Abstract
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.
Highlights
AC transmission becomes inefficient at distances longer than approximately 60 km [2], it is necessary to switch to High Voltage Direct Current (HVDC) transmission for sea-based wind farms
The temperature dependency of ester oil/Oil Impregnated Pressboard (OIP) conductivities are lower, which causes the transformer to be less sensitive to temperature variations during energization or variable loading conditions
The design aspects regarding a combined DC/AC transformer for a cost-effective integration of wind farms to land without the need for a huge and expensive power transformer and converter stations have been presented
Summary
The potential to harness energy from wind is enormous and offshore wind power generation is one of the most rapidly developing methods to utilize this potential. Today’s HVDC concept of offshore wind farms require a transformer and a converter station platform, which serves as the hub for the collection network and as a connection point of the HVDC cable to land. This solution is economically inefficient, and a reduction in costs of platforms is highly desirable
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