Multiport Power Electronic Transformer Research Articles

Analysis and Design of Independent DC Bus Structure Multiport Power Electronic Transformer Based on Maximum Power Transmission Capability of Low-Voltage DC Ports

Owing to the diverse connection configurations of dual active bridge converters, a multiplicity of low-voltage DC port structures are anticipated to emerge in the independent DC bus structure multiport power electronic transformer (IDBS-MPET). An inadequate low-voltage DC port structure exacerbates the power imbalance in IDBS-MPET, presenting a risk of overmodulation even when transmitting relatively low levels of power. To overcome this limitation, a design scheme of IDBS-MPET topology based on the maximum power transmission capability of the low-voltage DC ports is proposed in this paper. Three topology design rules are derived from the maximum power transmission capability results of more than 80 typical IDBS-MPET topologies. The symmetrical triple cross-phase connection structure, the symmetrical double cross-phase connection structure and the single-phase connection structure are sequentially identified as the three most optimal structures of low-voltage DC ports. By employing the proposed design methodology, each low-voltage DC port achieves its maximum power transfer capability relative to other configurations. The effectiveness of the proposed design scheme is validated by an optimal designed IDBS-MPET topology with six low-voltage DC ports.

Optimal operation of the green port system considering the multiport power electronic transformer in day-ahead markets

Increasing operation costs and carbon emissions have brought great challenges to the development of port systems, which are regarded as prosumers that comprise various renewable energy sources and diversiform electric loads. In this paper, an energy transaction framework for the green port with a multiport power electronic transformer (PET) is established to optimize the operation of the port system in day-ahead energy and reserve markets. The market behavior of the port system is formulated as a bi-level stochastic optimization model. The energy source schedule, berth allocation, and quay crane (QC) assignment are optimized to minimize the total operation cost at the upper level, whereas energy and reserve market clearing problems are settled at the lower level. The aforementioned nonlinear bi-level optimization problem is solved as a mathematical program with equilibrium constraints (MPECs). Karush–Kuhn–Tucker conditions and duality theory convert the proposed nonlinear bi-level problem into a linear single-level problem. Numerical simulations show that the proposed strategy can achieve the lowest total operation cost for the green port.

Investigating and Analyzing the Impact of a Bidirectional Multiport Power Electronic Transformer Interface on the Power Quality and Stability of Interconnected Microgrids

Investigating and Analyzing the Impact of a Bidirectional Multiport Power Electronic Transformer Interface on the Power Quality and Stability of Interconnected Microgrids

Low-voltage ride through of multi-port power electronic transformer

A low-voltage ride-through (LVRT) control strategy for the multi-port power electronic transformer (PET) based on power co-regulation is proposed. During the sag and recovery of the grid-side voltage of the medium-voltage ac (MVac) port, the grid-connected active power of the low-voltage ac (LVac) port, rather than the power from external renewable energy sources (e.g., photovoltaic (PV)), is adjusted quickly to rebalance the power flowing across all ports, thereby preventing overcurrent and overvoltage. Moreover, a power-coordinate-frame-based LVRT mode classification is designed, and a total of six LVRT modes are classified to meet the LVRT requirements in all power configuration scenarios of the PET. In this way, the PET is endowed with the LVRT capability in both power-generation and power-consumption states, which is significantly different from traditional power generation systems such as PV or wind power. Furthermore, by optimizing the active power regulation path during LVRT transition, the overcurrent problem caused by the grid-voltage sag-depth detection delay is overcome. Finally, the effectiveness of the proposed control scheme is verified by experiments on a hardware-in-the-loop platform.

Power Management of AC/DC Hybrid Distribution Network With Multi-Port PET Considering Reliability of Power Supply System

In the current distribution network, photovoltaic, wind power, energy storage, and other distributed energy are widely connected, and the proportion of generalized DC load is rapidly increasing. With the development of power electronics technology, using multi-port power electronic transformer (PET) to achieve high-efficiency access of large-capacity AC/DC source and load is the current research hotspot, and AC / DC hybrid power supply is inevitable. The introduction of a large number of power electronics and the flexible coordinated control and complementary fault-tolerant advantages of PET bring challenges to the operation and maintenance management of the AC/DC hybrid power system. In this paper, the structure of the AC/DC hybrid power system with the multi-port PET cluster is introduced. Based on the idea of hierarchical and partitioned control, a three-layer and multi-time scale control mode of integrated automation system, PET cluster and PET controller is proposed; Aiming at the problem of reliability evaluation, a sequential Monte Carlo simulation method is proposed to simulate the AC/DC hybrid power system with the multi-port PET cluster. The influence of multi-port PET parallel cluster mode and port capacity limitation on the reliability of the power system is analyzed, and the effectiveness of the model and algorithm is verified. Finally, an example is given to verify that the proposed structure is conducive to improving the utilization level of renewable energy in the distribution network.

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 MMC-Based Multiport Power Electronic Transformer With Shared Medium-Frequency Transformer

A modular multilevel converter (MMC)-based multiport power electronic transformer (PET) topology with shared medium-frequency transformer (MFT) is proposed in this brief, where the proposed PET is suitable for the hybrid AC/DC distribution network because of the existence of four essential voltage ports, including medium-voltage AC and DC ports, low-voltage AC and DC ports. Moreover, a multi-frequency modulation and the design of voltage decoupling circuit are introduced. One MFT is shared by three-phase MMC arms, which contributes to the compact structure and reduced cost of PET. The operating principle, applied control strategy and parameter design of proposed PET are introduced in detail. The experimental results are provided to verify the feasibility of PET.

Optimal operation of AC–DC distribution network with multi park integrated energy subnetworks considering flexibility

In order to improve the utilisation rate of renewable energy and flexibility of the system, the AC–DC distribution network with multiple park integrated energy subnetwork (PIESN) based on multi-port power electronic transformer (MPPET) is proposed. First, a power-sharing model of interconnected PIESNs based on MPPET is built, in which the port power of MPPET can be controlled. Then the flexibility demand and supply of electricity, heat and cooling are analysed and a flexibility-sharing model among multi-PIESN is established. Multi-energy flexibility conversion mechanism is proposed and the flexibility index is established. Moreover, the optimal operation problem is formulated and solved. The objective function is the comprehensive operation cost, which includes generation cost, energy cost of PIESN, network loss cost, MPPET loss cost, carbon emission cost and electricity storage system operation cost. Finally, simulation on the modified IEEE33 system shows that the proposed model can make the system be in an adequate flexibility, improve the utilisation rate of renewable energy, reduce load shedding and realise the power sharing among multi-PIESNs.

Reliability evaluation method for AC/DC hybrid distribution power network considering cascaded multiport power electronic transformer

With the advantage of high-power supply capacity, low loss in power transmission and distribution process, strong power flow control ability, direct current (DC) distribution network has been one hotspot of the future distribution network. The cascaded multiport power electronic transformer (PET) has been proposed as key equipment for future distribution grid with multi-voltage level and alternating current (AC)/DC hybrid feature. Based on the typical topological structure and engineering reliability theory, a reliability evaluation model for PET considering the device redundancy is established. After that, the reliability evaluation method of the AC/DC hybrid distribution network with a cascaded multiport PET is proposed. Finally, case studies are provided in a ‘hand in hand’ AC/DC hybrid power distribution network structure. Case 1 analyses the PET reliability level under different design patterns and the redundancy level. Case 2 gives the reliability evaluation results of the AC/DC hybrid distribution network with PETs, which proves the correctness and validity of the proposed algorithm. Case 3 compares the reliability level of the AC and DC system under different scenarios and proposes the reliability promotion advice for the DC power distribution network.

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.

Reduced-Order DC Terminal Dynamic Model for Multi-Port AC-DC Power Electronic Transformer

As new electric power conversion equipment, a multi-port power electronic transformer (MP-PET), including a power electronic converter, high-frequency transformer, and multiple ac or dc interconnection interfaces, has a broad application in the hybrid distribution network. However, high integration and a large number of energy storage devices has led to very a high-order model of the system. To address this issue, a reduced-order small signal model of MP-PET is established in this paper. By taking the participation factors of the system mode to the state variables, the reduced-order model is derived based on the state variables, which are highly correlated with the dc voltage dominant mode. Compared with the full-order model, the proposed reduced-order model is accurate enough and simplified, and the validity of the simplified model is verified against simulations on a 10 kV/3 MVA MP-PET. The simulation results indicate that the proposed reduced-order model coincides well with the dynamic performance of the MP-PET.

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.

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.

Modeling and Control of a Multiport Power Electronic Transformer (PET) for Electric Traction Applications

This paper proposes a multiport power electronic transformer (PET) topology with multiwinding medium-frequency transformer (MW-MFT) isolation along with the associated modeling analysis and control scheme. The power balance at different ports can be controlled using the multiwinding transformer's common flux linkage. The potential applications of the proposed multiport PET are high-power traction systems for locomotives and electric multiple units, marine propulsion, wind power generation, and utility grid distribution applications. The complementary polygon equivalent circuit modeling of an MW-MFT is presented. The current and power characteristics of the virtual circuit branches and the multiports with general-phase-shift control are described. The general current and power analysis for the multiple active bridge (MAB) isolation units is investigated. Power decoupling methods, including nonlinear solution for power balancing are proposed. The zero-voltage-switching conditions for the MAB are discussed. Control strategies including soft-switching-phase-shift control and voltage balancing control based on the power decoupling calculations are described. Simulations and experiments are presented to verify the performance of the proposed topology and control algorithms.

A Modular Multiport Power Electronic Transformer With Integrated Split Battery Energy Storage for Versatile Ultrafast EV Charging Stations

This paper proposes a power converter architecture for the implementation of an ultrafast charging station for electric vehicles (EVs). The versatile converter topology is based on the concept of the power electronic transformer. For the direct transformerless coupling to the medium-voltage grid, a cascaded H-bridge (CHB) converter is utilized. On the level of each submodule, integrated split battery energy storage elements play the role of power buffers, reducing thus the influence of the charging station on the distribution grid. The power interface between the stationary split storage stage and the EV batteries is performed through the use of parallel-connected dual-half-bridge dc/dc converters, shifting the isolation requirements to the medium-frequency range. By choosing several different submodule configurations for the parallel connection, a multiport output concept is achieved, implying the ability to charge several EVs simultaneously without the use of additional high-power chargers. All possible charging station operating modes among with the designed necessary control functions are analyzed. The state-of-charge self-balancing mode of the delta-connected CHB converter is also introduced. Finally, the development of a downscaled laboratory prototype is described, and preliminary experimental results are provided.