Engineer Explains Three Key Issues in Renewable Grid Design | WSJ Pro Perfected

Key Concepts

• Inherent Rotational Inertia
• Grid-Forming Inverters
• Grid-Following Inverters
• Synchronous Condensers
• Battery Energy Storage Systems (BESS)
• Frequency (60 Hertz in the US)
• Microgrid
• Alternating Current (AC)
• Direct Current (DC)

The Hidden Infrastructure Problem of Renewable Energy

The increasing reliance on renewable energy sources like wind and solar power is creating a significant challenge for existing power grids. These grids were originally designed for conventional power plants (coal, natural gas) and are not well-suited for the characteristics of renewable energy. This mismatch contributed to the severity of the blackout across Spain and Portugal in April.

Inherent Rotational Inertia: The Missing Piece

Conventional power plants possess inherent rotational inertia, which provides stability to the grid. This inertia comes from the large, spinning machinery (e.g., gas turbines) connected to generators. These generators produce electricity at a consistent frequency (60 Hz in the US) and remain synchronized. If a conventional power plant goes offline, the inertia acts as a buffer, providing time for other energy sources to compensate. This is analogous to the momentum of a bicycle, helping it continue moving even when pedaling stops.

Renewable energy sources, particularly solar and wind, lack this inherent rotational inertia. Solar panels do not rotate, and wind turbines do not rotate at speeds synchronized with the grid frequency. This absence of inertia increases the risk of frequency fluctuations and potential blackouts. The grid can only tolerate small frequency changes; drastic deviations can cause power plants to disconnect to protect themselves.

Solution 1: Grid-Forming Inverters

Grid-forming inverters offer a solution by providing renewable energy with synthetic inertia. Unlike grid-following inverters, which rely on the grid's voltage and frequency as a reference, grid-forming inverters can operate independently and support the grid during disruptions.

• How they work: Solar panels produce direct current (DC), which needs to be converted to alternating current (AC) at 60 Hz for grid compatibility. Grid-forming inverters perform this conversion and can react to frequency changes faster than conventional generators. They provide a continuous supply of synthetic inertia.
• Microgrids: Grid-forming inverters are particularly effective for building microgrids, such as those used in military installations.
• Limitations: Managing synchronization across large, interconnected regions is more challenging with grid-forming inverters compared to systems relying on inherent rotational inertia. They also require additional measurements (voltage, frequency, temperature) and instrumentation across the grid.

Solution 2: Synchronous Condensers

Synchronous condensers are large, rotating machines that provide rotational inertia to the grid. They are similar to the rotating machinery in conventional power plants but do not burn fuel. Instead, they use electricity from sources like solar panels to maintain their rotation, providing additional rotational inertia.

• Pairing: Ideally, synchronous condensers should be paired with grid-forming inverters for optimal performance.
• Trade-off: A portion of the electricity generated is used to keep the synchronous condenser spinning, representing a cost similar to an insurance premium. This cost provides risk adjustment benefits and ensures grid reliability.

Solution 3: Battery Energy Storage Systems (BESS)

Battery Energy Storage Systems (BESS) store excess energy and can deploy it to stabilize the grid during unexpected outages.

• Operation: Batteries store power and then feed it to inverters, which then supply power back into the grid when needed.
• Cost Reduction: The increasing adoption of BESS is driven by the dramatic decline in battery costs in recent years.

Conclusion

The integration of renewable energy requires significant investment in grid infrastructure to address the lack of inherent rotational inertia. Grid-forming inverters, synchronous condensers, and battery energy storage systems are three key solutions for improving grid resilience and reducing the risk of widespread outages. All energy sources involve trade-offs, and a balanced approach is crucial for designing a reliable and sustainable energy system. It's critical to keep thinking about how to best balance all of these trade-offs.