Research On Topology And Control Of Mixed Frequency Conversion Cascaded Power Electronic Transformer

The modern power grid is entering a post-carbon era characterized by "New Energy+Internet".Forming the Energy Internet(EI)and developing the distributed generation(DG)is the key to"Clean Alternative" of electrical energy.Distribution network(DN)is a main scenario for DGs.With power consumption rises,China's DN is gradually using 10kV and above medium voltage,forcing equipment to higher voltage.Power Electronic Transformer(PET)is one of the core equipment of EI.It may partially replace the line-frequency one in DN.Cascade type PETs can withstand high voltage.They have MVAC,LVAC and LVDC,providing interface for DGs.However,the existing topologies of the cascade type PETs have high hardware costs,low compactness,and engineering difficulties.To settle the problems above,this thesis proposes "mixed frequency conversion" concept,and focuses on the topology and control of Mixed Frequency Conversion Cascaded PET(MFCC-PET)as follows:Concept of mixed frequency conversion is proposed as well as its two implementation methods.Then,topology of MFCC-PET is first proposed,topology construction method is formed.MFCC-PET solves the problem of large number of high frequency transformers(HFT)and power switches in existing cascade type PET topologies.Saving hard ware costs,improving structural compactness,and good for engineering.On the two methods of mixed frequency conversion,hard switching(HS)and ZVS modes of MFCC-PET are proposed respectively.Function principle of MFCC-PET is revealed and design method of main circuit parameters is established,supporting MFCC-PET's stable and reliable operations.A well selected and designed HV-side multilevel modulation strategy is the key to high-performance of MFCC-PET in HS mode.Based on the sub-module(SM),by pure delay,zero-order holder,and Pade approximation,fundamental-wave approximation models of bridge arm are established for CPS-PWM under asymmetric regular sampling,which more accurately reveal the delay mechanism of CPS-P WM than the existing models.Then,applicability of CPS-PWM in MFCC-PET is evaluated.CPS-PWM and nearest level-PWM(NL-PWM)are compared,further,a sawtooth carrier NL-PWM is proposed.First,for traditional modular converters-MMC and CHB,the Double-Fourier Transform and Jacobi-Anger series based harmonic analysis shows,the proposed modulation has lower line voltage THD and higher line current quality than the triangular carrier one.Based on the proposed modulation,a switching-frequency-reduced SM balancing rotating method is proposed;Then,for MFCC-PET,achieving of sawtooth carrier NL-PWM is proposed;Finally,power-switch loss when bridge arm current has both low-and high-frequency components is calculated.The power-switch loss in HS mode is slightly higher than that of the traditional cascaded PET.But the number of HFTs and power switches are reduced,making MFCC-PET lower costs,compact structure and easy implementation.To comprehensively and deeply study the HS mode,a MFCC-PET experimental platform was developed:The main circuit design meets the parameter constraints of HS mode.For the controller,since the relatively small number of SMs in DN applications,a high-performance controller with multiple IOs and low latency is developed on the concept of centralized control.Utilizing the platform,experiment of MFCC-PET's steady-state and dynamic-state is carried out.The parameter design and the theoretical analysis of HS mode were verified.The advancement of MFCC-PET was proofed.This platform provides strong support for further research on the system-level applications,etc.of MFCC-PET.By modal analysis,high frequency conversion stage current modes and their constraints in ZVS mode are established.Expressions of keen current are derived.Further,the influence of the related PET parameters on the ZVS are concluded.Utilizing the freedom of LV-side modulation index of MFCC-PET,wide-load-rate-range ZVS parameter design and online setting methods are proposed,ensuring a wide-load-rate-range ZVS of power switches both on HV-and LV-sides of HFT.To avoid changing the on-off time of power switches,which destroys ZVS,a novel SM balancing control is proposed,solving the contradiction between ZVS and voltage balancing.In ZVS mode,current spectrum is more abundant than HS mode,a method for power-switch loss calculation is proposed.Based on SiC MOSFET,in ZVS mode,power-switch loss is slightly higher than that of the traditional cascaded PET with ZVS,but the number of power switches is further greatly reduced than HS mode,thus being low cost,compact structure and easy implementation.For MFCC-PET in ZVS mode,the large inrush current at start-up due to the high-frequency conversion stage is approximately inductive and the reactance is small.Based on the analysis of the existing control,an extended dead-band high frequency conversion stage start-up control is proposed as well as its parameter design method.Experimental verifications are carried out.Compared with the existing control,the proposed control is very universal and easy implementation.This study supports the efficient and safe start-up of MFCC-PET in ZVS mode,as well as PETs based on the single-phase-shift control.