A Modified DC Power Electronic Transformer Based on Series Connection of Full-Bridge Converters
Abstract: This paper proposes a novel DC power electronic transformer (DCPET) topology for locomotive, AC/DC hybrid grid, DC distribution grid and other isolated medium-voltage and high-power applications. Compared with conventional PET topology, the proposed DCPET has fewer power semiconductor devices and highfrequency isolation transformers, which can improve the power density and reliability. Fault handling or redundancy design can be achieved to further improve the reliability when some DC-DC modules break down. Also, input voltage sharing (IVS) control can be omitted to simplify the control system and improve the stability. Meanwhile, soft switching is guaranteed for all the switches, which is beneficial to increase switching frequency and improve power density. In this paper, the principle, evolution, and control of the proposed DCPET are respectively presented and studied in detail. Finally, a prototype of the proposed DCPET is built and the experimental results verify the validity and superiority of the proposed topology. Existing system:
This paper is organized as follows: Section II introduces the basic DCPET topology. The principle and characteristics of this topology are analyzed in detail. Section III discusses the evolution of the basic topology. Some derivative topologies are proposed and studied, which can be used for various applications. Section IV proposes the control strategies of the mentioned DCPET topologies, which mainly covers the voltage and power control strategy and fault-handling strategy. Section V verifies the validity of the proposed DCPET topology and the control strategy by experiments. Section VI makes a comparison between the proposed topology and the existing topologies. Finally, Section VII concludes this paper. Proposed system: However, when the DC input capacitor is bypassed, there will be a large shortcircuit current, and a normal by-pass switch cannot withstand the surge current. In, a resistance is used to be series with the by-pass switch to restrain the surge current. When the voltage and power level are high, the cost and loss of the resistance will be large. To improve the ability of fault handling, a high frequencylink dc transformer based on the switched capacitor (SCDCT) is proposed] and a multilevel high-frequency-link dc transformer (MDCT) is proposed. They all have a good performance of fault handling, but the power density (include the number of transformers, switches, and DC capacitors) still need to be further improved like conventional DCPET with ISOP structure. Advantages: However, when the DC input capacitor is bypassed, there will be a large shortcircuit current, and a normal by-pass switch cannot withstand the surge current. In, a resistance is used to be series with the by-pass switch to restrain the surge current. When the voltage and power level are high, the cost and loss of the resistance will be large. To improve the ability of fault handling, a high frequency-link dc transformer based on the switched capacitor (SCDCT) is proposed in and a multilevel high-frequency-link dc transformer (MDCT) is proposed. Disadvantages:
Moreover, DCPET with ISOP structure also has a problem of power balance. According, to achieve power balance of the DC-DC modules, an input voltage sharing (IVS) control strategy should be applied. The auxiliary control loop will make overall control system more complex, and the final output variables of the controller for each DC-DC module will be different, which will not only lead inconsistency of each DCPET module but also reduce the stability of the control system. Modules: Input voltage sharing: Moreover, DCPET with ISOP structure also has a problem of power balance. According to, to achieve power balance of the DC-DC modules, an input voltage sharing (IVS) control strategy should be applied. The auxiliary control loop will make overall control system more complex, and the final output variables of the controller for each DC-DC module will be different, which will not only lead inconsistency of each DCPET module but also reduce the stability of the control system. Based on the research introduced before, a novel DCPET topology is proposed in this paper. Power electronic transformer: POWER electronic transformer (PET) is is a kind of power conversion device with the characteristics of high frequency, bidirectional power flow and electrical isolation based on power electronics technology, which can also be named as solid state transformer (SST). In recent years, PET is widely used in AC/DC hybrid grid, DC distribution grid, new traction converter for locomotive, which is usually named as power electronic traction transformer (PETT) and other medium-voltage and high-power applications. According to the different requirements of the power conversion, there are many cascaded structures of PET, such as AC-AC, DC-DC, AC-DC-DC, and AC-DC-DC-AC. Except for AC-AC structures; other cascaded structures of PET usually contain a DC-DC stage. The DC-DC stage is used to achieve DC voltage conversion, bidirectional power flow and high frequency and electrical isolation, which can be regarded as the core of PET. Therefore, the DCDC stage of PET can also be named as DCPET.
Input – series output – parallel: At present, the commonly used topology of DCPET for MVDC applications is a kind of input-series output-parallel (ISOP) DC-DC converter, which has been deeply studied and published. ISOP DC-DC converter has the characteristics of good modularity and simple operation, but with the increasing of the voltage and power level, many DCDC modules are needed. Thus, many medium- or high frequency transformers and power semiconductor devices will be applied and the performance of DCPET, such as power density, cost and reliability will not be further improved. To reduce the number of transformers, a three-level DC-DC converter is used to replace two DC-DC modules. However, the number of transformers only can be reduced by half. To further reduce the number of transformers and switches, the multilevel structure is proposed switches-series scheme is discussed in, for these structures, only one transformer is needed. Switched capacitor: To improve the ability of fault handling, a high frequency-link dc transformer based on the switched capacitor (SCDCT) is proposed and a multilevel highfrequency-link dc transformer (MDCT) is proposed. They all have a good performance of fault handling, but the power density (include the number of transformers, switches, and DC capacitors) still need to be further improved like conventional DCPET with ISOP structure. Moreover, DCPET with ISOP structure also has a problem of power balance. According to, to achieve power balance of the DC-DC modules, an input voltage sharing (IVS) control strategy should be applied. The auxiliary control loop will make overall control system more complex, and the final output variables of the controller for each DC-DC module will be different, which will not only lead inconsistency of each DCPET module but also reduce the stability of the control system.