Xiangning Lin

A Novel Scheme to Identify Symmetrical Faults Occurring During Power Swings

A novel, fast unblocking scheme for distance protection to identify symmetrical faults occurring during power swings has been proposed in this paper. First of all, the change rate of three-phase active power and reactive power being the cosine function and the sine function with respect to the phase difference between the two power systems during power swings has been demonstrated. In this case, they cannot be lower than the threshold of 0.7 after they are normalized. However, they will level off to 0 when a three-phase fault occurs during power swings. Thereafter, the cross-blocking scheme is conceived on the basis of this analysis. By virtue of the algorithm based on instantaneous electrical quantities, the calculation of the active and reactive power is immune to the variation of the system power frequency. As the integration-based criterion, it has high stability. Finally, simulation results show that this scheme is of high reliability and fast time response. The longest time delay is up to 30 ms.

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.

Analysis of Electromagnetic Transient and Adaptability of Second-Harmonic Restraint Based Differential Protection of UHV Power Transformer

An autotransformer is used as the main type of UHV transformer, but the internal fault model of autotransformer provided by most simulation software is not available. To find out the existing problems of the application of the differential protection of UHV transformer, a three winding autotransformer model is firstly built according to the existing Unified Magnetic Equivalent Circuit transformer model provided by EMTDC software. Furthermore, the internal fault model of UHV transformer is set up. By virtue of above models, the energization and internal faults are simulated to validate the well-applied differential protection using second-harmonic blocking. It can be proven that the second-harmonic ratios of three phase inrushes of the UHV power transformer are all below 10% in some energization scenarios. In this case, the maloperation of the differential protection by means of 15%~20% second-harmonic restraint ratio cannot be avoided. On the other hand, the second-harmonic ratio of the fault current is relatively high in the beginning of fault inception for some light faults, leading to the short time delay of operation of protection. This discovery will encourage the research and development of the novel scheme of the main protection of UHV transformer.

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

An Adaptive Operating Characteristic to Improve the Operation Stability of Percentage Differential Protection

Based on the comprehensive analyses of the existing current differential protection criterion, a new type of adaptive restraint coefficient-based criterion is put forward in line with a variety of fault conditions. The restraint coefficient of this criterion can be adaptively adjusted according to the operation and fault conditions of the system. In this case, this criterion can guarantee the stability of the protection during external faults. Taking into account the increasing applications of current differential protection in the extremely high voltage or even ultra-high voltage transmission systems, the work performed in this paper is provided with better performance in the case of external faults and various high-resistance internal faults than the existing criterion. Hence, it is promising to apply the current differential protection for 750-kV and above transmission lines. Using Electromagnetic Transients Program software, a simulation model based on 750-kV transmission lines in the Northwest China power grid is established to validate the proposed scheme. All of the real data are provided by the utility of the Northwest China power grid and China Northwest Electric Power Design Institute. The simulation results show that the proposed scheme has satisfactory operation performance.

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.

Theoretical Fundamentals and Implementation of Novel Self-Adaptive Distance Protection Resistant to Power Swings

To deal with the maloperation of distance protection in the case of power swings, the criterion of “concentric circle” was widely used. This criterion is implemented based on the time difference that the apparent impedance locus passes through boundaries of these two impedance circles. In many cases (e.g., a very fast power swing), it will fail to block. Actually, with regard to different moving loci, the time difference staying within the gap between two circles and the corresponding time staying within the internal operating circle have an inherent relationship. In this paper, the aforementioned relationship is deliberately assumed and demonstrated. The worst condition for the correct action of this criterion is presumed and proven, leading to the invention of novel distance protection which can operate rapidly during power swings. The necessary issues on the implementation of the proposed criterion are discussed. Based on the result of the discussion, complete self-adaptive distance protection is put forward. On the basis of the aforementioned analysis, the availability and the feasibility of this novel scheme are verified with the results of EMTDC simulations.