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Grid-Connected Advanced Power Electronic Systems (GRAPES)

University of Arkansas Fayetteville

University of South Carolina

Last Reviewed: (not done)

Research focuses on design, development, evaluation, control, and standardization of grid-connected power electronic equipment on both the supply and load side of power systems.

Center Mission and Rationale

The mission of this center is to accelerate the adoption and insertion of power electronics into the electric grid in order to improve system stability, flexibility, robustness, and economy. We expect to accomplish that mission by focusing on the following main objectives

  • To develop new technologies for advanced power electronic systems in the areas supporting grid connected distributed energy resources, power steering and routing devices, and intelligent load-side devices,
  • To develop the software and tools for controlling embedded- and grid-connected power electronics to benefit the grid as well as controlled loads,
  • To educate engineers who understand the power electronic technologies important to the member companies.

The electric power industry is ultra-critical to the economy and security of the United States. Without electric power, everything stops - literally everything - and these interruptions have severe economic consequences. The demand for electrical energy is increasing and political and environmental pressures are forcing adoption of new distributed generation resources, such as wind, solar, and tidal, that do not fit well into the traditional architecture of the electric power grid. Robustness of the national power infrastructure is threatened by aging equipment, lack of integration between generation, transmission, distribution, and utilization, and by terrorism. Paradoxically, one of the direst near-term threats is imposed by increases in local, environmentally-green generation technologies that can undermine the traditional safety mechanisms. This threat occurs partly because the mathematical and physical structure of the power grid is evolving from a paradigm of "a few controls on systems governed by the laws of physics" to "many independent controls on systems being governed by independent digital controls". Our ability to predict the behavior of the system, and thus to control it, will rapidly erode without greater standardization of the control mechanisms embedded in power electronics. We currently have a narrow window of opportunity to re-define the power grid to improve its robustness. This is a focus of our research and is precisely where university research and industrial partnering will pay huge dividends.

Research program

Analysis of Subsynchronous Control Interactions (SSCI) in DFIG-based Wind Farms: ERCOT Case Study

The SSCI is mainly due to the interactions between DFIG wind turbine controllers and the series-compensated transmission line, to which the wind farm is connected. Unlike the other types of the SSR such as induction generator effect (IGE), the SSCI does not have well-defined frequencies of concern due to the fact that the frequency of oscillation in SSCI depends not only on the configuration of the series compensated transmission line and induction generator parameters, but also on the wind turbine controller configuration parameters. Moreover, the oscillations caused by the SSCI may grow faster compared to other SSR types, since the undamped oscillation in SSCI completely depends on the interactions between the electrical system and the controller, which have a smaller time constant.
The SSCI has come into prominence since the ERCOT event of 2009. A faulted line and subsequent outage in the network caused a large DFIG wind farm to become radially connected to the series compensation network, resulting in a rapid increase of sub-synchronous frequency oscillations, leading to damage to both the series capacitor and the wind turbine.
In this project, SSCI in DFIG-based wind farms is studied. The study includes 1) a comprehensive review on the SSCI phenomenon 2) small-signal modeling of the ERCOT power network for SSCI studies 3) design of a SSCI damping controller and insertion of the designed controller in the DFIG converters 4) large-signal time-domain simulations in MATLAB/PSCAD in order to validate the design process.

Future Hybrid Microgrids

The microgrid may be one of the best concepts for enabling integration of distributed energy resources and increasing the reliability of the power grid. This project includes the design and construction of hybrid microgrid prototypes, both in the MVA power range and in the scaled-down power range (kVA). Power electronic interfaces are the key components of the proposed microgrid, which includes: dc sub-transmission converter, distributed resources, circuit breakers, critical and non-critical loads. As the power ratings of the converter increase (to the MVA range), the stability of the microgrid becomes more challenging. This project will verify control algorithms for mitigating the unstable problems (e.g., various active damping controls), first at the scaled-down prototype, and then at the MVA power prototype. Optimal schemes of microgrid operating mode (normal ac grid parallel connected mode, transition-to-island mode, islanded mode, and ac grid reconnection mode) will be studied for the future power energy management.

Optimization and Reliability Assessment of Power Electronic Modules

With the power density in power electronics (PE) increasing rapidly, PE modules offer attractive solutions to improve performance. Several issues must be investigated in order to achieve the current and voltage specifications required by typical grid-tied PE applications. In the first phase of this project, researchers achieved substantial improvements in the breakdown voltage capacity and module architecture of silicon carbide (SiC) modules. In the second phase of this project, researchers will be investigating parasitic inductance minimization techniques and performing an EMI compatibility and reliability assessment of various layout techniques. A 3-D multilayer approach will be used, as it is more suitable both in terms of reducing parasitic inductance and EMI emission of PE modules. These tasks are critically important to the development of 3-D power modules and are needed to allow broader usability of SiC as a power module material.

PV Inverter Control to Sustain High Quality of Service

High penetration levels of PV power generation can produce significant undesired effect on distribution networks. The focus of this project is to define strategies for the planning, control, and coordination of PV plants that take in consideration quality of service requirements. Changes in feeder voltage profiles, including voltage rise and unbalance, change in feeder loading, including potential equipment overloading, frequent operation of voltage-control and regulation devices, reactive-power flow fluctuation due to operation of switched capacitor banks, overcurrent and overvoltage protection misoperation and change in electric losses and power factor are some of the main consequences that can arise due to high levels of PV power production. This project will focus on aspects that affect voltage quality and the related equipment.
We are developing a two-stage hierarchical control structure based on a top-level day-ahead control and on a fast on-line control. The goal of the day-ahead control is to optimize energy losses and voltage deviations by defining capacitor bank switch status, line regulator status, storage management plan, DG reactive power reference point for the day after on the base of load and weather forecast. The optimization of this product is influenced by DG limits, storage units limits, line limits, upper and lower limits as well as by the maximum number of operations of line regulators. The fast on-line control compensates for generation and load variability dispatching reactive power injection of the distributed generator and correcting the day-ahead schedule.

Reducing Short-Circuit Current Levels Using Fast-Acting Solid-State Fault Current Limiters

High short-circuit current levels cause adverse effects ranging from high interrupting capabilities required for protecting equipment to increased arc-flash protection. Developing a protecting piece of equipment which is able to isolate faults under a quarter cycle of the fundamental frequency should not only minimize safety hazards due to arc flash but bring economic benefits because of (i) reduced interrupting capabilities required for protecting equipment, and (ii) not needing to size the equipment to sustain higher short-circuit current levels. Drawing upon previous experience in solid-state fault current limiters (SSFCLs), the main goal of this project is to develop fault current limiter (FCL) technology which is able to interrupt fault current under 4 ms for 1kVac and 15kVac power systems. The objective of the first task is to evaluate whether a solid-state fault current limiter (SSFCL) or hybrid fault current limiter (HFCL) is a better option to reduce fault current levels than existing industry practice using mechanical circuit breakers. The objective of the second task is to select a case study of a large industrial load for evaluating the SSFCL/HFCL benefits and identify potential drawbacks associated with using power semiconductor devices, such as on-state power losses and fast di/dt’s that lead to over-voltages. The objective of the third task is designing a scaled-down prototype to carry out several experiments to verify mathematical and simulation models. It is envisioned that a full-size prototype will be built and tested at the National Center for Reliable Electric Power Transmission (NCREPT) test facility at a later stage. Utilities and industrial users can benefit significantly from a cost-effective SSFCL/HFCL. Manufacturers of distribution equipment can develop a new product using the generated FCL ideas.

Solid State Transformer

The traditional fundamental-frequency power transformer is a key component in many applications where it is necessary to step up or step down from one voltage level to another. This operation is done efficiently but at the expense of needing a large size/volume. There are several new applications where size or volume is critical. A solid-state transformer brings desired size or volume reductions as the expense of lower efficiencies and greater system complexity. The main goal of this project is to develop a modular solid-state transformer for applications characterized by space limitations, interconnection of solar or wind farms, or high fault currents. Modules are connected in series on the high-voltage (HV) side and in parallel or in series on the low-voltage (LV) side depending on the selected application. The HV DC side of the SST module consists of a three-level full bridge topology switching under zero-current and zero-voltage switching. The LV DC side could be a two- or three-level full bridge topology depending on the applications. It is envisioned that the high-frequency (HF) link operates at 20 kHz and a HF transformer provides the required voltage ratio for the selected application. Initially, the research team will consider silicon carbide 1.7kV MOSFETs for the HV side that will be packaged by UA. The LV side of the SST could use the same devices but their voltage rating will depend on the selected application. Applications for this prototype are not only in distribution systems but also for electric train traction. So, the potential for a product in this space is also important.

Special Activities

Associated personnel have extensive research programs in: 

Power systems, including off-grid systems such as in transportation systems (ships, planes, trains, and automobiles)

Power electronics devices, characterization, modeling

Control systems and motor drives

Integrated and power electronic circuit design and testing

Simulation  methods and environments for multidisciplinary dynamic systems

Facilities and Laboratory

National Center for Reliable Electric Power Transmission

University of Arkansas Fayetteville

  • $4.5 M facility opened in 2008
  • 7,000 sq. ft. with four test bays
  • Tractor trailer accommodation for large equipment under test
  • Variable Voltage to 15 kV
  • Variable Frequency
  • Reconfigurable
  • Programmable
  • Uniquely Capable

   

Virtual Test Bed Facility

University of South Carolina Columbia

  • Power Routing
  • Power Electronics
  • Realtime Electro Mechanics
  • Virtual Test Bed Development

Locations

University of Arkansas Fayetteville

3217 Bell Engineering
1 University of Arkansas

Fayetteville, Arkansas, 72701

United States

(479) 575 - 4838

(479) 575 - 7967

http://grapes.uark.edu

University of South Carolina

Swearingen Engineering Center
301 South Main Street Room 3A80

Columbia, South Carolina, 29208

United States