Methodology for the analysis of dc-network resonance-related instabilities in voltage-source converter-based multi-terminal HVDC systems

A method to investigate dc-network instabilities in multi-terminal high-voltage DC (HVDC) systems is presented. The method consists of defining subsystems, called in this Letter Z and Y, which can be interpreted as a dc-network impedance matrix and as set of converter admittances, respectively. Through the analysis of the Z and Y terms, it is shown that resonances originated from the system's dc network can be amplified if the converters show a negative dc-side conductance. Moreover, it is also shown that converters located at different terminals can contribute to instability if the resonances to which they are exposed are of the same frequency. The analysis method is applied to a four-terminal HVDC system as an example and, using the theory developed in this Letter, mitigation measures are applied. The results are verified through time-domain simulations.