Research On Calculation And Evaluation Methodology Of DC Bias Effects Under Peninsular Geological Conditions

In recent years, HV/UHVDC transmission technology has been developed rapidly in many regions of China. When HVDC project operates in monopolar mode or violent geomagnetically induced currents (GICs) appear, huge direct current will flow through the ground. Severe DC bias effects will be brought to the pivotal electromagnetic devices of power systems such as power transformers and current transformers, and the safety, stability and reliability of the power system will be affected. So, it has important practical significance and application value to study the occurrence, mechanism, calculation method, evaluation criterion and effective restrain measures of DC bias effect.Connected with the ongoing±660kV Ningdong-Shandong HVDC project, especially the DC bias effects under peninsula geological conditions, exploration and innovation research was done. The main aim is to research the fundamental problems including determination of the calculation method of earth-surface potential (ESP) and underground DC current distribution, the effects of DC bias on the transfer characteristics of both metering and protective current transformers, the transformer magnetized modeling theory with DC bias, the evaluation criteria of power transformer to withstand DC bias and so on.Against the peninsula geology circled by sea with any angle, based on the construction of peninsula geology model, extended images method was proposed. Using positive power transformation, points of 3d space can be mapped into 2d planar, and then the corner domain with any angle can be transferred into upper plane of another space with conformal mapping. After the above transform, ESP expressions of peninsula geology, and the ESP and DC current of peninsula can be determined effectively. Based on the specific structure and parameters of Shandong peninsula grid, the neutral point current of 500kV and 220kV systems were calculated when Ningdong-Shandong HVDC project operated with monopolar mode. The above results were compared with literature and the measured ones and it showed that this method is correct and effective. Further analysis of the key influence factors of the potential distribution on ESP and the feasibility of co-electrodes in the peninsula geological conditions were done.The transfer characteristics of electromagnetic current transformer (EMCT) relate to the fairness of power energy metering and the reliability of protective relays. Based on electromagnetic relation, transfer characteristics of EMCT with DC bias were studied. The general formula of EMCT time-to-saturation with the co-impacts of DC bias and hysteresis were deduced, and compensating coefficient was defined to comprehensive characterize their impacts. Based on the method of harmonic balance, the phase displacement and ratio error of EMCT with DC bias were derived, and they were verified by simulation results. This research provided the basis for the relay protection setting and accurate energy metering of EMCT under DC bias conditions.Physical description and exact math expression of iron core hysteresis model are premise of the model validity of electromagnetic device such as power transformers. Based on the magnetized physical mechanism of materials, unreasonable and wrong parts of the classical Jiles-Atherton theoretical kernel parts were pointed out, the correct energy conservation equation was presented through detailed analysis, and the correct J-A theory was deduced. The simulation results of the modified model were in good agreement with those of measurement, that mean the modified model can analog magnetized physical process of magnetic materials more accurately. Simulation model of single-phase power transformer considering hysteresis can be constructed according to the rectified J-A model and the influence of DC bias on the electromagnetic transfer characteristics of transformer can be studied.The evaluation method of the power transformer stability to withstand DC bias was also carried out in this paper. Combined with the output of the DC biased power transformer, effective evaluation criteria of power transformer to withstand DC bias based on the power quality index constrains including voltage adjustment rate and voltage harmonic distortion rate. It was shown that both saturating factor of the power transformer iron core and the load power factor were key elements to influence the power transformer ability to withstand DC bias. Based on series simulation of WSCC three-machine nine-node system, the validity of the theoretical analysis was proved. The relevant conclusions can be used to guide transformer operation, and it also provided effective scientific method to evaluate the power transformer stability to withstand DC bias.Connected with the Shandong peninsula grid, the method of DC bias suppressing with neutral string capacitor was optimized by paralleled lightning arrester and spark gap, which makes the device more reliable. Design parameters were given and suppressing device can be developed.The research achievements in the dissertation have further developed the fundamental theory, analysis method and Innovation techniques of DC bias effects. It provided important reference value for the development of the design of HV/UHVDC projects of China.