Inertia is the stored kinetic energy in a rotating mass that helps it maintain its rotational speed. Put another way, inertia helps a rotating mass resist changes in its rotational speed and remain stable. Think about bike wheels. Once a bike is pedaled up to speed, it takes very little pedaling to keep it moving at constant speed and considerable brake force to slow it down. This is because the energy stored in the bike’s wheels (their kinetic energy) keeps the wheels rotating near constant rotational speed (their frequency of rotation) and the bike keeps moving forward at a stable speed with little pedal input. This is inertia in action.
In the power system, inertia is the grid’s ability to maintain a stable electrical frequency by using the collective kinetic energy stored in the rotating generators on the grid. Maintaining near-constant frequency is critical to ensuring safe and secure operation of the power system.
Conventional generators (generators driven by gas turbines, steam turbines, and hydro powered turbines) are “synchronized” to the grid, which means that they are directly electrically coupled to the grid and operate at the same nominal frequency as the grid. For this reason, these types of generators are called “synchronous” generators. In Canada, our nominal grid frequency is 60 Hz which means that all synchronous generators operate at 60 rotations per second or 3600 rotations per minute (RPM). In Europe, 50 Hz (3000 RPM) was chosen as the nominal grid frequency, which is why need all those adapter thingys for our electronic devices when we travel to Europe.
The inertial strength of a power system is directly proportional to the number of synchronous generators in operation on the system. The greater the number of synchronous generators, the stronger the power system and the greater its ability to resist frequency disturbances, such as power generators and transmission lines suddenly going out of service, or “tripping”. Inertia enables the power system to maintain a stable frequency of 60 Hz.
Historically, since almost all generators were synchronous, we essentially got inertia for free because almost all generators provided both energy and inertia. In addition, many industrial and commercial loads connected to the grid provide some inertia – motors, pumps, and fans are also synchronous machines, but they consume energy instead of supplying it, while also adding a bit of inertia to the power system. All of these synchronous machines helped keep the power system extremely robust and stable.
However, the nature of power systems is changing. As we transition to renewable energy with greater numbers of wind and solar generators connecting to the power system, the inertial strength of power systems is declining. This is because wind and solar generators are not synchronized to the grid like conventional generators. They provide energy, but they don’t provide inertia. Instead of being synchronized, or directly electrically coupled to the grid, they are connected to the grid indirectly through electronic devices called inverters. This means they are “asynchronous” generators since they are not directly electrically coupled to the power system. These types of generators are commonly known as inverter-based resources (IBRs).
As more IBRs are added to the power system and the relative proportion of synchronous generators in operation decreases for a given amount of energy on the system, the overall inertial strength of the power system decreases. This means the power system becomes less stable and less able to maintain its target frequency of 60 Hz when generators or transmission lines trip.
And this is why power system operators are so concerned about the recent rapid increase in IBRs on power systems. While IBRs are fine for providing energy, the fact that they do not provide inertia poses a risk to the operability and frequency stability of the power system. Going back to the bike analogy, its kinda like taking away one of the wheels of the bike which makes it harder to balance and to maintain constant speed because you’ve reduced the available inertia. For this reason, the transition to higher levels of IBRs on power systems must be very carefully planned and paced.
But it’s not all doom and gloom – there are engineering solutions available. However, we will need sufficient time to thoughtfully design and plan each power system to ensure they have sufficient inertia for their unique mix of generating resources, interconnections, and transmission topology to enable safe operation while increasing the proportion of renewable energy (IBRs). There are ways to design IBR control systems to help frequency response and there are solutions, such as synchronous condensers, that can compensate for the loss of synchronous generators.
The paper titled Overview of frequency control techniques in power systems with high inverter-based resources: Challenges and mitigation by Dlzar Al Kez et al. provides an excellent overview of frequency control challenges and mitigations. These mitigations will be crucial as we evolve our power systems to greater proportions of IBR. And a great example of innovative thinking on the procurement of inertia services is the UK National Grid’s Stability Pathfinders initiative.
Hopefully this explanation of inertia was helpful to those who may not be familiar with it and helpful in providing context for some of the debate over the pace of renewable energy (IBR) additions and conventional (synchronous) generator retirements.
