Hanli Weng

A Fault Diagnosis Method of Power Systems Based on Improved Objective Function and Genetic Algorithm-Tabu Search

Based on the improved optimization objective function of fault diagnosis, a genetic algorithm-Tabu search (GATS) method is introduced for the purpose of fault diagnosis of power systems. The genetic algorithm has good global search ability, while the Tabu search algorithm has good local search ability. Integrating the advantages of these two algorithms, a new hybrid algorithm, called GATS, can be obtained. Using the function of Shcaffer, the advantages of GATS are proven in this paper. The results of case studies show that GATS is superior to conventional methods in terms of the sensitivity of original solution and the dependency of original parameters. The satisfactory results can be obtained when the GATS method is applied in the event of abnormal operations of protective relays and circuit breakers or the multiple-fault scenarios.

A series multiresolution morphological gradient-based criterion to identify CT saturation

The differential relay possibly mal-operate due to the inconsistent transforming characteristics of current transformers (CTs) when experiencing heavy through fault. According to the investigation of CTs of differential protection, the time interval between the occurrence of differential current and the sudden change of phase current can be utilized to distinguish between the external faults and the internal faults. A new CT saturation-blocking scheme for differential protections, which aims at identifying this time interval by virtue of series multiresolution morphological gradient (SMMG) is proposed. Using the excellent singularity detecting together with the noise-repressing abilities of SMMG, this time interval can be identified in real time with high accuracy. The EMTDC-based simulation results show that the scheme can effectively prevent the differential protection from mal-operating meanwhile guarantee the high-speed response to the internal fault.

A novel adaptive single-phase reclosure scheme using dual-window transient energy ratio and mathematical morphology

It is well known that most adaptive autoreclosure schemes are dependent on the voltages sampled from the transmission lines to distinguish between the transient faults and the permanent faults; therefore, the transforming characteristics or required mounting position of the voltage transducer will limit the applications of voltage-based schemes. To overcome this disadvantage, a criterion of a dual-window transient energy ratio (ER) based on mode current to implement the adaptive autoreclosure of transmission lines is proposed in this paper. It is demonstrated that the ER approaches 1 during the steady-state where it increases greatly during some moments; for instance, for the fault occurrence, breaker opening and closing, the existences of primary and secondary arcs, and arc extinguishing. Therefore, the setting of the criterion is easy. To effectively extract the high-frequency energy of the mode current, a novel concept of generalized multiresolution morphological gradient (GMMG) is put forward on the basis of the multiresolution morphological gradient (MMG). The design of GMMG is more flexible and the relative factors of the structure element can be regulated according to the desirable effects. Electromagnetic Transients Program-based simulation results show that the GMMG-based autoreclosure scheme improves the reliability of fault discrimination and promises to be applied to the real power systems

Universal Wavefront Positioning Correction Method on Traveling-Wave-Based Fault-Location Algorithms

A universal wavefront positioning correction method on traveling-wave-based fault-location algorithms is proposed. According to the location accuracy analysis for the faults of which the fault positions change successively, it is noted that the real fault distance generally cannot be divided exactly by the distance corresponding to an integer sampling interval. It will lead to unavoidable truncated error when the discrete sampling points of traveling-wave-based fault-location algorithms are applied. Furthermore, it is disclosed that the fault-location error can be limited within a single sampling interval by the simple wavefront identification method in the event of low sampling rate. In view of the aforementioned two scenarios, an appropriate correction strategy in the event of the appropriate sampling rate is put forward. The fault-location accuracy can be improved greatly with the aid of the proposed correction strategy. The effectiveness of the proposed correction strategy is verified with various typical EMTDC-based simulation test results.

Principles and Implementations of Hierarchical Region Defensive Systems of Power Grid

A hierarchical region defensive system (RDS) based on optical fiber communications is proposed in this paper. This system comprises three layers. The bottom layer is the local measuring unit (LMU), which is equipped in those corresponding substations within this region. The intermediate layer is region decision layer comprising region decision units (RDUs). Every RDS has one RDU, which is connected with all LMUs within this region by means of optic fibers. The system monitor center on the top layer is mainly responsible for harmonizing and monitoring every RDU, recording the fault occurrences and adjusting the protection settings. This system offers safeguard to key region where energy converges inside the power network utilizing multipoint information, which cooperates with conventional main and backup protections. The hierarchical structure and operating principle has been detailed elaborated, especially the working process of measuring elements and decision-making elements. Various complicated fault scenarios within and out of the protected region are simulated using Electromagnetic Transients Program software to validate the proposed defensive system, and the results approve the correction of this system. Finally, the implementation of prototype of region protection system and the scenarios of dynamic simulation tests are provided.

A Dual-Window Transient Energy Ratio-Based Adaptive Single-Phase Reclosure Criterion for EHV Transmission Line

A criterion of dual-window transient energy ratio (ER) to improve the adaptability of the self-adaptive autoreclosure of transmission line is proposed. The redundancy of setting of the criterion can be enhanced in comparison with the criterion of single-window transient energy. To effectively extract the high-frequency energy of the voltages, a novel concept of close-opening - open-closing morphological gradient (COOCG) is put forward. Compared with the multiresolution morphology gradient transform, COOCG has a lower unbalanced output when processing the steady-state signal; therefore, the threshold of ER can be lowered to improve the successful rate of fault discrimination. EMTP-based simulation results show that the COOCG-based autoreclosure scheme improves the reliability of discrimination and is promising to be used in the real power systems.

Studies on the Operation Behavior of Differential Protection During a Loaded Transformer Energization

Recently, several cases of maloperation of the differential protection with second harmonic blocking due to transformer switching on with load have been reported. Quite a few maloperation occurred at the nonrestraint region of percentage restraint plane. The previous theory cannot be utilized directly to analyze this phenomenon. Therefore, a novel model for analyzing the transient course of the loaded transformer energization, together with the CT model involving the magnetic hysteresis effect, is proposed to explain above-mentioned category of maloperation. It is disclosed that the magnetic linkage of CT core can be pushed into the region nearby the saturation point by the aperiodic component of through inrush. When the aperiodic component decays rapidly, the magnetic linkage formed by the periodic component of primary current with low amplitude is not enough to pull the operating point of the magnetic linkage back to the linear region. This phenomenon is named as CT local transient saturation, which results in the big measuring angle error and relative smooth waveform. In this case, the transformer differential protection using second harmonic blocking inevitably maloperates. This point of view is verified with the simulation tests.

Analysis of a Sort of Unusual Mal-Operation of Transformer Differential Protection Due to Removal of External Fault

Several cases of mal-operation of transformer differential protection with second-harmonic blocking after clearance of external fault are reported. These mal-operations all occurred at the nonrestraint region of percentage restraint plane. The previous theory cannot be utilized directly to analyze this phenomenon. Therefore, a mathematical model for analyzing the transient course of external fault inception and removal, together with the CT model involving the magnetic hysteresis effect, is proposed in this paper. It is proved that the magnetic linkage of one CT core can be pushed into the region nearby the saturation point by the high fault current with aperiodic component. As soon as the external fault is removed, the magnetic linkage formed by the primary current with low amplitude is not high enough to pull the operating point of the magnetic linkage back to the linear region. This phenomenon is named as CT local transient saturation, which results in the big measuring angle error and relative smooth waveform. In this case, the transformer differential protection using second harmonic blocking inevitably mal-operates. This point of view is verified with the simulation tests.

Studies on the Unusual Maloperation of Transformer Differential Protection During the Nonlinear Load Switch-In

Several cases of abnormal maloperation of the transformer differential protection with second harmonic blocking scheme are reported, which occurs when a type of nonlinear load, such as furnace, is switched-in to the transformer involved power systems. Different from well-known maloperation due to unloaded transformer energization, this type of maloperation does not occur on the time of load switching-in, but with a considerable time delay. The previous theories, including that interpreting sympathetic inrush, cannot be utilized directly to analyze this phenomenon. In this paper, a model for analyzing the transient course of the nonlinear load switching in, together with the description of the simplified characteristics of the transformer core and the nonlinear load is proposed to explain above-mentioned category of mal-operations. A set of nonlinear differential equations are formed using this model and a 4-order Runge-Kutta algorithm is utilized to solve the flux linkages and currents. The waveform and harmonic characteristic of the differential current are analyzed. It is disclosed that the maloperation of the transformer differential protection during the nonlinear load switching in mainly results from the saturation of transformer core caused by the interaction of the two nonlinear elements. This point of view is verified with the simulation tests.

The Frequency Closed-Loop Control Strategy of Islanded Power Systems

A novel power system frequency control strategy is designed in this paper, which is mainly used to improve the stability of the islanding power system. Detecting the change of circuit breaker status, it is feasible to identify the serious accidents, for instance, a generator outage or disconnection from main grids, and so on. The balanceable power before disturbance and the capacity of the removed power can be used to calculate the power deficiency. Then, appropriate load can be shed adaptively. In the subsequent regulating process, adapting to the various stages of frequency variation, the concepts of rough-adjusting step and fine-adjusting step are proposed to obtain a balance between shedding loads quickly and refining the load shed in every step. The concept of proportional-integral-differential (PID) is introduced to design a closed-loop control strategy to trace frequency variation, with which a balance between restraining the sharp drop of frequency effectively and releasing the reserved system capacity thoroughly can be obtained. Moreover, the frequency recovery curve can be optimized and the dead zone of control during the frequency rising stage can be avoided. Compared with the conventional under-frequency loading shedding (UFLS) schemes, the effectiveness of the control strategy presented has been verified by EMTDC simulation tests.