الفهرس 
Introduction
Types of HVDC convertersOnly 14 pages are availabe for public view
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Abstract
With the fast development of power semiconductor switches, voltage source converters based high voltage direct current transmission systems (VSC-HVDC) has attracted more attention in the last few years due to their high dynamic performance and flexible control features. The use of pulse width modulation PWM with voltage source converters VSCs provides separate and independent control of active and reactive power. First of all, a simplified model of a VSC-HVDC transmission system connecting two strong AC grids through a long DC link cable is presented. As the basis of the investigation, a proposed nonlinear control scheme is adopted for the design of the system controllers to achieve fast and decoupled control of controlled signals. An over/under voltage protection strategy is proposed to protect the system from the hazards of overvoltages accompanied with different abnormal conditions. Also, an over current protection algorithm is proposed to limit the value of the large transient currents resulting from different faulty cases. Furthermore, the performance of the proposed nonlinear controller has been compared with that of conventional PI controller. Steady state as well as transient state performance under various operating conditions has been examined. A robust multivariable H-infinity H∞ control scheme for a weak AC grid connected to a VSC-HVDC system is also presented in this thesis. The purpose is to achieve robust stability and superior dynamic performance under large uncertainty in the short circuit ratio SCR of the weak grid. Small-signal modeling and eigenvalues analysis are used to confirm the stability of the robust control scheme under wide range of operating conditions and SCR variation.