Active filtering of AC harmonics from HVDC converters

Abstract

HVDC converter stations, being non-linear loads for the AC system, have been relying on the use of shunt AC passive filters to reduce the harmful effects that the harmonic AC currents impose on the AC system. AC passive filters, however, present several characteristics that redeem their performance to levels far from excellent.

Concerns about harmonic-related problems caused by the use of large-capacity non-linear loads have led to the use of active filters in the industry. Active filters cancel the aimed harmonic content of a non-linear load by injecting harmonic currents with the same magnitude but in complete phase opposition. Active filters present superior performance than that offered by the traditional AC passive filters.

Although there have been proposals of the application of AC-side active filtering in HVDC systems, the present ratings of the power semiconductor switches commercially available do not permit the straightforward implementation of these proposals yet. Besides the high power rating that characterizes HVDC systems, some of their operation characteristics have been posing further obstacles to the use of active filters.

This thesis proposes an active-filter topology that decouples the reactive-power supply and filtering functions that characterize the traditional HVDC AC passive filters and all the active-filter proposals presented to date. The complete devotion to the filtering function offer several advantages that might overshadow its potentially higher initial costs.

The application of modern control theory is proposed as the basis of the control scheme. This technique suits more adequately the complexity of the control problem at hand than classical methods do, and provides a common framework for comparison of different control approaches. The optimal-control technique is the approach used in this thesis.

The CIGRE HVDC benchmark system has been chosen as the platform on which to test the performance of the proposed AC active filter. A simplified version of this system, that still maintains the weak AC system and the AC- and DC-frequency resonances of the original system, has been used, and poses a good degree of difficulty to the proposed active-filter operation.