近地空间与电网相关的谐波辐射现象研究

Research on Harmonic Radiation Related to the Power Grid inthe Near-Earth Space

大量卫星探测结果表明近地空间存在与地面电网谐波频率一致的辐射现象,但相应的谐波辐射源仍不确定。通过分析与晋东南—南阳—荆门特高压交流输电工程走向接近的DEMETER卫星轨道上的电场强度探测数据,发现空间存在2~5 kHz频带内的谐波辐射事件,且事件的数量在该特高压输电工程投入运行后急剧增加。地面实测结果表明,特高压输电工程首端确实存在数千赫兹的谐波辐射。利用偶极子等效地面谐波辐射源,基于全波模型,分析了其产生的电磁波在半球空间电离层中的传播特点,并与卫星探测结果进行了对比,表明近地空间谐波辐射现象与地面变电站的谐波辐射具有一定关联。此外研究了甚低频(VLF)电网谐波辐射在南北地磁共轭空间传播的机理,并利用卫星探测实例进行了初步验证。

A large amount of satellite-based observations show that the radiation corresponding to the power grid harmonic frequencies with the strength of more than 10dB greater than background noise exits in the near-Earth space.Some references believe that it is originated from power lines and name it power line harmonic radiation(PLHR).This paper studies the specific source of the power grid corresponding to PLHR by comparing the ground-based measurements with the satellite-based measurements and discusses its propagation mechanism in the ionosphere The spectrogram of the electric field strength observed along satellite DEMETER orbits that point in the same general direction as the Jindongnan–Nanyang–Jingmen ultra–high voltage(UHV)AC transmission line has been analyzed first.The results show after the UHV project went into operation at the end of 2008,the number of PLHR events in the frequency range of 2~5 kHz occurring in the ionosphere increased sharply.The ground-based measurement results show that the harmonic radiations at 2.5~3 kHz and 4~4.5 kHz do exist at the head end of the UHV project.The above results indicate the source of PLHR is closely connected with the UHV project.The full-wave model is used to explore the propagation characteristics of the UHV project's electromagnetic wave in the ionosphere.A horizontal dipole with the length of 200 m and the height of 12 m above the ground is used to equivalent the harmonic radiation source and the ionosphere is regarded as a stratified,linear,anisotropic and uniform plasma.The numerical results show that the electric field strength reaches the maximum values 0.15μV/m of 2.8 kHz at(660 km,530 km)in which 660 km is the altitude of DEMETER and 530 km is the distance away from DEMETER in the horizontal direction and 0.07μV/m of 4.25 kHz at(660 km,540 km),respectively.These values are close to but generally lower than the electric field strength of 0.09~0.27μV/m observed by DEMETER which is 551 km away from the UHV line in the horizontal direction.The difference between the theoretical analysis results and satellite-based measurement results indicates that there is a nonlinear amplification physical process during the propagation of PLHR in the ionosphere.The propagation mechanism of very low frequency waves between the North-South geomagnetic conjugate regions is then investigated to explain the propagation mechanics of PLHR.The propagation path of the wave in the ionosphere follows Snell's law.Lower hybrid frequency f_(LHF) is the critical frequency that only those waves with the frequencies below it can propagate between the hemispheres.f_(LHF) is calculated as 4.9 kHz by using the electron density and ion density observed by the satellite.The frequencies of all detected PLHR events are below f_(LHF),indicating that PLHR might theoretically cross the equator and propagate to the conjugate hemisphere.This procedure enables PLHR to be enhanced near the equator.Actually,PLHR events at approximate 4 kHz have been detected over the UHV project and its conjugate region near western Australia and the strength of events at the southern hemisphere is close to or even larger than that at the northern hemisphere,but the radiations at approximate 50 Hz have been detected only over the UHV project.Due to the complexity of the ionospheric plasma environment,it is challengeable to figure out the formation mechanism of PLHR and its resulting electromagnetic effects at present.However,with the development of UHV power transmission and transformation projects,its resulting radiations in the near-Earth space deserves more attention.