Abstract-Resonance instabilities in power systems can be assessed with the positive-net-damping stability criterion. This criterion is a review of the complex torque coefficients method but it does not provide the frequency of the closed-loop oscillatory modes. This paper presents an alternative approach of the positive-net-damping stability criterion to analyze electrical resonance instability. In this approach, resonance instabilities are identified in feedback systems derived from impedance-based equivalent circuits. The proposed approach is used to characterize the frequency of closed-loop oscillatory modes and identify the physical and control parameters of the system that increase or reduce the damping of these modes. The extension of the proposed approach to study the stability of Single-Input Single-Output and Multiple-Input Multiple-Output feedback systems is analyzed and the approach is also compared with other stability methods in the literature. An example of an offshore wind power plant illustrates the theoretical study and compares the proposed approach with different methods to evaluate stability. Time-domain simulations in PSCAD/EMTDC are shown to validate the stability study.

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Quasi-Z-source inverters (qZSIs) are becoming a powerful power conversion technology in photovoltaic (PV) power systems because they allow energy power conversion in a single stage operation. However, they can cause system resonances and reduce system damping, which may lead to instabilities. These stability problems are well known in grid-connected voltage source converter systems but not in quasi-Z-source inverter (qZSI)-based PV power systems. This paper contributes with Matlab/Simulink and PSCAD/EMTDC models of qZSI-based PV power systems to analyze transient interactions and stability problems. These models consider all power circuits and control blocks of qZSI-based PV power systems and can be used in sensitivity studies on the influence of system parameters on stability. PV power system stability is assessed from the proposed models. The causes of instabilities are analyzed from numerical simulations and possible solutions are proposed.

Wind turbine harmonic emissions due to power electronics are a well-known power quality concern in wind power plants (WPPs). This problem may be increased by resonances because they amplify harmonic distortion around resonance frequencies. Hence, the resonance phenomenon is widely analysed in the literature. Resonance studies usually consider wind turbines (WTs) as ideal current sources, which can lead to inaccurate results. The study explores the effect of WT converter control on WPP harmonic response. It contributes expressions of WT equivalent harmonic impedance including current control of the grid side converter. This impedance allows WT converter control to be considered in WPP modelling to analyse its effect on harmonic response and resonances. PSCAD and Matlab/Simulink simulations were performed to validate the analytical expressions and illustrate the usefulness of the work in predicting harmonic distortions at WT terminals in actual WPPs.

Multi-terminal high voltage direct current (HVDC) systems, together with AC transmission systems and voltage source converters (VSCs), form hybrid AC/DC grids with complex dynamic and transient interactions. VSCs characterization taking into account DC-and AC-side dynamics in order to study these interactions is not yet well solved. This paper presents a three-port transfer admittance-based matrix model of VSCs that can be applied for such purpose. It is derived from dq-complex space vectors and characterizes both AC-and DC-side dynamics by relating AC-and DC-side current and voltages in a three-dimensional admittance transfer matrix which considers the VSC outer control loops. The paper also proposes a systematical procedure for studying multi-terminal HVDC hybrid AC/DC transmission grids by the Norton admittance method, where the proposed VSC model can be easily included in the Norton admittance matrix. This procedure allows the study of grid dynamics using impedance-based stability criteria. The proposed model and procedure are applied to a stability study in a multi-terminal HVDC hybrid AC/DC transmission grid. PSCAD/EMTDC simulations are used to validate the application.