Abstract : This paper analyzes in depth the key issues in the control and design of the interleaved parallel chopper for direct-drive wind turbines. First, an analytical expression for the chopper input current command based on the maximum power point tracking (MPPT) algorithm is derived. Second, a unified expression for the ripple current of the N-fold interleaved parallel boost chopper is given, and based on this, the design criteria for the filter inductor are obtained. Finally, a comprehensive simulation of a 1.2MW direct-drive wind turbine full-power grid-connected converter unit is conducted. The simulation results verify the correctness of the control and design scheme proposed in this paper, demonstrating its practical value. Keywords : Direct-drive wind turbine, permanent magnet synchronous generator, interleaved parallel boost chopper, ripple analysis 1 Introduction Currently, the mainstream variable-speed constant-frequency wind turbine grid-connected converter systems include two types: doubly-fed induction generator (DFIG) based units and direct-drive units based on permanent magnet synchronous generators (PMSG). Compared with doubly-fed induction generators, direct-drive turbines eliminate the need for gearboxes and avoid generator slip rings and brushes, resulting in advantages such as improved efficiency, reduced maintenance, and increased reliability, and thus have broad application prospects [1-2]. Their topology is shown in Figure 1a. Direct-drive wind turbine grid-connected converters consist of two parts: a grid-side converter and a generator-side converter. The grid-side converter typically uses a three-phase voltage-type PWM rectifier circuit to achieve unity power factor grid connection and can also perform reactive power regulation of the grid when needed. The generator-side converter provides excitation power through the generator and adjusts the generator speed according to wind speed changes to achieve maximum power point tracking (MPPT). There are two main topologies: uncontrolled rectifier with boost chopper circuit (passive rectification) and three-phase voltage-type inverter circuit (active rectification). Although passive rectifier topologies have disadvantages compared to active rectifier topologies, such as low generator stator current with high harmonic content and low generator internal power factor, they are relatively inexpensive, simple and reliable to control, and can avoid the negative effects of active rectifier topologies on generators. Therefore, they still have a wide application market [3-4]. The main circuit of the system is shown in Figure 1b. The performance of passive rectifier topologies is closely related to the control of the boost chopper. In megawatt-level direct-drive wind turbine generators, the chopper generally controls the input current to regulate the load torque of the generator [align=center] Figure 1 Direct-drive wind turbine generator grid-connected converter system[/align]. Therefore, the calculation of the input current command is crucial for the accurate realization of MPPT. There are many research results in this area, such as obtaining the current command by using the hill climbing method and its improved methods [5], or obtaining the current command by using the simplified mathematical relationship between the input current and the uncontrolled rectifier voltage [6], but all lack in-depth analysis of the working characteristics of the passive rectifier topology itself. On the other hand, to increase output power and reduce total current ripple, choppers often adopt a multi-interleaved parallel structure [7-8]. The current ripple amplitude will change depending on the number of parallel elements or the duty cycle, making the design of the input filter inductor very complicated. Based on these considerations, this paper first deeply analyzes the mathematical relationships within the passive rectifier topology and gives an analytical expression for the input current command based on the MPPT algorithm, providing a basis for the current tracking control of the chopper; secondly, this paper derives a unified expression for the total current ripple of an N-fold interleaved parallel boost chopper, and based on this, obtains the calculation formula for the input filter inductor, greatly simplifying the system design process; finally, this paper establishes a simulation model of a 1.2MW direct-drive wind power generation full-power grid-connected converter unit based on Matlab 7.3, and gives the simulation waveforms of the main system variables, verifying the above control and design schemes. 2. Current control strategy As shown in Figure 1b, the chopper input voltage U[sub]r[/sub] is determined by the uncontrolled rectifier, and the output voltage U[sub]dc[/sub] is constant by the grid-side converter. Therefore, the chopper achieves maximum power tracking by controlling the input current I[sub]r[/sub]. 2.1 Maximum power tracking principle of wind power[9] The input power of the wind turbine satisfies the Betz principle. When the generator speed reaches the rated speed, as the wind speed increases, the speed becomes constant, the tip speed ratio decreases, the power coefficient decreases, but the output power of the wind turbine continues to increase. Until the wind speed reaches the rated wind speed, the output power of the wind turbine reaches the rated value. 2.2 Calculation of input current command Let the no-load potential of each phase of the permanent magnet synchronous generator be E, the terminal voltage be U[sub]s[/sub], and the phase current be I[sub]s[/sub] (all refer to the fundamental component, the same below), X[sub]d[/sub] and X[sub]q[/sub] are the DC and AC synchronous reactances, respectively. Considering the generator output is connected to a diode rectifier circuit, the current vector will be aligned with the terminal voltage vector. Ignoring the generator's internal resistance, the generator operating condition vector diagram is shown in the figure. P[sub]omax[/sub] is the limit of the active power output of the passive rectifier topology at a certain speed. The physical meaning of this value can be explained as follows: Since the motor output current and terminal voltage are in the same direction, when ω[sub]e[/sub] is constant and I[sub]r[/sub] changes, the trajectory of point A in Figure 3b is a semicircle with the no-load potential vector as its diameter. As I[sub]r[/sub] increases, point A moves clockwise, the angle δ increases, and the internal power factor of the generator gradually decreases. When δ=45[sub], the active output reaches its maximum. After that, if the current increases further, the generator's power factor decreases further, reactive power becomes the main component, and the active output decreases instead. 3. Current Ripple Analysis and Inductor Design 3.1 Current Ripple Analysis The single boost chopper topology, inductor current waveform and ripple expression are as follows, where D is the duty cycle and Ts is the switching period. Two interleaved parallel circuits are adopted, with pulses interleaved by 180 degrees. The current superposition can be divided into two cases according to the range of the duty cycle. Assuming that the two filter inductors are equal, the total current ripple expression can be obtained as shown in equation (14) after simplification. Observing equations (13) to (15), it is not difficult to find that the ripple current expressions all satisfy the following rules: As the duty cycle and the number of parallel circuits change, the constant term in the ripple current is u1Ts/L. The denominator of the changing part contains 1-D, and the numerator contains the product of the distance from the duty cycle to the two ends of the interval, and then multiplied by the number of interleaved parallel circuits. Therefore, the unified expression for the total current ripple after N-fold interleaved parallel connection can be derived as follows, providing a basis for the design of the filter inductor. 3.2 Filter Inductor Design The design of the filter inductor is analyzed using a triple interleaved parallel topology as an example. The steady-state duty cycle satisfies the relationship D=1-u[sub]r[/sub]/u[sub]dc[/sub]. Substituting it into equation (15) and rearranging the formula, we can obtain the relationship between the current ripple and the input and output voltages. In this equation, U[sub]dc[/sub] is constant by the grid-side converter. Therefore, as the ripple current changes, the variation law of the ripple current is shown in Figure 6. As can be seen from the figure, when the input voltage is 1/3 or 2/3 of the output voltage, the total current ripple can be completely canceled. If the corresponding 4 System Simulation To verify the above design and control methods, this paper builds a simulation model of a 1.2MW direct-drive wind power generation full-power grid-connected converter unit based on Matlab7.1, as shown in Figure 7. Among them, the generator-side converter adopts a passive rectification topology, and the boost chopper adopts a triple interleaved parallel structure; the grid-side converter adopts voltage-oriented control[11]. The main circuit parameters are listed in Table 1. [align=center] Figure 7 Simulation model of the full power grid-connected converter unit of the direct-drive wind turbine Table 1 List of main circuit parameters of the 1.2MW direct-drive wind turbine[/align] Figure 8 shows the waveform of the main variables of the unit when the wind speed changes. Among them, Figure 8a shows the corresponding waveform of the generator speed, and Figure 8b shows the corresponding chopper input current waveform. It can be seen that the generator speed changes with the wind speed to ensure the optimal tip speed ratio, indicating that the adjustment of the generator load torque, i.e. the adjustment of the chopper input current, is correct and effective. Figure 8c shows the intermediate DC voltage waveform, and the DC voltage is constant at 1100V in steady state. Figure 8d shows the voltage and current waveform of the grid-side a phase at rated power, the total harmonic distortion of the grid-connected current is 3.69%, and the power factor is close to 1. Table 1 shows that the chopper filter inductance is 0.37mH and the switching frequency is 2kHz. From equations (1), (6), and (12), the current command at a wind speed of 8m/s is 57.9A and U[sub]r[/sub] is 78.5V. Substituting the above parameters into equation (16), we can obtain that the amplitude of the single current ripple is 30.4A and the pulsation frequency is 2kHz; the amplitude of the total current ripple is 61A and the pulsation frequency is 6kHz. To verify the calculation results, Figure 9 shows the waveforms of the uncontrolled rectified voltage, single current, and total current at this time. It is not difficult to find that the actual value of the ripple matches the calculated value very well, proving the correctness of the calculation formula. [align=center] [/align] 5 Conclusion This paper conducts a relatively in-depth analysis and research on the key issues in the control and design of the interleaved parallel chopper of the direct-drive wind turbine. Overall, compared to active rectification, passive rectification topologies are simpler to control. To output the same active power, passive rectification topologies require a larger generator stator current but a lower terminal voltage, and they also do not have the du/dt problem of inverter-driven motors. However, passive rectification topologies have an inherent active power output limit, which should be carefully considered during system design. In addition, while using interleaved parallel chopper circuits reduces current ripple, the system cost will also increase accordingly. References[1] Kazmierkowski MP, Krishnan R, Blaabjerg F. Control in power electronics: selected problems[M]. USA: Academic Press, 2002. [2] Hansen AD, Lov F, Blaabjerg F, et al. Review of contemporary wind turbine concepts and their marked penetration[J]. Wind energy, 2004, 28(3):247-263. [3] Chen Z, Spooner E. Grid interface options for variable-speed, permanent-magnet generator[J]. IEE Proceedings-Electric Power Application, 1998,45 (4):273-283. [4] Chinchilla M, Arnaltes S, Burgos J C. 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Proceeding of the 8th International Conference on Electrical Machines and Systems ,2005,2:1046-1049. [9] Datta Rajib ,Ranganathan V T. A method of tracking the peak power points for a variable speed wind energy conversion system[J].IEEE Transactions on Energy Conversion, 2003, 18.（1）: 163-168. [10] Wang Zhengtong, Luo Qianchao, Diao Yuanjun. Research on DC/DC converter interleaved parallel connection technology[J]. Communication Power Supply Technology, 2006,23（5）:3-4. [11] Chen Yao , Jin Xinmin .Modeling and control of three-phase voltage source PWM rectifier [C].CES/IEEE 5th International Power Electronics and Motion Control Conference, 2006,3:1459-1462. [12] Heier S Grid integration of wind energy conversion systems [M].UK: John Wiley and sons Ltd, 1998. [align=center]Design and control of Parallel Interleaving boost chopper in Directly-Drive Wind Power Generation System YANG Guo-liang LI Hui-guang (College of Electrical Engineering, Yanshan University, Qinhuangdao 066004, China)[/align] Abstract: This paper profoundly analyze the key problem during the design and control of parallel interleaving boost chopper in directly-drive wind power generation system. First, analytic expression of input current order based on MPPT algorithm is obtained; secondary, uniform expression of ripples current in N parallel interleaving boost chopper is given, and based on which the design criterion of the filter inductance is derived; last, 1.2MW directly-drive wind power generation power interconnected converting unit is emulated, the results shows the utility and the truthness of the proposed control and design scheme. Key words : directly-drive wind power generation system; PMSG; parallel interleaving boost chopper; ripples analysis About the author : Yang Guoliang (1973-), male, from Gongzhuling, Jilin Province, is a doctoral candidate whose main research interests are the application of modern control theory in power electronics technology and distributed generation. Li Huiguang (1947-), male, from Qiqihar City, is a professor and doctoral supervisor whose main research interests include sampling theory, robot vision, distributed generation, and renewable energy.