Harmonics Reduction in Current Source Converters Using Fuzzy Logic

High power ratings gate turn-off (GTO) thyristors are usually used with low switching frequency in high power applications current source converters (CSCs), i.e., rectifiers and inverters. Space vector modulation (SVM) technique for CSC is established by dividing ac-side line current cycle into six sectors. Each sector is divided into certain number of space vector (SV) cycles. SV cycle is divided into three states, two active and one zero state. SVM technique generates fifth and seventh harmonics (HD_5-7 ) in the CSC ac-side current when operated with low switching frequencies, i.e., low number of SV cycles. Minimal reduction in HD_5-7 was achieved by using certain states sequence inside SV cycle, and by calculating states on times at once in the middle of each SV cycle. In this paper, fuzzy logic-dependent technique for calculating states on times in SVM-CSC is proposed. First, two straightforward techniques are discussed. One calculates each state on time from SVM equations in the middle of each state effecting time period. The other calculates all states on times from SVM equations at once when the effective state is changing from one active state to the other. Both techniques give high reduction in HD_5-7. Fuzzy logic technique adjusts states on times to give the least possible HD_5-7 with a compromise between both straightforward techniques, fuzzy logic tune states on time calculating instants in SVM-CSC. The target for fuzzy logic is to achieve fast states on times adjustment with fixed least HD_5-7 in steady state for states on times. States on time´s adjustment with fuzzy logic is applied also to a CSC with a small size ac-side filter. An experimental investigation is introduced.