Publications

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Inverters using phase-locked loops for control depend on voltages generated by synchronous machines to operate. This might be problematic if much of the conventional generation fleet is displaced by inverters. To solve this problem, grid-forming control for inverters has been proposed as being capable of autonomously regulating grid voltages and frequency. Presently, the performance of bulk power systems with massive penetration of grid-forming inverters has not been thoroughly studied as to elucidate benefits. Hence, this paper presents inverter models with two grid-forming strategies: virtual oscillator control and droop control. The two models are specifically developed to be used in positive-sequence simulation packages and have been implemented in PSLF. The implementations are used to study the performance of bulk power grids incorporating inverters with gridforming capability. Specifically, simulations are conducted on a modified IEEE 39-bus test system and the microWECC test system with varying levels of synchronous and inverter-based generation. The dynamic performance of the tested systems with gridforming inverters during contingency events is better than cases with only synchronous generation.

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Most inverters for use in distribution-connected distributed energy resource applications (distributed generation and energy storage) are tested and certified to detect and cease to energize unintentional islands on the electric grid. The requirements for the performance of islanding detection methods are specified in IEEE 1547-2018, and specified conditions for certification- type testing of islanding detection are defined in IEEE 1547.1. Such certification-type testing is designed to ensure a minimum level of confidence that these inverters will not island in field applications. However, individual inverter certification tests do not address interactions between dissimilar inverters or between inverter and synchronous machines that may occur in the field. This work investigates the performance of different inverter island detection methods for these two circumstances that are not addressed by the type testing: 1) combinations of different inverters using different types of islanding detection methods, and 2) combinations of inverters and synchronous generators. The analysis took into consideration voltage and frequency ride- through requirements as specified in IEEE 1547-2018, but did not consider grid support functionality such as voltage or frequency response. While the risk of islanding is low even in these cases, it is often difficult to deal with these scenarios in a simplified interconnection screening process. This type of analysis could provide a basis to establish a practical anti- islanding screening methodology for these complex scenarios, with the goal of reducing the number of required detailed studies. Eight generic Groups of islanding detection behavior are defined, and examples of each are used in the simulations. The results indicate that islanding detection methods lose effectiveness at significantly different rates as the composition of the distributed energy resources (DERs) varies, with some methods remaining highly effective over a wide range of conditions.

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Development of efficient non-overcurrent based protection schemes is a prerequisite for significantly increasing microgrids renewable energy penetration. In this paper a novel communication-assisted impedance-based protection scheme is proposed. For the sake of protection design, we partition microgrids into protection zones based on the availability of fault interruption devices. The proposed scheme depends on monitoring impedance trajectories at different feeder relays to detect the occurrence of faults and utilizes directional elements to determine the direction of faults. Communications between feeder relays are utilized to exchange permissive and blocking signals in order to locate the fault and trip the least part of the microgrid to clear the fault. Simulation results are presented to demonstrate the effectiveness of the proposed scheme.

This letter presents a new frequency control strategy that takes advantage of communications and fast responding resources such as photovoltaic generation, energy storage, wind generation, and demand response, termed collectively as converter interfaced generators (CIGs). The proposed approach uses an active monitoring of power imbalances to rapidly redispatch CIGs. This approach differs from previously proposed frequency control schemes in that it employs feed-forward control based on a measured power imbalance rather than relying on a frequency measurement. Time-domain simulations of the full Western Electricity Coordinating Council system are conducted to demonstrate the effectiveness of the proposed method, showing improved performance.

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As the penetration of renewables increases in the distribution systems, and microgrids are conceived with high penetration of such generation that connects through inverters, fault location and protection of microgrids needs consideration. This report proposes averaged models that help simulate fault scenarios in renewable-rich microgrids, models for locating faults in such microgrids, and comments on the protection models that may be considered for microgrids. Simulation studies are reported to justify the models.

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This study describes a cyber security research & development (R&D) gap analysis and research plan to address cyber security for industrial control system (ICS) supporting critical energy systems (CES). The Sandia National Laboratories (SNL) team addressed a long-term perspective for the R&D planning and gap analysis. Investment will posture CES for sustained and resilient energy operations well into the future. Acknowledgements The authors would like to acknowledge the funding and technical support from the Department of Energy Office of Electricity Delivery & Energy Reliability for the development of this report. The authors are very appreciative of the key contributions by other SNL personnel in supporting the analysis, particularly from Jennifer Depoy, Abraham Ellis, Derek Hart, Jordan Henry, John Mulder, and Jennifer Trasti. The authors would also like to thank the following government and non-government organiza- tions for their invaluable input to this study: Government Massachusetts Institute of Technology Lincoln Laboratory Construction Engineering Research Laboratory (CERL) Idaho National Laboratory Marine Corps Air Ground Combat Center, Twentynine Palms, California National Renewable Energy Laboratory National Institute of Standards and Technology Pacific Northwest National Laboratory U.S. Army Corps of Engineers U.S. Army Cyber Command U.S. Navy Installations Command Non-Government Customized Energy Solutions Electric Power Research Institute Enchanted Rock ICETEC Integrated Energy Solutions NEC Energy Solutions OpenADR Alliance PJM POWER Engineers Schweitzer Engineering Laboratory Southwest Research Institute Typhoon HIL, Inc. Executive Summary This study describes a long-term cyber security R&D plan to address ICS cyber security for CES. Long-term goals for ICS were assumed to be those that would require significant action and R&D to achieve, as opposed to being addressable by applying existing technology and best practices. Long-term R&D would roughly fall into the window of 5-10 years out. Investing in the identified R&D will posture CES for sustained resilient energy operations well into the future. The gaps were identified using a conventional gap analysis process. The current state of cyber security R&D was surveyed and summarized. Then, the desired future state of ICS cyber security was characterized, in terms of required capabilities for a secure and resilient ICS. Afterward, gaps were identified by comparing the current state of cyber security to the desired end-state. Finally, the gaps were prioritized and paired (where important) with the appropriate communities (industry, vendors, academia, etc.) suitable to address them. The baseline survey of the existing R&D focused on efforts in government, academia, feder- ally funded research and development centers (FFRDCs), and industry (including vendors). One primary source was existing DOE, Department of Homeland Security (DHS), and Department of Defense (DoD) programs, including Cybersecurity for Energy Delivery Systems (CEDS) and Defense Advanced Research Projects Agency (DARPA). Crucial documents from the National In- stitute of Standards and Technology (NIST) were also surveyed. On the academic side, the group included work from the Institute for Information Security & Privacy (IISP) and Trustworthy Cyber Infrastructure for the Power Grid (TCIPG) research consortiums. Numerous other smaller efforts were cataloged as well. Overall, the results show significant attention on the cyber security issues faced by ICS, but with a definite tendency toward near-term solutions, and less defined long-term goals, particularly in terms of needed R&D. The surveyed concepts and goals were used to develop the desired state for long-term ICS cyber security. These were complemented by concepts and frameworks previously used for ICS cyber security. The overall result was the development of a matrix of needed technical capabilities for secure and resilient ICS in the long term. Eighteen cyber security concepts (referred to as "topics" for gap analysis) were identified and sorted according to their positions in the security lifecycle (secure design, reinforced implementation, operation and deployment, or cross-cutting capabilities) and security category (protect, detect, react, or recover). For each topic, a description was provided, as well as other discussion, including a comparison to existing work. The comparisons formed the basis for the gap analysis. Some security topics, although an essential part of a desired secure ICS state in the future, have significant R&D resources alieady working to realize the goal. Others, however, are only partially addressed. Besides the severity of the R&D gap, an important consideration is that perfect security is unattainable; therefore, strong security engineering must be complemented with additional security monitoring. The final rankings for long-term R&D, including specific opportunities and challenges, along with suggestions about which group or groups should be targeted for funding opportunities, are in Chapter of the report. Some of the key results include: 1. Trusted monitors, which act as out-of-band security sentinels, and security analytics, which fuse weak indicators to detect security anomalies, have very high priority for R&D. As men- tioned previously, no system can be completely trusted (or, given the potential ramifications, even reasonably trusted); therefore, monitoring is essential. 2. Virtualization is a key capability for many aspects of ICS cyber security; potential applica- tions include training environments, pre-deployment change testing, red/blue engagement, evaluating tactics-techniques-procedures (TTPs), and others. Virtualization capability would be greatly enhanced with better support for ICS field devices (like relays, programmable logic controllers, etc.) and automated model generation from design or operational system information. 3. Field devices have unique cyber security issues, and are critical to cyber risk given their application: straddling the cyber/physical domains Addressing these issues in an organized fashion (including their virtualization) is a priority R&D gap. This is also an example where industry (particularly vendors) must complement other R&D organizations.

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In this report we focus on analyzing current-controlled PV inverters behaviour under faults in order to develop fault detection schemes for microgrids with high PV penetration. Inverter model suitable for steady state fault studies is presented and the impact of PV inverters on two protection elements is analyzed. The studied protection elements are superimposed quantities based directional element and negative sequence directional element. Additionally, several non-overcurrent fault detection schemes are discussed in this report for microgrids with high PV penetration. A detailed time-domain simulation study is presented to assess the performance of the presented fault detection schemes under different microgrid modes of operation.

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As PV and wind power penetrations in utility balancing areas increase, it is important to understand how they will impact net load. We investigate daily and seasonal trends in solar power generation, wind power generation, and net load. Quantitative metrics are used to compare scenarios with no PV or wind, PV plus wind, only PV, or only wind. PV plus wind scenarios are found to have a larger reduction in maximum net load and smaller ranges between maximum and minimum load than PV only or wind only scenarios, showing that PV plus wind can be a beneficial combination.

In this report we address the challenge of designing efficient protection system for inverter- dominated microgrids. These microgrids are characterised with limited fault current capacity as a result of current-limiting protection functions of inverters. Typically, inverters limit their fault contribution in sub-cycle time frame to as low as 1.1 per unit. As a result, overcurrent protection could fail completely to detect faults in inverter-dominated microgrids. As part of this project a detailed literature survey of existing and proposed microgrid protection schemes were conducted. The survey concluded that there is a gap in the available microgrid protection methods. The only credible protection solution available in literature for low- fault inverter-dominated microgrids is the differential protection scheme which represents a robust transmission-grade protection solution but at a very high cost. Two non-overcurrent protection schemes were investigated as part of this project; impedance-based protection and transient-based protection. Impedance-based protection depends on monitoring impedance trajectories at feeder relays to detect faults. Two communication-based impedance-based protection schemes were developed. the first scheme utilizes directional elements and pilot signals to locate the fault. The second scheme depends on a Central Protection Unit that communicates with all feeder relays to locate the fault based on directional flags received from feeder relays. The later approach could potentially be adapted to protect networked microgrids and dynamic topology microgrids. Transient-based protection relies on analyzing high frequency transients to detect and locate faults. This approach is very promising but its implementation in the filed faces several challenges. For example, high frequency transients due to faults can be confused with transients due to other events such as capacitor switching. Additionally, while detecting faults by analyzing transients could be doable, locating faults based on analyzing transients is still an open question.

High proliferation of Inverter Interfaced Distributed Energy Resources (IIDERs) into the electric distribution grid introduces new challenges to protection of such systems. This is because the existing protection systems are designed with two assumptions: 1) system is single-sourced, resulting in unidirectional fault current, and (2) fault currents are easily detectable due to much higher magnitudes compared to load currents. Due to the fact that most renewables interface with the grid though inverters, and inverters restrict their current output to levels close to the full load currents, both these assumptions are no longer valid - the system becomes multi-sourced, and overcurrent-based protection does not work. The primary scope of this study is to analyze the response of a grid-tied inverter to different faults in the grid, leading to new guidelines on protecting renewable-rich distribution systems.

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During FY15, Sandia National Laboratories executed research and development (R&D) work on a portfolio of 16 SunShot Program Systems Integration (SI) agreements, with a total FY15 budget of $13.2 million. This document summarizes the impact of the Sandia contributions based on Sandia’s direct contributions by DOE.

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Since solar PV power generation is growing rapidly, it is important to accurately model solar power production in renewable generation integration studies which look at the impact of variable renewable generation on electric grid operations. However, solar irradiance or power measurements are often sparse both spatially and temporally, making it difficult to simulate PV power output. Here, we describe the technique used to simulate generation from up to 40 utility-scale PV plants and 9 areas of distributed PV in the state of New Mexico given only five hourly irradiance measurements plus a sixth irradiance measurement at 1-minute resolution that was used as a lookup library. Spatial smoothing based on the plant size was applied, then this area-average irradiance was converted to PV power output using irradiance to power models. In this way, PV power output profiles for each location, and for the aggregate of all locations, were produced and supplied to the integration study. Also for use in the study, day-ahead solar power output forecasts were created by adding errors representative of the current state of solar forecasting to the actual power output values.

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Through the memorandum of understanding between the Depratment of Energy (DOE), the New Jersey Transit Authority (NJ Transit), and the New Jersey Board of Public Utilities, Sandia National Labs is assisting NJ Transit in developing NJ TransitGrid: an electric microgrid that will include a large-scale gas-fired generation facility and distributed energy resources (photovoltaics [PV], energy storage, electric vehicles, combined heat and power [CHP]) to supply reliable power during storms or other times of significant power failure. The NJ TransitGrid was awarded $410M from the Department of Transportation to develop a first-of-its-kind electric microgrid capable of supplying highly-reliable power.

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This document is intended to serve as a specification for generic solar photovoltaic (PV) system positive-sequence dynamic models to be implemented by software developers and approved by the WECC MVWG for use in bulk system dynamic simulations in accordance with NERC MOD standards. Two specific dynamic models are included in the scope of this document. The first, a Central Station PV System model, is intended to capture the most important dynamic characteristics of large scale (> 10 MW) PV systems with a central Point of Interconnection (POI) at the transmission level. The second, a Distributed PV System model, is intended to represent an aggregation of smaller, distribution-connected systems that comprise a portion of a composite load that might be modeled at a transmission load bus.

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It is important to be able to accurately simulate the variability of solar PV power plants for grid integration studies. We aim to inform integration studies of the ease of implementation and application-specific accuracy of current PV power plant output simulation methods. This report reviews methods for producing simulated high-resolution (sub-hour or even sub-minute) PV power plant output profiles for variability studies and describes their implementation. Two steps are involved in the simulations: estimation of average irradiance over the footprint of a PV plant and conversion of average irradiance to plant power output. Six models are described for simulating plant-average irradiance based on inputs of ground-measured irradiance, satellite-derived irradiance, or proxy plant measurements. The steps for converting plant-average irradiance to plant power output are detailed to understand the contributions to plant variability. A forthcoming report will quantify the accuracy of each method using application-specific validation metrics.

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In some situations involving weak grids or high penetration scenarios, the variability of photovoltaic systems can affect the local electrical grid. In order to mitigate destabilizing effects of power fluctuations, an energy storage device or other controllable generation or load can be used. This paper describes the development of a controller for coordinated operation of a small gas engine-generator set (genset) and a battery for smoothing PV plant output. There are a number of benefits derived from using a traditional generation resource in combination with the battery; the variability of the photovoltaic system can be reduced to a specific level with a smaller battery and Power Conditioning System (PCS) and the lifetime of the battery can be extended. The controller was designed specifically for a PV/energy storage project (Prosperity) and a gas engine-generator (Mesa Del Sol) currently operating on the same feeder in Albuquerque, New Mexico. A number of smoothing simulations of the Prosperity PV were conducted using power data collected from the site. By adjusting the control parameters, tradeoffs between battery use and ramp rates could be tuned. A cost function was created to optimize the control in order to balance, in this example, the need to have low ramp rates with reducing battery size and operation. Simulations were performed for cases with only a genset or battery, and with and without coordinated control between the genset and battery, e.g., without the communication link between sites or during a communication failure. The degree of smoothing without coordinated control did not change significantly because the battery dominated the smoothing response. It is anticipated that this work will be followed by a field demonstration in the near future.

The variability of solar PV power plants has led to some utilities imposing ramp limitations. For example, the Puerto Rico Electric Power Authority (PREPA) includes a 10% of capacity per minute limit on ramp rates produced by PV power plants in its minimum technical requirements for photovoltaic generation projects. However, it is difficult to determine storage requirements to comply with ramp limitations for plants in the planning or construction phase since the variability of the plant output is not known. In this paper, we use the wavelet variability model (WVM) to upscale irradiance measured in Mayaguez, PR to simulate various sizes of PV power plants. The results show that ramps will often exceed 10%, even for the largest plants (60MW) that benefit the most from in-plant spatial smoothing, meaning significant amounts of storage will be needed to meet the PREPA requirement. The results from Puerto Rico are compared to sites in San Diego and Oahu, Hawaii. Significant differences are seen in the ramp rate distributions of the three locations, demonstrating the importance of performing location-specific simulations. © 2013 IEEE.

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Distributed photovoltaic (PV) projects must go through an interconnection study process before connecting to the distribution grid. These studies are intended to identify the likely impacts and mitigation alternatives. In the majority of the cases, system impacts can be ruled out or mitigation can be identified without an involved study, through a screening process or a simple supplemental review study. For some proposed projects, expensive and time-consuming interconnection studies are required. The challenges to performing the studies are twofold. First, every study scenario is potentially unique, as the studies are often highly specific to the amount of PV generation capacity that varies greatly from feeder to feeder and is often unevenly distributed along the same feeder. This can cause location-specific impacts and mitigations. The second challenge is the inherent variability in PV power output which can interact with feeder operation in complex ways, by affecting the operation of voltage regulation and protection devices. The typical simulation tools and methods in use today for distribution system planning are often not adequate to accurately assess these potential impacts. This report demonstrates how quasi-static time series (QSTS) simulation and high time-resolution data can be used to assess the potential impacts in a more comprehensive manner. The QSTS simulations are applied to a set of sample feeders with high PV deployment to illustrate the usefulness of the approach. The report describes methods that can help determine how PV affects distribution system operations. The simulation results are focused on enhancing the understanding of the underlying technical issues. The examples also highlight the steps needed to perform QSTS simulation and describe the data needed to drive the simulations. The goal of this report is to make the methodology of time series power flow analysis readily accessible to utilities and others responsible for evaluating potential PV impacts.

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Ota City, Japan and Alamosa, Colorado present contrasting cases of a small rooftop distributed PV plant versus a large central PV plant. We examine the effect of geographic smoothing on the power output of each plant. 1-second relative maximum ramp rates are found to be reduced 6-10 times for the total plant output versus a single point sensor, though smaller reductions are seen at longer timescales. The relative variability is found to decay exponentially at all timescales as additional houses or inverters are aggregated. The rate of decay depends on both the geographic diversity within the plant and the meteorological conditions (such as cloud speed) on a given day. The Wavelet Variability Model (WVM) takes into account these geographic smoothing effects to produce simulated PV powerplant output by using a point sensor as input. The WVM is tested against Ota City and Alamosa, and the WVM simulation closely matches the distribution of ramp rates of actual power output. © 2012 IEEE.

Existing screening procedures contained in state and federal interconnection rules are designed to balance the need for efficiency and technical rigor for all Distributed Generation (DG). The interconnection of DG that pose no risk of system impacts based on the screens can be expedited without the need for further studies. While the interconnection screening procedures have served the industry well, they also need to evolve in order to remain relevant with respect to evolving standards, technology, and practical experience. This is particularly important considering the large and increasing volume of DG applications, particularly photovoltaic (PV) generation. This paper discusses the application of two screens from the point of view of PV: the 15% penetration on line sections and the 20 kW aggregate capacity screen for single-phase secondary circuits. We discuss extensions to the existing interconnection screens that allow for a more rigorous upfront technical evaluation to identify potential system impacts, based on the characteristics of PV generation. More effective and efficient screens will allow utilities to focus the interconnection study effort for PV systems on the cases most likely to impact the electric distribution system and avoid unnecessary interconnection study costs and delays. © 2012 IEEE.

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This report describes an algorithm, implemented in Matlab/Simulink, designed to reduce the variability of photovoltaic (PV) power output by using a battery. The purpose of the battery is to add power to the PV output (or subtract) to smooth out the high frequency components of the PV power that that occur during periods with transient cloud shadows on the PV array. The control system is challenged with the task of reducing short-term PV output variability while avoiding overworking the battery both in terms of capacity and ramp capability. The algorithm proposed by Sandia is purposely very simple to facilitate implementation in a real-time controller. The control structure has two additional inputs to which the battery can respond. For example, the battery could respond to PV variability, load variability or area control error (ACE) or a combination of the three.

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Voltage on the North American bulk system is normally regulated by synchronous generators, which typically are provided with voltage schedules by transmission system operators. In the past, variable generation plants were considered very small relative to conventional generating units, and were characteristically either induction generator (wind) or line-commutated inverters (photovoltaic) that have no inherent voltage regulation capability. However, the growing level of penetration of non-traditional renewable generation - especially wind and solar - has led to the need for renewable generation to contribute more significantly to power system voltage control and reactive power capacity. Modern wind-turbine generators, and increasingly PV inverters as well, have considerable dynamic reactive power capability, which can be further enhanced with other reactive support equipment at the plant level to meet interconnection requirements. This report contains a set of recommendations to the North-America Electricity Reliability Corporation (NERC) as part of Task 1-3 (interconnection requirements) of the Integration of Variable Generation Task Force (IVGTF) work plan. The report discusses reactive capability of different generator technologies, reviews existing reactive power standards, and provides specific recommendations to improve existing interconnection standards.

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As increasing numbers of photovoltaic (PV) systems are connected to utility systems, distribution engineers are becoming increasingly concerned about the risk of formation of unintentional islands. Utilities desire to keep their systems secure, while not imposing unreasonable burdens on users wishing to connect PV. However, utility experience with these systems is still relatively sparse, so distribution engineers often are uncertain as to when additional protective measures, such as direct transfer trip, are needed to avoid unintentional island formation. In the absence of such certainty, utilities must err on the side of caution, which in some cases may lead to the unnecessary requirement of additional protection. The purpose of this document is to provide distribution engineers and decision makers with guidance on when additional measures or additional study may be prudent, and also on certain cases in which utilities may allow PV installations to proceed without additional study because the risk of an unintentional island is extremely low. The goal is to reduce the number of cases of unnecessary application of additional protection, while giving utilities a basis on which to request additional study in cases where it is warranted.

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Sandia National Laboratories (Sandia) and SunPower Corporation (SunPower) have completed design and deployment of an autonomous irradiance monitoring system based on wireless mesh communications and a battery operated data acquisition system. The Lanai High-Density Irradiance Sensor Network is comprised of 24 LI-COR{reg_sign} irradiance sensors (silicon pyranometers) polled by 19 RF Radios. The system was implemented with commercially available hardware and custom developed LabVIEW applications. The network of solar irradiance sensors was installed in January 2010 around the periphery and within the 1.2 MW ac La Ola PV plant on the island of Lanai, Hawaii. Data acquired at 1 second intervals is transmitted over wireless links to be time-stamped and recorded on SunPower data servers at the site for later analysis. The intent is to study power and solar resource data sets to correlate the movement of cloud shadows across the PV array and its effect on power output of the PV plant. The irradiance data sets recorded will be used to study the shape, size and velocity of cloud shadows. This data, along with time-correlated PV array output data, will support the development and validation of a PV performance model that can predict the short-term output characteristics (ramp rates) of PV systems of different sizes and designs. This analysis could also be used by the La Ola system operator to predict power ramp events and support the function of the future battery system. This experience could be used to validate short-term output forecasting methodologies.

Sandia National Laboratories has developed a modeling approach to simulate time-synchronized, 1-minute power output from large PV plants in locations where only hourly irradiance measurements are available via satellite sources. The approach uses 1-min irradiance measurements from analogue sites in a similar geographic area. PV output datasets generated for 2007 in southern Nevada are being used for a Solar PV Grid Integration Study to estimate the integration costs associated with various utility-scale PV generation levels. Plant designs considered include both fixed-tilt thin-film, and singleaxis- tracked polycrystalline Si systems ranging in size from 5 to 300 MWAC. Simulated power output profiles at 1-min intervals were generated for five scenarios (149.5 MW, 222 WM, 292 MW, 492 MW, and 892 MW) each comprising as many as 10 geographically separated PV plants. Copyright© (2011) by the American Solar Energy Society.

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This report describes in-depth analysis of photovoltaic (PV) output variability in a high-penetration residential PV installation in the Pal Town neighborhood of Ota City, Japan. Pal Town is a unique test bed of high-penetration PV deployment. A total of 553 homes (approximately 80% of the neighborhood) have grid-connected PV totaling over 2 MW, and all are on a common distribution line. Power output at each house and irradiance at several locations were measured once per second in 2006 and 2007. Analysis of the Ota City data allowed for detailed characterization of distributed PV output variability and a better understanding of how variability scales spatially and temporally. For a highly variable test day, extreme power ramp rates (defined as the 99th percentile) were found to initially decrease with an increase in the number of houses at all timescales, but the reduction became negligible after a certain number of houses. Wavelet analysis resolved the variability reduction due to geographic diversity at various timescales, and the effect of geographic smoothing was found to be much more significant at shorter timescales.

The 1.2-MW La Ola photovoltaic (PV) power plant in Lanai, Hawaii, has been in operation since December 2009. The host system is a small island microgrid with peak load of 5 MW. Simulations conducted as part of the interconnection study concluded that unmitigated PV output ramps had the potential to negatively affect system frequency. Based on that study, the PV system was initially allowed to operate with output power limited to 50% of nameplate to reduce the potential for frequency instability due to PV variability. Based on the analysis of historical voltage, frequency, and power output data at 50% output level, the PV system has not significantly affected grid performance. However, it should be noted that the impact of PV variability on active and reactive power output of the nearby diesel generators was not evaluated. In summer 2011, an energy storage system was installed to counteract high ramp rates and allow the PV system to operate at rated output. The energy storage system was not fully operational at the time this report was written; therefore, analysis results do not address system performance with the battery system in place.

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This report investigates the effects that increased distributed photovoltaic (PV) generation would have on the Kauai Island Utility Co-op (KIUC) system operating requirements. The study focused on determining reserve requirements needed to mitigate the impact of PV variability on system frequency, and the impact on operating costs. Scenarios of 5-MW, 10-MW, and 15-MW nameplate capacity of PV generation plants distributed across the Kauai Island were considered in this study. The analysis required synthesis of the PV solar resource data and modeling of the KIUC system inertia. Based on the results, some findings and conclusions could be drawn, including that the selection of units identified as marginal resources that are used for load following will change; PV penetration will displace energy generated by existing conventional units, thus reducing overall fuel consumption; PV penetration at any deployment level is not likely to reduce system peak load; and increasing PV penetration has little effect on load-following reserves. The study was performed by EnerNex under contract from Sandia National Laboratories with cooperation from KIUC.

We present an approach to simulate time-synchronized, one-minute power output from large photovoltaic (PV) generation plants in locations where only hourly irradiance estimates are available from satellite sources. The approach uses one-minute irradiance measurements from ground sensors in a climatically and geographically similar area. Irradiance is translated to power using the Sandia Array Performance Model. Power output is generated for 2007 in southern Nevada are being used for a Solar PV Grid Integration Study to estimate the integration costs associated with various utility-scale PV generation levels. Plant designs considered include both fixed-tilt thin-film, and single-axis-tracked polycrystalline Si systems ranging in size from 5 to 300 MW{sub AC}. Simulated power output profiles at one-minute intervals were generated for five scenarios defined by total PV capacity (149.5 MW, 222 WM, 292 MW, 492 MW, and 892 MW) each comprising as many as 10 geographically separated PV plants.

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High frequency irradiance variability measured on the ground is caused by the formation, dissipation, and passage of clouds in the sky. If we can identify and associate different cloud types/patterns from satellite imagery, we may be able to predict irradiance variability in areas lacking sensors. With satellite imagery covering the entire U.S., this allows for more accurate integration planning and power flow modeling over wide areas. Satellite imagery from southern Nevada was analyzed at 15 minute intervals over a year. Methods for image stabilization, cloud detection, and textural classification of clouds were developed and tested. High Performance Computing parallel processing algorithms were also investigated and tested. Artificial Neural Networks using imagery as inputs were trained on ground-based measurements of irradiance to model the variability and were tested to show some promise as a means for predicting irradiance variability.

We propose and examine several statistical criteria for characterizing time series of solar irradiance. Time series of irradiance are used in analyses that seek to quantify the performance of photovoltaic (PV) power systems over time. Time series of irradiance are either measured or are simulated using models. Simulations of irradiance are often calibrated to or generated from statistics for observed irradiance and simulations are validated by comparing the simulation output to the observed irradiance. Criteria used in this comparison should derive from the context of the analyses in which the simulated irradiance is to be used. We examine three statistics that characterize time series and their use as criteria for comparing time series. We demonstrate these statistics using observed irradiance data recorded in August 2007 in Las Vegas, Nevada, and in June 2009 in Albuquerque, New Mexico.

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Expanded testing capabilities at Sandia National Laboratories Distributed Energy Technologies Lab (DETL) now include a single phase μGrid research test bed platform. This reconfigurable μGrid topology test bed platform is being utilized to evaluate control strategies and communication algorithms and associated issues applicable to high penetration of distributed resources on the grid. To demonstrate coordinated μGrid functionality, battery based Xantrex inverters were integrated in a μGrid configuration along with custom centralized LabVIEW generated virtual Energy Management System (EMS) software to provide system wide control. Enhanced μGrid cooperation was implemented by invoking control schemes based on existing Xantrex inverter command sets issued over a standard communication interface. Inverter cooperation was achieved without additional modifications to embedded software. This paper outlines test configuration and results for cooperative storage management and voltage support scenarios. ©2009 IEEE.

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