Danfoss AHF005 Design Manual

Also see for VLT AHF 005: Instruction manual Operating instructions Operating instructions Design guide

Contents

Danfoss AHF005 Design Manual Manual pdf 9 page image

PWHD = ∑h =1440(IhI1 )2× 100 %3.1.3 The Effect of Harmonics in a PowerDistribution SystemThe figure below shows an example of a small distributionsystem. A transformer is connected on the primary side to apoint of common coupling PCC1, on the medium voltagesupply. The transformer has an impedance Zxfr and feeds anumber of loads. The point of common coupling where allloads are connected together is PCC2. Each load is connectedthrough cables that have an impedance Z 1, Z 2 , Z 3 .Harmonic currents drawn by non-linear loads cause distortionof the voltage because of the voltage drop on the impedancesof the distribution system. Higher impedances result in higherlevels of voltage distortion.Current distortion relates to apparatus performance and itrelates to the individual load. Voltage distortion relates tosystem performance. It is not possible to determine thevoltage distortion in the PCC knowing only the load’s harmon-ic performance. In order to predict the distortion in the PCCthe configuration of the distribution system and relevantimpedances must be known.A commonly used term for describing the impedance of a gridis the short circuit ratio R sce , defined as the ratio between theshort circuit apparent power of the supply at the PCC (S sc) andthe rated apparent power of the load (S equ):Rsce =SceSequwhereSsc =U 2ZsupplyandSequ =U ×IequThe negative effect of harmonics is twofold:• Harmonic currents contribute to system losses (incabling, transformer)• Harmonic voltage distortion causes disturbance toother loads and increase losses in other loads3.2 Harmonic Limitation Standards andRequirementsThe requirements for harmonic limitation can be:• Application specific requirements• Requirements from standards that have to beobservedThe application specific requirements are related to a specificinstallation where there are technical reasons for limiting theharmonics.For example on a 250 kVA transformer with two 110 kWmotors connected. One is connected direct on-line and theother one is supplied through a frequency converter. If thedirect on-line motor should also be supplied through afrequency converter the transformer will, in this case, beundersized. In order to retrofit, without changing thetransformer, the harmonic distortion from the two drives hasto be mitigated using AHF filters.There are various harmonic mitigation standards, regulationsand recommendations. Different standards apply in differentgeographical areas and industries. The following fourcommonly encountered standards will be presented:• IEC61000-3-2• IEC61000-3-12• IEC61000-3-4• IEEE 519• G5/4Introduction to Harmonics a... AHF005/010 Design GuideMG.80.C3.02 - VLT® is a registered Danfoss trademark 93 3