| As the global climate continues to deteriorate and carbon emissions remain high,how to build a clean,low-carbon,safe and efficient energy system has become a common concern of the world.For this reason,China has put forward the development strategic goal of "carbon peak and carbon neutral".By vigorously increasing the installed capacity of solar power generation,speed up the construction of new power system with new energy as the main body;In order to meet the load demand and reduce the carbon emissions of the power generation side.However,due to the strong randomness and fluctuation of distributed power supply,large-scale grid connection poses a severe challenge to the stability and reliability of traditional AC power system.Compared with AC power grid,DC power distribution network has no problems such as voltage phase,frequency and reactive power,which makes grid connection of renewable energy represented by photovoltaic and wind power more convenient and economical.Therefore,the development of DC distribution network is an important direction of the future development of new power system.As the core equipment of energy exchange and multi-voltage level transformation,the function and performance of multi-port DC converter directly affect the stable operation of DC distribution network and the efficient consumption of new energy.Among them,Three Active Bridge(TAB)is an important direction of multi-port DC converter because of its advantages of electrical isolation,diverse working modes,high integration and flexible control.At present,most of the researches on TAB converters focus on efficiency optimization and soft switching capability,but few research on the operation control method of the converter in the transient process.In the transient process,the interaction between ports is aggravated by the coupling power of TAB converter.On the one hand,when a port has heavy load switching,not only the voltage and power stability of its own port will be affected,but also the voltage and power of its coupling port will be significantly affected.On the other hand,transient overcurrent induced by short circuit fault will not only damage the power device of the failed port,but also impact the voltage and power of the coupling port when the failed port is out of operation.In view of the above deficiencies,this paper carried out research on TAB converter’s dynamic response capability optimization under heavy load switching condition and operation reliability improvement under short circuit condition,as follows:(1)The modeling of TAB converter under steady-state condition is studied theoretically,and the working mode and power transmission characteristics under single phase-shift modulation are analyzed.Firstly,the switch on/off state of power device and the current path of inductor are analyzed in different operating modes of TAB converter under single phase shift modulation.The Y equivalent circuit model of TAB converter is constructed,and the voltammetric characteristics of inductance current are analyzed with modal analysis method,which provides theoretical support for the analysis of inductance current bias under port short-circuit condition in Chapter4.The Δ equivalent circuit model of TAB converter is constructed,and the power transmission expression of each port is deduced.A small current signal model is established to analyze the power coupling mechanism of TAB converter.Diagonal matrix decoupling control method is adopted to achieve the purpose of port power decoupling.Finally,it is verified by simulation.(2)For heavy load switching,a TAB load current feedforward control method based on decoupled network is proposed.Firstly,the dynamic characteristics of TAB converter with or without decoupling control are analyzed.Secondly,the principle of load current feedforward control to dynamically adjust the lifting speed is analyzed.Based on the analysis of power coupling mechanism of TAB converter in Chapter 2,the concept of virtual DC voltage component is introduced.The diagonal matrix decoupling network was added into the feedforward control loop of load current,and a TAB load current feedforward control method based on the decoupling network was proposed.The effect of coupling power between ports is eliminated while the dynamic response speed is improved.Bode diagram was used to analyze the response characteristics.Finally,it is verified by simulation.(3)Aiming at the short-circuit condition of output port,the fault transient characteristics of TAB converter are analyzed and a fault tolerant operation control method is proposed.Firstly,the influence of circuit inductance on short-circuit current is considered.The transient impact characteristics of the dc bias peak of the internal inductor on the switch tube after a short circuit fault are analyzed.The transient impact characteristics of the external short-circuit current on the reverse parallel diode are analyzed.Whether the controllable power devices are affected by fault current impact is discussed.A fault tolerant operation method of series setting inductor is proposed for fault ports.A phase shift Angle compensation method based on load current feedforward is proposed for non-fault ports.The analysis shows that the fault-tolerant operation of output port under short-circuit condition can be realized without locking controllable power device,which simplifies the control complexity of the system to a certain extent.Finally,the rationality of the theoretical analysis is verified by simulation and the effectiveness of the control method.(4)TAB experimental prototype development and experimental verification.A5 kW TAB converter experimental prototype is designed,and the design scheme of hardware circuit such as main circuit,drive circuit,power supply circuit and sampling circuit is analyzed in detail.The main program frame of the software system is constructed.The experimental platform verifies the correctness of the theoretical analysis of diagonal matrix decoupling control principle and the feasibility of load current feedforward control method based on decoupling network. |
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