China possesses abundant offshore wind energy resources, with offshore wind power offering advantages such as high operational efficiency, shorter transmission distances, centralized power consumption, and minimal land use. Under the global "dual carbon" goals, offshore wind energy is poised to become a significant focus for China's renewable energy development.
However, the operational frequency of offshore wind power systems is relatively low, which reduces transmission line impedance and significantly enhances power transmission capacity. Compared to onshore wind power, offshore systems operate in harsher environments, facing challenges such as high humidity, extreme temperatures, mold, and salt spray corrosion. These conditions lead to increased maintenance and repair costs, making the selection of suitable transformer materials critical.
As the core equipment in power transmission and distribution systems, transformers play a pivotal role in offshore wind power projects. Transformer cores are typically made of laminated silicon steel, whose properties—such as electrical conductivity, magnetic permeability, core loss, and magnetic flux density—vary with changes in temperature and frequency. These variations directly impact the transformer's operational efficiency. Consequently, selecting appropriate transformer materials is crucial for offshore wind power applications. To investigate the general patterns of transformer losses under low-frequency and low-temperature operating conditions in offshore wind power systems, researchers led by Li Xiang at Shanghai University of Electric Power conducted an in-depth study. This research was supported by the National Key Research and Development Program (2021YFB2401100) and the State Grid Corporation of China Science and Technology Project.
1. Magnetic Property Variations of Silicon Steel Sheets Under Different Temperatures and Frequencies at Low-Frequency Excitation
The researchers employed Epstein frame testing and finite element analysis to measure the magnetic properties of silicon steel sheets. They evaluated the magnetization and loss curves of silicon steel sheets at different temperatures (-40°C, 0°C, 23°C, 40°C, 80°C) and frequencies (50 Hz and 20 Hz).
The findings reveal:
The magnetic field strength at magnetic saturation increases with rising temperatures. However, at the same magnetic field strength, magnetic flux density decreases with increasing temperature. In the unsaturated state, temperature variations have a relatively minor impact on magnetization performance.
The specific core loss of silicon steel sheets is influenced by transformer core operating frequency, magnetic flux density, and temperature. As temperature increases, specific core loss gradually decreases, with the temperature-dependent effect being most pronounced at magnetic flux densities between 1.25 T and 1.75 T.
2. Loss Variations of Transformers at Different Temperatures and Frequencies
Using the finite element method, the researchers simulated the hysteresis loss and eddy current loss proportions of transformers under varying operational frequencies and temperatures. Stray losses were excluded from this study, focusing solely on hysteresis loss proportions.
The results show:
At the industrial frequency of 50 Hz, hysteresis loss accounts for a major proportion of the no-load core loss of silicon steel sheets. At the lower frequency of 20 Hz, the hysteresis loss proportion decreases.
The proportion of hysteresis loss decreases with rising temperatures.
At the low frequency of 20 Hz, hysteresis loss proportions are more significantly influenced by temperature changes compared to higher frequencies.
This research provides essential insights into transformer loss mechanisms under low-frequency and low-temperature conditions, offering guidance for reducing transformer losses and optimizing future designs for offshore wind power systems.
