Minimum SCR and Impact of PLL Gain on Flat Start Capability on a Point to Point VSC-HVDC Interconnection

This paper investigates the minimum grid strength requirements to the interconnection between two islands using HVDC, which are considered as weak or very weak networks, e.g., Sumba and Sumbawa islands in Indonesia. Renewable energy source power is transmitted from one island to the other one through an HVDC interconnector. However, the electricity network in the landing island is underdevelopment, with non-uniform 70 - 150 kV long transmission lines between generators and loads; therefore, its grid strength can be considered weak or very weak grid depending on transferred power. Simulations will consider different scenarios of power transfer through the HVDC link. The minimum AC-side grid strength in the form of short circuit rating (SCR) required to achieve a flat start for each scenario is investigated. It is found that minimum SCR is not constant, but rather follows a pattern of the minimum SCR increasing linearly when the amount of power transfer increases. It is also confirmed that the phase-locked loop (PLL) plays an influential role in the interconnection where a reduced proportional gain for the PLL can turn a marginally unstable into a marginally stable system.