Gangyao Wang

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.

Analysis and Comparison of Medium Voltage High Power DC/DC Converters for Offshore Wind Energy Systems

Offshore wind farm with an internal medium-voltage dc (MVDC)-grid collection connected HVDC transmission may be an option to harvest offshore wind energy. High-power MV dc/dc converters with high-step-up conversion ratios are the key components for the internal MVDC grid. In this paper, a high-efficiency step-up resonant switched-capacitor converter for offshore wind energy system is studied, which is characterized by the soft-switching condition for all switches and diodes. This significantly reduces switching losses and higher switching frequency is feasible to reduce the overall system volume and weight. The comparisons with other two kinds of topologies are also presented; moreover, the possible specification requirements of high power MV dc/dc converters are analyzed and set. The operation principle of the proposed converter has been successfully verified by simulation and experiment results.

Design and development of Generation-I silicon based Solid State Transformer

The Solid State Transformer (SST) is one of the key elements proposed in the National Science Foundation (NSF) Generation-III Engineering Research Center (ERC) “Future Renewable Electric Energy Delivery and Management” (FREEDM) Systems Center. The SST is used to enable active management of distributed renewable energy resources, energy storage devices and loads. In this paper, the Generation-I SST single-phase 20kVA, based on 6.5kV Si-IGBT is proposed for interface with 12kV distribution system voltage. The SST system design parameters, overall system efficiency, high frequency transformer design, dual active bridge converter, auxiliary power supply and gate drives are investigated. Design considerations and experimental results of the prototype SST are reported.

Multi-function bi-directional battery charger for plug-in hybrid electric vehicle application

A new multi-function bi-directional battery charger for plug-in hybrid electric vehicles (PHEV) is proposed based on the power circuitry configuration of an American house. This bi-directional charger can achieve three functions including battery charging, vehicle to grid (V2G) and vehicle to home (V2H), all of which are the major research areas of PHEVs integration with the power grid. The integration infrastructure and practical design issues are analyzed. The multiple control loop designs are presented for the three operation modes. Simulation and experimental results verify the functions and performance of the proposed charger. With the capability of achieving multiple functions, the bi-directional charger will contribute and enhance grid related research of PHEVs.

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.

Short-circuit capability of 1200V SiC MOSFET and JFET for fault protection

The short-circuit capability of power switches is crucial for the fault protection. In this paper, 1200V SiC MOSFET and normally-off SiC JFET have been characterized and their short-circuit capabilities have been studied and analyzed at 400V DC bus voltage. Due to different physics in the channels, SiC MOSFET and SiC JFET show different types of temperature coefficient. During the short-circuit operation, the saturation current, Isat, of SiC MOSFET increases for several microseconds before the gentle decreasing while that of SiC JFET decreases drastically from the very beginning. The SiC MOSFETs failed after short-circuit operations of 80μs and 50μs at 10V and 15V gate bias respectively while the SiC JFET could survive a short-circuit time more than 1.4msec.

Voltage and power balance control for a cascaded multilevel solid state transformer

In this paper, a 20kVA Solid State Transformer (SST) based on 6.5kV IGBT is proposed for interface with 7.2kV distribution system voltage. The proposed SST consists of a cascaded multilevel AC/DC rectifier stage, a Dual Active Bridge (DAB) converter stage with high frequency transformers and a DC/AC inverter stage. Based on the single phase d-q vector control, a novel control strategy is proposed to balance the rectifier capacitor voltages and the real power through the DAB parallel modules. Furthermore, the power constraints of the voltage balance control are analyzed. The SST switching model simulation demonstrates the effectiveness of the proposed voltage and power balance controller. A 3kW SST scale-down prototype is implemented. The experiment results verify the single phase d-q vector controller for the SST cascaded multilevel rectifier.

Coupling Effect Reduction of a Voltage-Balancing Controller in Single-Phase Cascaded Multilevel Converters

This paper presents a new voltage-balancing controller for cascaded multilevel converters, especially for single-phase cascaded multilevel converters. It proposes a control algorithm that devotes itself not only to balancing the floating dc capacitors but also to eliminating the coupling effect between the voltage-balancing controller and the original system controller (controller without additional voltage-balancing controllers). Specifically, the average model in the d-q coordinate frame is derived and the control law is established. Then, the coupling effect between the voltage-balancing controller and the original system controller is identified and a new expression for duty cycle modification is proposed thus to eliminate the effect. Furthermore, this paper gives the design considerations of the pro- posed method, including the derivation of key transfer functions and effective voltage-balancing area, for the completeness of the discussion. Moreover, the reference generation techniques of the voltage-balancing controller are also discussed. This paper investigates the voltage imbalance in the soft-start process caused by an unsuitable reference, and presents a simple modified reference generation solution. Finally, both simulation and experimental results verify the performance of the proposed control system.

Design and hardware implementation of Gen-1 silicon based solid state transformer

This paper presents the design and hardware implementation and testing of 20kVA Gen-1 silicon based solid state transformer (SST), the high input voltage and high voltage isolation requirement are two major concerns for the SST design. So a 6.5kV 25A dual IGBT module has been customized packaged specially for this high voltage low current application, and an optically coupled high voltage sensor and IGBT gate driver has been designed in order to fulfill the high voltage isolation requirement. This paper also discusses the auxiliary power supply structure and thermal management for the SST power stage.

Comparisons of 6.5kV 25A Si IGBT and 10-kV SiC MOSFET in Solid-State Transformer application

A 6.5kV 25A dual IGBT module is customized and packaged specially for high voltage low current application like solid state transformer and its characteristics and losses have been tested under the low current operation and compared with 10kV SiC MOSFET. Based on the test results, the switching losses under different frequencies in a 20kVA Solid-State Transformer (SST) has been calculated for both devices. The result shows 10kV SiC MOSFET has 7–10 times higher switching frequency capability than 6.5kV Si IGBT in the SST application.