الفهرس | Only 14 pages are availabe for public view |
Abstract The growing load demand globally necessitates increasing the penetration of renewable energy sources (RES) into electrical grids as well as interconnecting grids from different countries and even continents through High voltage direct current (HVDC) transmission systems. HVDC transmission provides superior advantages; among them the ability to transmit enormous amounts of electrical power over great distances at low cost. As a result, planners of power systems consider it as a viable choice for power transmission and interconnection of asynchronous networks. Depending on HVDC grids, continental/super grids have been recently constructed to promote global economic development. Since these applications rely on power electronics devices, several power quality issues arise namely voltage sags and swells. As HVDC-VSC transmission system is primarily used to interconnect two AC systems through a HVDC link, it may be subjected to several faults namely AC faults (at each AC side), DC faults (on the connecting DC transmission system) and internal faults (inside the converter itself).This study focuses on the behaviour of a voltage sourced converter (VSC) based HVDC transmission system comprising three arms- neutral point clamped (NPC) converters interconnecting two asynchronous AC networks. The thesis gives a brief introduction to the necessitating HVDC technology. Then HVDC transmission features as well as the fields of application are mentioned. The numerous technologies of HVDC such as the line commutated current (LCC) and voltage sourced converter (VSC) are comprehensively compared. The available HVDC configurations are also illustrated. Furthermore, the VSC different topologies like 2-level, 3-level, and multimodular level converter (MMC) are discussed. Different mathematical models of VSC-HVDC system are then compared. The detailed model of the VSC-HVDC control system is simulated using MATLAB/Simulink explaining the function of each control block. Recent research has not deeply discussed the system response to the prevalent power quality issues. Thus, the prevalent power quality issues in VSCHVDC transmission system are studied clarifying the causes and the consequences of each disturbance. Additionally, the thesis is distinguished by addressing the common faults in HVDC transmission. Finally, the VSC-HVDC system is simulated using MATLAB/Simulink. Additionally, the vector control strategy of active/reactive powers and DC bus voltage are simulated under varying situations by adjusting the controller’s settings. The performance analysis case studies are classified to the following: under small control perturbations, due to instantaneous power quality issues (voltage sags and swells), and the impact of the AC fault on the transmission link. The study records and analyses AC/DC voltages and active/reactive powers at two converter stations under varying power and voltage conditions to evaluate the system stability. The results of the study provide key performance indicators, such as settling time (tsett), steady state error (SSE), overshot/undershoot (OS% / US%), and correlation factor (CF). The obtained results reveal that the system control performs well in terms of stability and robustness during the small disturbances. It resumes its steady state quickly within 0.7 sec. However, the system hardly withstands voltage variation for a short period. It withstands the maximum sag limit (0.9 pu) for only 160 msec. Nevertheless, the system hardly sticks to the stable operational conditions during the maximum swell perturbation (0.8 pu) for only 40 msec. In addition, both the voltage and current of the DC link are immediately affected by the imposed AC faults on the inverter AC side. The system properly sticks to the stable operational conditions once again after it is affected by either symmetrical or asymmetrical ac faults. |