The interharmonics produced by some classes of nonlinear loads limit the effectiveness of classical passive filter compensation and active compensation must be considered. In high-voltage, high-power applications hybrid filter systems are technically justified and cost-effective solutions. Hybrid filter control strategies successfully employed in the compensation of high-power rectifier loads based on the detection of the total current distortion are less effective in the compensation of interharmonics. A frequency selective control system for a voltage controlled active filter connected to the power system through a multistage passive filter is described. A bank of second-order IIR digital filters with interharmonic tracking ability is employed for high-resolution spectral estimation and control. This approach is verified by simulation. It is shown for example that the active filter can compensate for supply interharmonics produced by a cycloconverter load. Different control strategies based on the selective control system are investigated and compared with an earlier strategy based on the detection of the total current distortion. By using individual detection and compensation of critical harmonics and interharmonics, hybrid power filters can be optimised to satisfy the distortion limits with the lowest active filter rating and switching frequency.
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Basic et al. (2000) studied this question.
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