Yusi Liu

Modular multilevel converter with high-frequency transformers for interfacing hybrid DC and AC microgrid systems

Recently, concepts of both ac and dc microgrids have been proposed for future energy systems. A novel converter topology which has three ports: 12 kV ac, 22 kV dc and 400 V dc for future hybrid ac and dc microgrid systems is presented in this paper. Each port has the capability of bi-directional power flow which is required in future hybrid ac and dc microgrids. The 400 V dc bus, which is connected to distributed energy resources, injects/draws power to/from both the ac grid and the dc grid through solid-state transformers. The topology avoids the use of line-frequency transformers which are considered to be expensive and bulky. Because the hybrid converter is connected to both the ac grid and the dc grid, enhanced reliability is achieved. A modular design of the sub-module of the modular multilevel converter is proposed for reducing the sub-module failure probability. Extensive time-domain simulations using Matlab/Simulink™ are presented demonstrating the feasibility of the proposed topology.

Optimized control of isolated residential power router for photovoltaic applications

This paper proposes an optimized control of the hardware of a residential power router for photovoltaic applications. The design of the hardware topology is described in this paper. Solar panels, which act as the green power input, are connected through a boost converter to an isolated dual-half-bridge and integrated with a full-bridge inverter to provide a 60 Hz voltage at the ac bus to supply traditional residential loads. The proposed maximum power point tracking control for photovoltaics begins with constant voltage tracking and then adopts a variable duty cycle step-size perturbation and observation to improve the accuracy of the photovoltaics power tracking. Furthermore, to significantly reduce the voltage error for the grid-tied inverter, the proposed direct quadrature (D-Q) rotating frame control provides much better performance over a normal proportional-integral-derivative (PID) control. Simulation and experimental results reported in this paper verify the functionality of the hardware topology of the power router system and its optimized controller.

A silicon carbide fault current limiter for distribution systems

Increasing load densities are leading to higher fault currents that may exceed the ratings of current circuit breakers. In addition, computer-controlled equipment is more susceptible to power supply disturbances of relatively long durations. So, there is a need for a new piece of equipment which is able to interrupt fault currents before reaching their first maximum peak isolating very fast faulted sections of a distribution system. Solid-state fault current limiters (SSFCL) have been proposed as a solution to accomplish the above, and thus, as a substitute for slow-operating electromechanical circuit breakers. The design of a silicon carbide fault current limiter with high voltage blocking capability and the subsequent testing at a 15-kV test facility are addressed in this paper. The semiconductor devices of this series-connected SSFCL are custom packaged silicon carbide super gate turnoff thyristors and SiC PIN diodes. The 4.16-kV experimental tests illustrate the performance of the proposed SSFCL and demonstrate the potential for deploying them in distribution systems.

Design of a 2 MW DC supply using a 4-stage interleaved DC-DC converter

An optimal design of a 2-MW dc supply with a variable output dc voltage range between 400 V and 1500 V, and intended for testing equipment under IEEE 1547 is presented in this paper. Trade-offs between size and system complexity must be taken into account because the main components (dc inductor, semiconductor switches, cooling system, etc.) of such a high power system are expensive and/or bulky. Several dc-dc converter topologies are evaluated and compared for the intended application, and all the components are selected based on current market availability. The 4-stage interleaved dc-dc converter topology is selected since it has a good performance-to-price ratio. Challenges of operation under both heavy and light loads are addressed. The control algorithm for this topology is presented and illustrated using simulation results. These simulations demonstrate that the proposed dc supply design provides an acceptable dc power source for testing distributed generation equipment under IEEE 1547 at not only relatively high power levels, but also light power levels.

A 13.8-kV 4.75-MVA microgrid laboratory test bed

Microgrid concepts and applications have become more promising during recent years because of the potential for improving system operation, particularly, under emergency conditions. The design and hardware realization of a microgrid laboratory test bed enabling research and development for microgrids, smart-grid systems, and distributed energy integration is presented in this paper. The state-of-the-art regenerative back-to-back voltage source converter topology is used for implementing and emulating the testing capabilities in the MVA range which are defined by IEEE Std 1547.7TM-2011. An overview of the proposed 13.8-kV microgrid test bed functions and goals are shown. Detailed power electronic circuit hardware, experimental prototype design, and control algorithms are addressed. Hardware test results of case study are shown which display the uniqueness of the proposed test bed which is planned for the research of future microgrids.

New power electronic interface combining DC transmission, a medium-frequency bus and an AC-AC converter to integrate deep-sea facilities with the AC grid

A new bidirectional dc-ac power electronic interface (PEI) consisting of a dc link, an HVdc terminal based on modular multilevel converters (MMCs), medium-frequency transformers (MF-XFMRs) - higher frequencies are also possible - and ac-ac converters is proposed for interconnecting two ac systems, or dc and ac systems. The advantages of each component make the proposed topology convenient for applications that require compactness, flexibility, and reliability. For example, integration of offshore and onshore wind farms with the power grid, ac and/or dc grids interconnections, and supplying power to conventional and future deep-sea oil and gas facilities. The PEI main overall topology, operating principles and design equations are described. Size and efficiency are evaluated for the proposed topology for different operating frequencies and compared with the conventional approach. A case study of a deep-sea electric power system (DEPS) is illustrated through timedomain simulations for different operating conditions to demonstrate the feasibility of the proposed ideas.

Topology, cost and efficiency comparisons of a 2 MW DC supply using interleaved DC-DC converter

An optimal design of a 2-MW dc supply which has a variable output dc voltage range between 400 V and 1500 V and is intended for testing equipment under IEEE 1547 is presented in this paper. Trade-offs between size and system complexity must be taken into account because the main components (dc inductor, semiconductor switches, cooling system, etc.) of such a high power system are expensive and/or bulky. Several dc-dc converter topologies are considered and compared for the application and all the components in this design were selected based on current market availability. The 4-stage interleaved dc-dc converter topology is finally selected since it has a good performance to price ratio. The control algorithm based on the selected topology is illustrated and simulation results are presented and described. These simulations demonstrate that the proposed dc supply design provides an acceptable dc power source for testing distributed generation equipment under IEEE 1547 at relatively high power levels.

Resonance propagation of ac filters in a large-scale microgrid

Research into microgrids has become more promising during recent years. It has the potential of leading to a more resilient electric power delivery system in the face of system disturbances. Low pass ac filters are frequently adopted in microgrid power electronic interfaces that convert dc power to ac power because most of todays distribution grids have ac voltages. Compared to a simple L filter, higher order filters, such as LC or LCL filters, are preferred due to their more effective reduction of switching-frequency harmonics and smaller sizes. However, the resonance problems caused by paralleling multiple converters with LCL filters, especially those converters having large power ratings (e.g., in the MVA range), must be accounted for when designing the entire microgrid. The analyses and designs should consider not only the resonant stability of a single converter but also the resonance propagation between multiple converters. A microgrid resonance propagation circuit model is built and investigated in this paper. Experimental waveforms validate the proposed analyses.

Securing DC and hybrid microgrids

Many security technologies for microgrids have been proposed in the literature, which must be rigorously tested in a realistic power platform before being transitioned to the energy sector. To address this need, a 13.8-kV microgrid security test bed is introduced in this paper towards the objective of securing dc and hybrid microgrids. Different from existing test beds that are based on simulated power flows, our test bed is built on a real power facility. The design of the test bed, including both the physical system and the cyber system, is described. Power electronics technology plays a major role in this test bed as it does in the microgrid infrastructure, and is an integral part of the testing instrumentation and methods. Potential security problems, from both software and hardware attacks, as well as security solutions, are considered and able to be emulated and evaluated using the test bed.

Interleaved Flyback based micro-inverter for residential photovoltaic application in remote areas

This paper presents a design of a solar micro-inverter. Solar panels are connected through an Interleaved Flyback Converter(IFC), with a flyback transformer playing the role of isolation, and integrated with a full-bridge inverter to provide a 60 Hz, 120 V ac voltage to home loads in remote areas. The proposed interleaved flyback converter for a solar micro-inverter has four stages and is regulated using the voltage loop control and phase shift control. In the proposed interleaved flyback converter, the Resistor, Capacitor, and Diode (RCD) snubber circuit is used for each stage. The Direct-Quadrature (D-Q) rotating frame control is considered for the full-bridge inverter, since it provides good performance. The simulation and experimental results reported in this paper verify the functionality of the designed topology for a solar micro-inverter.