Power Electronic Transformer Research Articles

A Fault Location Method for DC Lines Connected With DAB Terminal in Power Electronic Transformer

Power electronic transformer (PET) composed of modular dual active bridge (DAB) blocks has a promising future in ac-dc hybrid distribution network because of its energy routing ability. The key element that restrains the development of PET is the vulnerability of power electronic devices during faults, especially dc faults. DC faults (mostly line faults) have very short transient process and no obvious fundamental frequency, which means fault location is difficult to be realized for dc lines in distribution network. Considering that fault location plays an important role in realizing fast fault clearance and recovery, this paper analyzed fault characteristics of dc line faults with respect to the basic structure of modular DAB block and proposed a two-terminal fault location method based on transient voltages measured in dc line boundaries. To evaluate the validity of proposed method, numerical simulations with different fault conditions were implemented in PSCAD/EMTDC. The simulation results indicate that the proposed fault location method located the faulted point quickly and correctly.

Fault Characteristic on DC Pole-to-Pole Short-Circuit in Distribution Electronic Power Transformers

The analysis on fault characteristic of Distribution Electronic Power Transformer (D-EPT) can help design its operation and protection scheme, hence improve the reliability of the D-EPT as well as the distribution power grid. The DC pole-to-pole short-circuit fault at DC-Link is analyzed with R-L-C resonant mode and free-wheeling mode, and the theoretical analysis on the short-circuit voltage and current is based on several influence factors and relevant system parameters, which indicates that the fault characteristics are related to the configuration, operation and system parameters of D-EPT.

Influencing mechanism of phenyl thioether group in the main molecular chain on high frequency electrical properties of the modified polyimide films

Polyimides (PIs) and their nanocomposites may render quite high dielectric loss under high frequency (HF) electrical stresses, which will significantly affect the insulation properties and operating lifetime of the equipment when the composite materials are used in the high-voltage and large-capacity power electronics transformers (HFPTs). In this paper, the phenyl thioether groups of various molar ratios were introduced into the diamine residue as to prepare modified PI films via alternating copolymerization method. Frequency-domain dielectric spectrum and high frequency surface discharge were adopted to study the HF electrical properties. The results indicated that with the increase of the phenyl thioether content, the dielectric loss factor firstly decreased and thereafter rose up again, while the lowest arrived at 9.29 × 10–4 for a PI film incorporating 40% phenyl thioether content, which case also presented the longest endurance lifetime against surface discharge tests at 15 kHz. Based the UV–vis tests on the charge transfer complexation, further analysis verified that for the 40% molar content of phenyl thioether in the PI molecule, it was the weak charge transfer characteristics that resulted in the smallest stacking structure of the ordered molecular chain in the film, which led to the weakest lossy polarization under the HF alternating electric field and manifested the minimal dielectric loss factor accordingly. The proposed research presents a feasible technological approach to regulate HF electrical properties of the modified PI films.

Energy storage device locating and sizing based on power electronic transformer

In this study, firstly, the bi-directional energy flow of grid-connected photovoltaic and energy storage system based on power electronic transformer is demonstrated. Based on this, a bi-level programming model is proposed for the location and capacity of energy storage. The optimisation of the location of the outer layer is based on the improved particle swarm algorithm. The energy storage location is a variable, and network loss as well as PET loss are objective functions. The improved particle swarm optimisation algorithm is still adopted to optimise the capacity in the inner layer. The cost of electricity from the main grid is taken as the objective function, and the economic dispatch is realised based on the energy routing strategy of power electronic transformer.

Design and control of a SiC‐based three‐stage AC to DC power electronic transformer for electric traction applications

Design and control of a SiC‐based three‐stage AC to DC power electronic transformer for electric traction applications

DAB‐based PET in MVDC traction and shipboard applications with distribution and redundant control

Medium-voltage DC (MVDC) grid has drawn more interest in the application of traction and large-scale ships, for its lower loss, higher reliability, higher controllability and simpler system architecture. DC breaker and DC distribution are the key elements in the MVDC grid, and the power electronic transformers (PETs) have shown great advantage in the MVDC grid for its relatively smaller volume and higher flexibility compared to the traditional line transformers and AC/DC converters. In this study, a distribution DC PET based on input series output parallel dual active bridge (DAB) is proposed in the application of power supply system in traction and large scale ships. The DC PET consists of 12 DABs with an input voltage of 10 kV and the output voltage of 380 V. Strategies are proposed for distributed voltage balancing and redundancy control in the proposed PET. Simulations were done in MATLAB/Simulink. In the simulation, waveforms in the normal working conditions and the module fault status is shown, and the result shows that the proposed DC PET has great controllability and flexibility and by adding redundant modules, it keeps great reliability while fault occurs.

A Decentralized Optimal Operation of AC/DC Hybrid Microgrids Equipped With Power Electronic Transformer

In the AC/DC hybrid microgrids equipped with power electronic transformer (PET), different AC grids and DC grids are connected through multiport PET to operate the entire network in a secure and economic manner. This paper proposes a decentralized optimal power flow model, considering the multiport coordinated control strategy of PET, for running autonomous AC/DC hybrid microgrids. To optimize the power flow in the network, the steady state model of PET is established, including voltage source converter power injection model and Kriging model. By introducing a secondary droop control layer can realize the autonomous operation of AC/DC hybrid microgrids and increase the utilization efficiency of distributed generations. Then using the analytical target cascading approach, each AC/DC sub-microgrid is regarded as different autonomous subjects. The distributed PET is as power coupling node and information interaction node to decompose the optimal problem of AC/DC hybrid microgrids equipped with PET which is solved in parallel. The obtained subproblems for these grid levels are solved with a nested solution process. The simulation results verify the correctness and effectiveness of the proposed model in this paper.

Techno-Economic Evaluation of Mixed AC and DC Power Distribution Network for Integrating Large-Scale Photovoltaic Power Generation

This paper presents a methodology for evaluating the techo-economic feasibility of mixed AC and DC distribution network to replace the existing AC distribution network with large-scale integration of photovoltaic energy sources. Economic dispatch of mixed AC/DC system is formulated in the presence of power electronic transformer (PET) for the flexible control of AC and DC power flows while maintaining the reliability constraints in terms of bus voltage, branch current carrying capacity, and the efficient operation of power electronic converters. Furthermore, the configuration of the PET and power losses of converters with a changing operation efficiency is incorporated into the optimal dispatch problem to assess the energy savings provided by the AC/DC power network. Two objectives for minimizing the operating cost and total power losses are also considered in the formulated economic dispatch. To obtain the Pareto-optimal solutions of this reliability constrained multi-objective optimization problem, the non-dominated sorting genetic algorithm-II (NSGA-II) associated with the interior-point method are applied in the proposed two-stage optimal algorithm to solve the optimization for PET configuration and operation. The method has been tested on the IEEE 33-bus radial distribution system with excessive PV installation and updated PETs. In comparison with the optimal dispatch of existing AC power distribution system, the joint optimization of PET AC/DC ports and PV inverters improves the flexibility of power flow control for both AC and DC networks and thus minimizing the PV curtailment and total operating cost.

System-Level Efficiency Evaluation of Isolated DC/DC Converters in Power Electronics Transformers for Medium-Voltage DC Systems

This paper presents system-level efficiency evaluation of the suitable isolated dc/dc converters in input-series-output-parallel power electronics transformers (PET) applied for medium-voltage dc systems and provides engineering insights into the design of PETs from a system perspective. To this end, three candidates-a single-stage dual active bridge (DAB) converter, a boost converter plus a DAB with a constant boost output voltage, and a boost converter plus a DAB with a constant DAB voltage gain-are selected and evaluated from a system-level aspect. Comprehensive system-level efficiency comparisons are done with the chosen candidates, considering numbers of normal and hot standby submodules, different working conditions, rated voltages and power of the PET, switching frequencies, and commercial devices. Analysis with detailed loss breakdowns clearly shows the distribution of losses and elaborates the profound reasons behind the efficiency differences. In the comparison, the boost and DAB with a constant boost output voltage have higher or similar efficiency than the other two candidates, verified by simulation and experimental results. Considering its advantages in redundancy in the PET, the boost and DAB with constant boost output voltage are a preferable choice for submodules in input-series-output-parallel PETs.

Virtual Submodule Predictive Control method for Power Electronic Transformer based on MMC Structure

In order to achieve flexible and efficient operation of intelligent power distribution, solving the problems of traditional distribution transformer such as large volume and weight, easy to generate harmonics when overload, and need supporting protection equipment to protect it when failure, etc. We propose a power electronic transformer structure based on modular multilevel converter (MMC). Firstly, we consider the multi-dimensional control target of MMC converter to establish a mathematical model. Then a virtual submodule predictive control method is proposed. The method introduces the concept of virtual submodule to realize the optimal switching state rapid mapping and reduce the switching loss of MMC. Finally, the experimental results show that the mmc-based power electronic transformer has excellent dynamic steady-state performance and can effectively overcome the high loss of traditional predictive control.

Novel Frequency-Doubling Modulation and Control Strategy for Three-Level Full Bridge based Power Electronic Traction Transformers

Power electronic traction transformers (PETTs) are widely investigated to substitute bulky line frequency transformers in railway traction system presently. This paper adopts the three-level topology for both grid-side cascaded converters and primary-sides of LLC resonant converters. The cascade number is reduced by half as well as isolated transformers compared to the traditional two-level configuration. Less cascade number of modular converters leaves relatively more space for electric insulation implementation, which is beneficial to PETTs in terms of the limited installation space in the train. A frequency-doubling modulation strategy for LLC resonant converters is proposed, which can enable the operating frequency of isolated transformers two times higher than the switching frequency to further reduce the transformer size without increasing the switching loss. Meanwhile the proposed modulation strategy can realize the capacitor voltage balance autonomously as well. Additionally, the control strategy of cascaded rectifiers and key parameter design guideline of LLC resonant converters are also introduced.

Individual DC Voltage Balance Control for Cascaded H-Bridge Electronic Power Transformer With Separated DC-Link Topology

For the electronic power transformer, the dc voltage balance is an important issue. Unbalanced dc voltages will degrade the performance of the device and may trigger the dc voltage protection. In this paper, an overall individual dc voltage (including high-voltage and low-voltage dc-link voltages) balance strategy is proposed for the electronic power transformer with separated dc-link topology. The strategy adjusts active powers flowing through the isolation and output stages in different power modules to enhance the dc voltage balance capability. Through the strategy, the high-voltage and low-voltage dc-links can be well balanced when unbalance occurs among different power modules (e.g., component parameter mismatches or some of the high-voltage or/and low-voltage dc-links are connected with renewable energy sources or/and dc loads). The proposed strategy is analyzed and supported by experimental validation.

The Regulation Characteristics of Bridge Modular Switched-Capacitor Ac-Ac Converter

The switched-capacitor (SC) converter has become a hot candidate to replace power electronics transformer (PET) in commercial and residential applications, especially in micro-grids. However, its poor regulation blocks it wide application. This paper presents an improved phase-shift control strategy for a resonant bridge modular switched-capacitor (RBMSC) ac-ac converter. On the basis of maintaining existing merits of the reported BMSC-based ac-ac converter, its output voltage can be adjusted continuously with a relatively wide margin, breaking through inherent difficulty in step-less regulation. Meanwhile, the bidirectional operation, including step-down and step-up modes, could be realized conveniently by changing the of range phase-shift angle. In addition, the partial switches are turned on under zero voltage switching (ZVS) and all switches realize ZVS turn-off, resulting in reduction of switching loss. Due to one switch in each bidirectional switch always works under synchronous rectification state instead of diode conduction, conduction loss will be reduced compared with other SC ac-ac converters. Most importantly, the safe-commutation issue in the introduced resonant branch is well solved and the voltage spike across switches is alleviated significantly. Finally, the experimental results are provided to demonstrate the validity of the theoretical principle of the converter.

Research on power electronic transformer applied in AC/DC hybrid distribution networks

The AC/DC hybrid distribution network is one of the trends in distribution network development, which poses great challenges to the traditional distribution transformer. In this paper, a new topology suitable for AC/DC hybrid distribution network is put forward according to the demands of power grid, with advantages of accepting DG and DC loads, while clearing DC fault by blocking the clamping double sub-module (CDSM) of input stage. Then, this paper shows the typical structure of AC/DC distribution network that is hand in hand. Based on the new topology, this paper designs the control and modulation strategies of each stage, where the outer loop controller of input stage is emphasized for its two-control mode. At last, the rationality of new topology and the validity of control strategies are verified by the steady and dynamic state simulation. At the same time, the simulation results highlight the role of PET in energy regulation.

Research on AC & DC hybrid power supply system with high‐proportion renewable energy of data centre

In the background of the energy Internet, the number of data centres is increasing, aiming at the shortcomings of power supply and reliability in power supply system of traditional data centre and the development trend of green data centres. This article proposes an AC & DC hybrid power supply system with high-proportion renewable energy. The system includes photovoltaic generation, wind power generation, photothermal power generation, thermal utilisation system, power storage system, and different types of loads. Therein, a multi-port power electronic transformer is used to realise AC & DC multi-level mixing system with high proportion of variety of distributed renewable energy. Thereafter, the power supply, power grid, and the load can efficiently complement each other by using the integrated energy storage system such as electricity storage and heat storage. In detail, the rationality and functionality of the AC & DC hybrid power system are analysed through the system simulation and power supply reliability calculation. Moreover, the results show that the proposed hybrid power supply can effectively improve the impact of high-proportion renewable energy access to the grid and provide a new design scheme for the green data centre.

Power flow control scheme for multiport power electronics transformers

Development of power electronics transformers (PETs) has made dc distribution and hybrid ac/dc systems a competitive solution for future network expansion. This study focuses on the functions of a multiport PET interfacing medium-voltage and low-voltage networks. A comprehensive power flow control scheme based on a globalised multiport transmission model of the PET and droop control is derived. This allows for a coordinated energy exchange between the ports of the PET, which enables autonomous operation. Simulations of the proposed approach in a hybrid ac/dc distribution network with different levels of distributed generation and loads verify the effectiveness of the method.

Model Predictive Control With Power Self-Balancing of the Output Parallel DAB DC–DC Converters in Power Electronic Traction Transformer

This paper focuses on the output parallel dual-active-bridge (DAB) dc–dc converters in power electronic traction transformers. A model predictive control with current-stress-optimized (MPC-CSO) scheme based on dual-phase-shift (DPS) control is proposed to improve the dynamic performance, balance the transmission power, and realize the current-stress optimization. The dynamic behavior of output voltage in the next horizon is predicted accurately under the input voltage fluctuation and load disturbance conditions by developing the prediction model. In addition, the proposed MPC-CSO scheme can track the output voltage with the desired value directly with no overshoot during the start-up process. Combining the MPC and CSO scheme, the fast dynamic response and high-efficiency performance of DAB converters can be achieved simultaneously, and the transmission power of each DAB cell can be self-balanced. Finally, three control schemes consisting of traditional voltage closed-loop control with single phase shift, traditional CSO control with DPS, and MPC-CSO with DPS schemes are compared in a scale-down three DAB dc–dc converter cells experimental prototype by using TMS320F28335+FPGA_6SLX45 as core controller. Extensive experimental results have verified the excellent performance of the proposed MPC-CSO scheme and associated analysis in this paper.

Multirate Digital Signal Processing and Noise Suppression for Dual Active Bridge DC–DC Converters in a Power Electronic Traction Transformer

Power electronic traction transformer (PETT) is a single-phase ac–dc conversional system, which is made up of a cascaded H-bridge converter and output-parallel dc–dc converters, such as dual active bridge dc–dc (DAB) converters. As the voltage fluctuation of the dc output voltage of DAB converters is very small and the gain cross-over frequency of the open-loop transfer function of DAB converters is generally high, the DAB closed-loop control system is very sensitive to the influence of noises. The typical noises include the equivalent noise due to the conflict of controller interrupts, the noise due to the mismatch of sampling frequency and switching frequency (which is denoted as switching noise), the noise due to the second-order voltage ripple in the dc input side, the noise due to the quantization process of the analog-to-digital converter (which is denoted as quantization noise), and the noise due to the measuring error of transducers and conditioning circuits (which is denoted as measurement noise). The generation mechanism of these noises and their impacts on the DAB converters are analyzed. On the basis, the corresponding noise suppression schemes are presented, especially for the switching noise, quantization noise, and measurement noise. Besides, the mathematical models toward the sampling and the phase-shift modulation under multirate circumstance are derived, and the multirate digital control strategy of the PETT is proposed. Finally, based on a five-cell PETT laboratory prototype with a rated power of 30 kW, further research is carried out and the experiment results all verify the effectiveness of the proposed algorithms.

Input Filter for a Power Electronics Transformer in a Railway Traction Application

The need for lighter and smaller railway traction drives and recent advances in power semiconductor technology have resulted in a new energy conversion paradigm, one without the classical heavy and bulky line-frequency transformer. Several multilevel topologies have been introduced over the past years, all having in common the use of medium-frequency (MF) energy conversion to reduce the weight and volume of the magnetic components. This paper is concerned with the design of the input filter for an MF traction converter, focusing on a solution that minimizes the filter's overall mass. Two filter topologies are investigated, namely a single-pole filter (inductor) and a passively damped $LCL$ filter. The paper first describes the requirements to be fulfilled, establishes the spectrum to consider for the design, and proposes a method to determine the initial parameters of the filter's components. The results show that an $LCL$ -type filter achieves considerable weight savings in comparison to the single-pole solution.

Benefit analysis of using multi‐port and multi‐function power electronic transformer connecting hybrid AC/DC power grids

This study proposes a generalised configuration of the multi-port and multi-function power electronic transformer (MPMF-PET) based mixed AC/DC power grids, and the equivalent circuit and mathematical model of the MPMF-PET are established. To illustrate the benefits of using MPMF-PET to improve the integration and power control ability of a distributed energy resource in an existing distribution network, an iterating evaluation algorithm is proposed for system power flow calculations. Case studies are carried out on a real distribution system modified with MPMF-PET. Results show that the performance of the hybrid AC/DC distribution system is increased by MPMF-PET through proper control and optimal operation, which realised the renewable energy in the AC system to power the DC system and increased the renewable energy acceptance.