Grid Stability - Integrating Renewables

Background:

While the electricity system began over a century ago, it's far from settled. Instead, it has been constantly evolving, which will continue as new low-carbon infrastructure is deployed to meet the net zero target.

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

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

• 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 coal, gas, nuclear, hydro, and biomass.
• Variable 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 has consequences for the system services they can provide. Variable renewables include offshore wind, onshore wind, and solar generators.

Emerging challenges:

Please refer to image 1. 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.

Inertia: (In a new dance party - That keeps the dance smooth even when the music skips)

System inertia is a measure of the 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, most commonly large thermal plants.

Short Circuit Level: (The bouncer)

Short circuit level (SCL) is a measure of the stability of the system. When it is high, the system is considered strong; when it is low, it is considered weak. It is one of the key measures of system stability alongside inertia. SCL measures the amount of current that will continue to flow on the network when a fault occurs. It indicates the amount of generation that can provide fast reactive power within the timescales of a voltage dip.12 These faults can be caused by a number of factors, such as a lightning strike, a tree hitting an overhead cable or an equipment failure. During a fault, the system experiences a direct connection to the earth or another part of the network.13 When this happens large amounts of current flow through the network to the fault. This can lead to equipment damage and, importantly, to safety protection failure. Like many operability issues on the network SCL is an inherently spatial issue and depends on the regional topology of the system. Traditional synchronous generators provide a lot of short-circuit support, but variable renewables do not. For example, coal and gas plants may provide around five to seven times as much fault current as wind farms.14 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 network, are unable to increase their output to compensate for the fault rapidly. Without the appropriate levels of short circuit on the system, generators will likely trip off in the event of a fault or system disturbance.

Voltage Control (The DJ):

Like frequency, voltage levels of the electricity networks must be controlled and maintained for the safe and efficient transport of power. The ability to control voltage focuses on the generation or absorption of reactive power (measured in MVA). This reactive power, contrasted with active power, is used to push the active power that consumers need along the network to its destination. 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.

Where voltage is too low, reactive power is needed to increase it, and where it is too high reactive power absorption is needed to lower it. If reactive power levels are low and voltage levels fall significantly outside the allowed limits this could lead to disconnections from the network or damage to critical equipment on the network. If sufficient reactive power is not available, then the System Operator will need to re-dispatch generation to ensure safety, which 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 the 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. It is important to have system restoration capabilities at the right locations on the 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:

 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. Image 2 is inspired by the previous work by the National Infrastructure Commission - which summarises existing and new technologies that can meet the system's needs. 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 Learnings 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 - 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.

Renewables operate differently from these 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.