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The task of controlling a power system is generally accomplished by several entities, each responsible for a distinct part of the system. For the enhancement of the system security, controllable devices such as FACTS devices are installed and operated by these entities. But in order to avoid conflicting behavior coordination is needed, leading to the concept of multiarea control. In this paper, it is assumed that the areas are determined independently from each other according to the influence of the control devices. As a consequence, overlapping areas may result necessitating an extension of the applied multiarea control method based on approximate Newton directions. In addition, it is taken into account that part of the grid might not be included in any of the areas. Index Terms--FACTS, limited area control, multiarea control, optimal power flow.

This paper presents an approach for combined optimization of coupled power flows of different energy infrastructures such as electricity, gas, and district heating systems. A steady state power flow model is presented that includes conversion and transmission of an arbitrary number of energy carriers. The couplings between the different infrastructures are explicitly taken into account based on the new concept of energy hubs. With this model, combined economic dispatch and optimal power flow problems are stated covering transmission and conversion of energy. A general optimality condition for optimal dispatch of multiple energy carriers is derived, and the approach is compared with the standard method used for electrical power systems. Finally, the developed tools are demonstrated in examples. Index Terms--Cogeneration, energy hub, multiple energy carriers, natural gas, optimization methods, power conversion, power generation dispatch, power system modeling, power transmission.

The problem of defining and classifying power system stability has been addressed by several previous CIGRE and IEEE Task Force reports. These earlier efforts, however, do not completely reflect current industry needs, experiences and understanding. In particular, the definitions are not precise and the classifications do not encompass all practical instability scenarios. This report developed by a Task Force, set up jointly by the CIGRE Study Committee 38 and the IEEE Power System Dynamic Performance Committee, addresses the issue of stability definition and classification in power systems from a fundamental viewpoint and closely examines the practical ramifications. The report aims to define power system stability more precisely, provide a systematic basis for its classification, and discuss linkages to related issues such as power system reliability and security. Index Terms--Frequency stability, Lyapunov stability, oscillatory stability, power system stability, small-signal stability, terms and definitions, transient stability, voltage stability.

Common industry practice assumes certain quasi-static conditions to determine the power stability of HVDC systems. Particularly, a constant Thevenin AC voltage source has become a de facto industry assumption. This work presents a dynamic approach to study the power stability of HVDC systems. Based on this approach, the impact of dynamic system modelling on the power stability limit of HVDC systems is examined. Consequently this provides an insight into whether the quasi-static assumptions are justified. The qualitative and quantitative impact of system dynamics and associated parameters, respectively, on the quasi-static maximum power curve are shown. These are shown using dynamic time-simulations and mathematical analysis, the close correspondence between their results also mutually verify the two approaches. Keywords: dynamic system modelling, dynamic and quasi-static maximum power, power stability, HVDC systems

Voltage instability in AC power systems has been widely studied from a nonlinear dynamical system perspective. In contrast, this approach for HVDC systems is of recent origin and rudimentary, therefore much basic work in this area remains to be developed. Thus this work presents a comprehensive and fundamental analysis of the nonlinear phenomena in HVDC systems. In particular the conditions for occurrence of nonlinear phenomena are derived analytically for a basic and enhanced dynamic HVDC system models. These results consequently identify: (i) the various system and HVDC control parameters which influence the nonlinear behaviour (ii) oscillatory instability in HVDC systems as a feasible voltage collapse mechanism (iii) the explicit relationship between the quasi-static and dynamic conditions for aperiodic voltage collapse. Keywords: nonlinear dynamics, bifuractions, voltage stability, HVDC systems

In this work, the well known concept of maximum power for analyzing the power stability of single-infeed HVDC systems, is extended to the multi-infeed HVDC situation. based on this concept, two approaches to analyze the power stability of multi-infeed HVDC systems are derived. One approach uses the standard maximum power curve technique while the other uses an eigenvalue decomposition technique. These approaches are not equivalent, thus, comparisons between them and other power/voltage stability analysis methods are also made. KEYWORDS Power stability, maximum power concept, maximum power curve, eigenvalue decomposition, multi-infeed HVDC systems

This paper explores the influence of load models on decisions of undervoltage load shedding in power systems. A controlled load rejection can be used as an emergency countermeasure to avoid widespread blackout when system voltages are unstable. In this paper, dynamic simulations of a small power system using both static and dynamic load models are presented. When using a static load model, the system includes an explicit model of a transformer with load tap changer. The aim is to demonstrate how different load models influence the analysis and calculation of the amount of load needed to be shed to stabilise the system voltage. Keywords: Load modelling, Load shedding, Long-term dynamics.

In recent works, a method based on the eigenvalue decomposition technique, known as modal analysis, had been applied to the voltage stability analysis of multi-infeed HVDC systems. In this method, not only the critical eigenvalues are useful in evaluating the voltage stability of the system, but their associated eigenvectors also contain information on the most severe disturbances to the system. In this paper, the use of these modal information for such analysis is further exploited. In particular, has participation factors are used to identify the critical AC commutation bus for effective remedial measures to be implemented. The significant influence of system parameters on the bus participation factors are also investigated. KEYWORDS modal analysis, bus participation factors, voltage stability, multi-infeed HVDC system

With the increased use of HVDC transmission systems in power systems, situations have arisen and will be even more frequent in the future, where several HVDC systems are located in the vicinity of each other. It is evident that interactions between the different HVDC systems will occur in such configurations, and it is of importance to analyze such systems in a systematic way to ensure that there are no risks of adverse interactions. A method of analysis to cope with this task is presented and motivated in this paper. This method is demonstrated by application to a system with dual HVDC converters, and using the identified factors influencing the interactions, its practical realization is also illustrated. KEYWORDS Voltage stability, multi-infeed HVDC system, eigenvalue decomposition.

Concepts of maximum power transfer of dc systems and associated ac voltage variations, particularly at inverter stations having low short - circuit ratios, have been extended to include various ac load models and an ac line in parallel with the dc line. The operating capabilities are shown to vary from those predicted from either a Thevenin ac source model or the corresponding short-circuit ratio. The study used an ac/dc load flow program. Keywords - HVDC, short-circuit ratio, power transfer, ac/dc interaction.

Controllable Series Devices (CSD), i.e., series-connected Flexible AC Transmission Systems (FACTS) devices, such as Unified Power Controller (UPFC), Controllable Series Capacitor (CSC) and Quadrature Boosting Transformer (QBT) with a suitable control scheme can improve transient stability and help to damp electromechanical oscillations. For these devices, a general model, which is referred to as injection model, is used. This model is valid for load flow and angle stability analysis and is helpful for understanding the impact of the CSD on power system stability. Also, based on Lyapunov theory a control strategy for damping of electromechanical power oscillations in a multi-machine power system is derived. Lyapunov theory deals with dynamical systems without inputs. For this reason, it has traditionally been applied only to closed-loop control systems, that is, systems for which the input has been eliminated through the substitution of a predetermined feedback control. However, in this paper, we use Lyapunov function candidates in feedback design itself by making the Lyapunov derivative negative when choosing the control. This control strategy is called Control Lyapunov Function (CLF) for systems with control inputs. Index Terms--CSC and CLF, FACTS, QBT, UPFC.

This paper examines the enhancement of power system stability properties by use of thyristor controlled series capacitors (TCSCs) and static var systems (SVCs). Models suitable for incorporation in dynamic simulation programs used to study angle stability are analyzed. A control strategy for damping of electromechanical power oscillations using an energy function method is derived. Using this control strategy each device (TCSC and SVC) will contribute to the damping of power swings without deteriorating the effect of the other power oscillation damping (POD) devices. The damping effect is robust with respect to loading condition, fault location and network structure. Furthermore, the control inputs are based on local signals. The effectiveness of the controls are demonstrated for model power systems. Index Terms--Control strategy, energy function, interaction, local variables, POD, power swings, SVC, TCSC.