The application of a Static Synchronous Series Compensator (SSSC) controller to improve the transient stability performance of a power system is thoroughly investigated in this paper. The design problem of SSSC controller is formulated as an optimization problem and Particle Swarm Optimization (PSO) Technique is employed to search for optimal controller parameters. By minimizing the time-domain based objective function, in which the deviation in the oscillatory rotor angle of the generator is involved; transient stability performance of the system is improved. The proposed controller is tested on a weakly connected power system subjected to different severe disturbances. The non-linear simulation results are presented to show the effectiveness of the proposed controller and its ability to provide efficient damping of low frequency oscillations. It is also observed that the proposed SSSC controller improves greatly the voltage profile of the system under severe disturbances.

I. INTRODUCTION

HEN large power systems are interconnected by relatively weak tie lines, low frequency oscillations are observed. These oscillations may sustain and grow to cause system separation if no adequate damping is available [1]. Recent development of power electronics introduces the use of flexible ac transmission system (FACTS) controllers in power systems. FACTS controllers are capable of controlling the network condition in a very fast manner and this feature of FACTS can be exploited to improve the stability of a power system [2]. Static Synchronous Series Compensator (SSSC) is one of the important members of FACTS family which can be installed in series in the transmission lines. With the capability to change its reactance characteristic from capacitive to inductive, the SSSC is very effective in controlling power flow in power systems [3]. An auxiliary stabilizing signal can also be superimposed on the power flow control function of the SSSC so as to improve power system oscillation stability [۴]. The applications of SSSC for power oscillation damping, stability enhancement and frequency stabilization can be found in several references [5]-[8]. The influence of degree of compensation and mode of operation of SSSC on small disturbance and transient stability is also reported in the literature [9]-[11]. Most of these proposals are based on small disturbance analysis that required linearization of the system involved. However, linear methods cannot properly capture complex dynamics of the system, especially during major disturbances. This presents difficulties for tuning the FACTS controllers in that the controllers tuned to provide desired performance at small signal condition do not guarantee acceptable performance in the event of major disturbances.

A conventional lead-lag controller structure is preferred by the power system utilities because of the ease of on-line tuning and also lack of assurance of the stability by some adaptive or variable structure techniques. The problem of FACTS controller parameter tuning is a complex exercise. A number of conventional techniques have been reported in the literature pertaining to design problems of conventional power system stabilizers namely: the eigenvalue assignment, mathematical programming, gradient procedure for optimization and also the modern control theory. Unfortunately, the conventional techniques are time consuming as they are iterative and require heavy computation burden and slow convergence. In addition, the search process is susceptible to be trapped in local minima and the solution obtained may not be optimal [12].

Recently, Particle Swarm Optimization (PSO) technique appeared as a promising algorithm for handling the optimization problems. PSO is a population based stochastic optimization technique, inspired by social behaviour of bird flocking or fish schooling [13]. PSO shares many similarities with Genetic Algorithm (GA); like initialization of population of random solutions and search for the optimal by updating generations. However, unlike GA, PSO has no evolution operators such as crossover and mutation. One of the most promising advantages of PSO over GA is its algorithmic simplicity as it uses a few parameters and easy to implement. In PSO, the potential solutions, called particles, fly through the problem space by following the current optimum particles [14]. In view of the above, PSO is employed in the present work to optimally tune the parameters of the SSSC controller. In this paper, a comprehensive assessment of the effects of SSSC controller has been carried out. The design problem of SSSC-based controller is transformed into an optimization problem. The design objective is to improve the transient stability performance of a single-machine-infinite-bus power system, subjected to severe disturbances. PSO based optimal tuning algorithm is used to optimally tune the parameters of the SSSC controller. The proposed controller has been applied and tested on a weakly connected power system under different severe disturbances.

استفاده از یک کنترلر جبرانساز سری سنکرون استاتیک (SSSC) برای بهبود عملکرد پایداری گذرای سیستم قدرت به طور کامل در این مقاله بررسی شده است. مساله طراحی کنترلر SSSC به صورت یک مساله بهینه‌سازی بیان شده و از تکنیک بهینه‌سازی ازدحام ذرات (PSO) برای جستجوی پارامترهای بهینه کنترلر استفاده شده است. با کمینه‌کردن تابع هدفِ حوزه زمان که شامل زاویه نوسانی روتور ژنراتور نیز هست؛ عملکرد پایداری گذرای سیستم بهبود می‌یابد. کنترلر ارائه شده در این مقاله، روی یک سیستم قدرت با اتصال ضعیف که در معرض انواع اغتشاشات شدید قرار دارد، مورد آزمون قرار گرفته شده است. نتایج حاصل از شبیه‌سازیِ غیرخطی بیان شده‌اند تا کارائی کنترلرِ ارائه شده و توانائی آن در میرائی نوسانات فرکانس کوچک نشان داده شود. همچنین مشاهده می‌شود که کنترلر SSSC ارائه شده در این مقاله، حین اغتشاشات شدید به خوبی پروفیل ولتاژ را بهبود می‌دهد.