Phasing out synchronous generators puts pressure on power system operators Synchronous generators, such as coal, nuclear and gas power plants, have been the driving force to keep the power system's frequency stable for decades. Frequency is the power system's heartbeat and needs to be constant around 50 or 60 Hz, depending on the geographical region, to keep the system up and running. The power system's frequency is determined by the rotational speed of large power plants of which the turbines are synchronously connected to the electrical energy system, and remains constant if demand and supply of electrical power are in balance. In case of an imbalance between demand and supply, the shortage or excess of power supply will be compensated for by accelerating or decelerating the turbines of the power plants, and consequently decreasing or increasing the frequency. The acceleration or deceleration is caused by a change in the kinetic energy of the turbine to provide the lack of energy to the load or to absorb the abundant energy generated. The effect is similar to the deceleration that you experience when releasing the pedal when driving a car. This resistance against frequency deviations due to imbalances between demand and supply for electrical power, reflected in the rate of change of frequency (ROCOF), is also denoted as the inertial response of the system. The more synchronous generators in the system, the higher the resistance against acceleration or deceleration of the turbines due to these imbalances, and thus the smaller the frequency deviations.
Figure 1. P. Tielens, “Operation and control of power systems with low synchronous inertia,” PhD Thesis
To reduce the emission of CO2 and counteract climate change, more and more renewable energy sources are replacing synchronous generators. However, this evolution causes a decrease in the inertial response in power systems. Renewable energy sources are connected to the system via power-electronic convertors, which prevent the natural provision of inertial response. Several system operators worldwide have reported on the challenges they are facing to ensure the frequency stability due to the decreasing levels of synchronous inertial response. Reduced inertial response levels cause the frequency to change faster, which gives the system operator less time to react before critical frequency levels are reached. Beyond these critical frequency levels, customers need to be disconnected from the system to avoid a system breakdown.
To prevent disconnection of customers during low-inertia conditions, system operators want to reduce the imbalance between demand and supply sufficiently fast through injection or consumption of additional power. Historically, high synchronous inertial response complemented with an increase or decrease in the power output of large generators upon a frequency deviation from 50Hz or 60Hz within around 10 seconds was sufficient to keep the frequency within limits. Today, events, such as the event on August 9, 2019 in the UK , and grid analyses performed by system operators worldwide have indicated the need for innovative solutions to deal with falling inertia conditions.
Different solutions to tackle the falling inertia have been put in place:
An open question remains, how to operate a system with almost no synchronous inertial response. Although fast frequency response products have already shown their effectiveness to maintain the stability in power systems with moderate inertia levels, current research is investigating if alternative flexibility products are needed to maintain the frequency stability in systems with almost no synchronous inertial response.
Ongoing pilot projects in Australia  and the UK  are investigating the usefulness and stability implications of virtual inertia provided by batteries, generation or demand to ensure the grid stability, and how this flexibility can be remunerated. Advanced invertor control is needed to emulate the inertial response of synchronous generators with a battery, PV installation or wind turbine.
In the EU Horizon 2020 project MAESHA, with the French island Mayotte as a pilot case, our R&D optimization team contributes by:
As geographical islands with a sustainable energy provision are facing low-inertia conditions already today, the outcomes of this project will help in steering the future energy landscape of continental Europe.
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