Xishan Wen

Analysis of Nonlinear Characteristics for a Three-Phase, Five-Limb Transformer Under DC Bias

Monopole HVDC systems create dc current flow in the earth, which can cause the grounding location potential to rise relative to infinite spot and alters the working point of the transformer core. Because of the nonlinearity of the transformer core, the exciting current produces the amount of harmonics under dc bias. Past research has focused almost on simulation and testing dc bias problems of single-phase transformers. However, due to the complicated magnetic circuit structure in a three-phase five-limb transformer, there are some difficulties in studying its dc bias problem. In this paper, a new model and algorithm are proposed to find the nonlinear characteristics for the three-phase five-limb transformer under dc bias. Maxwells equations are used to replace the magnetic circuit model of the transformer. By combining an electrical circuit with the nonlinear characteristic curve of the core, dc bias problems for a three-phase five-limb transformer are studied. Results show that the waveform of the no-load current and magnetic field intensity of the transformer are distorted under dc current inrush, and low-order harmonics increase with increasing dc current. Finally the interior and exterior nonlinear curve characteristics under dc bias are discussed.

HVDC Ground Return Current Modeling in AC Systems Considering Mutual Resistances

Ground return currents (GRCs) in ac systems generated by HVDC monopolar operations can result in half-cycle saturation of transformers. Considering the ground potential rises of substations, which is represented by mutual resistances in this paper, an improved model of GRC flow in aca systems is presented. This model is verified by the IEEE benchmark test case for geomagnetically induced current (GIC) and the measured neutral direct current and voltage on neutral blocking devices (NBDs). Based on this model, how the dc bias current in ac systems is affected by mutual resistance is analyzed using measured earth resistivities and representative ac systems. Furthermore, the impact of mutual resistances on GICs is discussed. In the end, an enhanced model for optimal configuration of GRC mitigation is also proposed. The actual application of the optimal NBD configuration has been proven to be cost-effective in mitigating the GRC flow.

Potential Compensation Method for Restraining the DC Bias of Transformers During HVDC Monopolar Operation

The earth return currents (ERCs) of high-voltage direct current transmission can cause severe half-cycle saturation of transformers. This study presents a novel and highly beneficial potential compensation method (PCM) to mitigate the detrimental effects of ERCs. The simulation model of the PCM was first derived and verified through a laboratory test. Analysis has suggested an association of network configuration and mutual resistance to the compensatory factor. The proposed method has been applied in two representative substations with different types of transformers in various operating modes. Results show that as opposed to the neutral blocking device (NBD), the PCM can adjust the dc voltage equilibrium of ac systems rather than intervene in the neutral path of transformers. Moreover, the PCM can eliminate the direct magnetomotive force of autotransformers with proper selection of the injected current. An application shows that the PCM can outperform NBDs and line-series capacitors in restraining the dc bias of autotransformers.

Impulse Characteristics of Tower Grounding Devices Considering Soil Ionization by the Time-Domain Difference Method

Knowledge of impulse characteristics of tower grounding devices is important for lightning protection of the transmission lines. In this paper, the time-domain difference method is proposed to analyze the impulse characteristics of tower grounding devices. Combined with the soil ionization model and the soil-critical breakdown field strength, the proposed method is capable of modeling nonlinear ionization of the soil. The voltage-response waveform calculated by the proposed method is smooth, which is superior to the waveform calculated by the frequency-domain numerical algorithm. The reduced-scale experiments of grounding devices with different structures were carried out in the laboratory to validate the accuracy of the proposed method. Furthermore, the critical breakdown field strength used in the soil ionization model is measured by the coaxial cylindrical electrodes.

Effect of vulcanization temperature and humidity on the properties of RTV silicone rubber

In order to study the difference in performance of room temperature vulcanized (RTV) silicone rubber in vulcanization environment with different temperature and humidity, static contact angle method, FTIR and TG is utilized to depict the properties of hydrophobicity, transfer of hydrophobicity, functional groups and thermal stability of RTV silicone rubber. It is found that different vulcanization conditions have effects on the characteristics of RTV silicone rubber, which shows that the hydrophobicity of RTV silicone rubber changes little with the vulcanization temperature but a slight increase with the vulcanization humidity. Temperature and humidity have obvious effects on the hydrophobicity transfer ability of RTV silicone rubber, which is better when vulcanization temperature is 5°C or vulcanization humidity is 95%. From the Fourier transform infrared spectroscopy, it can be concluded that humidity and temperature of vulcanization conditions have great effect on the functional groups of silicone rubber, and vulcanization conditions also have effect on thermal stability of RTV silicone rubber. When vulcanization temperature is 5°C or vulcanization humidity is 15% or 95%, the thermal stability of silicone rubber becomes worse.

Calibration of excitation function measurement based on corona cage test results

Corona cage approaches are crucial for research on the corona characteristics of conductors. Calibration is an indispensable task for determining excitation functions, which are used to predict corona performance of long transmission lines through extrapolation from measurements of short lines in corona cages. In this paper, the amplification factor G is calculated through a frequently adopted method, propagation analysis of high-frequency corona current along a short line. Another convenient calibration method, based on distributed parameter equivalent circuits, is established. The results for G obtained through propagation analysis and equivalent circuits are compared. To verify the rationality of calculation parameters in propagation analysis and equivalent circuits, a calibration experiment based on the excitation caused by a simulated monopulse current was performed. The results of the proposed calibration method and the calibration experiment are in good agreement.

Study on the Effective Ionization Rate of Atmospheric Corona Discharge Plasmas by Considering Humidity

Atmospheric corona discharge plasmas are the most common diversely occurrence in the electric power system. In this paper, four elementary processes occurred in corona discharge are taken into account, including elastic collision, excitation, attachment, and ionization. As one of the most important parameters in corona characteristics analysis, the effective Townsend ionization coefficient is completely calculated by the Boltzmann equation under the consideration of humidity. This paper also derives the electron energy distribution function (EEDF) in water vapor and dry air, respectively. With reference to previous researches and the existing literature, comparisons and validations on ionization and attachment coefficient are conducted. Besides, the effects of humidity, pressure, and temperature on the effective ionization coefficient are also discussed in this paper. It is concluded that humidity can enhance both ionization and attachment. The results show that the ionization coefficient in this paper is well consistent with traditional ones, while there are variations for attachment coefficient. In normal condition, the effective ionization coefficient varies slightly with humidity, but it increases significantly with a decreased pressure or an increased temperature, which cause the change in critical electric field correspondingly.

Statistical characteristics of breakdowns in long air gaps at negative switching impulses

Discharge characteristics in long air gaps follow certain statistical regularities. Statistical methods and finite element simulation were both employed to deeply study discharge characteristics. The statistical results indicate that the height of grounding rod has a slight influence on the 50% breakdown voltage of rod-rod air gaps and that the breakdown time is longer than the wave front time when 20/2,500 μs switching impulse is applied and shorter than the wave front time when 80/2,500 μs switching impulse is applied. Furthermore it can be shown that the distribution of the breakdown voltages in 10 m rod-plane gap follow the normal distribution and that the V-t characteristics of rod-rod gaps is lower than that of rod-plane gaps. Calculation results indicate that the non-uniform coefficient of the electric field increases with increasing gap distance and that the ratio of gap distance to the non-uniform coefficient also increases with the gap distance. The work described in this paper can be used for the insulation coordination of UHV transmission lines.

Electron swarm parameters and Townsend coefficients of atmospheric corona discharge plasmas by considering humidity

Humidity is a critical factor in atmospheric corona discharge. Fluid dynamics models have become a common method to explore the detailed corona discharge characteristics in humid air. However, the models require the specification of some key parameters, such as electron swarm parameters and Townsend coefficients, which strongly depend on the electron energy distribution function (EEDF). In this paper, the EEDFs of dry air and water vapor are compared by solving the electron Boltzmann equation using classical the two-term approximation. Moreover, electron drift velocity in dry air and water vapor are compared and validated. Finally, effects of humidity on the electron swarm parameters and Townsend coefficients are also discussed. The results show that the electron drift velocity in dry air and water vapor in this paper is well consistent with the previous experimental results for a wide range. It is concluded that the humidity could increase the electron mobility coefficient and decrease the electron diffusion coefficient in low reduced electric field, which are insensitive to humidity in high reduced electric field. The strength of ionization and attachment reactions are both enhanced by humidity, and the corona onset electric fields increase with the increase in humidity.

Influence of Deep Earth Resistivity on HVDC Ground-Return Currents Distribution

During monopolar operation or commissioning of high-voltage direct-current (HVdc) links, ground-return current (GRC) leads to power transformer dc bias issues. A GRC model is proposed in this paper, and the penetration depth, penetration ratio, and earth surface potential of various earth models are simulated. The result indicates that the influence of deep earth resistivity increases rapidly by the increasing distance from substations to HVdc electrodes. The simulation results are compared with measured data of GRCs distribution in the Hubei power grid of China. The result demonstrates that the simulation considering deep earth resistivity is credible and the shallow earth model could lead to poor accuracy. Furthermore, earth resistivity measurements are carried out at different geological regions and the deep earth characteristics and the GRCs distribution are quite different. Thus, it can be concluded that to calculate GRCs distribution using uniform earth structure without measurement is not appropriate. Finally, the relationship between accurately measured earth depth and the accuracy ratio of GRCs calculation is analyzed. The result of this paper contributes to determine the depth of earth model in GRCs simulation modeling and earth resistivity measurements.