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What is Enhanced Frequency Response and Which Are Its Benefits?

Updated: Feb 27, 2019

In order for a power grid to be a reliable source of energy, it must provide a stable voltage and frequency output. Significant deviations in either variable can have negative effects on power grid operation, potentially damaging equipment used by customers.

Figure 1. Events that Can Destabilize the Power Supply

Frequency stability has traditionally been achieved with the inertia of rotating machinery, especially the turbine and generator sets used at power plants driven by fossil fuels, nuclear reactors or hydraulic power. When there are hundreds of tons of mass rotating at constant speed, grid frequency tends to remain stable!

However, with the rise of variable renewable energy sources like wind and solar power, and the gradual decommissioning of power plants driven by fossil fuels or nuclear energy, the total system inertia of power grids is declining. As a result of this, utility companies are becoming more environmentally friendly, but also more susceptible to sudden variations in power generation or consumption.

Types of Frequency Response

To keep the power supply stable, utility companies have traditional deployed primary frequency response, which responds in 10 seconds; and secondary frequency response, which has a timeframe of 30 seconds. However, these response times are becoming too slow for the needs of modern power grids, and Enhanced Frequency Response (EFR) has been proposed, which is capable of responding to grid fluctuations in less than one second.

Figure 02. Power Grid Response to Frequency Fluctuations

How Does EFR Stabilize Grid Frequency?

There are two main ways in which power grid frequency can deviate from its nominal value, which in the case of the United Kingdom is 50 Hertz.

  • A sudden surplus in generation tends to elevate frequency.

  • On the other hand, a sudden deficit tends to reduce frequency.

The way in which EFR systems respond to these scenarios is very simple: energy storage systems absorb power during generation peaks, and provide power during demand peaks. It is important to note that EFR achieves full output in less than a second, and only stays in operation until primary frequency response takes over. This is very different from energy arbitrage, where storage systems absorb energy during low-demand periods and supply it during high-demand periods – the basic operating principle is similar to that of EFR, but timeframes are much longer.

Enhanced frequency response is only possible if the system can reach full output or consumption within one second or less, making lithium-ion batteries a solid choice to provide the service. EFR also depends on a reliable control system that monitors power grid frequency in real-time and ensures the system responds in less than one second.

Large-Scale Enhanced Frequency Response in the UK

The UK National Grid recently carried out its first EFR auction, and the results were published in late August: a total of 201 MW will be commissioned by March 2018, distributed among eight projects. Competition was fierce, and the total capacity of all bids placed added up around 1400 MW. The proposed EFR systems were required to have a response time of between 0.4 and 1 seconds, and to be able to operate for at least 15 minutes.

The market for EFR will only continue to increase: the UK grid is expected to lose between 15 and 20% of its power grid inertia by 2020, and up to 40% by 2025. Other than battery arrays, flywheels and DSM systems show promise for the delivery of EFR services.

Commissioning large-scale projects is one viable approach for electric utility companies to deploy EFR, but they can also rely on distributed capacity at customer premises, using data aggregation and control to operate it as a single energy storage system. The Rocky Mountain Institute in Colorado, USA, has carried out research on battery storage and determined that all services available with bulk storage, including EFR, are also possible with distributed storage. In addition, distributed storage also allows demand-side management, backup power during blackouts, and enhanced integration between the power grid and distributed renewable generation.

The Economics of Battery Energy Storage, from Rocky Mountain Institue

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