inventory that identifies potential options – and associated implementation costs – for enhancing system flexibility. A system flexibility strategy should identify the steps necessary to create an environment that facilitates investment in flexibility; this may take the form of centralised procurement strategies for new infrastructure, increased co-ordination across balancing areas, creation of new market products, or a range of other nearor long-term policy options (see IEA [2018a]).

Planning for distribution grids

When considerable amounts of DER, such as VRE, are expected to be integrated into local grids within the planning horizon of a distribution utility, additional and potentially more complicated planning studies typically need to be completed. This is to ensure the continued safe, reliable and cost-effective operation of the interconnected distribution system (IEA PVPS, 2014). Depending on local circumstances, system operators are likely to pursue a combination of additional planning activities (Table 8).

Table 8. Additional planning activities to integrate DER

Source: Seguin et al. (2016), High-Penetration PV Integration Handbook for Distribution Engineers.

Improved screening/study techniques

Local utilities use “screens”43 to assess the impact that DER, such as behind-the-meter solar PV projects, will have on the local network. Screening forms part of distribution planning activities and guides the approval process for DER projects.

The precise distribution of DER on a distribution circuit (e.g. near the start of the circuit or near the end of the line) can strongly determine their impact on the circuit. More accurate grid planning can inform system operators as to which connection requests merit more in-depth analysis, which can be approved without further study, and which projects cannot be connected within the immediate planning horizon without significant circuit modification or upgrade (Box 22).

Many of the grid “screens” in place today were designed in a context of low DER penetration. To improve the accuracy of local grid screening, planning efforts must include future scenarios for DER penetration at a relatively high spatial resolution, such as neighbourhood or even street level. Planning tools could use socio-economic data to determine the likelihood of DER adoption in certain areas (Sigrin et al., 2016). An alternative approach to grid impact assessment assumes random placement of DER (Smith and Rylander, 2012). As such, improved screens require considerable development to account for the rising complexity of the required analysis at high penetration (Rylander et al., 2015).

Box 22. Beyond 15% penetration: New technical DER interconnection screens for California

In the past, DER projects in California required a full interconnection study if aggregate DER capacity amounted to 15% of the peak load of the circuit. With the growth of PV deployment, this limit was reached more often, leading to burdensome administrative processes, which slowed down further deployment. Today, a more refined analysis is performed to determine which DER projects are more likely to saturate local circuits and require in-depth analysis by the utility. This has made project assessment less burdensome and more effective, saving time and money for the

utility and project developers alike.

After screening, system operators can apply quasi-static time-series (QSTS) analysis, or alternative study methods, to assess PV interconnection requests (NREL, 2014). QSTS analysis incorporates more precise load and solar irradiance models, and allows for a more refined understanding of the expected impacts of DER on combined circuit operation and voltage regulation equipment. However, even with modern-day consumer-grade computing equipment, a comprehensive assessment can take several days to complete. Less timeintensive study methods and tools are needed to enable VRE interconnection. The German local system operators, EWE Netz, is leading efforts to reduce this delay, so that project developers or home owners can receive the outcome of the screening exercise in as little as a few minutes.

Including local flexibility requirements in planning techniques

In addition to basic screening practices, planning processes on the distribution grid increasingly factor in the evolution of local flexibility requirements (Box 23).

Box 23. Planning for local flexibility requirements

In order to accommodate increasing shares of distributed generation, UK Power Networks – the DNO for South East England – is in the process of transitioning to revised status as a DSO. As part of its long-term flexibility roadmap it has identified potential opportunities to defer infrastructure investment through use of DER, with a total potential volume of up to 206 MW by 2023. This potential refers to UK Power Network’s high-voltage and extra-high-voltage networks, up to 132 kilovolts, where larger DER such as industrial customers, commercial customers, EV fleets, larger generation and battery storage tend to connect.

Within this market, UK Power Networks has identified four use cases for DER that can be deployed to further support the grid. The table below presents two of these: deferral of grid reinforcement; and management of planned maintenance. For a more detailed discussion of these and the remaining two, refer to UKPN (2018).

UK Power Networks’ product development recognises the fact that local flexibility requirements are very location specific and that, while competitive procurement might be advantageous, DSOs should consider alternative procurement strategies in case of limited liquidity.

An additional important point in UK Power Networks’ strategy is to first engage with the highand very-high-voltage sections of its network, while excluding its lowand medium-voltage networks.