Study On Control Strategies Of Cascade DSTATCOM With Star Connection

Distribution Static Synchronous Compensator(DSTATCOM) is a the coreequipment for Custom Power system, which is a very good solution to many powerquality problems, such as low power factor,harmonic pollution, voltage fluctuationand flicker, three-phase voltage unbalance.Thus, this thesis choose cascadedDSTATCOM with star connection as the main object, the key technology such asPWM modulation strategy, phase-locked technology,blance strategy for DC-voltageblance, control strategies for reactive power compensation when the load is balance orimbalance are studied.Carrier phase modulation strategies for cascade H-bridge inverter were studied inthe beginning, pointed out the the link between CPS-SPWM and unit H-bridgeSPWM and Multiple technologies. Comparative analysis of the bipolar CPS-SPWMand the unipolar multiplier CPS-SPWM, the results of double Fourier analysis wasdeduced and the harmonic characteristics was analysed, pointing out the thesuperiority of unipolar multiplier CPS-SPWM.On the basis of traditional SRF-PLL, an improvement strategies based onDSC/CDSC algorithm was studied in the thesis, the effects of negative and harmoniccomponents of grid voltage was effectively inhibited. And the discretization methodsfor DSC/CDSC-PLL have also been studied, its discrete error was high qualityanalysed and an optimization strategy based on Lagrange interpolation algorithm wasstudied, the accuracy and speed of phase detection was effectively improved.Control strategies of cascaded DSTATCOM with star connection underequilibrium conditions were studied, and its mathematical model was established.Then, this thesis focused study on the problems of DC bus capacitor voltageimbalance for cascade DSTATCOM, analyzed the reasons of the imbalance.Hierarchical control strategy based on integrate split phase control with active voltagevector control was studied, effectively solved the problem of DC bus capacitorvoltage imbalance, and got a good effect in reactive power compensation.Besides, when the load is unbalanced, control strategies of cascadedDSTATCOM with star connection was also studied in the thesis.First, the reasons ofpower imbalance for its three-leg was analysed, then the principles of three powerbalance methods were analyzed and compared, pointing the the advantages ofzero-sequence voltage injection method.In the end, a simplify method of solving zero sequence voltage was studied, combinated with the split phase control based on PRcontroller and the detection method of the reactive and the negative load currentthrough notch filter, the negative sequence current and reactive current generated bythe unbalanced load was effectively compensated.