K.R. Padiyar

Design and performance evaluation of subsynchronous damping controller with STATCOM

A long transmission line needs controllable series as well as shunt compensation for power flow control and voltage regulation. This can be achieved by suitable combination of passive elements and active FACTS controllers. In this paper, series passive compensation and shunt active compensation provided by a static synchronous compensator (STATCOM) connected at the electrical center of the transmission line are considered. It is possible to damp subsynchronous resonance (SSR) caused by series capacitors with the help of an auxiliary subsynchronous damping controller (SSDC) on STATCOM. The objective of this paper is to investigate the SSR characteristics of the system and propose a new design procedure for SSDC based on nonlinear optimization to meet the specifications on the damping torque in the range of critical torsional frequencies. The SSDC uses the Thevenin voltage signal to modulate the reactive current reference of STATCOM. The Thevenin voltage signal is derived from the locally available STATCOM bus voltage and reactive current signals. The STATCOM configurations considered in this paper are 12 pulse, two- and three-level voltage source converter with Type-2 and Type-1 control, respectively. The controller regulates either reactive current (supplied by the STATCOM) or the bus voltage. The 3-phase model of the STATCOM is based on switching functions. By neglecting harmonics in the switching function, D-Q model is derived which is combined with similar models of the other system components for linear analysis. The results of the linear analysis are validated by carrying out transient simulation based on the detailed nonlinear models. The study is performed on the system adapted from the IEEE First Benchmark Model.

Damping torque analysis of static VAR system controllers

The use of a damping torque technique to examine the efficacy of various control signals, for reactive power modulation of a midpoint-located static VAr system (SVS) in enhancing the power transfer capability of long transmission lines is considered. A new auxiliary signal, the computed internal frequency (CIF), is proposed which synthesizes the internal voltage frequency of the remote generator from electrical measurements at the SVS bus. It is demonstrated that this signal is far superior to other conventional auxiliary control signals in that it allows full utilization of the network transmission capacity. The damping torque results are correlated with those obtained from eigenvalue analysis. >

Control design and simulation of unified power flow controller

The unified power flow controller (UPFC) is a solid-state controller which can be used to control active and reactive power flows in a power transmission line. In this paper, the authors propose a control strategy for UPFC in which they control real power flow through the line, while regulating magnitudes of the voltages at its two ports. They design a controller for this purpose which uses only local measurements. The control strategy is evaluated using digital simulation for a case study.

Online detection of loss of synchronism using energy function criterion

Maintaining dynamic security of a power system subjected to large disturbances is of utmost importance. Fast and accurate online detection of instability is essential in initiating certain emergency control measures. The techniques reported in the literature involve mainly the application of global phasor measurements and heuristic algorithms. In this paper, an accurate technique for the online detection of loss of synchronism based on voltage and current measurements in a line is presented. The technique makes use of the concept of potential energy in a line. The conditions for the system instability are derived from energy function analysis. However, no assumptions are made regarding the power-angle relationship in a line, nor are any data on the system equivalents necessary in implementing the detection scheme.

Investigation of Subsynchronous Resonance With VSC-Based HVDC Transmission Systems

The HVDC converter control can destabilize torsional modes of nearby turbogenerators. The first experience of HVDC-turbine generator torsional interaction was observed in 1977 during field tests at Square Butte. The development of power semiconductors, specially insulated-gate bipolar transistors and gate turnoff thyristors (GTOs) has led to the small power HVDC transmission based on voltage-source converters (VSCs). The self-commutated VSC-based HVDC installations have several advantages compared to conventional HVDC-based online-commutated current source converter. The main objective of this paper is to investigate and present the detailed analysis of subsynchronous resonance (SSR), arising from a VSC-based HVDC system connected close to generating units. The analysis considers different operating modes of the converters. Based on a case study, it is shown that the dc voltage control mode of VSC operation (rectifier/inverter) close to the generator units can contribute positive damping in the torsional-mode frequency range of interest. The investigations of SSR with VSC-based HVDC is carried out based on linear (damping torque and eigenvalue) analysis and nonlinear transient simulation. While the damping torque, eigenvalue analysis, and controller design are based on the D-Q model, the transient simulation considers the D-Q model and the three-phase detailed model of VSC using switching functions. The influence of the operating modes of the converters and effects of some important parameters, such as effective short circuit ratio and generator rating are investigated.

Direct stability evaluation of power systems with detailed generator models using structure-preserving energy functions

Energy-based direct methods for transient stability analysis are potentially useful both as offline tools for planning purposes as well as for online security assessment. In this paper, a novel structure-preserving energy function (SPEF) is developed using the philosophy of structure-preserving model for the system and detailed generator model including flux decay, transient saliency, automatic voltage regulator (AVR), exciter and damper winding. A simpler and yet general expression for the SPEF is also derived which can simplify the computation of the energy function. The system equations and the energy function are derived using the centre-of-inertia (COI) formulation and the system loads are modelled as arbitrary functions of the respective bus voltages. Application of the proposed SPEF to transient stability evaluation of power systems is illustrated with numerical examples.

Bifurcation analysis of a three node power system with detailed models

This article presents a comprehensive study of bifurcations in a realistic power system model. The two-axis model for the generator with the field winding on the d-axis and a damper winding on the q-axis (1.1 model), along with the excitation system is considered to represent the dynamics of the generator. The load is described by a dynamic load model. The dynamics of the resulting system is studied using (i) input power to the generator, (ii) both active and reactive power demand at the load bus, (iii) reference voltage to the AVR as bifurcation parameters. It is found that model refinement results in significant qualitative changes in the system behaviour. Quasiperiodic behaviour is shown to result from a torus bifurcation. The system also exhibits chaotic behaviour resulting from cascades of period-doubling bifurcations. Detailed numerical simulations are presented to illustrate the types of dynamic behaviour and attractors encountered

Modeling and control of unified power flow controller for transient stability

FACTS controllers are emerging as viable and economic solutions to the problems of large interconnected ne networks, which can endanger the system security. These devices are characterized by their fast response, absence of inertia, and minimum maintenance requirements. Thyristor controlled equipment like Thyristor Controlled Series Capacitor (TCSC), Static Var Compensator (SVC), Thyristor Controlled Phase angle Regulator (TCPR) etc. which involve passive elements result in devices of large sizes with substantial cost and significant labour for installation. An all solid-state device using GTOs leads to reduction in equipment size and has improved performance. The Unified Power Flow Controller (UPFC) is a versatile controller which can be used to control the active and reactive power in the Line independently. The concept of UPFC makes it possible to handle practically all power flow control and transmission line compensation problems, using solid-state controllers, which provide functional flexibility, generally not attainable by conventional thyristor controlled systems. In this paper, we present the development of a control scheme for the series injected voltage of the UPFC to damp the power oscillations and improve transient stability in a power system

Tuning and performance evaluation of damping controller for a STATCOM

The main objective of this paper is to present the tuning and performance evaluation of a damping controller recently proposed for a STATCOM. A detailed eigenvalue analysis is carried out to study the effectiveness of the control law and the location of the damping controller. With the objective of damping tie line oscillations two methods (Method-1 and -2) are proposed for tuning the controller parameters. The performance of the controller is evaluated for these methods of tuning by carrying out the simulation of a four-machine system for small and large disturbances. Case study shows that tie line oscillations are better damped when controller is tuned by Method-1 and is located at the sending end of the tie line.

Discrete control of series compensation for stability improvement in power systems

Changes in the network such as connecting a shunt resistor, inserting a series capaciitor, reducing of generation, load etc., when accomplished under a suitable control scheme can improve transient stability. Switching of series capacitors in a transmission line can also help to damp oscillatory power transients. In this paper a detailed analysis and study of a discrete control strategy for the Thyristor Controlled Series Capacitor (TCSC) to improve stability is presented. A single machine infinite bus system is considered to illustrate the development of the control strategy. The energy function is used in determining the switching instants. The control philosophy is later extended to the multimachine system where the energy of the transmission line is used in determining the control strategy.