Abstract

With the rapid increase of electricity demand, the possibility of overloading the existing power transmission lines increases. Building new transmission lines can be avoided or delayed, provided that fast and accurate means for controlling active and reactive power are made available. There are a number of Flexible AC Transmission System (FACTS) devices that can help meeting these goals. Among them is the Interphase Power Controller (IPC), which presents an important characteristic: Active power regulation in a transmission line with a highly variable transmission angle. Its original implementation is based on mechanically controlled phase-shifting transformers, which are relatively slow and only allow active and reactive power flow control in a discrete range. Static IPCs can be realized with three Voltage Source Converters (VSCs) in a configuration called dual Unified Power Flow Controller (UPFC). However, this Unified IPC (UIPC) has been controlled as a Static Phase Shifter (SPS) and does not make use of all the flexibility offered by the dual UPFC.
This thesis discusses the operation of the dual UPFC of the UIPC with all its features: Phase shifting and reactive power compensation, series and shunt. The complexity of the control logic increases since the number of control variables increases from two to four. To address this issue, a control strategy based on the sharing of the real and imaginary components of the desired transmission line current among the capacitive and inductive branches of the UIPC is proposed. The control variables are then computed with an optimization algorithm that minimizes the apparent power of the VSCs of the capacitive and inductive branches for increased efficiency. The superior performance of the proposed scheme over the conventional one is demonstrated based on analytical expressions.
The performance of the proposed control scheme is also investigated in the time domain. For that, dynamic models of the system required for designing the control loops of the currents in the inductive branch, capacitive branch, shunt branch and DC bus voltage are derived. Proportional plus Integral (PI) type controllers in the dq (rotating reference) frame and Proportional Resonant (PR) controller in the abc (stationary reference) frame are designed. The dynamic performance of the system is verified by means of simulation using PSCAD/EMTDC. Besides, a reduced-scale prototype controlled with a rapid prototyping real-time control hardware was built to further validate the performance of the system experimentally.