Renewable Energy Sources (RESs) particularly photovoltaic (PV) and wind are becoming important sources for power generation. Frequently varying output of PV and wind caused by clouds movement, weather condition and wind speed make them an intermittent and unreliable sources when connected to grid. Connecting intermittent sources to grid introduces challenges in various technical aspects such as power quality, protection, generation dispatch control and reliability. In this context, leveling intermittent source׳s output is necessary inorder to maintain grid׳s stability. This paper is aimed at bringing out the latest comprehensive literature review on problems associated when the intermittent PV is connected to grid and the methods of smoothing the output power fluctuation from PV. This paper also briefly discusses control strategy built for battery energy storage pertaining to this issue.

Mitigating methods of power fluctuation of photovoltaic (PV) sources – A review

http://dspace.uniten.edu.my/jspui/bitstream/123456789/13396/1/Gridconnected-renewable-energy-sources-Review-of-the-recent-integration-requirements-and-control-methodsJournal-of-Cleaner-Production.pdf

Grid-connected renewable energy sources: Review of the recent integration requirements and control methods

The high variability of solar irradiance, originated by moving clouds, causes fluctuations in Photovoltaic (PV) power generation, and can negatively impact the grid stability. For this reason, grid codes have incorporated ramp-rate limitations for the injected PV power. Energy Storage Systems (ESS) coordinated by ramp-rate (RR) control algorithms are often applied for mitigating these power fluctuations to the grid. These algorithms generate a power reference to the ESS that opposes the PV fluctuations, reducing them to an acceptable value. Despite their common use, few performance comparisons between the different methods have been presented, especially from a battery status perspective. This is highly important, as different smoothing methods may require the battery to operate at different regimes (i.e., number of cycles and cycles deepness), which directly relates to the battery lifetime performance. This paper intends to fill this gap by analyzing the different methods under the same irradiance profile, and evaluating their capability to limit the RR and maintain the battery State of Charge (SOC) at the end of the day. Moreover, an analysis into the ESS capacity requirements for each of the methods is quantified. Finally, an analysis of the battery cycles and its deepness is performed based on the well-established rainflow cycle counting method.

Comparative Study of Ramp-Rate Control Algorithms for PV with Energy Storage Systems

The microgrid vision contains several aspects, and a commonly admitted one is a portion of grid with its own means of production and energy flow controls. Photovoltaic (PV) generation is geographically the most distributed means of electricity production. In this sense, the integration of PVs in microgrids seems natural. The intermittency of PV generation can be compensated not only by using energy storage technologies but also by demand-side management and exchanges with other power networks: the main grid and surrounding microgrids. Many aspects still have to be investigated in the fields of power electronics, information communications technologies (ICTs), protections, and power quality (PQ) issues, to make this association a reality.

Photovoltaics in Microgrids: An Overview of Grid Integration and Energy Management Aspects

The installation of large scale photovoltaic power plants connected at transmission level has increased during the last years. There are some challenges that these power plants have to overcome regarding the operation and control while dealing with the solar energy variability and uncertainty. Today, few countries are aware of the importance of this source of energy as part of the utility system and how it can affect the operability. Thus, this paper discusses about the trend of large scale photovoltaic power plants around the world and the importance of the development of grid codes for its integration. Then, the paper addresses a comparison of the grid codes of Germany, US, Puerto Rico, Romania, China and South Africa considering: fault ride through capability, frequency and voltage regulation, as well as active and reactive power support. In addition, a broad discussion about the challenges that the large scale photovoltaic power plants have to overcome is presented together with the compliance technology and future trend.

Review of advanced grid requirements for the integration of large scale photovoltaic power plants in the transmission system

The U.S. Department of Energy (DOE) and the National Renewable Energy Laboratory (NREL) have partnered with the Government of Puerto Rico to assist in addressing barriers to the deployment of energy efficiency and renewable energy in Puerto Rico. This document provides an overview of the minimum technical requirements (MTR) for interconnection of wind power and photovoltaic generation developed by the Puerto Rico Electric Power Authority (PREPA). Integrating a large amount of variable renewable generation such as wind and solar into an electrical grid presents several potential challenges for operating a power system, particularly with small island grids like the Puerto Rico electrical system. Establishing valid technical requirements for interconnection of variable renewable generation to the electric grid is an important step in overcoming such challenges. The interconnection requirements need to address several important aspects of renewable integration such as safety, impacts on power system reliability and performance, and costs. Being an isolated island system, PREPA does not fall under Federal Energy Regulatory Commission (FERC) jurisdiction, and therefore FERC interconnection standards are not required to be applied to generators in Puerto Rico. In this regard, PREPA has developed its own set of interconnection requirements with which all transmission-level wind and solar PV generators shall comply. In this report, NREL provides an impartial third-party review of PREPA’s MTRs, drawing comparisons with other similar requirements and best engineering practices. Over the course of the collaboration on the MTR review (2012-2013), PREPA revised the MTRs based on the draft review. As such, PREPA’s responses to the recommendations are also included in this report. The PREPA MTRs are anticipated to constantly evolve as the level of renewable penetration increases, modeling tools are improved, and experience with system performance increases. This review is intended to assist PREPA in this process. Acknowledgements The authors would like to acknowledge the substantial contributions to this report provided by staff at PREPA, the Governor’s Office, and the Puerto Rico Energy Affairs Administration (PREAA). NREL worked closely with staff from PREPA, PREAA, and the Governor’s Office throughout the technical review of PREPA’s MTRs to ensure accurate understanding of Puerto Rico’s electricity grid and the status of renewable energy integration. The authors would like to particularly thank Martin Perez, Luis Francis, Alvin Roman (PREPA) and Efrain O’Neill (Governor’s Energy Affairs Advisor) for their significant contributions and support of this effort, especially for reviewing the recommendations and providing a summary of PREPA’s responses. The authors would also like to thank Jennifer DeCesaro and Steve Lindenberg from DOE who sponsored this work, this effort could not have been completed without their support and involvement. This report is available at no cost from the National Renewable Energy Laboratory (NREL) iv at www.nrel.gov/publications. List of Abbreviations and Acronyms AGC automatic generation control DER distributed energy resources DFIG double-fed induction generator ERCOT Electric Reliability Council of Texas FESTIV Flexible Energy Scheduling Tool for Integration of Variable Generation FRT fault ride-through HECO Hawaiian Electric Company HVRT high-voltage ride-through Hz hertz IEC International Electrotechnical Commission IEEE Institute of Electrical Engineers & Electronics Engineers kV kilovolt LVRT low-voltage ride-through ms millisecond MTR minimum technical requirements MVA mega volt ampere MVAR mega volt ampere reactive MW megawatt NERC North American Electric Reliability Corporation PFR primary frequency response PSLF Positive Sequence Load Flow POI point of interconnection PQ power quality PPA power purchase agreement PRC protection and control PREPA Puerto Rico Electric Power Authority PSCAD power systems computer aided design PSS/E power system simulation for engineering PV photovoltaic RR ramp rate SCADA supervisory control and data acquisition SCR short circuit ratio STATCOM static synchronous compensator SVC static VAR compensator VAR volt ampere reactive VRS voltage regulation system WPP wind power plant WTG wind turbine generator ZVRT zero-voltage ride-through This report is available at no cost from the National Renewable Energy Laboratory (NREL) v at www.nrel.gov/publications. Table of

/pdf/review-of-prepa-technical-requirements-for-interconnecting-131a1nmdr5.pdf

Review of PREPA Technical Requirements for Interconnecting Wind and Solar Generation

University fleets represent an enticing opportunity to explore the near-term feasibility of achieving net-zero-carbon emissions in transportation. In many instances, universities operate much like a small, self-contained ecosystem with all the same transportation needs as a larger municipality, but with a smaller geographic footprint. Their fleets often include a wide variety of vehicle types serving the campus, including low-speed vehicles (e.g., golf carts), light-duty sedans, SUVs, and pickups, as well as medium-duty trucks and delivery vehicles. The mix of vehicle and operational needs combined with broader activities related to net-zero campuses makes universities and colleges unique microcosms to determine the feasibility of and path to achieving net-zero fleets. As the availability of electric drivetrains expands beyond light-duty sedans, fleets need to understand when it will be operationally and financially appropriate to start adding electric drivetrains to their fleets. To better understand these opportunities, NREL contracted Sawatch Labs to analyze the role electric vehicles (EVs) can have in helping universities meet net-zero emissions and fleet sustainability goals they have instituted. 

/pdf/identifying-electric-vehicles-to-best-serve-university-fleet-2jos6ys5.pdf

Identifying Electric Vehicles to Best Serve University Fleet Needs and Support Sustainability Goals

State fleets represent an enticing opportunity to explore the near-term feasibility of fleet electrification. In many instances, state fleet operations encompass a wide geographic area with fleet locations for many vehicles. Serving these wide areas will require a significant amount of energy and, in the case of electric vehicles (EVs), a significant level of charging power. The peak demand as a result of this charging demand is of interest for fleets, with impacts on both utility bills and installation costs ranking among some of the greatest concerns. The combination of a wide operational area and multiple fleet locations positions state fleets as ideal candidates to understand the impacts of vehicle charging on fleet operations. As the availability of electric drivetrains expands beyond light-duty sedans, fleets need to understand when it will be appropriate operationally and financially to start adding electric drivetrains to their fleets. Throughout this process, it will also be important to understand the charging implications of fleet electrification and the resulting impacts to facility electrical systems. To better understand these considerations, NREL contracted Sawatch Labs to analyze the role that increasing state fleet electrification may have on the charging demand at fleet parking facilities. 

/pdf/impacts-of-increasing-electrification-on-state-fleet-8vjscmqs.pdf

Sarah Booth

Papers

Review of PREPA Technical Requirements for Interconnecting Wind and Solar Generation

Identifying Electric Vehicles to Best Serve University Fleet Needs and Support Sustainability Goals

Impacts of Increasing Electrification on State Fleet Operations and Charging Demand