Wei Cong

New approach for online fault resistance estimation

In order to estimate fault resistance accurately in real-time, a new approach is proposed based on the emerging synchronized measurement technology, using synchronized data sampled at both line terminals. This new approach uses n type equivalent model and time-domain signals during the fault to estimate fault resistance online by solving differential equations of the circuit. The approach does not require the information of fault location and would be more robust, accurate, flexible, and cost-effective than those approaches that do require the information of fault location. Simulations on a 500kV two-end system are run by PSCAD/EMTDC to verify the effectiveness and accuracy of the proposed approach.

Study on Electrical Digital Simulation System Following IEC61850

Power system real-time digital simulation system, which is widely used in China, plays an important role in stability analysis, security assessment, relay protection equipments test, etc. With electronic transformers, D/A conversion and power amplification are no longer needed, output data by simulation software can be directly sampled. The schematic diagram of the new digital simulation system with electronic transducer is proposed in this paper. The interface converter connecting simulation system and digital equipment secondary and its modules are presented. A method to test the validity of the new digital simulation system is proposed. GTNET has been developed to provide real time communication to and from the simulator via Ethernet. The suggestion has a primary theoretical significance and has important values for promoting the application of electronic transducer, the RD digital simulation; interface; data transformation.

Analyses and judgment methods of single-phase broken-line fault for loaded distribution line

The frequency of broken-line fault occurred in distribution network is increasing over the past years. Broken-line fault will create negative sequence and zero sequence voltages, which will seriously affect the normal work of loads. However, distribution network has not equipped with special protection against broken-line fault yet. In this paper, single-phase broken-line fault in distribution network is studied, features of the voltages at both ends of the fault line and the current flowing through the fault line are analyzed by using symmetrical component method combined with phasor diagrams when various single-phase broken-line fault occurs, and it proposes the main criterion and the additional criterions based on the theoretical analyses, as well as a method to judge the section that the broken-line fault located. Finally, simulation analysis is carried out through PSCAD/EMTDC, which ensures the correctness of the theoretical analysis and the protection criterions.

Distribution substation planning method based on weighted Voronoi diagram

The characteristics of weighted Voronoi diagram and its application in power network planning introduced, according to target annual total load, existing substation capacity and candidate substation capacity type, determine the maximum and minimum number of the new substations, set loop variable, use integer programming optimization technique to obtain the combination of the a new station capacity. According to with or without the existing station, produce the initial location of the new substation respectively based on the regular Voronoi diagram method and the coordinate geometry method given the comprehensive consideration of planning region terrain features, regional area and load distribution; combine with the weighted Voronoi diagram and alternative location-allocation algorithm to determine the new substation site and the scope of power supply, find out the minimum annual cost correspondingly as the final plan.

Operational analysis of zero-sequence inverse-time overcurrent protection in parallel double-circuit transmission lines

Operational characteristic of zero-sequence inverse-time overcurrent protection when grounding fault occurs within parallel double-circuit lines is analyzed. Results show that in some specific circumstances, protections in adjacent line may malfunction, and sensitivity of protections in both sides of fault line may conflict. An improved method that increases the value of Time Dial Setting of adjacent line protection to avoid malfunction is proposed. However, this will also bring adverse effect to the speed of adjacent line protection. Besides, operation time of protections in both sides of fault line may differ greatly due to system parameters. Therefore, two acceleration schemes based on zero-sequence power direction and zero-sequence current variation are put forward, which can ensure the priority operation of protections in fault line. Then the selectivity and sensitivity of all protections can be ensured. Simulation results have proved the validity of this acceleration program.

Distributed power service restoration method for smart distribution grid

In view of the problems existing in the traditional centralized power service restoration method a distributed power service restoration method for smart distribution grid is proposed. The method is based on the smart terminal unit (STU) arranged at each measuring point of the distribution grid, and through the information interaction and cooperation between the STU, the power service restoration task is completed. Combined with characteristics of distributed system and requirements of service restoration, distributed management of network topology information, network topology equivalence method, searching method of network topology and the simplified calculation method of the constraint conditions are analyzed and studied. The principle of reciprocating search is put forward, and the determination of the theoretical maximum restoration range and the determination of the scope of the actual restoration are completed. A simplified method for solving the constraint conditions of restoration is proposed based on Thevenin equivalent.

Research on distributed wide-area back-up protection system for high-voltage transmission system

At present, the high-voltage back-up protection system is facing the problems of setting difficulty and the complexity of protection coordination causing too long delay. As for the primary system device-dependent configuration principle, it aggravates the complicacy of the secondary circuit which does not have the interchangeability and interoperability, reducing the reliability of the relay protection system. As a result, this article proposes a high-voltage wide-area distributed back-up protection system using circuit breaker IED configuration principle so as to provide comprehensive back-up protection for the circuit breaker-associated primary system. In the article, a protection zoning method based on three typical system structures is demonstrated which is able to simplify IED protection logic and redistrict the protection domain into efficient fault isolation procedures in the substructures. The principle is analyzed under the typical IEEE5 nodes model, and the reliable conclusion is obtained.

Transformer inrush current restraining scheme based on closing voltage amplitude controlling method

A scheme of restraining transformer inrush current based on control by closing voltage amplitude is proposed based on analyzing the influence factors of transformer inrush current and deriving the mathematical relationship between inrush current and influence factors. In this scheme, in order to making sure that the magnetic flux produced by the transformer no-load closing does not exceed saturated magnetic flux, the voltage amplitude should be controlled by selecting the appropriate curve, thus attaining the goal restraining the transformer inrush current. The control function of voltage amplitude is derived from the magnetic flux inequality and the attenuation law of the transient flux component, and the upper and lower limitation of initial value and slope in the voltage amplitude control function is deduced.

Methodology to differentiate type of single-phase line break fault in 10kV ungrounded distribution networks

10kV distribution network is always a neutral point ungrounded system. Single-phase line break and grounding fault frequently occurs in 10kV distribution networks, threats the safe and stable operation of power grid and damages peoples lives and properties. A methodology to differentiate type of single-phase line break fault is proposed in this paper, which uses sequence network and symmetrical components method to analyze and summarize the voltage and current variation when various single-phase line break fault occurs, then draw the voltage phasor diagram and current phasor diagram. The voltage and current variation and phasor diagrams will provide criteria for the detection and identification of single-phase line break fault in ungrounded distribution networks. Simulation results show the accuracy of the theoretical analysis in this paper.

A fault detection and isolation algorithm for distribution systems containing distributed generations

For the smart distribution system with distributed generation, a new fault detection and isolation matrix algorithm based on the fault overcurrents and their directions is proposed. Using the correlation matrix to describe the network topology, and the fault overcurrents and their directions from the grids different fault detect points to form the fault information matrix, the fault section can be diagnosed quickly and precisely through the calculation of the two matrices. With the current network structure and the breaker status, the fault isolation can be realized. On this basis, a modified algorithm suitable for network topology changes is proposed by modifying the fault information matrix and the breaker trip vector.