Electric distribution system planning supports investment decisions and operations strategies. It... more Electric distribution system planning supports investment decisions and operations strategies. It is a broad subject with many facets. The increasing number of distributed energy resources (DERs) 1 connected to the grid is changing how utilities perform their distribution planning process. The adoption of DERs is based on a combination of several factors including state policy incentives, favorable economics, consumer preferences, and the provision of reliable and resilient power, for example through the application of energy storage and microgrids. This report assesses the planning tools and methods available for utilities performing distribution system planning with DERs and identifies the gaps that exist, as well as future functionality that will be needed. The report is part of a larger ongoing effort through the U.S. Department of Energy's Grid Modernization Laboratory Consortium and is a companion to the report, Summary of Electric Distribution System Analysis with a Focus on DERs, which was published in April 2017. i Information in this report can assist stakeholders (including utility personnel, regulatory bodies, technology manufacturers, product and software vendors, think tanks, and research organizations) as they plan for and begin to participate in integrated distribution system planning activities, regulatory proceedings, or tool development. Growing numbers of DERs on the distribution grid are increasing the speed of grid dynamics, which in turn requires better grid observability (sensing and measurement) and faster controls. This process requires higher-performance communications networks with more complex interface requirements. Some early-adopter states are taking a methodical approach for conducting a high-level assessment of needs that accounts for the structure of the grid, and tools needed to help prioritize and evaluate options. The increasing adoption of DERs is changing utilities' distribution system planning processes. Utilities must account for increased uncertainty due to DERs and need to evaluate potential future scenarios. To remain flexible and adaptable to changing conditions and technology development, scenario analysis, granular forecasting, and options analysis are becoming increasingly important in integrated distribution system planning. Utilities need accurate, granular data to develop system models, validate results, and perform analysis. Collecting the data needed for integrated distribution system planning analysis means utilities must install new measurement, sensing, and communication technology, as well as capture ongoing field changes to make sure that electronic models are up to date and represent the current state of operation. Obtaining the necessary data can be a significant challenge for utility planners and other decision makers, and it can require significant time and cost. Without granular, accurate data of the distribution system all the way to the premise level, analysis is limited, and results can be inaccurate. Therefore, it is essential that utilities and regulators make strategic, targeted data collection a priority including identifying data gaps 1 The National Association of Regulatory Utility Commissioners (NARUC) defines a DER as "a resource sited close to customers that can provide all or some of their immediate electric and power needs and can also be used by the system to either reduce demand (such as energy efficiency) or provide supply to satisfy the energy, capacity, or ancillary service needs of the distribution grid. The resources, if providing electricity or thermal energy, are small in scale, connected to the distribution system, and close to load. Examples of different types of DER include solar photovoltaic (PV), wind, combined heat and power (CHP), energy storage, demand response (DR), electric vehicles (EVs), microgrids, and energy efficiency (EE)." NARUC, Distributed Energy Resources Rate Design and Compensation, 2016. PNNL-28138 iv associated with specific analyses of interest and developing strategies for filling those gaps. This can be done in a proportional manner according to need (spatially and/or temporally). The more advanced analysis also necessitates new tools and approaches, and it requires new expertise to perform the analysis. Forecasting the number of future DERs deployments is important for distribution system planning, but it is challenging in part because DER deployment is dependent on policies and tariffs, which can be influenced by the distribution system planning process itself. While there are emerging tools that can provide utilities with granular load and DER forecasts, a tool capable of evaluating different DER deployment scenarios and policies and then automatically updating projections based on actual deployment patterns would be helpful for utility planners and regulators. Power flow analysis plays a key role in traditional distribution system operation and planning, but it is evolving to consider changing operating conditions, as more DERs are connected to the distribution system. DERs create more variable supply and demand, so it is no longer enough for utility planners to consider only peak-load conditions at periodic instances in time. Utilities are beginning to utilize time series power-flow analysis (TSPFA) to understand the impact of DERs. TSPFA is a time-series grid simulation composed of multiple steady-state power-flow calculations with user-defined time step sizes between each calculation. TSPFA at short time steps can require significant computational times and iterative solvers. While some utilities do not have the granular system data necessary for TSPFA, other utilities do not have the expertise or computing power. In research domains, a variety of proposed "fast TSPFA" approaches have emerged, but more work is needed. Several tools are needed that could assist utility planners, including commercial tools that move beyond hosting capacity screens and automatically generate solutions when hosting capacity or interconnection analyses identify an issue. Utilities have numerous options for mitigating issues, including controls, capacitors, battery storage systems, reconfiguring circuits, or demand-side solutions, and these can be used in a multitude of combinations. Traditional distribution system analysis tools (CYME, Synergi, Milsoft), as well as research tools (GridLAB-D and OpenDSS), can be used to manually perform analysis and explore different solutions. But no tools exist that can provide an automated review and assess potential solutions to interconnection studies or hosting capacity analyses and then recommend the best resource or combination of resources to meet system needs. A tool that could automate this process and help utilities identify and evaluate options would allow for a quicker assessment of solutions and could inform conversations with developers and customers. More robust tools are needed that can simulate distribution systems with multiple devices, such as smart inverters and energy storage, simultaneously operating autonomously. Tools are needed that can co-simulate the distribution and transmission systems, as well as DER, under different scenarios to determine reliability, protection, and the suitability of non-wires alternatives. Although tools exist-such as DER-CAM, ReOpt, and HOMER-that optimize DERs at the campus or microgrid levels, they do so based on balance of energy equations rather than engineering power flow analysis. Engineering tools that incorporate power flow analysis are needed for selecting optimum type, sizing, and placement of DERs at the distribution system scale. Table S.1 contains a summary of key gaps and challenges for distribution system planning with DERs.
OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), Jun 2, 2018
In Hawai'i, the relatively high cost of electricity costs coupled with various incentives have ma... more In Hawai'i, the relatively high cost of electricity costs coupled with various incentives have made it cost-effective to install solar photovoltaics (PV) on residential homes and larger central-station PV plants. On some of the islands, PV has reached over 50% of the installed generation capacity base. To make sure these inverter-based PV plants can maintain stable and safe operations, new smart inverter functionality is being evaluated and demonstrated at significant scale across the islands. This paper describes research conducted to validate high PV penetration scenarios with smart inverters and recent progress on the use of these advanced inverter grid support functions in actual power grids in Hawai'i.
This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.n... more This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. U.S. Department of Energy (DOE) reports produced after 1991 and a growing number of pre-1991 documents are available free via www.OSTI.gov.
2018 IEEE Power & Energy Society General Meeting (PESGM), 2018
Inverter-coupled generation is slowly displacing synchronous generators in the power system, resu... more Inverter-coupled generation is slowly displacing synchronous generators in the power system, resulting in reduced system inertia and primary frequency response, which makes the system susceptible to larger frequency deviations. Such deviations may result in load shedding or tripping of inverters, thus reducing the reliability. Frequency-watt control of inverters is an autonomous approach to support grid frequency at a fast timescale by adjusting the inverter output with changes in frequency. This study investigates the dynamics of a power system with a high penetration of distributed PV generation with frequency-watt droop control enabled. A simple aggregated reduced-order two-source model of the O'ahu Island (Hawaii) power system is developed and validated against a full dynamic model, and the small-signal stability of the reduced model is then examined the under various PV penetrations. The impact of frequency-watt parameters on the system frequency dynamics is investigated by observing trajectories of small- signal eigenvalues. The results of the eigenvalue analysis are validated through time-domain simulation. Considerations for frequency-watt parameter selection are discussed.
2021 IEEE Kansas Power and Energy Conference (KPEC), 2021
This paper proposes single-port equivalent and two-port equivalent network reduction methods to r... more This paper proposes single-port equivalent and two-port equivalent network reduction methods to respectively reduce single-port and two-port areas in a large power network. Parameters of the reduced systems are rigorously derived, which guarantees that the electrical quantities at the port(s) remain unchanged over the reduction, including voltage magnitude and phase and active and reactive power injections into the area to be reduced. The proposed techniques are applied to reduce a practical Maui grid, where the total numbers of buses, lines and transformers are respectively reduced from 212, 106 and 108 to 45, 30 and 13. Dynamic behaviors between the full model and the reduced model are compared in detail to illustrate the efficacy and accuracy of the proposed network reduction.
This letter develops an analytical frequency nadir prediction method that allows for the consider... more This letter develops an analytical frequency nadir prediction method that allows for the consideration of three potential forms of fast frequency response (FFR) provided by inverter-based resources. The proposed method provides fast and accurate frequency nadir estimation after N-1 generation tripping contingencies. Our method is grounded on the closedform solution for the frequency nadir, which is solved from the second-order system frequency response model considering the governor dynamics and three types of FFR. The simulation results in the IEEE 39-bus system with different types of FFR demonstrate that the proposed method provides accurate and fast prediction for the frequency nadir under various disturbances.
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018
Penetration levels of residential rooftop solar photovoltaic (PV) generation on the Hawaiian isla... more Penetration levels of residential rooftop solar photovoltaic (PV) generation on the Hawaiian island of O`ahu have increased significantly in recent years, with many circuits now at the limits of their PV hosting capacity. NREL and Hawaiian Electric have used this opportunity to deploy advanced inverters at new customer installations that would not otherwise have been approved, as a pilot study to gain first-hand operational field data and experience on the impact of inverter-based grid support functions. At each customer location, the deployed advanced inverters were remotely controlled and monitored. Additionally, customer AMI (smart meter) and secondary distribution transformer data was also available for the analysis. The analysis focused on quantifying the customer behind-the-meter voltage rise due to PV production, determining the customer voltage rise with advanced inverters in unity power factor as well as advanced grid support function modes, and estimating the curtailment w...
As utilities increasingly require inverters to perform grid-supportive functions, it is challengi... more As utilities increasingly require inverters to perform grid-supportive functions, it is challenging to ensure that those functions are properly configured in the field. This paper presents details of inverter testing related to Hawaiian Electric’s latest rule 14H interconnection requirements to validate that field installation procedures result in the intended advanced inverter behavior. The latest Volt-VAR, Volt-Watt, and frequency-Watt curves in Hawaiian Electric’s Source Requirement Document (SRD) V1.1 for UL 1741 Supplement A were tested for two inverter manufacturers used in the utility territory. Both inverters were found to be largely compliant, though several discrepancies were identified and reported to the manufacturers. The results are presented and discussed here.
Momentary cessation refers to an inverter control mode. When the inverter terminal voltage falls ... more Momentary cessation refers to an inverter control mode. When the inverter terminal voltage falls below (or exceeds) a certain level, the inverter ceases to output any current, but attempts to maintain (or quickly regain) phase-locked loop synchronization to allow for quick reinjection of current when the voltage recovers to a certain point. This paper presents a photovoltaic (PV) momentary cessation model developed in PSS/E. Simulations are presented for a high voltage transmission line fault contingency in the Hawaiian island of Oahu power system on a validated PSS/E model, modified to include a custom distributed PV inverter model, and different near-future distributed PV penetration levels. Simulations for the island power system include different penetration levels of PV, and different recovery times (ramp rates and delays) after momentary cessation. The results indicate that during low voltage events, such as faults, momentary cessation can produce severe under frequency events, causing significant load shed and shortly thereafter, in some cases, over frequency events that cause generation to trip offline. The problem is exacerbated with higher penetration levels of PV. If momentary cessation is used (as is typically the case for distribution-connected resources), the recovery process after momentary cessation should be carefully considered to minimize impacts to bulk power system stability.
2021 IEEE Kansas Power and Energy Conference (KPEC), 2021
Department of Energy (DOE) reports produced after 1991 and a growing number of pre-1991 documents... more Department of Energy (DOE) reports produced after 1991 and a growing number of pre-1991 documents are available free via www.OSTI.gov.
Electric distribution system planning supports investment decisions and operations strategies. It... more Electric distribution system planning supports investment decisions and operations strategies. It is a broad subject with many facets. The increasing number of distributed energy resources (DERs) 1 connected to the grid is changing how utilities perform their distribution planning process. The adoption of DERs is based on a combination of several factors including state policy incentives, favorable economics, consumer preferences, and the provision of reliable and resilient power, for example through the application of energy storage and microgrids. This report assesses the planning tools and methods available for utilities performing distribution system planning with DERs and identifies the gaps that exist, as well as future functionality that will be needed. The report is part of a larger ongoing effort through the U.S. Department of Energy's Grid Modernization Laboratory Consortium and is a companion to the report, Summary of Electric Distribution System Analysis with a Focus on DERs, which was published in April 2017. i Information in this report can assist stakeholders (including utility personnel, regulatory bodies, technology manufacturers, product and software vendors, think tanks, and research organizations) as they plan for and begin to participate in integrated distribution system planning activities, regulatory proceedings, or tool development. Growing numbers of DERs on the distribution grid are increasing the speed of grid dynamics, which in turn requires better grid observability (sensing and measurement) and faster controls. This process requires higher-performance communications networks with more complex interface requirements. Some early-adopter states are taking a methodical approach for conducting a high-level assessment of needs that accounts for the structure of the grid, and tools needed to help prioritize and evaluate options. The increasing adoption of DERs is changing utilities' distribution system planning processes. Utilities must account for increased uncertainty due to DERs and need to evaluate potential future scenarios. To remain flexible and adaptable to changing conditions and technology development, scenario analysis, granular forecasting, and options analysis are becoming increasingly important in integrated distribution system planning. Utilities need accurate, granular data to develop system models, validate results, and perform analysis. Collecting the data needed for integrated distribution system planning analysis means utilities must install new measurement, sensing, and communication technology, as well as capture ongoing field changes to make sure that electronic models are up to date and represent the current state of operation. Obtaining the necessary data can be a significant challenge for utility planners and other decision makers, and it can require significant time and cost. Without granular, accurate data of the distribution system all the way to the premise level, analysis is limited, and results can be inaccurate. Therefore, it is essential that utilities and regulators make strategic, targeted data collection a priority including identifying data gaps 1 The National Association of Regulatory Utility Commissioners (NARUC) defines a DER as "a resource sited close to customers that can provide all or some of their immediate electric and power needs and can also be used by the system to either reduce demand (such as energy efficiency) or provide supply to satisfy the energy, capacity, or ancillary service needs of the distribution grid. The resources, if providing electricity or thermal energy, are small in scale, connected to the distribution system, and close to load. Examples of different types of DER include solar photovoltaic (PV), wind, combined heat and power (CHP), energy storage, demand response (DR), electric vehicles (EVs), microgrids, and energy efficiency (EE)." NARUC, Distributed Energy Resources Rate Design and Compensation, 2016. PNNL-28138 iv associated with specific analyses of interest and developing strategies for filling those gaps. This can be done in a proportional manner according to need (spatially and/or temporally). The more advanced analysis also necessitates new tools and approaches, and it requires new expertise to perform the analysis. Forecasting the number of future DERs deployments is important for distribution system planning, but it is challenging in part because DER deployment is dependent on policies and tariffs, which can be influenced by the distribution system planning process itself. While there are emerging tools that can provide utilities with granular load and DER forecasts, a tool capable of evaluating different DER deployment scenarios and policies and then automatically updating projections based on actual deployment patterns would be helpful for utility planners and regulators. Power flow analysis plays a key role in traditional distribution system operation and planning, but it is evolving to consider changing operating conditions, as more DERs are connected to the distribution system. DERs create more variable supply and demand, so it is no longer enough for utility planners to consider only peak-load conditions at periodic instances in time. Utilities are beginning to utilize time series power-flow analysis (TSPFA) to understand the impact of DERs. TSPFA is a time-series grid simulation composed of multiple steady-state power-flow calculations with user-defined time step sizes between each calculation. TSPFA at short time steps can require significant computational times and iterative solvers. While some utilities do not have the granular system data necessary for TSPFA, other utilities do not have the expertise or computing power. In research domains, a variety of proposed "fast TSPFA" approaches have emerged, but more work is needed. Several tools are needed that could assist utility planners, including commercial tools that move beyond hosting capacity screens and automatically generate solutions when hosting capacity or interconnection analyses identify an issue. Utilities have numerous options for mitigating issues, including controls, capacitors, battery storage systems, reconfiguring circuits, or demand-side solutions, and these can be used in a multitude of combinations. Traditional distribution system analysis tools (CYME, Synergi, Milsoft), as well as research tools (GridLAB-D and OpenDSS), can be used to manually perform analysis and explore different solutions. But no tools exist that can provide an automated review and assess potential solutions to interconnection studies or hosting capacity analyses and then recommend the best resource or combination of resources to meet system needs. A tool that could automate this process and help utilities identify and evaluate options would allow for a quicker assessment of solutions and could inform conversations with developers and customers. More robust tools are needed that can simulate distribution systems with multiple devices, such as smart inverters and energy storage, simultaneously operating autonomously. Tools are needed that can co-simulate the distribution and transmission systems, as well as DER, under different scenarios to determine reliability, protection, and the suitability of non-wires alternatives. Although tools exist-such as DER-CAM, ReOpt, and HOMER-that optimize DERs at the campus or microgrid levels, they do so based on balance of energy equations rather than engineering power flow analysis. Engineering tools that incorporate power flow analysis are needed for selecting optimum type, sizing, and placement of DERs at the distribution system scale. Table S.1 contains a summary of key gaps and challenges for distribution system planning with DERs.
OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), Jun 2, 2018
In Hawai'i, the relatively high cost of electricity costs coupled with various incentives have ma... more In Hawai'i, the relatively high cost of electricity costs coupled with various incentives have made it cost-effective to install solar photovoltaics (PV) on residential homes and larger central-station PV plants. On some of the islands, PV has reached over 50% of the installed generation capacity base. To make sure these inverter-based PV plants can maintain stable and safe operations, new smart inverter functionality is being evaluated and demonstrated at significant scale across the islands. This paper describes research conducted to validate high PV penetration scenarios with smart inverters and recent progress on the use of these advanced inverter grid support functions in actual power grids in Hawai'i.
This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.n... more This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. U.S. Department of Energy (DOE) reports produced after 1991 and a growing number of pre-1991 documents are available free via www.OSTI.gov.
2018 IEEE Power & Energy Society General Meeting (PESGM), 2018
Inverter-coupled generation is slowly displacing synchronous generators in the power system, resu... more Inverter-coupled generation is slowly displacing synchronous generators in the power system, resulting in reduced system inertia and primary frequency response, which makes the system susceptible to larger frequency deviations. Such deviations may result in load shedding or tripping of inverters, thus reducing the reliability. Frequency-watt control of inverters is an autonomous approach to support grid frequency at a fast timescale by adjusting the inverter output with changes in frequency. This study investigates the dynamics of a power system with a high penetration of distributed PV generation with frequency-watt droop control enabled. A simple aggregated reduced-order two-source model of the O'ahu Island (Hawaii) power system is developed and validated against a full dynamic model, and the small-signal stability of the reduced model is then examined the under various PV penetrations. The impact of frequency-watt parameters on the system frequency dynamics is investigated by observing trajectories of small- signal eigenvalues. The results of the eigenvalue analysis are validated through time-domain simulation. Considerations for frequency-watt parameter selection are discussed.
2021 IEEE Kansas Power and Energy Conference (KPEC), 2021
This paper proposes single-port equivalent and two-port equivalent network reduction methods to r... more This paper proposes single-port equivalent and two-port equivalent network reduction methods to respectively reduce single-port and two-port areas in a large power network. Parameters of the reduced systems are rigorously derived, which guarantees that the electrical quantities at the port(s) remain unchanged over the reduction, including voltage magnitude and phase and active and reactive power injections into the area to be reduced. The proposed techniques are applied to reduce a practical Maui grid, where the total numbers of buses, lines and transformers are respectively reduced from 212, 106 and 108 to 45, 30 and 13. Dynamic behaviors between the full model and the reduced model are compared in detail to illustrate the efficacy and accuracy of the proposed network reduction.
This letter develops an analytical frequency nadir prediction method that allows for the consider... more This letter develops an analytical frequency nadir prediction method that allows for the consideration of three potential forms of fast frequency response (FFR) provided by inverter-based resources. The proposed method provides fast and accurate frequency nadir estimation after N-1 generation tripping contingencies. Our method is grounded on the closedform solution for the frequency nadir, which is solved from the second-order system frequency response model considering the governor dynamics and three types of FFR. The simulation results in the IEEE 39-bus system with different types of FFR demonstrate that the proposed method provides accurate and fast prediction for the frequency nadir under various disturbances.
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018
Penetration levels of residential rooftop solar photovoltaic (PV) generation on the Hawaiian isla... more Penetration levels of residential rooftop solar photovoltaic (PV) generation on the Hawaiian island of O`ahu have increased significantly in recent years, with many circuits now at the limits of their PV hosting capacity. NREL and Hawaiian Electric have used this opportunity to deploy advanced inverters at new customer installations that would not otherwise have been approved, as a pilot study to gain first-hand operational field data and experience on the impact of inverter-based grid support functions. At each customer location, the deployed advanced inverters were remotely controlled and monitored. Additionally, customer AMI (smart meter) and secondary distribution transformer data was also available for the analysis. The analysis focused on quantifying the customer behind-the-meter voltage rise due to PV production, determining the customer voltage rise with advanced inverters in unity power factor as well as advanced grid support function modes, and estimating the curtailment w...
As utilities increasingly require inverters to perform grid-supportive functions, it is challengi... more As utilities increasingly require inverters to perform grid-supportive functions, it is challenging to ensure that those functions are properly configured in the field. This paper presents details of inverter testing related to Hawaiian Electric’s latest rule 14H interconnection requirements to validate that field installation procedures result in the intended advanced inverter behavior. The latest Volt-VAR, Volt-Watt, and frequency-Watt curves in Hawaiian Electric’s Source Requirement Document (SRD) V1.1 for UL 1741 Supplement A were tested for two inverter manufacturers used in the utility territory. Both inverters were found to be largely compliant, though several discrepancies were identified and reported to the manufacturers. The results are presented and discussed here.
Momentary cessation refers to an inverter control mode. When the inverter terminal voltage falls ... more Momentary cessation refers to an inverter control mode. When the inverter terminal voltage falls below (or exceeds) a certain level, the inverter ceases to output any current, but attempts to maintain (or quickly regain) phase-locked loop synchronization to allow for quick reinjection of current when the voltage recovers to a certain point. This paper presents a photovoltaic (PV) momentary cessation model developed in PSS/E. Simulations are presented for a high voltage transmission line fault contingency in the Hawaiian island of Oahu power system on a validated PSS/E model, modified to include a custom distributed PV inverter model, and different near-future distributed PV penetration levels. Simulations for the island power system include different penetration levels of PV, and different recovery times (ramp rates and delays) after momentary cessation. The results indicate that during low voltage events, such as faults, momentary cessation can produce severe under frequency events, causing significant load shed and shortly thereafter, in some cases, over frequency events that cause generation to trip offline. The problem is exacerbated with higher penetration levels of PV. If momentary cessation is used (as is typically the case for distribution-connected resources), the recovery process after momentary cessation should be carefully considered to minimize impacts to bulk power system stability.
2021 IEEE Kansas Power and Energy Conference (KPEC), 2021
Department of Energy (DOE) reports produced after 1991 and a growing number of pre-1991 documents... more Department of Energy (DOE) reports produced after 1991 and a growing number of pre-1991 documents are available free via www.OSTI.gov.
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