HVDC grid can be considered as an alternative option to HVAC grid when it comes to the transmission of power over long distances. Utilizing HVDC grid brings the benefit of lower costs for long transmissions as it has less losses than HVAC grid. Also different renewable energy sources as well as consumption areas can be connected via HVDC grid. Different challenges appear in HVDC grid especially regarding the HVDC technologies. One of the main challenges is the HVDC grid protection. Fault phenomena imposes a great constraint on the HVDC technology and operation. The fault current can reaches tens of kiloampere in a short period of time. To protect the equipment, ensure the stability of HVDC grid and interrupt the fault current, fully selective fault clearing strategies (FCS) is studied in this thesis. Accordingly, an appropriate DC circuit breaker (DCCB) should be selected to interrupt the fault current. In fully selective FCS, the idea is to isolate only the faulty line or faulty cable after fault occurrence so that the power flow can be maintained in healthy part of DC grid. The objectives of the thesis is to identify the main challenges of applying fully selective FCS in HVDC grid taking into account the technologies utilized in the grid as well as presenting the methods to cope with such challenges. The thesis consists of three parts. In PART 1, introduction of HVDC grid, DCCBs and protection strategies are presented and the considerations for the thesis are provided. PART 2 targets the challenges related to the fully selective FCS considering the components utilized in HVDC grid. PART 2 is divided into three chapters, each of which is dedicated to a specific challenge. In CHAPTER I, different operating mode of DCCBs and its impact on the protection strategy and DCCB sizing is studied. In CHAPTER II, the protection strategy for fault between inductors and DCCB, and impact of fault location on inductor sizing and DCCB topology are studied. Then possible configurations for inductor and DCCB architecture are presented. In addition, the cost analysis of inductors and DCCB topology is provided. In CHAPTER III, a method is presented to calculate the fault current. This method is further used in inductor sizing optimization taking into account the constraints related to the components and protection strategy. In PART 3, the challenges related to the control of modular multilevel converter after fault occurrence is investigated.
Keywords: Faults, protection, MMC, MTDC, inductor sizing
Director of thesis: Professor Seddik BACHA
Co-director of thesis: Abdelkrim BENCHAIB