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The mechanism of oscillations that occur in the system connected to renewable energy sources is complex, generally including negative damping oscillation, limit induced oscillation etc. Combining with the dynamic characteristics of the low‐dimensional system after limit working, this paper analyses the mechanism of a type of limit induced oscillation that occurs when the output of the DC voltage outer loop continuously touches the limit for single‐machine infinite‐bus system of permanent magnet synchronous generator with positive damping under a large disturbance. Specific study as follows: firstly, a non‐smooth state space model of single‐machine infinite‐bus system of permanent magnet synchronous generator with nonlinear state limit (clarified as a Filippov system) is established. Secondly, the small disturbance characteristics of the system are analysed when the limit does not work. Thirdly, the piecewise dynamic description of the system is analysed with the limit working, and the reason that the final contraction of the system to an eight‐dimensional manifold is explained based on mathematical derivation and physical characteristics. Finally, the characteristics of equilibrium point of the system in a low‐dimensional manifold are analysed when the limit continuously working, revealing that the existence of a pair of conjugate unstable eigenvalues in the low‐dimensional system is the reason of system oscillation. [ABSTRACT FROM AUTHOR]

Phasor measurement units (PMUs) are important data source for parameter identification of the transmission line (TL). However, the "seesaw" phenomenon exists in the identified susceptance and reactance (B&X) based on real‐world PMU data, that is, the accuracy of X increases with the increase of the active power transmitted in the TL (APT), while the accuracy of B decreases. Recognizing the above, with the approximations of the opposite influence characteristics of relative errors of the identified B&X with respect to the APT, this paper proposes a method of selecting the steady‐state PMU data to improve the accuracy of the identified B&X. In detail, firstly, the "seesaw" phenomenon of the identified B&X is demonstrated based on the real‐world and the simulated PMU data. Secondly, the approximate relationship between the relative errors of B&X and the APT is derived and summarized, which can explain the "seesaw" phenomenon theoretically. Thirdly, combining the above relationships and the principle that B/X identified with the low/high APT respectively, the practical thresholds of APT for B&X identification are determined, and then, the method for selecting the steady‐state PMU data is proposed. Finally, results using both the simulated and the real‐world data demonstrate the effectiveness and practicality of the proposed method. [ABSTRACT FROM AUTHOR]

Dynamic security region (DSR) for security assessment and preventive control is defined in terms of variables that are under the control of the dispatcher prior to a fault. The boundary of DSR can be expressed using the function describing the stable manifold of the controlling unstable equilibrium point. The function is shown to be the solution of a partial differential equation. Quadratic and linear approximations to the function characterizing the boundary of DSR are derived. Numerical tests of the approximations are conducted. Potential applications of the method are discussed. Index Terms--Direct method, dynamic security region (DSR), stability region, transient stability.

Accurate parameters of transmission line (ParTL) are the basis of power system analysis. For high‐voltage transmission line (TL), remote terminal units (RTUs) are widely installed to provide supervisory control and data acquisition (SCADA) measurements. Based on multiple snapshots of SCADA data at both ends of the TL, a four‐step method to identify ParTL is proposed in this paper. In detail, based on the π‐model of TL, the relationship between voltage phase angle difference (VPADs) and parameter identification is presented. And then, the general estimation formulation for VPADs is established. Furthermore, four steps to simplify and solve the formulation are detailed, that is, firstly after the approximated relationship between VPADs and reactance are derived, the estimation of VPADs at multiple snapshots is converted into the optimal problem with reactance of TL, just one variable, and the VPADs are estimated roughly; secondly based on the median estimation, the ParTL are identified roughly; thirdly, combined with Taylor series expansions and the alternating current power flow model, the VPADs are estimated accurately; fourthly, the ParTL are identified accurately. Finally, the results with simulated and field data verify the effectiveness and practicality of the proposed method. [ABSTRACT FROM AUTHOR]

The synchronised phasor measurement units (PMUs), serving as 'GridEye', provide data that improve the observability and controllability of the power grid due to its high accuracy and high‐upload rate. However, missing or abnormal data may seriously affect its applications. This study proposes a method to recover the missing or abnormal amplitude data in PMU measurements (i.e. the active power, reactive power, positive sequence current, and voltage amplitude), based on the historical PMU data obtained from both ends of the line, which is independent of the transmission line parameters and the phase angle that may be influenced by synchronisation. First, the issues in the quality of measured PMU data are analysed, and the motivation of the proposed method is stated. Then, a method to screen out the bad data from the historical data is proposed based on the density‐based spatial clustering of applications with noise. Furthermore, the model of data recovery is established, and the recovery method, which recovers the voltage amplitude, active power, reactive power, and current amplitude in sequence, using historical amplitude data to calculate related recovery coefficients is proposed. Finally, the effectiveness and practicability of the proposed method are verified by examples of simulation and measured data. [ABSTRACT FROM AUTHOR]

Phase angle measurements from phasor measurement unit (PMU) data are sensitive to several issues, such as timing error, global positioning system (GPS) signal spoofing attack, transducer sensitivities and latency, and phasor computation algorithms. In some applications, highly accurate phase angle values are essential. In this paper, two methods are developed for correcting the phase angle difference (PAD) between the PMUs at both ends of a line. The method is based on the assumption that the line parameters are unchanged during the measurement period. The proposed scheme does not require knowing the parameters of the transmission line connecting the two PMUs, and uses multi-point measurements to estimate possible PAD bias. An additional benefit of the methods is that the parameters of the transmission line can also be calculated. [ABSTRACT FROM AUTHOR]

Phase angle measurements from phasor measurement units (PMUs) may have errors that can significantly affect line parameter calculation. In this paper, approximate expressions for the influence of phase angle difference errors on line parameter calculation are derived. Specifically, first the PMU phase angle error at both ends of a line is analyzed and an equivalent synchronization error is defined; second, approximated expressions for the influence of current and voltage phase angle difference error on the line parameters are derived. The voltage amplitude ratio on the relative error of line reactance, resistance, and capacitance measurement through the π-type equivalent model are considered. The influence of voltage and current phase angle difference error on reactance and resistance is shown from the perspective of relative sensitivity. The relative sensitivity expressions in active power form are given and the identifiable conditions of line reactance parameters based on PMU data are given. In addition, this paper presents the influence mechanism of voltage magnitude and discusses the case of synchronization issue. Finally, the accuracy of the sensitivity equations is verified by simulations, and the influence of the voltage amplitude ratio and the line flow on the identification result is quantitatively analyzed. [ABSTRACT FROM AUTHOR]

When the electric power system is disturbed, the transient frequency deviation may be large and harmful to its stable operation, especially in some small power systems. However, there is a lack of transient frequency acceptability margin (TFAM) which could be directly used by dispatchers. In this paper, a new TFAM is proposed based on the transient frequency acceptability index (TFAI). First, based on the frequency trajectory and the philosophy of "different weights to the different frequency offset levels", a new TFAI is proposed combined with frequency thresholds and time duration limits. The effectiveness of the TFAI is verified, and the critical acceptable disturbance is determined by using the TFAI. Then, a new TFAM is proposed based on the critical acceptability disturbance. The proposed TFAM can quantitatively describe the distance of the operation point from the critical frequency acceptability point, and distinguish the transient frequency acceptability of different disturbances. Finally, with different simulations, the effectiveness and applicability of the proposed TFAM are verified. The TFAM can be used for disturbances with single-parameter and multiple parameters. [ABSTRACT FROM AUTHOR]

The series compensation capacitor and HVDC are widely used in Chinese power grids to increase the transmission capability of long distance lines, which might cause subsynchronous oscillation (SSO) between the turbine generators and power systems. The torsional mechanical damping coefficient of a turbine generator reflects the inherent damping level of the shaft and, thus, is a critical factor in SSO analysis. However, the torsional mechanical damping coefficient under practical operating conditions is difficult to obtain from the design parameters. In this paper, a novel approach is developed for online torsional mechanical damping coefficient measurements. The torsional interaction mechanism of identical parallel connected turbine generators is investigated, and then the characteristics of in-phase mode (IPM) and anti-phase mode (APM) are described. From this analysis, an active stimulating method for APM is presented, and a practical scheme for online torsional mechanical damping coefficient measurement is designed. fThis approach takes advantage of the APM features and decouples the generators from the grid. Moreover, the proposed approach has been implemented in the Yimin power plant in Inner Mongolia to realize online measurement of the torsional mechanical damping coefficient. Field test results are presented to demonstrate the effectiveness of the proposed approach. [ABSTRACT FROM PUBLISHER]

Abstract: This paper presents an approach to estimate the critical clearing time (CCT) of the multi-machine power systems based on the quadratic surface which approximates the boundary of stability region relating to the controlling unstable equilibrium point. While obtaining the coefficient matrix of the second order term of the quadratic approximation, we partition the coefficient matrix into four blocks to reduce the computation burden. The CCT is estimated by the crossing point of the quadratic approximation and the continuous faulted trajectory. Simulations in IEEE 9-bus and New England system show the effectiveness of the proposed approach. [Copyright &y& Elsevier]

Allocation of reactive power equipment can relieve the transient overvoltage, which is a big threat to the sending-end electric power system of ultrahigh-voltage direct current (UHVDC). However, the dynamic reactive power allocation mostly depends on the trial-and-error method, lacking in an optimal allocation method based on the quantitative evaluation index. To deal with the abovementioned problem, in this study, a dynamic reactive power optimal allocation method is proposed based on the reactive power compensation sensitivity. In detail, first, based on the existing transient overvoltage assessment index, the general form of reactive power optimization problem is proposed. Then, taking the sending-end power system of UHVDC as an example, the reactive power allocation location is determined based on a reactive power compensation sensitivity. Furthermore, combined with the sensitivity, the compensation capacity of each place is determined by particle swarm optimization (PSO). The simulation results show that the proposed method can effectively allocate the dynamic reactive power and suppress the transient overvoltage after fault. [ABSTRACT FROM AUTHOR]