Western Interconnection Research Articles

What is different about different net-zero carbon electricity systems?

In deeply decarbonized electricity systems with significant shares of variable renewable energy, the additional availability of at least one firm electricity generating technology can overcome reliability challenges and substantially reduce electricity costs. Firm resources can operate at any time of the year and for as long as needed to maintain electricity system reliability. Low- and zero-carbon firm technologies include flexible resources with high variable and low capital costs, such as biogas or hydrogen combustion, capital-intensive resources with low or zero variable cost, including nuclear and geothermal, as well as intermediate resources such as natural gas plants with carbon capture and sequestration (CCS). This paper explains the distinct roles of nuclear, CCS, and combustion of zero-carbon fuels in decarbonized electricity systems as examples of each class of firm resources. We analyze and compare results from three long-term electricity system capacity expansion models for California and the U.S. Western Interconnection, demonstrating robustness of our conclusions to different model assumptions and domains. Individually, each firm technology delivers substantial cost reductions relative to portfolios restricted to wind, solar, and energy storage alone. Additionally, because each technology occupies a distinctive functional niche in the electricity system, having all of these technologies available optimizes the utilization rate of each resource and reduces system costs by up to 10% relative to cases with just one class of firm resource. The analysis highlights the benefits of an expansive range of technology options to meet emissions reductions goals for the power sector while maintaining operational reliability and affordability.

Synchrophasor spoofing detection and remediation for wide-area damping control

Evolving cyber-attack threats put at risk automatic closed-loop systems to be incorporated in the smart grid. Wide-area control systems are particularly vulnerable to signal spoofing attacks due to sensor remoteness and dependence on satellite communication for time synchronization. A successful cyber-attack on a wide-area controller has the potential to reduce relative stability of the power system or worse, destabilize it. As such, detection algorithms must be deployed as defense against such attacks with the ability to autonomously correct for detected tampering or misoperation. The Spoof Catch and Restore Routine (SCR2), a combination of three real-time spoof detectors, each requiring limited information about the plant, is presented. Nonlinear simulations of a compromised wide-area control system deployed in the Western Interconnection show the effectiveness of SCR2 in detecting both delay-type and counterfeit-type spoofing attacks on wide-area sensors.

Solving the Interconnection Problem

The distinct three energy grids as such exist in the United States (the Western Interconnection, the Eastern Interconnection, and the Texas Interconnection or ERCOT) unnecessarily constrain the United States in addressing its future energy needs, as electricity flow between the three grids is very limited. As our country’s reliance on traditional nonrenewable energy sources like coal continue to decline and the reliance on renewable energy sources continues to grow, the United States should focus on replacing the existing three-grid system with a national energy grid that will benefit energy companies and consumers, as well as serve our future energy needs. This Article provides both an overview of the current energy system in the United States and evidence in support of a solution—creation of a national energy grid.

The curtailment paradox in the transition to high solar power systems

Summary Rising penetrations of variable renewable energy (VRE) in power systems are expected to increase the curtailment of these resources because of oversupply and operational constraints. We evaluate the effect on curtailment from various flexibility approaches, including storage, thermal generator flexibility, operating reserve eligibility rules, transmission constraints, and temporal resolution, by using a highly resolved realistic system. Results reveal two aspects of a curtailment paradox as the system evolves to higher solar penetration levels. First, thermal generator parameters, especially in restricting minimum operating levels and ramp rates, affect VRE curtailment more in mid-PV penetration levels (∼25%–40%) but much less at lower (∼20%) or higher (∼45%) PV penetration levels. Second, although allowing VRE and storage to provide operating reserve results in significant operating costs and curtailment benefits, the price suppression effect from these resources reduces incentives for PV to provide operating reserves with curtailed energy.

Wind and Solar Resource Droughts in California Highlight the Benefits of Long-Term Storage and Integration with the Western Interconnect.

As reliance on wind and solar power for electricity generation increases, so does the importance of understanding how variability in these resources affects the feasible, cost-effective ways of supplying energy services. We use hourly weather data over multiple decades and historical electricity demand data to analyze the gaps between wind and solar supply and electricity demand for California (CA) and the Western Interconnect (WECC). We quantify the occurrence of resource droughts when the daily power from each resource was less than half of the 39-year daily mean for that day of the year. Averaged over 39 years, CA experienced 6.6 days of solar and 48 days of wind drought per year, compared to 0.41 and 19 for WECC. Using a macro-scale electricity model, we evaluate the potential for both long-term storage and more geographically diverse generation resources to minimize system costs. For wind-solar-battery electricity systems, meeting California demand with WECC generation resources reduces the cost by 9% compared to constraining resources entirely to California. Adding long-duration storage lowers system costs by 21% when treating California as an island. This data-driven analysis quantifies rare weather-related events and provides an understanding that can be used to inform stakeholders in future electricity systems.

Bulk Electric Power System Risks From Coordinated Edge Devices

As smart load adoption grows on the electric power system, potential for losing load diversity increases, possibly in ways that impact system stability. Cloud computing resources are able to coordinate large amounts of behind-the-meter loads and resources. Inadvertent or malicious actions could potentially result in gigawatts of load, distributed across large regions, acting nearly simultaneously. We study the resulting impacts of such a perturbation, which were previously recognized, with improved fidelity and granularity using a physically-based power system and demand model. The ResStock tool was used to calculate residential air conditioning load at more than 3,000 locations across the Western Interconnection, corresponding in time to heavy summer and light spring loading. Under an assumption that one cloud platform managed smart thermostats controlling 10%, 15%, or 20% of residential air-conditioning, calculated load steps could be injected into Positive Sequence Load Flow dynamic simulations. These load-driven effects were coupled with two classes of distributed generation ride-through to evaluate the potential for cascading outages. We found frequency deviations in the spring case far exceed the credible contingency event, leading to widespread distributed generation loss, while voltage depressions during the summer loading lead to widespread distributed generation loss and system separation.

Transfer Learning for Event Detection From PMU Measurements With Scarce Labels

Event detection in electrical grids is a challenging problem for machine learning methods due to spatiotemporally nonstationary systems and the inability to automate event labeling in high-volume data such as PMU measurements. As a result, the existing historical event logs created manually do not correlate well with the corresponding PMU measurements due to scarce and temporally imprecise labels. Trying to overcome this problem by extending event logs to a complete set of labeled events is very costly and often infeasible. We focused on utilizing a transfer learning model to reduce the need for additional data labeling by leveraging some labeled data instances available from a small number of well-defined event detection task. To demonstrate the feasibility, we tested our approach on a large dataset collected by 38 PMUs from the Western Interconnection of the U.S.A. over two years. The model evaluation performed based on varying percentages of labeled source data corresponding to $\sim 20$ -700 characteristic events on different sizes of time windows ranging from 2-seconds to 1-minute demonstrates that the developed method can significantly improve automated event detection based on PMU measurements when extensive labeling is costly or impossible to obtain. When compared to the state-of-the-art machine learning algorithms (unsupervised, semi-supervised, and supervised), the results show that the transfer learning method has significantly better performances when detecting events by learning from as low as 20 representative labeled data instances.

Smart Sampling for Reduced and Representative Power System Scenario Selection

With increasing penetration of renewable energy and active market participation, power system operation scenarios and patterns have increased exponentially. This has led to challenges in identifying a good subset of scenarios for routine planning, operation, and emerging machine learning applications. To address these challenges, we develop an approach integrating comprehensive exploratory data analyses and smart sampling techniques to identify and select a small subset of representative power system scenarios that maintain the coverage of system scenarios and operation envelope, therefore, leading to very efficient, yet representative studies and analysis. We propose a hierarchical Latin Hypercube Sampling (LHS) technique for smart sampling, which allows free-form distributions of system load and considers generator commitment status along with generation levels. A set of performance metrics are also defined for systematic evaluation of the adequacy and efficiency of the sampled cases. The developed approach and metrics are demonstrated using the Texas 2000 bus system in this paper and will be extended to the more complex real world systems such as Western Interconnect System.

Benefit Analysis of Long-Duration Energy Storage in Power Systems with High Renewable Energy Shares

The integration of high shares of variable renewable energy raises challenges for the reliability and cost-effectiveness of power systems. The value of long-duration energy storage, which helps address variability in renewable energy supply across days and seasons, is poised to grow significantly as power systems shift to larger shares of variable generation such as wind and solar. This study explores the system-level services and associated benefits of long-duration energy storage on the 2050 Western Interconnection (WI). The operation of the future WI system with 85% renewable penetration is simulated using a two-stage production cost model. The impact of long duration energy storage on systemwide operations is examined for the 2050 WI system, using a range of round-trip efficiencies corresponding to four different energy storage technologies. The analysis projects the energy storage dispatch profile, system-wide production cost savings (from both diurnal and seasonal operation), and impacts on generation mix, and change in renewable generation curtailment.

Stabilising transient disturbances with utility‐scale inverter‐based resources

This paper presents a trajectory tracking control strategy that modulates the active power injected by geographically distributed inverter-based resources to support transient stability. Each resource is independently controlled, and its response drives the local bus voltage angle toward a trajectory that tracks the angle of the center of inertia. The center-of-inertia angle is estimated in real time from wide-area measurements. The main objectives are to stabilize transient disturbances and increase the amount of power that can be safely transferred over key transmission paths without loss of synchronism. Here we envision the actuators as utility-scale energy storage systems; however, equivalent examples could be developed for partially-curtailed photovoltaic generation and/or Type 4 wind turbine generators. The strategy stems from a time-varying linearization of the equations of motion for a synchronous machine. The control action produces synchronizing torque in a special reference frame that accounts for the motion of the center of inertia. This drives the system states toward the desired trajectory and promotes rotor angle stability. For testing we employ a reduced-order dynamic model of the North American Western Interconnection. The results show that this approach improves system reliability and can increase capacity utilization on stability-limited transmission corridors.

Coupled transmission and distribution simulations to assess distributed generation response to power system faults

The historical simulation approach of aggregating distributed generation (DG) with load for transmission simulations is proving inadequate at capturing these DG dynamics. Presented is a methodology that uses interfacing data sets to couple disparate simulations in an iterative manner. This methodology was implemented with a Positive Sequence Load Flow (PSLF) dynamic model of the Western Interconnection with 9 GW of DG. A transmission fault leading to a fault-induced delayed voltage recovery (FIDVR) is analyzed with this iterative coupled simulation method, with more than 120 substation voltage profiles extracted, representing more than 4 GW of DG, and simulated in OpenDSS to capture the ride-through dynamics. OpenDSS simulations of feeders populated with photovoltaic DG and associated time-dependent tripping and momentary cessation criteria based on the IEEE 1547 standard are performed. Coupled simulation results show significant differences in DG responses, as well as overall system responses, between single PSLF and the coupled simulations.

Cost implications of increased solar penetration and time-of-use rate interactions

Abstract Electricity-grid operators are facing new challenges in matching load and generation due to increased solar generation and peak-load growth. This paper demonstrates that time-of-use (TOU) rates are an effective method to address these challenges. TOU rates use price differences to incentivize conserving electricity during peak hours and encouraging use during off-peak hours. This strategy is being used across the USA, including in Arizona, California and Hawaii. This analysis used the production-cost model PLEXOS with an hourly resolution to explore how production costs, locational marginal prices and dispatch stacks (type of generation used to meet load) change due to changes in load shapes prompted by TOU rates and with additional solar generation. The modelling focused on implementing TOU rates at three different adoption (response) levels with and without additional solar generation in the Arizona balancing areas within a PLEXOS model. In most cases analysed, implementing TOU rates in Arizona reduced reserve shortages in the Western Interconnect and, in some cases, very substantially. This result is representative of the interactions that happen interconnection-wide, demonstrating the advantage of modelling the entire interconnection. Production costs were decreased by the additional solar generation and the load change from TOU rates, and high response levels reduced the production costs the most for high-solar-generation cases. Load change from TOU rates decreased locational marginal prices for a typical summer day but had inconsistent results on a high-load day. Additional solar generation decreased the usage of combustion turbines, combined cycles and coal-fired generation.

Primary Frequency Support Through North American Continental HVDC Interconnections With VSC-MTDC Systems

This paper proposes a frequency control strategy for voltage source converter based multi-terminal HVDC systems (VSC-MTDC) to facilitate the exchange of primary frequency reserves among asynchronous AC systems, thus providing frequency support from each AC system to the others. The proposed frequency control utilizes a reference signal calculated from global measurements which reflects the overall frequency dynamics of all connected asynchronous AC systems. The proposed control outperforms the traditional frequency droop scheme by reducing impact on the DC voltage profile and improving frequency nadir. The performance of the designed frequency control with different DC voltage droop controls is evaluated. The adaptation of the proposed frequency control for converter outages is analyzed. The robustness of the proposed control to communication latency and the sensitivity of frequency support to power limits are also investigated. The VSC-MTDC model and the proposed control are implemented in the commercial grade software PSS/E and thus is suitable to study large-scale realistic systems and can be incorporated into the power system planning process at utilities and ISOs. The effectiveness of the proposed control is illustrated on a developed AC-MTDC test system and also on a large-scale realistic model combining the North American Western Interconnection (WI) and Eastern Interconnection (EI) with continental HVDC interconnections.

Grid-Scale Life Cycle Greenhouse Gas Implications of Renewable, Storage, and Carbon Pricing Options.

Models that characterize life cycle greenhouse gases from electricity generation are limited in their capability to estimate emissions changes at scales that capture the grid-scale benefits of technologies and policies that enhance renewable systems integration. National assumptions about generation mixes are often applied at annual time steps, neglecting spatiotemporal resolutions that provide insights on impacts from time-variable emissions. Our grid-scale model incorporates details of transmission and generation planning that allows a geographically and temporally textured and more realistic assessment of the life cycle greenhouse gas emissions outcomes, using a case study of the Western Interconnection of North America. Results from a co-optimized model of generation, transmission, and operations-the Johns Hopkins Stochastic Multistage Integrated Network Expansion Model-provide a detailed characterization of twenty-one scenarios with different configurations of storage additions, new renewable capacity, and carbon prices. Life cycle results suggest that optimization models that focus on generation alone may underestimate emissions by 18-29% because only emissions from power generation are quantified (i.e., supply chain emissions are omitted) but also that carbon pricing is the predominant driver of reducing emissions in the scenarios we examine. Life cycle assessment of electricity generation should move beyond individual technologies toward capturing the influence of policies at the system level to better understand technology-policy dynamics for the grid.

A Graph Theoretic Approach to Power System Vulnerability Identification

During major power system disturbances, when multiple component outages occur in rapid succession, it becomes crucial to quickly identify the transmission interconnections that have limited power transfer capability. Understanding the impact of an outage on these critical interconnections (called saturated cut-sets) is important for enhancing situational awareness and taking correct actions. This paper proposes a new graph theoretic approach for analyzing whether a contingency will create a saturated cut-set in a meshed power network. A novel feature of the proposed algorithm is that it lowers the solution time significantly making the approach viable for real-time operations. It also indicates the minimum amount by which the power transfer through the critical interconnections should be reduced so that post-contingency saturation does not occur. Robustness of the proposed algorithm for enhanced situational awareness is demonstrated using the IEEE-118 bus system as well as a 17,000+ bus model of the Western Interconnection (WI). Comparisons made with different approaches for power system vulnerability assessment prove the utility of the proposed scheme for aiding power system operations during extreme exigencies.

Advances on power system oscillation

Power system is one of the largest man-made systems, which over the years has become the vital infrastructure for modern life. The foundation of today’s power system is three-phase alternating current (AC), which is adopted as the main stream technology for its advantages in voltage transforming, wide-area networking and long-distance power transmitting. The AC power system is a self-sustained oscillatory system operating at nominal frequency, i.e., 50 or 60 Hz. However, the multi-time-scale dynamics in a power system, mechanical, electrical or their coupled are prone to cause the exchange of energy at wide range of frequencies other than the nominal one, which is collectively known as the oscillation issues in a power system. Such oscillations would lead to power quality degradation, equipment failure, or even worse, system instability. Actually, the development history of power system is also a history of continuous “battle” against the oscillation problems. This article reviews the basic concepts and classification of the classical as well as emerging power system oscillations. The mechanism, origin and research advances of various oscillations are re-visited and discussed, from the earlier and long-standing low frequency oscillation (LFO), and then subsynchronous oscillation/resonance (SSO/SSR) associated with turbo-generators and wind turbine generators, to the latest wide-band oscillations involving the sophisticated dynamics of increasing renewables and power electronics in a power system. LFO incident was reported for the first time in 1964 in American Western Interconnection, where the formation of a larger power system by combining two smaller one, i.e., Northwest and Southwest systems, caused power oscillation at about 0.1 Hz. Later studies showed that the multiple machines with mechanical inertial but electrically connected through networks function like a mass-spring system, leading to LFOs at a frequency ranging from 0.01 to 2.5 Hz, depending on the size of the system. Initially, excitation control was improved by adding the function of power system stabilizer (PSS) to mitigate LFO. Later, advanced control, including linear optimal control, nonlinear control and optimal decentralized coordinated control, were developed to solve the problem. Since 2000, the continuous increase of system size and coverage causes very-low-frequency oscillation issues, to address which, several new technologies, for instance, wide-area monitoring and control, flexible AC transmission system (FACTS) control, has been commissioned in China and around the world. SSR/SSO is induced by the interactions between the multi-mass shaft of a turbo-generator and the series-compensated AC or HVDC transmissions. The frequency of power oscillation is subsynchronous, or less than the nominal frequency but generally above the 2.5 Hz. The first SSR incident was captured at Mohave Power generating Station in 1970. In China, however, SSR/SSO has been a severe stability concern since 2000, the time witnessing the widely usage of series compensation and HVDC to transfer electricity from remote coal-fueled power plants. After more than ten years’ continuous research, we have developed effective tools and countermeasures, for instance, multi-dimensional analysis, optimized control and coordinated protection, to solve practical SSR/SSO problems in China and some other countries. The integration of wind power introduces new interactive dynamics that involve wind turbine converters and AC grids. It is called sub-/super-synchronous control interaction (SSCI), which would result in a different type of oscillation from several Hz to double nominal frequency. Such sub/super-synchronous oscillations associated with wind power has entered people’s vision since 2009 and gained widespread attention after the occurrence of several actual incidents in US and China. To address this new issue, we have developed novel frequency-domain modeling and analysis methods to quantify the risk first, and then control strategies on either generator/converters side or grid side to modify the interaction to stabilize SSCI. These methods and strategies have been put into practice, for instance in Guyuan and Hami systems in China, to verify their effectiveness. At this moment, power system is undergoing profound changes, one of which is the rapid growth of renewables and power electronics in all sections, including generation, transmission and consumption of electricity. Such a change is triggering power system oscillations in a very wide band, i.e., from several Hz to several thousand Hz. The underlying reason is the multi-time-scale dynamic interactions among the huge number of converters and the increasingly complicated power grid. This leads to open challenges that are yet to be solved, including the need of: (1) Wide-spectrum models to represent the multi-time-scale dynamics; (2) advanced analysis methods to quantify the stability of wide-band oscillations; and (3) adaptive and coordinated controls to mitigate the oscillation under varying system conditions. In a nutshell, systematic and in-depth researches are required to establish new theories and methodologies for this emerging issue so as to promote the sustainable growth of renewable energy and the low-carbon development of power system.

Fuel supply chains in the Western Interconnect: Evaluating availability during extreme weather events

During the winter of 2017, the eastern continental United States experienced an extreme weather event resulting in below average temperatures and consistently overcast skies. This event, referred to as a bombogenesis or “Bomb Cyclone” (BC), is one of a handful of extreme weather events experienced in the U.S. in recent years in which signif- icant strain is placed on U.S. electric interconnections. A U.S. Department of Energy study summarizing the impact of the BC indicated that coal and fuel oil/dual-firing plants provided sufficient reserves to prevent severe electricity shortages which would have otherwise resulted in significant widespread outages in the Eastern Interconnect (EI). A large regional transmission organization issued several responses to address the report’s concern regarding fuel supply security, highlighting the importance of timely fuel supply policy, and initiating a cross-institutional conversation to this end. The resultant policy conversations are not unique to the EI and generalizations are therefore of great interest to stakeholders in the western United States; a similar analysis is used to investigate the impact of a BC-like event placing strain of similar magnitude on the Western Interconnect, while introducing a resilience metric associated with the storability of fuel.

Modelling and simulation of ternary pumped storage hydropower for power system studies

As the deployment of wind and solar energy increases in the USA, energy storage (ES) will play an important role in future electric power grids to help manage the variability from high penetration levels of renewable generation, ES can provide promising power/energy demand to coordinate with renewables generation, as a virtual power plant (VPP). There is an industry need for the capability in power system studies to model ternary pumped storage hydropower (T-PSH), a pumped storage technology that offers increased system benefits. This study presents a comprehensive vendor-neutral dynamic model of T-PSH in GE's commercial software positive sequence load flow. A new governor model is developed with detailed gate valve modelling and a shared-penstock function. Specifically, this model is designed to simulate the seamless transition among three operation modes: generation mode, pumping mode, and hydraulic short-circuit mode. The developed T-PSH model has been tested and validated on a 10-bus test system. A comparison study of T-PSH to conventional pumped storage hydropower has also been conducted on the Western interconnection system.

Design of the Pacific DC Intertie Wide Area Damping Controller

This paper describes the design and implementation of a proof-of-concept Pacific dc Intertie (PDCI) wide area damping controller and includes system test results on the North American Western Interconnection (WI). To damp inter-area oscillations, the controller modulates the power transfer of the PDCI, a ±500 kV dc transmission line in the WI. The control system utilizes real-time phasor measurement unit (PMU) feedback to construct a commanded power signal which is added to the scheduled power flow for the PDCI. After years of design, simulations, and development, this controller has been implemented in hardware and successfully tested in both open and closed-loop operation. The most important design specifications were safe, reliable performance, no degradation of any system modes in any circumstances, and improve damping to the controllable modes in the WI. The main finding is that the controller adds significant damping to the modes of the WI and does not adversely affect the system response in any of the test cases. The primary contribution of this paper, to the state of the art research, is the design methods and test results of the first North American real-time control system that uses wide area PMU feedback.

A Generalized PSS Architecture for Balancing Transient and Small-Signal Response

For decades, power system stabilizers paired with high initial response automatic voltage regulators have served as an effective means of meeting sometimes conflicting system stability requirements. Driven primarily by increases in power electronically-coupled generation and load, the dynamics of large-scale power systems are rapidly changing. Electric grids are losing inertia and traditional sources of voltage support and oscillation damping. The system load is becoming stiffer with respect to changes in voltage. In parallel, advancements in wide-area measurement technology have made it possible to implement control strategies that act on information transmitted over long distances in nearly real time. In this paper, we present a power system stabilizer architecture that can be viewed as a generalization of the standard Δω-type stabilizer. The control strategy utilizes a real-time estimate of the center-of-inertia speed derived from wide-area measurements. This approach creates a flexible set of trade-offs between transient and small-signal response, making synchronous generators better able to adapt to changes in system dynamics. The phenomena of interest are examined using a two-area test case and a reduced-order model of the North American Western Interconnection. To validate the key findings under realistic conditions, we employ a state-of-the-art co-simulation platform to combine high-fidelity power system and communication network models. The benefits of the proposed control strategy are retained even under pessimistic assumptions of communication network performance.