High-performance Regulation Of T-type Three-level Inverters For Power Routers

Vigorous development of the energy internet is an inevitable choice for China to achieve the "double carbon" goal and promote energy transformation.As the core equipment of the energy internet,the multi-port power router based on advanced power electronic conversion technology and advanced information technology enables flexible conversion,transmission and routing of electrical energy.The AC port inverter is not only the key port connecting the PR to the grid or load,but also the core equipment for efficient and stable exchange of electrical energy.The T-type three-level inverter(3LT2I)has the advantages of fewer passive components,low harmonics,high power density and efficiency,which can effectively improve the performance of the AC port of the PR.At present,in response to the requirements of the regional PR for high efficiency and power at AC ports,there are still several problems to be improved:(1)Efficiency enhancement problem:As the PR integrates multiple port converters inside,the increase of the power conversion link will reduce the efficiency of the whole system.Therefore,improving the efficiency of the single-port converter is a necessary way to improve the overall efficiency.Among them,optimising the modulation strategy of 3LT2I is the main method of efficiency improvement.The traditional discontinous pulse width modulation(DPWM)strategy has been widely studied for its ability to reduce switching losses by having one phase of the switch clamped at a level at any given moment.However,the increase in efficiency leads to an increase in output current harmonics and the method of clamping by injecting zero-sequence voltages(ZSV)is coupled with neutral-point(NP)potential balancing control,resulting in large NP voltage fluctuations and poor power quality.(2)Control problems in parallel systems:To increase the capacity of the AC ports of PR,parallel machines are routinely connected in parallel.However,the loop current can lead to distortions in the output waveform,reduce efficiency,and affect the NP potential,causing oscillations.The poor dynamic performance of traditional NP control methods can further reduce system losses and deteriorate waveform quality.Therefore,based on the summary of the research results at home and abroad at this stage,this paper aims at the "high efficiency,low harmonic and high reliability" operation of the AC port of the PR,and carries out research on the modulation and control strategy of the 3LT2I:Firstly,the background and research significance are clarified,and the research status of inverter topology,modulation strategy and parallel system control strategy is analysed.Topology and basic principles of 3LT2I are introduced.The mathematical models under different coordinate systems are established to reveal the relationship between the four types of voltage vectors and the NP potential,laying the foundation for the later research.Secondly,to address the problems of high losses and low efficiency of a single inverter,an improved DPWM strategy is proposed.This strategy uses a low harmonic adaptive DPWM strategy,which effectively reduces the harmonics injected into the ZSV and injects the opposite offset component into the ZSV according to the NP voltage deviation.The NP potential is adjusted by varying the modulating waveform’s positive and negative clamping time to change the number of P/N vectors.Compared to conventional DPWM strategies,this method reduces output harmonics and quickly balances the NP potential.Simulations have shown that this method is effective in reducing switching losses,balancing the NP potential and improving the output power quality.Thirdly,for the NP potential imbalance and zero-sequence circulating current(ZSCC)problems caused by the parallel 3LT2I,it proposes a control strategy with fast NP potential balancing and ZSCC suppression.Especially,for the NP potential imbalance problem,a NP potential balancing strategy based on a finite-time controller(FTC)is proposed.Due to the fractional power terms,it has better dynamic performance than PI controller and can achieve fast balancing of the NP potential and suppress its fluctuation.The "P+feedforward"circulator is designed for the ZSCC problem,and the controller parameters are reasonably designed to effectively suppress the fluctuations and spikes of the ZSCC without affecting the NP potential balance.This method has been verified by simulation analysis.It can quickly balance the NP potential,suppress the ZSCC,improve the quality of the output current waveform and ensure the safe and stable operation.Finally,based on the "DSP+CPLD" framework,a 10kW 3LT2I experimental platform is built,including the main power circuit,driver circuit,sampling and conditioning circuit of the inverter,and the feasibility and effectiveness of the strategy proposed in Chapter 3 are verified.Based on the HIL hardware-in-the-loop system experimental platform,the main circuit model of the parallel inverter was built and loaded into a real-time simulator.The conditions of the simulator were observed in real time by the host computer to verify.the feasibility and effectiveness of the strategy proposed in Chapter 4 for the parallel system.