Research On Related Problems Of 6kV Neutral Point Clamped Three-level Variable Frequency Drives

Media and High Voltage Variable Frequency Drives (VFDs) can output a variable AC voltage/frequency to provide continuous over the full speed range and save energy efficiently, which catch the worldwide attention. The 6kV Neutral Point Clamped (NPC) Three-level VFD studied in this paper is aimed at our nation's voltage level , which has been passed the 6kV/2500kW tests and is about to run on the spot.With the analysis of currently worldwide Media and High VFDs, this paper gives a general introduction and analysis of topology of the lab's series IGCTs three level VFD, including 24-pulse Rectifier, DC-Link, series IGCTs three-level Inverter and output LC filter, and the feasibility of which is proved by the simulation and experiment. Contrapose the VFD device, the paper makes specific research about several issues. The paper discusses the contribution of the DC-Link to the VFD safe operation. Through the analysis on the turn-on and turn-off transience of NPC three-level VFD, the corresponding mathematical models are developed, based on which the DC-Link choke value is deduced. Synthetically thinking about the switching transient state, the DC-Link chock and the VFD over-voltage capability, the influence of clamped capacitor to inverter's permanent DC voltage is studied, and the design of DC-Link capacitor and resistance is deduced. Based on the National Nature Science Funds of China (No.60274037), the Rogowski coil sensor theory is applied in IGCT's current measure and for which the coil is made. Both simulation and experiment results show that the Rogowski coil can be used for generic IGCT's current measure, and very probably for the research of IGCT's turn-on and turn-off. For the power IGCTs, the VFD forced air cooling heat-sink system is analyzed and designed, which includes the power losses calculation of IGCTs, design and type-choose of radiator and fan, and the scheme design of air conduit. And due to the much better heat-sink efficiency of the water cooling choice, which a basic introduction and discussion is made for, and is expected to apply in 6kV three-level VFD in future.All above researches are applied for the 6kV/2500kW NPC Three-level VFD device, which has passed the 6kV tests.