Protection of multi-terminal HVDC systems: Algorithm development and performance verification by EMT simulations

In recent decades, the electrical power system has evolved into a new phase, in which the renewable energy resources are massively integrated into the grid. This change is mainly inspired by global policies that intend to reduce greenhouse gas emissions and decrease the society’s reliance on fossil fuels by replacing them with sustainable energy sources. The good examples are the European Network of Transmission System Operators for Electricity (ENTSO-E) that intends to integrate a high degree of renewables in Europe’s energy system, and the West-East Electricity Transmission Project that delivers wind energy from the northwest to the southeast of China. One important technology used to connect renewable energy resources is the high voltage direct current (HVDC) system based on the voltage source converter (VSC). Aside from the simple point-to-point HVDC link, the multi-terminal HVDC (MTDC) system is another option to connect these remote energy resources. In the MTDC system, the generation units are usually unsynchronized turbines that are interfaced with powerelectronic- based converters. As such, the responses of theMTDC system after faults occur are drastically different fromthe conventional AC systems that are based on synchronized generators. Since the development of an MTDC system is an important process, the research on the matter must be carried out. In an electrical power system, the transient events refer to a system’s response shortly after disturbances occur, such as the generation loss, the load shedding, the transmission line tripping, and the fault. This thesis focuses on the MTDC system’s protection based on the system’s transient events after faults. Due to the low impedance of the DC system and the low inertia of the HVDC converter, a fault in the DC system can spread quickly throughout both the DC and AC sides. Usually, the transient behavior of the HVDC system must be observed within severalmilliseconds, and it is a challenge to simulate the transient phenomena of a large HVDC system. The reason is that the accuracy of the electromagnetic transient (EMT) simulation heavily depends on how detailed the modeling system is: an extremely detailed system, such as one based on physical features of the semiconductor, cannot be modeled smoothly in the EMT application, while a too much simplified system cannot ensure accurate simulation results. Therefore, it means that a compromise must be made between modeling efficiency and accuracy. Consequently, this thesis implements an efficient method that ensures the efficient simulation of large-scaleMTDC system and its accurate transient phenomena. By using this method, the responses of an HVDC link after faults occur can be determined. More importantly, they can be classified into different stages, and the thesis explains themechanism of each stage. Furthermore, the thesis discusses the impact of grounding methods on the HVDC converter’s post-fault responses.