Abstract:
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.
