Abstract

In this paper a new method for power system stability analysis is introduced. The method is based on injection of a small voltage or current in an arbitrary point of the system. The apparent impedance is defined as the ratio between the voltage and current in the injection point. It is shown that the apparent impedance can be used to estimate the eigenvalues of the system that are observable from the injection point. The eigenvalues are obtained by applying the Vector Fitting algorithm to the measured set of apparent impedances. The proposed method holds some advantages over the well established impedance-based analysis method: It is no longer needed to estimate the source and load impedance equivalents separately, and it is not necessary to make any assumption regarding where the source and load are located. This reduces the required measurements and data processing. Furthermore, the stability analysis is global in the sense that the resulting stability margin does not depend on the injection point location. Finally, the method is well suited for real-time implementation due to low computational requirements. The method is outlined for DC-systems, while further work will extend the theory to cover single-phase and three-phase AC systems.

Highlights

  • Stability analysis of power systems is often conducted by small-signal methods

  • This paper proposes a new method for stability analysis called the apparent impedance method

  • The apparent impedance stability analysis method has been defined in this paper along with an example case study

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Summary

INTRODUCTION

Two branches of small-signal methods exist: impedance-based analysis and state-space analysis. Stability can be analyzed by applying the Nyquist Criterion to the ratio between source and load impedance. The main advantage of the impedance-based method is that stability can be analyzed from measurement/simulations in a single point in the system. This can be viewed as a black-box approach. Instead of applying the Nyquist Criterion, the method identifies the system eigenvalues based on the measured impedance values. A major advantage with the apparent impedance method is that the resulting stability estimate does not depend on the measurement location and provides a global stability margin. The number of measurements required for stability analysis is lower than the traditional impedancebased analysis

APPARENT IMPEDANCE DEFINITION
STABILITY ANALYSIS BY THE APPARENT IMPEDANCE
Estimating state-space model by the Vector Fitting method
SIMULATION RESULTS
Obtaining apparent impedances by simulation
Eigenvalue comparison
Nyquist criterion analysis
A B Unit circle
Transient response example
CONCLUSIONS
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