Hierarchical Primary–Secondary Control of HVDC Grids: Challenges of Droop Control Schemes in Ensuring Safe Operation under Large-Scale Renewable Integration

The response time of secondary control has typically been assumed to be in the range of tens of seconds. According to operating experience to date, offshore wind turbine generators generally do not provide sufficiently fast dynamic active power response to support DC voltage control during contingencies in MTDC systems. As a result, the offshore HVDC converters connected to them cannot effectively contribute to DC voltage droop control under such conditions. In MTDC systems with several terminals connected to offshore wind farms, this limitation forces the remaining onshore terminals to bear the burden of maintaining DC voltage levels within acceptable limits following severe disturbances. Under such conditions, primary control action alone may be inadequate, necessitating secondary control intervention.

Another driver for the need for faster secondary controller intervention is the recent trend of configuring onshore stations as Grid-forming (GFM) converters to provide inertial support to the AC grid. The dynamic response of GFM controllers is generally slower than that of grid following (GFL).

The purpose of this paper is to investigate these challenges and their impact on the transient and quasi–steady state DC voltage in MTDC systems. The results indicate that reduced response time for the secondary control layer could be required to mitigate the consequences of the limited response speed associated with GFM requirements and the limited number of converters contributing to DC system voltage regulation.