Overvoltages and coupling effects on an ac-dc hybrid transmission system
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Abstract Adding a dc circuit to an existing transmission line is one method of significantly increasing the power transfer capability of a transmission corridor. The resulting hybrid system has significant coupling between the ac and dc circuits, not only because of the proximity of the circuits, but also from the fact that they may share the same sending end or receiving end ac systems. The resultant interaction produces overvoltages on the dc system which can be somewhat higher than for a conventional dc scheme. This thesis investigates the overvoltages on a hybrid ac-dc transmission system and suggests some design considerations which could be taken into account to reduce stresses on certain critical components which result from such an arrangement. Blocking filters consisting of a parallel L-C combination in series with the dc converter were included to limit the flow of fundamental frequency current in the dc line. This thesis also investigates the proper blocking filter configuration to be used as an incorrectly chosen blocking filter can cause resonance overvoltages on the dc line at fundamental frequency. A method of eliminating dc components of the currents in the transformer windings of a dc converter is presented. The method uses the technique of firing angle modulation. It is shown that merely eliminating the fundamental frequency component on the dc side may not remove this dc component. The impact of such control action at one converter on the other converters in the dc transmission system is also presented. It is also shown that the undesirable side effects of such a scheme include increased generation of non-characteristic harmonies on both the ac and dc sides. The study is performed using an electromagnetic transients simulation program and theoretical calculations.