*Article* **Comparative Analysis of Bearing Current in Wind Turbine Generators**

#### **Ruifang Liu 1,\*, Xin Ma 1, Xuejiao Ren 1, Junci Cao 1 and Shuangxia Niu 2**


Received: 3 April 2018; Accepted: 16 May 2018; Published: 20 May 2018

**Abstract:** Bearing current problems frequently appear in wind turbine systems, which cause wind turbines the break down and result in very large losses. This paper investigates and compares bearing current problems in three kinds of wind turbine generators, namely doubly-fed induction generator (DFIG), direct-drive permanent magne<sup>t</sup> synchronous generator (PMSG), and semi-direct-drive PMSG turbines. Common mode voltage (CMV) of converters is introduced firstly. Then stray capacitances of three kinds of generators are calculated and compared through the finite element method. The bearing current equivalent circuits are proposed and simulations of the bearing current are carried out. It is verified that the bearing currents of DFIGs are more serious than the two kinds of PMSG, while common mode current (CMC) of the direct-drive PMSG is much greater than the other two types of wind turbine generators.

**Keywords:** bearing current; common mode current; doubly fed induction generators; permanent magne<sup>t</sup> synchronous generators; wind turbine generator
