Harmonic currents in electrical networks affect power quality and are the cause of many disturbances such as overloads, vibrations, ageing equipment, etc. This chapter deals with the origins of harmonics, their effects, how to measure them, and solutions for mitigating them.
Why detect harmonics and combat them ?

Disturbances caused by harmonics
Harmonics circulating in the networks deteriorate the quality of the energy, and are therefore the cause of
numerous nuisances  :
 Overloading of distribution networks by increasing the active / effective current,
 Overloading of the neutral conductors, where the current may exceed the current in the phases,
 Overloading, vibrations and ageing of alternators, transformers and motors, humming of
transformers,
 Overloading and ageing of reactive energy compensation capacitors,
 Distortion of the supply voltage that can disturb sensitive receivers,
 Disruption to communication networks or telephone lines.
A major economic impact
All these disruptions have an economic impact :
 Premature ageing of equipment means that it has to be replaced sooner, unless it is oversized from the outset,
 Overloading the distribution network means a higher installed capacity, an increase in the subscribed capacity for industrial customers, and increased power losses,
 Unexpected current distortion can lead to nuisance tripping and production stoppage.
production. Origin of harmonics Harmonic-generating devices are present in all industrial, tertiary and domestic sectors. Harmonics are caused by a non-linear load (the current it absorbs does not have the same shape as the voltage supplying it).

Examples of non-linear loads :
 Industrial equipment (welding machines, arc furnaces, induction furnaces, rectifiers),
 Variable speed drives for induction motors or DC motors,
 Inverters,
 Office equipment (computers, photocopiers, fax machines, etc.),
 Domestic appliances (TVs, microwave ovens, neon lighting, etc.),
 Certain equipment with magnetic saturation (transformers).

Solutions for attenuating harmonics  :
1- Basic solutions for attenuating harmonics
To limit the propagation of harmonics in the network, a number of measures can be taken and should be observed
particularly in the case of a new installation.
 Position pollutant loads upstream of the network
 Combining pollutant loads
 Separating sources
 Use transformers with special couplings
 Installing chokes in the installation
 Choosing the right earthing system

2- Harmonic filtering
If the above preventive actions are insufficient, it is necessary to equip the polluted installation with a filtering device.
with filtering equipment.
There are three types of filter :
 the passive filter,
 the active filter,
 the hybrid filter.

3- Expertise in attenuating harmonics
The choice of the most appropriate solution, both technically and economically, is the result of an in-depth study. MV and LV diagnostics Using an expert provides a guarantee of the effectiveness of the proposed solution (e.g. a guarantee of maximum THD u).
maximum THD u).
This harmonic diagnosis is carried out by an engineer specialising in the field of electrical network disturbances, equipped with analysis resources, simulation equipment and software.
The diagnosis stages are :
 Simple and composite measurement of current and voltage disturbances at polluting receivers
disturbed outgoing feeders and power sources.
 Modelling of phenomena using software to provide a precise explanation of their causes and to optimise possible solutions.
 A full diagnostic report highlighting :
 current levels of disruption,
 maximum permissible disturbance levels (IEC 61000, IEEE519, etc.).
 A performance guarantee for the solutions presented.
 Then the implementation, using appropriate equipment.

The service is ISO 9002 certified.