Reports
Microgrid Ramp Rates and Stability
(March 2014)
Variable power sources, such as solar or wind power, are becoming increasingly common in low-inertia microgrids, but inadequate combinations can result in poor service quality and operational instability.
To facilitate the effective adoption of these emerging power sources, the authors present new stability margins to predict how long microgrids can withstand time-varying power ramps caused by variable power sources before remedial action becomes necessary. Although microgrids can ride through ramped disturbances using local inertia, it is not clear for what ramp rate magnitudes or how long disturbances can be sustained. The stability margins presented herein yield available reaction times as a function of ramp rate magnitude and local inertia; their use is demonstrated as a run-time calculator that reports reaction times as disturbances take place on an actual microgrid model.
This work highlights how a modern power system communication and control system can change the nature of a grid. In a grid, at any scale, it is necessary to keep the load and the generation in balance or the system will shut itself off to prevent catastrophic damage to the system. In many microgrids, however, the geographic scale is sufficiently small so that the system can be designed to take other remedial action, bringing stored energy on line or selectively shedding or adding load. This action avoids the expense of system shutdown.
To engineer this new type of system, one needs to understand the interaction among the factors that are necessary for stability. This paper shows the three critical inherent factors are the inertia of the rotating machines, the rate of change (ramp rates) of the sources or loads, and latency in the control system. The effective inertia can be augmented by storage if necessary. The size of the microgrid is limited by the requirement that the reaction time be much longer than the latency of the control system.
So, this approach can add value in the design of new systems, in designing upgrades in existing systems, or in improving the control of new systems.
Report
Not available.
Video:
Frequency Stability in Nome, AK
(2014)
The city of Nome, Alaska, receives its power from a microgrid powered primarily by diesel generators augmented by wind power and a very small amount of solar power. It is now proposed to add geothermal source to this mix of sources.
The geothermal plant could provide 40% or more of the base load, reducing dependence on diesel fuel. The novel aspect to this situation is that the existing microgrid is quite compact with the sources and loads contained in a circle with a radius of a few miles.
The geothermal plant will be approximately 60 miles from the rest of the microgrid. During steady-state operation, this was expected to be a negligible concern. In a loss-of-tie-line or fault scenario, the situation was less clear. So, three scenarios were investigated. The first two examined the effect of the system if the line between the rest of the system and the geothermal plant was suddenly lost under two different loading conditions. The third examined the effect of a fault prior to the line loss.
Citation:
[1] F. M. Uriarte and R. E. Hebner, "Nome's Geothermal Interconnection: Sizing Diesel-Off Inertia and Remedial Response Time," Available [online] http://sites.google.com/site/fabianuriarte/
Report
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Slides:
Smart Grid in Austin, Texas
(2013)
Pecan Street Inc., with the cooperation of the residents of the Mueller development in Austin, is generating an extensive database of electricity usage with good time resolution and spatial resolution to the individual residence.
With funding from Pecan Street, Inc. (Pecan Street) and the cooperation of Austin Energy, one of Pecan Street's utility partners, researchers from the University of Texas at Austin have developed a model of the electrical distribution system. This model uses the available data to assess the state of the distribution system under existing conditions.
With the confidence developed from this effort, predictions of operation under other scenarios can be performed. This capability is particularly important in areas like the Mueller development, in which growth and the addition of new technologies, like rooftop photovoltaic systems and plug-in vehicles, are dramatically changing the demands on the electrical distribution system.
Citation:
Report
[1] F. M. Uriarte and R. E. Hebner, "Impact of Residential Photovoltaic Generation and Electric Vehicles on Distribution Transformers," Available [online] http://sites.google.com/site/fabianuriarte
Slides
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This video shows the impact of photovoltaic arrays and electric vehicles (Chevy Volts) on the distribution transformers. The smart grid is provided by Pecan Street (pecanstreet.org) out of the Mueller Community in Austin, Texas.
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