Haoyu Yuan

A comparative study of measurement-based Thevenin equivalents identification methods

With the development of phasor measurement units (PMU), real-time voltage stability monitoring techniques using phasor measurements have been widely discussed in the past decade. For various measurement-based techniques, the fundamental idea behind them is to identify the Thevenin equivalents (TE) of the outer system seen from the nodes/areas of interests, and then assess the voltage stability margin based on the equivalent circuits. Therefore, fast and accurate identification of the TE is crucial for such online monitoring applications. Though several identification methods have been proposed claiming to achieve good performance, they have not been explicitly compared with each other. This work presents an analogous comparative study of four different methods. After a brief introduction, the four methods are compared in the aspects of time complexity and measurement needed by using an algorithm analysis. Then they are tested on the measurements generated from time domain simulations of the Northeast Power Coordinating Council (NPCC) 140-bus system. Following the case study, a detailed performance analysis is given, and the results can serve as a general guidance on choosing the TE identification methods and the corresponding parameters.

Projection Pursuit: A General Methodology of Wide-Area Coherency Detection in Bulk Power Grid

This paper presents a general approach for coherency detection in bulk power systems using the projection pursuit (PP) theory. Supported by the concept of center of inertia (COI) in power systems, the PP theory is employed to model the wide-area coherency detection as an optimization problem. In the proposed method, the optimal projection direction in high dimensional orthogonal space is explored in order to detect the coherent groups via the data from synchronous phasor measurement units (PMUs). Two quantitative indices constructed with projection assessment index (PI), the objective of the optimization model, are then defined in order to determine the critical coherent group and the dominant coherent groups. The coherency detection criterion and the implementation framework for the proposed approach are also presented. Simulation data from the 16-machine 68-bus test system and China Southern power Grid (CSG), along with actual field-measurement data retrieved from WAMS database in the CSG, are employed to demonstrate the effectiveness and applicability of the proposed algorithm under different disturbances. It is shown that the proposed methodology successfully detects the dominant coherent groups of generators and buses in bulk power system via the wide-area field-measurement data.

Novel Linearized Power Flow and Linearized OPF Models for Active Distribution Networks With Application in Distribution LMP

The locational marginal price (LMP) methodology has been discussed for distribution networks/systems under the smart grid initiative. In this paper, a new distribution LMP (DLMP) formulation is presented which includes reactive power prices and voltage constraints. To solve DLMP, three modeling tools, namely, linearized power flow for distribution (LPF-D), loss factors for distribution (LF-D), and linear optimal power flow for distribution (LOPF-D) are proposed. LPF-D solves not only voltage angles but also magnitudes through linear expression between bus injections and bus voltages, specifically for distribution systems. LF-D is solved recursively based on the radial topology of typical distribution systems. With the integration of LPF-D and LF-D, conventional optimal power flow (OPF) can be reformulated as LOPF-D which is essentially a linear programming model. Test results on various systems show that: 1) LPF-D efficiently yields very close results if compared with AC power flow; 2) LOPF-D provides very close dispatch results in both real and reactive power if compared with ACOPF; and 3) the proposed DLMPs calculated with LF-D and LOPF-D give accurate price information if compared with the prices from ACOPF. Further, these three tools are not limited to DLMP but can be potentially applied to other distribution analyses.

Real-time wide-area loading margin sensitivity (WALMS) in power systems

Loading margin sensitivity (LMS) has been widely used in applications in the realm of voltage stability assessment and control. Typically, LMS is derived based on system equilibrium equations near bifurcation and therefore requires full detailed system model and significant computation effort. Availability of phasor measurement units (PMUs) due to the recent development of wide-area monitoring system (WAMS) provides an alternative computation-friendly approach for calculating LMS. With such motivation, this work proposes measurement-based wide-area loading margin sensitivity (WALMS) in bulk power systems. The proposed sensitivity, with its simplicity, has great potential to be embedded in real-time applications. Moreover, the calculation of the WALMS is not limited to low voltage near bifurcation point. A case study on IEEE 39-bus system verifies the proposed sensitivity. Finally, a voltage control scenario demonstrates the potential application of the WALMS.

A Novel Dominant Mode Estimation Method for Analyzing Inter-Area Oscillation in China Southern Power Grid

This paper proposes a new approach to estimate dominant mode for monitoring inter-area oscillation in the China Southern power grid (CSG) by the use of phasor measurement units (PMUs) under both ringdown and ambient conditions. The state space model is identified by the data driven stochastic subspace identification (Data-SSI) algorithm. The canonical variate algorithm is used first to construct the weighted projection matrix of the Data-SSI. Then, the criterion for model order selection is developed to estimate the model order, and the linear model of power system is built with Data-SSI. The dominant oscillation modes are calculated by eigenvalue analysis. To accurately identify the dominant modes, repetitive results are calculated with model order variation, and then clustering analysis and stepwise refinement are applied to discriminating the dominant modes from trivial ones to improve the estimation accuracy. Field-measurement data collected by PMUs in CSG is used to validate the proposed algorithm. The comparison between existing mode estimation techniques and the proposed approach demonstrates its accuracy and robustness under both ringdown and ambient conditions.

LMP step pattern detection based on real-time data

Locational marginal pricing (LMP) methodology has been widely adopted by most independent system operators (ISOs) and regional transmission organizations (RTOs) in todays electricity markets. Previous studies show that LMP has a step change characteristic with varying load. This can be used by market participants to predict the future electricity price and potential step change of LMP. In this paper, an effective algorithm using quality threshold (QT) clustering is proposed to detect the step change pattern of the hourly LMP. A set of indices to differentiate various patterns is introduced. Furthermore, a web-based tool is built to demonstrate the price behavior of different locations based on the 5 minutes real-time LMP data from ISOs/RTOs. The user friendly design with clustering functionality ensures easy statistical study over a large amount of historical data.

Mitigate overestimation of voltage stability margin by coupled single-port circuit models

Wide-area measurement-based voltage stability assessment (VSA) by coupled single-port circuit models has been widely discussed recently. This method models the coupling effects of load buses within a meshed network into extra impedance of a single-port model for each load bus. In simulation studies, overestimations of voltage stability margin using this approach have been observed when critical load bus or buses are decoupled from other load buses. In this paper, the overestimations are reported for the first time through examples and are further analyzed in details. Moreover, to mitigate such overestimations, two methods are proposed: one method uses a mitigation factor based on actual system reactive power response; the other method changes the types of certain weak generation buses when forming the coupled impedance. Both approaches are applied to a sample 4-bus system as well as the IEEE 118-bus system and successfully mitigate the overestimations.