International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering
MenuISSN ONLINE(2278-8875) PRINT (2320-3765)
ISSN ONLINE(2278-8875) PRINT (2320-3765)
Nan Li1, Bing YU2, Lei Liu1, Bing Kong2
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Through systematically analyzing the mathematical theory knowledge of the small and medium-sized direct-drive permanent magnet wind power system, this paper has designed a rated power of 3kW wind power system under the environment of PSIM9.0 software. The wind power system model has been built, the back-to-back double- PWM control circuit has been designed and the simulation analysis has been completed. In the circumstance of wind speed changes, the simulation results show that the output power of the generator side is stable and the DC voltage of the inverter side is constant, which suggests the correct control strategy, the favorable system stability, and the achieved design goal. The work already done in this article provides a good platform and infrastructure for systematically analyzing the small and medium-sized wind power system.
Keywords |
| PSIM; wind power system; PMSG; full power converter; simulation analysis. |
INTRODUCTION |
| With the increasing depletion of fossil energy, and the continuing deterioration of global environment, the wind power generation, as one kind of new energy power generations, has been paid further attention by the governments and researchers with its unique advantages. Recently, along with the small and medium-sized independent power generation achieving strong support by the relevant national policies, small and medium-sized wind turbines have been vigorously promoted and applied [1]. It is mainly used in remote areas where there is wind but no electricity and it is used as the self-provided power plant by some enterprises, such as remote regions, border posts, lonely islands, largescale agriculture, animal husbandry, fisheries and oil fields, etc. Therefore, the research on small and medium-sized wind turbine system has great significance. The direct-drive permanent magnet wind turbine system is a research hotspot in recent years, and the permanent magnet synchronous generator (PMSG) has been widely used with its good control precision, high efficiency, low maintenance and many other advantages. |
| Study in this paper is based on PSIM9.0 simulation software. PSIM9.0 is a dedicated simulation software for power electronics and motor control study, it has the main features of user interface simple, learning and understanding easy, operation convenient, simulation fast, simulation waveforms clear and intuitive, etc [2]. Compared to the previous version, PSIM9.0 has newly added the solar cell model and wind turbine module which are related to the new energy power generation, thereby providing a superior platform for wind power generation system design and analysis in this article. |
| Utilizing MATLAB/Simulink and PSCAD/EMTDC respectively, literature [3] and [4] have completed the WECS related research work. These two softwares are so powerful that they can realize the complex simulation analysis, however, they launch slowly, have longer simulation time and more complex component modules and control loops while compared with PSIM. Although the simulation software in literature [5] has perfect function, it is not universal and it is not suitable for widespread utilization. Literature [6] and [7] have established the permanent magnet synchronous motor (PMSM) control system simulation model and Maximum Power Point Tracking (MPPT) control system model of WECS respectively, but they did not establish complete WECS models and do simulation analysis. Using PSIM software which can be used to build the simulation system model quickly and intuitively, this paper has set up a system model and finished the small and medium-sized wind power system simulation, and the generator speed and grid voltage being controlled accurately has been realized, which provides new ideas for WECS researches. |
PERMANENT MAGNET WIND POWER GENERATION SYSTEM OVERVIEW |
| Wind power generation system consists of wind turbine, PMSG, converter and control system, the basic structure is shown in Fig.1. Differ from traditional asynchronous wind power generator system, PMSG system eliminates the need for heavy overdrive gearbox, which improves the reliability of power plant operation and the efficiency of wind energy utilization. Moreover, PMSG system absorbing reactive power barely from the power grid and full-power converter achieving low voltage ride through (LVRT) provide a reliable guarantee for the increasingly stringent wind power integration demands. In short, the direct-drive permanent magnet wind power generation system has the following salient features [8]: 1) Light weight and small size generator; 2) Small electromagnetic interference and good electromagnetic compatibility; 3) Simple structure, high reliability and long service life; 4) High efficiency and significant energy saving effect; 5)Good social adaptability under harsh environments; 6)Good voltage waveform quality and adaptable to a variety of load changing circumstances. |
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WIDN POWER GENERATION SYSTEM MATHEMATIC MODEL |
3.1 Power Characteristics of Wind Turbine |
| Wind turbine is a key part of the whole wind energy conversion system, its main function is to capture wind energy and converter it to the mechanical kinetic energy effectively. So, wind turbine not only affects the output efficiency and stability of the entire wind power generation system, but also directly determines the system operation status. According to fluid mechanics knowledge, the kinetic energy produced by airflow sweeping wind turbine is the input power of the wind turbine, which can be expressed as follows. |
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| Where, m is the air quality, S is the swept area of blades, v is the wind speed, ρ is the air density. According to Betz limit theory [9], the maximum power that the wind turbine can export theoretically is shown in the following equation (2). |
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| Where, pmax C is equal to 0.593, however, in the actual operation, it is almost impossible to achieve the value and is generally taken to be 0.4. The output torque formula of wind turbine can be expressed as follows. |
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| Where, P is the output power, Cp is the power conversion factor and ω is the angular velocity of wind turbine blade rotation. |
3.2 Permanent Magnet Synchronous Generator Model |
| During the operation process of PMSG, the rotor and stator keep a relative movement state, and there are mutual influences between windings and permanent magnet and windings. Coupled with the nonlinear factors such as magnetic circuit saturation, the electromagnetic relation is very complicated. Hence, it is difficult to establish accurate mathematical model of PMSG. Therefore, in order to facilitate analysis, the following assumptions are made: 1) Yconnected stator windings; 2)Sinusoidal back-EMF, no considering the effect of space harmonic and magnetic circuit saturation; 3)Excluding eddy current and hysteresis losses; 4)No dynamic response process of excitation current. In the d-q axis rotation coordinate systemïüÃÅthe voltage and electromagnetic torque mathematical relationships of PMSG are expressed as follows [10]. |
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CONTROL STRATEGIES AND MODELING |
| Back-to-back double PWM full power converter consists of generator side converter, DC link and grid side inverter. The generator side converter receives active power firstly, then this part of active power is sent to the grid side converter through the DC link and finally to the grid. Due to the speed of wind and generator changes unceasingly, the output of generator voltage and frequency varies, so, in order to achieve grid voltage and frequency requirements, control method of dual PWM converter is the key. |
4.1 Control Strategy and Simulation Model of Generator Side |
| The purpose of generator side converter control while wind speed changes, is to regulate the generator torque and rotating speed, and achieve maximum power tracking control. In the rotating coordinates, as formula (6) shows, when Ld ïÃâ¬Ã½ Lq , torque is only related to q-axis current, therefore, the torque control is equal to current control. In short, the control strategy of generator side is double closed loop control which puts the current control as inner loop control and the speed control as the outer loop control. |
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4.2 Control Strategy and Simulation Model of Grid Side |
| The grid-side three-phase voltage converter in fact is the rectifier which works in inverter state, controlling the inverter makes DC bus voltage stability and grid-side unit power factor operation. Traditional small and medium-sized wind power system is controlled by single current control method, aiming at the shortcomings of the method, this paper adopts double closed-loop controlled voltage oriented vector control method [11]. Voltage oriented vector control is divided into indirect control method and direct control method, this paper adopts the latter which has a better dynamic performance, including closed-loop current control, quick current response and good robustness [12-13]. In the control process, the outer loop voltage signal is affected by the given DC voltage signal Vdc and the feedback DC voltage signal Vdc , while the inner loop current signal is controlled by the coordinate transformed current on the grid side. The simulation model built utilizing PSIM software is shown in Fig.3. |
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SIMULATION RESULTS ANALYSIS |
| In order to verify the feasibility of the small and medium-sized permanent magnet wind power system, a rated power of 3kW system has been designed, the main parameters of this system is shown in Tab.1. The simulation system model is built utilizing PSIM9.0 software, and the simulation time is set to be 0.5s. When wind speed mutates, in order to verify the system characteristics of maintaining stable operation, it is set from the rated wind speed of 12m/s to 9m/s, the waveform is shown in Fig.4. At this point, the actual change of generator speed and electromagnetic torque is shown in Fig.5. Fig.6 expresses the output power waveform when the wind speed changes, while Fig.7 shows the DC voltage waveform. Fig.8 and Fig.9 indicate the output line-voltage value and the three-phase current value respectively, while Fig.10 illustrates the grid reference voltage. |
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| The pitch angle transformation is not considered in this paper, and the system is operated under or below rated wind speed. Referring to Fig.5, when the wind speed decreases, the generator speed and the electromagnetic torque decreases accordingly, the response speed is very fast for the signal keeps stability after tiny fluctuations. Especially, from Fig.6, the output power signal keeps steady after wind speed changes, which fully meets the requirements of the control system. The grid side DC voltage is a critical parameter which determines whether WECS can incorporate into the power grid or not. In Fig.6, the DC voltage signal transits smooth and recovers quickly and it won’t cause big impact and interference to power grid, which satisfies the power grid requirements. The inverter output voltage and current signals in Fig.8 and Fig.9 are good for there are almost no harmonics. In a word, the simulation result indicates that the simulation effect under PSIM environment is good and the output parameters are very standard. At the same time, it demonstrates the system is reasonable and satisfies the design requirements. |
CONCLUSIONS |
| The simulation model of small and medium-sized permanent magnet wind power generation system has been designed based on PSIM9.0, each module parameters has been calculated accurately. The final simulation results are in conformity with the theoretical calculation values, showing that the model design of the small and medium-sized permanent magnet power generation system is reasonable. In PSIM software environment, the system model is clear and intuitive, the control circuit is simple, the simulation waveform is easy analytical, which provides the experimental and investigative basis for small and medium-sized wind power generation. |
References |
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