Showing papers by "Chen-Ching Liu published in 2011"

Anomaly Detection for Cybersecurity of the Substations

Abstract: Cybersecurity of the substations in a power system is a major issue as the substations become increasingly dependent on computer and communication networks. This paper is concerned with anomaly detection in the computer network environment of a substation. An anomaly inference algorithm is proposed for early detection of cyber-intrusions at the substations. The potential scenario of simultaneous intrusions launched over multiple substations is considered. The proposed detection method considers temporal anomalies. Potential intrusion events are ranked based on the credibility impact on the power system. Snapshots of anomaly entities at substations are described. Simulation results using the modified IEEE 118-bus system have shown the effectiveness of the proposed method for systematic identification. The result of this research is a tool to detect cyber-intrusions that are likely to cause significant damages to the power grid.

Optimal Generator Start-Up Strategy for Bulk Power System Restoration

Abstract: During system restoration, it is critical to utilize the available black-start (BS) units to provide cranking power to non-black-start (NBS) units in such a way that the overall system generation capability will be maximized. The corresponding optimization problem is combinatorial with complex practical constraints that can vary with time. This paper provides a new formulation of generator start-up sequencing as a mixed integer linear programming (MILP) problem. The linear formulation leads to an optimal solution to this important problem that clearly outperforms heuristic or enumerative techniques in quality of solutions or computational speed. The proposed generator start-up strategy is intended to provide an initial starting sequence of all BS or NBS units. The method can provide updates on the system MW generation capability as the restoration process progresses. The IEEE 39-Bus system, American Electric Power (AEP), and Entergy test cases are used for validation of the generation capability optimization. Simulation results demonstrate that the proposed MILP-based generator start-up sequencing algorithm is highly efficient.

PMU-Based Monitoring of Rotor Angle Dynamics

Abstract: Online monitoring of rotor angle stability in wide area power systems is an important task to avoid out-of-step instability conditions. In recent years, the installation of phasor measurement units (PMUs) on the power grids has increased significantly and, therefore, a large amount of real-time data is available for online monitoring of system dynamics. This paper proposes a PMU-based application for online monitoring of rotor angle stability. A technique based on Lyapunov exponents is used to determine if a power swing leads to system instability. The relationship between rotor angle stability and maximal Lyapunov exponent (MLE) is established. A computational algorithm is developed for the calculation of MLE in an operational environment. The effectiveness of the monitoring scheme is illustrated with a three-machine system and a 200-bus system model.

Computation of Milestones for Decision Support During System Restoration

Abstract: System restoration involves status assessment, optimization of generation capability, and load pickup. The optimization problem needs to take complex constraints into consideration, and therefore, it is not practical to formulate the problem as one single optimization problem. The other critical consideration for the development of decision support tools is its generality, i.e., the tools should be portable from a system to another with minimal customization. This paper reports a practical methodology for construction of system restoration strategies. The strategy adopted by each power system differs, depending on system characteristics and policies. A new method based on the concept of “generic restoration milestones (GRMs)” is proposed. A specific restoration strategy can be synthesized by a combination of GRMs based on the actual system conditions. The decision support tool is expected to reduce the restoration time, thereby improving system reliability. The proposed decision support tool has been validated with cases based on a simplified Western Electricity Coordinating Council (WECC) 200-Bus system and Hawaiian Electric Company system.

Short-Term Congestion Forecasting in Wholesale Power Markets

Abstract: Short-term congestion forecasting is highly important for market participants in wholesale power markets that use locational marginal prices (LMPs) to manage congestion. Accurate congestion forecasting facilitates market traders in bidding and trading activities and assists market operators in system planning. This study proposes a new short-term forecasting algorithm for congestion, LMPs, and other power system variables based on the concept of system patterns - combinations of status flags for generating units and transmission lines. The advantage of this algorithm relative to standard statistical forecasting methods is that structural aspects underlying power market operations are exploited to reduce forecast error. The advantage relative to previously proposed structural forecasting methods is that data requirements are substantially reduced. Forecasting results based on a NYISO case study demonstrate the feasibility and accuracy of the proposed algorithm.

Black start capability assessment in power system restoration

Abstract: Black start (BS) capability assessment is important for system planners to prepare the power system restoration (PSR) plan. To achieve a faster restoration process, installing new BS generators can be beneficial in accelerating system restoration. While additional BS capability does not automatically benefit the restoration process, power systems have to update the PSR plan and quantify the benefit based on appropriate criteria. In this paper, a decision support tool, developed in an EPRI project, using Generic Restoration Milestones (GRMs)-based strategy is utilized to provide a quantitative way for assessing the optimal installation location and amount of BS capability. Based on the proposed criteria, the benefit from additional BS capability is quantified in terms of system restoration time. The IEEE Reliability Test System (RTS) 24-bus test system is used for validation of the proposed strategy, using the System Restoration Navigator (SRN) restoration tool. It is shown that power systems can benefit from new BS generators to reduce the restoration time. However, there is a threshold beyond which system restoration time cannot be further reduced from additional BS capability. Economic considerations should be taken into account when assessing additional BS capabilities.

Cyber intrusion of wind farm SCADA system and its impact analysis

Abstract: The increased and concentrated penetration of wind power makes the power network more dependent on, and vulnerable to, the wind energy production. The dynamic performance of power system can be affected by the wind farm operations. Cyber attacks on the cyber systems of wind farm present a potential threat for power system dynamics. The cyber security for the SCADA system of a wind farm is studied in this paper by incorporating the impact on the power system dynamics. The vulnerabilities of a wind farm SCADA system are identified. Credible attack scenarios are developed consequently. The simulation results show that cyber attacks can cause major problems for a power system, including economy loss, overspeed of a wind turbine, and equipment damage.

An intrusion and defense testbed in a cyber-power system environment

Abstract: The proposed testbed of the cyber‐power system consists of power system simulation, substation automation, and the SCADA system. Scenarios for substation cyber security intrusions and anomaly detection concepts have been proposed. An attack tree method can be used to identify vulnerable substations and intrusions through remote access points. Specific substation vulnerability scenarios have been tested. Temporal anomaly is determined by data and information acquired at different time points. This is a metric to determine the anomaly between two snapshots. In a distributed intrusion detection algorithm, distributed agents are trained with a large number of scenarios and intended for real‐time applications. In a distributed environment, if an anomaly is detected by one agent, it is able to distribute critical information to other agents in the network.

A european supergrid: present state and future challenges

Abstract: The construction of the North Sea Super Grid is the major step towards meetingthe future demand for electric power transmission in northern Europe. This gridwill likely also extend onshore towards the load centres, and eventually form theEuropean Super Grid.Large-scale electric power generation at remote locations will lead tosignificant long distance power flows with a preferred flow direction. A methodto identify these unidirectional flows has been developed and applied on a casestudy, indicating the importance to consider unidirectional flows when designinga super grid.Voltage source converter based HVDC appears to be the best technicalsolution for the implementation of long distance transmission in such an offshoresuper grid. AC technology appears to be the most convenient choice for offshorenodes, but DC might also gain importance in this field, if reliable and affordableDC protection systems become available. A meshed DC grid offers significantadvantages towards a solution with many independent point-to-point HVDClinks, but also here protection is an unsolved issue that has to be overcome first.Reliability assessment of HVDC-based super grids is still very difficult,because operational experience with new technologies like the modularmultilevel converter is limited. This leads to a lack of data to calculate thefailure probabilities.A test system with a DC grid and the connected AC grids has been developedto serve as a common reference for a variety of DC grid studies.Unlike classical AC grids, DC grids will be dominated by power electronicsand the system behaviour will be determined to a large extent by the controllersof those power electronic systems. Large-scale implementation of powerelectronics with inappropriate control design has led to problems in AC systemsbefore. Photovoltaic generation systems in Germany are a good example forthis.A simplified AC frequency model has been developed to assess how powerelectronic systems influence the grid frequency. This model has been used to simulate how photovoltaic generation systems in Germany can endanger systemstability. A ‘grid-friendly’ charging controller for plug-in electric vehicles withbattery storage has been developed, and simulations have indicated that thiscontrol can contribute significantly to system stability.Even though AC and DC grids have some significant differences, some ofthe general concepts and lessons regarding balancing are true for both, andtomorrow’s DC grids can learn from today’s AC challenges.The balance in a DC grid should be defined as a current balance rather thanan active power balance (as it is used in AC grids), and the voltage can serve asa balance indicator, similar to AC frequency in AC grids. The control base forcontrolling the voltage should also be current instead of active power, leadingto linear system behaviour and a linear control task.HVDC converter control methods can be regarded as cases of droop controlwith one or more linear segments in the characteristic control curve. Withinone linear segment of the control curve, a HVDC converter can be representedby the Thevenin or Norton equivalent circuit.To unify a variety of proposed control concepts, Undead-band droop controlhas been proposed as a general piece-wise linear voltage control, which includesall other proposed methods as special implementations of undead-band droopcontrol. This concept could also be applied for other tasks than DC voltagecontrol like AC frequency control.

Detecting cyber intrusions in SCADA networks using multi-agent collaboration

Abstract: Current SCADA (Supervisory Control and Data Acquisition) system architecture increases the interconnectivity to/from other distributed networks and services. In addition, within the SCADA networks there are different types of sub-networks and protocols that are used to monitor and control industrial operations. This complex expansion increases the productivity of SCADA networks; however, it also increases security risks and threats. The state-of-the-art Intrusion Detection Systems (IDSs) are not capable enough of detecting anomalies and intrusions that may be aimed to disrupt the SCADA operations. This paper proposes a Distributed Intrusion Detection System (DIDS) based on a community collaboration between multiple agents of anomaly detectors to identify anomaly behaviors in SCADA networks. The proposed architecture for DIDS incorporates the SCADA network topology and connectivity constraints. In this paper, detailed architecture, components, and functions of DIDS are described and attack scenarios are developed to validate the effectiveness of the proposed methodology

Computation of milestones for decision support during system restoration

Abstract: System restoration involves status assessment, optimization of generation capability and load pickup. The optimization problem needs to take complex constraints into consideration and, therefore, it is not practical to formulate the problem as one single optimization problem. The other critical consideration for the development of decision support tools is its generality, i.e., the tools should be portable from a system to another with minimal customization. This paper reports a practical methodology for construction of system restoration strategies. The strategy adopted by each power system differs, depending on system characteristics and policies. A new method based on the concept of “Generic Restoration Milestones (GRMs)” is proposed. A specific restoration strategy can be synthesized by a combination of GRMs based on the actual system conditions. The decision support tool is expected to reduce the restoration time, thereby improving system reliability. The proposed decision support tool has been validated with cases based on a simplified Western Electricity Coordinating Council (WECC) 200-Bus system and Hawaiian Electric Company system.