Positive- and zero-sequence impedance estimation of YNyn0+d-connected main power transformers in wind power applications

With the increased penetration of non-conventional renewable energy resources, such as wind power plants (WPPs), into the power system, a lot of interest has been placed on studying their impact for system protection, planning and operations to ensure continued reliability of the electric grid. The impact of WPPs on the grid is typically identified during generation interconnection studies. One of the challenges faced by protection and planning engineers, at an early stage in this process, is that many of the studies involving these power plants must be performed without specific data available because the equipment is not built yet. Among all the needed data, a particular interest in this study is placed on the positive- and zero-sequence impedances of the main power transformer (MPT) that interconnects a wind power plant to the transmission system due to its contribution to short-circuit current during ground faults. The most commonly found in practice MPT winding configurations is a high-voltage winding in wye with neutral bushing; a low-voltage winding in wye with neutral bushing without phase shift between these windings, and a stabilizing tertiary winding connected in delta that is not designed for external loading designated as YNyn0+d. Thus, this paper proposes a methodology to estimate the sequence impedances of YNyn0+d transformers from a statistical point of view applying clustering and regression analyses on existing MPT test report data.