Development of a Novel Polarity Reversal Inhibitor for Future MT-HVDC

System Operators (TSOs) toward the realisation of meshed High Voltage Direct Current

(HVDC) grids. In such networks, the availability of robust HVDC circuit breakers (DCCBs) will play a crucial role in enabling selective and fast DC fault clearing, to enhance the system flexibility. However, the operation of DCCBs introduces fast transient overvoltages and, more critically, voltage polarity reversal on the DC link. This phenomenon poses a significant threat to solid-insulated components like XLPE-insulated cables, bushings, and transformer terminations, which are typically not qualified for long duration voltage tests for polarity reversals when intended only for VSC-MMC half-bridge systems.

This challenge is especially relevant for future HVDC projects, such as those under development by the Italian TSO, that foresee the realisation of new ±525 kV HVDC links, including both overhead lines and submarine cables. These links will initially operate as pointto-point connections, with the long-term goal of evolving into fully meshed and multi-terminal

HVDC grid. While metal-oxide surge arresters (MOSAs) can effectively limit overvoltages peaks of the same polarity, they are inherently symmetrical and unable to prevent negative voltage swings introduced by fast switching events in DCCBs. As a result, solid insulation equipment may be subjected to non-validated stress profiles that compromise long-term reliability and service life.

To mitigate this issue, a novel protective device called Polarity Reversal Inhibitor (PRI) has been developed through a joint collaboration between the Italian TSO and an Italian power electronics manufacturer. The PRI is a modular system combining metal-oxide surge arresters and unidirectional power diodes, designed to clamp negative transients and suppress polarity reversal following DCCB activation. Its architecture ensures simplicity, robustness, and suitability for outdoor installation in ±525 kV HVDC systems, without the need for control drivers or active thermal cooling.

A key feature of the proposed PRI is its modularity and scalability. The 24 kV prototype module has been specifically designed to enable series assembly, making it suitable for operation up to

±525 kV by stacking multiple identical units. This modular approach simplifies manufacturing, testing, and maintenance, and allows straightforward adaptation to different voltage levels across the HVDC network. Moreover, it supports standardisation of components while maintaining mechanical and thermal robustness suitable for harsh outdoor environments.

The PRI’s design has been validated through a detailed analysis of electromagnetic and thermal stresses. Electromagnetic transient simulations, performed using a widely adopted EMT simulation environment, show that the PRI effectively limits negative voltage peaks and damped oscillations during typical pole-to-ground fault scenarios. Thermal simulations confirm that commercially available power diodes and surge arresters are sufficient to withstand fault energy levels without exceeding critical temperature thresholds. Additionally, the PRI improves overall system behaviour by reducing converter-fed fault current duration and busbar voltage drop, thereby contributing to improved protection coordination and possible optimisation of converter station design.