Key Technology Research On Thermal Stability Analysis And Control Of High-Power Three-Level Inverter

Inverter is a kind of electric energy conversion device. During the conversion process from DC power source to variable frequency power supply, part of energy was consumed in the inverter itself, and its main existence form is heat energy. The management and control of this heat energy has a crucial influence on the operation stability of inverters. What’s more, inverter as a collection of many power electronic components in a highly complex, high precision, sophisticated equipment, has a high thermal sensitivity. All the components have their corresponding reasonable temperature areas for normal working, which means higher or lower than the temperature areas will affect the working characteristics of these components. Then the so called Buckets Effect will appear in inverter system, which indicates that when certain component breaks down, it may cause a chain reaction, and even cause a failure to the entire system. Therefore, a reasonable and sufficient cooling is necessary for the inverter to ensure a moderate system temperature, which is in the normal operating temperature ranges of all components.In this dissertation, the main circuit is the three-level inverter topology structure and the research object is mine hoist, on which basis, the NPC three-level inverter, power loss calculation and analysis of power devices, cooling system analysis and design, laminated busbar analysis and calculation, and thermal equilibrium were studied further.For modulation strategies of diode clamping NPC three-level inverter, the comparison and analysis of simplified three-level SVPWM strategy based on reference voltage translattion and discontinuous pulse width modulation(DPWM) algorithm were made. A neutral voltage balancing strategy based on DPWM was proposed, which has double effectivenesses in power-loss reduction of power devices and neutral point potential control.For the lack of theoretical analysis and calculation in internal thermal field of high-power inverter system, a fast power-loss algorithm based on SVPWM modulation strategy was proposed, and by combining the special condition of mine hoist, a fast dynamic power-loss algorithm was deduced to realize the online calculation of high-power inverter’s power losses for mine hoist.For the problem of overlapping abstract modeling during high-power laminated busbar design, a laminated busbar power-loss algorithm of high-power three-level inverter based on 3D cubical thermal model was brought in. Using this model, the complicated abstract laminated busbar consist of components and heteromorphism laminated copper bars can be decomposed equivalently into 3D coordinates in accordance with commutation loops to calculate stray inductance, equivalent thermal resistance,power loss independently under different current paths, which will support the stray inductance analysis,thermal analysis,and structure optimization.For the lack of theoretical support in high-power inverter with limited space during cooling system design, an analysis and design theory of cooling system based on maximum power range dynamic power-loss calculation was proposed. According to the special working condition of mine hoist, the dynamic calculation of maximum power-loss range under multicycle operation was made to complete the cooling theory capacity calculation of forced-air cooling system. The result will offer references to optimization analysis during structure design. What’s more, the cooling limit analysis of inverter under specific size was conducted to improve the power density.For the special phenomenon caused by instantaneous thermal accumulation damage under overloading in entire lifting process, large overloading in start period, and multicycle operation, a thermal equilibrium dynamic control based on DPWM dual-mode modulation was put forward. This control strategy can dynamicly switch the two modulation strategies with different control performance advantages in accordance with the power loss change of power devices in inverter to realize the dynamic control of power losses and thermal equilibrium dynamic control.Finally, the proposed theory analyses and control strategies were proved by experiments. What’s more, from september, 2012, the power loss calculation and cooling system analysis method have been used successfully in the power-loss analysis and cooling system design during process of inverter design of Hemei 5th coal mine, Hemei second coal mine, Chensilou coal mine, Pingdingshan 4th coal mine, Chiyu coal mine, Dongda coal mine, and Dongguashan mix well, etc. The application effectivenesses show good thermal stability, and achieve good social and economic benefits.