GridStab News: Regular posts to demystify power system dynamics and stability

Topic 1: Inertia and frequency dynamics

Did you say "inertia"?

In classic power systems, inertia can be defined as the kinetic energy stored in rotating machines (including mainly synchronous generators and industrial motors).

Historically, inertia is hence provided by classic thermal units (e.g., gas or nuclear power plants) and usually cannot be provided by Renewable Energy Sources (RES) connected to the grid through power electronic converters, like Wind Turbines and Solar PV, for instance.

Why do power systems need inertia?

Inertia is extremely important for the stability of the power system. Inertia acts as an energy buffer in the event of power imbalance (sudden loss of a generator, for instance) which smooths out and slows down frequency variation.

Frequency is a signal that reflects the balance of the system, as long as Production = Consumption (losses neglected) the frequency will be equal to the nominal value (e.g. 50 Hz in Europe). Still today, frequency is mainly imposed by classic generators which rotates at their nominal speed to set frequency of voltage and current at nominal frequency in the electrical grid.

In the event of a sudden loss of a generator, the power system will face a power imbalance, the kinetic energy stored in rotating machines (mainly generators) is intrinsically released into the form of electrical energy to the grid leading generators to slow down (by energy conservation), hence leading to a drop of frequency.

The release of kinetic energy slows down the frequency decrease and give enough time for generators speed governors to act and change their power output to bring frequency back up. This stage is usually referred to as the "primary frequency response" of the system. A typical frequency evolution in this kind of scenario is shown in Fig. 1 as an example:

• Inertial response: release of kinetic energy by rotating machines to the grid.

• Governor response is the primary frequency response of the system.

• Automatic generation control (also called secondary frequency response), is a centralized, coordinated control of generators that will bring back the frequency to its nominal value.

Figure 1: Figure 1: Example of frequency response in the event of a sudden loss of generation.

What is the impact of low inertia?

With the energy transition, classic thermal units are progressively replaced by RES which brings the level of power system inertia down. Indeed, while the automatic release of kinetic energy in case of power imbalance is a natural response of classic generators, RES such as solar PV and Wind Turbines cannot provide such kinetic energy. This is not without consequences.

The loss of sudden generation will involve more and more faster frequency deviations (commonly referred to as the rate of change of frequency, or RoCoF in short) which can have multiple consequences:

• More stress on power system equipment leading eventually to some damages.

• More frequent triggering of RoCoF-based protections, that can in some cases lead to cascading trips/failures that can further increase the power imbalance, hence the frequency deviation.

• If the RoCoF is too high, speed governors of classic generators will have less and less time to react, which increases the chance to reach unacceptable frequency value in short amount of time, that could eventually lead to under-frequency load shedding (i.e., a controlled disconnection of consumers to reduce the power imbalance), or even worst, partial or total black-out.

As an illustration, frequency responses to the same event for a high and low inertia system can be seen in Fig. 2. It is seen that low inertia as two major consequences on frequency evolution, a faster decrease and a lower minimum frequency (usually referred to as the frequency nadir).

Figure 2: Fig. 2: Frequency response, high vs low inertia

What now?

Now it should be understood that a minimum amount of inertia is required to guarantee the frequency stability of the power system.

But with the ongoing energy transition and the ambition of multiple countries to reach high level of RES, there are more and more frequent situations where the minimum critical threshold of inertia is reached (due to low amount classic generators online in high RES production scenario) and the stability of the power system is threaten.

There are multiple ways to guarantee a stable operation of the grid. Some region, like in South Australia have a minimum amount of classic generators that must remain all the time online to guarantee a minimum level of inertia to face any unplanned generator disconnection. South Australia is also more and more relying on Battery Energy Storage System (BESS) to provide inertia to the grid [https://www.pv-magazine-australia.com/2022/07/27/hornsdale-big-battery-begins-providing-inertia-grid-services-at-scale-in-world-first/]. Note that the provision of inertia with BESS is only possible using converter in Grid-Forming Control mode. The difference and advantages to rely on Grid-Forming control with respect to the classic grid-following control will be the topic of one of the next article.

Another way to bring additional inertia to the system is to invest in Synchronous condensers. A Synchronous condenser is basically a generator that is not coupled to any turbine and hence cannot provide any active power (i.e., power that can be converted into useful work). Yet, as a rotating mass, it can provide inertia to the system. In addition, modern Synchronous condensers can be coupled to a flywheel [https://www.engineeringtoolbox.com/flywheel-energy-d_945.html] to further increase the inertia contribution.