Different from traditional concentrated power-collection and boosting-voltage (CPCB) topology adopted by HVDC station, this paper adopts a distributed power-collection and cascaded boostingvoltage (DPCB) topology for offshore DC station to collect DC power and boost DC voltage, which consists of multiple cascaded single active bridges (SABs). It has merits of strong DC-voltage boosting ability, low design difficulty and cost, and low system error rate. Three aspects are studied as follows. First, DPCB topology with its circuit equations is deduced step by step, and its operational mechanism is analysed under phase-shifting control method further. Second, PI control strategy is designed based on system small signal model, and system performance is analyzed by Bode diagram. Third, fault tolerance strategy with predicted power ratio is designed to keep DPCB station from breakdown when some SABs and generators are out of order, and enormous power difference occurs due to the factors of wind turbine location and windward. Finally, the feasibility and effectiveness of designed DPCB station with its strategy are verified by experiment results.
Doubly fed induction generator (DFIG) system suffers from complex control structure, slow dynamic response speed and cumbersome parameter design due to its traditional cascaded dual-loop control strategy. A single-loop finite control set model predictive control (SLMPC) is proposed in the paper for DC-based DFIG in DC grid, which simplifies its control structure and parameter design, and enhances system dynamic response. There are three improved aspects. First, the single-loop control structure is proposed to eliminate intermediate link in dual-loop cascaded structure, and enhance system dynamic response. Second, system reduced-order discretization algorithm is proposed by differential and integral discretization method to reduce finite control set model predictive control strategy (FCS-MPC) design difficulty. Third, cost function with nonlinear additional current limiting function is designed to protect system from overcurrent effectively. Finally, the feasibility of proposed strategy is verified by simulations and experiments.
Traditional double-loop PI control strategy (DLPI) is adopted in PMSG system, which has slow response speed and complex parameter design. In this paper, an improved single-loop model predictive control strategy (SLMPC) is proposed for PMSG system, which improves system response speed, and reduces parameter design difficulty. Two improvements are as follows. First, a single-loop control structure is established based on the deduced second-order speed model, which simplifies cascaded double-loop control structure and improves system transient performance. Second, an integral and differential discretization method is adopted to simplify design difficulty for FCS-MPC without measuring dTm/dt and tracking d 2 ωr/dt 2 . Finally, simulation and experiment are carried out to verify the proposed strategy.
Conventional finite control set model predictive control (FCS-MPC) outputs one voltage vector per period by vector traversal algorithm, which leads to low control accuracy, unnecessary high computational burden and unfixed switching frequency. In this paper, an improved FCS-MPC based on virtual vector expansion and sector optimization is proposed to improve system performance and reduce computational burden for 2L-VSCs. Three aspects are improved as follows. First, a sector optimization method based on effective vector radiation range is proposed to directly determine the optimal vector without vector traversal. Second, a virtual vector expansion method based on vector synthesis algorithm with fixed 1/4 & 1/2 & 3/4 duty cycle is proposed to expand optional vectors from 8 to 44, and improve vector accuracy and system performance. Third, a modulated vector output mode is adopted per period to fix switching frequency and decrease current harmonics. Finally, the advantages of proposed MPC are verified by simulation and experiment.INDEX TERMS 2L-VSCs, FCS-MPC, sector optimization, virtual vector expansion, modulated vector output.
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