Selective compensation of cycloconverter harmonics and interharmonics by using a hybrid power filter system

High power variable speed cycloconverter loads are source of variable frequency interharmonics which are difficult to reduce using classical passive filters. Hybrid power filters with series connection of a small rating active filter (AF) with control systems based on total source current distortion are less effective in suppression of interharmonics, In order to improve hybrid filter compensation for interharmonics a novel selective active filter control system with tracking of interharmonics is proposed and tested. The tracking algorithm is based on adaptive notch filters with separate detection of cycloconverter input and output frequencies. Based on this information a bank of PI controllers shapes the AF compensation voltage until full compensation of distortion at targeted frequencies is achieved. Test results show good compensation of the targeted harmonics and interharmonics with a simple capacitor used as coupling element.