Research On Doubly-fed Wind Generation System Based On Nine-switch Converter

With the development of wind power generation technology，the penetration of windpower generations in power systems is becoming larger and larger．To ensure the safety andstability of power systems，modern grid codes require the grid-connected wind powergenerators having the abilities of low voltage ride-through(LVRT)．Because doubly fedinduction generator(DFIG) has occupied a bigger market share， it is an urgent andchallenging research focus that how to improve the LVRT capability of DFIG．In aspect of LVRT research on DFIG，the excitation control strategies of DFIG，whichdo not require more hardware cost and changing the system topology，are better solutionsfor doubly-fed wind power system to accomplish LVRT．But these solutions demand theconverter to have bigger power rating，which is unnecessary under normal conditions ofpower system．The back-to-back dual-PWM voltage source converters (VSC) have beenwidely used in the doubly-fed wind generation system．This paper proposes to replace theconventional Back-to-Back VSC by the novel Nine-switch Converter (NSC) for LVRT，andin normal condition，the power rating of NSC can be made full use.Nine-switch converter is a new AC/AC circuit which saves three power electronicswitches through switch shared-use in back-to-back VSC．The proposed configuration usesless switches but still grantees the same control performance as the original system with theback-to-back VSC．Because of its special topology，the DC voltage of NSC is higher，andthe switch rating bigger，compared with back-to-back VSC．This is a disadvantage forNSC，but it can become a superiority when it is used in doubly-fed wind power generationsystem．Because the higher DC voltage and bigger switch rating make it more easy torealize LVRT for DFIG，and in normal operation the rating can be full-used by modulatingits nine switches．So NSC has more advantages than back-to-back VSC in doubly-fed windpower generation system． By analyzing the unique SPWM，this paper analyzes the application feature andequivalent form of the NSC in the doubly-fed wind generation system，dividing the systeminto grid-side equivalent part and rotor-side equivalent part． In grid-voltage-orientedsynchronous rotating coordinate system，this paper gives modeling analysis of grid-sideequivalent converter and rotor-side equivalent converter，and also gives their controlstrategy respectively，concerning the special SPWM modulation of NSC，obtains thecontrolling-modulating block diagrams of every part under normal grid voltage．The resultsof simulation and experiments claim the controlling scheme correct and appropriate．On the occasion of grid-voltage sags，this paper analyzes the NSC advantage ofDC-voltage and power electronic switches voltage-rating and current-rating based on atypical megawatt-level DFIG．Through analysis，gives a simple excitation control strategyto prevent rotor-side over-current．Based on this strategy，simulation results of two systemsdemonstrate the advantages of NSC in doubly-fed wind power generation system．...