Mitigation of Harmonics Produced by Nonlinear Loads in Industrial Power System

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1 Mitigation of Harmonics Produced by Nonlinear Loads in Industrial Power System Muhammad Abid 1, Tehzeeb-ul-Hassan 2, Tehseen Ilahi 3 1 National University of Computer and Emerging Sciences, Lahore, Pakistan; muhammad.abid@nu.edu.pk 2 University of Lahore, Lahore, Pakistan; tehzibulhassan@gmail.com 3 University of Engineering & Technology, Lahore, Pakistan; engineer_tehseen@yahoo.com Abstract----Harmonics are present in power systems due to the rapid switching of nonlinear loads. These harmonics have several adverse effects on a power system infrastructure, such as saturation of transformers, overheating of neutral cables, reduction of torque in rotating machines, malfunctioning of switchgears and protective relays, etc. Thus, it is important to analyze and mitigate these harmonics. Several harmonic mitigation techniques (HMTs) have been proposed in the research literature. This paper presents the simulation and analysis of a phase shifting technique for harmonic mitigation. Results from MATLAB simulations of the proposed technique have been included as part of this paper. Keywords---total harmonic distortion; harmonic mitigation techniques; phase shifting techniques; triplens. I. INTRODUCTION Harmonics in power systems are sinusoidal components of periodic waveforms that have frequencies equal to an integral multiple of the fundamental frequency. The frequency of the harmonic component, f h, is related to the fundamental frequency by the following formula: f h = (h) (fundamental frequency) (1) where h is an integer. Due to the use of power electronic devices such as MOSFETs, IGBTs, GTOs, etc., the nonlinear load on a power system is increased. Nonlinear loads are those in which the current waveform does not resemble the applied voltage waveform [1]. Single-phase nonlinear loads produce harmonics of odd multiples of the fundamental frequency in which the most severe are the "triplen". Odd multiples of the 3 rd harmonic of the fundamental frequency are defined as triplen harmonics. Examples are the 3 rd, 9 th, 15 th, 21 st, etc., harmonics. Triplen harmonics are zero sequence harmonics, unlike the fundamental, which is a positive sequence. As a result, the magnitude of these currents on the 3 phases are additive in the neutral. Unless the neutral is sufficiently oversized, theses large currents can present a fire hazard because of overheating [2]. Three phase nonlinear loads primarily produce 5 th and 7 th harmonics. These harmonics are the main cause of distortion, overheating of the neutral conductor and malfunctioning of control devices [3]-[4]. Three phase harmonics also cause serious problems in the power system equipment, for example, increased losses in motors and transformers, poor power factor, skin effect in conductors and unexpected tripping of protection equipment, etc. II. RELATED WORK There are different harmonic mitigation methods available, each with technical advantages and disadvantages. Examples are harmonics filters (passive, active and hybrid), line reactors, and K-factor transformers. Passive filters are the cheapest form of filters applicable for fixed frequency [5]. Active filters are difficult to design and more expensive than passive filters, but have several advantages over passive filters [6]. Hybrid power filters are a combination of active and passive filters. They provide a suitable solution to mitigate harmonics and are suitable for heavy loads [7]. Line reactors and K-factor transformers may be used to overcome the problem of harmonics [8]. The connection type of a feeding transformer also helps in reducing the harmonics in a power system. The harmonics injected by nonlinear 123

2 loads may be canceled at the point of common coupling (PCC). The concept is to connect nonlinear loads through phase shifting transformer such as Dy 1, Dy 5, Dy 11 that are used with harmonic loads [9]-[10]. An excellent review of existing mitigation techniques can be found at [11]. Another interesting and relevant discussion is available at [12]. III. PHASE SHIFT CONCEPT FOR MITIGATION OF HARMONICS Fig. 2: Phasor diagram of a PST with load Mitigation of harmonics by employing phase shifting transformers (PSTs) has also been proposed in the literature [13]. One way is to connect the primary winding in delta and the secondary winding in star with a -30 phase shift (called a Dy11 transformer) [14]. Similarly a Dy1 transformer having primary winding connected as delta and the secondary as star connected with 30 phase shift, and a Dz0 transformer with primary winding delta and secondary winding in zigzag connected with 0 phase shift can be used as PSTs [13]. In Figure 2 above, the parameters are as follows: I Load current V s (a) Source voltage (advance) V s (r) Source voltage (retard) V L Load voltage when loaded * V L Load voltage (no-load condition) β Transformer load angle -α Phase angle shift (retard) +α phase angle shift (advance) The load phase angle β can be calculated using the following equation: U U - (3) 30 0 The load phase angles of the transformers α* ( a ) and α* ( r ) can be obtained as phase angle advance, defined as: V V W Fig. 1: Phase shift between source and load Figure 1 shows the vector representation of a phase shifting transformer with a 30 phase shift to eliminate harmonics. In this figure, U, V and W are the voltage vectors of a three phase balanced system and U, V and W are the vectors after a 30 phase shift. W α * ( a) = α- β (4) The load angle, as phase angle retard, is defined as: α * (r ) = -(α β ) (5) To achieve an advanced phase angle α * ( a) under load, the no load phase angle α has to be chosen properly taking into consideration the phase angle of the phase shifting transformer. A. Mitigation of +ve & -ve sequence harmonics Harmonics such as the 7 th, which rotate with the same sequence as the fundamental, are called positive sequence. Harmonics such as the 5th, which rotate in the opposite sequence as the fundamental, are called negative sequence. Triplen harmonics which do not rotate at all 124

3 because they are in phase with each other, are called zero sequence [15]. Positive sequence harmonics 7 th, 13 th and 19 th act against negative sequence harmonics 5 th, 11 th and 17 th to mitigate lower order harmonics. These positive sequence harmonics require a phase shift of 180 /7= 26, or approximately 30 to mitigate the 7 th order harmonics. In case of the 13 th order harmonics, 180 /13=14, or approximately a 15 phase shift is required. In the same way, a 30 phase shift is required for 5 th order harmonics of the negative sequence and a 15 phase shift is required for the 11 th order harmonics of the negative sequence.. B. Mitigation of zero sequence harmonics All triplens (3rd and multiple of third harmonics) are zero sequence harmonics. These harmonics act against each other in a three phase system to achieve mitigation. A 60 phase shift (180 /3=60 ) is required to mitigate lower order zero sequence harmonics in three phase power system. TABLE I. HARMONICS AND THEIR PHASE SEQUENCES IN A THREE PHASE SYSTEM Harmonics Order Fundamental (50Hz) Phase-I (R) 3 rd R th R th R th R th R th R 13 0 Phase-II (Y) Phase-III (B) R Y B Y (360 = 0 ) Y ( = -120 ) Y ( = 120 ) Y (1080= 0 ) Y ( = Y ( = 120 B (720 = 0 ) B = 1200 ( = -240 B = 1200 ( = 240 B = 0 B ( = 240 B ( = 240 Phase Sequence R-Y-B No Phase Rotation B-Y-R (-Ve) R-Y-B (+ Ve) No Phase Rotation B-Y-R (-Ve) R-Y-B (+ Ve) Table I illustrates that the three phases R-Y-B are apart from each other in balanced three phase power system network. In a balanced three phase system, zero sequence harmonics have no phase rotation, 5 th & 11 th harmonics have negative phase sequence and 7 th & 13 th harmonics have positive phase sequence. So negative and positive sequences act against each other and cancel the unnecessary harmonics. Table I also shows that each phase R-Y-B acts against the similar phase with opposite angle to mitigate unnecessary harmonics. For example, phase Red of the 5 th harmonic acts against phase Red of the 7 th harmonic to cancel the undesired effects. The same applies to the Yellow and Blue phases. Figure 3 shows the schematic arrangement of the technique under study. Four motors are fed through variable frequency drives (nonlinear loads) and these VFDs are fed from the utility supply through phase shifting transformers or PSTs. Winding connections of phase shifting transformer are arranged as follows: PST-1 primary winding is delta connected and secondary winding is ig ag with phase shift. PST-2 primary winding is star connected and secondary winding is zigzag with a zero phase shift. Similar connections are required for PST-3 and PST-4. The phase shift between these transformers is 15 degrees. Fig. 3: Schematic diagram of proposed technique 125

4 IV. SIMULATION AND RESULTS Fig. 4: Simulation schematic of the proposed technique Figure 4 shows the MATLAB simulation model of the phase shifting technique under discussion. Four nonlinear loads are fed through four phase shifting transformers each having an appropriate phase angle. The desired phase shift is achieved by changing the vector group of the associated PST in the MATLAB model. Different configurations of phase shifting transformers for harmonic mitigation have been simulated such as Y-Δ, Y-Y/ Y-Δ, Zig-Zag/4Y & Zig-Zag/2Y- 2Δ. From this simulation model, it is clear that the implementation of a single phase shifting transformer with a particular load does not reduce harmonics. Harmonics are reduced when two or more phase shifting transformers are employed with similar nonlinear loads. The phase shifting is achieved by taking harmonics from different sources, shifting one source of harmonics from other and then combining them; if the amplitude of harmonics is equal then the harmonics are cancelled. As shown in Figure 5 and Figure 6, the total harmonic distortion, or THD, is reduced up to 3.79%. All triplen harmonics are cancelled. 126 Fig. 5: Results of proposed simulation model Fig. 6: Graphical representation of harmonic spectrum

5 When two transformers with a -30 and 0 phase shift are used, the 3 rd harmonic currents are cancelled and the voltage distortion is reduced. TABLE II. COMPARISON OF RESULTS AT DIFFERENT PHASE ANGLES Harmonic THD-I THD-III Order (No Phase (Appropriate (Odd) Shift) Phase Shift) THD-II (180 0 Phase Shift) % 0.01% 0.00% % 12.69% 0.01% % 8.69% 0.01% % 0.01% 0.0.1% % 6.91% 0.01% % 5.16% 0.01% % 1.94% 0.01% % 1.42% 0.01% According to the analysis of the simulated results, it is clear that the most dominant harmonics, the 5 th and the 7 th, are cancelled at the point of common coupling (PCC) and not transferred to the power system network. Besides, since there are no 5 th & 7 th harmonics, the occurrence of the system resonance at these frequencies is avoided. Moreover, the results are the same for 17 th and 19 th harmonics, etc. The remaining harmonic components are below the recommended standard of IEEE 519 limits and IEC V. CONCLUSIONS In this research paper, the phase shifting technique for mitigation of harmonics is explored. The simulation results illustrate that mitigation of the lower order odd harmonics (5 th, 7 th, 11 th & 13 th ) is achieved in the system of Figure 4. The results also show that harmonics produced by nonlinear loads can be cancelled at the PCC by combining the waveforms after phase shift. The simulation results concur with the IEEE standards for harmonic limitation, i.e., THD V 3% for special application, 5% for general systems and 10 % for dedicated systems. REFERENCES [1] WAI-KAI CHEN EDITOR The Electrical Engineering Handbook [2] harmonics.html 127 [3] Hussein A. Attia, M. El-Metwally and Osama M. Fahmy Harmonic Distortion Effects and Mitigation in Distribution Systems Journal of American Science, vol 6, pp 10, [4] Uma P Bala Raju, Bala Krishna Kethineni, Rahul H Shewale & Shiva Gourishetti, Harmonic Effects and its Mitigation Techniques for a Non-Linear Load International Journal of Advanced Technology & Engineering Research (IJATER), Volume 2, Issue 2, May [5] Sindhujah L. J. Parthasarathy and V. Rajasekaran, Harmonic mitigation in a rectifier system using hybrid power filter, International Conference on Computing, Electronics and Electrical Technologies (ICCEET), Kumaracoil,pp , March,2012. [6] Sekar, T. C. and Rabi, B.J. A review and study of harmonic mitigation techniques International Conference on Emerging Trends in Electrical Engineering and Energy Management (ICETEEEM), Chennai, pp 93-97, December, 2012 [7] Akagi H, (2005) "Active harmonic filters", Proceedings of the IEEE, vol. 93, No.12. [8] Xiaodong Liang Influence of Reactors on Input Harmonics of Variable Frequency Drives Industrial and Commercial Power Systems Technical Conference (I & CPS) IEEE, Baltimore, MD, vol. 47, No. 5, November, [9] G. M. Carvajal, G O Plata, W. G. Picon, J. C. C. Velasco, Investigation of Phase shifting transformers in distribution systems for harmonics mitigation, IEEE Power System Conference (PSC), pp 1-5, March [10] viewthread.cfm?qid= [11] Hussein A. Kazem, Harmonic Mitigation Techniques Applied to Power Distribution Networks, Advances in Power Electronics,

6 vol. 2013, Article ID , 10 pages, doi: /2013/ [12] Skjong, E.; Molinas, M.; Johansen, T.A., "Optimized current reference generation for system-level harmonic mitigation in a diesel-electric ship using non-linear model predictive control," in Industrial Technology (ICIT), 2015 IEEE International Conference on, vol., no., pp , March [13] Malagon Carvajal, G, Ordonez Plata, G.; Picon, W.G and Chacon Velasco, J.C. Investigation of Phase shifting transformers in distribution systems for harmonics mitigation IEEE, Power System Conference (PSC), Clemson, SC, [14] understanding-vector-group-transformer-1. [15] textbook/alternating-current/chpt- 10/harmonic-phase-sequences/ 128