Xunwei Yu

A High Step-Up Three-Port DC–DC Converter for Stand-Alone PV/Battery Power Systems

A three-port dc–dc converter integrating photovoltaic (PV) and battery power for high step-up applications is proposed in this paper. The topology includes five power switches, two coupled inductors, and two active-clamp circuits. The coupled inductors are used to achieve high step-up voltage gain and to reduce the voltage stress of input side switches. Two sets of active-clamp circuits are used to recycle the energy stored in the leakage inductors and to improve the system efficiency. The operation mode does not need to be changed when a transition between charging and discharging occurs. Moreover, tracking maximum power point of the PV source and regulating the output voltage can be operated simultaneously during charging/discharging transitions. As long as the sun irradiation level is not too low, the maximum power point tracking (MPPT) algorithm will be disabled only when the battery charging voltage is too high. Therefore, the control scheme of the proposed converter provides maximum utilization of PV power most of the time. As a result, the proposed converter has merits of high boosting level, reduced number of devices, and simple control strategy. Experimental results of a 200-W laboratory prototype are presented to verify the performance of the proposed three-port converter.

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.

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.

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.

Hierarchical power management for DC microgrid in islanding mode and Solid State transformer enabled mode

A hierarchical power management scheme is proposed in this paper for a typical DC Microgrid. Different from other Microgrids, the DC Microgrid can interface to the distribution system by Solid-State transformer (SST). The hierarchical power management strategy includes three control levels: 1) primary control for DC Microgrid to implement distributed operation 2) secondary control for the DC Microgrid bus voltage recovery to achieve seamless mode switch 3) tertiary control to manage the battery charge and discharge. The DC Microgrid can operate in islanding mode, including the individual control for distributed renewable energy source (DRER) and distributed energy storage device (DESD). In addition, the DC microgrid can operate in SST-enabled mode to interface to the distribution system. The DC Micorgrid can seamlessly switch between islanding mode and SST-enable mode. The consideration of state of charge (SOC) for battery is also involved into the tertiary control. To this end, a lab test-bed is constructed to verify the system performance. Lastly, several typical case studies are carried out and the experimental results verify the proposed power management strategy.

Droop controller design methods for isolated DC-DC converter in DC grid battery energy storage applications

Droop control is a potential solution to assign the power share for multiple DC power sources such as battery energy storage systems in a DC micro-grid, without demanding communication requirement. The design objective of the droop controller is to regulate the DC-DC converter virtual output impedance. As the major contribution of this paper, two types of DC droop controllers are proposed for the isolated DC-DC converter in battery energy storage applications in DC grid. One method provides cost effective solution by only designing the output voltage compensator without current sensor. The other method requires a current-feedback loop but provides accurate and linear output droop. An isolated DC-DC converter prototype was built and the test results were reported for both droop controller designs.

A fully autonomous power management strategy for DC microgrid bus voltages

A typical DC microgrid is investigated in this paper. Two unidirectional DC/DC converters for photovoltaic (PV), and two bidirectional DC/DC converters for batteries are included in the proposed DC microgrid system embodying multiple renewable energy sources and energy storage devices. In order to manage the system operation, a fully autonomous power management strategy, namely adaptive DC bus voltage signal, is proposed. In the proposed control algorithm, the DC microgrid system can operate in islanding mode, DC source-connection mode, featuring seamless transitions between these two modes. Experimental results verify that the proposed power management strategy can be applied to a DC microgrid stably and achieve good performance.

Power management strategy for plug and play DC microgrid

DC microgrid is getting more and more attention as an effective and efficient solution to integrate different kinds of renewable energy storage and energy resources with DC loads. A typical DC microgrid envisioned for future DC powered residential homes is investigated in this paper, including distributed power management strategy design, plug and play function implementation, communication ports to monitor the system performance. In this power management strategy, each module in the system is in the distributed control and can be plugged and unplugged into the system without affecting the system performance. Furthermore, the communication ports guarantee all modules information can be sent to the control center to monitor the whole system information.

A new nonisolated three-port DC-DC converter with high step-up/down ratio

A new nonisolated three-port dc-dc converter integrating PV and battery power is proposed in this paper. The topology includes two coupled inductors and two active-clamp circuits. The coupled inductors are used to achieve high step up/down gain and to reduce the voltage stress on both primary and auxiliary switches. Two sets of active-clamp circuits in the proposed converter are used to recycle the energy stored in the leakage inductors. Therefore, the system efficiency can be improved. Charging/discharging transition of the battery is autonomous while tracking the maximum power of PV source and regulating the output voltage simultaneously. When the charging voltage is too high, the control loop of PV port will be switched from maximum power point tracking (MPPT) to battery voltage regulation. Experimental results of a 200W laboratory prototype are presented to verify the performance of the proposed three-port converter.

LCL filter utilized in battery charging applications to achieve compact size and low ripple charging

In this paper, an LCL filter is proposed to replace L-type filter in a dc/dc converter for battery charging applications. The merits of using an LCL filter are compact size, and extremely low ripple of charging current. The ripple free charging can reduce the heat generated by the ripple of the charging current and improve the battery lifetime. Filter based controller is proposed to solve the control loop instability issue caused by LCL filter. Low-pass filter and notch filter based controllers are designed and compared. The controller robustness to the variation of the filter parameters is investigated. The proposed system and control methods are verified by simulation and experiment.