Preventive assessment and enhancement of power system voltage stability: an integrated approach of thermal and voltage security

This thesis is devoted to the preventive assessment and enhancement of voltage
stability and security in electric power systems. However, in the course of
deriving all the proposed methods we have paid attention to keeping them
compatible with the (more traditional) handling of thermal overloads, thereby
providing a unified treatment of voltage and thermal security.

The approaches presented in this work apply to both deregulated environments
and classical, vertically integrated ones.

The heart of most methods developed in this thesis is : (i) the derivation of
sensitivities indicating the relative efficiency of the various bus injections
to restore voltage stability or increase an insufficient voltage security
margin, and (ii) the use of these sensitivities in linearized security
constraints that can be incorporated to various optimization problems.

Using this formulation, we deal with three different problems of interest in
preventive security analysis:
1) Congestion management. We propose two optimization-based
approaches to manage congestions due to voltage instability and/or
thermal overload. The control variables are either power injections (generation
rescheduling and load curtailment) or power transactions;
2) Computation of Available Transfer Capabilities (ATCs). We determine
the simultaneous ATCs of multiple transactions by means of a single
optimization-based computation;
3) evaluation of security margins interval. To face the uncertainty
affecting power transfers, we present an optimization-based computation of the
minimal and maximal margins under the assumption that individual injections
vary within specified bounds.
Besides this main theme, the thesis offers additional reflections on the:
1) Filtering of contingencies. We propose a simple and reliable technique
to filter out harmless contingencies when computing voltage security margins of
a large set of contingencies;
2) Evaluation of reactive reserves with respect to a contingency, an
important topic for voltage security reasons as well as within
the context of a deregulated market where providing reactive reserves is an ancillary
service which should be properly paid.

Most of the methods proposed in this thesis were successfully tested on
realistic power system models.

From a practical viewpoint all the above computations have been coupled to the
fast time-domain quasi steady-state simulation used in the ASTRE software
developed at the University of Liège.