The Celtic Interconnector Project will deploy a 700 MW, 575 km HVDC link connecting the Irish transmission system to the continental European grid. It will be the first European HVDC interconnection between two asynchronous grids to operate with grid-forming (GFM) control mode enabled simultaneously at both converter stations.

In this project, a dedicated test programme has been conducted to assess the GFM capabilities of the converters and the dynamic behaviour of the link. To support an objective and transparent evaluation, analytical envelopes, derived from typical GFM requirements such as effective impedance, inertia constant, and damping, have been used. These formulations, based on the expected response of a grid-forming converter unit, enable assessment of key functionalities including synchronizing power, voltage stiffness, and inertial support during normal operation, considering the converter hardware limits on current, voltage, and stored energy.

In the latest power electronic based systems employing GFM converters, the analysis typically focuses on a single injection point, as these applications mainly concern generation facilities. In the context of an HVDC interconnector, the situation is fundamentally different: the HVDC link is a transmission asset that connects two distinct power systems and has very limited intrinsic energy reserves. Consequently, the GFM behaviour of one terminal inevitably influences the operation of the other terminal, and vice versa.

The paper first provides an overview of the Celtic Interconnector Project and the motivations for adopting GFM and then introduces the GFM control principles implemented in the HVDC converters of the scheme. The second part presents the framework used to evaluate GFM behaviour, including the test descriptions and the analytical envelope methodology. The third part illustrates and analyses representative test results focusing on GFM behaviour. Finally, the coupling effects introduced by GFM HVDC converters between asynchronous grids are examined in the context of the HVDC interconnector.