Investigation On The Operation And Control Of DFIG-based Wind Power Generation System Under Unbalanced And Harmonically Distorted Grid Voltage Conditions

Doubly-fed induction generator (DFIG) has been widely used in large-scalevariable-speed constant-frequency wind power generation systems due to its manyexcellent operational performances. However, owing to direct connections between thestator windings and the power grid, the DFIG system is very sensitive to griddisturbances, degrading the operational performance under distorted grid voltageconditions and coexisting conditions of grid voltage unbalance and harmonic distortions.Thus, how to further improve the safe operation capabilities of DFIG system underdistorted grid voltage conditions and coexisting conditions of grid voltage unbalanceand harmonic distortions has attracted more and more attentions recently. The proposedDFIG system with series grid-side converter (SGSC) where the inspiration for SGSCstems from dynamic voltage restorer (DVR) is with characteristics of flexible controlledstator voltage, which can achieve zero voltage ride-through and has excellent lowvoltage ride-through (LVRT) ability. Therefore, the domestic scholars have deeplyinvestigated the operation and control of DFIG system with SGSC under grid voltageunbalance and grid faults. However, the operation and control of DFIG system withSGSC under distorted grid voltage conditions and coexisting conditions of grid voltageunbalance and harmonic distortions has not been reported worldwide. This dissertationintends to investigate the operation and control of DFIG system with SGSC underdistorted grid voltage conditions and unbalanced and harmonically distorted gridvoltage conditions. The main work is as following:1) The mathematic model of the DFIG system with SGSC is established. Based onthis model, a steady-state control strategy for this novel system during normal gridcondition is proposed and validated by the simulation results. Simulation resultsindicate that the DFIG system with SGSC and the tranditional DFIG system have thesame control effect, which lays the foundation for the operation and control of the DFIGsystem with SGSC under distorted grid voltage conditions and coexisting conditions ofgrid voltage unbalance and harmonic distortions.2) The operational behaviors of the DFIG system with SGSC under grid voltagedistortion of fifth-and seventh-order harmonics involved is analyzed. Based on theanalysis, two alternative enhanced control strategies for the DFIG system with SGSCunder aforementioned grid conditions are proposed to restrain the adverse effects of harmonic voltage on the DFIG and improve the operational performance of the wholesystem, i.e. no oscillations at six times the grid frequency in the total active and reactivepower simultaneously, or no harmonic current injected to grid. As for the controlstrategy, i.e. no oscillations at six times the grid frequency in the total active andreactive power simultaneously, the reactive power limitation of the parallel grid-sideconverter (PGSC) supported to power grid is derived in detail. Simulation resultsindicate that the proposed control strategies can realize generator symmetrical andstable operation. In the meanwhile, two alternative operational functionalities includingno pulsations at six times the grid frequency in the total active and reactive powersimultaneously, or no harmonic current injected to grid can be nicely achieved, whicheffectively improve the operational performance of the whole DFIG system underdistorted grid voltage conditions and the stability of the connected power grid. While forthe proposed control strategy, i.e. no pulsations at six times the grid frequency in thetotal active and reactive power simultaneously, the reactive power limitation of PGSCsupported to the grid under such grid voltage conditions can be precisely obtained bythe proposed derivation of the reactive power limitation, while the PGSC’s current iswithin its rating.3) Based on deeply analyzing the operational behaviors of DFIG system withSGSC during network unbalance and fifth-and seventh-order harmonics distortion,three selective enhanced control strategies for the DFIG system with SGSC underaforementioned grid conditions are proposed, viz., no oscillations at twice and six timesgrid frequency in the total active or reactive power, or no negative-sequence andharmonic components in the total current injected to the grid. From the perspective ofthe impact on PGSC’s current rating to the proposed control strategies, the allocationprinciples for PGSC’s calculated current references under its maximum and theallocated current references are studied. Simulation results indicate that the proposedcontrol strategies can realize generator symmetrical and stable operation. Meanwhile,the three selective operational functionalities including no oscillations at twice and sixtimes grid frequency in the total active or reactive power, or no negative-sequence andharmonic components in the total current injected to the grid can be effectivelyaccomplished. However, with the consideration of the PGSC’s current rating, the controleffect of the proposed control strategies will be appropriately reduced during PGSC’scalculated current references are with the proposed allocation principles, viz., no thepulsations at twice and six times grid frequency in the total active or reactive power. In addition, the positive-sequence current closed-loop control of PGSC has been taken asan example to analyze the closed-loop characteristic of the control system in thisdissertation. Analysis results indicate that the whole system can work stably.