The theory and design of a new cycloconverter for operation in high frequency links

Abstract

The naturally commutated cycloconverter (NCC), compared to force commutated inverters and frequency changers, is still a viable solution for high power converters and conditioners. A new circuit configuration of thyristors (or more specifically silicon controlled rectifiers (SCR's» has been developed for the NCC. The new NCC, designated the "flywheel SCR" NCC, achieves significantly improved input/output operating characteristics compared to the standard three-pulse NCC. These improvements are obtained without the need for many extra thyristors or input/output transformers, which are necessary if conventional higher pulse number NCC's are used. All of the "flywheel SCR" NCC advantAges are not only beneficial for certain high power frequency chanqer applications, but are particularly favourable for NCC's in high frequency AC (HFAC) links or interties. A theoretical analysis of the "flywheel SCR" NCC is performed, indicating the differences compared to the three-pulse NCC. Decreased output voltage distortion, reduced input reactive loading, and reduced input RMS current are some of the advantages of the new NCC. The design and construction of a new NCC prototype is presented. The design problems encountered are discussed, and the sizing of the thyristors and their protective components is given. The improved performance of the new NCC compared to the three-pulse NCC is gained at the expense of an increased number of thyristors, (but less than the six-pulse NCC), together with a slight increase in control complexity. However, the new NCC characteristics easily justify the extra expense. The experimental measurements, obtained using the prototype, verify the theoretical analsyis. The measurements also indicate that the new NCC is better suited for driving induction machines than standard NCC's because of improved bank selection or zero current cross-over, and reduced discontinuous output current problems.The benefits of using the new NCC in HFAC links compared to standard NCC's is also discussed. The new NCC characteristics ~ enhanced in the link, whilst the disadvantages are effectively masked. The methods of controlling the NCC's in an HFAC link are described, thus providing a suitable interface between the link and a power system. Sizing of the HF tank circuit components and other link power components is also discussed. One link control method is practically implemented using the new NCC prototype to interconnect an HF synchronous machine to a threephase power system. The results of the tests are presented, and illustrate the operation of an HFAC link and its input/output characteristics. Finally, some of the link applications (some novel) are discussed. These applications show the versatility of the link, together with its few, relatively minor limitations. The applications include AC/DC machine control, reactive and harmonic power compensation, power syst~m interconnection, and general power control and conditioning. The applications are so wide ranging that the HFAC link must be of considerable interest wherever power at one voltage and frequency is to be converted to power at another voltage and frequency.