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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 *School of Electrical and Computer Engineering Curtin University of Technology GPO Box U1987, Perth, Western Australia 6845 Australia 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 three different approaches applied to harmonic mitigation on PMSG WECS. The studied techniques are: a) harmonic trap filters, b) single-switch three-phase boost rectifier and c) three-phase boost type PWM rectifier; Keywords: Harmonic mitigation; PFC; PWM rectifier; permanent magnet synchronous generator; wind energy converters system; variable speed operation. Corresponding author: Fernando Soares dos Reis School of Electrical and Computer Engineering Curtin University of Technology GPO Box U1987, Perth, Western Australia 6845 - Australia E-mail: [email protected] HARMONIC MITIGATION IN WIND TURBINE ENERGY CONVERSION SYSTEMS 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 three different approaches applied to harmonic mitigation on PMSG WECS. The studied techniques are: a) harmonic trap filters, b) single-switch three-phase boost rectifier and c) three-phase boost type PWM rectifier; Keywords Harmonic mitigation; PFC; PWM rectifier; permanent magnet synchronous generator; wind energy converters system; variable speed operation. 1. 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 converters 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 sensorless scheme developed by Tan et al in [1] will be used in this work for extracting desired output power from the WTG over a wide range of wind speeds. 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 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 full bridge rectifier 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. Figure 1. Wind Energy Conversion System 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 three well-known approaches to harmonic mitigation in three-phase AC-DC energy conversion systems [2, 3]: a) harmonic trap filters, b) single-switch three-phase boost rectifier and c) three-phase boost type PWM rectifier; Using these approaches is possible to minimize or to eliminate the current harmonic content. A software simulation model developed in [1] using PSIM software, which allows easy performance evaluations is used to estimate the behaviour of these three different schemes associated with the PMSG WECS. Simulation results showed the possibility of achieving maximum power tracking, output voltage regulation and harmonic mitigation simultaneously. 2. 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 power factor rectifiers; and a VS-CCI. The entire system is shown in Fig. 1. A brief description of each element of the system is given below. 2.1. 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 . This relationship is usually provided by the turbine manufacturer in the form of a set of non- dimensional 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 [r/sec]. 1 3 P ρC AU Watts m 2 p w λ rω m (1) (2) Uw may, however, interfere with the control system modelled in this paper. 2.2. PMSG model Theoretical models for generator producing power from a wind turbine have been previously developed [6, 7]. The outer rotor 20kW CRESTA PMSG described in [8] 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) [7]: (4) v (R pL )i ω L i ω λ q q q r dd r m v d (R pLd )i d ω r L q i q Figure 2. Power coefficient vs. Tip seed ratio with =0 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. Cp max 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. dω m dt 1 J Tm Te (5) 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 ρ = d/dt. r=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 2.3. p 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 Cp max. 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. Therefore, the controller has to be designed to keep the operating point inside the desired region. For a variable speed wind turbine with pitch control, optimum power can easily be obtained using appropriate control. However, for small machines that use a fixed pitch, this mechanism is not possible. The current paper looks at fixed pitch machines. The use of pitch machines control Figure 3. Implemented sensor-less VS-CCI WECS 3. 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 12m/sec, 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. 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. Figure 4. Predicted DC power characteristics the WECS. 4. HARMONIC ANALISYS 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 using a conventional six-pulse rectifier shown in Fig. 3 which, is normally employed in PMSG WECS is presented, 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 threephase PFC presented previously will be presented in the final paper. 4.1. Figure 6. Harmonic content of the PMSG output current. In Fig. 7 a detail of the harmonic content amplitude of the PMSG output current in percent of the fundamental is shown. The fundamental component was omitted in this figure, in order to, remark the harmonic content. Three-Phase Full Bridge Rectifier A detail of the PMSG WECS output currents, for the maximum power deliver situation at 12 m/sec wind speed, is shown in Fig. 5. In order to evaluate the quality of these currents an objective study was made using the Fourier analysis, the harmonic content and the total harmonic distortion (THD) of the output PMSG current were obtained, the results are summarized in Fig. 6 and Fig. 7. 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 = 13.47 % which is quite high. Figure 7. Harmonic content of the PMSG output current. The amplitude of the 5th harmonic is 12.4% of the fundamental, which is greater than the 10% allowed by IEEE 519 standard. Of course, the IEEE 519 standard it is not applicable to this situation but it is a guideline. 5.0. HARMONIC MITIGATION The classical passive trap filters are always associated to 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 high frequency filter design criterion will also be presented. 5.1. Figure 5. Three phase PMSG output current at 20 kW. At full load, the harmonic content of the output current is minimized by the influence of the machine stator Passive Harmonic Trap Filters Looking for this classical power electronics problem the first solution is to use passive harmonic trap filters 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 at the best wind condition the trap filters could be used. 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 have different names as: factor correctors (PFC), power factor rectifiers (PFR), power factors preregulators (PFP), PWM rectifiers or resistor emulators 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. 5.2. Single-Switch Three-Phase Boost Rectifier (AC-DC converter) Figure 8. Passive harmonic trap filters. A design of two trap filters was made to minimise the 5th and 7th harmonic the results are shown in Figs. 9 and 10. Figure 9. Three-phase PMSG output current at 20 kW using 5th and 7th harmonic trap filters. A remarkable improvement in the current wave form was obtained using this simple solution. A THD less than 3% was obtained as shown in Fig. 10. The fundamental component was omitted in this figure, in order to, remark the low harmonic content. Note that this solution implies in bulky components and is matched only for the maximum power condition, which occurs at 12 m/s wind speed. One of the selected power factor corrector (PFC) approaches, suitable to implement the input rectifier, the black block shown in Fig. 3, was the three-phase DCM boost rectifier proposed by Prasad and Ziogas [2]. Because this converter presents many advantages such as: a very low THD in the input current, a simple structure using just one controlled device and could be controlled using a simple control strategy easily implemented with general-purpose PWM integrated circuits (IC) controllers. The transistor can operate at constant switching frequency. The control of the DC output power is easily made by duty cycle control. Only a single active device such as a MOSFET or IGBT is needed. The main disadvantages of this implementation are the increasing of the power losses in the devices in comparison with the conventional three-phase AC-DC converter and the need of an additional input filter to remove the high-frequency harmonic components of the input currents [2, 4]. Therefore, the implementation of the input rectifier using a single-switch three-phase boost rectifier resulting in harmonic mitigation on the PMSG output currents. The complete schematic diagram of the three-phase DCM boost rectifier is shown in Fig. 11. Figure 11. Single-switch three-phase boost rectifier. Figure 10. Harmonic content of the PMSG output current using 5th and 7th harmonic trap filters. Unfortunately in the real WECS, the wind speed is constantly varying and hence the PMSG produces variable-voltage and variable-frequency output. Therefore, an infinity number of the trap filters would be Inductors L1, L2, and L3 have the some small value, such that they operate in the discontinuous conduction mode in conjunction with diodes D1 – D6. At the end of the transistor Q1 conduction subinterval, the inductor currents reach peak values, which are also proportional to the applied three-phase line-to-neutral voltages. When transistor Q1 turns off, then diode D7 becomes forwardbiased and the inductors release their stored energies to the DC output. Since the peak input currents are proportional to the applied input line-to-neutral voltages, then the average values of the input currents are also approximately proportional to the input line-to-neutral voltages. Approximate three-phase input resistor emulation is obtained. The three-phase DCM boost rectifier does generate a modest amount of low frequency input current harmonics. But, increasing the DC output voltage is possible to reduce the THD [4]. 5.3. Three-Phase Boost type PWM Rectifier (AC-DC converter) As mentioned before the behaviour of the two PFC associated to PMSG WECS are 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 in order to overcome the disadvantages presented by the single-switch three-phase boost rectifier. The converter topology has six two-quadrant controlled switches each one implemented by an active device 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. 12 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 bi-directional power flow. This converter also has some disadvantage like: requirement for six active devices when compared with the previous circuit shown in Fig. 11 which uses just one active device, the active semiconductor utilization is low, since the rectifier has a boost characteristic and requires a much more complex control scheme using either a multiplying controller scheme employing average current control, or with some other approach like estimation [3, 4, 5]. 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. According to the paper proposition, the filter will be designed considering only the differential mode EMI noise produced by the converter. For this filter type, not only the required attenuation but also other restrictions must be taken into account. For example, VDE standard specifies a maximum x-type capacitance of 2.2 F in order to limit the line current (50/60Hz component) even at no load situation. The maximum capacitance should be used to minimise the inductance value. Let us consider the damped second order filter topology shown in Fig. 10. This filter attenuates 40 dB/dec. For a given necessary attenuation, the cut-off frequency is given by: fx fc A1 10 A2 (8) 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 (9) The maximum C1+C2 value is 2.2 µF [8], and for a proper damping effect, C2=10C1. The damping resistance can be calculated as: Rd Lf C2 (10) Fig. 13. EMI differential mode filter. 6. CONCLUSION Figure 12. Three-phase boost type PWM rectifier. 5.4. EMI Filter Design Considerations The design of a suitable EMI filter can be done according different approaches [7-9]. 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 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. The full paper will present design considerations and a complete harmonic mitigation study. 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. 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. The best technical solution is obtained using three-phase boost type PWM rectifier which can mitigate the harmonic generation over the entire range of wind speed. A very low EMI level is generated since this converter work in CCM. Now, the high stator inductance value of the PMSG is desirable and EMI filter could be strongly minimized or unnecessary. In spite of this, this is the most expensive solution and requires more complex control circuits. An EMI filter design criterion is in addition presented. 7. REFERENCE [1] K. Tan, S. Islam, and H. Tumbelaka, "Performance comparison of a current controlled and line commutated inverter in maximum energy conversion," presented at International Power Engineering Conference IPEC'2003, Singapore, 2003. A. R. Prasad, P. D. Ziogas, and S. Manias, “An active power factor correction technique for three-phase diode rectifiers,” in PESC’89 Rec., pp. 58–65. T. Ohnuki, O. Miyashita, P. Lataire, and G. Maggetto, “Control of a Three-Phase PWM Rectifier Using Estimated AC-Side and DCSide Voltages” IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 14, NO. 2, MARCH 1999, pp. 222-226. 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. 1 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 A. J. G. Westlake, J. R. Bumby, and E. Spooner, "Damping the power-angle oscillations of a permanent-magnet synchronous generator with particular reference to wind applications," IEE Proc.-Electr. Power Appl, Vol. 143, No. 3, May 1996, pp. 269-280. B. S. Borowy and Z. M. Salameh, "Dynamic response of a stand-alone wind energy conversion system with battery energy storage to a wind gust," IEEE Transactions on Energy Conversion, Vol. 12, No. 1, March 1997, pp. 7378. J. Y. Chen and C. V. Nayar, "A low speed, high torque, direct coupled permanent magnet generator for wind turbine application.," presented at Proceedings of Solar'97 Australian and New Zealand Solar Energy Society, 1997, pp. 1-6. [2] [3] [4] [5] [2] [3] [4] [5] [6] L. J. Borle, M. S. Dymond, and C. V. Nayar, "Development and testing of a 20kW grid interactive photovoltaic power conditioning system in western Australia," IEEE Transactions on Industry Applications, Vol. 33, No. 2, March/April 1997, pp. 502-508. L. J. Borle and C. V. Nayar, "Zero average current error control power flow for AC-DC power converter," IEEE Transactions on Power Electronics, Vol. 10, No. 10, November 1995, pp. 725-732.