - The addition of new equipment in the installation

• Measurement devices installed in the distribution network prepare and facilitate the diagnosis of the experts, thus reducing the number and duration of their visits
• Permanent measurement devices detect any new disturbances arising following the installation of new equipment, the implementation of new operating modes or fluctuations in the supply network

Take advantage of built-in measurement and detection devices

Measurement and detection devices built into the electrical distribution equipment:

• For an overall evaluation of network status (preventive analysis), avoid:

  - Renting measurement equipment
  - Calling in experts
  - Having to connect and disconnect the measurement equipment.
For the overall evaluation of network status, the analysis on the main low-voltage distribution switchboards (MLVS) can often be carried out by the incoming device and/or the measurement devices equipping each outgoing circuit

• For corrective action, are the means to:

  - Determine the operating conditions at the time of the incident
  - Draw up a map of the distribution network and evaluate the implemented solution
The diagnosis is improved by the use of equipment intended for the studied problem.