The rising power demand has forced power systems all over the world to operate very close to their stability limits. When power systems are overloaded, faulty, or in lack of reactive power, voltage collapses would ensue. The capacity of a power system to keep the voltage of every bus constant under disturbances is called voltage stability. This dynamic phenomenon hinges on the load features. It is commonly known that flexible AC transmission systems (FACTS) can improve voltage stability. This paper puts forward a load flow model with the unified power flow controller (UPFC), and relies on the model to investigate the voltage stability of a power system through continuation power flow (CPF) method. The validity of the model was verified through a simulation, using the power system analysis toolbox (PSAT) in MATLAB/Simulink environment.

The modular multilevel converter (MMC) is a recently developed converter architecture with the potential for high-voltage direct current (HVDC) transmission applications. Due to the large number of IGBTs and diode devices, modeling a modular multilevel converter (MMC) for electromagnetic transient type (EMT-type) simulations might be difficult. HVDC-MMC transmission system detailed models take a long time to compute. To solve this issue, simplified and averaged models have been suggested. In order to efficiently and accurately depict MMC-HVDC systems, several kinds of models are developed and compared in this study using EMTP-rv software. The results suggest that the kind of model to be used will rely on the study performed and the level of accuracy needed.