| Power quality problems such as reactive power and harmonic pollution in distribution system have been attracting more and more attention because of their harmfulness.Dynamic capacitor(D-CAP)is an economical and effective shunt power quality apparatus.Research on D-CAP mainly focus on reactive power compensation,harmonic suppression,reactive current reshaping,series and parallel resonance damping,imbalance suppression and other aspects of power quality control.However,when D-CAP is under control of harmonics suppression,coupling exists between the many control loops with PI controllers,which makes it difficult to design appropriate parameters to ensure the global control performance.In order to decouple the control loops of D-CAP in harmonic suppression,model predictive control of three-phase Buck-type D-CAP is studied in this paper.Firstly,the couplings between the harmonic modulation terms and the compensation current of DCAP is analyzed according to the simplified time-domain transient differential equations.Then,current prediction models for single and three-phase D-CAP are established based on the steady-state mathematical model in frequency-domain.Based on the principle of even harmonic modulation(EHM),two control sets of three-phase D-CAP are designed firstly,and then the optimization algorithms are designed for single and three-phase D-CAP respectively to realize the complete decoupling of modulation terms.Then,the opened-loop current control strategy of model predictive control(MPC)is designed.Recursive discrete fourier transform(RDFT)algorithm is taken to extract the specified harmonics of load current in the complex frequency domain.The abc/dq transform and inverse transform are taken to extract the point of common coupling(PCC)voltage and generate the modulation waveform of the two control sets respectively.The slope input of modulation waveform is taken to reduce the impulse current during switching states.Finally,the closed-loop current control strategy of MPC is designed,which is based on the negative feedback of compensation current.In addition,control cycle and time sequence are designed.One control cycle is set by two fundamental cycles.The optimization of duty function once in each control cycle is carried out offline by the main function,while the extraction of PCC voltage,load current and compensation current with generating the modulation waveform is implemented online by the interrupt program,so as to realize closed-loop MPC of DCAP for reactive power compensation and harmonic suppression.The above contents are cooperated with simulation by Matlab/Simulink platform and experimental verification by a 33 k Var/220 V three-phase Buck-type D-CAP prototype,which verifies the correctness of the established model and the effectiveness of the designed control strategy.The designed MPC system can effectively reduce the designing of controller parameters and improve the global performance of D-CAP in harmonic suppression. |
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