Introduction of Adaptive Active Filters in HVDC Systems

Abstract: When converting electric power from AC to DC, a 12-pulse converter is often used. Unfortunately, the converter creates harmonics superimposed on the DC voltage. These are removed by filters. A passive filter is normally used, but active filters may be added for improved performance. However, the drift in component values due to fluctuations in temperature and the change in load, makes it difficult to obtain filters with consistently good performance. Initially, tests and measurements are required to adjust the filters to the environment. To circumvent these drawbacks, adaption and adjustment during operation is needed. Different adaptive control concepts are investigated in this report. Two of them, feedforward with estimation using the MIT rule and feedback using an indirect self-tuning regulator, are also tested in simulations. The ability to estimate the whole system, including the effect of reflections from the transmission net, is partially analyzed. Estimation is done using the recursive least-mean-square algorithm and the stochastic approximation algorithm. Many problems can arise in designing adaptive controllers for a HVDC system. In this report, some of them are discussed and different solutions are suggested.