Controller Design And Performance Analysis Of Back-to-back Voltage Source Converter

Back-to-back voltage source converter is increasingly used in new fields such as VSCF wind power generator system and HVDC light. When back-to-back VSC is applied to the mentioned fields, its integral working performance depends on not only the stability, rapidity and accuracy of the controller, but also the structure and parameters of the main circuit. So research for control and operation performance of back-to-back VSC has important directive significance to its practical operation.In this paper, the mathematical modeling, design of controller and the effects on performance of system from main circuit parameters of back-to-back VSC are studied. The main research contents are as follows:The dynamic mathematical model for back-to-back VSC under dq synchronous reference frame is developed, the power exchange control principle between converters and two side ac systems is analyzed,the problem of power exchange control is converted to the problem of tracking of active and reactive current reference values under satisfying operational constraint. A dual closed loop controller based on direct current control strategy is designed for active power and reactive power exchange between the converter and two side ac systems, active power control and reactive power control are decoupled. According to the system transfer function, the direct quantitative relationship is established between the desired performance targets and PI parameters based on pole-assignment for PI parameter′s design method. Anti-disturbance performance of the system is improved by introducing load current feed-forward.The influence of the inductance on ac side of back-to-back VSC to the system operation performance is analyzed from three aspects: power transfer capability, response speed of instantaneous current and harmonic characteristic, and the influence of the capacity on dc side to the fluctuation quantity of the dc-link voltage is analyzed.Finally, power exchange control simulation model for back-to-back VSC based on PSCAD/EMTDC is set up, various function modules are introduced. Power exchange control between back-to-back VSC and two side ac systems is simulated, the validity of the proposed controller is demonstrated by the simulation results.