DYNAMIC PERFORMANCE ENHANCEMENT OF SERIES COMPENSATION TRANSMISSION LINES USING STATIC VAR SYSTEM

Abstract

Flexible Alternating current transmission system (FACTS) devices are used widely for the dynamic control of voltage, impedance and phase angle of high voltage AC lines. Thus with Facts controllers the existing transmission facility can be fully utilized and hence minimizing the gap between stability limit and thermal stability. Series Compensation has been widely used to enhance power transfer capability. However, series compensation gives rise to the problems of dynamic instability and sub synchronous resonance (S.S.R) [1, 2, 3]. Mitigation of low frequency oscillations is one of the most important and challenging tasks in power industry as the stability of these oscillations is a prerequisite for secure and stable operation of power system. Many preventive measures to cope with dynamic instability problem in series compensated lines have been reported in literature [7]. Among these the application of Static Var Systems(SVS) controller has gained the importance in recent years in modern power systems due to its capability to work as Var generator and absorption systems[8,9,10].Besides voltage control and improvement of transmission capability SVS in coordination with auxiliary signals can be used for damping of power system oscillations effectively[11,12].The selection of location of an effective location and feedback signal is an important aspect to employ FACTS controllers[14].It is often seen that the control of SVS has not been demonstrated over a wide operating range[15,16].It requires an effective SVS controller to be developed which is applicable over a wide operating range and under large disturbance conditions. The controlled series compensation is one of the novel techniques proposed under FACTS concepts. But investigations on how to coordinate these devices for improving stability and damping sub synchronous resonance are still in implementation stage and needs investigation. A detailed system model which can reflect system dynamics accurately over a wide operating range has to be developed. Proper location of SVS and Series compensation play a very important role for enhancement of dynamic and transient stability of power system[18].Thus there is need to determine the optimal location of static var system and the amount of series compensation in the power systems. Also the rating of SVS should be based on steady state analysis because SVS may cause high power transfer level over long distance. Different types of SVS auxiliary signals and their combination have been tried to find out the most effective auxiliary signal or effective combination for enhancement of dynamic and transient performance over a wide operating range. A detailed system model has been developed for the dynamic and transient performance of the system (Chapter -2). The study system consists of a generator supplying power to an infinite bus over a long transmission line. In the detailed machine model, the stator is represented by a dependent current source in parallel with the inductance. Its rotor flux linkage is expressed in terms of currents which are defined wit respect to the machine reference frame. To have a common axis of representation with the network and SVS, these flux linkages are transformed to synchronously rotating D-Q frame of reference. A SVS auxiliary controller is then developed using Fortran Power Station , which is an advanced software for analyzing and simulating a variety of dynamic systems. The performance is evaluated for enhancement of dynamic stability of a series compensated transmission lines by computing the Eigen value of the linearised system model. The dynamic performance of the series compensated power system has been evaluated by performing the eigen value analysis .A linearised power system model has been developed and system eigen values have been computed with and without incorporating the combined reactive power and voltage angle SVS auxiliary controller.It is seen that the unstable modes of the system are effectively stabilized and the dynamic performance is greatly improved.