Danfoss VLT AHF005 Design Manual

Contents

Danfoss VLT AHF005 Design Manual Manual pdf 13 page image

4 Introduction to Advanced Harmonic Filters4.1 Operation PrincipleThe Danfoss Advanced Harmonic Filters (AHF) consist of amain inductor L0 and a two-stage absorption circuit with theinductors L1 and L2 and the capacitors C1 and C2. Theabsorption circuit is specially tuned to eliminate harmonicsstarting with the 5th harmonic and is specific for thedesigned supply frequency. Consequently the circuit for50Hz has different parameters than the circuit for 60Hz.L0L1L2 C2C1MAHFSupply Motor130BB578.11FrequencyconverterAHFs are available in two variants for two performancelevels: AHF005 with 5% THiD (total current harmonicdistortion) and AHF010 with 10% THiD. The strategy behindthe two levels is to offer a performance similar to 12 pulserectifiers with the AHF010 and a performance similar to 18pulse rectifiers with AHF005.The filter performance in terms of THiD varies as a functionof the load. At nominal load the performance of the filtershould be equal or better than 10% THiD for AHF010 and 5%THiD for AHF005.At partial load the THiD has higher values. However, theabsolute value of the harmonic current is lower at partialloads, even if the THiD has a higher value. Consequently, thenegative effect of the harmonics at partial loads will be lowerthan at full load.Example:An 18.5kW frequency converter is installed on a 400V/50Hzgrid with a 34A AHF010 (type code AHF-DA-34-400-50-20-A).Following values are measured for different load currents,using a harmonic analyzer:I line RMS [A] Fundamentalcurrent at 50HzI1 RMS [A]THiD [%] Total harmoniccurrent Ih RMS[A]19.6 9.59 5.45 0.5215.24 15.09 13.78 2.0720.24 20.08 12.46 2.525.17 25 11.56 2.8930.27 30.1 10.5 3.1534.2 34.03 9.95 3.391)The total harmonic current has been calculated. The THiDvs. load plot is shown in the following figure.AHF-DA-34-400-50-20-A024681012141610 15 20 25 30 35Iline [A]THiD [%]00,511,522,533,54Harmonic current Ih [A]130BB579.10THiD [%]Harmonic current Ih [A]It can be observed that at partial load, 15A, the THiD isapproximately 14%, compared to 10% at the nominal load of34A. On the other hand, the total harmonic current is only2.07A at 15A line current against 3.39A harmonic current at34A line current. Thus, THiD is only a relative indicator of theharmonic performance. The harmonic distortion of thevoltage will be less at partial load than at nominal load.Factors such as background distortion and grid unbalancecan affect the performance of AHF filters. The specific figuresare different from filter to filter and the graphs below showtypical performance characteristics. For specific details aharmonic design tool such as MCT 31 or HarmonicCalculation Software (HCS) should be used.Background distortion: The design of the filters aims toachieve 10% respectively 5% THiD levels with a backgrounddistortion of THvD = 2%. Practical measurements on typicalgrid conditions in installations with frequency convertersshow that often the performance of the filter is slightlybetter with a 2% background distortion. However, thecomplexity of the grid conditions and mix of specificharmonics can not allow a general rule about theperformance on a distorted grid. Therefore we have chosento present worst-case performance deterioration character-istics with the background distortion.Introduction to Advanced Ha... AHF005/010 Design Guide12 MG.80.C4.02 - VLT® is a registered Danfoss trademark44