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PWM Boost Rectifier versus Trap Filters Applied to Harmonic Mitigation in Wind Turbine Energy Conversion Systems Fernando Soares dos Reis*, Member, IEEE, Kelvin Tan Student Member, IEEE, and Syed Islam, Senior Member, IEEE Department of Electrical and Computer Engineering Curtin University of Technology GPO Box U1987, Perth, Western Australia 6845 AUSTRALIA e-mail: [email protected] Abstract—Permanent magnet synchronous generators (PMSG) wind energy conversion system (WECS) using variable speed operation is being used more frequently in low power wind turbine application. Variable speed systems have several advantages over the traditional method of operating wind turbines, such as the reduction of mechanical stress and an increase in energy capture. To fully exploit the last mentioned advantage, many efforts have been made to develop maximum power point tracking (MPPT) control schemes for PMSG WECS. To allow the variable speed operation of the PMSG WECS a conventional three-phase bridge rectifier with a bulky capacitor associated with voltage source current controlled inverter (VS-CCI) is used. This simple scheme introduces a high intensity low frequency current harmonic content into the PMSG and consequently increases the total loses in it. Subsequently, decreases the power capability of the system. This paper presents a comparative simulation study between two different harmonic mitigation approaches applied to PMSG WECS. The studied techniques are: a) harmonic trap filters and b) three-phase boost type PWM rectifier. I. * FENG - Department of Electrical Engineering Pontifical Catholic University of Rio Grande do Sul 90619-900, Av Ipiranga, 6681, Porto Alegre, RS BRAZIL e-mail: [email protected] two conversion stages: the first one an AC-DC stage and the second one a DC-AC stage. To realize the first one a classical three phase bridge rectifier (BR) associated to a bulky capacitor is used and the second stage could be implemented by two types of converters schemes Voltage source current controlled inverter (VS-CCI) and Line commutated inverter (LCI) as shown in Fig. 1. This paper has the main focus in the first energy conversion stage the AC-DC converter, which is responsible by an injection of a high harmonic current content into the PMSG. The circulation of these currents into the machine will generate losses. This work applies two well-known approaches to harmonic mitigation in three-phase AC-DC converters to WECS [2, 3]: a) harmonic trap filters (HTF) and b) threephase boost type PWM rectifier. Using these approaches is possible to minimize the current harmonic content. INTRODUCTION The amount of energy capture from a WECS depends not only on the wind at the site, but depends on the control strategy used for the WECS and also depends on the conversion efficiency. Permanent magnet synchronous generators (PMSG) wind energy conversion system (WECS) with variable speed operation is being used more frequently in low power wind turbine application. Variable speed systems have several advantages such as the reduction of mechanical stress and an increase in energy capture. In order to achieve optimum wind energy extraction at low power fixed pitch WECS, the wind turbine generator (WTG) is operating in variable-speed variable-frequency mode. The rotor speed is allowed to vary with the wind speed, by maintaining the tip speed ratio to the value that maximizes aerodynamic efficiency. The PMSG load line should be matched very closely to the maximum power line of the WTG. MPPT control is very important for the practical WECS systems to maintain efficient power generating conditions irrespective of the deviation in the wind speed conditions. To achieve optimal power output, a sensor-less scheme including a wind turbine model was developed by Tan et al in [1]. The developed wind turbine model will be used in this work in order to evaluate the different harmonic mitigation approaches. In spite of, all this complex control theory to get MPPT on PMSG WECS the standard way to implement a grid connected PMSG WECS at variable speed is using Fig. 1. Wind Energy Conversion System. A software simulation model developed in [1] using PSIM software, which allows easy performance evaluations is used to estimate the behaviour of these two different schemes associated with the PMSG WECS. Simulation results showed the possibility of achieving maximum power tracking, output voltage regulation and harmonic mitigation simultaneously. II. WECS MODEL The WECS considered in this work consists of a PMSG driven by a fixed pitch wind turbine; an AC-DC energy conversion stage implemented using two different approaches and a VS-CCI. The entire system is shown in Fig. 1. A brief description of each element of the system is given below. A. Power from wind turbine The output mechanical power of the wind turbine is given by the usual cube law equation (1). Where Cp is the power coefficient, which in turn is a function of tip speed ratio and blade angle . This relationship is usually provided by the turbine manufacturer in the form of a set of nondimensional curves, the Cp curve for the wind turbine used in this study is shown in Fig. 2. The tip speed ratio is given by equation (2). A= wind turbine rotor swept area [m2], Uw= wind speed [m/s], = air density [kg/m3], r= radius of the rotor [m], m= mechanical angular velocity of the generator [rad/s]. 1 3 P ρC AU [Watts] m 2 p w (1) and λ rω m (2) Uw Where, R and L are the machine resistance and inductance per phase. vd and vq are the 2-axis machine voltages. id and iq are the 2-axis machine currents. m is the amplitude of the flux linkages established by the permanent magnet and r is the angular frequency of the stator voltage. The expression for the electromagnetic torque in the rotor is written as: Te 3 P 2 2 Ld Lq iqid λ miq (6) The relationship between r and m may be expressed as: ωr Figure 2. Power coefficient vs. Tip seed ratio with =0. C. It can be seen that if the rotor speed is kept constant, then any change in wind speed will change the tip-speed ratio (), leading to change of Cp as well as the generated power out of the wind turbine. If the rotor speed is adjusted according to the wind speed variation, then the tip-speed can be maintained at the optimum points, which yield maximum power output from the system. C pmax is the maximum torque coefficient developed by the wind turbine at the optimum tip-speed ratio max. The rate of the rotor speed is proportional to the inverse of the inertia and difference between mechanical torque (T m) produced by the wind turbine and the electrical torque (T e) load from the generator (3). dω m dt 1 J Tm Te 2 ωm (7) Input Bridge Rectifier (AC-DC converter) The complete grid connected sensor-less PMSG WECS scheme using a well-known three-phase six-pulse bridge rectifier and two bulky capacitors are shown in Fig. 3. (3) The wind turbine output mechanical torque is affected by the Cp. In order to maximize the aerodynamic efficiency, the Te of the PMSG is controlled to match with the wind turbine Tm to have maximum possible Cpmax. With a power converter, adjusting the electrical power from the PMSG controls the Te; therefore, the rotor speed can be controlled. For the system to operate at maximum power at all wind speeds, the electrical output power from the power converter controller must be continuously changed so that under varying winds speed condition the system is matched always on the maximum power locus. From the power curve of the wind turbine, it is possible to operate the wind turbine at two speeds for the same power output. In practice, the operating range at region 1 is unstable as the rotor speed of the WTG belongs to the stall region. Any decrease in the tip speed region will cause a further decrease until the turbine stops. B. p Figure 3. Implemented sensor-less VS-CCI WECS. D. Power Variation of the PMSG Wind Turbine The loading characteristic of the PMSG WECS can be easily simulated by connecting an adjustable load resistor to the PMSG and rectifier terminal. Fig. 4 shows the calculated corresponding output power of the PMSG for wind speeds ranging from 4 to 12 [m/s], where the generator maximum power curves show the different operating DC voltages and currents over a range of wind speeds. In order to extract the peak power from the WTG at a given wind speed, the WECS has to match closely to the maximum power curve. PMSG model Theoretical models for generator producing power from a wind turbine have been previously developed. The outer rotor 20kW CRESTA PMSG is used in this WECS mathematical model. The model of electrical dynamics in terms of voltage and current can be given as (4) and (5) [1]: v di q Ri L ω L i ω λ q q q dt r d d r m (4) Figure 4. Predicted DC power characteristics the WECS. III. HARMONIC ANALYSIS di d v d Ri d L d ωr Lqiq dt (5) First of all it is necessary to understand why this study is important. Therefore, a briefly remark of the problem is presented. To do this job a study case is presented showing the PMSG output currents at full load condition (20 kW resistive load) using a conventional BR shown in Fig. 3, which is normally employed in PMSG WECS. The wind speed in this case is 12 m/s. Harmonic characterization of these abnormal currents is obtained and the results are presented in the following section. A complete harmonic analysis of the two three-phase harmonic mitigation approaches mentioned previously will be presented in the following sections. IV. greater than the 4% allowed by IEEE 519 standard. Of course, the IEEE 519 standard it is not applicable to this situation but it is a guideline. THREE-PHASE FULL BRIDGE RECTIFIER A detail of the PMSG WECS output current and line-toline voltage (divided by 4), for the rated power deliver situation at 12 m/sec wind speed, is shown in Fig. 5. Figure 7. Harmonic content of the PMSG output voltage. V. HARMONIC MITIGATION The classical passive trap filters are always associated with the idea of harmonic mitigation. Also the PFC schemes will need high frequency filters to mitigate the high frequency current ripple generated by them selves. A. Figure 5. PMSG output currents and line to line voltage div. by 4. In order to evaluate the quality of current and voltage an objective study was made using the Fourier analysis, the harmonic content and the total harmonic distortion (THD) of the output PMSG current and voltage were obtained, the results are summarized in Fig. 6 and Fig. 7. Passive Harmonic Trap Filters Looking for this classical power electronics problem a first design could be to use passive HTF as shown in Fig. 8 in fact a reduction of the 5th and 7th harmonic could be enough to turn the THD to acceptable levels. If the main idea is to found a compromise solution to track maximum power point at the best wind condition, the performance of the trap filters must be investigated. A design of two trap filters was made to minimise the 5th and 7th harmonic at nominal power. The designed components are C5 = 135F, C7 = 69F and L5 = L7 = L_F1 = 3mH. As the focus of this work is in the field of the losses minimization is important to remark that there is no ideal components. Figure 8. Passive harmonic trap filters. Figure 6. Harmonic content of the PMSG output current. The fundamental components were omitted in these figures, in order to, remark the harmonic content. From these figures it is possible to observe that the 5th, 7th, 11th, 13th, 17th and 19th harmonics are significant. The obtained total harmonic distortion was THD = 10.68 % and 29.15 % for current and voltage respectively, which are quite high. At full load, the harmonic content of the output current is minimized by the influence of the machine stator equivalent inductance and resistance which are L_F1 = 3 mH and R_S1 = 0.432 respectively. Unfortunately this effect is not so noticeable when the available wind decreases and therefore, the maximal output power decreases and the THD increases. The amplitude of the 5th current harmonic is 9.2% of the fundamental, which is Hence, the resistance series equivalent (RSE) of each filter component must be considered. The RSE of each branch were estimated equal to the armature resistance (RSE = 0.432 ) once the branch inductances are equal to the armature inductance. The simulated results are shown in figures 9 to 12. A remarkable improvement in the current and voltages waveforms was obtained using this simple solution. Figure 9 shows the PMSG output currents and the line to line output voltage (divided by 5), which are almost sinusoidal. A current THD less than 2.5% was obtained as shown in Fig. 10. However, this solution implies in bulky components and is matched only for the maximum power condition, which occurs at 12 m/s wind speed. In figure 11 the PMSG output line to line voltage div. by 5 is shown in the frequency domain. Figure 9. Three-phase PMSG output currents and output line to line voltage div. by 5 using 5th and 7th harmonic trap filters. Figure 11. Harmonic content of the PMSG output line-to-line voltage using 5th and 7th harmonic trap filters. Harmonic amplitude in percent of the fundamental component 1.6 1.4 1.2 1.0 THD = 2.26 % 0.8 0.6 0.4 0.2 0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Figure 10. Harmonic content of the PMSG output current using 5th and 7th harmonic trap filters. From these figures is possible to observe an improvement in the PMSG output line-to-line voltage waveform in relationship with the first case without any filter shown in figure 5. The voltage THD was drastically reduced from 29.15% to 9.20%. On the other hand its RMS value has increased from 293 V to 343 V respectively. This occurs because the chosen comparison parameter, which was: the constant output power (20 kW). The resistive load is adjusted in order to keep the output power constant. Once the harmonic core losses are related with the harmonic content of the voltage [5, 6] this characteristic can not be neglected. In order to evaluate the system losses is necessary to know the RMS value of the currents in PMSG output ia, HTF currents ia5th and ia7th and the BR diodes current i_Ret_a. Figure 12 shows the time domain evolution of these currents. The high current values will generate important losses in the system. On the other hand the copper losses inside the PMSG will be slightly reduced since the HTF has change the WECS operation point increasing the line-to-line voltage and decreasing the line current. Unfortunately in the real WECS, the wind speed is constantly varying and hence the PMSG produces variablevoltage and variable-frequency output. Therefore, an infinity number of the trap filters would be necessary. To solve this problem an active solution is needed, there are basically two actives approaches to solve this power electronics problem the first solution is to use the factor correctors (PFC) and the second possible solution is to use active power filters. The study of the active power filters applied to PMSG WECS is out of the scope of this paper. However, is under study at the moment. Figure 12. PMSG output, bridge rectifier and 5th and 7th harmonic trap filters current at 20 kW. B. Three-Phase Boost type PWM Rectifier (ACDC converter) As mentioned before the behavior of the Three-Phase Boost type PWM Rectifier associated to PMSG WECS is studied in this work focused in losses reduction on PMSG by harmonic mitigation. The three-phase boost type PWM rectifier has the capability of to work as an ideal PFC, with very low THD at any operation point. Hence, it is investigated in this work. The converter topology has six two-quadrant controlled switches each one implemented by an IGBT associated to a diode. The inductors and capacitor filter the high-frequency switching harmonics, and have little influence on the low frequency AC components of the waveforms. The switches of each phase are controlled to obtain input resistor emulation. To obtain undistorted line current waveforms, the DC output voltage V must be greater than or equal to the maximal peak line-to-line AC PMSG output voltage. The three-phase boost type PWM rectifier shown in Fig. 11 has several attributes the AC input currents are non-pulsating because the converter works in continuous conduction mode (CCM), and hence very little additional input EMI filtering is required. The converter is capable of bidirectional power flow. This converter also has some disadvantage like: requirement for six active devices, since the rectifier has a boost characteristic working in CCM it requires a complex control scheme using either a multiplying controller scheme employing average current control, or with some other approach like estimation [3, 4]. Where fc is the cut-off frequency, fx is the frequency in which the required attenuation (A1) is determined. A2 is the filter characteristic attenuation. The inductance value is given by: 1 Lf 2 4 C1f c2 (11) Fig. 12. Three-phase boost type PWM rectifier. VI. PMSG POWER LOSSES CALCULATIONS The maximum C1+C2 value is 2.2 µF, and for a proper damping effect, C2=10C1. The damping resistance can be calculated as: Lf C2 Rd Basically the total power losses generated into the machine can be divided into two big groups: a) copper losses and b) core losses. The copper power losses are produced in the stator winding as function of RMS current in it according to equation (8), where Ia_I is the RMS value of the ith harmonic component of the current Ia and Ra is stator equivalent resistance. [9]. However, operating at higher current level resulted in temperature rise. The change of Ra due to temperature rise was not included in the calculations. The high frequency flux changing generated by the harmonics causes hysteresis and eddy current power losses in the core the equation (9) represents these losses [9, 10]. PCu 3 R a I2 a_i i 1 Pcore Pe Ph (k e f 2 B 2 max k h f B 1.5 to 2.5 (9) In order to evaluate the influence of the different harmonic mitigation approaches in the PMSG power losses a normalized study is presented in the final paper. VII. EMI FILTER DESIGN CONSIDERATIONS The design of a suitable EMI filter can be done according different approaches. Usually a high-order filter is used to reduce inductance and capacitance values. Ideally it could be considered that no harmonic components exist in the frequency range between the line and the switching frequencies. This fact would allow centring the filter resonance at a suitable frequency in order to guarantee the required attenuation. In this ideal situation the filter could be undamped. In the Continuous Conduction Mode the input rectified voltage is usually used to create the current reference waveform. In this case the presence of instabilities in the filter output (converter side) can lead to severe converter malfunction. As the input voltage waveform is not so important for the proper operation of the PFC in the Discontinuous Conduction Mode, some damping effect is necessary to avoid oscillations in the average input current produced by transient situations, like load or wind changes. The maximum capacitance should be used to minimise the inductance value. Let us consider the damped second order filter topology shown in Fig. 13. This filter attenuates 40 dB/dec [11]. For a given necessary attenuation, the cut-off frequency is given by: fx fc A1 10 A2 Fig. 13. EMI differential mode filter. VIII. (8) max ) Weight (10) (12) CONCLUSION This digest presents a review on harmonic mitigation in three-phase rectifiers using trap filters and PFC applied to PMSG WECS and preliminary simulation results of a prototype variable speed PMSG WECS. Three possible harmonic mitigation solutions were discussed. Using harmonic trap filters is possible to maximize the PMSG WECS efficiency by mitigation of the harmonics losses. Regrettably, this solution is effective just at one specific operation point since the wind is constantly changing as well the frequency. The single-switch three-phase boost rectifier requires a very simple changing in the three phase rectifier topology as shown in Fig. 11, once the converter work in the DCM the control is as well very simple. However, the conduction losses in the switch Q1 Fig. 11 are high since the high RMS current value caused by the DCM operation. The problem could be minimized using proper switches like IGBT or GTO. The full paper will present design considerations and a complete harmonic mitigation study for this PFC. An EMI filter must be used for EMI reduction and for proper operation of the converter, to avoid CCM once the PMSG stator inductance is high. An EMI filter design criterion is in addition presented. IX. [1] [2] REFERENCE K. Tan and S. Islam, "Optimum Control Strategies in Energy Conversion of PMSG Wind Turbine System Without Mechanical Sensors", IEEE Transactions on Energy Conversion, Vol. 19, No. 2, June 2004, pp. 392-400. Phipps, J.K.;"A transfer function approach to harmonic filter design", Industry Applications [3] [4] [5] [6] [7] Magazine, IEEE , Volume: 3 , Issue: 2 , MarchApril 1997, pp.:68 – 82. Robert W. Erickson, “Some Topologies of High Quality Rectifiers” Keynote paper, First International Conference on Energy, Power, and Motion Control, May 5-6, 1997, Tel Aviv, Israel, pp. 1-6. Indirect current control of a unity power factor sinusoidal current boost type three-phase rectifier Dixon, J.W.; Boon-Teck Ooi; Industrial Electronics, IEEE Transactions on , Volume: 35 , Issue: 4 , Nov. 1988, Pages:508 - 515 Kaboli, Sh.; Zolghadri, M.R.; Homaifar, A., “Effects of sampling time on the performance of direct torque controlled induction motor drive”, IEEE International Symposium on Industrial Electronics, 2003. ISIE '03, Volume: 2 , June 911, 2003, pp.:1049 – 1052 Yao Tze Tat, "Analysis of Losses in a 20kW Permanent Magnet Wind Energy Conversion System", in the Department of Electrical and Computer Engineering. Western Australia: Curtin University of Technology, October 2003. pp. 101. Dos Reis, F.S.; de Lima, J.C.M.; Canalli, V.M.; Pomilio, J.A.; Sebastian, J.; Uceda, J.; “Matching Conducted EMI to International Standards”, Power Electronics Specialists Conference, 2002. PESC 02. 2002 IEEE 33rd Annual, Volume: 1, 23-27 June 2002, pp.:388 – 393, vol.1.