Application of coherency-based dynamic equivalents in small perturbation stability studies

Coherency-based dynamic equivalent is a technique commonly employed for power system reduction. The key step of the technique which will mostly affect the accuracy of resultant equivalents is coherency identification. Traditionally, coherency is identified by using swing curve or similar approaches and the accuracy of the resulting equivalent is also verified by transient plots. It is well known that the swing curve is often associated with transient stability study. Not much attention has been drawn to analyze how the dynamic equivalent is applied to small perturbation stability studies. A new coherency identification technique is, therefore, introduced to suit this purpose. This new approach is different from the conventional one in that it employs the mode shape of electrical power to identify coherency of a power system. The approach is tested on a typical interconnected system and the results show that the proposed technique provides good approximation. Moreover, the effect of system reduction to dynamic stability study is also discussed