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This paper proposes a simple procedure to identify the onset of long-term voltage instability from the time evolution of the distribution voltages controlled by load tap changers. The moving average of sampled measurements is computed and used to trigger local emergency signals. The method is validated on voltage signals obtained from time simulation of a realistic test system. The ability to identify instability in the critical area is demonstrated in three test cases. Index Terms--Emergency detection, load tap changer, long-term voltage stability, overexcitation limiter, voltage collapse.

Analytical description of the design of individual load-shedding schemes against voltage collapse in the Hellenic System is presented in this paper. Analysis and simulation results that validate the efficiency of these protection schemes are provided. This information may prove helpful for the design of similar load-shedding schemes in the future. Two event-driven load-shedding schemes based on design strategies proposed in this paper have been already implemented in the Hellenic System. Index Terms--Emergency control, load shedding, security assessment, system protection schemes, voltage stability.

In this paper, the problem of maximizing power-system loadability with multiple complementarity constraints representing generator limits is examined. The structure of the loadability surface is investigated, the various type of limits are classified and a general algorithm to optimize settings of control variables, in order to maximize loadability margin is developed. The importance of nonsmooth corner points (CPs) of the loadability surface that are due to the simultaneous occurrence of several loadability conditions is discussed. At such limits, maximization of loadability margin is performed based on techniques provided by nonlinear optimization theory. For this purpose an algorithm to identify multiple binding constraints on a CP is developed. Illustrative examples on small, but realistic systems are included. Index Terms--Complementarity constraints, corner point (CP), loadability limit, loadability surface, optimization.

This paper proposes a new method for the incorporation of the unit unavailability and the uncertainty of the load forecast in the solution of the short-term unit commitment problem. The above parameters are taken into account in order to assess the required spinning reserve capacity at each hour of the dispatch period, so as to maintain an acceptable reliability level. The unit commitment problem is solved by a simulated annealing algorithm resulting in near-optimal unit commitment solutions. The evaluation of the required spinning reserve capacity is performed by implementing reliability constraints, based on the expected unserved energy and loss of load probability indexes. In this way, the required spinning reserve capacity is effectively scheduled according to the desired reliability level. Numerical simulations have proven the efficiency of the proposed method. Index Terms--Probabilistic reserve assessment, reliability constraints, simulated annealing (SA), unit commitment (UC).

On July 12, 2004 at 12:39 local time, the south part of the Hellenic Interconnected Transmission System (including Athens) was split from the rest of the system and collapsed, driving consumers with a total load of 4500 MW into blackout. Technical information about the causes, the evolution, and the consequences of the blackout are presented in this paper. Some qualitative conclusions drawn immediately after the blackout are presented, as well as the results of further studies, including detailed simulations with corrected data, generator adjustments, and protection considerations. Index Terms--Armature current limit, blackout, loadability limit, voltage collapse, voltage instability.

This paper extends some previously obtained results on load tap changer (LTC) stability and derives detailed conditions for the stability of a dynamical system consisting of discrete LTC transformers based on Lyapunov's direct method and its extension according to LaSalle's Invariance Principle. The exact region of attraction of a two LTC system is constructed based upon tap position and controlled voltage values. Generalizations and extensions are finally discussed in the framework of extracting rules for post-load-shedding stability. Index Terms--Discrete systems, load shedding, load tap changers (LTCs), Lyapunov methods, power systems.

This paper discusses a few aspects of short-term voltage instability: modeling of transient overexcitation limiters of synchronous generators; possible outcomes of instability, such as induction motor stalling, or generator loss of synchronism leading to a local blackout; eigenvalue tracking and use of eigenvector to identify and characterize the instability mode; and finally, induction motor disconnection to counteract short-term voltage instability. Results on a system with large penetration of induction wind generators, as well as on a small test system, commonly used in voltage stability studies, are included. Index Terms--Eigenvalue tracking, eigenvector analysis, induction motor stalling, loss of synchronism, overexcitation limiter, voltage stability.

A new simulated annealing (SA) algorithm combined with a dynamic economic dispatch method has been developed for solving the short-term unit commitment (UC) problem. SA is used for the scheduling of the generating units, while a dynamic economic dispatch method is applied incorporating the ramp rate constraints in the solution of the UC problem. New rules concerning the tuning of the control parameters of the SA algorithm are proposed. Three alternative mechanisms for generating feasible trial solutions in the neighborhood of the current one, contributing to the reduction of the required CPU time, are also presented. The ramp rates are taken into account by performing either a backward or a forward sequence of conventional economic dispatches with modified limits on the generating units. The proposed algorithm is considerably fast and provides feasible near-optimal solutions. Numerical simulations have proved the effectiveness of the proposed algorithm in solving large UC problems within a reasonable execution time. Index Terms--Dynamic economic dispatch (DED), ramp rate constraints, simulated annealing (SA), unit commitment (UC).

Wind-power penetration in many cases faces significant barriers due to limited transmission capability. In Greece (and in most countries in Europe), the guaranteed access to the grid offered to wind parks, in conjunction with the existing planning practices, has led to severe limitations in wind power installed in certain congested areas. This paper presents some simple ideas that are currently being applied to the Hellenic Interconnected System, in order to increase the wind-power penetration. The main concept is the introduction of interruptible generator contracts and the control of the power flow through congested corridors by issuing power reduction commands, whenever security limits are violated. The proposed control is implemented using common PLCs of low cost and the existing SCADA system, and is both simple and transparent. The paper presents also a methodology to assess the impact of the proposed techniques on the wind penetration achieved. Index Terms--Grid integration, on-line monitoring and control, security assessment, transfer capacity, wind-power penetration.

This paper presents a Simulink-based educational tool developed for the purpose of illustrating power system control and stability notions as well as introducing students to realistic, though tractable in size, design problems. Relevant courses are taught to last-year undergraduate as well as graduate students at the University of Liege, Belgium, and the National Technical University of Athens, Greece. After a brief description of the corresponding curricula, the paper describes the simulation tool and gives examples of problems and assignments given to the students. Index Terms--Computer simulation, Matlab, power system control, power system stability, Simulink.

This paper reviews the general methodology of analysing voltage stability in the mid-term and in the transient time scale. It shows how a simplified simulation of the mid-term dynamics only can be combined with the analysis of 'snapshots' using a linearised model for the transient dynamics. Finally it points out how the stability of mid-term dynamics can be predicted using constant power loads in a transient time scale modelling.