الفهرس 
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Abstract
Abstract
DC power transmission began in 1954. The advent of the thyristor valves has been responsible for the acceleration of the growth of the HVDC systems. The HVDC technology has reached a maturity level and is contributing to improve the AC systems performance.
This thesis comprises the studies and the results from the application of fuzzy logic theory to the control of the high voltage direct current (HVDC) systems. The study is verified using MA TLAB/SIMULINK simulation software. The results demonstrated successful performance for single mode control (either constant extinction angle or constant current) using a fuzzy based controller. The results using the fuzzy logic based controller showed many distinct improvements compared to the conventional proportional plus integral (PI) controller.
In this research, the performance of capacitor commutated converter was studied using MA TLAB/SIMULINK simulation software, and showed its advantages over the conventional HVDC system such as improving the commutation performance of the converter and operating at lower firing angle (i.e. higher power factor).
This thesis deals with the control of voltage source converters (VSC). A model of a VSC based dc link using pulse width modulation Technology and Insulated Gate Bipolar Transistors (IGBT) semiconductors is designed. A mathematical model of the control system based on the relationships between voltage and current is described for the voltage source converter. A control system is developed combining an inner current loop controller and a number of outer controllers.
A developed technique to maintain constant power transfer during voltage sag at the sending end of a DC link is investigated. This technique depends upon a constant reference power which determines the value of reference current in the link. When using constant current control (C.C) at the sending end and constant extinction angle control (C.E.A) at the receiving end with a distinct voltage dip or sag in the sending end voltage, a run down will occur. On the application of minimum ignition angle (a min) to the rectifier side (sending end) and by controlling the voltage at the inverter side (receiving end) a dc current in the DC link can be controlled to maintain constant power. This will sustain until the transformer’s onload tap changer can restore the predetermined needed voltage. The problem of constant power control is that the maximum available power (MAP) of the link is dependent on the magnitude of the sending end voltage, so the constant power control mode can be used only if the dc power reference is lower than the maximum available power (MAP) of the link. This has been verified in this thesis.