Research On Key Techniques Of Medium Voltage High Power NPC Three Level Converters Based On IGCTs

Focus on "IGCT" device and "NPC three-level" topology, key techniques of medium voltage high power converters are discussed in this dissertation, involves device characteristics and drive techniques of IGCT, design method of PEBB, modulation of NPC-3L and resolution of neutral point balance problem.Firstly, drive techniques of4000A/4500V series asymmetric IGCT devices are discussed. The basic structure and operation mechanism of IGCT are analyzed, the design requirements of gate unit to achieve basic functions such as turn-on, turn-off and conducting are proposed. The variation rules of gate-to-cathode voltage according to different working conditions in NPC-3L topology are analyzed. How to detect these working conditions and related logical control requirements such as status detection, back-porch current limitation and retrigger are explained. A4000A/4500V asymmetric GCT gate unit is developed. Finally, through circuit basic function test, single pulse switching test and three-level operation mode test, the drive theory of IGCT and the performance of designed gate unit are verified.Secondly, multiple objective SVPWM strategies is equivalent realized by simple SPWM principal. According to the principal of time-voltage product conservation, line voltage space vector diagram and switching window shift diagram are established to derive equivalent algorithms between the two modulation strategies. The two diagrams bridged the gap of duty cycle and vector sequence between the two modulation strategies. Based on the analyses, using zero sequence injection and modulation waveforms decomposition, a few different modulation algorithms are deduced to reduce switching loss, optimize output waveforms quality and balance the neutral point voltage. Simulation and experiments indicated that, using zero sequence injection and modulation waveforms decomposition to equivalent realize multiple objective SVPWM strategies, not only simplify the modulation algorithms, but also realize the smooth switching of different modulation strategies with each objectives.Thirdly, output AC voltage compensation and neutral point voltage feedback control under neutral point voltage unbalanced conditions are discussed. Based on zero sequence injection or modulation waves decomposition, two neutral point feedback control strategies are proposed. Using the two strategies, not only output voltage can be guaranteed no distortion but also neutral point voltage can be adjusted at will. Both the two modulation strategies are based on SPWM principal, so they are very simple to realize, and suitable for enhancing neutral point balance control or applications where need to control up and down DC link voltage separately. The neutral point current control abilities of the two strategies are given separately, and the neutral point voltage low frequency fluctuation amplitude is also quantized when using zero sequence injection way.Fourthly, a novel design method of the clamping circuit in IGCT converters is proposed and the calculation process has been greatly simplified. After a detailed analysis of the clamping circuit commutating process, an equivalent circuit is presented and its response analysis is made. According to the design rule of the clamping circuit, an optimal parameter design method is introduced, and it reduces the minimum pulse width and speed up the dynamic recovery process. Finally, a design example is presented. Simulations and experiments are made to verify the goodness of the result.Fifthly, power loss estimation method for IGCT NPC-3L converter is discussed. The undetermined coefficients in switching loss calculation formulas are determined through double pulse test. Traditional loss estimation method is restricted by one particular modulation strategy and there is estimation error due to idealized working conditions. Aim at this problem, a universal NPC-3L power loss simulation estimator suitable for any modulation strategies is presented. Output current ripple and DC link voltage fluctuation are considered in the estimator. Through PEBB half bridge power test, the accuracy of estimator is verified to meet the engineering requirements. Through estimator calculation, the power capabilities of PEBB under different modulation strategies and critical thermal conditions are analyzed.