| In China’s industrialization and urbanization, the development of electric power industry has made a great stride. However, due to the extensive use of power electronic equipments, a large number of nonlinear loads have emerged in electricity generation and transmission as well as power utilization terminals. Power quality problems, which affect the secure and stable operation of the power system, have already become a problem which needs to be solved urgently. Shunt active power filter (APF) is a kind of power quality compensation equipments, which is able to implement comprehensive treatment of power quality problems, such as harmonics, reactive power and grid unbalance. Compared with the traditional passive power filters, APF is featured as high precision steady-state compensation, fast dynamic response, flexible compensation manners, and low probability of causing resonance with the grid impedance. This dissertation is focused on some key technologies of the shunt APF with LCL-filter.Firstly, the continuous-time model of shunt APF with LCL filter has been built. The delay of the three-phase PWM converter has been analyzed in details with different sampling and modulation methods.Secondly, the characteristics and principles of the LCL filter are analyzed. Then, characteristics of the passive damping method and its related improvement measures are introduced. With an equivalent control block transform method, a corresponding active damping approach based on state variables feedback is derived and the relationships between state variables are analyzed. On the basis, considering that conventional control methods of APF with LCL filter are featured as many current sensors and low reliability, a double-loop current control method is proposed without additional filter capacitance current sensors, which is based on gird-source current harmonic compensation outer loop and converter-side current inner loop. In this method, source-side current control method is adopted to effectively improve compensation accuracy for harmonic currents while converter-side current control method is used to suppress LCL filter resonance, reduce current sensors and improve the reliability. In addition, the stability of converter-side current and grid-side inductor current feedback are analyzed in discrete domain. As a result, constraint conditions between stability of LCL filter and its resonance frequency are obtained. Utilizing the inherent delay effect of digital PWM converter, a no damping LCL control method based on only gird-side inductor current feedback is subsequently proposed. This method effectively avoids producing resonance current, thus implementing stable and reliable operation of APF.Thirdly, considering that compensation current of APF consists of multiple harmonics, a repetitive control method with high steady-state accuracy is introduced in the current control loop. Then stability criterion and control performance of the repetitive controller are analyzed. For improving dynamic response of current control, the repetitive internal model is subsequently corrected aiming at characteristic harmonics, thus proposing a fast-transient repetitive control strategy which only contains one sixth of the delay in the conventional method. In the designing of compensation corrector, a zero phase shift notch filter is adopted to suppress the resonance peak of LCL filter and the corresponding designing method of corrector is also given.In order to reduce the computational consumption of APF control algorithms, this paper proposes a selective harmonic compensation (SHC) current control method based on reduced order generalized integrators (ROGI). The ROGI is deduced through complex vector signal model and improved to implement independent adjustment of bandwidth and gain. In the stationary reference frame, decoupling characteristics of frequency and sequence in ROGI units are considered and respective ROGI units are thus set for fundamental harmonic and specific harmonics, which implements selective compensation for harmonics with different frequencies and sequences. Then, the relationship between the current controller parameters and compensation system performance is analyzed. As a result, the optimization design of the controller parameters is given.Finally, in order to deal with the grid synchronization problem of power electronic converter under adverse grid voltage conditions, three-phase voltage vector under distorted gird voltage conditions and PLL based on synchronous rotating frame are analyzed. A synchronization method based on frequency adaptive moving average filter phase locked loop (MA-PLL) is subsequently proposed in this dissertation. A moving average filter is inserted in the conventional synchronous rotating frame PLL in order to eliminate the influence of unbalanced and harmonic distortion voltage components in adverse grid conditions. In addition, magnitude normalized method and frequency-adaptive algorithm are used to improve the performance of MA-PLL under the condition of voltage magnitude fluctuation and frequency variation. The PLL coefficients are optimized by the small-signal model analysis method. On the basis, considering that MA-PLL is featured as a slower dynamic response, an improved MA-PLL is proposed to improve its dynamic response. |
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