Operation Control Of Multiple Power Converters For Direct Driven PMSG-based Wind Energy Conversion System

With the increasing power level of direct-driven permanent magnet windgeneration, the converter that is composed of custom power device by traditionaltopology is difficult to satisfy the demand of high power wind energy conversionsystem (WECS) based on permanent magnetic synchronous generator (PMSG), whichis become one of the important reasons to restrict further development of wind powertechnology. Multiple conversion technology is that the novel converter isreconstructed of original topology by parallel, series or cascaded manner to reduce thetransmission power of single circuit unit and improve the whole power level ofsystem. From the two aspects of generator-side and grid-side, the topology andcontrol strategy of multiple converter are analyzed and studied in the paper.In WECS based on PMSG, the AC/DC converter composed of diode rectifierand boost chopper is widely adopted for its low cost, high reliability and convenientfor control. The operation characteristic of the structure as generator-side converter isanalyzed. From the analysis, the serious stator current distortion is caused by thediode rectifier and the narrow speed range is originated from the boost chopperstructure. Therefore, a switchable-cascaded-mode boost chopper (SCMBC) isproposed by adopting the multi-pulse rectifier technology that can provide severalisolated dc-sources. Compared with the traditional multiple boost chopper built up ina fixed cascaded manner, not only the transmission power of single circuit unit isreduced by the novel converter, but also the speed range is extended by changing theoperation of the proposed converter. The mathematic model and the operationprinciple of the new converter are analyzed. The sufficient condition of speed rangecontinuity is given. And the speed control strategy for the converter is proposed and itis applied to a prototype of permanent magnetic wind generation system for theexperimental verification. The experimental results demonstrate that speed range isextended by using the new converter as generator-side device in PMSG-based WECS.A kind of multiple converter composed of traditional two-level PWM voltagesource converter (VSC) by line-voltage cascaded (LVC) manner as the grid-sideconverter of permanent magnetic wind generation system has been introduced in the paper. Considering that the isolated dc-source actually provided by wind generationsystem is limited, the triple line-voltage cascaded VSC is the major object of researchin the paper. According to the structural features of triple LVC-VSC, a modulation,which combines the SVPWM of three-phase six-switch VSC with carrierphase-shifting theory, is applied to triple LVC-VSC in the paper. Because the multipleconverter is always composed of several units, the control strategy is independentlydesigned for each unit, which will make the control algorithm more complicated,increases the hard-ware cost of the system and unable to achieve the close-loopcontrol. Thus, the multiple converter is equivalent to an switching circuit model byanalyzing the operation manner according to the external circuit characteristic of theconverter. The proposed control strategy not only can ensure the stability of the wholetransmission power of the multiple converter, but also can solve the problem of thedifferent dc-side capacitor voltage and the asymmetrical ac-side line-current causedby the unbalanced transmission power of each VSC unit through introducing thepertinent compensation control algorithm.