Decentralized Energy Management and Robust Decisions for Networked Microgrids in Next-Generation Distribution Systems (GR-17-14)


Principal Investigator: Dr. Lingfeng Wang

A growing amount of distributed energy resources is being integrated into both electricity distribution systems (DS) and microgrids (MGs). The efficient coordination between DS and MGs becomes increasingly critical for DS operators and MG operators to achieve effective energy management, considering the high uncertainties contributed by the intermittency of renewable energy resources (e.g., wind and solar) and random variations of load demands. In this project, a novel, decentralized energy management framework will be developed to efficiently coordinate the power exchange between DS and MGs based on a fully decentralized optimization algorithm termed alternating direction method of multipliers (ADMM). Individual energy management model for each entity (DS or MGs) will be built based on the two-stage robust optimization theory in order to address all the potential uncertainties. Mathematically, the formulated problem will be handled with a second order cone programming method based on a relaxed distflow model. Moreover, the developed robust model will be solved by the column and constraint generation (CCG) algorithm, where cutting planes are introduced to ensure the exactness of second order cone relaxation. The proposed method will be tested on a number of IEEE test systems and practical systems with multiple interconnected MGs. If successful, this work will be highly beneficial to more effectively coordinating interconnected microgrids and distribution grids to ensure higher renewable energy integration, higher economic efficiency, and more reliable power system operations.

Presentation Materials

Inverter technologies; Cuzner/Nasiri; Extensive Comparative Study on High Power Inverters Using Various Switching Devices (GR-17-12)

Principal Investigator: Rob Cuzner and Adel Nasiri

Conventional medium voltage inverters with Silicon IGBT have 5-8 kHz switching frequency and hence require extensive filtering at the output to meet the THD requirement of the grid. Newly stablished WBG device technologies offer the same power rating of traditional IGBT with the capability to switch at higher frequencies. High switching frequency improves the inverter output voltage quality and reduces the filter to meet a THD requirement. This research develops the detailed model of classic Silicon IGBT and some Silicon-Carbide MOSFET in Ansys Simplorer and estimates the power loss of IGBT-based and MOSFET-based inverters. It will suggest a higher switching frequency for MOSFET to have a comparable losses of low frequency IGBT. Control system, driver circuit and EMI challenges of SiC MOSFET will be addressed and hence a smaller filter will be suggested. The tasks of the project include derivation of the detailed model of IGBTs and MOSFETs in Ansys Simplorer, loss analysis comparison of IGBT and MOSFET based inverters, optimal filter size and switching frequency determination for a certain power rating, control circuit mitigation from the IGBT to MOSFET, EMI challenges at high switching frequency and how to address them.

Profs. Robert Cuzner and Adel Nasiri at UWM will collaborate to perform this project. Both investigators have worked extensively on inverters and EMI related work and integration of power electronics-based sources. The proposed work will create expertise within GRAPES on SiC devices and design know-how. The developed SiC device model and analysis can also be applied to other systems, e.g. motor drive, energy storage etc. The work will lead to more efficient energy conversion system.

Distributed Energy Resources; Nasiri; Multi-Port Solid State Transformer Design and Implementation for Microgrids and Distribution Systems (GR-17-11)


Principal Investigator: Adel Nasiri 

Recent developments in the design of power electronic elements with higher voltage and power ratings and medium/high frequency enable the use of solid state transformer at different voltage levels for distribution system. In this project, the concept of a Multi-Port Solid State Transformer (MPSST) is introduced. MPSST enables compact, integrated, and galvanically isolated multi-port node for multiple AC and/or DC for the design of efficient smart distribution system. The developed concept interconnects different voltage types and levels using one compact converter with a centralized control logic. During year one study under GRAPES, simulation model is implemented verifying the outcome of a state space model for the MPSST. A hardware prototype implementation is underway for experimental testing. System testing will be performed at multiple voltage levels.  Hardware design involves high frequency transformer design, converter design, heat transfer, and filter design.

The proposed work will create expertise within GRAPES on high power, high voltage, multi-port SST and high frequency magnetics. The tasks of the project includes hardware prototype implementation of a 40kW MPSST, complete implementation and testing of a 480V, 4-port MPSST including converters and transformer, perform hardware evaluation of the concept to migrate to medium voltage using SiC modules, complete studies on application of the proposed system in different distribution systems.


Distributed Energy Resources: A Testbed for Distributed Autonomous Control Concepts for High-Power Microgrids (GR-17-10)


Principal Investigators:Dr. Juan Balda & Dr. Adel Nasiri

Both the University of Wisconsin-Milwaukee (UWM) and the University of Arkansas (UA) have worked on several microgrid controls projects including high-power microgrids, hierarchical control, virtual droop control, and central control.  There are several research tasks within Project GR-17-10 to be performed jointly by UWM and UA, (i) to develop the concept for distributed microgrid controls, (ii) to evaluate the reliability improvement using distributed controls, (iii) to build an HIL setup to test and implement microgrid control, (iv) to implement a high-power microgrid  testbed (MGTB) at the UA National Center for Reliable Electric Power Transmission (NCREPT), and (v) to develop autonomous and predictive concept in a microgrid with higher penetration of renewables.

Tasks i, ii, and v will be performed at UWM, led by Prof. Adel Nasiri.  The concept of the distributed control system is based on installing fast and low cost controllers at each distributed source or smart load.  The reliability assessment will be conducted using Markov Chain theory.  Both UWM and UA will perform task iii on different platforms, with UWM on NI CompactRIO-based system and UA on Typhoon-based system.  UA will perform task iv using the existing three back-to-back voltage-source converters, the so-called regen benches that will be connected in parallel to the point of common coupling in order to emulate different type of generators and loads.

These regen benches would emulate wind power, photovoltaics arrays and other generators to determine their interaction and stability problems in high-power microgrids.  The regen benches would work in two  modes: the grid-connected and island modes.  The UWM controller will be implemented on a system with real renewable sources and loads.  The controller will take into account the forecast for renewable energy generation and load to minimize the stress on energy storage and improve power quality in the microgrid.  The ultimate goal of this project is to compare the performances and differences between the UA high power testbed and the UWM testbed with high renewable penetration so a set of guidelines could be produced.

Coordinated Optimal Voltage Regulation for the Next-Generation Distribution Grids with High Penetration of PV Generation (GR-17-08)

Principal Investigator: Dr. Yue Zhao


Photovoltaic (PV) generation has been extensively deployed in the modern distribution systems.  However, high penetration of PV generation also brings about severe challenges to the grid operations.  Among all the challenges, voltage violation is the most critical, since the current voltage regulation schemes are designed to manage on-way power flow and cannot easily accommodate the fast changing dynamics in the distribution grids.  In addition, the existing grid infrastructures are ill-equipped to gain real-time visibility of distributed PV generations, since the data acquisition and monitoring systems typically do not extend beyond substations and/or distribution feeders and are not designed to handle real-time processing of large volumes of data.  To address these issues, a coordinated optimal voltage regulation (COVoR) framework is proposed to enable high penetration of PV generations.

To accomplish this goal, three specific objectives are expected to be achieved.  Firstly, we envision a self-sensing network enabled by the sensing and communication capabilities of smart inverters.  Based on these measurements, a scalable and optimal scheme will be developed to partition the distribution grid into dynamic voltage regulation (VR) zones.  Secondly, we will develop an advanced multi-agent system based cooperative control method for reactive power sharing among PV inverts within a local VR zone.  Thirdly, we will fully exploit and upswing the advanced grid supportive capabilities of smart inverters by using model predictive control.

Fig.1. An illustration of the proposed coordinated voltage regulation framework

Optimized Gate Drivers for High Voltage Power Devices (GR-17-04)


Principal Investigator: Dr. Alan Mantooth

This project’s main focus is to develop a gate driver with an integrated power supply to drive high-voltage silicon carbide (SiC) devices. In particular, the focus is on the 10 kV SiC MOSFET, which is available and has been tested in some literature studies. The capability of commercially available gate drivers do not meet the requirements needed to efficiently drive SiC devices at the 10kV voltage level. However, the use of high-voltage SiC devices in power electronics is increasing. This calls for the development of research techniques and growth in this area. Thus, this project aims to develop and optimize a gate driver board for the 10 kV SiC MOSFET with the goal of optimizing the performance, cost, and size.  The scope of this project addresses the main issues inhibiting the development of the SiC device gate drivers, such as isolation, dv/dt EMI tolerance, and protection. The small collection of research which analyzes the performance and characterizes the high-voltage SiC MOSFET is used to determine the gate driver’s needs. Ongoing research and industry needs are considered in the optimization of the gate driver board design. A PCB will be fabricated for the design, and the testbed for the module will be created. The design cycle will consist of both simulated and physical testing, including the development of a double-pulse test at high-voltage to be done at NCREPT (National Center for Reliable Electric Power Transmission). This testbed development will also serve as a standard for future research projects in this area. In addition to the optimization of the main driver functions, alternate laminate technologies will be considered including the use of LTCC (low-temperature co-fired ceramic), which would increase voltage isolation.

Presentation Materials

SiC-Based Direct Power Electronics Interface for Battery Energy Storage System into Medium Voltage Distribution System (13.8 kV) (GR-17-03)

Principal Investigator: Dr. Alan Mantooth

This project involves the design and construction of a SiC-based direct power electronics interface for a battery energy storage system (BESS), which is to be integrated into a 13.8 kV medium-voltage distribution system.  Normally, to interface a BESS to a medium-voltage distribution line, a step-up transformer is required to boost the inverter ouput voltage.  The use of the transformer provides convenient isolation, however using a transformer to meet medium-voltage inverter insulation requirements leads to substantially higher leakage inductance, increased switching losses and limited transformer power transfer capability.

Fig.1. Development of SiC-based transformerless interface for battery energy storage system

Recent advances in high voltage power semiconductor devices, medium-voltage (>10 kV) SiC power modules present an opportunity to realize a transformerless interface, shown in the Fig.1 below.  Transformerless topologies, and the use of wide bandgap devices, have the potential for reducing cost and size of passive components for the medium-voltage inverter.  To satisfy the medium voltage basic insulation level (BIL) requirements for the power electronics interface, modular multilevel cascade (MMC) inverters provide a better solution.  This battery energy storage system interface will also include fault protection circuitry and communication protocols.  The performance of the control algorithms for a BESS equipment will be tested through an experimental prototype at the National Center for Reliable Electric Power Transmission (NCREPT) using the 13.8 kV distribution system.

Fault Detection and Management Needs Development Protective Relaying Methods for Microgrids (GR-17-02)

Principal Investigator: Dr. Rob Cuzner

Because the microgrid is a dynamically changing mesh that will respond differently to faults depending on its configuration, achievement of reliable fault discrimination drives complexity and cost. When short circuit faults occur within a microgrid multiple sources of energy can feed the fault, including adjacent electronic loads with front-end filter/storage capacitors–this is particularly the case with DC microgrids where sudden fault inception is characterized only by connected capacitors and cable inductances. An array of additional corner case scenarios exist each of which must be handled in a different way.

This project is a collaborative effort between UWM (Cuzner) and USC (Ginn, Benigni) to develop Hardware in the Loop (HiL) and Power Hardware in the Loop (PHiL) test platforms to develop protective relaying approaches for AC, DC and hybrid AC/DC microgrids. Presently, UWM has developed a Controller-Hardware in the Loop (CHiL) system that enables the study of timing propagation delays between distributed controllers embedded within Distributed Energy Resources (DERs), reliability of a decentralized microgrid control architecture and demonstration of scalability concepts. The UWM CHiL consists of Compact RIO units used to collect feedback information and interface with a Tertiary controller implemented in LabView.  USC has developed an Integrated Grids Laboratory (InteGraL) that supports combined simulation of power and communication grids for testing distributed solutions for control and monitoring in distribution grids. The InteGraL system uses OPAL RT for power system simulation, NS3-RT and Apposite N-91 for communication network emulation, Compact RIO to emulate distributed control and data collection interfaces and multi-purpose ARM based processors to augment the HiL real-time simulation capability, 10Gbit communication between nodes is available. The plan is to augment the CHiL and PHiL systems at UWM and USC to add high speed serial communications for protective relaying.  Common FPGA-based high speed serial communications implementations will be incorporated into the communication network emulations in order to research distributed and centralized schemes for achieving fault discrimination within the microgrid and to develop self-healing systems having autonomous fault detection, isolation and reconfiguration capabilities. This effort is part of a wider vision to enable collaborative research encompassing all levels of microgrid systems control and application to various industries.

Physics-based Analytical and Compact Modeling of GaN Power Devices for Advanced Power Electronics (NSF-15-06)


Principal Investigator: Dr. Alan Mantooth

Advances in wide bandgap materials such as SiC and GaN have led to substantial advances in power semiconductor devices and are now positioned to dominate the next generation of power electronics replacing silicon devices. This research focuses on the creation and validation of analytical models for state-of-the-art GaN power devices.

The market share of GaN power devices is expected to reach a staggering $15.6 billion by 2022, mainly due to the growing demands in the power and energy sector, the communication infrastructure sector, and the power electronics market. GaN devices are expected to reduce overall energy conversion losses down to 1%, resulting in annual savings of nearly $40 billion in US revenues. A high-efficiency and green energy infrastructure is vital for reducing overall expenditures and reducing the carbon footprint of the electronics industry and the environment.The expected outcome from this fundamental research focuses on developing physics-based compact device models for circuit simulations that will help electronics engineers rapidly develop circuit designs and prototypes based on GaN devices. Impacts of this model will enable a side-by-side comparison of GaN and silicon devices at the design and analysis phase. This in turn will likely promote increased usage of GaN semiconductor technology. The models generated in this research will be open access and made publicly accessible on the NSF Industry/University Co-Operative Research Center website under the Grid-Connected Advanced Power Electronics Systems (GRAPES) center site.

Conventionally, the GaN device is a normally-on device. The device is shown in Fig.1 with its corresponding band diagram. GaN devices for power electronics applications are modified as shown in Fig. 2 with a p-type GaN gate and an AlGaN buffer layer. The discontinuity in polarization between the AlGaN barrier layer and p-GaN cap layer brings about the desired normally-off operation by lifting the conduction band above the Fermi-level.

Presentation Materials