He Jinliang

Reconfiguration and capacitor placement for loss reduction of distribution system

There are many means in reducing the active power loss of a distribution network, two important methods are feeder reconfiguration and capacitor placement. The minimum nodal voltage method (MNV) and genetic algorithms (GA) are chosen to solve the network reconfiguration problem and capacitor placement problem, respectively. These two means are combined together so that a much better effect of loss reduction can be achieved than either method alone. The methods presented in this paper have been testified in real-time applications in an actual distribution network.

Influence of grain size on distribution of temperature and thermal stress in ZnO varistor ceramics

The nonuniformity of temperature distribution within ZnO varistor ceramics would decrease its energy absorption capability. In this paper, the distributions of current, temperature and thermal stress within the microstructures of ZnO varistor ceramics are simulated using Voronoi diagram models. The results show that the current concentrates through a few paths in ZnO varistor due to the nonuniformity of ZnO grain size and the variety of electrical characteristics of grain boundaries, which induces local high temperature and great thermal stress when injecting impulse current into ZnO varistor, and leads to melting puncture or cracking failure. The influence of the ZnO grain size on the distributions of temperature and thermal stress within ZnO varistor ceramics is analyzed in detail. The energy absorption capability of ZnO varistor ceramics can be greatly improved by increasing the uniformity of ZnO grain size or decreasing the average size of ZnO grains.

Impulse characteristics of grounding systems of transmission-line towers in the regions with high soil resistivity

The lightning protection effects of power transmission lines are related to the impulse characteristics of grounding systems of transmission-line towers. The influences of several main factors on impulse grounding properties are systematically analyzed by simulation experiments according to the simulation theory in this study. Soil resistivity was changed in the region between 100 to 5103 /spl Omega//spl middot/m. The influential regularities of different parameters on impulse grounding resistance and impulse coefficient of different grounding devices for transmission lines in high resistivity region are discussed. By analyzing the experimental results, the authors conclude that the structure of grounding systems should be selected according to soil resistivity, geological structure and technical economy analysis. The fitting calculating formulae of impulse coefficients and power frequency grounding resistances of different grounding systems are acquired. The simulation results are in good agreement with those of field experiments in the literature.

Effects of manganese dioxide additives on the electrical characteristics of Al-doped ZnO varistors

The effects of manganese dioxide (MnO2) additives on the electrical characteristics of the Al-doped ZnO varistors are investigated. The leakage current densities of the samples decrease first and then increase again with the increase of the amount of MnO2 additives. Correspondingly, the nonlinear coefficients of the samples keep on increasing and then decrease. In addition, the donor densities increase monotonously with the amount of the doped MnO2 increasing. The effects of MnO2 additives on the electrical characteristics of the samples are different from the conclusion of previous literatures. The reasons of the distinct effects are supposed to be related with the donor and interface state densities of the samples.

Loss transmission-line model of grounding electrodes considering soil ionization of lightning impulse

A loss transmission-line model with distributed time-variable parameter for the calculation of transient characteristic of a grounding system under lightning to consider the dynamic and nonlinear effect of soil ionization around the ground electrode is proposed. The influences of the length of grounding electrodes and location of feed point on the transient characteristic of ground electrodes are analyzed. Grounding electrodes have impulse effective length under impulse current. The model has been validated by comparing the numerical results with experimental tests.

Electromagnetic Interference on Secondary Systems of Substation Caused by Short-circuit Fault

This paper analyzed the influences of different factors on the electromagnetic interferences applied on the secondary systems due to the short-circuit faults in substation. The secondary cables with sheath grounded in two terminals can effectively suppress the potential difference between the core and sheath of secondary cable. The influence of different factors on the potential difference between the core and sheath of secondary cable and sheath current are systematically studied in power frequency short-circuit fault. Analysis results show, when the secondary cable sheath is grounded in two terminals, the maximum potential differences applied on the insulation layer of the secondary cable can reach about 40% of the ground potential rise.

Lighting electro magnetic field generated by grounding electrode considering soil ionization

Abstract A circuit model with lumped time-variable parameter is proposed to calculate the transient characteristic of grounding electrode under lighting current, which takes into consideration the dynamic and nonlinear effect of soil ionization around the grounding electrode. The ionization phenomena in the soil are simulated by means f time-variable parameters under appropriate conditions. The generated electromagnetic field in the air is analysed by using electrical dipole theory and image theory when the lighting current flows into the grounding electrode. The influence of soil ionization on the electromagnetic field is investigated. *Supported by National Natural Science Foundation of China (Grant Nos. 50407002), and the major state basic research development program of china(Grant No. 2004CB217906)

Equivalent simplified parallel multi-conductors model of shielded control cable in substation

The electromagnetic coupling between the control cable with metal shielding sheath and metal conductor of grounding grids in the substation of a power system threatens the safe operation of an electronic system connected with the control cable. In order to simplifying the electromagnetic coupling analysis of this system, the core-sheath model of a control cable is replaced by a parallel multi-conductor model with theoretical analysis and numerical computation in this paper. From analysis of the resistance and the self-inductance of the sheath, the mutual inductance and the capacitance between the core and the sheath in both models, four nonlinear equations to solve the four unknown parameters in the parallel multi-conductors model are established. A program has also been coded to solve the equations. The results of the parallel multi-conductors model are compared with the analyzed ones of the actual control cable by the PSCAD software package. They have good agreement with each other.

Adopting line surge arresters to increase lightning-withstand levels of transmission-lines

Line surge arresters are applied to increase the lightning-withstand levels of power transmission lines. Here, the lightning-withstand levels, lightning discharge currents and absorbed discharge energies of power line surge arresters are analyzed.<<ETX>>

Impulse characteristics of grounding systems of transmission-line towers

Lightning faults on power transmission lines occur very frequently. Decreasing the impulse grounding resistance of grounding systems of transmission line towers is an effective method to lower lightning incidents. A simulation experiment was adopted to study the impulse characteristics of grounding systems in this paper. The influences of several main factors on impulse properties are discussed from a vast amount of experimental data.<<ETX>>