Continuous Operation of Meshed HVDC Grids During DC Faults: HVDC Circuit Breaker Integration & Design from a System Perspective

Name: Mudar Abedrabbo

Supervisor: Dirk Van Hertem

Co-supervisor: Willem Leterme

Introduction / Objective
High Voltage Direct Current (HVDC) grids have been identified as a promising solution for the massive integration of renewable energy resources. These grids will be protected by HVDC Circuit Breakers (DCCB) to avoid the prolonged power interruption in case of DC-side short circuits (i.e. faults). The design of DCCBs is challenging due to the complex interaction between grid components and DCCBs. This thesis proposes a new framework to integrate and design DCCBs with a wide range of operating times in large-scale meshed HVDC grids, which maps the interactions and enables the continuous operation of HVDC grids under DC-side faults. Within the content of this framework, the fundamental concepts, approaches and tools are provided to develop, investigate and evaluate the protection scheme using DCCBs.

Research Methodology
In this thesis, the developed framework is divided into two stages:

1st stage: During DC-side fault clearance
• Proposing high level operational performance scenarios of HVDC grids during DC-side faults (DC-FRTS).

•Developing systematic approaches to DCCB sizing (DCCB Operational Graphs) and fault current estimation in HVDC grids.

• Developing an enhanced mechanical DCCB. 

2nd stage: During post-DC-side fault recovery
• Developing novel methods to analyze and enhance the post-fault Recovery performance of meshed HVDC grids, which restores the power flow in a stable manner with an adequate speed.

Results & Conclusions
- DC-FRTSs describe the expected behavior of the grid operation during DC-side faults, which could be used in future HVDC grid codes.

- The operational graphs help vendors and system operators in characterizing DCCBs, through mapping the interactions between grid components and considering the system performance.

- The fault current estimation approach facilitates the construction of the operational graphs without the need for time-domain simulations.

- A stable post-fault system recovery is achieved using the proposed methods with an adequate speed, and consequently reduces the impact of the fault on connected AC systems.

- The enhanced mechanical DCCB allows for a shorter interruption time and consequently reduced DCCB requirements.