A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering

Thesis (M.Eng.) - Asian Institute of Technology, 1997

Conventionally, the optimal operation of the power system networks have been based on the economic criterion. Economic load dispatch is widely being used by most of the power utilities as· part of their Energy Management System (EMS). However, recent concern of the utilities about power quality, specially in terms of improved voltage profile, requires to consider additional objective in the Optimal Power Flow (O.P.F.) formulation. Optimal Reactive Power Dispatch (ORPD) is a subproblem of OPF, which has two fold objectives of improving the system voltage profile and minimizing the system transmission loss. This is achieved through redistribution of reactive power in the network through optimal setting of generator terminal voltage or reactive power outputs, transformer tappings and output of other compensating devices such as shunt capacitors, reactors, synchronous condensers etc. The fast progress in the field of power electronics, microprocessors, microelectronics and communication technology has resulted into introduction of new devices for more flexible operation of the power systems known as Flexible AC Transmission System (FACTS) devices. These devices include Static Var Compensators (SVC), Thyristor Controlled Series Compensators (TCSC), Thyristor Controlled Phase Angle Regulators (TCP AR), Static Condensers (STATCON), Unified Power Flow Controllers (UPFC), etc. These devices have been mainly studied and applied for improving the dynamic behavior of the power system, avoiding loop flow and subsynchronous resonance problems. Only little study have been done to explore the capability of these devices as a candidate control in the optimal power flow and specially the loss minimization. Therefore, the objective behind the present study has been to explore the relative impact of some of the FACTS devices in improving the system voltage profile and minimizing the transmission losses. Three FACTS devices namely SVC, TCSC and TCP AR have been considered in the present study. Static models of these devices have been utilized along with their operating limits. The present study has considered the following two subproblems: I. Optimal Placement and sizing of FACTS devices. II. Optimal Reactive Power Dispatch to determine optimal setting of FACTS devices along with other control variables. A sensitivity based method has been developed to determine optimal placement of the three FACTS devices. Sensitivity of total real power transmission loss (PL) at a base operating point with respect to the parameter of SVC, TCSC and TCP AR have been used for siting of these devices. Reactive power injection Qsvc by SVC, series reactance (Xc) of TCSC and phase shift (Üa) of the TCP AR have been considered as their operating variables. After the placement of these FACTS devices in the system, ORPD have been modified to determine their optimal settings along with the setting of generator voltages (VG), transformer taps (t). The ORPD formulation have been used to minimize total system loss. The limits of FACTS devices were determined through peak load and off peak load simulations. The studies have been conducted on an IEEE 30 bus test system and also a practical system representing the 500kV and 230kV network of the Electricity Generating Authority of Thailand (EGAT). A comparison of the voltage profile and transmission losses have been made from ORPD results without considering FACTS devices and considering the three FACTS devices one at a time. It has been observed that all the three devices help in improving the voltage profile and also reducing the transmission losses. The result also clearly bring out the relative performance of these three FACTS devices in the ORPD.