Grid Outages in Texas Spark Discussion on the Challenges of Renewable Energy Integration
Freezing temperatures and rolling blackouts have spurred discussion on whether renewable energy grid integration may be a safety net for future outages. Design-level challenges, however, make this endeavor more complicated than it may seem.
With the slogan, "Everything is bigger in Texas," one would assume the state’s electrical power grid would be well-equipped to handle the high power demands associated with extreme weather. Yet, Texas is currently in a state of emergency because state-wide power grids are unable to meet the current power and heat demands during winter storm Uri.
Diagram of a conventional power distribution system in which if "the grid" were to fail, there would be no power for customers. Image used courtesy of the Electric Power Research Institute
The Electric Reliability Council of Texas (ERCOT), a nonprofit organization controlled by the state legislature, is responsible for Texas’ electrical load, and it claims this disaster could not have been avoided—it's infrastructural in nature. ERCOT has shed nearly 11,000 MW of its power load in order to protect the power grid, leaving millions without power amidst freezing temperatures.
This event has spurred increased conversation on the role of renewable energy in grid systems. While some critics have falsely blamed the blackouts in Texas on frozen wind turbines, others have posited that the ERCOT could have buffered the extent of the outages by having backup sources of power generation.
Currently, Texas relies on coal, natural gas, and nuclear plants to provide power (with the exception of some wind turbines). In general, however, renewable energy (RE) sources do not play a significant role in the state's total power infrastructure.
Five Obstacles to Renewable Energy Grid Integration
In 2016, wind turbines generated 487 GW of power and solar energy generated 303 GW of power worldwide. Proponents of renewable energy (RE) systems claim that these forms of power generation not only fight against climate change; they also provide support to traditional low- and high-voltage electrical networks.
Diagram of a PV system. Image used courtesy of Varun Kumar et. al
Why, then, is it difficult to integrate RE systems into current struggling networks, like those in Texas?
RE Systems are Bidirectional
Implementing a new RE system into a traditional high-to-low power distribution system introduces many challenges. Power grids rely on frequency and voltage parameters to ensure the flow of power is moving in one direction. RE integration doesn’t play by the same rules. Because RE systems are bidirectional, integrating these energy sources into a traditional power grid could cause unfamiliar voltage and frequency variations.
Grid integration of renewable systems typically happens at the distribution level (as opposed to the transmission level for larger systems). Image used courtesy of Dr. K.V. Vidyanandan and Balkrishn Kamath
The Challenges of RE Generators
At the distribution level, RE generators could be introduced to help support all levels of the new system. Traditionally, a subsystem would operate at one voltage and step down to lower voltages or step up—but not when both are at the same location. Using RE generators would require that voltage levels at each substation be monitored and protected.
If a solar photovoltaic (PV) source were to be implemented at a single-phase source, it may cause an imbalance in the system. With an unbalanced system, the interconnecting three-phase system will follow suit, leaving RE generators to overheat and shutdown.
Fault Detection Becomes Complicated
Another hurdle that arises when introducing RE to traditional power grids is potential blind protection. Distribution systems are balanced with devices such as relays, fuses, and transformers that help protect the network against faults from any abnormal voltage, current, or frequency inputs.
Fluctuation of fault currents based on distributed generation interconnection. Image used courtesy of Dr. K.V. Vidyanandan and Balkrishn Kamath
By integrating new RE sources, the grid will experience a reduction of fault currents. A reduction should be a good thing—but in this case, it may actually allow short-circuit faults to go under the radar. This can lead to unprotected devices malfunctioning and overheating.
Higher Component Counts
Grid integration of RE sources like variable-speed wind turbines generates more energy than fixed-speed technology. These variable-speed RE sources incur less mechanical stress and fewer power fluctuations, allowing for more kinetic energy to be stored. The problem with these turbines is that they require power converters, increasing component count and making controls more complex.
Variable-speed wind turbines often have higher harmonic distortion, which requires robust power inverters to mitigate harmonics. In addition, RE generators require adaptive filters to handle frequency and voltage regulation. However, this also increases the cost of added components at every RE generator, which could stretch into the hundreds for one major city.
Issues at the Circuit Level
Bringing wind and solar energy to the power grid presents challenges at the circuit level, too. RE integration can notably affect voltage regulation, harmonic distortion, power quality, and system stability. These issues can manifest in reverse power flow, fluctuating short circuit levels, grid congestion, and undetectable device issues.
Because bidirectional power flow can be tough to analyze, developers may need to improve voltage-regulating circuits. Some researchers are introducing grid-interfacing architecture with fuzzy logic controllers to improve voltage quality. This will help with controlling and organizing bidirectional power flow.
An Uncertain Path Forward With Renewable Energy
While the U.S. has a similar power deployment layout as Germany, Germany has been increasing solar PV and wind into its traditional power networks since 2003.
RE grid modernization is a worthwhile goal, but it is not without its challenges as we've outlined. This integration would require developers to reconduct current power infrastructure to incorporate smart technology such as sensors, comms, generators, and energy storage. Introducing smart inverters that can operate with two-way power flow may make RE systems more appealing.
The U.S. statistics of PV capacity as a percentage of the total energy capacity compared to Germany. Image used courtesy of the Electric Power Research Institute
Despite these challenges, an integrated grid distributing balanced energy can assist during outages and minimize downtime in the event of a threatening circuit malfunction. While the infrastructural- and even circuit-level issues associated with integrating RE systems into a traditional grid are worth consideration, slowly integrating solar- and wind-based generators may at least ameliorate devastating outages in the future.
Featured image used courtesy of David J. Phillip/AP