Showing papers by "Chen-Ching Liu published in 2014"

Distribution System Restoration With Microgrids Using Spanning Tree Search

Abstract: 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 emerging microgrids embedded in distribution systems enhance the self-healing capability and allow distribution systems to recover faster in the event of an outage. This paper presents a graph-theoretic DSR strategy incorporating microgrids that maximizes the restored load and minimizes the number of switching operations. Spanning tree search algorithms are applied to find the candidate restoration strategies by modeling microgrids as virtual feeders and representing the distribution system as a spanning tree. Unbalanced three-phase power flow is performed to ensure that the proposed system topology satisfies all operational constraints. Simulation results based on a modified IEEE 37-node system and a 1069-node distribution system demonstrate the effectiveness of the proposed approach.

Integrated Anomaly Detection for Cyber Security of the Substations

Abstract: Cyber intrusions to substations of a power grid are a source of vulnerability since most substations are unmanned and with limited protection of the physical security. In the worst case, simultaneous intrusions into multiple substations can lead to severe cascading events, causing catastrophic power outages. In this paper, an integrated Anomaly Detection System (ADS) is proposed which contains host- and network-based anomaly detection systems for the substations, and simultaneous anomaly detection for multiple substations. Potential scenarios of simultaneous intrusions into the substations have been simulated using a substation automation testbed. The host-based anomaly detection considers temporal anomalies in the substation facilities, e.g., user-interfaces, Intelligent Electronic Devices (IEDs) and circuit breakers. The malicious behaviors of substation automation based on multicast messages, e.g., Generic Object Oriented Substation Event (GOOSE) and Sampled Measured Value (SMV), are incorporated in the proposed network-based anomaly detection. The proposed simultaneous intrusion detection method is able to identify the same type of attacks at multiple substations and their locations. The result is a new integrated tool for detection and mitigation of cyber intrusions at a single substation or multiple substations of a power grid.

Distance Protection of AC Grid With HVDC-Connected Offshore Wind Generators

Abstract: When a transmission line close to points of common coupling (PCCs) experiences a short-circuit (SC) fault, the fast reactive power control of voltage-source converter-HVDC (VSC-HVDC) is likely to affect the protective relay operation of transmission lines. To study the performance of distance relays on an ac grid with an offshore wind HVDC network, this paper presents an apparent impedance calculation method, which utilizes the bus impedance matrix (Zbus) to calculate the impedances viewed by distance relays during a three-phase SC fault. The proposed method is used to identify the potential miscoordinated Zone 2 relays in the proposed combined ac/dc system. The analysis is verified by software simulation results. It is shown that the proposed method results in accurate impedances viewed by distance relays. It also identifies the protective device settings on the ac grid that need to be adjusted due to HVDC control of offshore wind generators.

Integrated Anomaly Detection for cyber security of the substations

Abstract: Cyber intrusions to substations of a power grid are a source of vulnerability since most substations are unmanned and with limited protection of the physical security. In the worst case, simultaneous intrusions into multiple substations can lead to severe cascading events, causing catastrophic power outages. In this paper, an integrated Anomaly Detection System (ADS) is proposed which contains host- and network-based anomaly detection systems for the substations, and simultaneous anomaly detection for multiple substations. Potential scenarios of simultaneous intrusions into the substations have been simulated using a substation automation testbed. The host-based anomaly detection considers temporal anomalies in the substation facilities, e.g., userinterfaces, Intelligent Electronic Devices (IEDs) and circuit breakers. The malicious behaviors of substation automation based on multicast messages, e.g., Generic Object Oriented Substation Event (GOOSE) and Sampled Measured Value (SMV), are incorporated in the proposed network-based anomaly detection. The proposed simultaneous intrusion detection method is able to identify the same type of attacks at multiple substations and their locations. The result is a new integrated tool for detection and mitigation of cyber intrusions at a single substation or multiple substations of a power grid.

Detection of cyber intrusions using network-based multicast messages for substation automation

Abstract: This paper proposes a new network-based cyber intrusion detection system (NIDS) using multicast messages in substation automation systems (SASs). The proposed network-based intrusion detection system monitors anomalies and malicious activities of multicast messages based on IEC 61850, e.g., Generic Object Oriented Substation Event (GOOSE) and Sampled Value (SV). NIDS detects anomalies and intrusions that violate predefined security rules using a specification-based algorithm. The performance test has been conducted for different cyber intrusion scenarios (e.g., packet modification, replay and denial-of-service attacks) using a cyber security testbed. The IEEE 39-bus system model has been used for testing of the proposed intrusion detection method for simultaneous cyber attacks. The false negative ratio (FNR) is the number of misclassified abnormal packets divided by the total number of abnormal packets. The results demonstrate that the proposed NIDS achieves a low fault negative rate.

The Healing Touch: Tools and Challenges for Smart Grid Restoration

Abstract: Effective system restoration is an important step toward a self-healing smart grid. in the future energy paradigm, with a high penetration of renewable resources and responsive demands, variability and uncertainty will affect power system operating and recovery technologies. Smart restoration provides an adaptive and optimized strategy with which to make restoration decisions, one that will reduce restoration time while maintaining system integrity. With the implementation of such decision support tools, the power grid will be better prepared and equipped to handle extreme events. They enable the streamlining of communication among all stakeholders, and they preserve knowledge and experience for future engineers.

Parameter Identification With PMUs for Instability Detection in Power Systems With HVDC Integrated Offshore Wind Energy

Abstract: Large-scale offshore wind farms can be integrated with onshore ac grids by the voltage source converter based high voltage direct current (VSC-HVDC) technology. The resulting impact on the security of ac grids can be significant. Therefore, it is important to develop an HVDC model for online stability monitoring of the integrated ac/dc systems. This paper proposes a new HVDC model based on a circuit-theoretic foundation. With the available phasor measurement units (PMUs) at VSC stations, the parameters of the HVDC equivalent model can be identified in real-time by synchronized voltage and current phasor measurements at VSC ac terminals. The proposed HVDC model is applied to online voltage instability detection for integrated ac/dc systems by the Thevenin impedance matching. The HVDC equivalent circuit enables the Thevenin equivalent impedance of the HVDC-connected offshore wind farm to be determined directly from the PMU measurements. Numerical simulations are performed on the IEEE 39-bus system with an HVDC-connected offshore wind farm to validate the effectiveness of the proposed HVDC model.

Distributed specification-based firewalls for power grid substations

Abstract: Cybersecurity issues have raised concerns as potential loopholes exist in the Supervisory Control And Data Acquisition (SCADA) system when the system architecture moved from a proprietary to an open system. As a result of the interdependency between the power grids and SCADA communication networks, it is important to further strengthen the defense of cyber networks against malicious intrusions. The focus of this research is to model and evaluate a novel cybersecurity protection scheme using network firewall technology operated at the application layer to enhance the information protection in SCADA networks. A quantitative analysis is performed based on the attack and defense simulations on the SCADA cybersecurity testbed available at UCD. The preliminary test results elucidate the effectiveness of the proposed scheme and demonstrate the feasibility of the proposed scheme for enhancing information security of the substations.

Temporal Causal Diagrams for Diagnosing Failures in Cyber Physical Systems

Abstract: Resilient and reliable operation of cyber physical systems of societal importance such as Smart Electric Grids is one of the top national priorities. Due to their critical nature, these systems are equipped with fast-acting, local protection mechanisms. However, commonly misguided protection actions together with system dynamics can lead to un-intentional cascading effects. This paper describes the ongoing work using Temporal Causal Diagrams (TCD), a refinement of the Timed Failure Propagation Graphs (TFPG), to diagnose problems associated with the power transmission lines protected by a combination of relays and breakers. The TCD models represent the faults and their propagation as TFPG, the nominal and faulty behavior of components (including local, discrete controllers and protection devices) as Timed Discrete Event Systems (TDES), and capture the cumulative and cascading effects of these interactions. The TCD diagnosis engine includes an extended TFPG-like reasoner which in addition to observing the alarms and mode changes (as the TFPG), monitors the event traces (that correspond to the behavioral aspects of the model) to generate hypotheses that consistently explain all the observations. In this paper, we show the results of applying the TCD to a segment of a power transmission system that is protected by distance relays and breakers.

Threat assessment and response for physical security of power substations

Abstract: Power grid facilities can be vulnerable with respect to malicious physical attacks. Preventive and/or remedial actions are essential to maintain the system integrity. This paper is focused on the physical security of power substations. The proposed methodology consists of an intrusion classification method, dynamic and static threat assessment methods, and decision support for the system operator's response in a contingency scenario. An important feature of the proposed method is to build the linkage between substation physical security monitoring and power system analysis. In addition, industry experience from two Transmission System Operators (TSOs) is incorporated in the development of the methodology.

Lyapunov exponents over variable window sizes for prediction of rotor angle stability

Abstract: A PMU-based method using Maximal Lyapunov Exponent (MLE) to determine the stability of a system following a disturbance has been established in previous research. This paper proposes a new method to determine the proper time window of MLE in an on-line environment. Spectral analysis is applied to the oscillation waveforms to calculate the variable window size of MLE. Two MLEs are calculated using the same window size but at different initial times. The consistency of the two MLEs indicates that sufficient information has been included in the first time window to characterize the system dynamics. Otherwise, the window size needs to be adapted for the operating condition. This method increases the accuracy of prediction given by MLE. A case study using a 200-bus system is presented to validate the feasibility of the proposed method.

A fast EMT simulation method for control and protection studies of microgrids

Abstract: In this work, ac-dc hybrid microgrids are studied, which contains inverter-interfaced distributed generators (DGs) and storages (DSs). As an effective way, electromagnetic transient (EMT) simulations are widely adopted for studies on control and protection strategies of these microgrids. However, detailed EMT simulations are time-consuming, especially for microgrids with many inverter-interfaced DGs/DSs. Based on model reduction, this paper proposes a fast EMT simulation method for microgrids. A piecewise averaged model (PAM) of inverter-interfaced DGs/DSs is established and integrated into the Electromagnetic Transient Program (EMTP). Using a microgrid with 2 PV generators and 1 battery energy storage system (BESS) as test system, the proposed method is verified by comparing simulation results with those obtained from a detailed model built in PSCAD/EMTDC. The results indicate that the proposed method can effectively speed up EMT simulation of microgrids with negligible lost in accuracy.

TWENTIES : conclusions of a major R&D and demonstration project on offshore DC grids

Abstract: European governments’ targets for renewable energy by 2020 will lead to large offshore wind power integration in the existing Power System. High-Voltage Direct Current (HVDC) provides the most suitable technology to enable massive integration of offshore wind farms into AC onshore grids over long distances, with great control on transmitted power. More specifically, DC Grids (DCG) based on Voltage Source Converters (VSC) are being widely investigated to integrate multiple offshore wind farms dispersed over wide areas into AC onshore networks. For three and a half years, the « DC GRID » demo within the TWENTIES European project was focussed on a wide range of challenging issues related to the DC grid benefits for connecting offshore intermittent power: offshore DCGs economic assessment and likely layouts; DCG control and protection; ancillary services provided by such grids to the mainland AC network. This paper presents major achievements of the TWENTIES project in these domains. In addition, two major outcomes of the « DC GRID » demo are physical demonstrators. One of them is a low scale DCG mock-up, on which some of the above controls, as well as DC grid protection algorithms were successfully tested. Last, a highly innovative DC Circuit Breaker (DCCB) prototype was developed to overcome a strong technological barrier: this new equipment was successfully tested in presence of an independent expert to establish the qualification of this technology in the HV domain.