The high-frequency magnetic link is gaining popularity due to its light weight, small volume, and inherent voltage balancing capability. Those features can simplify the utilization of multilevel converter (MLC) for the integration of renewable energy sources to the grid with compact size and exert economic feasibility. The modulation and control of MLC are crucial issues especially for grid connected applications. To support the grid, the converter may need to operate in over-modulation (OVM) region for short periods depending upon the loading conditions. This OVM operation of the converter causes increased harmonic losses and adverse effects on overall system efficiency. On top of that, the size and cost of filtering circuitry become critical to eliminate the unwanted harmonics. In this regard, a modified OVM scheme with phase disposed carriers for grid connected high frequency magnetic link-based cascaded H-bridge (CHB) MLC is proposed for the suppression of harmonics and the reduction of converter loss. Furthermore, with the proposed OVM technique, the voltage gain with modulation index can be increased up to the range which is unlikely to be achieved using the classical ones. Extensive simulations are carried out with a 2.24 MVA permanent magnet synchronous generator-based wind energy conversion system which is connected to the 11 kV ac grid through a high-frequency magnetic link and a 5-level CHB MLC. A scaled down laboratory prototype is implemented to validate the performance of the converter.
- Multilevel converter
- Over modulation
- Grid connection
- High-frequency magnetic link
- Wind energy
To improve the system performance, a modified OVM technique is presented in this paper with grid connected and islanded operation. With the proposed modified carrier signal based BCPWM techniques, the overall loss and THD are decreased for both the islanded and grid connected modes compared with the traditional OVM techniques. Moreover, the voltage gain can be increased and remains approximately constant in the proposed method, which may not be possible to obtain using the traditional OVM methods. In this paper, a high-frequency magnetic link-based fully-rated CHB converter is developed for wind energy applications and the behavior of the system under rated and overrated load conditions are investigated.The use of magnetic link for the generation of isolated and balanced dc sources of the MLC inherently overcomes the voltage imbalance problem of CHB MLC and hence effectively simplifies the system control complexities. The core loss of high-frequency magnetic link is also measured to identify the overall loss of the system. The effectiveness of the proposed technology is confirmed by the simulation and experimental results.
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