Investigation on the magnetic bias current and electromagnetic force of power transformer in multi-layer soil area

Abstract

When HVDC transmission lines operate in the monopole ground return mode or asymmetric bipolar mode, DC current flows in the earth and DC magnetic bias will arise in neutral point grounding (NPG) transformers.In this work, the model to calculate the potential distribution for multi-layer soil structure is set up, then the magnetic bias current (MBC) is calculated. The magnetic field distribution in the NPG transformer is obtained according to MBC, and electromagnetic energy and force in the transformer are calculated. Finally, the influence on MBC, electromagnetic energy and force caused by soil parameters are investigated. Results indicate that thickness of the first layer and resistivity of the second layer have a greater contribution to MBC, electromagnetic energy and force. To avoid excessive vibration of the NPG transformer, the soil at grounding electrode with a greater thickness in the first layer and lower resistivity in the second layer is preferred.through the neutral grounding points and DC magnetic bias arises. Secondly, geomagnetic storm caused by the interaction of geomagnetic field and solar plasma wind, generates an induced current, which flows through the NPG power transformers, finally DC MBC will come up [6-9].Many researchers have been concentrated on the studies of DC magnetic bias, and great achievements have been obtained. In [10], DC bias experiments on power transformers are conducted. The experimental results show that the vibration and audible noise of the transformer will change under the DC magnetic bias current. Siti et al. also conducted some experiments to investigate the effect of DC magnetic bias in power transformer, the results show that harmonic arises with the DC current, and the even harmonics are significant in DC magnetic bias rather than in normal AC supply [11]. Baguley et al. measured the core loss, and the results show that there is a strong correlation between core loss and the magnetostrictive vibration caused by the DC magnetic bias [1]. Hasan et al. researched the magnetizing current waveforms of single phase power transformer under DC bias by simulation and experiment, and results show that the waveforms will distort when the DC bias exist [12]. In [13], the influence of transformer DC magnetic bias was analyzed using the equivalent coupled electric circuit and magnetic circuit method, which was verified by tests. In [14], Li et al. used Wavelet Packet Transform (WPT) to analyze the vibration signals which were obtained by experiment, and the energy eigenvectors were obtained. Results show that the energy eigenvector of vibration can reflect DC magnetic bias level in the transformer, hence it can be used as the criterion to determine the relationship between vibration levels and influence degree of DC magnetic bias to power transformers. In [3,6,15], the mechanism and influence of DC magnetic bias were analyzed, then some suppression measures were introduced, for example, using resistors or capacitors in transformer grounding wire; connecting compensation device;...