The Rules Governing the Grid – I

Grid Forming

The Five Pillars of a Grid Forming Converter

In recent years, the term Grid Forming has evolved from being confined to the realm of research to becoming one of the most relevant concepts in the energy transition. As conventional generation is overtaken by power electronics-based technologies, the stability of the power system depends less on the natural inertia of large synchronous machines and increasingly on the intelligence embedded in converters.

This evolution has brought about a paradigm shift. It is no longer sufficient for a converter to merely exchange power with the grid efficiently. It is now also expected to actively contribute to maintaining the grid’s stability, respond correctly to disturbances, and even be capable of participating in the system’s recovery following a blackout.

But what is the meaning of designing a Grid Forming converter? 

The answer goes far beyond implementing a particular control algorithm. A true Grid Forming converter must possess a set of capabilities ranging from its electrical behavior under normal conditions to its response during short circuits, its interaction with other converters, and the accuracy of the models used to study its integration into the grid. 

In this article, we review five key elements that, from an engineering perspective, define the expected behavior of a converter with Grid Forming capabilities. 

1. Stability begins long before the first disturbance 

When considering the stability of a power grid, we tend to imagine major disturbances: a short circuit, a voltage dip, or the tripping of a line. However, in most cases, the grid operates very close to equilibrium. 

The primary feature that distinguishes a Grid Forming converter is its behavior as a voltage source. This means that it does not constantly wait for the grid to tell it how to act but rather establishes its own electrical reference and actively participates in maintaining the system’s balance. 

But generating voltage is not enough. How it is generated also matters. 

From the grid’s perspective, a Grid Forming converter must exhibit behavior that is predictable and consistent with the physical laws of the electrical system. To achieve this, the control system replicates the effect of an internal impedance that determines how the converter exchanges power with the grid and how it responds when operating conditions change. This impedance—even though it results from the combination of an algorithm and a physical component—allows the converter to respond naturally to load variations or minor disturbances. 

That same philosophy extends to power distribution amongst multiple converters. As the use of power electronics-based resources grows, it is becoming increasingly common for multiple Grid Forming units to operate together at a single point in the grid or even to supply a power island. In such situations, each converter must be able to regulate its active and reactive power using only the information available at its terminals. 

However, power sharing alone is not sufficient if every minor disturbance ends up turning in persistent oscillation. A Grid Forming converter must naturally dampen the electromechanical oscillations that arise in the grid. Some of these oscillations are part of the control algorithm’s inherent behavior; others require specific damping functions. In both cases, the goal is the same: to prevent the converter from amplifying dynamics that are already complex by nature. 

It is imperative that these targets are met in conjunctions with maintaining the ability to regulate voltage and frequency. Because, ultimately, the stability of an electrical system does not begin when a disturbance occurs. It begins much earlier, in the way each piece of equipment contributes to the overall balance, even when, on the surface, nothing seems to be happening. 

2. Speed Is Useless Without Stability

If steady-state behavior defines the character of a Grid Forming converter, disturbances are when it is truly put to the test. 

The connection of a large load, the opening of a transmission line, a protective trip, or the disconnection of a generator instantly alters the state of the grid. Within milliseconds, voltage changes, variations in the phase angle, or frequency deviations can occur, which all connected equipment must manage in a coordinated manner. 

A Grid Forming converter cannot simply endure these events. It must proactively contribute to stabilizing the system from the very moment they occur. 

One of the most demanding scenarios involves sudden changes in the grid’s phase angle. While these  issues may not be immediately apparent to the end user, they can have a significant impact on synchronization systems. A converter designed to operate in Grid Forming mode must maintain synchronization during these events and respond by exchanging power to help dampen the disturbance. 

A similar phenomenon occurs when there is a sudden change in voltage. Instead of reacting solely from a protection standpoint, the converter must almost instantly adjust its reactive power exchange to assist in restoring the voltage level. 

Frequency is another key indicator of an electrical system’s health. Traditionally, the inertia of synchronous generators naturally dampened frequency variations caused by an imbalance between generation and consumption. In a system dominated by converters, this contribution can no longer be taken for granted. For this reason, Grid Forming algorithms incorporate mechanisms capable of replicating this behavior in a controlled manner, supplying or absorbing power to slow the change in frequency and provide a margin of stability while the rest of the system regains balance. 

What is notable is that all these responses share the same philosophy. The goal is not to react as quickly as possible, but to do so in a way that is beneficial to the grid. A slow response offers minimal support, while an overly aggressive response can generate new oscillations or intensify the initial disturbance. 

3. When the Converter Reaches Its Own Limits

There comes a point when all the sophistication of a control algorithm becomes irrelevant. It is the moment when a fault occurs in the grid and the converter reaches the current limit that its hardware—that is, its power electronics—can support. 

From that point on, it is no longer just a matter of controlling voltage or frequency. The objective becomes much more complex: protecting the equipment while continuing to support the electrical system. 

It may seem like an impossible balancing act. On the one hand, the converter must limit the current to prevent damage to its semiconductors. On the other hand, the grid needs it to maintain voltage stability and ensure the proper operation of protective devices. 

For many years, the standard strategy involved prioritizing certain components during overcurrent events. More recent approaches take a different tack. Instead of deciding which variable should be compromised initially, the converter aims to preserve the electrical behavior it would exhibit in the absence of limitations, reducing only the total current to a safe level. 

The speed with which a fault is detected, and support is provided is also critical. In a modern electrical system, a few milliseconds can mean the difference between a local disturbance and a problem with much broader consequences. Therefore, in addition to protecting itself, a Grid Forming converter must be capable of providing support current from the very first moments of the disturbance; this will promote both voltage stability and the proper operation of grid protections. 

This is particularly crucial in weak grids, where the available short-circuit power is limited and each converter has a significant impact on the system’s overall behavior. In these scenarios, maintaining synchronization during a fault and quickly restoring normal operation is no longer an advanced feature but rather an essential operating requirement.

4. Much More Than Just Generating Power

There is a fundamental difference between maintaining an existing grid’s stability and establishing a new grid from the beginning. For decades, this latter task was reserved for a small number of conventional power plants capable of gradually energizing lines, transformers, and loads following a widespread blackout. 

One of the greatest potentials of Grid Forming technology is its ability to serve as the starting point for an electrical grid. In applications where necessary, a converter can independently establish a stable voltage and frequency reference, gradually energize a facility, and allow other equipment to connect progressively and safely. This capability, known as Black Start, assumes strategic importance as the penetration of renewable resources and storage systems increases. 

The energization of transformers, power lines, or large loads can generate very high transient currents if not managed properly. Therefore, the converter must be capable of gradually increasing the voltage, adapting to the conditions of the installation and avoiding unnecessary stress on both its own components and the rest of the connected equipment. 

Once the grid is established, a new challenge arises keeping it operational even if an external reference is lost. This is exemplified in island operation, where the converter assumes complete control of the local system’s voltage and frequency. While this scenario may appear unusual, it is becoming increasingly relevant in microgrids, hybrid systems with storage, or installations that require a high level of supply continuity. 

Achieving this behavior requires paying attention to a lesser known but equally important aspect: the interactions between converters. When multiple electronic devices share a grid, oscillations or resonances may occur that do not depend on any single device individually, but rather on how they all interact with one another. For this reason, Grid Forming design aims not only to ensure that each converter is stable on its own, but also that it remains stable when operating alongside other devices, even in very weak grids. 

Taken together, all these capabilities reflect an increasingly evident reality: converters are no longer merely devices designed to exchange energy. They are becoming elements capable of creating, maintaining, and restoring entire electrical systems, taking on functions that, until just a few years ago, could only be performed by large synchronous machines.

5. From the Lab to the Power System: Demonstrating That the Above Statements are True 

Developing a Grid Forming converter is not just about designing an algorithm capable of responding correctly to any situation. It also involves demonstrating—before the equipment is deployed in the field—that it will behave as expected regardless of the grid to which it will be connected. 

In such cases, the use of simulation models plays a key role. 

Today, virtually no project involving the integration of renewable energy, energy storage, or power electronics is developed without extensive preliminary simulation work. Prior to installing any equipment, standard practice is to analyze how the system will respond to disturbances, study its interaction with the rest of the grid, and verify compliance with connection codes. The quality of these conclusions depends, to a large extent, on how accurately the models represent the converter’s actual behavior. 

However, no single model can answer all questions. Some simulations focus on phenomena that evolve over seconds or minutes and allow for the study of the power system’s overall stability. Others need to capture transients lasting just a few milliseconds to analyze the converter’s behavior during a fault, a switching operation, or an interaction between control systems. 

Just as important as the model itself is the confidence that can be placed in it. A useful model is not one that merely approximates the converter’s behavior, but one whose relationship to the actual equipment is thoroughly documented. Each functional block, each relevant parameter, and each control strategy must align clearly with the implementation that the firmware will ultimately execute. This traceability ensures that the results obtained during grid integration studies remain valid throughout the product’s entire lifecycle, even as new versions of the control system are released or the hardware evolves. 

Naturally, simulation is not substitute for experimental testing either. Both are part of the same validation process. The results obtained in the laboratory and in the field make it possible to verify the behavior predicted by the models, adjust any aspects that require it, and progressively increase confidence in the tools used to design future generations of converters. 

In essence, models serve as the initial operational environment for converters. It is there that its behavior in weak grids is studied, scenarios that would be difficult to reproduce experimentally are analyzed, and its interaction with other equipment is evaluated. For this reason, a well-constructed model is not merely a simulation tool; it is an extension of the design process itself. 

Ultimately, developing Grid Forming technology means combining physics, power electronics, control, and validation into a unified engineering process. A converter cannot be considered ready for the grids of the future simply because it performs correctly in the laboratory. It must be able to demonstrate that performance with the same reliability in the models utilized by grid operators, project developers, and integration engineers.

Conclusion

For decades, the requirements that a grid-connected converter had to meet were relatively well defined: efficiently injecting power, complying with grid connection codes, and remaining connected during certain disturbances. Today, that approach is no longer sufficient. 

The increasing use of power electronics-based resources is changing the role of converters within the power system. They are no longer merely devices responsible for converting energy, but active contributors to grid stability. They must be capable of establishing voltage and frequency references, damping oscillations, assisting during faults, operating in weak grids, coordinating with other converters, and even participating in system restoration following a blackout. 

The good news is that power electronics provides the tools to meet this challenge. But this capability does not stem from a single algorithm or a specific feature. It is the result of a design philosophy in which every decision—from control architecture to model validation—pursues the same goal: to ensure that the converter is a reliable element on which the grid can rely. 

In the coming years, we will continue to discuss new functions, new network codes, and new requirements for Grid Forming technology. However, the real challenge will likely not be adding more features but rather ensuring that thousands of converters—manufactured by different suppliers and connected in increasingly complex networks—are able to behave as a single power system. That will likely be one of the major challenges facing grid engineering in the coming decade.