Direct Power Contorl Strategy Of Grid Connected Voltage Source Inverter In Distributed Generation System Under Unbalanced And Harmonic Grid Conditions

Along with the extensive research on renewable energy generation technologies, the control strategies of voltage sourece inverter (VSI) during grid-connected operation mode have been investigated deeply. The voltage distortion in public of common coupling has great impact on the operation of VSI, which are directly connected with DPGS. Since the conventional control strategies can no longer adapt to the grid voltage deterioration, this paper is intended to propose improved control strategies which are able to implement the flexible control targets.In this paper, the improved SVM-DPC is investigated. The main contributes of this paper are include three aspects.1) The mathematical model of VSI under unblanaced and harmonic grid conditions in synchronous dq frame is built. The VPI controller is applied to track the sinusoidal error signals in order to implement two alternative control targets①to obtain smooth instanenous active and reactive power or②to obtain balanced and sinusoidal current output. The design principle of VPI parameter is investigated and the control capability of the proposed DPC strategy on the alternative control target is analyzed. The experimental results are given out to verify the effectiveness of the proposed control strategy.2) In order to enhance the operation performance of VSI without the influence of PLL, the mathematic model of VSI in the stationary αβ frame is deducted and based on which, the improved SVM-DPC for VSI under the unbalanced and harmonic grid voltage is proposed to implement the two alternative control targets,①to obtain smooth instanenous active and reactive power or②to obtain balanced and sinusoidal current output. The reduced order generalized integrator (ROGI) is applied to quickly decompose the positive sequence component of the grid voltage which is used to calculate the power reference compensations. The transfer function of the control frame is given out for the analyses of control performance. Finally, the experiment system of VSI is built to validate the availability and effectiveness of the proposed DPC strategy.3) Since these two improved control methods get limited control targets, this paper is then to investigate the compromise control between the instaneous power and current output. Based on the mathematical expressions of power fluctuations and current distortion, the power reference compensation expression is deducted to implement the flexible control targets. Finally the experimental result and simulation result are given out to verify the correctness of the proposed control method.