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Abstract
DC power transmission began in 1954 .The Energy transmission by means of direct current is an attractive alternative to ac transmission for a number of special applications .The advent of the thyristor valves has been responsible for the growth of the HVDC systems. The HVDC technology has reached a maturity level and is contributing to improve the AC systems performance.
In this thesis the study of the HVDC links as a method of power flow control is investigated and the study is verified by means of MATLAB/SIMULINK software package, where two models are simulated:
•A simplified model of the LCC-HVDC links
•A detailed model of the LCC-HVDC links
Besides the capacitor commutated converters were studied also using MATLAB/SIMULINK software package and showing its advantage 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).
Also the thesis investigates the advantages of the VSC_HVDC links over the conventional HVDC links, The best support from a VSC transmission system is for many grid configurations and stability problems a mixture of active and reactive power control .A VSC DC transmission system is able to support the AC grid with a suitable power factor and hence improve stability .One can say that it works as a ’distributed’ FACTS-unit connecting two )or more (points in the grid.
The use of pulse width modulation (PWM) for voltage source converters enables independent control of real and reactive power within the equipment limits.
The thesis then deals with the Variable Frequency Transformer (VFT) as an alternative method of power flow control. The VFT is controllable bi-directional transmission device that can transfer power between asynchronous networks .The construction of VFT is similar to conventional asynchronous machines .Electrical power is exchanged between the two networks by magnetic coupling through the air gap of the VFT.
The VFT has advantages over the HVDC link including its lower cost, better technical attributes for the application. smaller site size, and conventional electrical machine design which is easy to operate and maintain.
The VFT improves grid stability by retaining the natural instantaneous response of AC connections for fast events .This inherent characteristic, an invaluable option for connecting a weak network by transferring short-circuits away from the system to increase the stability.
Then the study of the VFT is verified by the simulation of a point to point system where the YFT is used to interconnect these systems together.
The simulation is performed using the MATLAB/SIMULINK software package to study both the steady state and dynamic performance of the YFT verifying the robustness of the YFT as a method of power flow control.