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The influence of DC transmission lines on the geomagnetic observation is the urgent problem. Based on the principle of geomagnetic observation instrument, the influence is analyzed, and it is believed that the magnetic field of current on DC transmission line which is attached to the observed magnetic field of geomagnetic observation instrument is the main reason. According to the direction of current on DC transmission line and the relative position between geomagnetic station and transmission line, the influence characteristic is researched. A calculation model for magnetic field of current on polar conductor and a model for ground current in infinite homogeneous soil and in layer soil around earth electrode field are established. Calculated results of the magnetic field strength at different distance from earth electrode and polar conductor show that main influence of DC transmission line on geomagnetic observation is its influence on Z component of geomagnetic, and the influential degree is related with imbalance current of DC transmission line. During the calculation of the influence, the soil conditions and the ground current could be ignored.

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... Earthquake observation is mainly composed of measurements of geomagnetic field, geoelectric field, and soil resistance [3], all of which could be interfered with by operating UHVDC transmission lines. The interference in geomagnetic observation from UHVDC lines has been studied rather well in China, as reported in [5][6][7], where calibration measurements were also proposed. However, there are only a few studies on the interference in geoelectric field observation (GFO) and soil resistivity measurement from UHVDC lines [8]. ...

The existence of ultrahigh voltage direct-current (UHVDC) transmission lines impacts geoelectric field observation (GFO), which further disturbs earthquake observation and prediction. We proposed a mathematic method based on a 3-dimensional layered soil model to calculate the interference and conducted GFO experiments at observatories and field observation sites during the live debugging of a ±800 kV UHVDC transmission line to verify the method. The results obtained by using the proposed method are in good accordance with the experimental results and the maximum error is no more than 15%. Moreover, the experiment proves that grounding current is a decisive factor in the interference from a transmission line to GFO.

In order to reduce the interference on geomagnetic field observation from DC power lines, the observation interference on Z component of geomagnetic field was solved accurately using mathematical calculation method. The geomagnetic observation point coordinate system with observation point as coordinate origin was constructed, and then the unit vector expression equations, which represented each axis of observation point coordinate system in geocentric coordinate system, were deduced. Based on the definition of space vector dot product and the three-dimensional current magnetic field calculation model of power lines, the calculation equation of space current magnetic field vector projected on the direction of Z component vector was acquired. Therefore, the "Interference on geomagnetic observation from DC power lines correction system" was developed. The geomagnetic field observation interference experiment was accomplished by station observation combined with field observation. The results showed that the error of proposed mathematical model and solution methods was less than 5%, while the distance between DC power line and geomagnetic observation station was no more than 10 km.

The interference on geoelectric field observation (GFO) from UHVDC power lines becomes increasingly prominent with the growth of UHVDC transmission projects. Thus, we analyzed the interference by the mechanism of GFO equipment, and found that the potential distribution between the electrodes in soil of GFO equipment was changed by the ground current from ground electrode and the synthesis electric field of power lines. Using the assumption method proposed by Deutseh, we researched the potential distribution in soil regarding to the synthesis electric field and ground ion flows and found that the ion flow in the earth affected GFO. Consequently, we proposed a complex soil model to study the interference on GFO from ground current, derived Green's function of the model, and calculated the interference. The results show that the ground current from grounding electrodes of power lines is the decisive impact factor of interference. Typically, if the allowable interference value is 0.5 mV/km, the effective distance of interference from an operating power line could be above 30 km.

A correction system for geomagnetic observation against interference from the UHVDC power line is developed. The system could offer accurate calculation of interference value and, thus, realize the real-time correction in the Z component observation of the geomagnetic field. Based on the 3-D current magnetic-field calculation model of the dc power line, in which the towers as points and the tower spans as line segments, the calculation equation of the pace current magnetic-field vector projected on the direction of the Z component vector is acquired to calculate the interference value from the power line. Using Microsoft Visual Basic 6.0, the system receives the values of imbalance current on the UHVDC power line and geomagnetic Z component observation through the PC serial port. Consequently the real-time calculation of interference value on the Z component from the power line is realized. Then, the actual geomagnetic observation curve is acquired. During the live debugging period of $pm $800~kV Xiangjiaba—Shanghai UHVDC power line, the high accuracy of the correction system is verified by the joint experiment of the current observation in converter stations and the indoor/infield geomagnetic observation.

The focus in this paper is mainly to characterize the fractal properties denoted by the Hurst exponent of geomagnetic field during quiet and active periods of geomagnetism activity. Analysis is made on the time series of the geomagnetic horizontal component data, H, which were acquired by the Magnetic Data Acquisition System (MAGDAS) developed by the Space Environment Research Center (SERC) of Kyushu University in Japan. The data set covers the quiet and active periods in August 2005 at the equatorial stations of Cebu and Davao in the Philippines. This month had very intense geomagnetic storm with the Dstmin, Kp and Ap indices for the quietest and active days of the month are -20 nT, 2, 6 and -216 nT, 9, 110 respectively. For both stations, the data sampling interval is 1 second for each time periods or one day giving a sample size of 86,400. Using the fast Fourier transform, the power law of the data time series was obtained in the shorter time periods of 10 minutes to 6 hours. This observed power spectrum law exhibit scaling with corresponding Hurst exponents of 0.3-0.5 for quiet periods and 0.5-0.7 for active periods for both stations. For comparison, other fractal techniques were performed which are rescaled range analysis (RS) and detrended fluctuation analysis (DFA) on the data set, and the same ranges of the Hurst exponents were obtained. The work also includes simulation of the similar sized data sets by generating the fractional Brownian motions (FBM) with the Hurst exponents of 0.3-0.7. Our results show that the H component at these equatorial stations is a fractal in nature and become more persistent during active periods of geomagnetism activity. Thus, the Hurst exponent could be used to characterize the geomagnetic time series during quiet and active periods.

The power frequency electric field generated by the transmission line currents was calculated by the quasi-Monte Carlo integration method. Compared with the control values of power frequency electric field, the minimum distances between the AC transmission lines and seismic observation stations were obtained under the typical operating current for different voltage level, different tower structure and different earth resistivity. The simulated results show that: (1) the minimum distances for the AC transmission lines below 500 kV are consistent with the proposed values in GB/T 19531.2-2004; (2) the minimum distances for the 750 kV and 1000 kV AC transmission lines are more rigorous and greatly affected by the earth resistivity.

This paper describes the threat of geomagnetic storms created by solar activity to EHV and UHV power grids based on the experience of the past. It is clear from the available data that the higher the voltage of the AC power system, the more vulnerable that system is to a particular geomagnetic storm. Given a description of the time and spatial variation of the geomagnetic field (past, present or hypothetical), it is possible to model the response of any specific high-voltage grid with a high level of accuracy. Models have been developed and validated in the past, and samples of that work are presented here. Through careful modeling, it is possible to design protection for high-voltage grids against this unique threat.

In most of the studies of electromagnetic effects performed for EMC purposes, the main approach is to study the local effects on equipment, systems or even a single building. It is not usual that analytic methods try to consider the impacts on a large extended network such as a national high voltage power grid. This paper describes such an approach that has successfully modeled several large power grids in Europe, North America and Asia with respect to their response to the very low-frequency transients (time scale of seconds to minutes) due to the threat of geomagnetic storms. This paper will describe the basic modeling accomplished and the validation of the models to geomagnetic storms

Power lines are exposed to low frequency electromagnetic fields produced by natural (geomagnetic disturbance) sources. The resulting geomagnetically induced currents (GIC) produce excessive harmonics, which affect power system operations. The analysis presented makes extensive use of GIC data from different measuring sites in North American power grids and of geomagnetic data from different geomagnetic observatories. At low temporal resolution hourly GIC index shows good correlation with hourly geomagnetic index provided there is close proximity of the recorded sites. For higher resolution (one minute) the difference between time variations in GIC and geomagnetic field is significant. Although it has been widely popular to use time derivative of geomagnetic field as a characteristic of GIC, we show that ground conductivity is a critical parameter that needs to be included. This is especially important for sharp changes in geomagnetic field, for example, during the beginning of a geomagnetic storm (storm sudden commencement)