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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. [ABSTRACT FROM AUTHOR]

In this paper the dynamical characteristics of a ferrofluid acceleration sensor are studied experimentally and theoretically. The stiffness coefficient and damping coefficient are key factors in dynamical characteristics of a ferrofluid acceleration sensor. The damping coefficient depends on the viscosity of ferrofluid. The experimental data is very close to theoretical calculation. [ABSTRACT FROM AUTHOR]