A Passivity-Based Small-Signal DEF Analysis for Low-Frequency Oscillation Source Characterization of VSC-HVdc

In bulk power systems, detecting the sources of low-frequency oscillations can be challenging. The energy-based approaches for oscillation source localization are known to have better accuracy compared to others. The dissipating energy flow (DEF) method is one such approach with notable success in localizing real-world oscillation cases. While the mathematical justifications for the method's characterization of a device as an oscillation source (or sink) is well-exposed for synchronous generators and standalone inverters, no insights have been developed for power-electronics-interfaced dc transmission systems. To fill this gap, this paper presents a theoretical analysis of the DEF in a voltage source converter-based high voltage direct current (VSC-HVdc) system. Passivity-based analysis is performed to explain why a VSC-HVdc system, operating at unity power factor, with the commonly used control strategy involving constant real power control, dc-link voltage control, and ac voltage-reactive power droop control is a source of oscillation energy. Supporting case studies are performed on the IEEE 4-machine and IEEE 16-machine 68- bus test systems.