Papers by Keyword: DC Bias

Abstract: The investigation of the direct current (dc) bias effect on single phase power transformer through their harmonics performance is present in this paper. The experimental study is done by the dc bias current injected simultaneously with alternating current (ac) source to primary winding of transformer. The input ac voltage to primary winding is maintained constant during investigation and the dc currents increased gradually. Compare to pure ac power applied, the results show that the waveforms distorted when the dc bias exist. The pulsated waveforms are pushed to half cycle in which the bias current is in the same direction as magnetizing current (unsymmetrical pulsated) and even harmonics are significant.

Abstract: In this paper, a method for earth potential computation of layered soil structure is discussed. To calculate thermal field of the transformer with dc bias, simulation models using MAXWELL and COMSOL are proposed. Based on the electromagnetic analysis, electric power loss resulted in temperature rise is calculated. Under the case of normal operation of transformer, a research was carried out to compare the abnormal changes of temperature of transformer. It worked out that if there are different DC parts in transformer, the loss will increase with DC bias and the temperature of transformer core will also increase. If it is increasing continuously and no preventive measures are taken soon, the thermal aging of some parts in transformer will speed up and gradually affect the normal operation of transformer. The research lays the groundwork for further research of transformer DC bias and aging.

Abstract: The study on characteristics of transformer vibration caused by GIC is the foundation of studies on transformer noise caused by GIC, location selection of monitoring vibration and noise. In this study, the characteristics of low-frequency vibration of the core of single phase transformer sets caused by GIC have been analyzed through building mathematical models and experimental analysis. The results show that when DC bias occurs the vibration signals contain 50 Hz and 150 Hz vibration. With the degree of DC bias the 50 Hz vibration grows by linear increase and 150 Hz vibration grows by square increase. Affected by GIC, the 50 Hz and 150 Hz vibration waveform’s phase changes with alternate 0 andπ. The characteristics of 50 Hz and 150 Hz vibration discriminate between core vibration caused by GIC and HVDC grounding currents. This study has significance for vibration monitoring of transformer faults, and can assist in determining the level of GIC.

Abstract: When calculating the grounding current under DC bias, there are many uncertainties. Then an efficient calculation method based on point estimate method is proposed. This method can make the results more reasonable because it takes into account the uncertainties of input quantity and conditions. This paper will value the grounding resistances as variables, and use the point estimate method for the calculation of the grounding current under DC bias. The results are proved to be more realistic and satisfactory.

Abstract: Transformer vibration research is an effective diagnosis of DC bias fault. The vibration signal acquired is actually the mix signal of vibration from both the core and windings, and this makes it difficult to analyze the above two separately. Therefore the paper presents a new approach to separate vibration signal of core and windings by Fast-ICA algorithm. The simulated results of core and windings vibration are given and compared with experiment test. Comparative analysis verify that transformer vibration in no-load is core vibration and vibration in short circuit is windings vibration. Transformer vibration signals under different DC bias are tested by experiment. Vibration signals are separated by Fast-ICA algorithm. Core vibration signal and windings vibration signal are respectively obtained. The research lays the groundwork for further research of transformer DC bias vibration.

Abstract: Solar storms and power engineering HVDC, make the earth there geomagnetic induced current and direct current. If the DC current into the neutral grounding transformer, the transformer will be caused bias magnetic phenomena, changing its excitation characteristics, interfere with the normal operation of power systems and equipment. This paper considers the impact on the excitation characteristics of hysteresis effects, establishing excitation characteristics calculation model, proposes an improved graphics corresponding method to calculate and analyze the open circuit excitation characteristics of single-phase transformer in three columns under DC bias, the experimental results prove the validity of the proposed method.

Abstract: As the voltage level to improve, DC bias problem of power transformers increasingly prominent. When the DC current affects the transformer, transformer laminated core magnetization characteristics inside will be affected by magnetic saturation increased. Aiming at this problem, the model of the load side and a smart experimental platform are established, for the measurement of related data such as the excitation current under DC bias. Based on MATLAB software, analysis summarized the laws of physics, to provide a reference for theoretical research.

Abstract: With the wide application of high voltage direct current, it also created some problems, such as transformers dc bias. This article describes the impact on the transformer of the operation of HVDC monopole mode, introducing the model used in the soil and the ground electrode potential distribution, it also calculates the ground voltage in the case of a single or double ring grounding .Besides this paper describes the potential level of the two layers of soil structure calculation method and uses this method to analyze the influence on 220kV station of the 500kV Tian-Guang HVDC monopole operation mode. The conclusion is that the operation mode has seriously affected the station.

Abstract: An important source of transformer core vibration is magnetostriction of the thin silicon-steel lamination. In the power grid, direct current (DC) may form by solar magnetic storms and DC transmission. Transformer core supersaturation and harmonic increase in current caused by DC bias will intensify magnetostriction and produce additional transformer core vibration. In order to obtain transformer core vibration under DC bias, the magnetization curve and magnetostriction characteristics of thin silicon-steel lamination are measured. Then, this paper establishes a numerical magneto-mechanical strong coupling model including magnetostrictive effect under DC bias for three-phase three-column dry transformers. On the basis of the proposed model, and the measured magnetic characteristics of the thin silicon-steel lamination, transformer core vibration under DC bias and no-load condition has been calculated. In order to verify the proposed model, transformer core vibration under DC bias and no-load condition is studied experimentally. Numerical calculation results together with experimental results confirm the validity of the proposed method.

Abstract: Based on PSCAD/EMTDC simulation platform, this paper analyses the harmonics caused by the process of rectifier and inverter and DC bias of transformer in Yindong ±660kV HVDC transmission system. The results indicate that, characteristic harmonics of 660kV HVDC system AC bus give priority to 12k±1 time harmonics and bring serious influence to the AC system of the rectifier and converter station; the characteristic harmonics of DC lines are mainly about 12k time and the voltage ripple factor is in the specified scope; both the odd and even harmonic components are increasing significantly under DC bias than that without DC bias,which thereby makes the power transformers become the severe harmonic sources.