Hvdc - a Means of Damping

AC SYSTEM OSCILLATIONS? J. D. Mountford During the 1960's many HVDC applications were developed throughout the world but these , were surrounded by a certain aura of evan, gel ism. Now HVDC has indeed become recognized as an invaluable, if very specialized, system design recourse. However, there still remains a tendency to overestimate its advantages in service. One of the conceptual advantages cited for HVDC is its ability to damp long-term oscillations in ac systems, an area of increasing concern to ac system planners. It would seem, for example, that an ideal means for damping oscillations in the receiving system would be th e ability to supply energy to an oscillating system and to modu late that energy in any of several ways. Yet, where and how that energy is applied (or absorbed) is very important. To illustrate, consider two relatively similar masses joined by a spring and set into oscillation. If energy is applied to one mass alone, it will be relatively ineffective in damping the oscillation. Energy would have to be applied or absorbed between the masses, i.e., in parallel with the spring, to cause significant damping. Electrical networks behave in the same way. AC systems whic h have long-term oscillation problems are obviously much mo re complex than the simple analogy cited above. For instance, six or more poles may be involved in the oscillation, and the likelihood that a dc link, whose termination point must be dictated by system economics, would fall at a point where it could effectively damp system oscillation is unlikely. In fact, even if it should terminate at one of the offending poles (machines or machine groups), it is probable that optimal energy modulation would only be able to reduce the number of poles involved in the operation, and not eliminate the problem altogether. Furthermore, if the HVDC link ties two interconnected ac systems together, energy modulation sufficient to damp oscillations on one system could instigate them on the other. PTI has run several example cases using the Power System Simulator, PSS/E, for the equivalent two-machine system shown below. System A is transmitting power through an ac transmission system to a re latively small system, S, having a relatively large load but very little local generating capacity. Part of the total system's power is provided via a dc link from a third remote system. The receiving system, S, is set into long-term oscillations relative to the larger system, A, following the disturbances in the transmission system. The effects on damping of the location of the dc inverter and several feasible dc power modulation signals were investigated.