Comparing Harmonics Mitigation Techniques Revision 2 – 2014-03-14 Jonas Persson Comsys AB Fältspatvägen 4, SE-224 78 Lund, Sweden jonas.persson@comsys.se Abstract — The document at hand compares harmonic mitigation techniques in a range of applications and settings. Theoretical and practical comparisons are made between active and passive series and shunt filters. The overall context is to reduce harmonic loading in a drive system. I. BACKGROUND The reader should be aware of the following concepts: harmonics, notching, voltage distortion, current distortion, and voltage unbalance. Harmonics in power systems are predominantly caused by various semiconductor-based loads. Most common loads are drive systems (typically transistor based variable frequency drives and occasionally also line commutated DC drive systems). Harmonics are simply multiples of the fundamental frequency. Hence, the 5th harmonic in a 50 Hz system is the 250 Hz frequency component. We will now consider a 3-phase rectifier. In the simplified case where the output of the rectifier is a constant DC-current the harmonic orders visible on the AC line can be written as ℎ =  ∗  ± 1, where k=1,2,3… The amplitude of the harmonics will depend on a number of factors. The grid strength (or stiffness) will interact with the semiconductor load, as well as the equivalent series line impedance, if present. In general, a stronger grid gives higher amplitudes on the current harmonics, but lower amplitudes on the voltage harmonics, all else being equal. In practical systems and applications, a discussion on reasonable goals for harmonic distortion are needed; for a treatment of this, please see [1]. II. HARMONIC ISSUES There are a number of reasons to limit the amount of harmonics in a system. The following is a non-exhaustive list of symptoms that may be caused by harmonics;  Notching  Motor vibration  Bearing current  Overheating  Nuisance tripping  Generator tripping/malfunction  Production stops  Electrical fires There is no point in reducing harmonic levels for its own sake; harmonics do not automatically mean problems like the ones mentioned above. This paper will not go into depth on the issues caused by harmonics, but will focus on the various ways of mitigating harmonics, along with both the advantages and disadvantages of those methods. III. OVERVIEW OF SOLUTIONS For the remainder of the discussion, compensation solutions will be divided into four broad classes, with two defining factors; (1) whether the solution is active or passive, and (2) whether the solution is used in shunt or in series with the load or device to be compensated. Using this classification, four classes are obtained, with several practical examples in each class; Passive Active Series Compensation Line Reactor Series Harmonic Filter Multipulse solution AFE DVR Hybrid Series Active Filter Shunt Compensation Fixed Capacitor bank Detuned Capacitor bank Fine Tuned Capacitor bank Thyristor Switched capacitor bank TCR/SVC Active Filter STATCOM Some of the solutions mentioned in the table above are not ideally suited or intended for harmonic mitigation, they are mentioned for the sake of completeness. IV. PASSIVE SOLUTIONS – SERIES The following section describes and compares passive series mitigation solutions. Line Reactor A line reactor is a 3-phase series choke placed in front of the rectifier on the line side of a drive. The line reactor will cause a voltage drop as seen from the rectifier; due to being inductive, the series impedance and hence voltage drop is larger the higher the frequency. Typical inductance values are 2-5%. Lower values than 2% have a very limited impact on the harmonics.