Transient stability study of the Hsin Yu Co-Generation plant in Hsin-Chu science based industrial park in Taiwan

The combined cycle plant consists of three gas-turbine generators and one steam-turbine generator. The total generating capacity is 215 MVA. The plant feeds via two 161/161 kV transformers into the 161 kV TPC (Taiwan Power Company, Taipower) network. The loads fed from the internal 161 kV and 22.8 kV busbars amount to 145 MW. So dependent from the actual load/generation conditions, both a large power export as well as a large power import is possible. The aim of the study was to examine the behaviour of the plant in case of internal and external faults. The determined critical clearing times provides general information about the influence of pre-fault load conditions, pre-fault active/reactive power generation and fault location on the transient stability of the generators. Besides, the critical clearing time calculations are used to find the settings of the decoupling scheme for the voltage relay and the frequency relay at the metering outfit (MOF). Both relays operate independently and must guarantee a safe island transfer of the plant in case of external faults in the 161 kV Taipower network that endangers the transient stability or the mechanical strength of the generator/turbine system. Because of large load variations in terms of possible power export and import a supporting load shedding scheme was studied in conjunction with the de-coupling logic. The loads are rejected in three steps depending on the frequency level at the internal 161 kV busbar. The simulations show that there is a close coherence between the de-coupling scheme, the settings of the load shedding stages and the frequency behaviour