Abstract-A new load shedding scheme against long-term voltage instability is proposed. It uses a set of distributed controllers, each monitoring transmission voltages in a zone and controlling a group of related loads. Each controller acts in closed-loop, shedding amounts that vary in magnitude and time according to the evolution of its monitored voltage. The whole system can operate without information exchange between controllers, the latter being implicitly coordinated through network voltages. The operation, design and robustness features are illustrated through simulations of a real system. Index Terms-distributed control, system protection scheme, undervoltage load shedding, voltage stability.

Abstract-An approach inspired by model predictive control is proposed to determine a sequence of control actions aimed at alleviating thermal overloads. The algorithm brings the line currents below their limits in the time interval left by protections while accounting for constraints on control changes at each step. Its closedloop nature allows to compensate for model inaccuracies.

Abstract-A new design of load shedding against long-term voltage instability is proposed. It uses a set of distributed controllers, each monitoring a transmission voltage, controlling a group of loads, acting in closed-loop, and adjusting its action to the voltage evolution. The whole system operates without information exchange between controllers.Index Terms-Distributed control, system protection scheme (SPS), undervoltage load shedding, voltage stability.

Abstract-This papers deals with the emergency control of Load Tap Changers (LTCs) to face low transmission voltages or voltage instability situations. The proposed simple control logic consists in reverting the tap movements once the voltage at a monitored transmission bus falls below some threshold. A deadband on this voltage allows the system to settle down in between the normal and reverse logic modes. In order to control a large number of LTCs, the latter are divided into clusters, each with its own monitored voltage. The paper also considers the control of two levels of LTCs in cascade, where proper coordination is required between the two levels. The proposed scheme has been tested on a detailed EHV-HV-MV planning model of the Western region of the French transmission system operated by RTE. Long-term time responses to major disturbances are shown to illustrate the performance of the proposed scheme.

Abstract-An approach inspired of Model Predictive Control is proposed to determine a sequence of control actions aimed at alleviating thermal overloads in emergency conditions. The algorithm brings the line currents below their limits in the time interval left by protections, while accounting for constraints on control changes at each step. Its closed-loop nature allows to compensate for measurement noise and model uncertainties.

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This paper is the continuation of Ref. [9], where the impact of the induction motor loads on a load shedding protection scheme designed to deal with long-term voltage instability, initialy proposed in [10], was under study. For the load shedding scheme to cope with the fast response of motor loads, additional information exchange was required in order to enable the protection action with a reduced time delay. Therefore, the protection scheme changed from a purely distributed to wide-area, although simplicity was maintained. Moreover, the latter preserved important features such as closed-loop operation and redundancy between controllers.The proposed scheme was successfully tested under severe situation, as it was assumed that only non-motor loads can be shed.B. Otomega (bogdan.otomega@yahoo.com) is senior lecturer in the Power Systems Dept. of the "Politehnica" University of Bucharest, Romania. Induction motors due to their ability to reaccelerate after a fault, play an important role in short-term voltage instability. The objective of the paper is to report on the extension of the protection scheme dealing with short-term voltage instability.Furthermore, this paper will also compare the effect of shedding motor vs. non-motor loads. II. PRINCIPLE OF THE COMBINED PROTECTION SCHEME A. Protection against long-term voltage instabilityThe most important settings of an undervoltage load shedding scheme are the voltage threshold V th below which the controller starts curtailing load, and the delay τ before loads are effectively disconnected.In long-term voltage instability scenarios, where voltage degradation is precipitated by generator field current limitations, the voltage threshold value V th LT has to be set high enough, typically in the range [0.8 0.9] pu, as illustrated in Fig. 1 with the rightmost curve. The main consequence is that the corresponding delay τ LT should be large enough in order not to shed load inadvertently in case of a normally cleared fault. This is illustrated in Fig. 2, showing the minimum delay τ min LT that could be chosen when V th LT is set to 0.90 pu. Shorter delays could be considered if, at the same time, V th LT was set to a lower value.In the presence of induction motor loads, the final voltage collapse can be very fast, as illustrated by the five curves in the middle of Fig. 1, corresponding to various proportions