Introduction

Power system voltage control has a hierarchy structure with three levels: the primary, secondary, and the tertiary voltage control. Over the past 20 yrs, one of the most successful measures proposed to improve power system voltage regulation has been the application of secondary voltage control, initiated by the French electricity company, EDF, and followed by some other electricity utilities in European countries.

The secondary voltage control closes the control loop of the references value setting of controllers at the primary level. The primary objective of secondary voltage control is to achieve better voltage regulation in power systems. In addition, it brings in the extra benefit of improvement of power system voltage stability, for this application, several methods to design secondary voltage controllers have been proposed.

The useful concept of secondary voltage control is explored for a new application-the elimination of the voltage violations in power system contingencies. For this particular application, the coordination of various secondary voltage controllers is proposed to be based on a multi agent request -and- answer type of protocol to between any two agents. The resulted secondary voltage control can only cover the location where voltage controllers are installed. This paper presents results of significant progresses in investigating this new application to eliminate voltage violations in power system contingencies via secondary voltage control.

A collaboration protocol, expressed graphically as finite state machine, is proposed for the coordination among multiple FACTS voltage controllers. The coordinated secondary voltage control is suggested to cover multiple locations to eliminate voltage violations in the adjacent locations to a voltage controller. A novel scheme of a learning fuzzy logic control is proposed for the design of the secondary voltage controller. A key parameter of the learning fuzzy logic controller is proposed to be trained through off-line simulation with the injection of artificial loads in the controller's adjacent locations.


FACTS (Flexible AC Transmission Systems)

Sudden changes in the power demands or changes in the system conditions in the power system are often followed by prolonged electromechanical oscillations leading to power system instability. AC transmission lines are dominantly reactive networks characterized by their per mile series inductance and shunt capacitances. Suitably changing the line impedance and thus the real and reactive power flow through the transmission line is an effective measure for controlling the power system oscillations and thereby improving the system stability.

Advances in high power semiconductors and sophisticated electronic control technologies have led to the development of FACTS. Through FACTS the effective line impedance can be controlled within a few milliseconds time. Damping of the power system oscillation is possible through effective changes in the line impedance by employing FACTS members (SVC, STATCOM, UPFC etc).