Integrating Renewables: Why grid stability will keep your music playing

By Adam Middleton & Vijay Shinde

With this evolution, comes a growing need to make the impact of these changes accessible for the wider population. In short, how do you explain the implications of a move to renewable energy upon the grid? Think of how you explain this to your great uncle and aunt: make the basics accessible to them and you will make it accessible to most of the population (who, let’s be clear, are not engineers). Also, to be honest, not everyone in our industry understands this either.  

OK: so here goes …

The electricity system has four main components: generation plants, which create electricity; storage, which allows the electricity to be used when needed; networks (power grids), which transport it; and consumers, who use the electricity.

From the perspective of electricity grid management, generators have historically been split into two categories:

Large coal, gas and nuclear powered (synchronous) generators: These plants produce power synchronised with the frequency of the electricity network. They generate power through rotating alternators in an electromagnetic field, which is connected to turbines linked to spin at the same speed. Synchronous generators include energy coming from coal, gas, nuclear, hydro, and biomass.

Planet friendly Variable (Asynchronous) renewables: These technologies depend on weather patterns to generate and are connected to the electricity networks via power converters. This means that they are not currently naturally linked to the frequency of the grid. This means these generation sources need to be managed in new ways. Variable renewables include offshore wind, onshore wind, and solar generators. Though they are variable, they are predictable, and system can be managed with new ways of operating grids.

Emerging challenges

Among many other technical challenges, four key system needs must be met to operate a safe and secure system today: inertia, short circuit level, voltage control and system restoration. These are not the only challenges in operating the electricity system, but they are the most important ones and are likely to remain important in a future system.

When trying to explain this to non-technical colleagues, we often revert to analogies. In this specific case, how does our energy system now relate to … a dance party. Keep with us …

Inertia: (what keeps the music and dance smooth even when the music skips)

System inertia is a measure of the AC system's inherent resistance to changes in frequency. It refers to the kinetic energy stored in the rotating masses of turbines in generators connected to the network. These rotating masses are linked to the frequency of the network and respond automatically if the frequency changes by instantaneously injecting or absorbing some power.

The level of inertia in the system is calculated in MVAs. Inertia is critical for a stable network as it provides the fastest possible injection of active power when there are disturbances on the system. The more inertia there is on the system, the slower the rate of change of frequency during a system disturbance, allowing more time for additional measures, such as frequency response and reserve, to be deployed before safety limits are breached. Systems with more inertia are, therefore, considered more inherently stable.

Most inertia is currently provided to the transmission network by synchronous generators of large   power plants and by synchronous condensers.

Short Circuit Level: (The bouncer)

Short Circuit Level (SCL) is the amount of current that flows in the electricity system during a fault, like one caused by a lightning strike or power system equipment failure. This value is crucial because a low SCL makes the system weaker, meaning any disturbance can cause bigger voltage changes and greater instability, potentially leading to generation trips or equipment damage. During a fault, the system experiences a direct connection to the earth or another part of the network.

A strong (high) SCL, provided for example by traditional (synchronous) generators like coal and gas, helps the system dampen disturbances quickly, ensuring protection equipment operates correctly and the network remains stable and reliable, whereas a system with insufficient SCL, for example due to higher proportions of renewable generation, can lead to undamped oscillations, voltage dips, and eventually to protection failure. To put this into context, coal and gas plants may provide around five to seven times as much fault current as wind farms.

Synchronous generators can rapidly increase their output in the event of a fault which can help maintain the network stability. Variable renewables, due to the way they are currently connected to the power network, are unable to increase their output rapidly to compensate for the fault.

Without the appropriate levels of short circuit capability in the system, generators will likely trip out in the event of a fault or system disturbance.

Voltage Control (The DJ)

A wide range of different technologies can provide or absorb reactive power. This includes some types of generation capacity alongside demand from electronic equipment such as computers and TVs. Network assets, such as capacitors or reactors, also contribute to reactive power management. Network assets and connected generators have provided much of the reactive power absorption and generation requirements for the network to date.

Voltage control involves actively managing reactive power across the transmission network to maintain stable voltage levels, using equipment and generators to inject or absorb reactive power as needed. Generators are required by the Grid Code to provide continuous voltage control and reactive power capability within defined limits to ensure the network remains safe and efficient, especially as renewable energy sources like wind and solar displace conventional generators. Grid operators forecast future requirements and use market-based tenders to procure these services, ensuring the system can handle changes in the generation mix and demand profiles. Hence this power management will come at a cost.

The need for reactive power depends on local conditions as well as what is happening on the rest of the network. When demand is low and so the flow of active power through the network is also low, network assets - such as overhead cables - will generate reactive power. This is when greater amounts of reactive power absorption are needed. Equally, when demand is high, and there is a lot of active power flowing through the network, then network assets will absorb reactive power. This is when the system needs additional provision of reactive power.

System Restoration (Party starts again after power outage)

System restoration is the procedure which the System Operator (ESO) would use to restore power if there is a total or partial shutdown of the system. This would be done by re-energising certain parts of the transmission network incrementally before bringing the whole system back online.

System restoration requires generators on the system that can turn on quickly and turn on without an external supply of electricity. Not all generators can do this as many require some electrical input from the network in order to turn on. But if the whole network is down, this source of power will not be available. Therefore, not all types of generators can provide system restoration services.

System restoration requirements are locational so it is important to have system restoration capabilities at the right locations on the power network. The network will need to be re-energised in a way that maintains stability as it is powered back up, which is a complex challenge. If assets are not in the right location this could lead to sections of the network being re-energised and then failing again.

Solutions and implications

In looking to make the issues of power system stability and restoration accessible to all through our analogy of a disco, there are some key takeaways and learning points which remain important and which we hope you will capture as part of this narrative.

There is a range of technologies that can be deployed alongside variable renewables to meet the system needs required to operate a safe and reliable electricity system. Evidence provides confidence that the cost of deploying them will be low.

Some technologies, such as synchronous condensers, have been deployed on electricity networks for decades. Others, such as virtual synchronous machines, are at the early stage of deployment.

Key Learning from various markets:

• Market design is a must to procure these services. Without the right market signals, these solutions won't be deployed.
• Repurpose existing plants – authorities can make use of assets which will be phased out
• The market design must not miss the opportunity to secure Inertia from Flywheels that can be mounted on Synchronous Condensers and many storage technologies.
• Know your market and investor preferences for longer contract lengths, as some assets could have a 30-year project life.
• Liaise with OEMs, as only a limited number of suppliers in the market can offer these technologies, and they have plenty of options to work with.

Summary

Managing the electricity system is becoming more complex. The increasing diversity of sources and the potential for the growing demand for electricity creates new challenges to maintaining a stable system, but these challenges can be addressed.

Renewable generation operate differently from traditional forms of generation, which introduces new operability challenges that need to be addressed. Often, these challenges are not accounted for in electricity system modelling, which has raised concerns that deploying lots of renewables will make a highly renewable system either unviable or that such a system would be prohibitively expensive to manage.

Maintaining stability in the system will require services to be purchased that were previously provided by traditional forms of generation.

Existing technologies can provide the system with what it needs to maintain a secure and reliable supply. Some technologies, such as synchronous condensers, have been deployed on electricity networks for decades. Others, such as virtual synchronous machines at the early stages of deployment, offer scope for a lower-cost solution depending on the market design in a particular country. So, while it is not clear at present what mix of technologies will best deliver the critical operability needs for the system, the evidence is clear that they can be met.

So, next time you are listening to music at a party, don’t forget about how grid stability is keeping the music pumping … 😊

About the Authors

References

• LinkedIn article by Vijay Shinde on Grid Stability
• What is Short Circuit Level? Article on neso.energy
• What is inertia? Article on neso.energy