Harmonic detection and filtering - Schneider Electric

The main effects of harmonicsin installationsEn brefHarmonics have a major economicimpact on installations in that theycause:b higher energy billsb premature ageing of equipmentb drops in productivity.4.1 ResonanceThe use of both capacitive and inductive devices in distribution systems leads toresonance phenomena, resulting in extremely high or low impedance values. Thesevariations in impedance modify the current and voltage in the distribution system.Here we will discuss only parallel-resonance phenomena, which are the mostfrequent.Consider the simplified diagram below, showing an installation made up of:b a transformer supplying powerb linear loadsb non-linear loads causing harmonic currentsb power factor correction capacitors.E95233For harmonic-analysis purposes, the equivalent diagram is shown below:E56976Ls: supply inductance (distributionsystem + transformer + line)C: power factor correction capacitanceR: resistance of the linear loadsIh: harmonic currentjLsωZ =1 − LCωs2when R is neglected.Resonance occurs when the denominator 1-LsCw 2 approaches zero. Thecorresponding frequency is called the resonant frequency of the circuit. At thisfrequency, the impedance is at its maximum value, resulting in considerable voltageharmonics and consequently major voltage distortion. This voltage distortion isaccompanied by the circulation of harmonic currents in the Ls + C circuit which aregreater than the injected harmonic currents.The distribution system and the power factor correction capacitors are subjected toconsiderable harmonic currents, resulting in the risk of overloads.19