Managing system frequency dynamics in small power system by hydro generating unit operated in Tail Water Depress (TWD) mode with use of Feed Forward Controls

This paper details the investigation of a large frequency excursion of 2.28 Hz, experienced in the New Zealand power system on 1^st August 2009 when the HVDC interconnector tripped, resulting in a power deficiency in South Island power system. A major portion of the spinning reserve to cover for this contingent event was provided by generating units operated in Tail Water Depressed (TWD) mode which were found to perform unsatisfactory. A system wide investigation into this frequency excursion event was initiated with a number of modelling and operation issues being identified. The errors in modelling information and delay in the TWD operation are the main factors that caused system frequency to fall more than what was predicted by the software tools. A brief explanation of TWD Feed Forward Control system and the models used in the Reserve Management Tool (RMT) are provided. There is also reconsideration of the modelling of inertia in a DC model using a simple first order equation. The limitations of such a model are demonstrated, as are the importance of modelling the correct number of synchronized generators, with calculated (not tested) inertia constants. A number of solutions to the problems discovered are discussed.