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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