FEASIBILITY OF UTILIZING HVDC TO IMPROVE SYSTEM DYNAMICS

By growing SCR industry, HVDC lines become not only feasible but also more economical than HVAC lines over long distances. Besides the economy question, an engineer must look at other impacts of any new technology on the existing well established ones. Concerning DC lines they are not new to the power industry. However, due to certain properties and conveniences of AC lines, almost from the beginning of the worldwide usage of electrical power, it became clear that AC is better. Now that HVDC systems are becoming available cheaper than AC counterparts, one is faced with the old question again "which one is better AC or DC". More and more researchers are becoming convinced that DC has certain characteristics that should not be overlooked. of course economy most of the time can justify most of the management decisions. But sometimes a good decision worth more than an economical one. When both economy and some other inherent characteristics or properties justify a decision, opposition to it becomes overwhelmed very easily. HVDC lines of course create a lot of problems for the well matured AC industry; incompatible technologies and harmonics, to name a few. However, improved stability of HV AC/DC system can justify-rendering some of above inconveniences. In the following it is demonstrated that certain type of controls on the firing angles of the thyristors of HVDC lines can have substantial effects on reducing dynamics and improving stability of the interconnected AC/DC system. In this paper a formulation of the problem based on successive linear programming technique is presented. This technique accomplishes by first computing an operating point for the operation of AC/DC interconnected system; and next linearizing all equations and inequalities (or operating constraints) the operating point and setting up a linear programming problem that, among others, yield the incremental firing angle for the thyristors. When this procedure repeated many time steps it will furnish the necessary control for damping system dynamics. of course, in reality, time steps are chosen in accordance with the speed by which SCRs can be controlled and also by the measurement speed of the SCADA system itself.