Harmonic assessments for filter design specifications at U.K. National Grid electricity transmission system for large wind farms

This paper presents the methods and issues when assessing harmonic effects of large wind farms in the U.K. The harmonic assessment is not only a licence obligation for a transmission operator but is vital to a wind farm developer for their filter design. It discusses the effects of the system background and wind farm on the harmonic assessment. The modelling aspects of the wind farm is affected by (1) the large array of medium (higher than 33kV) and low voltage (below 33kV) cables, (2) compensation equipments and (3) the active components which generate harmonics that propagate into the transmission and distribution systems. The importance in understanding this process is vital in obtaining the correct performance from any filtering requirements identified by the transmission company, not only in meeting compliance but to minimize the direct and associated cost of installing the filters. Any harmonic assessment will also have to consider the interaction between other connections in the same area. The paper will discuss this interaction and describe how they are currently dealt with when performing harmonic assessments.