Small-signal stability analysis of the asymmetrical DC operation in HVDC networks

Abstract

• Stability of interconnection of monopolar and bipolar HVDC stations is analysed. • A small-signal stability model for asymmetric DC operation studies is developed. • The model is validated with EMT dynamic simulation. • The findings are useful for systematic design of the HVDC grounding systems. • A controller is designed to enhance stability during asymmetric DC operation. The increase in High Voltage Direct Current (HVDC) projects favors their future interconnection to form regional networks, but this interconnection can be hindered by the lack of standardization in station's configuration and converter's differing grounding schemes. Whilst other studies have studied the effect of Direct Current (DC) asymmetrical operation, they are limited in their scope. This paper proposes a more realistic perspective, considering stations with different topologies, permanent DC asymmetries and the influence of the grounding systems. This more complex point of departure requires the re-examination of the modeling methods, from time-domain to frequency-domain. This paper proposes a small-signal stability model of the system, which is validated with the electromagnetic transient (EMT) dynamic simulations using PowerFactory. This model provides guidelines for designing the grounding impedance and allows the proper design of a specific controller to dampen the asymmetrical DC operation mode.