Introduction to Transformer Bushings

The following is a detailed introduction about transformer bushing:

1. Function

  Insulation isolation: Isolate the high and low voltage winding leads inside the transformer from the external circuit, ensuring insulation between the leads and between the leads and the transformer casing, preventing current from directly passing through the box and grounding, and ensuring the safe operation of the electrical system.

Mechanical fixation: The lead wire is fixed to the transformer box through a sleeve to prevent it from loosening or falling off during transportation, installation, and use.

  Current carrying transmission: As one of the current carrying components of transformers, the load current passes through the transformer for a long time during operation. When a short circuit occurs outside the transformer, the short-circuit current also needs to be passed through.

2. Classification

  Classified by main insulation type：

  Capacitive bushing: Install a conductive or semiconductive layer inside the main insulation to achieve the required potential gradient. According to the different types of main insulation capacitor cores, they can be divided into adhesive paper type, adhesive impregnated paper type, oil immersed paper type, and gas impregnated type.

Non capacitive conduit: According to the main insulation medium, it can be divided into liquid insulation conduit, gas insulation conduit, inorganic material conduit such as porcelain or glass, cast or molded resin conduit, and composite insulation conduit.

  Classified by voltage level：

  Low voltage level bushing: For transformers of 10kV and below, pure ceramic bushing is commonly used, which has a simple structure, low cost, and can meet the insulation and fixing requirements of lower voltage levels.

Medium voltage level bushing: Oil filled bushing is commonly used in 35kV transformers. By filling the porcelain bushing with oil, the insulation performance can be improved, which can effectively meet the insulation and current carrying requirements at this voltage level.

High voltage level bushing: High voltage transformers above 100kV usually use capacitive bushings, whose capacitor core structure can better evenly distribute the electric field and withstand the electric field stress under high voltage.

  Classified by external insulation medium：

  Oil air bushing: The lower part of the bushing is in the transformer oil inside the transformer oil tank, and the upper part is in the air, which is the most common combination of external insulation media.

Oil SF sleeve: Utilizing the excellent insulation properties of SF gas, combined with transformer oil, it can be used in some special requirements.

  Oil oil sleeve: The upper and lower parts of the sleeve are both immersed in transformer oil, and it is used as a structure for cable outlet terminals.

  SF ₆ - SF ₆ casing: The entire casing uses SF ₆ gas as the insulating medium and is generally used in specific high-voltage or ultra high voltage equipment.

3. Connection method

  Cable through type: The lead wire of the transformer winding passes through the center hole of the bushing and is connected to the conductive rod or cable inside the bushing. This method is suitable for transformers with high current and high voltage levels, and can reduce losses and heat generation at the connection point.

Guide rod current carrying type: can be divided into direct type and through rod type. Direct type refers to the direct connection between the wiring terminal in the oil and the sleeve; The pole through type is achieved by passing a conductive pole through a bushing to connect with the internal leads of the transformer, and is commonly used for small and medium-sized transformers.

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