Power Electronic Transformer Topology Research Articles

Research on Optimization of Distribution Network based on Power Electronic Transformer

As the core equipment of hybrid microgrid, power electronic transformer is a new type of multifunctional transformer which integrates power electronic technology, information communication technology and control protection technology. On the background of the Beijing Low-Carbon Winter Olympics Smart Grid Demonstration Project, this paper proposed an AC/DC hybrid microgrid topology with dual-end power supply based on power electronic transformer. Moreover, a coordinated optimization control for AC/DC hybrid microgrid based on power electronic transformer is studied. The content of the paper mainly includes: The common topology of power electronic transformer is studied, and a three stage four-port power electronic transformer is proposed according to the demand of demonstration project. The interface converter topology structure and control method of distributed generation and energy storage in hybrid microgrid are analyzed, and the system model of hybrid microgrid is established. Simulation results show that MPPT control can maximize the power output of distributed generation, and piecewise droop control can realize seamless switching between power control and voltage control in the grid-tied or autonomous mode.

Energy Balance Control for Improving Transient Performance of DC Bus Voltage in Power Electronic Transformer for the Voltage-Sensitive Loads

The power electronic transformer (PET), as a main topology for the energy router in the energy internet, consists of the rectifiers, the dual active bridge (DAB), and the inverter, and these three parts are connected by two dc buses. So, the performance of the dc bus voltages is very important because it can totally affect the output waveforms of the dc and ac voltage, especially for the voltage-sensitive loads. Compared with the proportion integration (PI) control scheme, the energy control method utilizes the energy as the control variable, and the control strategy derived from the energy relationship, including the passive elements and all the interfaces, is more direct and explicit. In this paper, considering the energy between the dc bus capacitors and the input inductor and the load and the source in the PET topology, the energy balance control (EBC) strategy is proposed. For the two dc bus voltages, the energy balance relationship of the different time scales is used to decouple the interaction in the control scheme. The EBC strategy can obviously reduce the fluctuation and the transient time of the two dc bus voltages when the load power or voltage reference is changed. Thus, under the limited voltage fluctuation, the EBC strategy can reduce the dc bus capacitance in order to reduce the volume and weight of the converter and enhance the reliability. The simulation and experimental results verify the effectiveness of the proposed control strategy.

Stability analysis and control for power electronic transformer with FRT capability

In this paper, a power electronic transformer (PET) topology with the fault ride-through (FRT) capability is presented. In the PET, modular multilevel converters (MMCs) are adopted to provide medium voltage DC (MVDC) ports. The positive and negative isolated modular multilevel DC converters (IMMDCs) are cascaded respectively with the positive and negative MVDC bus. Under pole-to-ground (PTG) fault condition, the healthy pole of MMC and the healthy IMMDC can operate as normal with the capability of half rated active power transmission. As a cascaded system, the stability of the PET may be affected by the impedance-mismatching of the cascaded converters. Therefore, based on impedance analysis method, the stability analysis of the proposed PET is developed. Then, the stability control method based on series virtual resistance is presented to improve the PET stability. Besides, to fast the dynamic respond speed, a coordinated control scheme based on low voltage DC port current feedforward is introduced. Finally, the presented PET topology and the control strategy is verified by simulations in MATLAB/Simulink.

A novel PET model based volt/var control of AC–DC hybrid distribution network

With the explosive growth of distributed renewable energy connected to the power system, the application of AC–DC hybrid distribution network with power electronic transformer (PET) becomes more and more extensive. An equivalent loss model is formulated from the perspective of the internal topology of PET. Then, a novel PET power flow model is established to obtain the relationship between the three ports. On this basis, reactive power of PV nodes connecting PET and distributed renewable generation is modified by establishing an equivalent impedance matrix, thus the forward–backward sweep load flow calculation method is improved for power flow calculation and volt/var control of distribution network. Different from traditional reactive optimization methods, a multi-objective reactive power optimization model considering the PET and two types of photovoltaic converters is established in this paper, with the targets of minimizing voltage deviation and power loss simultaneously. Finally, the numerical simulation shows the accuracy, rationality, and feasibility of the proposed method.

DAB Based DC-DC High Frequency Link PET for Interconnecting MVDC-LVDC Grids

This paper proposes dual active bridge (DAB) based high frequency power electronic transformer (PET) for interconnecting medium voltage dc (MVDC) and low voltage dc (LVDC) grids for dc power distribution. The above proposed concept works on dual active phase shift principle and square wave HF modulation technique for bidirectional power transfer. Compared to the traditional dc transformer scheme, The proposed power electronic transformer (PET) can disconnect from LVDC distribution grid effectively as a dc breaker when a short circuit fault occurs in the distribution grid. The isolated DC-DC PET topology with a wide range of voltage conversion ratio is useful for High Voltage DC tapping. The DAB based on switched capacitor is connected to the medium voltage DC side and acts as an inverter. The proposed topology has the ability to transfer higher power, and lower circulating power, lower high frequency link voltage, and RMS current and peak values with the same transmission power in the MVDC side. This paper analyzes the topology, voltage and power characterization, control strategy in detail. Increase in the intermediate AC frequency will reduce the size of the transformer and other passive elements significantly in the circuit. The theoretical analysis is supported by MATLAB simulation.

Design and Implementation of Four-Port Megawatt-Level High-Frequency-Bus Based Power Electronic Transformer

With high penetration of various distributed energy resources and dc loads in modern power system, ac/dc hybrid distributed grid attracts a lot of attention due to its compatibility. Aiming for interface of the ac/dc hybrid distributed grid, a four-port megawatt-level power electronic transformer (PET) is focused on in this article. A novel high-frequency-bus based multiport PET topology is proposed, which is found to have superiorities such as mutual isolation, mutual independence among different ports and fewer power stages, compared with the conventional dc-bus based solution. Design and implementation of the four-port PET prototype is presented in this article. Modular structure is adopted, and similar power cell is used for all the four ports for easy manufacture. The parameter design principle based on the port-equality is proposed to guarantee the suitable operating range of all the ports. The functions of this four-port high-frequency-bus based PET are tested and verified from the cell-level, port-level to the system level with maximum efficiency over 96%, which confirms the practicability of this industrial prototype.

A three-phase integrated power electronic transformer with capability of DC fault clearance

This paper presents a three-phase integrated PET topology with capability of DC fault clearance based on the investigation of power electronic transformer topology (PET). The proposed PET has three ports of high-voltage AC, high-voltage DC and low-voltage DC, which can achieve the flexible access of distributed energy resources. The power unit of the PET adopts a three-phase integrated topology, and the single-phase fluctuating power in the DC-link capacitor is reduced by a 4-port DC/DC converter, thereby reducing the capacitance of the DC-link capacitor; Meanwhile, the power unit uses the clamp double sub-module(CDSM) to allow the self-clearing of the short-circuit fault on the high-voltage DC side. Finally, the paper verifies the proposed PET through EMT simulation and experimental prototype.

A Multiport Power Electronic Transformer Based on Modular Multilevel Converter and Mixed-Frequency Modulation

Power electronic transformer (PET) is a key apparatus in smart grid system, due to the advantages of power controllability and ac/dc grids interconnection. The four-stage PETs have multiple output ports, good controllability, and flexibility, but are limited by multiple power conversion stages, complex structure, and low efficiency. Therefore, a novel modular multilevel converter (MMC)-based PET topology with mixed-frequency modulation is proposed to reduce the power conversion stage. The power decoupling circuits are designed to realize multiple output ports, including medium voltage ac (MVAC) port, medium voltage dc (MVDC) port, and low voltage dc (LVDC) port. The equivalent circuit and controllability of proposed PET are analyzed. The proposed PET topology provides the same multiple output ports by only three power conversion stages, therefore, the PET structure is simplified. In addition, simulation results are provided to verify the effectiveness of proposed PET.

Research on Unbalanced-Load Correction Capability of Two Power Electronic Transformer Topologies

Compared with conventional power transformer, power electronic transformer (PET) has many merits and an increasing possibility to replace it in the future. In this paper, the unbalanced-load correction capability of two H-bridge based three-phase three-stage modular PET topologies, the separate-phase-connection (SPC) one and the cross-phase-connection (CPC) one, are analyzed and compared. Both of the SPC and CPC consist of three stages: a modular multilevel ac–dc input stage, an isolation stage with several independent modular dual active bridge (DAB) dc–dc converters, and a dc–ac output stage with single-phase-inverter parallel-connected structure. Based on the control strategy, it is found that the SPC is suitable for dealing with full-range unbalanced load under the condition of increasing the input-stage current stress. For the SPC feeding, one load or three loads with identical power polarity, the current stress shall be theoretically increased up to 115% of the rated one, while the percentage is 153% when the SPC feeds three loads with different power polarity. On the contrary, the CPC is only suitable for dealing with partial unbalanced-load conditions. Extendedly, the third so-called full-range-power auto-balance PET is proposed to deal with unbalanced load, the performance of which is verified by simulation results. A downscaled prototype for SPC and CPC is built and tested. Experimental results verify the unbalanced-load correction performance of SPC and CPC.

Analysis and Design of Power Electronic Transformer based Power Quality Improvement

A new type of transformer based on Power Electronics (PET) has been proposed in this paper, which realizes voltage transformation, galvanic isolation, and power quality improvements in a single device. The PET provides a fundamentally different and more complete approach in transformer design by using power electronics on the primary and secondary sides of the transformer. Several features such as instantaneous voltage regulation, voltage sag compensation and power factor correction can be combined into PET. This paper presents a novel topology of power electronic transformer. In the design process, the AC/DC, DC/AC, AC/AC converters and high frequency transformer have been used. One matrix converter operates as AC/AC converter in power electronic transformer. The designed power electronic transformer performs typical functions and has advantages such as power factor correction, voltage sag and swell elimination, voltage flicker reduction and protection capability in fault situations. The proposed power electronic transformer has been modeled using MATLAB/ SIMULINK and Power quality improvement with proposed model has been verified by the simulation results. KeywordsPower electronic transformer; AC/AC Matrix converter; Power Quality; High Frequency Transformer; Space vector Technique