G. Durga Prasad

Design of a robust load—frequency controller for interconnected power systems based on the singular-value decomposition method

Abstract A robust control scheme is presented for the load—frequency control of an interconnected power system with uncertain parameters. A combination of the singular-value decomposition (SVD) technique and the Riccati equation approach is adopted to design a robust controller. Stability analysis of the closed-loop interconnected system for all admissible uncertainties is considered. The effectiveness of the proposed robust control scheme has been verified through simulation studies on a two-area power system model. The performance robustness of the proposed control algorithm is compared with recent results and found to be much better.

A new approach to dynamic state estimation of power systems

This paper presents a new method of dynamic state estimation (DSE) based on Kalman filter, rather than extended Kalman filter. The complex line flow measurements are used to obtain the complex element voltages of the power system, which are then treated as the effective system measurements at the filtering stage. Since, the complex element voltages are linearly related to the complex bus voltages, use of the Kalman filter provides more accurate and faster estimation of the unknown complex bus voltages. At the prediction stage, Holts two parameter exponential smoothing technique has been adopted for its reliable performance. The proposed new dynamic state estimation (NDSE) has been tested on 5-bus, 14-bus and 30-bus test systems and the numerical results are presented. The performance of the proposed NDSE has been compared with the existing DSE methods for various simulated test conditions. The error analysis of the methods studied reveals the superiority of the proposed NDSE method particularly under sudden change in operating conditions of the system.

A heuristic method for the real-time load flow solution of integrated multiterminal AC-DC power systems

Abstract This paper presents a heuristic method for the real-time load flow solution of integrated multiterminal AC-DC power systems based on the substituted power injection (SPI) method at the terminal busbars. The proposed method requires calculated real and reactive powers at the terminal busbars obtained from the standard AC-DC load flow algorithm. Using these calculated powers as specified power injections at the terminal busbars, the proposed method models the integrated AC-DC system under the specified operating conditions of load, generation and DC system control strategy to an equivalent AC system and uses the fast decoupled AC load flow algorithm to solve it. An alternative heuristic method, based on the concept of replacing the existing DC links by an equivalent AC line and named the substituted line parameter (SLP) method, has also been investigated. The proposed heuristic SPI and SLP methods are tested on 5-bus, 14-bus and 30-bus multiterminal AC-DC test systems having link and mesh connections and the results are compared with the load flow solution obtained by the sequential AC-DC load flow algorithm. The performance of the proposed SPI method was also tested for random load variations up to ±25% at all the buses of these systems and the results obtained are found to be in good agreement with those obtained by the sequential AC-DC load flow method for the same random load variations.

Design of a load frequency regulator based on the Schur approach

A linear-quadratic regulator for the load frequency control of a two-area power system has been developed based on the Schur method. The proposed method places open-loop poles successively in prescribed locations by solving a number of Riccati equations of order not more than two. The performance of the proposed method is tested by considering a two-area power system model and the results are presented. The proposed method may be used in the same way to design an observer with desired eigenvalues.

Modifications to Newton-Raphson load flow for ill-conditioned power systems

Abstract This paper presents the effects of five different modifications to the standard Newton-Raphson (NR) method for the solution of load flows in ill-conditioned power systems. The proposed modifications to the NR method are: (1) NR method with decelerated convergence; (2) accelerated method for Newtons method at singular points (two-step algorithm and three-step algorithm); (3) Broydens Jacobian-updating procedure for the NR method; (4) constant positive shift algorithm; (5) increasing positive shift algorithm. All the proposed modifications are based on the numerical technique to get a stable and feasible solution at singular points. The results of the proposed algorithms are presented for 11-, 13- and 43-bus ill-conditioned test systems and for a 25-bus test system while performing single-line contingency studies, for those cases where the NR algorithm fails to converge.

Dynamic stability assessment of wind turbine generators using the Lyapunov function approach

Abstract This paper presents the application of the Lyapunov direct method (LDM) for dynamic stability assessment of a single wind turbine generator (WTG) system connected to an infinite bus under widely varying loading conditions. The proposed method is used to assess the dynamic stability for a given operating condition of P and Q very easily without calculating the overall system eigenvalues. In fact, the use of dominant complex eigenvalue loci plotted in the s -plane as constant P and Q contours to obtain the dynamic stability information for a given operating condition of the WTG system may not give the true picture because of complex surface shapes. The proposed method can be used to obtain the optimum reactive power loading for a given real power loading condition, corresponding to which the transient response of the system would be the best, by calculating a parameter defined as the dynamic stability measure. The system performance curves presented ( V - Q curves for constant P ) enable not only the optimum reactive power loading for a particular real power loading to be identified but also give some indication of the total optimal range in which it can be chosen from the point of view of fast transient response. Several cases are examined and the effects of the system and control parameters on the dynamic stability of the wind turbine generator are discussed. It is observed that the proposed LDM is computationally more efficient than the conventional eigenvalue analysis for dynamic stability assessment of the WTG system.

A heuristic linearized line outage contingency ranking of integrated multiterminal AC-DC power systems

Abstract This paper presents an efficient noniterative algorithm for the line outage contingency ranking of integrated multiterminal AC-DC systems. The proposed method can be used to obtain the contingency ranking based on the real power overload performance index. The proposed method uses the converter real and reactive powers, obtained by the base-case solution of AC-DC load flow, to transform the original AC-DC system to an equivalent AC system. The post-contingency voltages and angles of the transformed system are obtained either by a hybrid method or by using the first iteration 1 P - 1 Q of the AC fast decoupled load flow method. The hybrid method is based on an improved linearized method and the sensitivity relationship of the equivalent AC power system. The proposed hybrid method was tested on 5-bus, 14-bus and 30-bus AC-DC test systems having a DC link and DC mesh connections. For all the test systems, contingency rankings based on the real power overload performance index and the voltage deviation performance index were obtained by the proposed method and compared with the results obtained by the full sequential AC-DC load flow algorithm. The capture ratios obtained by the proposed method are excellent.

Optimum compensation to improve line loadability

Abstract This paper presents ‘optimum compensation requirement’ (OCR) curves to select the optimum series and shunt compensation to be employed for different loading conditions of EHV AC transmission systems. The OCR curves enable improvements to be made to the system loadability from the points of view of compensation effectiveness, total compensation MVAR requirement, line voltage profile and efficiency of power transmission. The results are reported for three compensation schemes selected from the point of view of voltage stability. The OCR curves are also shown for voltages of 760 and 1100 kV under different operating conditions.

STABILIZATION OF A CLASS OF INTERCONNECTED SYSTEMS USING AGGREGATION MODEL VIA SCHUR APPROACH

ABSTRACT An output feedback controller based on the aggregation model via Schur approch is presented. Equivalent aggreation models for global and each decoupled subsystem are obtained via Schur decomposion technique in order to generate global and local output feedback control signals. The effect of sub-sysyem state interactions is reduced by employing a global signal while the transient response of the system is regulated using a local control signal. Stability analysis of the proposed control scheme is discussed. The effectiveness of the proposed control scheme based on the aggregation model is studied by considering an interconnected power systems.

An improved linearized method for evaluation of bus voltage for line outage contingency

Abstract This paper presents an improved linearized method for contingency analysis which computes load-bus voltage following line outage. This method requires one base case AC load-flow and subsequently one DC load-flow for each contingency to determine the bus voltages. This proposed non-iterative method is applied to 14 and 30 bus IEEE test systems and compared with the existing method to show its superiority over it.