Based On Double Coordinates Transform Phase Locking Of Direct Drive Wind Power Grid-connected Inverter Research

As a kind of new green energy, wind power has developed rapidly in recent years. Thedirect-drive wind power network convert the electric energy output by blower into the electricenergy which can satisfy the requirements of the network by two back to back PWMconverters. The part which connected to the electric network is called the grid-side converterwhich plays an important role in the wind power grid-connected system and determines thequality of the electric energy output by the wind turbine and the effect on the electric network.This paper studies the control strategy of wind power grid side converter, and analyzes thecontrol system when Grid voltage is balanced or unbalanced.The main goal of grid side converter control is making the voltage of the DC side stable,making three phase current is sine wave and three-phase symmetrical, and making thethree-phase current be synchronized to track the Grid voltage and can independently controlthe output power factor. This paper firstly established the mathematical model ofgrid-connected inverter., and adapted the voltage and current double closed-loop feed-forwarddecoupling control strategy, combined with space vector pulse width adjustment method(SVPWM) to carry out effective control on the AC output current and the DC side voltage, sothat it can independently control the active component and reactive component of electriccurrent and can independently control active power and reactive power.On the basis of phase-locked loop, this paper introduces two kinds of methods:SSRF-SPLL and DDSRF-SPLL. It describes their corresponding scope of the application andestablishes the mathematical model of DDSRF-SPLL. The DDSRF-SPLL can detect thepositive sequence component and negative sequence components of unbalanced Grid voltageand eliminate the influence of negative sequence components by the decoupling method,which can accurately detect the phase angle of Grid voltage to realize parallel.This paper used different control algorithms for different control objectives underunbalanced Grid voltage. Through the establishment of the mathematic model of Grid sideinverter under the condition of unbalanced Grid voltage. Secondly, I analyzed the effect onGrid side inverter made by unbalanced Grid voltage. Last, I listed control objectives: suppressthe AC side of negative sequence current and the DC side voltage fluctuation. The paper usedcontrol algorithm to suppress negative sequence current and double current closed-loopcontrol of unbalanced algorithm. Due to the double current closed-loop control algorithmcontains4PI controllers, causes the control structure is relatively complex, on-linecomputation is relatively large, this is a potential risk for a real-time system, the author putsforward a control strategy for suppression of DC voltage fluctuation of two-phase stationarycoordinate system, which is based on the exponential reaching law of variable structurecurrent control algorithm, this algorithm avoids the use of a large number of PI regulator, ACside current can be very good to follow the instruction current, complete control objectives, and variable structure control than the PI control robustness is better, more easy to implementin engineering. And according to the principle of DDSRF-SPLL phase-locked loop, a newmethod extracting voltage positive and negative sequence components is proposed, namelydual synchronous coordinate decoupling method.Based on the above theoretical research, this paper finally used the matlab/simulinksimulation software and did detailed simulation of above control system. The simulationresults show that the voltage and current double closed-loop feed-forward decoupling controlstrategy can be well applied in Grid-connected inverter when the network is balanced. AndDDSRF-SPLL can eliminate the influence of unbalanced grid and detect the phase angle ofelectric network. The exponential reaching law control variable structure current twofluctuation algorithm can restrain the DC voltage on the DC voltage, no overshoot, reduceharmonic pollution, realize the effective integration.