Incorporation of detailed HVDC dynamics into transient energy functions

A common approximation used in the derivation of the Transient Energy Functions (TEFs) for AC/DC power systems is the omission of HVDC dynamics. The differential equations that represent HVDC dynamics are used to find the fault-on trajectory but not to derive the TEF. This leads to an error in TEF-based stability prediction. In this paper, a method is presented to derive the TEF with HVDC dynamics included. By using this improved TEF, the accuracy of the stability prediction in AC/DC systems is improved. In addition, due to the inclusion of differential equations that represent HVDC dynamics in the TEF, the calculation of the post-fault Stable Equilibrium Point (SEP) must be done only once. This reduces the CPU time required to find the Transient Energy Margin (TEM). The method is validated using several test systems by comparing results with those obtained from time simulation. The new transient energy function is used in a number of studies to establish its usefulness.