What is the function of transformer bushings?

What is the function of transformer bushings? What performance characteristics do we need?

Transformer bushings are a common accessory. Whether it’s an oil-immersed transformer or a dry-type transformer, you’ll see this component on the outside. Today, we’ll mainly introduce their performance and function, and also provide some related information: Regarding bushings,

There are generally two types: capacitive and non-capacitive. Bushings with a rated voltage of not less than 35kV are generally capacitive bushings. Why this distinction? Mainly because their materials differ, resulting in different voltage withstand capabilities.

Therefore, we’ll discuss the function of transformer bushings:

1. Function

Bushings need to withstand voltage because the windings need the bushing to pass through the transformer tank. Therefore, if the bushing fails, the transformer cannot operate directly, and the failure could also lead to fires, etc.

Let’s explain in detail: The oil tank of an oil-immersed transformer is made of steel and grounded. The winding leads pass through the tank to connect to the power grid; and the winding leads must rely on bushings to pass through the tank. Bushings are part of the winding leads and are a component of the winding circuit.

2. Performance Introduction:

Generally, oil-immersed transformer manufacturers focus on the performance of bushings, primarily insulation and thermal performance. These two properties determine the stability of its operation and ensure its proper functioning. Below, we will introduce these two points:

A. Insulation Performance
A. Key performance of internal insulation. The key insulation performance requirements for capacitive bushings are: good sealing performance, no leakage; partial discharge level less than 10 pC; stable tan.

B. Key performance of external insulation. The external insulation of a bushing refers to the insulation of the part of the bushing’s outer surface in contact with the atmosphere. The outer surface in contact with oil or SF6 gas is not considered external insulation. The key performance of porcelain bushing external insulation is that the creepage distance of the bushing is compatible with the pollution level of the application environment and has a margin.

(2) Thermal Performance

A. Thermal performance of current-carrying conductors. The thermal performance of a current-carrying conductor refers to whether the conductive rod, cable, and its terminals may overheat when subjected to rated operating current for a long period or overcurrent for a short period. The temperature rise of the cable or conductive rod is related to the current density used; no faults have occurred when used at the rated current. The temperature rise of the terminals is related to various factors such as apparent current density, plating quality (including tin plating, silver plating, etc.), and fastening force. Overheating faults in terminals are generally not caused by structural irregularities, but rather by improper operation during connection.

B. Thermal performance of bushing core insulation. The thermal performance test of bushing core insulation is called the thermal stability performance test in national standards. During the test, the bushing is immersed in transformer oil at (90 ± 2) °C, and the relationship between the bushing core insulation and the applied voltage is measured to ensure the moisture content of the image meets the requirements.