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Research on the Control Scheme of Hybrid PWM Rectifier under Unbalanced Voltage

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Tutor: WangMingYan
School: Harbin Institute of Technology
Course: Electrical Engineering
Keywords: unbalanced voltage,PWM rectifier,hybrid rectifier,non-characteristic harmonic
CLC: TM461
Type: Master's thesis
Year:  2008
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Abstract:
The performance of PWM rectifier designed based on the condition of balanced voltage will be poor under unbalanced power supply. The problems such as negative sequences current, voltage and current harmonics and un-constant power flow from ac to dc side will come out, so it becomes the researching hotspot in the field of power electronics that the control strategies of the PWM rectifier under unbalanced power supply.Nowadays, the study of the control strategies under unbalanced power supply is mostly based on the conventional three-phase PWM rectifier which has only one unit. Although this kind of rectifier can realize bi-direction power flow and adjustable power factor, its output DC voltage is not high enough so that it can¡¯t be used in high voltage occasion. For the purpose of having the PWM rectifier more widely used, the topology should be improved.A new topology of PWM rectifier is proposed in this paper. The rectifier is consisted of two units which are controllable unit and uncontrollable unit. Because of the unit serial structure the hybrid PWM rectifier can be used in high voltage frequency conversion region. Compared with the traditional multiple structure, it integrates the advantage of controllable multiple rectifier and uncontrollable multiple rectifier and has better performance.The control strategies of the hybrid PWM rectifier under unbalanced power supply are researched in the paper. Firstly, the mathematical models are set up separately in abc and dq reference frame,and the latter is verified more valuable for the unbalanced system through the analysis of asymmetrical power supply. Secondly, according to the characteristic of the unbalanced voltage, an improved harmonic detecting method is put forward. The fundamental positive sequence voltage can be extracted effectively through this detecting method, so that the harmonic current can be detected accurately. Thirdly, the control strategy will be studied for the purpose of sinusoidal AC current and constant power. Finally, simulation has been done under Matlab/Simulink. The control method can reduce THD of the AC current from 9.59% to 3.52% with the unbalanced degree of 24%. THD of AC current will accordingly go up while the unbalanced degree increases, but when load changes in the definite scope, the output of rectifier remains constant DC voltage. The correctness of the topology and theory has been verified by the simulation result.
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