If the research of Samantha Molnar interests to you, the CU Energy Club is hosting Energy Talks throughout the Fall semester and YOU can come! To join us, meet this Friday at noon in Koelbel 375. Next up: Matthew Butner is an environmental economist who will discuss how market structure can influence competition in electricity markets.

Renewable energy was once a topic whose most serious audiences sat in science fiction book clubs. Today, power system operators around the world cannot ignore the trend of renewable energy sources moving into their networks. Yet the operation of a wind or solar farm differs from operating the sources that currently power our grid, and those differences may make future power systems unstable.

Cascading failures on the power grid (more popularly known as “blackouts”) are the bane of system operators. When one part of a transmission network fails, power is redistributed to neighboring components. If those components also fail, a chain reaction could entirely eliminate the connections between generators and consumers of electricity.

Renewable energy has many benefits—the energy is generated by a less pollutant process and avoids using any of Earth’s finite resources. However, it has an important failing: renewable sources of energy weaken a power system’s response to failures because they rely on different methods of generation than conventionally produced energy. PhD student Samantha Molnar may yet help us understand what installation pitfalls to avoid as the push for renewables continues to grow year by year.

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Molnar studies Computer Science at CU Boulder, or as she says, “uses computers for science”. Due to her background in physics, Molnar has an interesting perspective on electricity generators. She knows that they function fundamentally as oscillators—devices which generate oscillating electric currents and voltages. So she builds computer simulations of power systems as interconnected grids of oscillators.

In the past this would only entail constructing a basic map of a power grid, but Molnar has increased the complexity of her models to more accurately simulate a real system. She now incorporates new information about the different sources of energy and how they operate. These variables address the ability of a system to restore operational equilibrium — or return the system to normal — after disturbances to supply and demand for energy in the network.

Energy needs to be in the form of AC current for transmission in modern power lines. With this in mind, Molnar identifies a crucial difference between renewable energy and conventionally produced energy (from generators running on traditional fuels like coal and natural gas). Energy produced by renewable energy generators must be converted into AC current with power electronics equipment. The current-producing mechanisms are not themselves tied directly to the grid and provide no dampening — or self-correction — to unexpected fluctuations in energy moving across it.


A 2017 paper illustrates the differences between how generators are connected to the power grid.

Generators fueled by conventional sources use spinning magnets within current-carrying wire loops to generate the AC current, and thus require no external conversion to AC current. They can instead be tied directly to the grid and oppose sudden changes in the quantities of energy flowing across it.

As Molnar puts it, “conventional generators form the grid, whereas energy produced by renewables only follow it”. In other words, our existing system relies heavily on technology designed for a system with a majority of conventionally-fueled generators. Due to their reliance on power electronics, renewables cannot be counted on to “pick up the slack” if another generator drops offline. Therefore, the more renewables in a given network, the less resilient the grid. This means system operators would have less time to respond to system disturbances that could trigger a network component failure.

Through her models, Molnar hopes to identify the conditions that make a power system susceptible to failure. While highly dependent on the shape of the given network and arrangement of generators in it, these findings can help determine the maximum threshold on installations of renewable generation in that network.

The next step is to then generalize these findings across similar network types. With Molnar’s findings, system operators will be better able to plan for disturbances within their system and ensure reliability of energy supply to consumers. Molnar hopes to one day apply her findings towards an assessment of reliability of the power system nationwide, and ask the question: what correlation is there between wealth and security of the power grid?

By Adrian Unkeles

Posted by Science Buffs

A CU Boulder STEM Blog

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