Renewable energy sources are typically interfaced to the grid using power electronics, which can cause their fault current characteristics to display significant low-frequency harmonics and unbalanced sequence impedances. Such current characteristics can lead to the operation failure of fault-component-based directional relays. To demonstrate the influence of inverter-interfaced renewable energy generators (IIREGs) on directional relays in detail, analytical expressions for the IIREG equivalent positive- and negative-sequence superimposed impedances are derived in this paper. Considering various factors, the angular characteristics of the sequence superimposed impedances are investigated. Based on these attributes, it can be concluded that fault-component-based directional relays may be unable to operate in some circumstances. A novel high-frequency impedance-based protection scheme is proposed to manage the adaptability problem by determining the fault direction due to a stable impedance angle. The theoretical analysis and the proposed scheme are tested and verified through real time digital simulation and field testing data.

With the increasing penetration of renewable energies, phasor measurement units (PMUs) play increasingly important roles in security control and mechanism studies for power systems. The testing and calibration of PMUs are key to their effectiveness in a power system; therefore, providing valid test reference values with sufficient precision under steady-state and dynamic conditions is critical. This paper proposes a non-linear fitting-based phasor measurement method for a PMU calibrator to provide reference measurements. Two universal phasor-fitting models are built to represent different signals in PMU steady and dynamic testing. An objective function is presented, which obtains the minimum value of the sum of squares of a non-linear real-valued function based on phasor-fitting models. The methods to set the initial points and bound constraints of the parameters to be fitted are stated to make the objective function convex. An iterative technique, finding the local minimizer of the built objective function in the restricted region, is proposed to calculate the phasor, frequency, and rate of change of frequency (ROCOF). A white noise restraining method is proposed to further improve measurement accuracy. Then, the proposed method is implemented into hardware to develop a real PMU calibrator. To verify the accuracy of the PMU calibrator, simulations and experimental testing are carried out. Results show that the effectiveness of the PMU calibrator can meet calibration requirements under typical static and dynamic conditions of power systems with high penetration of renewable energies. The developed PMU calibrator can thus be used to calibrate PMUs.

The paper provides a short history of the phasor measurement unit (PMU) concept. The origin of PMU is traced to the work on developing computer based distance relay using symmetrical component theory. PMUs evolved from a portion of this relay architecture. The need for synchronization using global positioning system (GPS) is discussed, and the wide area measurement system (WAMS) utilizing PMU signals is described. A number of applications of this technology are discussed, and an account of WAMS activities in many countries around the world are provided.

Distributed generations (DGs) with power electronic devices and their control loops will cause distortion to the fault currents and result in errors for power frequency measurement based fault locations. This might jeopardize the distribution system fault restoration and reduce the grid resilience. The proposed method uses high frequency (up to 3 kHz) fault information and short window measurement to avoid the influence of DG control loops. Applying the DG high frequency impedance model, faults can be accurately located by measuring the system high frequency line reactance. Assisted with the DG side recorded unsynchronized data, this method can be employed to distribution systems with multiple branches and laterals.

The phasor measurement unit (PMU) provides synchrophasor measurements for wide area dynamic security monitoring and control. Synchronism of measurements is the essential feature of PMU, whose precision directly affects the accuracy and reliability of phasor measurements. In this paper, the impacts of time synchronization deviation (TSD) on PMU measurements and wide area damping control are analyzed, respectively. Both PMUs in substations and in power plants are included in the analysis. And for each kind of PMU, the impact is elaborated systematically under steady and dynamic states. Furthermore, by studying the TSD's impact on the input signal of a wide area power system stabilizer, the influence of TSD on the effect of wide area damping control based on these two kinds of PMUs is presented. Finally, the validity of theoretical analysis is confirmed through a series of hardware-in-the-loop experiments and simulation results.

With the development of power grid interconnection, disturbance propagation phenomena are more and more apparent in the form of electromechanical waves. Once a disturbance occurs, its propagation might cause cascading events and even large blackouts. With the help of advanced communications networks in smart grids, new event-based protection systems can be devised by the reveal of disturbance propagation mechanisms. In this paper, a widely used measurement-based protection system is analyzed and the idea of event-based protection is proposed. By introducing the inverse process of lumped mass method, the impact scope of generator inertia is proposed. Then, the structural frame model of power networks for disturbance propagation is built, which is sectionalized homogeneously. The spatial heterogeneity of propagation velocity of the electromechanical waves is depicted. According to the constant velocity characteristic of the segments in the structural frame model, a weighted undirected graph is formulated. Then, the Dijkstra algorithm based shortest path searching method for arrival time of electromechanical wave is proposed. The simulations are carried out to demonstrate the effectiveness of the proposed electromechanical wave propagation model and disturbance arrival time estimation algorithm. The research work is one of the essential steps for realizing event-based protection strategies.

Phasor measurement units (PMUs) are taking an increasingly important role in power system dynamic security monitoring and control. However, traditional discrete Fourier transforms (DFTs) used by PMUs cannot obtain accurate phasor measurements during frequency excursion and transient events, being limited by its static phasor model. In this paper, a synchrophasor estimation algorithm for M-class PMUs based on the dynamic phasor model is proposed. In this algorithm, the dynamic phasor within an observation data window is approximated by the second-order Taylor expansion. The linear relationship between the second-order coefficients of the Taylor expansion and the errors caused by the DFT averaging effect is explored. Then, the linear relationship is used to compensate the raw measurement errors under a dynamic condition. Two digital filters are applied not only to eliminate the spectrum leakage caused by the dynamic inputs, but also to make the PMU immune to the out-of-band signals. The algorithm is investigated through simulation, laboratory experiments, and real data from the power system according to IEEE C37.118.1. The test results demonstrate that the proposed algorithm can meet the requirements of the standards.

The dynamic behavior of phasor measurement units (PMUs) is not only the key point to dynamic phasor algorithm improvement but also the basis for the successful implementation of PMUs in power system dynamic security monitoring and control. In this paper, the dynamic performance of PMUs is evaluated, and their dynamic behaviors are analyzed. PMUs from the four largest Chinese manufacturers are tested through the established PMU test system, and their dynamic performance is comprehensively evaluated. The dynamic behaviors of the PMUs are also analyzed. Analysis shows that the spectral leakage and the averaging effect of the discrete Fourier transform (DFT) under dynamic conditions are the key reasons that lead to their dynamic behaviors. The theoretical results are consistent with the experimental data and the simulation. Finally, to eliminate the spectral leakage, the DFT with a digital low-pass filter is adopted to obtain more precise phasor measurements.

A novel distance protection algorithm for the first-zone distance relay of parallel transmission lines is proposed for cross-country grounded faults. The algorithm calculates the fault impedance using one-end sampling voltages and currents from a single line. The theoretical basis of the proposed algorithm is described in this paper, which is based on the symmetrical component method for parallel lines. And an attempt is made to express the zero-sequence current of the adjacent parallel line at the relay location by the zero-sequence current of the concerned line in combination with the fault characteristics of the system model, by which the impact of the mutual coupling between the lines can be eliminated. And, thus, the distance algorithm for cross-country grounded faults is presented. The simulations show that the proposed algorithm can obtain the accurate fault reactance when cross-country bolted faults occur, which is superior to the existing distance protection algorithm based on one-end data from a single line.

State estimator is crucial for on-line power system monitoring, analysis and control. With the increasing use of synchronized phasor measurement units (PMU) in power grids, how to utilize phasor measurements to improve the precision of state estimator becomes imperative. Since there are lots of traditional measurements in SCADA system and it is hard for phasor measurements to replace them in the near future, the best way is to develop hybrid state estimator which includes both phasor and traditional measurements to get better behavior. In this paper, a novel state estimator for including voltage phasors, branch current phasors and traditional measurements is proposed. The detailed model and how to calculate covariance matrix of PMU measurements are described in detail. New England 39-bus system and IEEE 118-bus system are used as test systems and the simulation results demonstrate that the proposed state estimation algorithm improves the precision greatly and gets better behavior as compared with other state estimators with or without phasor measurements.