The novel system protection concepts applied to the future low-inertia power system

Due to the technical excellence of flexibility and reliability, the MMC-based MTDC systems are regarded as one of the most cost-effective and promising solutions for long-distance, bulk power transmission and the integration of large-scale renewable energy sources and building hybrid HVDC/HVAC systems.

Despite the benefits of the MMC-based MTDC system, many new emerging technical challenges need to be resolved to accelerate the practical feasibility of this system. The HVDC transmission lines are generally long-distance and exposed to complicated and wicked environments, which makes the phenomenon of DC faults not unusual. To this end, in MTDC systems, the fault currents should be promptly disconnected. Otherwise, the MMC converter will be blocked, preventing it from being damaged by the huge energy resulting from the fault current. The AC system will also crumble due to the uncontrollable MMC converter. The conventional control and protection technologies are insufficient to ensure the safe operation of the system. Therefore, the control and protection for the MMC-based MTDC transmission system in the future is a challenging subject.

Therefore, the main objective of this Ph.D. work is to develop a comprehensive control and protection scheme against DC faults for an MTDC system, as described in Fig. 1, which typically consists of protection algorithms for the DC fault discrimination, and control strategies for the system fault recovery and fault current limitation. The major questions raised in this project are:

•    How to detect DC faults promptly and ensure selectivity, sensitivity, and reliability?
•    How to determine the exact fault location for long-distance transmission lines?
•    How to realize the fault current limitation through advanced nonlinear control strategies of MMCs?
•    How to realize the control and protection interactions in different system transient scenarios?

MTDC system, DC fault detection, HVDC fault location, MMC nonlinear control

•    An robust HVDC protection algorithm;
•    An accurate HVDC fault location method;
•    An robust MMC controller using a backstepping sliding mode control method;
•    Interoperability study of different MMC nonlinear controllers; 

China Scholarship Council (CSC)

About PhD Researcher

Le Liu was born on March 29th, 1996 in Xi’an, Shaan Xi province, China. He received his B.Sc  in field of Electrical Engineering, from the Xi’an University of Technology, Xi’an, in 2017. He completed his MSc in field of Electrical Engineering, from the Xi’an Jiaotong University and Polytechnic University of Milan in 2020. Then, he joined the IEPG group in the Faculty of EEMCS, Delft University of Technology to continue his study to Ph.D. in the field of Electrical Engineering in 2020.
His research interests lie in the area of HVDC protection, HVDC fault location, and MMC nonlinear control.