Research Of Three-Level Neutral Point Clamped Type Converters And Their Modulation Strategies

The high-voltage high-power converter plays an important role in the generation,transmission,distribution and of the power system.The multi-level structure is a topology solution with low voltage stress and flexible modulatin strategy.The neutral point clamped(NPC)multi-level topology was first reported in the 1980 s,which has a history of 40 years development,but the fileld of its applications is still expanding,new applications also put forward new requirements of the converter,so there still many problems to be explored and solved.Based on the NPC three-level leg,this paper explores the improvement of the topology and the optimization modulation strategies in three important applications: unidirectional DC-DC converters,bidirectional DC-DC converters and inverters.The main research contents of this paper are as follows:For the high-voltage input three-level full-bridge(TLFB)DC-DC converter,the cause of the deviation on the neutral point voltage is revealed and the mechanism of the self-balance ability provided by the flying capacitors is also analyzed.Under ideal conditions,the power supply stage in which an individual input capacitor provides the load current dose not exist in the TLFB converter,so the neutral point voltage will not deviate from the balance state.However,there may be slight discrepancies in the switching behavior,when the two diagonal switches are not turned off simultaneously,an individual input capacitor will supply the load current alone,then the voltages of the input capacitors diverges from each other.Two solutions are proposed to solve this problem: at the modulation level,the neutral point voltage can be controlled by modifying the pulse width with a close loop regulation;at the topology level,the self-balance of the neutral point voltage can be realized by adding the flying capacitors.After that,a detailed mode analysis of the TLFB converter with flying capacitors is carried out.It is found out that the introduction of flying capacitors can provide extra compensation current branch for the input capacitors,which explains the mechanism of the self-balance ability on the neutral point voltage.The influence factors of the steady state voltage error are also analyzed,and the concrete expression of the voltage error is derived.Finally,the correctness of the theoretical analysis is verified on the experimental platform.In high-voltage output application,the rectifier diodes in the TLFB converter have to bear the whole secondary voltage of the transformer,in order to improve the performance of the converter,the low-voltage rating diodes are expected to be used in this application.In this paper,a NPC type three-level structure with flying capacitors is introduced in the secondary rectifier stage to construct a new converter topology,which has the same modulation strategy as the conventional full-bridge converter,but the operating performance has been improved in many aspects: ?1 the voltage stress of the rectifier diode is reduced by half,moreover,the flying capacitors can guarantee the voltage distributed equally on the series-connected diodes;?2 not only can the leading switches realize zero voltage switching(ZVS)as the same as the conventional converters,but also the lagging switches can realize zero current switching(ZCS)in discontinuous conduction mode(DCM)and ZVS in continuous conduction mode(CCM);?3 due to the secondary flying capacitors,there are no voltage spikes on the rectifier diodes even if no snubber circuits are attached;?4 the rectified voltage has no zero-state voltage,it has been raised to half of the input voltage,so the voltage fluctuation is lower than the conventional converter under the same modulation strategy,and the current ripple of the filter inductor can be reduced by half.The operation principle of the proposed rectifier structure is analyzed in details,and a prototype is fabricated to verify the features of the proposed converter.For the three-level dual-active-bridge(DAB)converter applied to bidirectional power transmission,a modulation with five control degrees of freedom is proposed,which is a trade-off solution between the control flexibility and implementation complexity.This modulation strategy can take the advantage of control flexibility in three-level structure,moreover,the implementation algorithm is relatively simple for ordinary microprocessors.Based on this modulation strategy,an optimization algorithm for the root-mean-square(RMS)value of the inductor current is proposed,which extracts the fundamental and third harmonic components from the high-frequency voltages generated by primary and secondary H-bridges,then obtains the optimal duty cycles by adjusting the amplitude of each component.Compared with the traditional optimization algorithms,the proposed strategy does not require detailed mode analysis,which simplifies the calculation process and reduces the implementation complexity.Moreover,the proposed optimization process is more intuitive and helps better understanding the operating characteristics of the converter.Finally,the performance of the modulation strategy and the optimization algorithm is verified by simulation and experimental results.Compared with the conventional single phase shifted(SPS)modulation strategy,the proposed method significantly reduces the RMS value of the inductor current and improves the system efficiency at ligh load.For the three-level NPC inverter,a double modulation waves carrier-based pulse width modulation(CBPWM)strategy is proposed,which features neutral point low-frequency voltage elimination and common-mode voltage reduction.In traditional CBPWM strategy,there is only one modulation wave corresponding to each phase,therefore,the NPC leg can only output two states in a switching cycle.The double modulation waves CBPWM strategy has two modulation waves for each phase,then three states can be generated in a switching cycle,so many complex optimization algorithms can be realized with this strategy.In this paper,the optimization objective is to eliminate the low-frequency voltage on the neutral point,based on the principle that the zero-state ducy cycles of each phase are the same,the expression of ducty cycles are deduced and the modulation signals are constructed.Based on this modulation strategy,the voltage vectors used in the modulation process are analyzed,and the switching sequence is rearranged optimally to reduce the common-mode voltage generated by the inverter within one-sixth of the input voltage.Finally,the performance of the modulation strategy and the optimization algorithm are verified by the simulation and experimental results: with the double modulation waves CBPWM method,the low-frequency voltage on the neutral point is completely eliminated;with the rearranged switching sequence,the common-mode voltage is reduced from 240 V to 120 V.