Harmonic detection and filtering - Schneider Electric

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GeneralUsing once again the model of non-linear loads reinjecting harmonic currents into thedistribution system, it is possible to graphically represent this phenomena(figure 5).E95232Figure 5 - Flow of harmonic currents in a distribution system.Note in this figure that certain loads cause harmonic currents in the distributionsystem and other loads are disturbed by them.8
1.2 Why harmonics need to be detectedand suppressed ?1.2.1 Disturbances caused by harmonicsIn distribution systems, the flow of harmonics reduces power quality andconsequently causes a number of problems:b overloads on distribution systems due to the increase in the rms currentb overloads on neutral conductors due to the summing of third-order harmonicscreated by single-phase loadsb overloads, vibrations and premature ageing of generators, transformers, motors,etc., transformer humb overloading and premature ageing of capacitors in power factor correctionequipmentb distortion of the supply voltage, capable of disturbing sensitive loadsb disturbances on communications networks and telephone lines.1.2.2 The economic impact of disturbancesHarmonics have a significant economic impact, in that:b premature ageing of equipment means that it must be replaced earlier, unless itwas oversized to begin withb overloads on the distribution system mean the level of subscribed power must beincreased, with additional losses, unless the installation can be upgradedb distortion of the current provokes nuisance tripping and shutdown of productionequipment.These extra costs in terms of equipment, energy and productivity all contributeto reducing the competitiveness of companies.1.2.3 Increasingly serious consequencesAs recently as ten years ago, harmonics were not considered a major problem,because their effects on distribution systems were, generally speaking, relativelyslight. However, the massive increase in the use of loads employing powerelectronics has significantly worsened the situation in all fields of activity.Harmonics are all the more difficult to reduce in that they are often caused byequipment that is vital to the operation of companies.1.2.4 Practically speaking, which harmonics must bemeasured and reduced ?The harmonics most frequently encountered (and consequently the mosttroublesome) on three-phase distribution systems are the odd-order harmonics (3 rd ,5 th , 7 th , etc.).Beyond the 50 th order, harmonic currents are negligible and measurements are notrequired.Sufficient accuracy of measurements is obtained by taking into account harmonicsup to the 30 th order.Utilities monitor harmonic orders 3, 5, 7, 11 and 13.It follows that conditioning of harmonics is imperative up to order 13 and ideallyshould include harmonics up to order 25.9
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