Study Of The Back To Back Three-level PWM Converter With A Low Switching Frequency

Medium voltage high-power converters have advantages in environmentalprotection and energy savings considering their small harmonic pollution for the loadand the grid, and the high efficiency in energy conversion. The switching frequency ofthe power semiconductors is usually limited to below500Hz to reduce the switchlosses and to improve the output power of the converters. However, a low switchingfrequency affects the control system and the modulation algorithm. Some detailedresearch mainly focusing on system modeling, modulation algorithm and controlstrategy, have been carried out in this dissertation by taking the diode clamped doubleback to back three-level high-power PWM converter as an example.Firstly, a complex model is established for the three-level PWM voltage sourcerectifier (VSR) considering the influence from low switching frequency. And then, anovel current controller with complex state variables is proposed to solve thecross-coupling between the idand iq, whose stability and effectiveness are bothverified by the modern control theory. Also, a new modulation strategy combined withasymmetric regular sampling and the space vector PWM (SVPWM) is studied as wellas its improvement, which ia called asymmetric space vector modulation (ASVM),after the first using of a novel double fourier analytic approach to deeply analyze theharmonic output performance of the three-level PWM topology.In order to make the output current of high-power grid-connecting convertersmeet the grid standards with a low switching frequency, a novel pulse pattern basedon the combination of model predictive control (MPC) and sliding discrete fouriertransform (sDFT) has been presented. Firstly, extract the current fundamentalamplitude as well as the harmonic components’, and which will be taken as a part ofthe cost function for MPC besides the consideration of switching frequency andneutral point potential balancing issues. This kind of pulse pattern not only has acharacteristic similar with the selective harmonic elimination (SHE), also can realize adynamic regulation. For the further algorithm simplification, a reduced-ordermodulation from three-level to two-level is adopted here by the reference coordinatestheory to realize a simple digital implementation.Taking an electrically excited synchronous machine (EESM) as a load ofthree-level converter, a novel complex matrix model based on the combination of thematrix and complex vector methods is proposed considering the rotor asymmetry of an EESM. Owing to the low switching frequency, the harmonic current componentsof the stator are increased and extracting the fundamental content by low-pass filtersproduces a signal delay and amplitude reducing, which can not meet systemrequirements of high dynamic control. To overcome these problems, a novel hybridfundamental current observer at different coordinates for EESMs is designed based onthe observer theory and the complex matrix model. The stability of the fundamentalobserver at different speed or with a parameters mismatch has been analyzed using theclassic control method. Then, a complex current controller is applied to realize thedecoupling between imand it.For the subsection modulation with a low switching frequency, the demarcationpoint is firstly given based on the analytical results from double Fourier approach. Forthe disadvantages that the conventional synchronous symmetry optimal PWMalgorithm only being calculated off-line, a novel dynamic performance schemecombining the receding horizon policy and SHEPWM is firstly rolled in the controlstrategy for EESMs. By taking stator flux as the tracking trajectory, the flux trackingerror can be converted to volt-second value in the receding horizon, and the switchangles calculated from SHEPWM can be modified dynamically to meet the suddenchanged conditions.In order to make a further performance improvement for the back to backthree-level PWM converters, an approach controlling the neutral point potential isresearched based on the optimal selection of the sub-bridges for low modulation index.This method doesn’t need the output current as a feedback signal, which avoids theeffect from the severe distortion of the output current at low switching frequency. Fornon-ideal grid environment, a new kind of phase locked loop (PLL) both suitable forthe single and three phases is studied. With the characteristics of SHE, the SHE theorycombined with square wave has been adopted to form the novel phase detector, andwhose feasibility is proved by the fourier analysis. Lastly, a control strategy to be usedunder non-deal grid environment is also given.Finally, an experimental platform has been set up to complete the verification ofthe cooresponding research contents in this dissertation.