| The energy transformation and energy revolution of renewable energy, distributed generation and other clean energy as the main energy development goals get more and more attention from academic circles and industrial circles, thus the energy Internet formed with intelligent management of power allocation is proposed in such a context.In the energy internet architecture, the power electronic interface technology not only can support distributed generation, energy storage, controllable load equipment plug is used, but aslo can dispatch management and fault isolation effectively for electricity, is one of the key technologies in the Internet of energy to achieve. In some literature, the interface equipment with the ability of energy management and control is suited for energy router. The power electronic transformer(also known as EPT, PET or SST) with“high frequency electrical isolation and adjustable power factor, fault isolation and high control degree of freedom” and other advantages is considered to be the ideal choice for energy router as the core equipment.Due to its modular symmetrical structure, high power density, zero-voltageswitching(ZVS), bi-directional energy transmission ability and simple Phase-Shift(PS)control and other inherent advantages, dual-active-bridge(DAB) DC-DC converter has been the most widely concerned since the time of proposed.In many literatures single-phase-shift(SPS) control strategy doesn’t consider that in the process of the active power transferred from the input side to the output side, a part of energy will flow back to the input side, reducing the efficiency of the converter.This part of the energy is defined as the backflow power or circulating power or reactive power. In order to reduce the reactive power, several improved PS control strategies are proposed. For example, based on the SPS control strategy, the extend-phase-shift(EPS)and dual-phase-shift(DPS) control strategies are proposed by inserting phase shift angle into advanced or lagging H-bridge. The purpose of reducing the backflow power is achieved by reducing the time interval in which the voltage and current of the leakage inductance are of the opposite transferred direction. However, there are two problems as follows in the above research contents.On the one hand, the transferred power and backflow power model are built based on the method of instantaneous power integration in the most leteratures to analyze the power transfer performance of DAB converter. However, this modeling method is computational complexity and complex process, especially when the degree of freedom of the control strategy increases, the calculation complexity of the instantaneous power integral modeling method is further improved. Besides, this modeling method can’t establish a unified power model for all PS control strategies. On the other hand, even the minimum phase angle combinations of the backflow power is given, nor does itmean that minimum transport current effective value and minimum conduction loss are achieved under the same transferred power.By analyzing the reasons why the problems exist, it’s found that the method based on instantaneous power integration is a pure mathematics. The calculate process is not of physical interpretation corresponding to that in electrical engineering major,especially the reactive power(or circling power or backflow power). First of all, the definition this part power is not clear; secondly, the existed description to transferred characteristics of the reactive power is too rough, which is not only unable to reveal the physical meaning of the power essence, but also cannot be applied to discuss the relationship between the power and the current effective value.Another research focus of DAB converter is to reduce the switching loss by soft-switching technology, so as to realize the objective to improve the converter efficiency and the power density. By ignoring soft-switching resonant process and assuming that leakage inductance current is linear variation when the switching action occurs, the expression of phase shift angle is obtained under soft-switching limited condition, and the corresponding control method is designed. However, this method not only ignores the specific process of soft-switching and causes the soft-switching condition inaccurate, but also the switching loss model cannot be established, and the characteristics of soft-switching, the design of circuit parameters cannot be analyzed. In addition, the established control method in the case of different PS control has not yet come to a unified rule, which is not widely used. In order to make converter switching actions in the light load condition also achieve soft-switching, series saturation inductance or shunt inductance is imported to ensure that the leakage inductance of the low current situation also owns enough energy to achieve soft-switching. However, the switching losses and the state losses of the converter are improved with the increase of the switches and resonant unit.The small-signal model and the dynamic performance analysis of DAB converter in existed literature achieved by state-space-averaging which is in the basis of instantaneous power integration. And the process needs to repeat respectively in SPS,EPS and DPS control. However, there are several limitations in this method. The dynamic model achieved by state averaging method becomes more and more accurate with the increasing of the expressions order. But for the different PS control strategies,the model needs to be separately derived, which the unified dynamic model cannot be applied to all kinds of PS control strategies. The existed dynamic model, selected dc voltage and leakage current as state variables, cannot meet the requirement of active power and reactive power as control target.This paper relies on National Natural Science Fund Project-“Study on key technology of cascaded multilevel converter without power frequency transformer(51077125)” and “Research on the key technology of the new generation of highfrequency isolation cascaded high voltage frequency converter(51577187)”. In this paper, the DAB converter is the main study object which acts as the power transferred stage in power electronics transformer. By deep analysis and overall comparison in ZVS resonant process, steady-state characteristics and dynamic properties of DAB converter under SPS, EPS and DPS control strategies, this paper intends to propose equivalent circuit in ZVS resonant process, circuit parameter optimization design method and unified power and small-signal model for all PS control strategies.When the switching action occurs in SPS, snubber capacitances of a pair of conducting switches on the diagonal of the H-bridge, snubber capacitors of switches on off-state, leakage inductance of high frequency transformer and equivalent dc voltage of transformer forms ZVS resonant circuit, it is defined as “the equivalent circuit of four-switch resonant soft-switching” in this paper. When switching action occurs instantly in the EPS control, snubber capacitances of the switch to be turned off,snubber capacitor in the same bridge leg to be switched on, leakage inductance and equivalent dc voltage of high frequency transformer forms resonant circuit to realize the soft-switching. This paper calls the soft-switching circuit as “the equivalent circuit of the two-switch resonant soft-switching”. In the DPS control, based on the SPS control,both sides of H-bridge are embedded within phase shift angle, therefore, soft-switching process of DAB converter under DPS control is “the equivalent circuit of the two-switch resonant soft-switching”. At this point, the ZVS process of DAB converter of multi-PS control is analyzed in this paper.In order to achieve soft-switching as far as possible, it is supposed to adopt realization condition of the full soft-switching to design of the circuit parameter of DAB converter, so according to the state expression of the soft-switching resonance process,the design range of the snubber capacitance and leakage inductance parameters of the equivalent circuit of the two-switch and the four-switch resonant soft-switching can be obtained. The parameters design of SPS control and DPS control circuit can be realized.However, in terms of the parameter design of the EPS control, the soft-switching range of the two resonant circuits still needs to be compared. Through the research and comparison in this paper, it is concluded that the soft-switching load range of the traditional SPS controlled DAB converter is smaller than that of the extended PS control and the DPS control strategy.By redefinition of inner phase shift angle and outer phase shift angle in PS control,the output square voltage waveform and trapezoidal square current waveform on both sides of the transformer are transferred to Fourier decomposition. As there is no work between different harmonic, the phasor expression of voltage and current are established in the same frequency. And then the complex power model is founded based on the phasor expressions, and the model can be used to analyze the performance of DAB converter under SPS, EPS and DPS control. On the condition that the voltages are equalon both sides of the high frequency transformer, the phasor diagram are shown based on the phasor expressions. The control area and boundary condition are drawn in the analysis in phasor diagram.Similarly the active power and reactive power distribution diagram are figured out on the basis of the control area and boundary condition obtained by the phasor diagram.The power corresponding to different PS controls are drawn in one figure which extremely simplifies the comparison and analysis of power transferred performance. It’s founded that the reactive power in SPS is the minimum when the active power is specified. The small-signal model suited for all PS control strategies is built based on the differential equation around the ac/ac stage in the DAB converter.In summary, this paper has the following three major innovation points:1) Two resonant equivalent circuits of the “four-switch resonant circuit” and“two-switch resonant circuit” in the PS control are proposed, and the analysis of soft-switching resonant process in different PS control is unified and simplified; On the basis of this, the mathematical model of switching loss of two kinds of resonant equivalent circuits is established. The concept of “soft-switching curve” and the optimization design method of soft-switching limitation and the hardware parameters(snubber capacitance and leakage inductance) are defined and presented. 2) A unified phasor analysis method for all PS control strategy is proposed, the definition of active power and reactive power in DAB converter is defined, and the physical meaning of phasor diagram is also available, which is easy to understand and analyze; On this basis,a unified dynamic small-signal model is established, which is suitable for three phase shift control strategies, and under the same transferred power, the current effective value of the leakage current of the SPS control strategy is the minimum, and the dynamic performance of the DPS control strategy is optimal. 3) Hybrid control strategy, the SPS used in the steady state and dynamic process used in DPS, is proposed to improve the dynamic performance of the converter, which reduces the power loss of the converter and improves the efficiency of the converter.At last, the correctness of the theory proposed in this paper is verified in the semi-physical simulation and test platform, which lays a foundation for further research. |
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