Xu She

Review of Solid-State Transformer Technologies and Their Application in Power Distribution Systems

The solid-state transformer (SST), which has been regarded as one of the 10 most emerging technologies by Massachusetts Institute of Technology (MIT) Technology Review in 2010, has gained increasing importance in the future power distribution system. This paper presents a systematical technology review essential for the development and application of SST in the distribution system. The state-of-the-art technologies of four critical areas are reviewed, including high-voltage power devices, high-power and high-frequency transformers, ac/ac converter topologies, and applications of SST in the distribution system. In addition, future research directions are presented. It is concluded that the SST is an emerging technology for the future distribution system.

Review of solid state transformer in the distribution system: From components to field application

The emergence of high power converters makes the modern power grid more active than it was before. One of the research directions in this area is the solid state transformer, which aims at replacing the traditional 50/60 Hz power transformer by means of high frequency isolated AC/AC solid state conversion techniques. This paper presents a systematical technology review essential for the development of solid state transformer in the distribution system, especially focusing on the following four areas: high voltage and high frequency power devices, high power and high frequency transformers, AC/AC converter topologies, and applications of solid state transformer in the distribution system. For each category, the state-of-art technologies are reviewed and possible research directions are presented. It is concluded that the solid state transformer is an emerging technology for the modernization of the future smart grid.

On Integration of Solid-State Transformer With Zonal DC Microgrid

The contribution of this paper has been focused on investigating a new microgrid architecture that integrates the solid-state transformer with zonal dc microgrids. By utilizing the dc and ac links of the solid-state transformer, both ac and dc networks can access the distribution system, which renders the coordinate management of the power and guarantees high power supply reliability. In addition, the presented system together with the proposed power management strategy can minimize the effect of newly established zonal dc microgrid to the existing power grid, of which promises a better stability. To this end, this paper takes the photovoltaic and battery as the typical renewable energy resource and energy storage device, and develops a simulation platform for system study. Simulation results are shown to demonstrate the feasibility of the proposal.

Design and Demonstration of a 3.6-kV–120-V/10-kVA Solid-State Transformer for Smart Grid Application

Solid-state transformer (SST) has been regarded as one of the most important emerging technologies for traction system and smart grid application. This paper presents the system design and performance demonstration of a high-voltage SST lab prototype that works as the active grid interface in smart grid architecture. Specifically, the designs of the key components of the system, including both power stage and controller platform, are presented. In addition, the advanced control system is developed to achieve high-performance operation. Furthermore, integration issues of SST with dc microgrid are presented. Lastly, tests under different scenarios are conducted to verify the following advanced features of the presented SST technology: 1) VAR compensation; 2) voltage regulation; 3) source voltage sag operation; and 4) microgrid integration.

Wind Energy System With Integrated Functions of Active Power Transfer, Reactive Power Compensation, and Voltage Conversion

As the power of wind energy system increases, the control of their active and reactive power becomes increasingly more important from a system standpoint given that these are typical frequency and voltage control parameters. In this paper, a family of wind energy systems with integrated functions of active power transfer, reactive power compensation, and voltage conversion is proposed. The proposed wind energy systems using solid-state transformer (SST) can effectively suppress the voltage fluctuation caused by the transient nature of wind energy without additional reactive power compensator and, as such, may enable the large penetration of wind farm (WF) into the power grid. To this end, a simulation study for WF driven by squirrel-cage induction generators is presented to verify the effectiveness of the proposed system. In addition, a modular-type high-voltage and high-power three-phase SST topology is presented for the proposed system, and its basic building block, which is a single-phase SST, is analyzed. The functions of SST in the presented wind energy system are verified in a single-phase laboratory prototype with scaled-down experiments. Lastly, cost issue of the proposed technology is analyzed with comparison to the traditional one.

3-D Space Modulation With Voltage Balancing Capability for a Cascaded Seven-Level Converter in a Solid-State Transformer

The solid-state transformer (SST) is an alternative for existing power transformer due to its advantage in low volume, bidirectional power flow, power factor control, and fault management capability. In this paper, a three-stage cascaded-type SST, which utilizes the cascaded seven-level rectifier as the interface with a 7.2 kV distribution system, is presented. In the described system, dc voltage balancing of the cascaded seven-level rectifier is a necessity for stable operation. Previous PI-based voltage balancing control has intrinsic disadvantage in compromise between fast regulation and PI saturation. Besides, the addition of a voltage balancing controller may also change the system loop and bring unexpected performance. This paper aims at giving an alternative view of voltage balancing mechanism in a cascaded multilevel converter. A novel 3-D space modulation technique with voltage balancing capability is proposed for a cascaded seven-level rectifier stage of SST. By choosing the most suitable switching pairs for dc voltage balance, this method has a very good voltage balancing capability that can realize fast regulation of dc voltages in all operation modes, including unity power factor operation, capacitive operation, and inductive operation. Simulation and experimental results in a scaled-down system are provided to verify the effectiveness of the proposed method.

Power Management for DC Microgrid Enabled by Solid-State Transformer

A novel distributed power management scheme is proposed in this paper for a DC microgrid system, which is enabled by Solid-State transformer (SST). The proposed system includes distributed renewable energy resource (DRER) and distributed energy storage device (DESD). The proposed distributed control algorithm, which only relies on the local information and guarantees full utilization of each module in the system based on their characteristics, is applied to both SST and DC microgrid. To this end, a simulation platform is developed in MATLAB/Simulink, in which Photovoltaic (PV), fuel cell and battery are selected as the typical DRERs and DESD, respectively. Lastly, several typical case studies are carried out and the simulation results verify the proposed distributed power management.

AC circulating currents suppression in modular multilevel converter

Modular multilevel converter is a next generation multilevel converters for medium to high voltage conversion applications, such as medium voltage motor drive and high voltage direct current transmission. One potential issue of this type of converter is the AC circulating current, which increases the current stress and brings additional conduction loss to the system. This paper proposes modified control architecture for modular multilevel converters, aiming at suppressing the AC components in the circulating current. Specifically, a proportional-resonant type minor loop is incorporated to regulate the most AC components of the circulating current to zero in addition to the DC regulation loop. The proposed minor loop can also be applied to single phase MMC, which is not available in previous methods. Simulation results for a three-phase MMC operating as an inverter are provided to demonstrate the feasibility of the proposed method.

Performance evaluation of solid state transformer based microgrid in FREEDM systems

A new concept of solid state transformer based microgrid system is presented in this paper. By utilizing 400V DC bus generated from Gen-I solid state transformer proposed by FREEDM systems center, integration issues of DC microgrid and solid state transformer are analyzed. Zonal DC microgrid concept is applied to this novel system with the consideration of burden minimization to the existing AC grid. The future grid architecture is described by using this solid state transformer based integrated microgrid system.

System Integration and Hierarchical Power Management Strategy for a Solid-State Transformer Interfaced Microgrid System

This paper investigates, and for the first time presents, the system integration of a novel solid-state transformer (SST) interfaced microgrid system. Accordingly, a hierarchical power management strategy is proposed for this system to enable islanding mode operation, SST enabled operation, and the seamless transfer between two modes. The proposed power management strategy includes three control levels: primary control for the local controller; secondary control for the dc microgrid bus voltage recovery; and tertiary control to manage the battery state of charge. The proposed system architecture and control strategies are detailed in this paper and a lab test bed is constructed to verify the system performance. Finally, several typical case studies are carried out. The experimental results verify the proposed system and distributed power management strategy.