An Overview of Distributed Energy Resource Interconnection: Current Practices and Emerging Solutions
The NREL technical report, An Overview of Distributed Energy Resource Interconnection: Current Practices and Emerging Solutions, serves as a central document summarizing considerations, practices, and emerging solutions across a broad set of topics related to distributed energy resource (DER) interconnection.
An Overview of Distributed Energy Resource Interconnection: Current Practices and
Emerging Solutions (Horowtiz et al. 2019)
With DER penetration growing increasingly in certain regions of the United States, utilities and regulators need to incorporate special considerations and solutions that encompass all aspects of the interconnection procedure. This report discusses current practices and standards while acknowledging the need to develop where practices are not yet in place.
In addition to a brief summary of the IEEE-1547.2018 standard itself, the report covers topics such as:
- Application management
- Technical screening
- Advanced inverters
- Cost allocation
- Forecasting DER deployment
- Storage interconnection
- Models for different levels of DER penetration.
Best Practices for Interconnection Processes
Start with a simplified application process that is transparent and easy to understand and manage for the utilities and customers alike.
Process improvements (may require regulator's approval) can enhance transparency, accountability, and speed of processing. The technical screening is used to determine the need for a supplementary review or a detailed impact study. The report explains the purpose of each of the ten screens as per Federal Energy Regulatory Commission Order No. 2006 which is followed by most of the states in the U.S. Additionally, depending on the DER penetration level, power flow modeling and hosting capacity analyses can be used to study the impact of DER on the network more accurately. A workflow of such an advanced model is illustrated by Pepco Holdings Inc.
In order to comply with the current IEEE Standard for DER interconnection (1547-2018), advanced inverter capabilities are necessary to ride through minor grid disturbances ("normal conditions"). The report describes different modes of voltage regulation and the curtailment caused by these settings. Regulators need to identify the levels of curtailment and the ways to compensate customers for these losses. It also provides the important interconnection standards and codes along with state rules that have formally adopted them in their interconnection procedure. The changes in the IEEE 1547-2018 revision are highlighted. State PUCs and non-regulated utilities need to review the existing states rules for interconnection and consider the impact of the revision on the existing rules and the operations parameters of each utility.
Working with Utilities on Interconnection Policies
Different strategies and upgrades are used by utilities depending on the local characteristics of the distribution system to address system violations that may occur due to DER deployment. The report delineates the traditional approaches typically used today and emerging solutions such as battery storage, reactive power support on distribution system (using D-STATCOM & D-SVC), preemptive upgrades, flexible interconnect capacity systems (to avoid upgrades) and advanced communication and control schemes. Understanding the current DER penetration levels and the ability to estimate the future deployment levels (forecast) is imperative to devise an effective strategy.
An important and longstanding issue with utilities and regulators has been about fair and equitable cost allocation for grid upgrades. Several alternatives with varying risk factors are being explored since the conventional cost-causer approach may not be equitable but clear best practices have not yet emerged. Hosting capacity maps and predicting the future DER growth are some tools that can add a level of certainty to the cost and anticipate the limitations of the distribution network to make decisions on potential grid upgrades. The report discusses two main approaches—top-down and bottom-up. It provides a reasonable comparison and data requirements for the two methods. However, some of these models are yet to be tested for accuracy.
The deployment of DER increases the number of devices on the distribution network exposing it to cybersecurity risks. The report provides some information about how these risks can manifest in different forms and in general, various protocols relevant to the cybersecurity concerns. However, standards and guidelines specific to DER are under development. The report provides the details of the cybersecurity working group convened by SunSpec Alliance. Additionally, Lawrence Livermore National Laboratory (LLNL), HECO and others are currently developing tools to assess the cybersecurity risks of various DER interconnection architectures with remediation strategies that would be presented with the best practice guidelines for DER cybersecurity in fall 2019.
The U.S. storage energy market is projected to grow to nearly 4GW (GTM Research 2018) as costs continue to decline. Storage is unique in that it can act as load and generation. Hence, states' interconnection procedures for storage needs to reflect both modes of operation. Some states are addressing this by special provisions that are detailed in the report. In some cases, storage interconnection applications have the possibility of qualifying for the expedited process.
Interconnection Maturity Model
The report concludes by presenting the interconnection maturity model that specifies the interconnection approach at low and moderate-to-high DER penetration levels. The key ongoing interconnection challenges are summarized.