Abstract:

The grounding current of HVDC(high voltage direct current)converter station causes the most significant interference in the observation of geoelectric field, which usually causes a step change as 0.5~100mV/km within a range of hundreds of kilometers near the grounding pole. However, it is difficult to determine which converter station’s grounding current causes the step change. Taking Hainanzhou-Zhumadian line, Zhalute-Qingzhou line and Baoji-Deyang line as examples, we obtain the response data of three typical disturbances, and calculate the step change using the data of 58 geoelectric stations around these three lines. To compare the interference situation, we referred the extremely low frequency data of Dashan station for comparison with their original curves.

First, we explained the different response modes of the stations at different locations to the ground current. This means that when the stations locate near a ground electrode, in the middle of the two poles and near to one side of the ground electrode between the two poles, the corresponding three response types are: step response, pulse response and pulse+half step response, respectively. In particular, stations located in the middle of two poles but close to one pole are mainly affected by the near pole and less importantly affected by the other pole. Consequently, step response appears in the near pole stations, step recovery appears in the far pole stations, and the final result is in the form of step+pulse.

Then, we use daily variation amplitude to correct the order variable of HVDC interference and locate the position of grounding pole by the principle that the potential difference of multiple stations has directivity, and then determine the source of grounding current and the approximate location of converter station. The step change after diurnal change correction shows a certain trend, which is shown as the quadratic attenuation of the source-station distance. The fitting of the step change observed by a wide range of geoelectric stations confirms this trend. The locating results have good directive effect on the grounding poles’ positions of the Hainanzhou-Zhumadian line, Zhalute-Qingzhou line and Baoji-Deyang line, and by combining the step change synthesis vector of multiple stations, we can simultaneously determine the approximate location of the converter station. In addition, the amplitude of step change after daily variation correction can suggest the site of the ground electrode, which can supplement the locating results.

Furthermore, we build the quantitative diffusion model of the grounding current to show the law of potential distribution of large input current, and determine the interference range and the variation trend. The simulation results show that the potential difference decreases rapidly within 50km near the grounding pole; the potential difference reduction effect is not strong in far-field exceeding 200km and basically maintains a gentle trend. Based on observation data of 58 geoelectrical stations and another station of extremely low frequency, the response characteristics of grounding current to the surrounding stations are identified, which may serve for the data correction of HVDC interference in the future.

Results of the influence of grounding current on geoelectric and geomagnetic field can be further extended to the study of seismic electromagnetic signal. Electromagnetic stations are usually set up near the active fault zone in an attempt to detect electromagnetic signals generated by strong earthquakes. Relying on the observation data, researchers can present a preliminary prediction of strong earthquakes under certain conditions, and provide a spatial range and time scale of the earthquakes. However, the explanation of how the electromagnetic signal near the source propagates to the observation stations is not very satisfactory. In particular, there are anomalies appearing in some distant stations, while no anomalies appear in the nearby stations. It means the differential response is obvious. Moreover, some prediction is generally not logical and physical, which means the abnormal signal may not come from earthquake activity but some other sources. Therefore, it is necessary to study how the signal propagates from the source to the station and why it causes differential response.

Key words: HVDC(high voltage direct current), grounding current, geoelectric field, locating grounding poles, extremely low frequency

摘要：

高压直流输电(HVDC)换流站的入地电流造成了地电场观测中的显著干扰, 通常在入地极附近数百千米范围内会引起很大的阶变。 但判断阶变来源于某个换流站的入地电流是较为困难的, 一般需要借助高压直流线路对地磁场的影响数据来识别。 文中以海驻线(海南藏族自治州—驻马店)、 扎青线(扎鲁特—青州)和宝德线(宝鸡—德阳)为例, 获取了3次典型干扰的响应数据, 对3条线路周边58个地电场台站的数据展开分析, 并使用山东大山台极低频数据作为对比案例。 首先, 解释了不同位置的台站对入地电流有不同的响应方式, 即台站分别位于1个入地极附近、 两极中间以及两极之间靠近一侧入地极这3种情况时, 对应的3类响应分别为台阶状阶变、 脉冲状响应和脉冲+半台阶状响应。 然后, 采用日变化幅度对高压直流输电干扰的阶变量进行校正, 基于多台的电位差具有方向性的原理对入地极进行定位, 判断入地电流的来源和换流站的大致位置。 定位结果对海驻线、 扎青线和宝德线的入地极位置都有较好的指向, 结合多台的阶变合成矢量能够判断换流站的位置; 此外, 经日变化校正后的阶变幅度能显示入地极所在, 可对定位结果进行补充。 进一步建立入地电流的定量扩散电流模型, 展示大电流入地时电位差的分布规律, 判断入地电流的干扰范围和台站响应的阶变量。 基于58个地电场台站和1个极低频台站的观测数据, 文中给出了入地电流对周边地电场台站干扰的特点, 可将其应用于实际观测中对HVDC干扰的数据校正。

关键词: 高压直流输电, 入地极, 地电场, 多台定位, 极低频