Power Oscillation Damping Using Expandable VSC-HVDC Transmission System

Nowadays, there is a huge potential for power generation from offshore wind farms, with the North Sea constituting the central point for offshore wind power deployment. In order to integrate the latter into the mainland transmission system, DC interconnection forming multi-terminal HVDC (MTDC) transmission systems can be used. Currently, only PtP HVDC links are in operation, including BorWin, DolWin, HelWin and SylWin projects. However, MTDC transmission systems are expected to be built in the near future. As an example, COBRAcable between Denmark and the Netherlands is planned to become an MTDC system, where a possible expansion can be realized with the addition of an offshore converter along its cable. The expansion introduces challenges in the operation of the three-terminal system, including its control strategies. As explained in CIGRE WG D2.35, HVDC systems can work together with the control function of Power Oscillation Damping (POD), which consists in modulating the transferred power through the HVDC link in order to provide damping to low frequency power oscillations within either or both of interconnected AC systems.

Although the POD control is well established for PtP schemes, there is room for further research regarding its application in MTDC systems, which create challenges in the design and tuning of the POD function. For example, when a PtP system is to be connected to an offshore wind terminal, the effect of the wind farm output power profile on the power oscillation damping capability has to be investigated and the POD function has to be adapted in order to provide adequate damping for the three-terminal system. This paper initially presents the design and application of a POD controller to a PtP system. Afterwards, the aforementioned issues are adressed: An AC two-area test system is used to introduce a three-phase fault and a PtP HVDC link is connected in parallel in order to damp its oscillations. An offshore wind farm is connected to it and the influence of the various wind turbine output profiles on the POD performance is analyzed for two control modes of the onshore converters, i.e. master-slave and DC voltage droop. The POD controller is shown to provide larger damping for operation in master-slave mode. It is observed that the decreased production of the traditional generators involved in the interarea power exchange results in increased damping. In addition, it is shown that the rate of change of the wind power output has negligible effect on the controller’s damping performance, which is proved to reduce when the existing controller is applied to the system operated in droop control mode. Thus, a modification in its tuning is proposed in order to damp the oscillations effectively. The results show that the damping performance is improved, opening more possibilities for use of MTDC-HVDC links during emergency operation.