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The future distribution system is envisioned to be a network of distributed energy resources (DER), being able to operate in both the grid connected and islanded modes. This paper presents an optimal power flow algorithm based on the generalized benders decomposition (GBD) method for scheduling DER and voltage regulation device operations to meet economic and network security objectives. Among the key model features, the proposed algorithm performs multi-interval scheduling studies on the unbalanced distribution system network model with conservation voltage reduction. It is also able to support the microgrid operation in the grid-connected and islanded modes. The GBD-based OPF algorithm has been successfully tested on the IEEE 123-node test feeder model and demonstrated potential for large-scale system applications.

When a major outage occurs on a distribution system due to extreme events, microgrids, distributed generators, and other local resources can be used to restore critical loads and enhance resiliency. This paper proposes a decision-making method to determine the optimal restoration strategy coordinating multiple sources to serve critical loads after blackouts. The critical load restoration problem is solved by a two-stage method with the first stage deciding the post-restoration topology and the second stage determining the set of loads to be restored and the outputs of sources. In the second stage, the problem is formulated as a mixed-integer semidefinite program. The objective is maximizing the number of loads restored, weighted by their priority. The unbalanced three-phase power flow constraint and operational constraints are considered. An iterative algorithm is proposed to deal with integer variables and can attain the global optimum of the critical load restoration problem by solving a few semidefinite programs under two conditions. The effectiveness of the proposed method is validated by numerical simulation with the modified IEEE 13-node test feeder and the modified IEEE 123-node test feeder under plenty of scenarios. The results indicate that the optimal restoration strategy can be determined efficiently in most scenarios., 13 pages, 7 figures, journal

This paper presents voltage regulation and maximum power point tracking (MPPT) control schemes for a class of DC-microgrids. The DC-microgrid under consideration consists of a photovoltaic (PV) panel, battery, along with constant resistance and constant power loads. In this study, a dynamic model of the DC-microgrid system is derived and described by a multi-input and multi-output nonlinear system with non-affine inputs. Based on the nonlinear model thus built, we design nonlinear controllers that globally regulate voltages to prescribe set points and maximize the power output from PV by virtue of passivity and Lyapunov stability. The effectiveness of the proposed control schemes is validated by simulations.

This paper presents a novel primary control strategy based on output regulation theory for voltage and frequency regulations in microgrid systems with fast-response battery energy storage systems (BESS). The proposed control strategy can accurately track voltage and frequency set points while mitigating system transients in the presence of disturbance events. Therefore, it overcomes the key weaknesses of droop-based control methods such as large steady-state voltage and frequency deviations and poor transient performance. Throughout this paper, four control schemes are derived with tradeoffs between communication requirement and system dynamic performance. Their effectiveness is validated through MATLAB Simulink simulation studies involving a medium-voltage microgrid with both synchronous generation resources and BESS. Although the proposed control schemes are centralized, practical implementation is possible with available communication links in microgrids and embedded hardware technologies.

The increasing penetration of renewable energy sources (RESs) can challenge both power system planners and operators to maintain system reliability. Potential power system stability issues may arise when a large amount of RESs are connected to a weak power system. The short-circuit ratio (SCR) with some modifications has been used to analyze power system strength. However, the existing SCR-based methods for system strength assessment neglect real electrical network connections among multiple RESs, which may not reflect the effect of the interactions among multiple RESs at different sites on system strength. To account for the interactions among electrically interconnected RESs, the so-called site-dependent SCR (SDSCR) is proposed in this paper by analyzing the relationship between system strength and voltage stability. The efficacy of the proposed SDSCR is demonstrated through numerical simulation case studies on the IEEE 39-bus system.

The number and locations of transmission lines that are likely to get simultaneously congested are useful information for analysis of power market. In this paper, we present a study that demonstrates the number of simultaneously congested transmission lines is very limited, as well as an algorithm for identifying a small set of transmission lines that may become simultaneously congested based on the current states of the system and market. To this end, we first investigate the maximum number of simultaneously congested lines. Then, a novel algorithm is developed to identify a smaller set using two scientific criteria – one is concerned with the absolute impact of generation dispatch decisions, and the other, the relative sensitivities of line flows with respect to the changes of generation outputs. The effectiveness of the proposed algorithm is demonstrated through two simulation studies, one based on IEEE 39-bus for illustration of concepts, and the other based on a large-scale, real-world power system for showing the practical application and significance of results.

This paper discusses the integration of a Security Constrained Economic Dispatch (SCED) algorithm with Generalized DC (GenDC) power flow model into an existing regional transmission organization (RTO) market clearing engine. Compared with conventional DC power flow-based SCED algorithms, sensitivity-based voltage security constraints that include voltage constraints and reactive power dispatches, are considered in the proposed algorithm. The effectiveness and efficiency of the GenDC-based SCED model has been validated using Southwest Power Pool (SPP) market data during a pre-contingency low voltage event. The results provide convincing evidence that the proposed GenDC-based voltage constraint model can effectively mitigate power system voltage issues through minor modifications of the existing market design. Therefore, it can overcome the shortcomings of current operational voltage mitigation strategies such as creating surrogate flow-based constraints and outside-of-market unit commitment decisions that are common practices among RTO markets today. Numerical experiments are conducted on a practical system model with greater than 18,000 buses to illustrate the effectiveness of the proposed approach.

This study proposes a novel algorithm to systematically identify a minimum set of external network elements to retain in the operator's internal model, to maintain targeted levels of accuracy in CA. The existing static network model reduction methods reduce the size of network preserving power flow in reduced model the same as in full model. None of the existing method reduce network with the objective to preserve results of contingency analysis. This paper introduces the first attempt to reduce the size of network by preserving results of power flow and enforcing the results of contingency analysis in reduced model deviate from the same results in the full model not more than the user selected thresholds for bus voltages and branch flow. The method can be applied for reduction of transmission and distribution networks. The effectiveness of the method is illustrated on 179-bus WECC system and actual ISO New England model.

The political rhetoric would have it that the dramatic decline in the use of coal in the U.S. since 2008 has been a result of ‘Obama’s war on coal’ – that is, the EPA’s Clean Air rules. But the data say otherwise. Rather, enabled by shale drilling, cheap natural gas is outcompeting coal markedly. An analysis that includes gas’s techno-economic pluses concludes the trend will continue.

This work proposes a novel control strategy based on the output regulation theory, for achieving voltage and frequency regulation in a standalone microgrid system. The distributed generation of the microgrids includes both conventional generation types and battery energy storage systems coupled with power electronic converters. The fast-response power electronic converters play an important role for both tracking voltage and frequency set point targets and ensuring the system dynamic performance. Simulation case studies shows that the proposed control schemes can effectively track system voltage and frequency set points while mitigating the system transients under disturbances.

The Active distribution system (ADS) is a promising operational framework for integrating and coordinating distributed energy resources (DERs) at high penetration levels. In this paper, the detailed electromagnetic model of an ADS has been constructed in PSCAD. The model is used to study the dynamic performances of the ADS under various operational scenarios. The simulation case studies have demonstrated the operational feasibility of DERs in performing economic dispatch and grid ancillary services for the bulk transmission and the local distribution systems. Our studies show that the ADS is able to maintain frequency and voltage response characteristics under both grid-connected and standalone operating conditions. In the meantime, the ADS presents itself with desirable operational characteristics to the upstream transmission system, such as frequency droop response and reactive power support capability.

As the electrical distribution system evolves towards an active operational management in both operation planning and real time control, economic dispatch will be a required function for the distribution system operator. This work presents an economic dispatch algorithm for the distribution energy resources and price-responsive demand under a market economic structure. Meanwhile, the physical operational characteristics of the medium voltage distribution system are fully considered, including the voltage dependency of load demand and unbalanced system network model. A Generalized Bender Decomposition based algorithm has been developed to solve the proposed nonlinear optimization problem with successful case study results.

This two-part study first proposes the Generalized DC (GDC) power flow formulation which makes critical enhancements to the DC model by addressing its model limitations, with abilities to approximate bus voltage magnitudes and model networks with high resistance to reactance (R/X) ratio. Then, the GDC is applied to the current RTO electricity market clearing algorithms to model bus voltage security constraints. Case studies have successfully validated the GDC model capabilities and the new market clearing algorithm with voltage security enhancement. In addition, the incorporation of voltage security constraints supports reactive power pricing that helps incentivize voltage support via the market mechanism.

Distribution systems in North America are typically radial and unbalanced, especially with the increasing penetration of single-phase distributed energy resources (DER). In the new technological environment, new modeling and analysis approaches need to be developed to study the reliability and economic impacts of DER on the electric distribution system operation. In this study, a novel method for approximating nodal voltage sensitivities is proposed, based on some important observations made about the radial distribution system. The proposed method can efficiently compute voltage sensitivities for all network nodes based on one topology search and one power flow solution. The results have been validated using the perturbation analysis performed on the IEEE 13-Bus Test Feeder model. The proposed method for voltage sensitivity approximation can be applied in the studies of large unbalanced radial distribution systems for supporting distribution system planning and operation decisions.

To enable the development of an autonomous power control system comprised of highly-distributed software agents, a distributed implementation of a power system state estimator must be developed. The estimator uses agents located at each bus to estimate the local state and boundary conditions that are then communicated to neighboring agents. Using an iterative approach, the agents can compute a near-optimal state estimate using only local information and the communicated boundary conditions. The algorithm is implemented and tested against a subset of the International Space Station's power system. The results are compared and validated with those of a centralized state estimator.

Penetration of advanced sensor systems such as advanced metering infrastructure (AMI), high-frequency overhead and underground current and voltage sensors have been increasing significantly in power distribution systems over the past few years. According to U.S. energy information administration (EIA), the aggregated AMI installation experienced a 17 times increase from 2007 to 2012. The AMI usually collects electricity usage data every 15 minute, instead of once a month. This is a 3,000 fold increase in the amount of data utilities would have processed in the past. It is estimated that the electricity usage data collected through AMI in the U.S. amount to well above 100 terabytes in 2012. To unleash full value of the complex data sets, innovative big data algorithms need to be developed to transform the way we operate and plan for the distribution system. This paper not only proposes promising applications but also provides an in-depth discussion of technical and regulatory challenges and risks of big data analytics in power distribution systems. In addition, a flexible system architecture design is proposed to handle heterogeneous big data analysis workloads.

This study examines the North American electricity wholesale market design concerning the Demand Response (DR) resources. It shows that the competitiveness of DR bids is underrepresented when the DR impacts on distribution network losses are neglected. The study further proposes changes to the current market design by incorporating the loss sensitivities of DR resources in the market clearing algorithms. The quantities and prices of the resultant market clearing outcomes will correctly represent the market's incentives for the DR resources. This study also describes an effective method for computing distribution network loss sensitivities, which can be applied to study large-scale practical distribution system networks. This method can help facilitate the practical implementation of the proposed market design changes, with necessary collaboration from the local distribution system operators.

In this study, a new method that handles multiple hypotheses is presented for fault diagnosis using sequence of event recorders (SERs). To quantify the certainty of hypotheses, a method to calculate their credibility is provided. The proposed techniques are integrated in a generalized alarm analysis module (GAAM) and have been tested with numerous scenarios from the Italian power system.

As the computational and analytical methods for voltage stability assessment become more mature, voltage control becomes a primary issue. Power systems are large, nonlinear, and dynamic. During the last three decades, the application of differential geometry in the area of nonlinear control has produced significant results for the controllability analysis. This paper is concerned with the controllability problem of power systems using the differential geometric methods. The modeled control devices include the mechanical power input to generators, VAr compensation devices, and tap settings of the on-load tap changers. Conceptually, the main result of this research is the characterization and construction of a complete controllability region, within which a power system can be steered from one state to another by use of piecewise constant controls. The results are obtained for two cases: (1) unbounded piecewise constant controls and (2) bounded piecewise constant controls. A complete controllability region identifies the limitations of the available controls of a power system. The proposed method is believed to be significant since it is a new and systematic approach to the analysis of power system controllability using a nonlinear control system model.

In the real time operation of a Regional Transmission Organization (RTO), the generation resource commitment plans are constantly revised for the look-ahead time horizon due to changes in system operating conditions. Optimization based resource commitment algorithms can most effectively help to minimize the costs of committing new resources. However, most optimization algorithms lack transparency on how commitment decisions are made internally. Such transparency is necessary for the RTO operators to understand and approve the commitment recommendations. This study provides an effective method for identifying the reliability and economic justifications behind the new resources recommended for commitment by the optimization algorithm. The method has been implemented for the RTO, and has been validated by real-time operational experience.

A thorough analysis is performed for the voltage controllability of a simple power system model involving load dynamics and a voltage control device. The main result of this letter is the complete controllability region inside which the system voltage can be fully controlled. >

As the main power system interconnections in North America have undergone changes toward the market economic structure, ensuring grid reliability has become the responsible of the Regional Transmission Organizations (RTOs). In market economy, operations planning works side-by-side with the market to achieve grid operation reliability. Operations planning identifies reliability requirements to be enforced in the market clearing processes. It also provides outside-the-market mitigation strategies for system conditions that are not effectively addressed by the market clearing process. The RTO must maintain a correct balance to avoid both the “under planning” and the “over planning”. The former jeopardizes the grid reliability. And the latter results in poor market economic efficiency. This paper reviews the Midwest ISO market and operations planning business processes. It also discusses sensitive areas of operation where the correct balance is harder to achieve, but its success would significantly impact the market economics, in terms of both operation cost and pricing.

Recent studies have shown that locational marginal pricing (LMP) model can be extended to the electric distribution system to create better economic and operational signals. Loss sensitivity calculation is an important requirement for calculating distribution LMP. This letter proposes a novel approximate approach for calculating nodal loss sensitivities for large unbalanced radial distribution networks. The method has been further validated through perturbation studies performed with the IEEE 8500-node model. The results from this work can help facilitate further studies and implementation of distribution LMP in practical electrical distribution systems.

This paper reports upon the security-constrained commitment and dispatch algorithms and their implementation for Midwest ISO's day-ahead co-optimized energy and ancillary service markets. The MIP method is utilized to solve the DA market and the RA commitment problems and the LP method solves the DA market clearing problem. Demand response resources are qualified for provision of energy and ancillary services. AS demand curves are introduced to allow for efficient pricing of energy and AS capacities by location. Efficient price signals are expected to induce new generation additions, demand response participations and thus improve grid reliability.

In this paper the complete controllability of an N-bus power system model is analyzed. The controls in this model are bounded, which is consistent with the limited capacities of actual control devices. The complete controllability region refers to a subset of the power system state space in which any pair of states is mutually reachable using available controls. The result shows that the complete controllability region exists and contains an open set of the state space. The computational task is nontrivial for high dimensional systems. In actual power systems, the proposed controllability region provides a tool to identify a set of states where the power system is completely controllable given the control limits.

This research is concerned with the power system voltage controllability issue. A voltage controllability region is obtained for a dynamic, n-bus power system model. The complete controllability region helps to determine if the trajectories of a large, dynamic power system can be fully controlled with the available control devices.

This paper summarizes our results of a study on the complete controllability of power systems. With a nonlinear dynamic power system model, it is shown that power systems are completely controllable within a region of the state space. In other words, with the available controls, a power system can be steered from one state to any other state within the complete controllability region. The study also provides a strategy for constructing the complete controllability region. The results imply that during emergencies, if the power system states are in the complete controllability region, the movement of the power system states can be directed toward a desired operating point so that events like a voltage collapse can be avoided.