De-risking RENEWABLE ENERGY: Frequency and Inertia

Essentially, just prior to the event, on a very sunny day, solar PV was supplying almost 60% of the electricity demand, and when combined with wind, made up almost 80% of the electricity generation capacity on the grid. At this point conventional generation, (fossil fuel-based electricity, as opposed to renewables) was providing just over 20% of capacity, and this fact (as we shall see later) is seen as one of the primary causes of the event.

According to the chronology of events, a ‘still under investigation’ interruption of electricity from two large arrays in southwestern Spain caused an instability which triggered a disconnection from the French grid (likely averting a much more widespread event) and precipitated a massive power outage. Within five seconds, Spain lost the equivalent of 60% of its electricity demand (about 15 GW of capacity). As the remaining capacity was insufficient to meet demand, the failure cascaded across the grid as equipment disconnected and power plants, including nuclear plants, shut down automatically to protect infrastructure.

The current consensus on the cause of the instability points to the fact that the grid did not have sufficient inertia to maintain a stable frequency of the electricity supply. In this context, we can understand inertia as a property of large and heavy equipment such as turbines and generators that are spinning at high RPM, which under normal circumstances, would continue spinning through any instantaneous event, and thereafter, only slow gradually over time. These equipment and the momentum that they possess when running would serve to stabilize the frequency of the electrical supply and resist and forestall any instability.

Conventional, fossil-based power generation and its large (exceeding 200 tons) turbines spinning at high speed (typically in excess of 3000 rpm) have long been acknowledged to have high inertia. Similarly, hydroelectric power, although regarded as a renewable energy source, is also a high-inertia energy source due to the large mass of flowing water, and the heavy spinning turbines and generators. Because they are resistant to change, high inertia sources that are not intermittent in nature are used to set the frequency of electricity in the grid using synchronous generators. This can be contrasted against wind energy, which although having high inertia due to the mass and momentum of the blades and generators, is nevertheless intermittent as winds can be variable or even cease altogether.

At the opposite end of the inertia spectrum, solar PV has extremely low inertia, meaning that generation can oscillate wildly on sunny days with fast-moving clouds. For this reason, solar PV arrays are connected to grid-following inverters in order to match the supply to the grid frequency. The same is true for wind turbines, not because they lack inertia, but because of their intermittent nature.

Some have postulated that the blackout could have been less severe had grid-forming (as opposed to grid-following) inverters been used for the solar PV arrays, as these could lend ‘synthetic inertia’ to the system to absorb instabilities, but the technological challenges of coordinating large numbers of grid-forming inverters, each trying, as it were, to set the frequency of the grid, have not been solved. With barely 20% Conventional Power generation on the day in question, the Spanish grid did not have sufficient inertia to stop the instability and the resulting cascade.

This episode has as number of stark lessons, in particular for developing countries such as China and Pakistan that are rapidly planting up solar PV in anticipation of quickly achieving net net-zero. As long as the technological barriers associated with grid-forming inverters remain, disregarding the need to maintain Frequency through Inertia can have dire consequences for the stability and resilience of the grid.

Fortunately, apart from solar PV and wind, there are other renewable, low-emissions energy sources such as biomass, biogas, and the potential for syn-gas from pyrolysis or gasification, that could constitute high inertia electricity generation as they too employ gas and steam turbines.

Malaysia, with its abundant biomass resources, should rapidly plan, develop and implement electricity generation from these high-inertia sources to keep pace with the rapidly-growing solar PV capacity. The foreseeable has occurred, and as the ongoing investigations in the coming weeks uncover additional facts, the Malaysian energy sector needs to note the key risk factors and areas of vulnerability in the Malaysian grid and plan accordingly.

Failing to learn from the mistakes of others would be too expensive a lesson to learn, and we would be duly held accountable.