Multimegawatt wind-turbine systems, often organized in a wind park, are the backbone of the power generation based on renewable-energy systems. This paper reviews the most-adopted wind-turbine systems, the adopted generators, the topologies of the converters, the generator control and grid connection issues, as well as their arrangement in wind parks.

Overview of Multi-MW Wind Turbines and Wind Parks

The steady growth of installed wind power together with the upscaling of the single wind turbine power capability has pushed the research and development of power converters toward full-scale power conversion, lowered cost pr kW, increased power density, and also the need for higher reliability. In this paper, power converter technologies are reviewed with focus on existing ones and on those that have potential for higher power but which have not been yet adopted due to the important risk associated with the high-power industry. The power converters are classified into single- and multicell topologies, in the latter case with attention to series connection and parallel connection either electrical or magnetic ones (multiphase/windings machines/transformers). It is concluded that as the power level increases in wind turbines, medium-voltage power converters will be a dominant power converter configuration, but continuously cost and reliability are important issues to be addressed.

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Power Electronics Converters for Wind Turbine Systems

Wind power is still the most promising renewable energy in the year of 2013. The wind turbine system (WTS) started with a few tens of kilowatt power in the 1980s. Now, multimegawatt wind turbines are widely installed even up to 6-8 MW. There is a widespread use of wind turbines in the distribution networks and more and more wind power stations, acting as power plants, are connected directly to the transmission networks. As the grid penetration and power level of the wind turbines increase steadily, the wind power starts to have significant impacts to the power grid system. Therefore, more advanced generators, power electronic systems, and control solutions have to be introduced to improve the characteristics of the wind power plant and make it more suitable to be integrated into the power grid. Meanwhile, there are also some emerging technology challenges, which need to be further clarified and investigated. This paper gives an overview and discusses some development trends in the technologies used for wind power systems. First, the developments of technology and market are generally discussed. Next, several state-of-the-art wind turbine concepts, as well as the corresponding power electronic converters and control structures, are reviewed, respectively. Furthermore, grid requirements and the technology challenges for the future WTS are also addressed.

Future on Power Electronics for Wind Turbine Systems

This paper presents a comprehensive study on the state-of-the-art and emerging wind energy technologies from the electrical engineering perspective. In an attempt to decrease cost of energy, increase the wind energy conversion efficiency, reliability, power density, and comply with the stringent grid codes, the electric generators and power electronic converters have emerged in a rigorous manner. From the market based survey, the most successful generator-converter configurations are addressed along with few promising topologies available in the literature. The back-to-back connected converters, passive generator-side converters, converters for multiphase generators, and converters without intermediate dc-link are investigated for high-power wind energy conversion systems (WECS), and presented in low and medium voltage category. The onshore and offshore wind farm configurations are analyzed with respect to the series/parallel connection of wind turbine ac/dc output terminals, and high voltage ac/dc transmission. The fault-ride through compliance methods used in the induction and synchronous generator based WECS are also discussed. The past, present and future trends in megawatt WECS are reviewed in terms of mechanical and electrical technologies, integration to power systems, and control theory. The important survey results, and technical merits and demerits of various WECS electrical systems are summarized by tables. The list of current and future wind turbines are also provided along with technical details.

High-Power Wind Energy Conversion Systems: State-of-the-Art and Emerging Technologies

Doubly fed induction generators (DFIGs), often organized in wind parks, are the most important generators used for variable-speed wind energy generation. This paper reviews the control systems for the operation of DFIGs and brushless DFIGs in wind energy applications. Control systems for stand-alone operation, connection to balanced or unbalanced grids, sensorless control, and frequency support from DFIGs and low-voltage ride-through issues are discussed.

Overview of Control Systems for the Operation of DFIGs in Wind Energy Applications

One of the main points in the design of a Multi-MW-class turbine is the size and weight of the total turbine. Gamesa has developed a converter system with a very high power density and low weight. The system is designed and tested to withstand very high short circuit currents, high vibrations and other wind turbine typical environmental aspects.

A high power density converter system for the Gamesa G10x 4,5 MW wind turbine

The present invention relates to a method for operating a converter system of a wind turbine, wherein the converter system includes converter modules capable of converting electric power produced by a generator to electric power applicable to a utility grid. The converter modules include generator inverters and grid inverters. The method determines the enabling/disabling of the converter modules in response to a parameter related to the variable amount of electric power being produced by the generator. Advantages of the present invention are optimisation of power efficiency of the converter modules and improved reliability of the converter modules. Another advantage is the capability of fast enabling and disabling of the converter modules.

Method for operation of a converter system

The converter system for the Gamesa G10x 4.5 MW wind turbine consisting of 6 parallel-connected converters and some of the converter system control algorithms are presented. The benefits of this solution compared with a solution with only one converter are higher reliability, higher efficiency and possibility of extremely low grid harmonics.

Parallel-connected converters for optimizing efficiency, reliability and grid harmonics in a wind turbine

Method for commutation of a brushless motor (3) being supplied with electrical energy from a DC intermediary circuit via a multi-phase inverter, by which a first current and a second current value are determined and the length of a commutation interval is set in dependence of these current values. With motors being exposed to heavily varying load torques during one rotation, e.g. motors driving a compressor, a stable operation must be reached. For this purpose a correction value is added to the pre-set commutation interval determined by the speed of the motor (3), said correction value being the result of the difference of the actual value of the intermediary circuit current and a filtered and delayed value of the intermediary circuit current, multiplied by a weighting factor.

Method for commutating a brushless motor and power supply for a brushless motor

Until recently, there were no specific requirements in the grid codes for connecting the wind turbines/farms to the electrical network. Nowadays the power system operators e.g. in Denmark, Germany and Spain, have started to issue new grid codes addressed to wind turbines/farms. The grid connection requirements are addressed both at distribution and transmission level and can be applied to on-shore units as well as to off-shore ones. The focus in these grid codes is on the power quality, grid stability, control capabilities and on the performances of the wind turbines/farms under fault conditions as well. These requirements can vary considerably from one specific grid code to another and one of the major goals for the wind turbine industry is to meet them without major changes in the control algorithms as well as in the hardware. The grid side converter is a key element in achieving the grid code requirements regarding the compatibility levels for voltages and currents in the Point of Common Coupling (PCC). An analysis of these compatibility levels for voltages and currents in the PCC for different control strategies of the grid side converter in large wind turbine applications is presented in this paper. Different grid conditions as well as different connection method to PCC are considered. The analysis is based on the existing recommendation within the grid codes.

Jens Birk

Papers

A high power density converter system for the Gamesa G10x 4,5 MW wind turbine

Method for operation of a converter system

Parallel-connected converters for optimizing efficiency, reliability and grid harmonics in a wind turbine

Method for commutating a brushless motor and power supply for a brushless motor

Grid Code Compliance of Grid-Side Converter in Wind Turbine Systems