Multi-vendor Protection Systems for Meshed HVDC Grids

Voltage source converter based high voltage direct current (VSC HVDC) transmission is one of the key solutions to integrate a large amount of renewable energy and to provide the increased reliability and flexibility required to handle the variable nature of renewable sources. Protecting HVDC grids against DC-side faults is essential to achieve the required reliability, yet very challenging due to the nature of the DC fault behaviour. To tackle the challenges associated with HVDC grid protection, various protection philosophies and technologies have been proposed by academia and industry in recent years. As a result, different converter and circuit breaker technologies can be implemented in the application of various fault clearing strategies, ranging from fully selective fault clearing to non-selective fault clearing.

In the meantime, multi-vendor HVDC grid development has become vitally important to allow building large-scale HVDC grids, as it is necessary to ensure competition between manufacturers which can in turn drive innovation and cost reduction. To-date, HVDC technology has mainly been used in point-to-point connections, which are typically provided by a single vendor in turn-key projects. Consequently, HVDC control and protection systems and components are often vendor-specific. The necessity to implement a multi-vendor approach has driven several international standardisation bodies to work on guidelines and standards for HVDC systems. Furthermore, in China, national standards on protection equipment and DC circuit breakers have been established along with fast development in a meshed HVDC grid. However, many technical barriers still exist blocking the implementation of multi-vendor HVDC grid protection. Particularly there is a great gap in the literature to address interoperability of the large variety of possible solutions and technologies emerged in recent years.

This thesis aims at systematically addressing multi-vendor interoperability of HVDC grid protection focusing on key components and multi-vendor protection system design. First, an overview of interoperability aspects in various multi-vendor schemes is provided and unique challenges of multi-vendor interoperability of HVDC grid protection are identified. Second, an in-depth analysis on the travelling wave behaviour during a DC fault in a meshed HVDC grid is performed, which provides a deeper understanding on the influencing parameters to the fault behaviour so that robust protection algorithms can be designed, suitable for a multi-vendor environment. Third, this thesis investigates various DC circuit breaker (DCCB) functions and their applications to achieve multi-vendor interoperability. Auxiliary DCCB functions, such as fault current limiting, self-protection, breaker failure internal detection are analysed to understand how and when to use these functions in a coordinated and beneficial way. A breaker failure backup protection algorithm, generally applicable to all DCCB technologies, is developed to deal with realistic component failures. In addition, a coordinated backup protection scheme is proposed to incorporate DCCB- and system-level protection functions. Fourth, requirements on pole rebalancing devices and necessary control and protection sequences are developed for safe and fast pole rebalancing in coordination with fault clearing in high-impedance grounded systems using DCCBs. The interactions of pole rebalancing and DCCB operation are investigated to achieve fast voltage recovery and low energy dissipating requirements. Fifth, a high-level framework to design HVDC grid protection is proposed to support a step-by-step multi-vendor grid development. A detailed protection system design is elaborated on a three-terminal system and the performance of the protection system is tested using hardware intelligent electronic devices (IED) prototypes. As a result, this work assists in facilitating future standardisation of the HVDC grids and key components, including DCCBs, DC protection IEDs and pole rebalancing equipment.