Grid-Forming Inverters for Renewable Energy

The transition to renewable energy sources like solar and wind is accelerating globally. However, the intermittency and variability of these resources present challenges for integrating them into the electric grid. Grid-forming inverters and virtual synchronous machines (VSMs) have emerged as key enabling technologies to overcome these challenges and facilitate greater renewable penetration.

Inverters are power electronic devices that convert direct current (DC) electricity into alternating current (AC) to interface with the grid. Traditional grid-following inverters simply inject power according to available renewable generation. This can lead to frequency fluctuations and instability when renewable output changes abruptly.

Grid-forming inverters and VSMs take a different approach. They mimic the inertial and governor-like responses of conventional synchronous generators to provide crucial grid-stabilizing functionality. This allows inverter-based resources to actively support grid frequency and voltage, enabling higher renewable penetration while maintaining reliability.

How Grid-Forming Inverters Work

Grid-forming inverters synthesize their own AC voltage waveform rather than relying on the grid voltage as a reference. This allows them to dictate frequency and voltage like a synchronous generator. The inverter controls the amplitude, phase angle, and frequency of its output voltage.

To maintain stability with the grid, grid-forming inverters measure the frequency and phase of the grid voltage. By comparing its internally generated waveform to the true grid waveform, the inverter can synchronize and match phase and frequency. This synchronization happens extremely fast, on the order of milliseconds.

The inverter can then adjust the phase and frequency of its output as needed to maintain synchronicity with the grid. For example, if grid frequency drops, the inverter will detect this change and accelerate its waveform appropriately to match. This provides inherent inertial and governor-like grid support.

Virtual Synchronous Machines for Added Functionality

While grid-forming inverters stabilize frequency and voltage, more advanced virtual synchronous machine (VSM) control adds additional grid services. VSM control mimics real synchronous generator physics and electromechanical dynamics.

VSM control introduces a virtual rotating mass, akin to the spinning mass inside a synchronous generator. This virtual inertia absorbs changes in power, slowing frequency deviations. VSM control also emulates governor droop characteristics, adjusting power output based on frequency to help restore normal frequency after a disturbance.

Together, these virtual inertia and governor behaviours provide critical short-term frequency stabilization. VSM control can also simulate other physical aspects like oscillation damping, voltage/reactive power control, and black start capability. This helps ensure seamless replacement of conventional generation with inverter-based resources.

Realizing the Potential of Grid-Forming Inverters

The crucial grid-supporting capabilities of grid-forming inverters and VSMs make them foundational for modern power grids. However, there are challenges to overcome before they become ubiquitous.

First, grid-forming functionality requires advanced power electronics and complex control systems not found in traditional inverters. This makes them more expensive. Costs are coming down with technological improvements, though. Second, interoperability issues can arise when integrating multiple inverters from different manufacturers. Standards are emerging to address these challenges and ensure coordinated response.

Finally, grid operators must gain operational experience with grid-forming inverters at scale. Large penetration can introduce control interactions that require careful study and planning. Pilot deployments are ongoing worldwide to enable learning and refinement of technology, standards, and procedures.

With these challenges being actively tackled, grid-forming inverter adoption is accelerating globally. These technologies are critical to transition electricity grids seamlessly to high renewable penetration while maintaining reliability. Unlocking their full potential will require contributions across manufacturers, operators, regulators, and other stakeholders.

Takeaway

Grid-forming inverters and virtual synchronous machines (VSMs) play a crucial role in providing essential services like inertia, governor response, and grid stabilization by mimicking the behaviour of traditional synchronous generators. This is key to integrating more renewable energy while ensuring the grid remains reliable and resilient. These technologies can be applied across various scales, from microgrids to larger generation sites. While there are challenges concerning costs, interoperability, and deployment, ongoing technological advancements, standards development, and operational experience are helping to address these issues. Understanding their capabilities and limitations is important for effectively incorporating them into energy systems.

If you have questions about what we can do for your renewable energy implementation, please don't hesitate to reach out. We're here to help and welcome your thoughts and comments.

Until then, keep shining bright like a solar panel on a sunny day!