| Renewable power generation systems are developing rapidly in order to solve theglobal energy shortage and environmental pollution in recent years. As the core memberof the whole power generation system, grid converter is used to exchangeenergy between renewable energy and power grid. In view of more and more seriouspollution of grid caused by power electronic devices, it’s necessary to study on controlstrategy of grid converter deeply under the nonideal state of power grid. The key issuesof grid converter studied in this paper are: synchronization with grid positive sequencevoltage, suppressing harmonics in grid currents, restraining fluctuations in active powerand dc voltage.The vector angle of grid positive sequence voltage is the most essential andimportant information for renewable power generation systems to connect to gridproperly. This paper analyzes the basic principle of phase-locked loop and the way toimprove phase-locked loop, then proposed two PLLs based on improved complex filterin two-phase stationary coordinate and sliding Goertzel Transform respectively. Thesimulation results verify that the improved complex filter can separate the positivesequence from three-phase grid voltage quickly and accurately while attenuatingharmonics of grid voltage to some extent; The PLL based on sliding Goertzel Transformcan quickly and accurately lock phase of distorted grid voltage, and it still works with asmall frequency offset of grid voltage.The currents of grid converter flow into (out of) the grid should meet standardrequirements and the harmonics of grid currents result from periodic disturbance. Thispaper firstly analyzes the source of currents harmonics in the three-phase PWM gridconverter and introduce the repetitive control applied to suppress the periodicdisturbance; then designs the repetitive controller according to the parameters of actualhardware; finally verifies that the repetitive controller in the2-phase stationaryreference can effectively suppress the periodic disturbance caused by harmonics of gridvoltage.The stability of dc voltage can ensure the safety of dc capacity and make dc loadwork reliably when three-phase grid converter works as a rectifier. For the three-phasevoltage source converter, this paper firstly calculates the instantaneous complex powerof ac side in the αβ coordinate and analyze the cause of dc voltage fluctuation accordingto the calculation of complex power; then derives the basic equations of power controlin the system to confirm the different expressions to satisfy different require for currentsand power in the system and applies the expressions to the actual system; Simulation and experimental results show that the derived expressions of power can effectivelysuppress the fluctuation of active power and dc voltage.A15kW experimental platform of three-phase PWM grid converter is set up tovalidates the practical feasibility of theoretical analysis. A series of experiments areimplemented, containing PLL, suppressing harmonics of grid currents and restrainingfluctuations in active power and dc voltage. The experimental results and analysis areshowed to verify the correctness and practical feasibility of theoretical analysis. |
