Research On Parallel Operation Of Converters For Permanent-magnet Direct-drive Wind Power Generation

With the rapid development of permanent-magnet direct-drive (PMDD) windpower generation technology, the capacity of wind turbine is close to10MW. Bycapacity constraints of modern power electronic devices, it is difficult to deliver thelarge energy to the grid through single converter, because the full power converter isneeded in the grid-connected operation of PMDD wind power generation system. Tosolve this problem, the parallel technology is becoming a research focus. The paralleloperation of PMDD wind power converters is studied in the paper and the specificcontents are as follows:The control strategy of circulating-currents for parallel PMDD wind powerconverters with common DC buses is proposed to solve the problem ofcirculating-currents. Its average-value models are derived in three-phase stationarycoordinate system and in synchronous rotating coordinate system respectively. Thecirculating-currents of this topology are analyzed and the fundamental reason ofcirculating-current generation is obtained. Besides, the generator-side and grid-sidecirculating-currents are independent with each other. Therefore, the suppression ofcirculating-currents on generator-side and grid-side can be achieved independently. Thesymmetrical SVPWM strategy with variable zero vectors is designed and thegenerator-side and grid-side circulating-currents are suppressed effectively without thehardware cost increase. The problems of current discrepancy, waveform distortion, andthree-phase unbalance are overcome well and the parallel operation of PMDD windpower converters with common DC buses is achieved.To solve the problem of loop delay in the digital control system, the current loopexpanding strategy of parallel converters with common DC buses is proposed. Thecurrent decoupling strategies of the generator-side and grid-side converters for theparallel topology with common DC buses are designed and the unified transferfunctions of current loops are derived. Its operating principle is analyzed and the twosampling and two duty cycle updating in one switching period are realized, whichreduces the sampling and control delay fundamentally. The inverse relation between thebandwidth of current loop and the total inertia time constant is derived by the transferfunction and Bode diagram. With the same switching frequency, the bandwidth ofcurrent loop expanding strategy is2.67times wider as that of typical control sequence.The system performance is improved and the stable operation of the system with highopen loop gain is achieved.To achieve the sensor-less vector control of PMSG, the model of PMSG based onthe parallel topology with common DC buses is derived. The equivalent circuits in synchronous rotating coordinate system are obtained and the sliding mode observer isdesigned. The1.5MW experimental platform has been set up. By integrating thesymmetrical SVPWM strategy with variable zero vectors, the current loop expandingstrategy, and the sensor-less vector control, the grid-connected operation with ratedpower is achieved, which make it possible to produce the wind power converters withhigh power rating.Due to the small circulating-current impedances and complicatedcirculating-current control of parallel topology with common DC buses, the controlstrategy of circulating-currents for parallel PMDD wind power converters with separateDC buses is proposed. The system is composed of multiple back-to-back PWMconverters with same structures, which are paralleled with each other and have separateDC buses. Through the average-value models, the circulating-current routes areanalyzed and it is found that the generator-side and grid-side circulating-currents arecoupled with each other. The fundamental reason of circulating-current generation forparallel topology with separate DC buses is obtained and the circulating-currentimpendences of parallel topology with separate DC buses are larger than one withcommon DC buses, which has good influences on the inhibition of circulating-currents.Moreover, the circulating-current suppressors based on SPWM method are designed tosuppress the circulating-currents. Besides, the control strategies of generator-side andgrid-side converters are designed and the parallel operation of multiple PMDD windpower converters with separate DC buses is realized.The parallel topologies with high circulating-current impedances and the gridcurrent harmonics are studied. Since the circulating-current impedances are small whileordinary three-phase inductors are used in the parallel system, the parallel topologieswith high circulating-current impedances composed of interphase inductors areproposed and their decoupling equivalent circuits are obtained. The reason of automaticcurrent sharing is analyzed and the short circuit current requirement of interphaseinductor is derived. It is validated that the interphase inductor can increase thecirculating-current impedance effectively, which has a positive influence on thesuppression of circulating-currents and improves the system operation performance. Toimprove the grid current quality further, the influences of parallel manners and carrierphase-shifted control on grid current harmonics are analyzed. It is validated to choosethe system parallel manner. Besides, the carrier phase-shifted control doubles theequivalent switching frequency, reduces the harmonic components of grid currents, andimproves the grid current quality.