Translated Abstract
In resent years, offshore wind power technology has become the hotspot of the world‘s major companies and scholars from various countries. Voltage rating of generator is usually 690V or 960V. In order to improve the voltage rating ,a high-power step-up transformer must be installed inside the cabin ,however by this not only increases the size, weight and cost, but also step-up transformer failure leads to a reduction of wind power system reliability, especially double-fed asynchronous induction generator (DFIG) needs to be equipped with a large gearbox, making this kind of problem is more prominent. According to these issues,we proposed a high-power direct-drive wind power scheme without a step-up transformer,which transmits power with high voltage direct current (HVDC) transmission line. This scheme uses a low-speed direct-drive polyphase permanent magnet synchronous generators and series of multi-level rectifier to raise voltage rating above 20kV. And then the power is directly transmitted to the converter stations on land with HVDC,which is connected to power grid through the inverter and the boost transformer. The program eliminated the bulky gearbox and transformer，lessen the volume and weight,cut down transportation costs and maintenance costs,and improved reliability. The design of grid-connected inverter system on shore converter stations is one of the key technologies of the program. Considered in terms of power rating, efficiency, reliability and cost,we select three-level neutral-point clamped convertors as a basic inverter topology unit to share the high voltage and power. however, voltage imbalance in the midpoint of serial capacitors is the inherent problems of 3L-NPC inverter. The imbalance problem is more obvious when the situation is of the low power factor,the high modulation and the low switching frequency. In this paper, a flying capacitor auxiliary leg is used to control the voltage balance of serial capacitor midpoint in 3L-NPC converter.First of all,it describes grid-side inverter system structure comprised of basic modular units in series with each other,which is made up of the 3L-NPC converter with a flying capacitor auxiliary leg. Then use the basic module as the study object, the operation principle of 3L-NPC converter and the techniques of space vector pulse width modulation(SVPWM) is analysed,as well as the root cause of the imbalance. The existing methods of suppression of low-frequency capacitor voltage fluctuation and DC voltage shifts are introduced and compared. Secondly, the four kind of proposed balance control strategy based on flying capacitor auxiliary leg was analyzed in detail, and compared their advantages and disadvantages. Then, a switch model of flying capacitor auxiliary leg 3L-NPC converter is established in the power system simulation software PSCAD and the balance strategies were simulated. Finally, a set of experimental system was builded and some experiements were studyed. Simulation analysis and experimental results show that the four control strategies based on flying capacitor auxiliary leg to control the voltage balance of the serial capacitor midpoint in a condition of low switching frequency, power factor 0 and modulation index 1 are feasible, particularly,the dual closed loop control of serial capacitor midpoint voltage and midpoint current shows a very good performance. In addition,a single closed-loop controller of capacitor middle voltage based on a proportion resonant(PR) controller and a single closed-loop controller of capacitor middle voltage based on a proportion integration(PI) and middle current forward controller can also achieve similar control performance.
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