What types of transformer bushings are commonly used? Explain its structure and performance. There are three commonly used bushings for transformers: pure porcelain bushings, oil-filled bushings, and capacitor bushings.

Pure porcelain bushing: This is a simple type of transformer bushing with a skirt on the outer surface and a cavity inside. When installed on a transformer, the interior is filled with oil. For 20~35kV cases, use large skirt plates near the flange to prevent flashover. The higher the voltage, the thicker the magnetic wall is required. However, manufacturers of thick ceramic sleeves face difficulties; it is difficult to reduce the electric field strength. Moreover, pure ceramic bushings are not suitable for use at higher voltages.

Oil-filled transformer bushings: These bushings mainly use transformer oil and edge paper pipes as barriers, rather than pure ceramic bushings. Thanks to these improvements, the distribution of electric field strength and other properties have greatly improved compared to the original pure ceramic bushings. Due to its simple structure and easy manufacturing, this type of bushing is used in voltage levels.