Practical Approach for representing Wind Turbine DFIG machines for IEC60909 Based Short Circuit Studies.

The Fault level determination through fault level studies is a basic requirement for the connection of any generation or load to the electrical network. Thereby, the standard IEC 60909 is internationally either directly used or referred to. The developed methods presented within the standard have not undergone major revision and are based upon traditional synchronous machine dominated networks. While other types of generators are discussed and an attempt is made to improve their representation through updates of the standard, there is currently no accurate representation of wind turbines with doubly fed induction generators (DFIG). These 'unconventional' machine types have fault characteristics dominated by power electronic control system operation rather than rotating machine principles. While suitable for representing upstream network faults, the existing IEC 60909 fault current representation becomes less accurate the closer the fault occurs to the wind turbine itself. The fault contribution of Wind turbines with DFIG generators is often overestimated, contributing to over specification of HV-equipment and erroneous grading of protection equipment. As such, recognizing the influence of the controlled fault in-feed from wind turbines is important for static fault level studies based on IEC 60909 which are commonly performed for protection designs. Specifically the initial symmetrical short circuit current, symmetrical short circuit breaking current and the short circuit impedance are of interest. This paper discusses the difficulties experienced with existing approaches for representing fault current in-feed when applied to wind turbines equipped with DFIG generators. Further, the paper proposes a new method to achieve meaningful values and results when performing IEC 60909 studies for this generator type. [ABSTRACT FROM AUTHOR]