SSR ANALYSIS OF POWER SYSTEM COMPENSATED BY SERIES FACTS CONTROLLERS

Abstract

At present India's industrial and infrastructure growth is increasing at rapid rate and hence to meet with this ever increasing electric power demand, mega power generation plants are coming up at different locations. This power supply shall be more reliable and efficient, to support rapid development. Series compensation has been recognized as a 'low cost' solution for transmission of bulk-power over a long distance. The simplest form of series compensation is known as fixed series compensation (FSC). The principle of series compensation consists of inserting a capacitor in the transmission line, which compensates the line inductive reactance. Series capacitor compensation improves the transient stability and voltage regulation of the system. However, line compensated with series capacitance is often severely restricted by concerns for the destructive effects of subsynchronous resonance (SSR). Two shaft failures at the Mohave Generating station in Southern Nevada, led to the understanding of SSR phenomenon and since then extensive research and development efforts have been devoted to the development of SSR mitigation methods. Subsynchronous resonance (SSR) is an electric power system condition where the electric network exchanges significant energy with a turbine-generator shaft system at one or more frequencies below the synchronous frequency of the system. Three distinct aspects of SSR analysis have been identified and are referred to as induction generator effect, torsional interaction, and transient torque amplification. SSR phenomena can take place due to other power system components also. Other potential sources include power system stabilizers, high voltage DC converters, high speed governor controls, variable speed drive converters, and control loop of FACTS devices. In general, any device that controls or responds rapidly to power or speed variations in the subsynchronous frequency range is a potential source for the excitation of subsynchronous oscillations. The development of semiconductor technology has made significant impact on AC transmission. Hingorani proposed the concept of flexible AC transmission systems or FACTS, which provides the needed corrections of transmission functionality in order to fully utilize existing transmission systems. Controllable series line compensation is a cornerstone of FACTS technology. There are two basic approaches to FACTS series compensators, one, which employs thyristor-switched capacitors and thyristor-controlled reactors to realize a variable reactive admittance, and the other, which employs a switching power converter to realize a controllable synchronous voltage source. Thyristor controlled series compensator (TCSC), which is a parallel combination of a fixed capacitor and thyristor controlled reactor. However, TCSC suffers from some disadvantages. It injects low order harmonic components into the power system because of thyristor phase control. Deriving a closed-loop model of TCSC is complicated. Static Synchronous Series Compensator (SSSC), proposed by Gyugyi, is a second generation FACTS controller. It injects synchronous ac voltages in series with a transmission line to allow fast continuous control of the flow of power in the line. It is a GTO based VSI and can provide controllable compensating voltage over an identical capacitive and inductive range, independently of the magnitude of the line current. An ideal SSSC is essentially a pure sinusoidal ac voltage source at the system fundamental frequency. Its output impedance at other frequencies is theoretically zero. The question of SSR will arise in all FACTS applications, for the basic reason that all high-speed high-power controllers have potential of enhancing or degrading subsynchronous phenomenon. In a practical SSSC, the dc side of VSI is terminated by a finite energy storage capacitor to maintain the desired dc operating voltage. Thus the dc capacitor in effect interacts with the ac system via the converter switches. This interaction may conceivably influence the subsynchronous behavior of a practical SSSC There is considerable opportunity for original research to explore the torsional characteristics of SSSC. It is also very likely that the SSSC can be controlled to be highly effective in the active damping of prevailing subsynchronous oscillations brought about by conventional series compensation. Based on the above considerations, this thesis focuses attention on the torsional characteristics of series compensated power system. SSR analysis of the fixed series capacitor and TCSC compensated power systems are re-examined to give a background to the studies of the later chapters in the thesis. Linearized models of the power system, with conventional and FACTS compensators, are derived for eigenvalue analysis. Nonlinear time domain simulations are used to validate the results of the eigenvalue analysis. Different topologies of VSI, namely multi-pulse, multi-level and hybrid multi -level are used for the realization of SSSC. The effect of dc link capacitor of the SSSC on torsional performance is studied. The effect of SSSC compensation levels on torsional behavior of the system is studied. To improve the torsional performance of the SSSC compensated power system, a derivative free optimization method, particle swarm optimization (PSO), is used. An objective function which maximizes the damping of critical torsional mode is proposed. PSO finds the best controller parameters of the dc link controller to maximize the objective function. The effect of excitation system that includes power system stabilizer (PSS) on torsional performance of SSSC compensated power system is studied. The control interaction study is carried out through eigenvalue analysis and time domain simulations. The analysis has shown that the adverse effect of PSS on torsional interaction can be eliminated in an SSSC compensated power system. The need of torsional filter can be eliminated by properly designing the controller of SSSC using Particle swarm optimization (PSO). The thesis also shows that multi-level and hybrid SSSC can improve the torsional performance of the compensated system.