Zhengtian Li

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.

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.

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.

Efforts on Improving the Performance of Superimposed-Based Distance Protection

Existing superimposed-based distance protection possibly maloperates due to some extreme conditions, for instance, an external earth fault occurring while the transmission system operates at a large phase difference between the sources at two sides of the protected line. To solve this problem, an adaptive distance relay using the superimposed component and the information of the opposite end is put forward. The shortages of the existing schemes are first investigated, and the scenarios leading to the maloperations of those schemes in theory are clarified. Furthermore, corresponding countermeasures by means of the floating threshold and criterion switching are introduced. In this case, the reliability and the sensitivity of the superimposed-based distance protection are improved. By means of a variety of comprehensive tests, the feasibility of the proposed scheme is verified.

Design of Wind Turbine Dynamic Trip-Off Risk Alarming Mechanism for Large-Scale Wind Farms

The wind power in China is being developed mainly in terms of large scale, long distance, and high clustering. In this scenario, the uncertainty of wind farm operation gradually becomes a significant factor that power grid dispatcher needs to deal with. Among which, trip-off risk of wind turbine results from system disturbance, fault and fault removal of wind farm, etc. Besides, the geographical distribution of wind turbine usually also has the impact on the trip-off risk of wind farm. In view of above factors, existing security analysis methodology is difficult to satisfy the requirement of real-time alarming due to extreme analytical difficulty and huge computing complexity. Therefore, reasonable design of trip-off risk measure index of wind turbine becomes a key issue to implement a dynamic trip-off real-time alarming system. In this paper, a wind turbine trip-off decision tree system is constructed with the discrete characteristics of wind turbine distribution in wind farms taken into account. The decision trees can perform data mining using online information and make fast prediction on wind turbine voltage out-of-limit and trip-off situations under anticipated accident sets. The trip-off risk measure indexes are then output in light of the prediction results, which can provide intuitive risk level information and decision references for operators in wind farms and power systems. The results of case studies show that the proposed method can meet the requirement of prediction accuracy and resolve the contradictions between the amount of modeled nodes, the diversity of anticipated faults and the computation time when performing over-voltage early warning and control strategies.

Mechanism analysis of DC bias increasing risk of commutation failure

The relation among DC bias, zero-crossing shifting resulting from voltage distortion and commutation failure is analyzed in this paper. It is pointed out that magnetizing current of converter transformer distorts when DC biasing, which not only may cause distortion of AC bus voltage, but increase risk of commutation failure. Based on PSCAD / EMTDC simulation software, China Southern Power Grid AC-DC Parallel Transmission System is established. By means of this model, the above hypothesis is verified with simulation tests. The correctness of theoretical analysis is proved with simulation results.

Similarity Comparison Based High-Speed Pilot Protection for Transmission Line

To solve the following problem: the action speed of protection based on a phasor is restricted by the length of the filtering algorithm time window and the vulnerability to false data, a new pilot protection principle for transmission lines based on waveform similarity is proposed in this paper. By comparing the discrete point sets of current sampling values, a waveform similarity-based algorithm called the Hausdorff distance distinguishes the difference between both side currents of a protected transmission line, realizing fast identification, and removal of the fault. Based on the characteristics of the algorithm, setting methods and protection criterion are discussed. Finally, an improved Hausdorff distance algorithm is proposed to cope with false data. The effectiveness and advantages of the proposed algorithm are validated with theoretical analysis and extensive simulation test results.

A New Distance Protection Method Considering TCSC-FCL Dynamic Impedance Characteristics

Ignoring the dynamic impedance characteristics of a thyristor-controlled series-capacitor-type fault current limiter (TCSC-FCL) may lead to maloperation/fail-to-trip or slow response of distance protection. To solve the above problem, the TCSC-FCL dynamic impedance analysis model was established. Taking the special operation mode of the TCSC-FCL into account, the time-varying characteristics of the TCSC-FCL during the whole process from the series compensation mode to the current-limiting mode were analyzed. Then, this paper proposed the function expression of dynamic impedance characteristics of the TCSC-FCL and put forward the distance protection setting strategy based on a dynamic offset impedance circle in order to eliminate the influence of TCSC-FCL dynamic impedance characteristics to the distance protection operation. By establishing a typical simulation model including the TCSC-FCL, the correctness of dynamic features analytic expression and the distance protection setting strategy is proved with extensive simulation results.

Novel DC Bias Suppression Device Based on Adjustable Parallel Resistances

For lack of the appropriate global distribution of dc currents, the conventional suppression method to suppress dc bias based on capacitive dc blocking device (BD) redirects current to the ground as much as possible, which predisposes to the exceeding neutral current of other transformers in the regional power grid and leads to the contradiction between the power grid corporation and other public enterprises. Therefore, this paper presents a flexible suppression method for dc bias based on a novel dc-bias suppression device. First, a current balancing device (CBD) based on adjustable parallel resistances is designed. The mathematical model for global optimal switching of CBDs is established by a field-circuit coupling method with the equivalent resistance network of an ac system along with the location of substations and ground electrodes. The optimal switching scheme to minimize the global maximum dc current is obtained by gravitational search algorithm. Based on the aforementioned work, we propose a suppression strategy considering electro-corrosion of metal pipelines. The effectiveness and superiority of suppression methods are verified by comparative case studies of the Yichang power grid.