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Today many of the worlds largest electric grids are facing new challenges in sustaining the levels of reliability that made electrification indispensable. In addition to those physical challenges of reliability have been challenges of imagination and policy. In the past, reliability often turned on the question of what happened if a key power plant or power line unexpectedly failed. The rapidly increasing share of power supply from sources such as wind and solar plants, and the build-out of interconnections between different grid regions, countries, or even continents using high voltage direct current (HVDC) cables introduce new reliability considerations related to weather conditions and faults in control software that need our careful attention.

For the last century nearly every modern grid has depended on large, centralized power plants with spinning turbinesfired with fossil fuels and, in some cases, large nuclear and hydro plants. Those turbines generate prodigious quantities of electricity along with huge amounts of inertia, helping to stabilize the grid. The bigger the volume of electricity supplied from such sources, the larger the inertia. Because grids with a lot of inertia can ride through shocks and disruptions, they are a lot more reliable than grids that depend on fewer spinning turbines.

In many countries, there are policy and technological pressures to reconfigure electric grids in ways that will lessen the role of large spinning turbines. Those changes include more decentralization of electric supplysuch as through a shift to microgrids and rooftop photovoltaics that operate locally. In tandem, many grids are shifting to wind and solar supplies that typically dont provide inertia. Wind turbines, while spinning, are rarely synchronized with the grid and thus dont offer inertia that stabilizes grids. Solar photovoltaic systems provide electricity via electronic processes that involve no turbines and no inertia. These two trendsdecentralization and much bigger roles for renewableshave also led many grid operators to install growing numbers of battery storage systems, which are electronic devices that also dont intrinsically provide inertia.

New technologies and procedures are emerging to replace some of services that turbine inertia used to provide. For example, electronic devices that can help stabilize grid voltage and frequency. But reliability remains the watchword for modern grids. And how these new electronic systems will perform at scale is still hard to fathom. Inertia remains essential.

Around the world some grid operators are now beginning to grapple with the consequences of declining inertia. This Energy Insight looks at this issue with a focus on the experiences of grid operators in Britain as well as in the Nordic regional group. The British grid is of special note because it has seen the most rapid shift to a more decentralized grid and toward much greater roles for intermittent renewable power (mainly wind, but solar as well). In the case of the United Kingdom, policies that decreased the use of generators and favored intermittent renewables pushed the grid in the direction of declining inertia. The loss of inertia was a somewhat unexpected and completely unintended byproduct of those market designs and policies.

The British experience is an important case study for other grid authorities and a reminder that policymakers can pursue new technologies for important reasons: the British shift to renewables has lowered pollution from coal and other fossil fuels. But in the case of the UK, reconfiguration with abundant intermittent power and other actions, including international interconnections and not adding synchronous-turbine-driven new generation, impact grid inertia negatively. Many other grid operatorsin the Nordic nations, parts of the United States such as California, and elsewhereare facing similar challenges. In the Nordic grid (which comprises Sweden, Norway, Finland, and half of Denmark), premature retirement of nuclear units alongside the expansion of wind power have lowered system inertia and, as a result, forced grid operators to develop and fund an entirely new type of supporting market, offering at the very least an interim mitigating action. The Nordic experience also suggests the need for much clearer system-wide awareness of how digitalized parts of the grid system can fail or affect reliability in ways that were previously unexpected.

These experiences suggest an agenda for many other countries that may be on the cusp of similar ones. Grid systems that move away from power plants with synchronous spinning turbines need a strategy for addressing the loss of inertia. Better situational awareness can help, as can incentives to encourage the retention and production of inertia. This paper looks at those experiences and responses, and outlines what to watch forso that the coming century, like the last one, is marked by a central role for reliable electric supply.

By: Staffan Qvist,Mohammad Al Hammadi,David G. Victor

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Meeting the Challenge of Reliability on Today's Electric Grids: The ... - Oxford Institute for Energy Studies