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Multi-functional grid-connected inverters (MFGCIs) not only interface renewable energy sources into the utility, but also provide ancillary power quality enhancement service. Therefore, extra installment of power quality conditioners can be partially avoided in a micro-grid including MFGCIs. Because the capacity of an MFGCI employed for power quality compensation is limited, how to balance the multiple functions and optimally utilise the limited capacity becomes a challenging for MFGCI application, and this is studied in details in this paper. First, to set up a benchmark for balancing the multiple functions of the MFGCI, a comprehensive power quality evaluation (CPQE) index is presented based on the catastrophe decision theory to quantify the power quality of a micro-grid. Then, for the strategic utilisation of the limited capacity, a multi-objective optimal compensation model is proposed in which the objectives are to optimise the CPQE index and minimise the occupied capacity of an MFGCI for power quality compensation. Finally, the solutions of the model are derived on the basis of Pareto approach. As a result, the MFGCI can flexibly customise the power quality of the micro-grid according to its available capacity margin and the users' requirement. Finally, the experimental results performed on a 10 kVA MFGCI prototype have confirmed the validity of the proposed model.

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This paper investigates the cost-effective ranges of gearbox ratios and power ratings of multibrid permanent-magnet (PM) wind generator systems by using a design optimization method. First, the analytical model of a multibrid wind turbine concept consisting of a single-stage gearbox and a three-phase radial-flux PM synchronous generator with a back-to-back power converter is presented. The design optimization is adopted with a genetic algorithm for minimizing generator system cost. To demonstrate the effectiveness of the developed electromagnetic design model, the optimization results of a 500-kW direct-drive PM generator and a 1.5-MW multibrid PM generator with various gear ratios are, respectively, compared with those from other methods. Then, the optimal design approach is further employed for a range from 750 kW up to 10 MW. The optimization results of PM generator systems including direct-drive and multibrid wind turbine configurations are obtained, and the suitable ranges of gear ratios for different power ratings are investigated. Finally, the detailed comparisons of the most cost-effective multibrid PM generator system and the optimized direct-drive PM generator system are also presented and discussed. The comparative results have shown that the multibrid wind turbine concept appears more cost-effective than the direct-drive concept.

Sub‐synchronous interaction (SSI) phenomenon is one of the dynamic system problems that have an adverse influence on the safety and stability of the system.The use of supplementary damping controllers (SDCs) in the double‐fed induction generator (DFIG) converter controllers is quite promising due to their simplicity,low costs, effectiveness, and easiness of tuning. This paper presents a new effective input control signal, rotor voltage, to the SDC for damping SSI in a series‐compensated DFIG‐based wind farm. The proposed SDC is embedded into both the q‐axis and d‐axis of the rotor‐side converter inner current loops. Particle swarm optimization algorithm is used to identify the optimum parameters of the SDC which maintain the system stability at various operation conditions. Both eigenvalue analysis and time‐domain simulations have been carried out to demonstrate the capability of the proposed SDC for enhancing the system stability and damping SSI. Compared to the conventional SDC, the proposed SDC has the best performance where it can quickly and robustly damp the SSI at different compensation levels, different wind speeds, and sub‐synchronous control interaction.