Showing papers by "Chen-Ching Liu published in 2016"

Resilience-Oriented Critical Load Restoration Using Microgrids in Distribution Systems

Abstract: A resilience-oriented service restoration method using microgrids to restore critical load after natural disasters is proposed in this paper. Considering the scarcity of power generation resources, the concept of continuous operating time (COT) is introduced to determine the availability of microgrids for critical load restoration and to assess the service time. Uncertainties induced by intermittent energy sources and load are also taken into account. The critical load restoration problem is modeled as a chance-constrained stochastic program. A Markov chain-based operation model is designed to describe the stochastic energy variations within microgrids, based on which the COT is assessed. A two-stage heuristic is developed for the critical load restoration problem. First, a strategy table containing the information of all feasible restoration paths is established. Then the critical load restoration strategy is obtained by solving a linear integer program. Numerical simulations are performed on the IEEE 123-node feeder system under several scenarios to demonstrate the effectiveness of the proposed method. The impacts of fault locations, available generation resources, and load priority on the restoration strategy are discussed.

Placement of Remote-Controlled Switches to Enhance Distribution System Restoration Capability

Abstract: A smart distribution system should restore service to interrupted customers as quickly as possible after an outage. Upgrading manual switches to remote-controlled switches (RCSs) enhances restoration capability. The placement of RCSs should consider both functional and economic requirements. This paper presents a systematic method to determine the set of switches to be upgraded for an existing distribution system. The maximum restoration capability is achieved by upgrading a near-minimum number of manual switches to RCSs. The RCS placement problem is formulated as a weighed set cover (WSC) problem. A greedy algorithm with a polynomial-time computational efficiency is designed to generate a near-optimal solution for the WSC problem. A 3-feeder 9-node test system and a 4-feeder 1069-node unbalanced test system with microgrids are used to validate the effectiveness of the proposed method.

Cybersecurity for distributed energy resources and smart inverters

Abstract: The increased penetration of distributed energy resources (DER) will significantly increase the number of devices that are owned and controlled by consumers and third-parties. These devices have a significant dependency on digital communication and control, which presents a growing risk from cyber-attacks. This study proposes a holistic attack-resilient framework to protect the integrated DER and the critical power grid infrastructure from malicious cyber-attacks, helping ensure the secure integration of DER without harming the grid reliability and stability. Specifically, the authors discuss the architecture of the cyber-physical power system with a high penetration of DER and analyse the unique cybersecurity challenges introduced by DER integration. Next, they summarise important attack scenarios against DER, propose a systematic DER resilience analysis methodology, and develop effective and quantifiable resilience metrics and design principles. Finally, they introduce attack prevention, detection, and response measures specifically designed for DER integration across cyber, physical device, and utility layers of the future smart grid.

Outage Management of Distribution Systems Incorporating Information From Smart Meters

Abstract: A critical function in outage management for distribution systems is to quickly detect a fault and identify the activated protective device(s). With ongoing smart grid development, numerous smart meters and fault indicators with communication capabilities provide an opportunity for accurate and efficient outage management. Using the available data, this paper proposes a new multiple-hypothesis method for identification of the faulted section on a feeder or lateral. Credibility of the multiple hypotheses is determined using the available evidence from these devices. The proposed methodology is able to handle i) multiple faults, ii) protection miscoordination, and iii) missing outage reports from smart meters and fault indicators. For each hypothesis, an optimization method based on integer programming is proposed to determine the most credible actuated protective device(s) and faulted line section(s). Simulation results based on the distribution feeders of Avista Utilities serving Pullman, WA, validate the effectiveness of the proposed approach.

Cyber-Physical System Security of a Power Grid: State-of-the-Art

Abstract: As part of the smart grid development, more and more technologies are developed and deployed on the power grid to enhance the system reliability. A primary purpose of the smart grid is to significantly increase the capability of computer-based remote control and automation. As a result, the level of connectivity has become much higher, and cyber security also becomes a potential threat to the cyber-physical systems (CPSs). In this paper, a survey of the state-of-the-art is conducted on the cyber security of the power grid concerning issues of: (1) the structure of CPSs in a smart grid; (2) cyber vulnerability assessment; (3) cyber protection systems; and (4) testbeds of a CPS. At Washington State University (WSU), the Smart City Testbed (SCT) has been developed to provide a platform to test, analyze and validate defense mechanisms against potential cyber intrusions. A test case is provided in this paper to demonstrate how a testbed helps the study of cyber security and the anomaly detection system (ADS) for substations.

Dynamic load shedding for an islanded microgrid with limited generation resources

Abstract: When an extreme weather event strikes a distribution system, the utility power may not be available for days or even weeks. As a result, a microgrid in the affected area will be operated in the islanded mode during that period. To continuously serve critical load with available generation resources, which are usually limited, load shedding actions can be performed to gradually disconnect load with low priority. This study proposes a method to obtain the dynamic load shedding strategy for an islanded microgrid with limited generation resources. First, dynamic load shedding is formulated as a stochastic optimisation problem, where the uncertainties induced by intermittent energy sources and load are incorporated. The objective is to maximise the economic performance of the microgrid. Limits on the generation resources and operational constraints are considered. Then, a model based on Markov decision process (MDP) is developed for the problem. A solution method for the MDP model is proposed to obtain the optimal load shedding strategy. Finally, numerical simulations are performed to validate the effectiveness of the proposed method. Impacts of available generation resources, uncertainties of wind power prediction, and load shedding time period on the load shedding strategy are discussed.

Smart Distribution Systems

Abstract: The increasing importance of system reliability and resilience is changing the way distribution systems are planned and operated. To achieve a distribution system self-healing against power outages, emerging technologies and devices, such as remote-controlled switches (RCSs) and smart meters, are being deployed. The higher level of automation is transforming traditional distribution systems into the smart distribution systems (SDSs) of the future. The availability of data and remote control capability in SDSs provides distribution operators with an opportunity to optimize system operation and control. In this paper, the development of SDSs and resulting benefits of enhanced system capabilities are discussed. A comprehensive survey is conducted on the state-of-the-art applications of RCSs and smart meters in SDSs. Specifically, a new method, called Temporal Causal Diagram (TCD), is used to incorporate outage notifications from smart meters for enhanced outage management. To fully utilize the fast operation of RCSs, the spanning tree search algorithm is used to develop service restoration strategies. Optimal placement of RCSs and the resulting enhancement of system reliability are discussed. Distribution system resilience with respect to extreme events is presented. Test cases are used to demonstrate the benefit of SDSs. Active management of distributed generators (DGs) is introduced. Future research in a smart distribution environment is proposed.

Smart Grid Handbook

Abstract: Comprehensive, cross-disciplinary coverage of Smart Grid issues from global expert researchers and practitioners. This definitive reference meets the need for a large scale, high quality work reference in Smart Grid engineering which is pivotal in the development of a low-carbon energy infrastructure. Including a total of 83 articles across 3 volumes. The Smart Grid Handbook is organized in to 6 sections: Vision and Drivers, Transmission, Distribution, Smart Meters and Customers, Information and Communications Technology, and Socio-Economic Issues.

Piecewise Average-Value Model of PWM Converters With Applications to Large-Signal Transient Simulations

Abstract: State-space average-value models (AVMs) of pulse width modulation converters are widely used in both small-signal frequency-domain analysis and large-signal time-domain transient simulations. This paper is focused on the latter category. The limitations of the traditional state-space AVM (T-AVM) are discussed. A piecewise state-space AVM, P-AVM, is proposed, which uses the actual duty ratio as the control input instead of the continuous duty ratio used by the T-AVM. In order to consider the effect of switching ripples, an approximate ripple function is obtained. An iterative algorithm is proposed for utilization of P-AVM and ripple function for large-signal time-domain transient simulations. A boost converter and a two-level three-phase ac–dc converter are used to validate the performance of the P-AVM in comparison with the T-AVM under large ripple and large disturbance conditions. The detailed models developed in PSCAD/EMTDC are used as benchmarks. Improvement in accuracy is demonstrated. The efficiency of the iterative algorithm is discussed. Experiments on a 50-kVA three-phase ac–dc converter are conducted to validate the proposed method.

Networked Microgrids Scoping Study

Abstract: Much like individual microgrids, the range of opportunities and potential architectures of networked microgrids is very diverse. The goals of this scoping study are to provide an early assessment of research and development needs by examining the benefits of, risks created by, and risks to networked microgrids. At this time there are very few, if any, examples of deployed microgrid networks. In addition, there are very few tools to simulate or otherwise analyze the behavior of networked microgrids. In this setting, it is very difficult to evaluate networked microgrids systematically or quantitatively. At this early stage, this study is relying on inputs, estimations, and literature reviews by subject matter experts who are engaged in individual microgrid research and development projects, i.e., the authors of this study The initial step of the study gathered input about the potential opportunities provided by networked microgrids from these subject matter experts. These opportunities were divided between the subject matter experts for further review. Part 2 of this study is comprised of these reviews. Part 1 of this study is a summary of the benefits and risks identified in the reviews in Part 2 and synthesis of the research needs required to enable networked microgrids.

Optimal Opt-In Residential Time-of-Use Contract Based on Principal-Agent Theory

Abstract: Installation of smart meters is increasing world-wide, opening the possibility to implement time-of-use (TOU) tariffs to moderate peak loads. This study is focused on the design of an optimal opt-in residential TOU tariff. Residential consumers are assumed to act in their private interests and maximize utility in response to tariffs. The regulated utility has a broader interest in maximizing societal welfare. However, the regulated utility cannot impose household behavior and cannot directly observe household type. Instead, the regulated utility must offer contract options including both a TOU tariff and current pricing to allow households to self-select the plan best suited to their interests. This paper proposes a simple flexible household utility function that can be calibrated with minimal data to describe diverse household behaviors and reveal household responses to different prices. An optimal pricing model is designed for the regulated utility, taking account of asymmetric information and potential household opportunistic behavior. Using economic constructs from principal-agent theory, the pricing model ensures participation among households most aligned with the regulated utility's desire. The pricing model can also be extended for competitive markets. A case study is performed to demonstrate the benefit the optional TOU tariff can realize.

A coordinated cyber attack detection system (CCADS) for multiple substations

Abstract: In recent years, the concern over cyber security of power grids has increased significantly due to the fast growing connectivity among power system facilities. Several cyber security measures, e.g., intrusion detection systems (IDSs) and anomaly detection systems (ADSs), have been proposed to (1) mitigate unauthorized access, (2) detect anomalies, and (3) block abnormal behaviors in the communication system of substations. However, due to the lack of capability to handle coordinated cyber attacks by existing cyber security solutions, there is a need for effective methods that can detect coordinated cyber attacks. This paper proposes a new method to detect coordinated cyber attacks on power systems by identifying the relations among detected events. Examples of the relations include (1) IDS alarms, (2) geographic location of the attack, (3) criticality of substations, (4) firewall logs, and (5) attack patterns. Time Failure Propagation Graph (TFPG) and Fuzzy Cognitive Map (FCM) are used for the detection algorithms. A cyber-physical security testbed has been used to simulate the coordinated cyber attacks and validate the methods of the proposed coordinated cyber attack detection system (CCADS).

Reliability assessment of distribution systems incorporating feeder restoration actions

Abstract: This paper proposes a computational methodology for the evaluation of the IEEE reliability indices for distribution systems considering distribution system restoration. The goal of the proposed methodology is to move from a reliability assessment based on historical data to a computational approach. The developed tool allows the evaluation of the Service Restoration benefits, in terms of customers interruption duration in case of fault occurrences. Distribution System Restoration (DSR) is aimed at restoring loads after a fault by altering the topological structure of the distribution network while meeting electrical and operational constraints. The Spanning Tree Search algorithm is used to identify a post-outage topology that will restore the maximal amount of load with a minimal number of switching operations. The goal of the proposed tool is to determine the optimal switching sequences for the restoration process. The reliability indices incorporates contributions of all possible faults effects.

Evaluating the feasibility to use microgrids as a resiliency resource

Abstract: Regulated electricity utilities are required to provide safe and reliable service to their customers at a reasonable cost. To balance the objectives of reliable service and reasonable cost, utilities build and operate their systems to operate under typical historic conditions. When abnormal events such as major storms or disasters occur, it is not uncommon to have extensive interruptions in service to the end-use customers. Society is becoming less tolerant of extensive interruption in services, and it is not cost effective to harden the existing electrical distribution architecture to ensure 100% reliable power; more utilities are examining the deployment of microgrids as a part of the a coordinated resiliency plan. This paper evaluates the feasibility of microgrids as a resiliency resource under three scenarios: as a local resource, a community resource, and as a black start resource. A method of nomograms is proposed, based on dynamic simulations and evaluations of an operational microgrid, to allow operators to quickly evaluate the feasibility of these difference scenarios in operational conditions.

Observability of nonlinear power system dynamics using synchrophasor data

Abstract: Summary The increasing availability of high-resolution, synchronized measurements from phasor measurement units (PMUs) presents an opportunity to monitor power system dynamics in real time. However, the degree of observability of power system dynamics depends upon the power system states monitored by the PMUs. This paper presents a new algorithm to assess the observability level of rotor angle dynamics, based on PMU locations and monitored states. An observability index is proposed, which can be tracked along the trajectory of system states over time. To overcome the difficulty in calculation of high-order derivatives, an automatic differentiation technique is applied for observability calculation of large-scale systems. The proposed observability algorithms and techniques are validated on a 179-bus system. The proposed method is also helpful in identifying locations where PMUs should be installed for effective observability of nonlinear power system dynamics. This synchrophasor based observability monitoring method serves as a basis for control schemes that require knowledge of power system dynamics. Copyright © 2015 John Wiley & Sons, Ltd.

Coordinated Voltage Control With Online Energy Margin Constraints

Abstract: Based on data available from phase measurement units (PMUs), this paper proposes an energy margin constrained online coordinated voltage control (CVC) algorithm. First, the energy function model is established, which is based on detailed device models that include the effects of shunt capacitor banks, reactive power output of generators and under-load-tap-changers (ULTCs). In order to improve the computational efficiency, a Lie derivative-based method is used to map the energy change into the system trajectory along the energy manifold. Then, the change of energy margin based on the Lie derivative is used as the objective function and the reactive power margin is used as a constraint to formulate a linear programming problem. A coordinated strategy is proposed to help achieve the optimal solution when the linear programming problem is solved. In comparison with the other control modes, the proposed method improves the stability margin and the control effectiveness of voltage profile. Numerical results are provided for a three-machine system, the IEEE-30 bus system and the New England-39 bus benchmark system.

Artificial Intelligence and Computational Intelligence: A Challenge for Power System Engineers

Abstract: Smart grid is an important application area for artificial intelligence (AI) and computational intelligence (CI), as solutions to complex problems in power system engineering and electric energy markets depend on logic reasoning, heuristic search, perception, and the abilities to handle uncertainties. AI is concerned with decision‐making capabilities such as knowledge representation, search methods, inference techniques, heuristic reasoning, and machine learning. CI techniques include expert systems, fuzzy logic, genetic algorithms (GAs), and artificial neural networks (ANNs). CI can further involve adaptive mechanisms for intelligent behaviors in complex environments, such as the ability to adapt, generalize, abstract, discover, and associate. AI and CI have been applied to address the challenges that arise from the increasing complexity and highly nonlinear nature of electric power systems. AI and CI techniques provide effective solutions to the design of nonlinear, adaptive, and optimal controllers for generator excitation systems, High‐Voltage Direct Current (HVDC), and Flexible Alternating Current Transmission System (FACTS) devices.

Cybersecurity of SCADA within Substations

Abstract: The cybersecurity of modern substations is becoming increasingly important as they continue to integrate more digital control and communication technologies. This chapter will explore the cybersecurity threats to substations and will then discuss various mechanisms to protect them. The chapter will demonstrate how the security protections, such as encryption, authentication, access control, and intrusion detection, should be applied to substation systems. The chapter will then discuss the emerging cybersecurity requirements for both product vendors and utilities. Keywords: substations; power grid; protocol; encryption; authentication

On‐line physical security monitoring of power substations

Abstract: Summary Power grid facilities can be vulnerable with respect to malicious physical attacks. An advanced system to monitor physical security of substations and other facilities is essential to maintain system integrity. This paper describes a substation physical security monitoring (SPSM) system for remote monitoring of power substations. The proposed methodology consists of video surveillance for outdoors, motion detectors for indoors, and the classification of intrusion events. An important feature of the proposed method is the ability to determine the response to a threat based on the result of physical security monitoring. In addition, industry experience from two transmission system operators (TSOs) is incorporated in the development of the methodology. To validate the proposed SPSM system, an experiment was performed at a critical substation involving two physical intrusion scenarios. Copyright © 2015 John Wiley & Sons, Ltd.

Defensing against measurement attacks on sub-transmission level

Abstract: This paper develops a correlation index for Anomaly Detection System (ADS) to detect and identify wide area measurement attacks at sub-transmission level. Inspired by forensic analysis, we study the pattern of measurement attacks at sub-transmission power systems by developing an index of correlated substation attacks. The attacking strategy is reconstructed as a one stage two-level optimization problem that models the interaction between falsified measurements and power system operation decisions. The proposed correlation index is computed as the “attacker's decision under limited resources” by solving the mixed integer problem transformed from the attacking model. We show that this correlation index may greatly improve ADS detection rate without increasing false alarms. With the proposed correlation index, the existing protection scheme is reinforced against correlated attacks at sub-transmission power systems. The effectiveness of the developed correlation index is demonstrated through numerical simulations of the IEEE 39-bus system.

Power system severe contingency screening considering renewable energy

Abstract: Power system contingency screening is aimed at detecting potential threats, such as tripped transmission lines, to support secure operation of power systems. The growing penetration of intermittent-renewable resources into the power system challenges the traditional paradigm of contingency screening. One particular concern is that different renewable-generation profiles may lead to changing severe contingencies. This paper presents a method for power system severe contingency screening considering renewable energy. By running the hourly economic dispatch (ED), network congestion patterns under different renewable-generation profiles are determined. Next, spectral clustering is used to determine the most severe or highly congested multiple-line transmission cutset. Additionally, boundaries in the renewable-generation profile space leading to different severe transmission cutsets are investigated. Moreover, cutset-based contingency management through redispatch of conventional generators is implemented to reduce the congestion of transmission lines within the cutset. Simulation results based on the IEEE standard 14-bus system show that the proposed method is promising.

PMU-based monitoring of power system stability incorporating load and voltage dynamics

Abstract: The availability of high-resolution, synchronized measurements from phasor measurement units (PMUs) presents an opportunity to develop advanced analytical techniques for evaluation of power system dynamic performance?? This paper proposes a PMU-based technique for online monitoring of power system stability. The algorithm of Maximum Lyapunov Exponent (MLE) is used to determine if a power system swing leads to instability. Advanced models are applied to consider the frequency dynamics of the loads and voltage dynamics of the generators. The effectiveness of the proposed techniques is illustrated on a 179-bus system model. The results show that the MLE technique predicts system stability in real-time. It is also illustrated that, with the extended models, the accuracy for stability prediction is further improved.