الفهرس 
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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.