Alexander Petroianu

Design of suboptimal H/sup /spl infin// excitation controllers

This paper presents a method to design power system suboptimal robust excitation controllers based on H/sup /spl infin// control theory. The suboptimal controller results from additional constraints that are imposed on the standard optimal H/sup /spl infin// solution. Global stability constraints are incorporated into the H/sup /spl infin// algorithm to ensure stability of the interconnected system under decentralized control. Furthermore, a Lyapunov-based index is used to evaluate the robustness properties of the closed loop. In order to obtain a reduced order controller, the method of balanced truncation is used. The suboptimal H/sup /spl infin// controllers are output feedback controllers. These controllers posses superior robustness as compared to a conventional PSS and optimal H/sup /spl infin// controllers.

Assessment of commercially available software tools for transient stability: experience gained in an academic environment

There are many power system simulation software tools for transient stability analysis that are commercially available today. The choice of the most adequate tool for a specific type of stability analysis is a difficult task, and is influenced by economical reasons. It is imperative that a comparative study of various tools be carried out to help users make informed decisions. This work evaluates DIgSILENT, PSS/E and PSCAD, which are commercial software tools used for digital simulation of power systems. As criteria for software assessment, the following features are investigated: (i) modelling capabilities - synchronous generator, generator saturation, transmission line representation, and external network, (ii) flexibility allowed to the user. A well known benchmark power system model from literature is used to validate the simulation results of the commercial tools.

ANALYTICAL INVESTIGATION OF THE EFFECT OF GENERATOR MODELLING ON ELECTROMECHANICAL MODE DAMPING

Abstract Power system analytical tools differ in their components modelling. The differences affect electromechanical modes damping. This paper investigates the effect of including rotor speed deviation in stator voltage calculation–with the stator transients neglected- and the modelling of turbine output, on electromechanical mode damping of a single machine infinite bus system. We use a sixth order generator model with different excitation control configurations. We analyse results obtained with EUROSTAG and compare these with results obtained with three other industrial-grade tools. Our results show that: (i) if rotor speed deviation is included in the stator voltage calculation, the results are more conservative than those obtained if speed deviation is neglected. (ii) if the turbine model output is torque, the results are more conservative than those obtained if the output model is power.

Effect of variation in modelling of generator saturation on electromechanical modes

Several industrial-grade power system simulation tools are commercially available in the market. They are expensive to acquire and time-consuming to learn. As a result, very few institutions (utilities, academic/research organizations) can afford to use more than one power system simulation tool. The simulation tools differ in their components modelling. Different simulation tools can give different results for the same system model. Understanding the reasons for the discrepancies in the results is not a trivial task. In this paper, we investigate the effect of variations in modelling generator saturation. We carried out an eigenvalue analysis of a single machine infinite bus (SMIB) system using PSS/E, PowerFactory, EUROSTAG, SSAT, and MatNetEig. The simulation results show that the effect of saturation on the local modes is erratic and there is a need for further research on saturation representation in power system stability simulation tools.

STABILITY ANALYSIS OF AN ISLANDED GENERATOR

Abstract The authors investigate the stability of an islanded generator. They examine the effect of a predominantly resistive load on small-signal (frequency) stability of an islanded generator. The authors derive a new linear generator model for the islanded power station, and perform small-signal analyses using this model. The analyses are also performed using two industrial–grade software packages, and the results are compared. The authors show that predominantly resistive loads lead to poor damping of the speed response, and that an increase in the L/R ratio further decreases the damping. The use of a power system stabilizer is proposed to increase the damping of the speed response by varying the generator terminal voltage in response to speed changes. In this sense the PSS is also contributing to frequency control. Finally, the authors study the effect of the turbine and its control system on stability.

APPLICATION OF AFFINE TRANSFORMATIONS TO REAL-TIME POWER SYSTEM EMS FUNCTIONS

Abstract The paper introduces the concepts of affine transformations in the methodology and in the algorithms for real-time power system EMS functions. It demonstrates the advantages of this new conceptual approach. The advantages are: i) a new formulation of the state estimation problem and related applications, ii) a faster and more reliable solution obtained through direct, non-combinatorial techniques.

AN AFFINE GEOMETRICAL APPROACH TO POWER SYSTEMS PROBLEMS

Abstract The paper introduces modern concepts and tools from affine geometry into power system analysis. It is shown that such an approach allows: i) a new non linear formulation of such classical problems as load flow and state estimation, ii) a more efficient way of solving such problems through non iterative methods. The new approach is illustrated for a small but representative example of a load flow for a two-bus network.

Tuning of PSS Based on Sub-Optimal H∞ for Natal System

Abstract PSS tuned using classical techniques often do not result in a robust or well-tuned PSS. In this paper, a tuning method is presented using sub-optimal H ∞ . This method has many of the benefits of optimal H ∞ and has the advantage of keeping the PSS structure and improving the overall damping of all weakly damped modes in the power system. In this paper the method will be applied to a model of the Natal Hydro-Electric Plant in South Africa. It will be shown that the PSS tuned using this method is more robust than PSS tuned using classical techniques.

Control of the Static Voltage Stability Margin Using Optimal Power Flow

Abstract This paper presents an optimal strategy for the control of the static voltage stability margin (SVSM) in stressed power systems. The optimal control strategy is determined by an Optimal Power Flow (OPF) using existing reactive power control devices. The objective of the OPF is the maximisation of a static voltage stability index (MVSI). A conceptual real-time implementation of the proposed optimal control strategy is outlined and the optimal control strategy is applied to a 16-bus network. It is demonstrated that the SVSM can be significantly enhanced by using a static voltage stability index instead of the reactive power losses as objective function of the OPF.

An Algorithm for Robust Estimation in Electric Power Systems

Abstract The properties of the robust estimation methods based on elemental sets are presented. Sampling of elemental sets is the important part of the methods. Few techniques for sampling are compared. The Linear Programming (LP) algorithm with re-weighting strategy is developed in order to limit the number of samples used. Extreme Studentized Deviate (ESD) method is applied for sample evaluation. Together LP with re-weighting strategy and ESD method are able to process bad data efficiently. The new approach succeeds in treating bad leverage points. The robustness of the method is demonstrated on the small linear regression example and on the state estimation problem in IEEE 14-bus power system