BASIC KNOWLEDGE Grid-forming inverters: How they work and why grids need them

The world is transitioning from the conventional grid to the renewable-dominated one, backed by solar, wind, and batteries. The problem with the renewable-backed grid is the lack of stability. Instead of synchronous generators, it uses inverter-based resources (IBR) to facilitate stable operation. However, when a disturbance hits, the inverter system has very little to hold it together. Times have changed as newer types, such as grid-forming inverters (GFMs), start supporting the grid.

Grid-forming inverters are defined by the software, rather than the hardware. The same way software-defined power electronics (SDPE) defines software-defined vehicles (SDVs).

Why grid-forming inverters?

The conventional grid is backed by synchronous generators, whose physics combats any rate-of-change-of-frequency (RoCoF) events. Whenever the frequency, whether 50 Hz or 60 Hz, shifts, the synchronous generators don’t stop. They provide inertia as a by-product of their spinning mass to prevent generators from stopping. You can learn about the missing physics of renewables here.

When grids become renewable, IBRs have no physics to generate inertia and prevent grid downtime because they act when they first sense a faulty frequency or voltage. The grid-forming inverter automatically holds its own voltage and frequency by instantly supplying the power the grid needs. This means that grid-forming inverters don’t wait to detect a fault; they automatically have the response ready. No blackout occurs.

There is no specific grid-forming inverter symbol; instead, it is represented as a “GFM” block in the diagram. Some sources can also use the inverter symbol ~ to depict grid-forming inverters.

There are several control strategies engineers use in grid-forming inverters. They add more to the GFM bill of materials (BOM files).

Droop control: The word “droop” means to bend or hang downwards because of a lack of strength. Similarly, droop control works by allowing all power sources to lower their frequencies or voltages up to a permissible limit. In droop control-based GFM inverters, multiple power sources are connected in parallel. Based on their power ratings, the load is divided among them proportionally.

Each of them is assigned a droop value. This means that they are only allowed to deviate from the grid frequency and voltage in accordance with the assigned values. Droop control is an established strategy in GFM inverters; however, it reacts to the event only in proportion to the values. So in a weak grid, sudden swings can push beyond safe limits until the droop settles them.

Virtual synchronous machine (VSM): When grid frequency or voltage alters, synchronous generators, by their physics, try to remain in the same state of motion. They provide inertia to prevent alteration of grid frequency and voltage. A virtual synchronous machine functions like a synchronous generator by providing virtual inertia, known as “synthetic inertia”, to stabilize the grid.

Simply put, the VSM control strategy pushes extra power when the grid frequency declines and draws when the grid frequency increases. VSM follows software algorithms, commonly the swing equation, to simulate the behavior of a synchronous generator connected to distributed power sources.

It simulates their physical and electrical characteristics, including rotating mass, ability to self-synchronize with the grid, high impedance, and large inertia. VSM control strategy is suitable for grids transitioning from conventional systems to renewable-heavy implementation. It holds backend compatibility with legacy grid infrastructure.

Virtual oscillator control: Instead of copying a synchronous generator, the virtual oscillator control (VOC) method mimics a self-sustaining nonlinear oscillator. In simple words, VOC GFM inverters are based on natural oscillator behavior. Voltage and frequency are regulated in accordance with internal states.

All connected power sources hold the grid’s nominal voltage and frequency. When a RoCoF event occurs, they snap back to the same nominal values. Each source is independent of the others, reducing the reliance on communication. However, coordinating a large number of sources can be problematic, which is why VOC performs the best in islands and microgrids.

Adaptive control: GFM inverters using an adaptive control strategy can dynamically adjust inverter parameters to combat faulty grid events. Adaptive algorithms need to continuously record values of grid voltage, frequency, power, current, and other important parameters to optimize inverter performance. It is suitable for islands and microgrids because data acquisition adds complexity and cost.

RENEWABLE ENEGRY

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Benefits of grid-forming inverters

At present, grid-following inverters, a predecessor of grid-forming inverters, dominate the renewable grid. Grid-forming inverters have shown excellent performance in practical deployments. The U.S Department of Energy funded an initiative known as Universal Interoperability for Grid-forming Inverters (UNIFI) to promote grid-forming inverters.

Benefits of grid-forming inverters include the following.

• 1. Regulate voltage: Grid-forming inverters provide stable output AC voltage for proper grid functioning.

• 2. Synthetic inertia: Grid-forming inverters provide synthetic inertia to resist frequency fluctuations in RoCoF events.

• 3. Autonomous operation: Grid-forming inverters can automatically regulate voltage and frequency without taking reference from the actual grid.

• 4. Instant response: Grid-forming inverters react as soon as grid parameters deviate.

• 5. Smooth operation: Grid-forming inverters don’t embed grid parameters in a feedback loop to generate their output. Such interactions can sometimes generate oscillations.

• 6. Black-start capabilities: If grid-forming inverters are deployed with energy storage solutions, they can restore grid operation from total blackouts and outages.

• 7. Interoperability: Grid-forming inverters are compatible with legacy grids. They enable integration of distributed energy resources (DERs), including PVs, batteries, and wind turbines.

• 8. Seamless islanding: Grid-forming inverters can transition smoothly, without causing surges, between grid-connected and islanded operations.

• 9. Facilitating renewable penetration: Grid-forming inverters enable the grid to run on more IBRs, about 60-70%.

• 10. Fewer curtailments: By stabilizing the grid, grid-forming inverters reduce the operator’s need to switch off clean energy.

ENERGY STORAGE

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Challenges for grid-forming inverters

In the faulty condition, GFM inverters have to supply whatever current, roughly 1.2-1.5 times the normal, to stabilize grid voltage. Switches must be able to handle such current surges. In general, IBRs in renewable-grid use IGBTs for their low cost and optimal performance.

Grid-forming inverters call for silicon carbide (SiC) MOSFETs for their fast switching speed, power handling capability, and heat management. SiC MOSFETs and other components increase the overall cost of GFMs.

The only downside of grid-forming inverters is that they’re new. However, with better features and outcomes, they cost more. Due to the performance benefits, the industry is commissioning more projects and inviting investment to promote GFM deployment in the renewable grid infrastructure.

References

https://in.mathworks.com/discovery/grid-forming-inverter.html

https://iea-isgan.org/grid-forming-inverter/

https://ieeexplore.ieee.org/document/6652456

https://in.mathworks.com/discovery/droop-control.html

https://ieeexplore.ieee.org/document/8662197

https://www.sciencedirect.com/science/article/pii/S2352484723002810

https://www.mdpi.com/2079-9292/15/5/1115

https://unificonsortium.org/#:~:text=The%20Universal%20Interoperability%20for%20Grid%2DForming,effort%20to%20advance%20grid%2Dforming%20(GFM

https://spectrum.ieee.org/electric-inverter

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