Feiyang Xu

The phasor measurement units (PMUs) are the eyes for the dynamic of the power system. However, the measured phase angle data from PMUs for a transmission line (TL) may coexist with different deviations, i.e., the deviations in voltage/current phase angle difference (DVPAD/DCPAD) between different ends of the TL, and the deviations in power factor angle (DPFA, i.e., the deviations in phase angle difference between voltage and current at the same end), which threaten the advanced application with PMUs. This article proposes a correction method for the phase angle data when the DVPAD/DCPAD and DPFA coexist for the TL with unknown line parameters. Specifically, first, the abnormal voltage phase angle differences and power factor angle anomalies are discussed, and the corresponding illustrative examples are presented. Second, the mathematical model for estimating phase angle deviations in a general case is presented. Third, a “two-step” method to correct the phase angle data of PMU for the TLs with unknown parameters is proposed, in which the DVPAD/DCPAD between two ends is estimated in advance, and then, the DPFA is estimated. Finally, simulation and examples with field data verify the feasibility and engineering applicability of the proposed method.

Errors in voltage and current transformer and compensation algorithms may cause errors in local voltage and current phase angle measured by phasor measurement units (PMU), whose difference is defined as the power factor angle (PFA). Consequently, the error of local PMU voltage and current phase angle difference, i.e., power factor angle deviation (PFAD), can cause reactive power measurement deviation, which leads to reactive power measurement inconsistency of symmetrical double-circuit lines, deteriorating the application of dynamic monitoring. In this paper, a three-step correction method for PFAD of double-circuit lines is proposed based on the operation characteristics and reactive power of double-circuit lines:1) estimate the difference between PFADs of different lines at the same end; 2) estimate the difference of PFADs at both ends of one line of the double-circuit lines; 3) estimate the local PFAD. The method does not require line parameters and can realize the correction of PFAD at each end of the double circuit line. Finally, the effectiveness and robustness of the proposed method are demonstrated by the simulation results using both simulated and field data.

The phase angle difference between local voltage and current phasors from the phasor measurement unit (PMU), which is defined as the power factor angle (PFA), is sensitive to voltage and current transformer errors and signal transmission delay. This paper proposes a robust PFA deviation (PFAD) detection and correction method using transmission line (TL) reactive power. Specifically, the root cause of PFAD and its influence on the calculated power measurements from PMU are analyzed. Then, an expression for TL reactive power loss independent of phase angle is derived. This allows the development of a detection method for PFAD that takes into account PMU measurement errors and TL parameter deviation. A method for estimating PFAD is also proposed to correct the PFAD at both ends of a transmission line. Simulation results using both simulated and field data demonstrate the effectiveness and robustness of the proposed method.

Phasor measurement unit (PMU), which could provide high-frequency phasors including the amplitude and phase angle synchronized with Global Positioning System (GPS)/BeiDou Navigation Satellite System (BDS) signal, plays a vital role in dynamic monitoring of power systems. However, if the GPS/BDS signal could not be received, the time of PMU, which depends on the crystal oscillator (CO), would cause measurement deviation due to frequency shifts of CO, and then the PMU data would engage deviations and deteriorate the relevant applications. In recognizing the above problems, this paper proposes a method to detect the frequency deviation of the crystal oscillator (FDCO) based on the correlation coefficient, according to its relation with the characteristic of the voltage phase angle difference(VPAD) between the two ends of the line. In detail, firstly, the quality of the VPAD of PMU between the two ends of the line is analyzed and the related problem is stated. Secondly, the characteristics of PMU data, when FDCO exists, are derived and analyzed, especially the VPAD between the line, and the suitability of the data for detecting the FDCO is discussed. Thirdly, the ideas and specific methods of detecting the FDCO in case of malfunction of GPS/BDS signal are proposed. Furthermore, the validity of the proposed method with simulation data and modified measured data is presented. Finally, the conjectures on the reasons for the approximately constant VPAD deviation in the previous section are discussed.

Accurate transmission line parameters are the basis of power system calculations. Aiming at obtaining accuracy online transmission line parameters in the case of large random noises even bad data in Phasor Measurement Unit (PMU) measurements, which occurs frequently in the practice, a new adaptive robust identification method combining adaptive linear neuron (ADALINE) and traditional robust IGG (Institute of Geodesy & Geophysics, Chinese Academy of Sciences) method is proposed. In detail, first, the identification model of transmission line parameters is presented based on the multi-period PMU measurements at both ends of the transmission line. Then, a parameter solving model based on ADALINE is established. Furthermore, to fully use measurement information, the adaptive robust ADALINE (ARA) are proposed, which applies the robust IGG weight function (Scheme I) to ADALINE to realize “three segments” robust identification. In addition, to improve the robustness, the expectation and variance of the equation residual sequence are estimated adaptively with the median principle to adjust the threshold for the IGG function to assure robustness (TAR), which is independent of the known information for the error of the measurement equipment. The cases based on PSCAD simulated data and measured data show the effectiveness and engineering practicality of the proposed method.

Accurate transmission line parameters are the basis of power system calculations. The measured phasor measurement unit (PMU) phase angle data at both ends of a line may contain large errors caused by synchronization problems, which can seriously affect the accuracy of parameter identification. This paper proposes a robust PMU-based method for calculating transmission line parameters from PMU data at both ends of a line in such a way that a synchronization error between the ends does not degrade the results. Specifically, a π-equivalent model for the transmission line is established, and a least square objective function with nonlinear equations for positive parameter identification is derived. Furthermore, to reduce the impact of noise and biased data, median estimation is used to obtain the final result. Finally, a simulation shows the effectiveness and robustness of the proposed method, and its practicality is demonstrated in a case study using measured PMU data.

Alzheimer’s disease is a chronic neurodegenerative disease that usually starts slowly and gradually worsens over time. Although there’s no cure for Alzheimer’s disease yet, a number of recent researches have shown that the early diagnosis and intervention could not only improve the quality of life but also help to slow the progression of the disease. Clock Drawing Test (CDT) is one of the commonly used clinical methods for screening cognitive impairment, due to its simplicity and convenience. In this paper, we’d like to introduce DCDT, a novel Clock Drawing Test system based on digital collection and intellectualized analysis. We first introduce the background of AD and CDT, and then describe the DCDT system from the external and internal aspects. Finally, the demonstration scenario is described briefly.