G. Yesuratnam

Comparison of Optimum Reactive Power Schedule with Different Objectives Using LP Technique

Three algorithms for reactive power optimization are proposed in this paper with three different objective functions. The objectives in the proposed algorithm are to minimize the sum of the squares of the voltage deviations of the load buses, minimization of sum of squares of voltage stability L-indices of load buses (?L2) algorithm, and also the objective of system real power loss (Ploss) minimization. The approach adopted is an iterative scheme with successive power flow analysis using decoupled technique and solution of the linear programming problem using upper bound optimization technique. Results obtained with all these objectives are compared. The analysis of these objective functions are presented to illustrate their advantages. It is observed comparing different objective functions it is possible to identify critical On Load Tap Changers (OLTCs) that should be made manual to avoid possible voltage instability due to their operation based on voltage improvement criteria under heavy load conditions. These algorithms have been tested under simulated conditions on few test systems. The results obtained on practical systems of 24-node equivalent EHV Indian power network, and for a 205 bus EHV system are presented for illustration purposes.

Congestion management for security oriented power system operation using generation rescheduling

In the literature many methods for overload relieving have been reported for determining a secure operating point. Most of these methods use conventional optimization techniques, which are generally time - consuming from a computation point of view, especially for large systems. Further the conventional optimization technique updates all the controllers for most of the operating conditions. Under emergency conditions the operator, has to make quick decisions, with little concern for the theoretical optimality of the operating point and also the operator can not move all the controllers to different settings within less time. In this context a simplified approach has been proposed in this paper for security oriented power system operation. The contribution of each generator for a particular overloaded line is first identified, then based on Relative Electrical Distance (RED) concept the desired proportions of generation for the desired overload relieving is obtained. Then based on the Generation Shift Sensitivity Factor (GSSF) concept the desired proportions of generation for the desired overload relieving is obtained. An attempt is also made to curtail the number of generators to be rescheduled based on GSSF for overload relieving Results obtained for network overload alleviation of IEEE 39-bus New England system are presented for illustration purposes.

Fuzzy-expert approach for voltage-reactive power dispatch

The voltage stability control problem has become an important concern for utilities transmitting power over long distances. This paper presents an approach using fuzzy set theory for reactive power control with the purpose of improving the voltage stability of a power system. To minimize the voltage deviations from pre-desired values of all the load buses, using the sensitivities with respect to reactive power control variables form the basis of the proposed fuzzy logic control (FLC). Control variables considered are switchable VAr compensators, on load tap changing (OLTC) transformers and generator excitations. Voltage deviations and controlling variables are translated into fuzzy set notations to formulate the relation between voltage deviations and controlling ability of controlling devices. The developed fuzzy system is tested on a few simulated practical Indian power systems and some IEEE standard test systems. The performance of the fuzzy system is compared with conventional optimization technique and results obtained are encouraging. Results obtained for a 24-node equivalent EHV system of part of Indian southern grid and IEEE New England 39-bus system are presented for illustration purposes. The proposed fuzzy-expert technique is found suitable for on-line applications in energy control centre as the solution is obtained fast with significant speedups

Monitoring the effects of on-load tap changing transformers on voltage stability

It is becoming increasingly important for power system planning and operating engineers to be capable of performing comprehensive voltage stability analyses of the systems. This need is largely due to recent trends towards operating systems under stressed conditions as a result of increasing system loads without sufficient transmission and/or generation enhancements. This paper discusses effect of the on-load tap changing transformers (OLTCs) on voltage stability and identifying critical OLTCs to avoid possibilities of voltage instability conditions. The developed approach has been tested on sample systems. Results obtained on an equivalent system of 24-node practical power system are presented.

Generators contribution towards loads and line flows-a case study

In many parts of the world, the goal of electricity supply industries is always the introduction of competition and a lowering of the average consumer price. Because of this it has become much more important to be able to determine which generators are supplying a particular load, how much use each generator is making of a transmission line and what is generators contribution to the system losses. In this paper a case study on generator contributions towards loads and transmission flows are illustrated with an equivalent 11-bus system, a part of Indian Southern Grid, based on the concepts of circuit flow directions, for normal and network contingency conditions

An expert system approach of congestion management for security and economy oriented power system operation

Alleviation of the emergency transmission line overload is a critical problem in the present day power system operation; hence a control strategy is necessary to effectively reduce the line overloads to the security limits in shortest time, by generation rescheduling and/or load shedding, not giving priority to the economic considerations. In the literature, many methods have been reported for determining a secure operating point. Most of these methods use conventional optimization techniques, which are generally time-consuming from a computation point of view, especially for large systems. Further the conventional optimization technique updates all the controllers for most of the operating conditions. Under emergency conditions the operator, has to make quick decisions, with little concern for the theoretical optimality of the operating point and also the operator can not move all the controllers to different settings within less time. In this context a simplified expert system approach has been proposed in this paper for security and economy oriented power system operation. The proposed approach is used to remove the congestion in the transmission lines by generation rescheduling with minimal cost involved in the rescheduling process. An attempt has been made for curtailing the number of generators to be rescheduled based on the incremental fuel cost of generators and the Relative Electrical Distance (RED) concept. Under emergency operating conditions the proposed approach helps the operator in the load dispatch centre for improving the system security with the rescheduling of few number of generators for overload relieving with less incremental fuel cost. The proposed approach has been illustrated with an IEEE 39 bus New England System.

Optimal Reactive Power Dispatch based on Voltage Stability Criteria in a Large Power System with AC/DC and FACTs Devices

An algorithm for optimal allocation of reactive power in AC/DC system using FACTs devices, with an objective of improving the voltage profile and also voltage stability of the system has been presented. The technique attempts to utilize fully the reactive power sources in the system to improve the voltage stability and profile as well as meeting the reactive power requirements at the AC-DC terminals to facilitate the smooth operation of DC links. The method involves successive solution of steady-state power flows and optimization of reactive power control variables with unified power flow controller (UPFC) using linear programming technique. The proposed method has been tested on a real life equivalent 96-bus AC and a two terminal DC system under normal and contingency conditions.

Optimum Reactive Power Dispatch and Identification of Critical On-load Tap Changing (OLTC) Transformers

Abstract The voltage stability problem has become a major concern in planning and operation of todays power system as a result of stressed conditions. It is known that the following disturbances, on-load tap changers (OLTCs), act to restore the load bus voltage and thus restore the load power to near pre-disturbance level. But the operation of OLTCs has a significant influence on voltage stability under some operating conditions. This article discusses the effect of OLTCs with different objectives of reactive power dispatch and identify critical OLTCs based on voltage stability criteria. Results obtained on an equivalent system of 24-node EHV system are presented.

Optimal reactive power dispatch based on particle swarm optimization and LP technique

Voltage stability enhancement is obtained by adjusting the control variables to nearer optimal solutions for the selected objective function. This paper presents the biological swarm behaviour of a bird flocking or fish schooling for solving the optimal reactive power dispatch (ORPD) problem using PSO algorithm. The main idea of this paper is to minimize the sum of the square of voltage deviations at load buses using fast decoupled load flow (FDLF) method and sensitivity parameters relating control and dependent variables. The on-load tap changing transformers (OLTC), Generator exciters, Static VAR Compensators (SVC) are used as control variables for improvement of system parameters like voltage deviation (Ve), L-index and real power loss. The effectiveness of the proposed method is demonstrated on IEEE New England 39-bus power system for near optimal solutions. The performance of the proposed method is compared with conventional optimization technique and presented for illustration purpose.

PSO based optimal reactive power dispatch for voltage profile improvement

This Paper presents an approach to solve the single objective optimal power flow problem considering the critical objective function of minimization of sum of squares of voltage deviations from desired voltages at all load buses while satisfying a set of constraints associated with the control variables such as generator excitations, tap positions of on-load tap changing transformers and switched var compensators and also dependent variable reactive power generation of all generators. This problem has been solved by using the Particle Swarm Optimization (PSO) method and the results obtained from this proposed method have been compared with the results obtained by the conventional Non-Linear Programming interior point method (NLP). To test the effectiveness of the proposed method, the results are obtained for a practical 24 bus EHV southern region equivalent Indian power system network. Analysis of results including effect on power loss, voltage stability indices as well as sum squared voltage deviations are done and superiority of the proposed method in comparison to NLP method is demonstrated.