CONTROL OF STATIC VAR SYSTEMS FOR IMPROVING POWER SYSTEM PERFORMANCE

Abstract

The transmission of electrical power from generating plants to distant areas of consumption is severely limited by the voltage regulation and stability problems. Due to electric and electromagnetic properties of transmission lines, AC lines generate a net amount of reactive power at light loads and absorb reactive power at higher loads. The direction and magnitude of reactive power flow together with the line inductances have a profound influence on the voltage profile of an AC transmission line network. Static VAR systems (SVS) have emerged as VAR generation and absorption systems. The function of SVS is to provide high speed reactive power load compensation, resulting in voltage stabilization and stability improvement Series compensation is an alternate means of enhancing power transmission capability economically. It also contributes, significantly to the improvement of system stability, voltage regulation, and reduction of transmission losses. However, the series compensation technique came into criticism because it gives rise to dynamic instability due to subsynchronous oscillations. Many countermeasures to damp out these subsynchronous oscillations have been reported in the literature. Among these, the application of static VAR system to damp out the subsynchronous oscillations has gained importance in recent years It is observed from the literature that the thyristor control devices such as SVSs and controlled series capacitors (CSCs) are finding increasing application in the modern power systems. Flexible AC Transmission systems (FACTS) is emerging as the advanced technology for the efficient utilization of the existing power systems. However, it is often seen that control of SVS has not been demonstrated over a wide operating range. It requires an effective SVS controller to be developed which is applicable over a wide xxi operating range and under large disturbance conditions. The Controlled series compensation (CSC) is one of the novel techniques proposed under FACTS concepts. But investigations on how to control these devices for improving stability of power systems are still in preliminary stages. A detailed model which can reflect the system dynamics accurately over a wide operating range is required to be developed and used. The omission of the line dynamics for series compensated lines can lead to erroneous predictions especially when the transmission network eigenvalues are close to those of electromechanical system. The quality of response in such cases may not correctly reflect the transients due to fast control of CSC. In system studies the proper location of series compensation with respect to the SVS is very important to effectively damp the system oscillations over a wide operating range. Therefore an SVS controller effective over the entire operating range is required to be developed. Also different types of controllers are to be tried over such a wide range of operation to check their efficacy. Based upon the extensive review of literature the major objectives and the scope of the thesis are described below: (i) To develop a suitable system model and to enhance the steady state and dynamic stability of the system using auxiliary control of SVS over a wide operating range. (ii) To enhance the steady state and dynamic performance of a series compensated AC transmission system and to study the effect of locating the series capacitance at various locations along the transmission system. Thus after determining the best location of series capacitor with respect to the SVS location, to improve the dynamic performance of the series compensated line over a wide operating range. (iii) To enhance the transient stability of the transmission system with and without series compensation by developing an effective SVS auxiliary controller over a wide operating range.

(iv) To study the effect of employing controlled series compensation in coordination with SVS for improving further the transient performance of the series compensated line and to assess their applicability in Flexible AC Transmission systems over a wide operating range. (v) To study the subsynchronous resonance and to damp it out by auxiliary control of SVS and to study the comparative performance of various auxiliary controllers and to determine the most effective SVS auxiliary control signal for damping the subsynchronous oscillations over a wide range of power transfer. A detailed system model has been developed- for the dynamic and transient performance study of the system. 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. The rotor flux linkages are expressed in terms of currents which are defined with respect to machine reference frame. To have a common axis of representation with the network and SVS, these flux linkages are transformed to the synchronously rotating D-Q frame of reference. The generator model includes field winding and a damper winding along d-axis and two damper windings along q-axis. In this study, the mechanical system is described by its simplified model. The detailed shaft torque dynamics, however, has been considered for SSR studies. IEEE type-1 excitation system is considered for the generator. The SVS of (SC-TCR) configuration, provides dynamic voltage support at the midpoint of the line. The detailed SVS model has been used which incorporates the TCR transients. Two types of network models are developed (i) using a lumped parameter T-circuit (ii) using a single lumped parameter n-circuit on either side of the midpoint located SVS. The midpoint location of the SVS is chosen with regards to its capability to transfer maximum power over the line.

The dynamic stability of the system is studied by incorporating the two network models over a wide operating range. The dimension of the system matrix in case of T-circuit model is reduced by 8, which significantly reduces the computational burden. Whereas, the n-circuit model gives accurately the system eigenvalues even of the higher frequencies. However the steady state reactive power flows in the two models are different. The model for the SVS auxiliary controllers are developed and applied in the SVS control system to enhance the dynamic performance and a comparison is made over a wide operating range. The study reveals that the damping of the rotor mechanical mode with the application of the line reactive power and the bus frequency auxiliary controllers is slightly higher with the system represented by the T-circuit network model as compared with the system represented by n-circuit network model. However, both the auxiliary controllers as demonstrated in Chapter 3 provide stable operation over the entire range. Thus the system model having the network model represented by lumped parameter T-circuit can also be used with fair accuracy for the dynamic and transient stability studies. An attempt has been made to study the effect of locating the series capacitor at various locations along the transmission system. The steady state and dynamic stability has been evaluated by locating the series capacitor at the locations of (i) both sending and receiving ends (ii) sending end only (iii) receiving end only. The steady state and dynamic performance of such a line has been compared with the line without series compensation over a wide operating range. The study reveals that the sending end location of the series capacitor with midpoint located SVS gives the best dynamic and steady state performance throughout the operating range. The performance of the reactive power auxiliary controller is better than that of the bus frequency auxiliary controller in series compensated line over the entire range considered.

A new SVS auxiliary controller, namely combined reactive power and frequency (C.R.P.F) auxiliary controller has been developed. The performance of this auxiliary controller is examined for enhancing the dynamic and transient performance of a long transmission line over a wide operating range using (i) T-circult and (ii) n-circuit models of the AC transmission network. The C.R.P.F. auxiliary controller in which the line reactive power and the bus frequency auxiliary controllers are used in combination, provides the excellent damping, as established in Chapter 5, to the voltage and power angle oscillations under large disturbance conditions and thus improves the dynamic and transient stability of the line over the wide operating range. The transient stability study reveals that the bus frequency auxiliary controller has more effect on the voltage and power angle oscillations than that of the reactive power auxiliary controller. But the best performance is achieved by the C.R.P.F. auxiliary controller. The performance of various SVS auxiliary controllers has been examined to enhance the dynamic and transient stability ofj a series compensated long transmission lines over wide operating conditions using the two network models. The performance of the C.R.P.F auxiliary controller is excellent in case of the series compensated line also. Thus, the dynamic and transient stability of the series compensated lines is improved over the wide operating range. A novel concept of coordinated application of the controlled series compensation (CSC) and the auxiliary controlled static VAR system to enhance the transient performance of the series compensated line over a wide operating range has been presented, in order to justify its application in the flexible AC transmission systems (FACTS). A model of CSC, in its bang-bang control characteristics, has been developed and is applied along with the SVS auxiliary controllers (Chapter 7). The transient stability with the coordinated application of CSC and SVS xxv auxiliary controllers has been evaluated using the T-circuit and n-circuit models of the transmission network. It is investigated that the coordinated application of CSC and the C.R.P.F. auxiliary controller developed for the SVS located at the middle of the series compensated line, greatly improves the transient performance of the line over a wide operating range, hence justifying the coordinated application for the flexible AC transmission systems. The results are greatly affected by the magnitude of CSC and SVS range. The concept of SVS auxiliary controller application for damping the subsynchronous resonance in the power systems, has been developed and demonstrated over a wide range of operating conditions. The new auxiliary controllers known as combined reactive power and machine internal frequency (C.R.P.I.F.) and combined reactive power and derivative of reactive power (R. P.D.R.P) auxiliary controllers are developed along with the C.R.P.F. auxiliary controller in order to stabilize the unstable torsional modes over a wide range of generator power. It is seen that the C.P.P.I.F. and C.R.P.F. controllers stabilize all the torsional modes over the wide operating range. However, in case of the C.R.P.I.F. auxiliary controller the electrical mode becomes unstable at lower level of generator powers. To illustrate the performance of various auxiliary controllers under large disturbance conditions, a digital computer simulation study using the nonlinear system, has been carried out for the system. The results obtained are in accordance with the eigenvalue analysis. The R.P.D.R.P. auxiliary controller improves the performance of the reactive power auxiliary controller by damping the higher frequency torsional oscillations. The performance of the C.R.P.F. auxiliary controllers is quite good and stabilize the unstable torsional modes over the entire operating range. The performance of the C.R.P.I.F. auxiliary controller is equally good except for lower generator power range.