Renewable energy generation is becoming a research hotspot due to the lack of fossil energy andenvironment pollution problem. Wind power generation systems have application significantadvantages. According to the prospects and development of the wind power generation systems, thispaper analyzes several high-power converters and typical converter topologies which are suitable fordirect-drive wind power generation systems and selects Three-Level (Three-Level, TL) Boostconverter (referred to as the TL-Boost converter) and three-level diode neutral point clamped(neutral-point-clamped, NPC) inverter cascaded topology. The structure of the pre-stage is a TL-Boostconverter, the converter of the last stage is NPC three-level inverter.This paper analyzes the operation principle of the two-stage three-level cascade system in detail.According to the former TL-Boost converter, the input-output relationship of the converter is derivedand the mathematical model is also established. For NPC three-level inverter, both low frequency andhigh frequency mathematical models are established considering the dc bus capacitor voltageunbalancing situation.By analysing the voltage balance of the voltage divided capacitors, a decoupling idea about theoutput-voltage control loop and the deviation-voltage control loop is analyzed and a specific controlscheme is given. The traditional SVPWM modulation strategy of three-level NPC inverter isintroduced. To simplify the control of the three-level NPC inverter, a SVPWM modulation strategybased on gh coordinate system is applied.This paper focuses on the neutral-point voltage balance problem of three-level NPC inverter. Theresult of the neutral-point voltage drifting and the effect of neutral-point voltage unbalance onswitching vector composition are analyzed. The change of control ability of traditional balance factoralgorithm under parameter variation is analyzed. On this basis, the control ability of TL-Boostconverter is analyzed. The neutral-point voltage balance of the inverter is achieved by the TL-Boostconverter to ensure neutral-point voltage balance at full range of power loads.On the basis of theoretical analysis, this paper builds two-stage three-level cascaded system ofSVPWM simulation model and proved the theory. Finally, a5kW two-stage three-level cascadedsystem principle prototype is made. Experimental results verify the theoretical analysis. |