13-02-2013, 12:33 PM
TRANSIENT STABILITY ENHANCEMENT OF POWER SYSTEM USING A THYRISTOR CONTROLLED SERIES CAPACITOR
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ABSTRACT
This paper presents the variable impedance capability of TCSC for enhancing the transient stability of power systems. For obtaining the varying impedance, two different controllers namely a speed deviation based bang-bang controller and a nonlinear controller are used. To validate the performance of the control schemes, the simulation studies are carried out on a single machine infinite bus system using PSCAD software package .The simulation results show that variable impedance TCSC provides improvement in transient stability limit when compared with fixed impedance TCSC. Also among the two controllers, nonlinear controller provides an improved damping of power system oscillations.
INTRODUCTION TO TRANSIENT STABILITY
TRANSIENT STABILITY
Power systems transient stability phenomenon are associated with the operation of synchronous machines in parallel, when subjected to a severe transient disturbance, and becomes important with long distance heavy power transmissions.
From a physical viewpoint, transient stability may be defined as the ability of a power system to maintain machine’s synchronous operation when subjected to large disturbances such as a short-circuit or line tripping.
Stability depends on both the initial operating state of the system and severity of the disturbance. Often, the system is altered so that the post-disturbance steady state operation differs from that prior to the disturbance. Disturbance of widely varying degrees of severity and probability of occurance can occur on the system. The system is, however, designed and operated so as to be stable for a selected set of contingencies. The contingencies are short-circuits of different types: phase-to-ground, phase-to-phase. They are usually assumed to occur on transmission lines, but occasionally bus or transformer faults are also considered.
FACTS CONTROLLERS
FACTS controllers may be based on thyristor devices with no gate turn-off or power devices with gate turn-off capability. FACTS controllers are used for the dynamic control of voltage, impedance and phase angle of high voltage AC transmission lines. The basic principles of the following FACTS controllers, which are used in the two-area power system under study, are discussed briefly.
Static Var Compensator (SVC)
Static var systems are applied by utilities in transmission applications for several purposes. The primary purpose is usually for rapid control of voltage at weak points in a network. Installations may be at the midpoint of transmission interconnections or at the line ends.
Thyristor Controlled Series Capacitor (TCSC)
TCSC is one of the most important and best known FACTS devices, which has been in use for many years to increase the power transfer as well as to enhance system stability. The main circuit of a TCSC is shown in Fig. 2.2 The TCSC consists of three main components: capacitor bank C, bypass inductor L and bidirectional thyristors SCR1 and SCR2. The firing angles of the thyristors are controlled to adjust the TCSC reactance in accordance with a system control algorithm, normally in response to some syste m parameter variations. According to the variation of the thyristor firing angle or conduction angle, this process can be modeled as a fast switch between corresponding reactances offered to the power system.
Unified Power Flow Controller (UPFC)
Among the available FACTS devices, the Unified Power Flow Controller (UPFC) is the most versatile one that can be used to enhance steady state stability, dynamic stability and transient stability. The UPFC is capable of both supplying and absorbing real and reactive power and it consists of two ac/dc converters. One of the two converters is connected in series with the transmission line through a series transformer and the other in parallel with the line through a shunt transformer. The dc side of the two converters is connected through a common capacitor, which provides dc voltage for the converter operation.
The power balance between the series and shunt converters is a prerequisite to maintain a constant voltage across the dc capacitor. As the series branch of the UPFC injects a voltage of variable magnitude and phase angle, it can exchange real power with the transmission line and thus improves the power flow capability of the line as well as its transient stability limit.
SYSTEM OVERVIEW
The fundamental operating principles of variable series reactance control to enhance the transient stability of the power system which may be summarized as:
(i) To decrease the overall line impedance (by increasing the magnitude of the dynamically controlled series capacitive reactance) so as to transfer excess accelerating power out of the affected generators; and
(ii) To increase the overall line impedance (by decreasing the magnitude of the dynamically controlled series capacitive reactance) so as to minimise excess decelerating power in the affected generators.
Fig.3.1 shows the single line diagram of the system that is being studied. The system consists of a 3-phase generator that is connected to an infinite bus bar via a transformer and two parallel transmission lines. Transmission line L1 is compensated by a TCSC, while line L2 remains uncompensated. A detailed model of the study system is developed in PSCAD [9]. The effect of AVR is not included in the simulation studies.
CONCLUSION
In this paper, to demonstrate the impact of TCSC on transient stability, a single machine connected to infinite bus system is used. All the simulation studies show that there is an improvement in transient stability limit with thyristor controlled series compensation. With fixed TCSC, it is found that the transient stability is improved with higher values of reactance order. Simulation results for TCSC with varying reactance order show that there is further improvement in transient stability limit when compared to TCSC with constant reactance order. In this paper in order to obtain the varying reactance order, two different controllers selected are a simple speed deviation based bang bang controller and a nonlinear controller. Simulation results show that nonlinear controller provides an improved transient stability and power oscillation damping compared to bang bang controller.