Impact of State-Space Modelling Fidelity on the Small-Signal Dynamics of VSC-HVDC Systems

The main purpose of the paper is to investigate the effect of state-space modelling fidelity on the small signal stability characteristics of HVDC systems based on Voltage Source Converters (VSCs). For this purpose, a small-signal state-space model of a point-to-point VSC-based HVDC connection, including the dynamics of the ac networks and the converter control loops is developed, and verified by comparison to time responses of a detailed model including the nonlinear effects. Critical modes of the system under worst case operating conditions are investigated by participation factor analysis to show the influence of different states and identify the physical effects associated with the various eigenvalues. The same testcase is investigated by using DigSilent Power Factory (P.F.), as an example of a commercial power system simulation tool, to assess small-signal stability and dynamic behaviour of the system. Thus, the effects of commonly used simplifications for large-scale power system studies on the stability assessment of a VSC-based HVDC transmission scheme are identified, and their influence on the location of critical poles in the system is analysed. The results are used as a basis towards general guidelines on how to assess the results from traditional power system stability studies of networks containing VSC-based transmission schemes and to shed light on the impact of the simplifications on the stability estimate.