The Study On Key Techniques Of Three-Phase Three-Line Grid-Connected Converter Based On LCL-filter

Grid connected converter (GCC) can operate in inverter and rectifier modes, which is widely used in Static Var Generator (SVG), Active Power Filter (APF) renewable & Distribution Generation (DG) etc. applications, and derives an important type of power electronics equipment. Therefore, main circuit design and current loop control of GCC based on LCL-filter are studied in this thesis.LCL-filter, with good performance, is suitable for ac-side filter of GCC. As three order one, LCL-filter is difficult for design. An LCL-filter design approach with suitable filtering-inductor voltage drop and good bypass effect for switching ripple provided by capacitor and grid-side inductor is proposed based on LCL-filter operation characteristic analysis. This design approach is simple and suitable for practice.DC-bus structure design is important to suppress turn-off voltage spike of IGBT. Laminated bus bar can reduce stray inductance effectively. A simple process of bas bar protected by patent is introduced.3D Software UG is adopted to improve design efficiency.LCL-filter, as a three order one, causes difficulty of stabilization. A grid-side current feedback control approach using inherent one-beat delay of digital control is proposed. Additional phase lag reducing-180°crossing frequency caused by one-beat delay can improve open-loop magnitude margin and ensure close-loop stability. Compared with active damping method, it is also low cost because of no additional sensor is required.To improve steady-state performance of current loop, one-beat delay control as inner loop and repetitive control as outer loop can be combined together to construct double-loop control, so that both fast dynamic response and small steady-state error can be obtained. Simulation and experimental results validate that the proposed double-loop control designed for LCL-filter based GCC can satisfy both dynamic and steady-state performance. The double-loop control can perform unbalanced grid control in synchronous rotating dq frame. Therefore, control system is simplified and fundamental, harmonic and unbalanced current control can be unified..Redundant parallel is suitable for high power and reliability application. Different with common cut-off whole parallel module, this thesis proposes a novel fault-tolerance method that just cut off failure leg to improve system flexibility. Double-loop control in ABC steady frame is used to ensure balanced output current. Principle analysis and experimental results prove that the current control method is robust and reserves good steady-state performance to guarantee balanced output current when the circuit parameter is changed.To simplify debugging of digital control system, an off-line close-loop debugging method is proposed, no main circuit is required. Through discreted control object is real-time computed in DSP, the DSP can simulate close-loop operation, so that control program can be tested fully and debugging efficiency is improved.