On-line Voltage Stability Assessment of Power System — An Approach of Black-box Modelling

The management of power systems has become more difficult than earlier because power systems are operated closer to security limits, environmental constraints restrict the expansion of transmission network, the need for long distance power transfers has increased and fewer operators are engaged in the supervision and operation of power systems. Voltage instability has become a major concern in many power systems and many blackouts have been reported, where the reason has been voltage instability. On-line voltage stability assessment is commonly based on a black-box modelling. Traditionally the model has been a two-dimensional security boundary, which is based on tabulation of data. The idea of the model-based approach presented in this study is to generalise the security boundary method and to take into account the uncertainty related to power system operation. The term “model-based approach” means in this thesis an approach which is based on black-box models. The model-based approach does not replace traditional analysis methods, rather these methods are urgently needed at the off-line computation step of the approach. The computation of the voltage stability margin is time-consuming and that is why these methods cannot be used in on-line applications. The objective of the study was to develop a method to be applicable in practice for on-line voltage stability assessment of long-term large-disturbance voltage stability based on black-box model. The basis of the approach is appropriate modelling of power system voltage stability and computation of the most critical voltage stability margins. A large number of power system operation points and the most critical voltage stability margins are computed to create the model. An essential part of model creation is the selection and extraction of model inputs, on which the accuracy of the model is mainly based. This thesis evinces new ideas for on-line voltage stability assessment of black-box model. The requirements and the proposed solution of all steps are presented to provide a step by step procedure for the use. The thesis describes first time an application of linear regression models to voltage stability margin approximation. The contributions of the thesis also include the improvement of maximum loading point algorithm, development of new voltage stability contingency ranking methods, and application of data analysis methods to voltage stability studies. The main results obtained in this thesis are an algorithm to compute the most critical voltage stability margin, a method to create a black-box modelling approach for on-line voltage stability assessment, and a method to approximate the most critical voltage stability margin accurately.