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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

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.

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.

Part II. Examines the influence of load characteristics on the stability margins of the three high voltage direct current (HDVC) system configurations. Functions of the maximum power and voltage sensitivity based methods; Identification of the three types of HDVC system configurations; Information on the reactive static load characteristics.

Part I. Examines the stability boundary conditions of various maximum power and voltage sensitivity based techniques, for several high voltage direct current (HVDC) configurations combining load characteristics. Reasons for the introduction of different concepts of maximum available power and voltage sensitivity; Information on single-infeed HVDC systems with a parallel alternating current line.