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Grid Planning, Integration, & Operations

The unique nature of distributed, grid-connected PV (DPV) systems challenges the way we typically plan and operate the distribution grid. When properly planned and integrated, DPV systems can be “good grid citizens,” contributing to grid reliability, line loss reduction, avoided fuel and infrastructure costs and more.

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Introduction

Distributed, grid-connected solar photovoltaic (PV) power poses a unique set of benefits and challenges. In distributed PV applications, systems generate electricity for on-site consumption and interconnect with low-voltage transformers on the electric utility system. Deploying DPV can reduce transmission line losses, increase grid resilience, avoid generation costs, and reduce requirements to invest in new utility generation capacity. With proper equipment and calibration, DPV systems can also mitigate reliability issues experienced by providing standby capacity during electric utility disturbances or outages.

Operational Considerations

In most electric utility systems, power flows in one direction: from centralized generators to substations, to end-use consumers. With DPV, power can flow in both directions. Most electric distribution systems are not designed to accommodate widespread DPV and a two-way flow of power. Common challenges include maintaining required voltage levels within regulated limits, coordinating protection system devices, and managing additional cycling—and associated wear and tear—of the voltage control equipment, especially critical for longer distribution feeder circuits in rural areas.

Planning Considerations

Traditional distribution and transmission planning does not adequately address the benefits and challenges of DPV systems. Traditional distribution planning procedures use load growth to inform investments in new distribution infrastructure, with little regard for distributed generation systems and for PV deployment. 

 

Example Interventions

Power systems can address the challenges associated with integrating DPV into the grid through a variety of actions. The following suggested actions may help improve system and distribution planning.

  • Incorporate DPV into integrated resource planning and modeling of system capacity expansion to optimize the amount of distributed PV on the system in the future.
  • Consider planning for higher PV penetration in designated areas—defined by regulatory criteria or created through targeted grid reinforcements and upgrades. These areas can be identified by utilities through a process administered by a regulatory body that considers stakeholder needs and is based on sound engineering principles.
  • Evaluate emerging approaches to distribution system planning (e.g., Integrated Distribution Planning), which proactively assesses the hosting capacity of distribution circuits, the ability of these distribution circuits to accommodate growth in distributed generation, and any necessary infrastructure upgrades in advance of receiving interconnection requests from generators.
  • Evaluate pricing mechanisms and rate designs and update them as needed to reflect the system-wide benefits and costs of distributed PV integration (e.g., by implementing Value of Solar tariffs).
  • Collect data on the load profile of distribution circuits (feeders) in order to assess hosting capacity and infrastructure needs.
  • At the transmission level, encourage system flexibility (e.g., through the use of generators that can change output quickly) to accommodate high levels of variable generation from DPV.

 

Reading List and Case Studies

Analysis of Indian Electricity Distribution Systems for the Integration of High Shares of Rooftop PV

Deutsche Gesellschaft für Internationale Zusammenarbeit GmbH (GIZ), and Energynautics, 2017

India has a target of 40 GW of rooftop PV by 2022. The study intends to provide an overview of the characteristics of the Indian power system and its challenges with regard to a significant increase in rooftop PV, as well as a template for distribution companies on how to manage rising PV shares, which studies to conduct and which technology options to select.

 

Grid-Integrated Distributed Solar: Addressing Challenges for Operations and Planning

National Renewable Energy Laboratory, 2016

Integrating DPV on a distribution system poses both unique challenges and opportunities. This factsheet reviews the barriers and provides best practices when operating and planning for distributed solar.


High-Penetration PV Integration Handbook for Distribution Engineers

National Renewable Energy Laboratory, 2016

When not properly addressed, high penetrations of DPV on a distribution system present issues like voltage overload, reverse power flow, and protection failure, which threaten system reliability. Drawing upon results from a five-year study of the Southern California Edison (SCE) distribution system, this handbook presents a detailed analysis of the potential impacts and mitigation techniques when integrating DPV. Written for distribution engineers, the handbook also provides a study guide for modeling and assessing PV impacts, covering topics such as model development, data validation and measurement, study criteria, and the steps involved in power flow and fault analysis. While the impacts and mitigation techniques described in this handbook are written for utility-scale PV systems (1-5 MW), much of the information is also relevant for the proliferation of smaller, DPV systems.


Emerging Issues and Challenges in Integrating Solar with the Distribution System

National Renewable Energy Laboratory, Sandia National Laboratories, and the Massachusetts Institute of Technology, 2016

As part of the U.S. SunShot Initiative—aiming to make PV electricity cost-competitive with conventional generation by 2020—this report analyzes the impact of high-penetration variable generation on the distribution grid, it demonstrates that in most cases DG can be safely integrated at much higher levels than interconnection standards allow. By streamlining interconnection processes, deploying advanced inverter functionalities, and coordinating DGPV, upwards of 350 GW can be hosted on the U.S. grid with little additional hardware. The report also outlines challenges to interconnection such as voltage regulation, power flow, and protection issues. It then studies the role of storage and complementary technologies to overcome reliability constraints. This research is applicable outside of the U.S. in demonstrating how to maximize an existing grid for DG.


Grid Integration of Distributed Solar Photovoltaics in India: A Review of Technical Aspects, Best Practices and the Way Forward

Prayas Energy Group, 2014

In anticipation of significant growth in DPV in India, this report reviews policies and regulations for DG; identifies technical challenges to significantly increasing grid-connected DPV; and makes recommendations for addressing power quality, safety, grid stability, and distribution system operation issues. The report provides an example of a country-specific review and synthesis of best practices to inform national and state-level technical and grid code specifications, advanced inverter functionalities, meter technology deployment, certification, and testing processes.


Distributed Solar PV for Electricity System Resiliency: Policy and Regulatory Considerations

National Renewable Energy Laboratory, 2014

DPV can be designed to supply electricity during grid outages. This paper presents approaches that specifically support resiliency through design of PV systems utilizing community energy storage, solar-diesel hybrid systems, and micro-grids. The paper also considers policies and regulations to support resiliency.


Advanced Inverter Functions to Support High Levels of Distributed Solar

National Renewable Energy Laboratory, 2014

Technological innovations are supporting increased DPV penetration levels. One important innovation involves the use of advanced inverter functionality to address PV grid integration challenges. In many cases these functionalities only require software and protocol updates to inverters currently in use. The report describes the use of advanced inverters to support voltage and frequency level control as DG comes on and off-line. Policy and regulatory considerations to support advanced inverter deployment are also presented in the paper.


Time in the Sun: The Challenge of High PV Penetration in the German Electric Grid

IEEE Power and Energy Magazine, 2013

Germany leads the world in deployment of DPV, with PV generation contributing approximately 40% of peak power demand during some hours of the year. This article outlines the impacts of high PV deployment in Germany on grid stability and power flows in the transmission and distribution system. It also highlights practical solutions that Germany has implemented to support frequency and voltage control and reduce congestion, and suggests new voltage-control strategies.


Integrated Distribution Planning Concept Paper: A Proactive Approach for Accommodating High Penetrations of Distributed Generation Resources

Interstate Renewable Energy Council, 2013

Traditional interconnection processes evaluate the impacts a distributed generator has on system safety, reliability, and power quality. This concept paper introduces “Integrated Distribution Planning,” an emerging alternative methodology designed to enable DPV. In the report, utilities and distribution system operators proactively study the hosting capacity of distribution circuits, the ability of these circuits to accommodate growth in DG, and any necessary infrastructure upgrades; all in advance of receiving interconnection requests from generators.


IEEE Draft Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces

Institute of Electrical and Electronics Engineers, 2003

IEEE 1547 was introduced in 2003 when DER penetration was low in most places. As some regions like Germany, California, and Hawaii reach higher levels of DG penetrations, alterations to IEEE 1547 are required. IEEE has fast-tracked the development of a full-revision of 1547 to maintain cross-system compatibility; drafts of the new version enhance the capabilities of DER to provide grid support, voltage ride-through, and reactive power modes for generators in high-penetration contexts. The updated version is currently an active draft, the full version will be released in 2018. 1547.1 provides engineers with interconnection system testing guidelines and will also be updated. Both 1547 and 1547.1 are recognized by UL 1741 and are to be used in conjunction.

 

Standard for Inverters, Converters, Controllers and Interconnection System Equipment for Use with Distributed Energy Resources

Underwriters Laboratories, 2001

UL 1741 provides certification requirements for DG equipment that operates according to the parameters established in IEEE 1547. The standard is used together with IEEE 1547 to provide comprehensive requirements for DG equipment and development.


Distributed Generation Interconnection Collaborative Website

National Renewable Energy Laboratory, Electric Power Research Institute, Solar Electric Power Association, Western Area Power Administration

To enable informed decision-making and planning related to increasing levels of DPV, the Distributed Generation Interconnection Collaborative (DGIC) facilitates knowledge sharing among its members about DPV interconnection practices, research, and replicable innovations. While the Collaborative primarily works with utilities and other stakeholders in the United States, its website hosts a collection of presentations, webinars, and reports that provide more broadly applicable information and case studies on interconnection practices.


Teaching the “Duck” to Fly, Second Edition

Regulatory Assistance Project, 2016

High penetration of DPV often leads to the infamous “duck curve,” the formation of two daily peaks in the morning and evening when PV is not available. This easy to read report assesses a variety of options to mitigate the “duck curve” from targeted efficiency, time of use rate design, storage, demand response, balancing, and complementing DPV with peak-oriented renewables. Implantation of these recommendations flattens the demand curve, allowing for additional DPV.

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