Qixun Yang

The Evaluation of Phasor Measurement Units and Their Dynamic Behavior Analysis

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.

Hybrid Non-linear State Estimation with Voltage Phasor Measurements

In this paper, a new method of hybrid non-linear state estimation with PMU is proposed. Firstly, the voltage phasor measurements are transformed into indirect equivalent branch current measurements, with which the conventional state estimator takes the impact of PMU into consideration. Secondly, the indirect measurement errors caused by the measurement transforming are analyzed and calculated in detail. Thirdly, the accuracy of hybrid estimator is analyzed in the paper. The simulation results show that the proposed method improves the accuracy of estimator and the convergence speed also.

The PMU dynamic performance evaluation and the comparison of PMU standards

Synchronized phasor measurement is one of the most important information sources for smart transmission grid, which provides a new path for power system dynamic monitoring, analysis and control. Therefore whether the PMU (phasor measurement unit) can track the status of the power system, especially under the dynamic condition, is crucial. In order to evaluate the performance of PMU, the latest Chinese PMU standard including dynamic specifications is outlined, and the comparison with the IEEE C37.118.1 is made. On the basis of this, the performances of PMUs from three main vendors in China are tested and evaluated. The PMU test system is presented and the detailed results are discussed, which reveals the problems existed and the suggestions are made to improve the algorithm of PMU.

Flow transferring identification algorithm with consideration of transient period

Cascading trips caused by flow transferring are crucial for lots of well-known blackouts. For a power system with heavy load flow, once there is a fault, the system might suffer flow transferring after the fault clearing. However, for relays, they cannot know the sensing overcurrent is caused by internal faults or flow transferring. The relays will trip the overcurrent after the predefined time delay, which might result in flow transferring again and worsen the system state. Therefore, if relays pickup due to flow transferring, the back-up relay should be blocked until the thermal limit of the line reaches. In this paper, Flow Transferring Relativity Factor (FTRF) is suggested to estimate the flow distribution after a line is tripped. Moreover, based on the relationship of node injection and branch currents, the impact of transient period on the flow distribution is analyzed. Finally, a flow transferring identification algorithm is presented to distinguish flow transferring from internal fault caused by fault clearing. The simulation results of 10-generator 39-bus New England system show that the proposed flow transferring identification algorithm works well.

Adaptive loss of field protection based on phasor measurements

Loss of field protection is of great importance for the security of large-scale generators. With the development of large-scale generators and corresponding loss of field protection, the complicated structure leads to the high possibility of loss of field faults. According to the statistical data, more than sixty percentages of the synchronous generator faults are the excitation system faults, which might cause loss of load, tie-line tripping, instability, collapse and etc. Therefore, it is crucial to identify loss of field as early as possible and take countermeasures. In loss of field protection, the impedance boundary criterion of steady state stability limit is widely used to distinguish loss of field and normal condition. However, the steady state stability limit circle is affected by the system operating modes, which might result in mis-operation of the relay. In this paper, an approach for on-line identifying system equivalent impedance is presented based on PMU measurements. The steady state stability limit circle can be adaptively fit for operating modes. The computer simulation results demonstrate that the proposed method works well.

Impact of transient response of instrument transformers on phasor measurements

The instrument transformer is the necessary equipment which connects the primary power system and PMU. Therefore, the measurement accuracy of PMU is directly affected by the transfer accuracy and the transient response of instrument transformer. In this paper, the dynamic signal models are abstracted from the phenomena of practical power systems. Under such kinds of dynamic signal models, the impact of instrument transformer on PMU is analyzed in detail, especially for the saturation problem of the current transformer and the ferromagnetic resonance and transient response of capacitor voltage transformer. Based on the analysis, the suggestions for the evaluation criteria of PMU dynamic behavior can be provided. Moreover, the principles of choosing instrument transformers in the wide-area measurement systems are suggested as well.

Ground Fault Location Using the Low-Voltage-Side Recorded Data in Distribution Systems

Ground fault location and isolation are challenging in compensated distribution systems due to the fact that the fault current is too small to be measured. Considering that massive intelligent electronic devices and “smart meter” are installed in the distribution networks with the development of the “smart grid,” this paper proposes a fault location method using the low-voltage-side smart metering unit recorded data. The developed method uses the faulted negative-sequence voltage and locates the faulted sections by applying the relationship between fault distance and the clustered measurement groups. The method is tested using multibranched distribution systems, and results show that it is suitable for systems with both passive loads and distributed generations and not influenced by the system harmonic distortions, measurement noises, and loss of communication from one or even more of the metering units. The proposed method uses the wide-area measured data but does not require any synchronization. Accompanied with the low-cost contactors, the method can locate and isolate the fault sections without interrupting the healthy system operations, and this makes it feasible for industry application.

Ground Fault Distance Protection for Paralleled Transmission Lines

Paralleled transmission lines on the same tower can increase the power transmission capacity but bring challenges to the protection unit. The conventional distance protection schemes can easily be influenced and malfunction. This paper investigated the most commonly occurred ground fault scenarios and proposed two first zone distance protection schemes based on the measurement from only the single line of one terminal for single-phase ground faults and cross-country ground faults separately. A 500 kV and 300 mk parallel transmission testing system was built in the PSCAD/EMTDC with the parameters referring to a real operating site. The results derived from the simulation show that the proposed method can provide good fault distance estimation accuracy for different ground fault scenarios with variable fault resistance. Compared with the traditional double-circuit line protection method, the proposed method which requires the same equipment investment and offers better performance is more suitable for industry application.

The sectionalized homogeneous model of power systems and its analytical solution

With the development of power grid interconnection, once a disturbance occurs, the propagation of the disturbance might cause cascading events and even large blackouts. Therefore, the reveal of disturbance propagation mechanism is of great importance for the power system security. Electromechanical wave (EMEW) theory attempts to explore the power system dynamics from the point of view of wave mechanics and open a new path to the mechanism of disturbance propagation. In this paper, a novel approach is presented to model the power network as a sectionalized homogeneous medium (SHM) and depict the process of disturbance propagation. The impact scope of the moment of inertia of a generator is analyzed to make the generators lump mass to be homogenized. Then the SHM of a chained inhomogeneous power system is proposed. Furthermore, the corresponding equation for electromechanical wave propagation is established. The analytical solution of the proposed equation is derived. The formulations and detailed derivations are given in the paper. The computer simulations are carried out in a chained power network and the results demonstrate the effectiveness of the proposed model and its analytical solution.

Dynamic state estimator with Phasor measurements for power system electromechanical transient process

Wide area measurement system (WAMS) has been widely used in power system dynamic monitoring and control, such as low frequency identification and control, stability assessment and preventive control, etc. However, as a measurement tool, Phasor Measurement Unit (PMU) has the measurement error and bad data unavoidably. If the raw measurements are applied in power system analysis and control directly, the unexpected consequence might be resulted in. Aiming at this problem, a novel dynamic state estimator for generator state variables is proposed in this paper. The corresponding mathematical model is given in detail. Bad data detection and elimination approach is presented as well. The simulation results show that the proposed method can provide the estimated trajectories of the generators in real-time, which eliminates the impact of measurement error and bad data and could give the backbone for WAMS based dynamic applications and real-time control.