Electrical conduction in HVDC mass-impregnated paper cable

HVDC cables start playing a more and more important role in interconnecting national grids. This paper deals with the calculation of electric fields in HVDC cables. The calculation of fields in an HVDC cable is far more complex than the equivalent case in HV ac cables. This is due to the fact that the conductivity of the cable insulation is temperature and field dependent and due to the fact that the electric fields under dc voltage may be time-dependent. The field distribution in an HVDC cable may be of a capacitive, intermediate (and time-dependent) or resistive nature. The kind of field depends on the stage the cable finds itself in: for instance, whether the voltage has just been applied, whether a polarity reversal has occurred or whether the field distribution has become stable. For each stage, the method of calculating, together with the computed results on a real HVDC cable are discussed. Usually, the effect of heating of the insulation by the leakage current may be disregarded. However, in certain cases, i.e. the cable temperature and applied voltage are high enough, the field distribution is influenced by these insulation losses. They even may lead to an instability that causes breakdown of the cable. A cable in service may be subjected to impulses superimposed on the dc voltage. The most severe case is that of an impulse superimposed on a dc voltage of opposite polarity. The calculation of the field distribution in this situation also is carried out.

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“…Calculate the distribution of the electric conductivity. 4. Calculate the electric field distribution.…”

Section: Fields Taking Into Account Ohmic Insulation Lossesmentioning

confidence: 99%

Electric fields in HVDC paper-insulated cables

Jeroense

Morshuis

1998

IEEE Trans. Dielect. Electr. Insul.

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113

106

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“…Moreover, insulation materials behave nonlinearly under high DC voltage stresses. In addition, additives such as inhibitors, pour-point depressants, antioxidants or antistatic agents that are added to the transformer oils, introduce large variations of the real conductivity values [33]. Even specific features of production facilities or the season of production and packaging can make an effect.…”

Section: Conductivity Measurementmentioning

confidence: 99%

Green Solution for Insulation System of a Medium Frequency High Voltage Transformer for an Offshore Wind Farm

et al. 2022

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High Voltage Direct Current (HVDC) transmission represents the most efficient way for transporting produced electrical energy from remotely located offshore wind farms to the shore. Such systems are implemented today using very expensive and large power transformers and converter stations placed on dedicated platforms. The present study aims at elaborating a compact solution for an energy collections system. The solution allows for a minimum of total transformer weight in the wind turbine nacelle reducing or even eliminating the need for a sea-based platform(s). The heart of the project is a Medium Frequency Transformer (MFT) that has a high DC voltage insulation towards ground. The transformer is employed in a DC/DC converter that delivers the energy into a serial array without additional conversion units. The insulation design methodology of an environmentally friendly HV insulation system for an MFT, based on pressboard and biodegradable oil, is introduced. The measurement method and results of the measurements of electrical conductivities of the transformer oil and Oil Impregnated Pressboard (OIP) are reported. The measurements show that the biodegradable ester oil/OIP conductivities are generally higher than the mineral oil/OIP conductivities. Numerical simulations reveal that the performance of the insulation system is slightly better when ester oil is used. Additionally, a lower temperature dependency for ester oil/OIP conductivities is observed, with the result that the transformer filled with ester oil is less sensitive to temperature variations.

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“…Additives like inhibitors, pour-point depressants, antioxidants or antistatic agents are added to the mineral or synthetic oils to make them as a reliable medium for a long-time application of power transformers as possible. Dissolution of these additives, or oil and cellulose oxidation products has a considerable effect on the conductivity of insulating oil or oil immersed cellulose [25]. That is why, even for the same type of oil, there is a vast spreading in the conductivity values depending on difference in producers, production facilities, markings or even periods of production.…”

Section: Importance Of Measurement Of Electrical Conductivitymentioning

confidence: 99%