Dynamic interactions of multiple functions of a power-angle controlled and vector-current controlled UPFC

The most important feature of the Unified Power Flow Controller UPFC is its multi-control functions to maintain the voltage and to regulate power flow at the substation where it is installed. There might exist dynamic interactions among the multiple control functions of the UPFC which could impose difficulty in the design of the control algorithm. This has been investigated previously for a phase-angle controlled UPFC. With the fast advance of high-voltage power electronics technology, the voltage source converter based on the MMC technology has found applications in power transmission systems. The first UPFC to be installed in Chinses power system is implemented by the MMC technology and controlled by the vector-current control algorithm. Dynamically, this vector-current controlled UPFC could be different with the phase-angle controlled UPFC. This paper investigates the dynamic interactions of multiple control functions of the vector-current controlled UPFC and carries out a comparative study with the conventional phase-angle controlled UPFC. In the paper, the method of relative gain matrix (RGA) is used to assess quantitatively the dynamic interactions between multiple control functions of both the phase-angle controlled and vector-current controlled UPFC. An example power system installed with the two different types of the UPFC separately is presented with computational and simulation results being presented. It is found that the dynamic interactions exist between the multiple control functions of a phase-angle controlled UPFC which have been reported before. However, they are hardly noticeable in a vector-current controlled UPFC. In the paper, the finding is discussed and explained.