Abstract
Simulating and modelling electric field dynamics in the insulation of medium- and high-voltage DC electrical systems is needed to support insulation design optimization and to evaluate the impact of voltage transients on ageing mechanisms and insulation reliability. In order to perform accurate simulations, appropriate physical models must be adopted for the insulating material properties, particularly conductivity, which drives the electric field in a steady-state condition and contributes to determining the field behavior during voltage and load transients. In order to model insulation conductivity, polarization, and conduction, mechanisms must be inferred through charging and discharging current measurements, generally performed at different values of electric field and temperatures in flat specimens of the material under study. In general, both mechanisms are present, but one of them may be predominant with respect to the other depending on type of material. In this paper, we showed that models based on predominant polarization mechanisms were suitable to describe impregnated paper, but not polymers used for HV and MV DC insulation. In the latter case, indeed, trapping–detrapping and conduction phenomena were predominant compared to polarization, thus conductivity models had to be considered, in addition to or as a replacement of the polarization model, in order to carry out proper electric field simulations.
Highlights
The trend towards High Voltage Direct Current (HVDC) transmission, hybrid grids, renewable integration, and electrified transportation may cause weaknesses in grid and asset component reliability. This will hold, for electrical insulation systems of existing components, such as cables, whose ageing is affected by voltage waveforms, DC and/or modulated AC, with repetitive or sporadic transients, harmonics, and ripples that might have not been accounted for the design stage
A basic need is the availability of accurate models for electric field calculation in insulation systems, which will help in gaining insights on the risk of triggering extrinsic ageing processes, such as partial discharges, especially during voltage transients [3]
Since conductivity drives the electric field under a DC steady state, and it contributes to determine the field behavior during transients, measurement and modelling of conductivity behavior as a function of electric field and temperature constitutes the basis of building up accurate models
Summary
The trend towards High Voltage Direct Current (HVDC) transmission, hybrid grids, renewable integration, and electrified transportation may cause weaknesses in grid and asset component reliability. From a mathematical point of view, interfacial and dipolar polarization is usually described by the extended Debye model, which postulates the existence of a number of parallel processes, each of which is accounted for by an equation of the form (2) This model is generally employed to fit the polarization and depolarization currents measured under DC excitation in both impregnated paper [6,7,8] and polymers [9,10,11,12,13]. While in impregnated paper there is good confidence that interfacial polarization is the dominant relaxation process, and the extended Debye model can be regarded as a suitable physical description of the material rather than just a mathematical tool, in the case of polymers, its validity may be questionable due to the presence of other mechanisms, such as charge trapping and detrapping. The purpose of this paper was to show that, while the extended Debye model can be a suitable physical and mathematical description for polarization and depolarization currents measured in impregnated paper under DC excitation, it is not as effective, in general, for polymers, for which a new modelling approach is needed
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