Two-Port Characterization and Transfer Immittances of AC-DC Converters–Part II: Applications

Abstract

This two-part paper presents a general method to model three-phase ac-dc converters as two-port networks for power system stability studies. Part I of the work has developed the two-port network models and identified two types of transfer characteristics that are unique to ac-dc converters and important for system stability. Part II presents the applications of the developed models in stability analysis of different power systems that use ac-dc converters, including grid integration of converterbased generation from wind, PV and other renewable sources, grid-connected dc microgrids and dc distributed power systems, high-voltage dc transmission, as well as hybrid ac-dc power systems. While impedance-based stability analysis has been studied for each of these systems, the new two-port models, especially the transfer immittance models, make it possible and easy to formulate more complete system models to include dynamics and coupling effects that have been ignored or impossible to consider in the past. The methods to study the stability of each type of systems are explained and illustrated by case studies that also highlight the effects of coupling modeled by the transfer immittances on system stability. Each case study also exemplifies a practical stability concern in converter-based power systems.

On the Applicability of SISO and MIMO Impedance-Based Stability Assessment of DC–DC Interlinking Converters

“…Still, in the existing literature on the SISO impedance based stability assessment [2], [4], [6], [10], [11], only Approach 2b is typically used, applied to one of the ports, i.e., a single L LAAT ii (s) = Z t gii (s)Y t ii (s) is assessed. However, this is not generally applicable, since it does not always guarantee accurate BIBO stability prediction (as elaborated in the previous subsection), due to possible, unstable port-level hidden dynamics 8 .…”

“…As a partial generalization, evaluation of all L LAAT ii (s) is sometimes performed (which corresponds to applying the SISO method at every port) [5], [6], [8], [16]. For example, in [5], stability of the voltage source converter in non-meshed HVDC system featuring unstable port-level hidden dynamics at the dc port could not have been accurately predicted by applying the SISO impedance based method to that port.…”

“…Some examples of this are reported in [5], [8], [16], [17] for radial (non-meshed) systems. Along the line of unstable port-level hidden dynamics, further limitations of the portlevel impedance-based method may arise when, to enhance reliability and flexibility, instead of radial, the system features a meshed structure [8], [18]. Moreover, for such systems, even in absence of any unstable port-level hidden dynamics, this method loses intuitiveness as the definition of the terminated immittances provides little physical insight when the ports are fully coupled.…”

“…This port-level impedance-based method is hereafter also referred to as the single-input single-output (SISO) one. Although widely used in the literature [2], [4]- [14], when applied to a single port only, this method may lead to a wrong prediction of stability [5], [8]. This occurs when at the considered port the system features unstable portlevel hidden dynamics, i.e., when the unstable poles do not appear in the transfer functions corresponding to that port [15].…”

“…This occurs when at the considered port the system features unstable portlevel hidden dynamics, i.e., when the unstable poles do not appear in the transfer functions corresponding to that port [15]. Some examples of this are reported in [5], [8], [16], [17] for radial (non-meshed) systems. Along the line of unstable port-level hidden dynamics, further limitations of the portlevel impedance-based method may arise when, to enhance reliability and flexibility, instead of radial, the system features a meshed structure [8], [18].…”