Voltage transformers (VTs) must be highly reliable as they are essential components of the electric power industry. VT failures can cause a significant impact for important facilities such as nuclear power plants. In fact, VTs which passed the international standard, were damaged and sometimes resulted in the shutdown of the nuclear power plants. In order to improve the reliability, VTs with various iron core was studied. The electromagnetic field simulation, a saturation characteristics test, and an overvoltage breakdown test were conducted. The result shows that the magnetic flux density significantly affects the reliability of VTs. To increase the reliability of VTs, it is required to reduce the magnetic flux density by increasing the knee-point and voltage factor. Owing to limited installation space, increasing the size of the iron core is difficult. Instead, VT by decreasing the thickness of the conductor and increasing the number of turns, it was possible to reduce the magnetic flux density without the size change. Therefore, improved VTs were obtained by decreasing the thickness of the conductor and increasing the number of turns, and the improved reliability was verified through a comparison test with existing VTs. The fabricated VTs shows a significantly higher saturation voltage and breakdown time than existing VTs. Thus, by proposed VTs can contribute the stable operation of the nuclear power plant and improve the manufacturer’s quality.
