Abstract
The potential distribution of windings under impulse voltage is very important for the design of transformer inter-turn insulation especially for large capacity transformers such as ultra-high voltage direct current (UHVDC) converter transformer. Quite a lot of equivalent circuit models for transformer winding have been proposed for the potential distribution calculation assuming that the influence of magnetic core is negligible at frequencies higher than 10 kHz. However, lightning impulse or VFTO waveforms usually contain abundant frequency components higher than 10 kHz. At above situations the magnetic core plays an important role during the transient procedure. To obtain a more comprehensive model and also to provide a more accurate potential distribution of transformer winding, in this paper, a wide frequency magnetic properties of silicon steel sheet were measured and the relationship between relative permeability of lamination stack and frequency is studied and implemented in the calculation of frequency-dependent parameters such as resistance, self- and mutual-inductances. Then the equivalent circuit model of UHVDC converter transformer is established considering the properties of core lamination stack. Coding the program in MATLAB to solve the matrix equation and the potential distribution properties are extracted from the calculation results under lightning situation. The inter-turn potential distribution is also analyzed and the results may provide more accurate information for transformer inter-turn insulation design.
Highlights
The rapid development of ultra-high voltage power grid, the popularity of gas insulated substations, the large-scale wind farm grid connection and the realization of AC/DC interconnection network, justifies the continuing trend toward higher transmission voltages and lager capacity of power transformers with increased complexity
The potential distribution of windings under impulse voltage is very important for the design of transformer inter-turn insulation especially for large capacity transformers such as ultra-high voltage direct current (UHVDC) converter transformer
Quite a lot of equivalent circuit models for transformer winding have been proposed for the potential distribution calculation assuming that the influence of magnetic core is negligible at frequencies higher than 10 kHz
Summary
The rapid development of ultra-high voltage power grid, the popularity of gas insulated substations, the large-scale wind farm grid connection and the realization of AC/DC interconnection network, justifies the continuing trend toward higher transmission voltages and lager capacity of power transformers with increased complexity. An UHVDC converter transformer was taken as the object of study and the broadband magnetic properties of silicon steel sheet were measured and applied to calculate the parameters of the equivalent circuit. The research results indicate the effects of the magnetic core on the transient oscillation and provide a more accuracy potential distribution prediction for transformer inter-turn insulation design. Transformer considering the effects of iron core, the broadband magnetic properties of the silicon steel sheet should be measured and modeled. It is obvious that the fitting error of the exponential fit is significantly smaller than the linear fit This non-linear relationship is hard to implement in the software to calculate the parameters of the equivalent circuit, but it can be used for theoretical analysis in future research. Eq (1) will be implemented in the material properties of the transformer model in the finite element calculation software to calculate the inductances and resistances of the equivalent circuit
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