## 1 Introduction  
The capacity of power converters in recent years has steadily grown in step with the increased size of large wind turbines; correspondingly, the load capacity of components of the converter have improved evidently and the electrical structure is more complex, which is bound to raise the failure rate greatly [1; 2]. Meanwhile, the wind farms are mostly built in areas with abundant wind resources and complicated climate [3; 4]. The operational environmentof the power converter is extremely harsh, and the failure rate is high [5]. The core device of the power converter of wind turbine is the power switching component--insulated gate bipolar transistor (IGBT) module [6]. A lot of harmonics and interharmonics will be produced if the IGBT module in a power converter has a short-circuit or open-circuit, which will impact the power quality and pollute the grid [7; 8; 9; 10]. Then, the wind turbine would be shut down and off grid immediately, in which case seriously impacting the stability of the grid and even threatening personal safety [11]. The existing research has matured for the short-circuit fault of the power converter, and there are corresponding modules for protection [12; 13]. Though there are a few achievements for the IGBT module open-circuit fault of the power converter over the years, there are few single and double IGBT modules open-circuit fault diagnosis methods producing negative results including erroneous judgment, omissive judgment and low accuracy. Moreover, due to the capability of the protection module, the short-circuit fault will turn into an open-circuit fault. Therefore, the open-circuit fault diagnosis of wind turbine power converter is crucially significant; its essence is to diagnose the power switching component, such as the IGBT module [14; 15]. Over the past decade, a number of studies have been made concerning open-circuit fault diagnosis of the IGBT module in the power converter [16; 17; 18; 19; 20; 21], which can be divided into qualitative fault diagnosis and quantitative fault diagnosis. Qualitative fault diagnosis includes fault tree analysis method and expert system method, whose basic idea is to build a knowledge base by using the effective experience and expertise accumulated by experts in the case of open-circuit fault of the power converter, and to determine the diagnosis results and fault causes according to certain logic reasoning for the fault status. Quantitative fault diagnosis includes the current detection method and the voltage detection method, which analyze and compare the current, voltage and other operating electrical parameters of power converter, and set thresholds when necessary, so as to make a decision and classification. The method of qualitative fault diagnosis is intuitionistic and easy to understand with clear thinking and strong logic. However, with the increasing complexity of power converter systems, the knowledge base is not comprehensive enough and the logic reasoning process is extremely complex, which makes the process of diagnosis and search difficult and the accuracy of fault diagnosis low. In order to improve the efficiency and accuracy of fault diagnosis, a combination of qualitative fault diagnosis and quantitative fault diagnosis is usually used. As stated in [22], the zero current periods are registered in each converter phase circuits. The open-circuit faults are identified calculating the average values of differences between predicted and measured phase currents. This method is insensitive to load changes but out of the high power application. [23] proposes an approach, which is based on the absolute normalized Park's current vector. This method can detect multiple open-circuit switch faults. But, this method is also prone to false alarm or failure alarm when the load changes abruptly. In [24], wavelet transform is used to preprocess load current signals, and open-circuit faults are diagnosed by the processed currents based on back propagation neural network (BPNN) and classification and regression trees (CART).