Contents I. Introduction
Pushing the power systems to their stability boundaries increases the risk of wide spread or partial blackouts. A robust electric power system is a well-planned system. The planning and implementation of a robust system is the long-term solution to the problems facing the power systems today. However, given the current state of the system, it is still possible to optimize the system performance by implementing automatic protection and control schemes [1] [2] [3]. A special protection system (SPS), or remedial action scheme (RAS), is designed to detect abnormal system conditions, and take preplanned, corrective actions (other than the isolation of faulted elements) to preserve system integrity and provide acceptable system performance. These actions include changes in system parameters, control settings or system configuration to restore the acceptable levels of system performance [4], [5]. Among the system wide disturbances, the important one is the voltage instability, which refers to the inability of a power system to maintain steady state voltages at all buses in the system after being subjected to a disturbance from a given initial operating conditions [6]. Voltage collapse is the process by which the sequence of events accompanying voltage instability leads to a blackout or abnormally low voltages in a significant part of the power system. The previous proposed schemes could be categorized as local and system wide schemes. In the local schemes each device decides and operates only based on the parameters that can be measured at the installation point of the device. The decision criteria for the device could be voltage magnitude or other voltage instability indicators that can be computed at the installation bus. Under voltage relays are the traditional common used devices to counteract voltage instability, locally [7]. Based on the magnitude and rate of voltage drop, these devices shed a predefined amount of load in several steps. A more advanced solution is to extract a voltage instability indicator. Given a simple two buses power system, one could extract an indicator by applying the required conditions at the voltage collapse point. The required conditions could be obtained from a voltage stability theory such as bifurcation theory [8] [9], energy function theory [10], or other approaches that are capable of analyzing stability of nonlinear dynamic systems. Some of local approaches for detection and evaluation of the voltage instability have been presented in [11] [12] [13] [14]. In [15], the change in the apparent power-line flow, during a time interval, is exploited for computing the voltage-collapse criterion. The second category contains wide area schemes that are designed to arrest voltage instability, globally. In [16], based on a wide-area network of phasor measurement units, the criticality of the current operating point has been determined using the estimates of the voltage-stability margin and the generator reactive-power reserves. In [17], based on the under voltage load shedding, a voltage stability protection scheme has been implemented. A more advanced scheme has been proposed for the Swedish network [18]. This scheme operates based on the combination of voltage magnitude data and reactive reserve data. Voltage stability protection scheme of the hydro Quebec is a comprehensive method for under voltage load shedding [19]. This scheme operates based on the voltage drop magnitude and its rate. Also an advanced version of this scheme has been proposed in [20]. In [21], the authors have implemented an emergency action in terms of secondary-voltage control and load shedding. Based on these remedies, the authors have designed two modes of co-ordination. Less effort has been done to implement remedial actions in such a way that the system cascading is reduced or eliminated, optimally. Based on the post contingency conditions, the here proposed scheme attempts to implement the most sensitive remedies in order to restore the power system from a cascaded collapse and immunes it against such severe contingencies. The present paper is organized as follows. In section 2, the overall structure and formulation of the proposed method is described. Also, in this section the formulation of the proposed voltage instability index is introduced. The numerical results of applying the proposed method over a test system are presented in Section 3. Finally a conclusion is provided in section 4.
