Voltage and power stability of HVDC systems
Abstract: This research work had been motivated by the emergence of multiple HVDC links in close proximity within a power system area, such as those in Southem Scandinavia and Northem Europe. Such system tonfigurations, generally known as multi-infeed HVDC systems, are expected to exhibit voltage and power stability problems when one or more of the constit uent AUDC interconnections are weak. The primary objective of this work is therefore to study the voltage and power stability of HVDC systems under these situations, with an emphasis on multi-infeed configurations. In the tirst part of this work, the voltage and power stability of multi-infeed HVDC systems are studied from a quasi-static approach. For this purpose, existing analytical concepts and methods for weak single ACfDC interconnections have to be extended or new ones developed. A mathematital technique known as modal analysis is the basic analytical tool employed. Modal anal ysis has been successfully applied to the voltage stability analysis of AC power systems but its use for analysis of multi-infeed HVDC systems is unprecedented. In the tontext of multi-infeed HVDC systems, there are salient differentes such as the incorporation of voltage sensitivity and maximum power concepts which are pertinent to HVDC applications. Here, the application techniques of modal analysis are fully developed into a comprehensive tool suitable for the voltage/power stability analysis of multi-infreed HVDC systems. These include determining the voltage/power stability margins, criticality of system locations, and effectiveness of remedial measures with respect to multi-infeed HVDC systems. The developed techniques are applied to a practical power system and has shown to be feasible and suitable for use by the HVDC industry. Sinte the developed concepts and methods take root from those for the single ACIDC interconnection, the study also tompares them to show their fundamen tal similarities and differentes.Voltage stability and loads of power systems are closely intertwined. In the tontext of AUDC interconnections, a recent study had also shown the influence of static load charactetistics on the power stability of a specific HVDC system model. An original intent was to show these similar influences for multi-infeed HVDC systems but it was also essential to consider other system contigurations to cast the investigations into a more general and fundamental tontext. Thus HVDC system models representa tive of typical system tonfigurations are used, viz. a single-infeed, single-infeed with a parallel AC line, multi-infeed system configuration. It is shown in this work that static load characteristics have a consistent pattem of influence on the power/voltage stability of these system tonfigurations. It is thus possible to introduce general concepts of Active Load Characteristics Index (ALCI) and Load Characteristics Sensitivity (133) to estimate a bound for and characterize these influences. An important ob jective of these investigations relating to static load characteristics is to show that their effects do not invalidate the analytical concepts and methods developed for the multi-infeed HVDC system.In the second part of this work, voltage and power stability of HVDC systems are studied from a dynamit approach. For power stability, the impact of dynamit system modelling on the power stability limit of ACIDC interconnections is examined. Com mon industry practice assumes certain quasi-static conditions pertaining to the voltage control in the receiving AC system. It is shown that voltage dynamits has a significant impact on the power stability limit based on the quasi-static assumptions. Quasi static analysis also ascribes voltage instability of AUDC interconnections to the singularny of the powerflow Jacobian. This may be viewed from a nonlinear dynamital system perspective, as the system having undergone a saddle-node bifurcation. Consequently, an aim is to establish the relationship between the quasi-static and dynamit conditions for voltage instability via saddle-node bifurcation. Within the framework of nonlinear dynamital system theory, another possible mechanism of voltage instability is the Hopf bifurcation. This oscillatory voltage collapse mechanism is shown to occur in ACIDC systems. In these tontexts, fundamental conditions for existence of these nonlinear phenomena are derived analytically for the single-infeed HVDC system. The ultimate aim is to extend these results to investigate the nonlinear phenomena in multi-infeed HVDC sys tems.
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