Electric Power Systems Research 52 (1999) 273 – 286 Real-time coordinated optimal facts controllers Guojie Li a , T.T. Lie a , G.B. Shrestha a, *, K.L. Lo b a School of Electrical and Electronic Engineering, Nanyang Technological, Singapore 639798, Singapore b Department of Electrical and Electronic Engineering, Uniersity of Strathclyde, Glasgow G11XW, Scotland, UK Received 18 December 1998; accepted 15 February 1999 Abstract A coordinated optimal (CDO) controller is designed to implement multiple Thyristor Controlled Series Compensation (TCSC) devices in a transmission network of interconnected power systems. The proposed controller is utilized to damp inter-area oscillations and to enhance power system damping during large disturbances. The effectiveness of the CDO controller to properly control such devices is demonstrated for a two-machine power system through real-time digital simulation studies using a PSCAD/RTDS. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Inter-area oscillations; Coordinated optimal (CDO) control; Thyristor controlled series compensation (TCSC). 1. Introduction Inter-area oscillations with frequencies ranging from 0.1 to 0.8 Hz [1] are common problems in large ex- tended power systems. They are usually spontaneous, i.e. they can be initiated by small disturbances, such as changes in load that take place continually. Such oscil- lations are associated with the linear response of the system and represent natural modes of oscillations. Inter-area oscillations can also be associated with the nonlinear response of the system. They appear when the system is subjected to large disturbances, such as sudden and large load changes or system faults. Inter- area oscillations involve all of the generators in a large-scale system. The system is split into two parts, with the group of generators in one part swinging against the group of generators in the other part. Such oscillations are very harmful and may cause a total breakdown in the power transfer, especially when there are weakly coupled transmission lines in the system carrying large loads. A common approach adopted to aid damping of these oscillations is the use of power system stabilizers (PSSs) to modulate the generator excitation control. This has been introduced and used in power industry for quite some time [2,3]. However, PSS is designed for each generator individually, which is likely to be af- fected by these oscillations. Due to the complexity of present day power systems, the number of modes of oscillation experienced by a particular generator has become large and the frequency of these modes has begun to vary over a wide range. Therefore, the design of an effective PSS for the generator has become ex- tremely complex and difficult. To overcome this problem, the use of new equipment such as Static Var Compensator (SVC), and various Flexible AC Transmission System (FACTS) devices, are being increasingly considered to aid damping of these oscillations. These techniques have become feasi- ble due to the recent advancement in power electronic technology. A number of SVCs have been installed in actual power systems [4,5]. Many investigators have studied the use of FACTS devices in the transmission system to aid damping of these oscillations and to enhance system stability [6 – 8]. A variety of controllers has been developed for this purpose [9 – 11]. However, only a few of them address the issue that these con- trollers are now required to operate satisfactorily in the presence of several modes of power swings and over a wide range of operating condition. The effectiveness of controllable series compensators is very much depen- dent upon the level of transmission line loading, load characteristics and inertia of the systems. The impact of individual FACTS controllers on control center opera- * Corresponding author. 0378-7796/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 8 - 7 7 9 6 ( 9 9 ) 0 0 0 3 0 - 9