Showing papers by "Pei Zhang published in 2011"

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.

Survey of tools for risk assessment of cascading outages

Abstract: This paper is a result of ongoing activity carried out by Understanding, Prediction, Mitigation and Restoration of Cascading Failures Task Force under IEEE Computer Analytical Methods Subcommittee (CAMS). The task force's previous papers [1, 2] are focused on general aspects of cascading outages such as understanding, prediction, prevention and restoration from cascading failures. This is the second of two new papers, which extend this previous work to summarize the state of the art in cascading failure risk analysis methodologies and modeling tools. The first paper reviews the state of the art in methodologies for performing risk assessment of potential cascading outages [3]. This paper describes the state of the art in cascading failure modeling tools, documenting the view of experts representing utilities, universities and consulting companies. The paper is intended to constitute a valid source of information and references about presently available tools that deal with prediction of cascading failure events. This effort involves reviewing published literature and other documentation from vendors, universities and research institutions. The assessment of cascading outages risk evaluation is in continuous evolution. Investigations to gain even better understanding and identification of cascading events are the subject of several research programs underway aimed at solving the complexity of these events that electrical utilities face today. Assessing the risk of cascading failure events in planning and operation for power transmission systems require adequate mathematical tools/software.

A New Unified Scheme for Controlled Power System Separation Using Synchronized Phasor Measurements

Abstract: Controlled power system separation, which separates the transmission system into islands in a controlled manner, is considered the final resort against a blackout under severe disturbances, e.g., cascading events. Three critical problems of controlled separation are where and when to separate and what to do after separation, which are rarely studied together. They are addressed in this paper by a proposed unified controlled separation scheme based on synchrophasors. The scheme decouples the three problems by partitioning them into sub-problems handled strategically in three time stages: the Offline Analysis stage determines elementary generator groups, optimizes potential separation points in between, and designs post-separation control strategies; the Online Monitoring stage predicts separation boundaries by modal analysis on synchrophasor data; the Real-time Control stage calculates a synchrophasor-based separation risk index for each boundary to predict the time to perform separation. The proposed scheme is demonstrated on a 179-bus power system by case studies.

Total supply capability and its extended indices for distribution systems: definition, model calculation and applications

Abstract: This study systematically proposes and defines a set of new quantitative indices named total supply capability (TSC) and its associated indices, such as available supply capability, substation supply capability, network transfer capability, maximum supply capability and expandable supply capability. The concepts in the TSC family provide new approaches to analyse and optimise distribution networks. TSC for distribution systems is comparable to the total transfer capability for transmission systems, but there are significant differences, such as the radial configuration and service restoration in TSC consideration. TSC is defined as the maximum load to serve under the expanded N−1 security guideline for the distribution system, taking into account the capacities of the substation transformers, network topology, link capacity and other constraints. Here, the expanded N−1 security means a successful, near-immediate service restoration to all loads except for the faulted section. A mathematical model for TSC is then set up and an accurate method is presented to calculate the TSC with lingo. Sample applications are provided to illustrate a new TSC-based approach to evaluate the link capacity and obtain the reconductoring proposal. The results from a real distribution system demonstrate the effectiveness of the proposed TSC modelling and TSC-based approach.

Risk assessment of cascading outages: Part I — Overview of methodologies

Abstract: This paper is a result of ongoing activity carried out by Understanding, Prediction, Mitigation and Restoration of Cascading Failures Task Force under IEEE Computer Analytical Methods Subcommittee (CAMS) The task force's previous papers are focused on general aspects of cascading outages such as understanding, prediction, prevention and restoration from cascading failures This is the first of two new papers, which extend this previous work to summarize the state of the art in cascading failure risk analysis methodologies and modeling tools This paper is intended to be a reference document to summarize the state of the art in the methodologies for performing risk assessment of cascading outages caused by some initiating event(s) A risk assessment should cover the entire potential chain of cascades starting with the initiating event(s) and ending with some final condition(s) However, this is a difficult task and heuristic approaches and approximations have been suggested This paper discusses different approaches to this and suggests directions for future development of methodologies The second paper summarizes the state of the art in modeling tools for risk assessment of cascading outages

An Adaptive Power System Equivalent for Real-Time Estimation of Stability Margin Using Phase-Plane Trajectories

Abstract: This paper studies real-time estimation of stability margin of a power system under disturbances by continuous measurement data. The paper presents a ball-on-concave-surface (BOCS) mechanics system as a power system's adaptive equivalent representing its real-time status and the stability region about a monitored variable. The parameters of the equivalent are adaptive to the operating condition and can online be identified from the phase-plane trajectories of the monitored variable. Accordingly, the stability margin and risk of instability can be estimated. Case studies on a two-generator system and a 179-bus system show that the BOCS system can be applied either locally or for wide-area stability monitoring in real-time calculation of stability margin. This paper proposes a new idea for using real-time measurements to develop and identify a power system's adaptive equivalent as a basis for online prediction of instability.

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.

An adaptive power system equivalent for real-time estimation of stability margin using phase-plane trajectories

Abstract: This paper studies real-time estimation of stability margin of a power system under disturbances by continuous measurement data. The paper presents a Ball-On-Concave-Surface (BOCS) mechanics system as a power system's adaptive equivalent representing its real-time status and the stability region about a monitored variable. The parameters of the equivalent are adaptive to the operating condition and can online be identified from the phase-plane trajectories of the monitored variable. Accordingly, the stability margin and risk of instability can be estimated. Case studies on a two-generator system and a 179-bus system show that the BOCS system can be applied either locally or for wide-area stability monitoring in real-time calculation of stability margin. This paper proposes a new idea for using real-time measurements to develop and identify a power system's adaptive equivalent as a basis for online prediction of instability.

A practical two-stage online voltage stability margin estimation method for utility-scale systems

Abstract: This work proposes a practical two-stage architecture for online voltage stability margin estimation using statistical models and classification techniques. The approach models the relationship between reactive power reserves and voltage stability margin using multi-linear regression models. In order to handle uncertainty related to variable loading conditions and network topology, a few regression models are required. A classification tool is then developed in order to discern which regression model should be employed at any given operating condition. Both the regression models and the classification tool are developed offline form a database generated through a comprehensive VSA. The methodology is implemented on a reduced case of the U.S. eastern interconnection, which contains around 21k buses. The studied area represents a large part of the state of Iowa and small portions of neighboring states. NERC category B, C and D contingencies have been considered in the study. Several load increase directions are used in order to account for uncertainty in load variation. Results have shown that the methodology can successfully estimate voltage stability margin in the presence of uncertainty related to variable loading condition and network topology.

Cumulant-based probabilistic load flow calculation in a market environment

Abstract: This paper proposes a probabilistic load flow (PLF) method considering market factors in power system analysis. Market-based power system brings a great many uncertainties and challenges to system operation and planning and market factors cannot be ignored in probabilistic load flow calculation. The method proposed in this paper is based on combined Cumulants and Gram-Charlier expansion and incorporates system participants' activity in power flow calculation. A real large power system Queensland network in Australia is used as an example to examine efficiency and accuracy of the proposed method. The results are compared with conventional Cumulants and Gram-Charlier based PLF method (CGC) without considering market level uncertainties and Monte Carlo simulation (MCS) based on the real power system. Differences among these methods are analyzed and discussed in detail. The proposed method improves the accuracy and reality of PLF results.