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Basic Knowledge of Transformer Bushings

Purpose of Transformer Bushings: The primary purpose of transformer bushings is to bring the leads from the transformer coils out to the external environment. These bushings serve as an insulating device for the leads from the oil tank and also function as a fixing device for the leads.

During transformer operation, bushings continuously carry load currents and, in the event of an external short circuit, carry short-circuit currents. Therefore, transformer bushings must meet the following requirements:

1. They must have the specified electrical strength and sufficient mechanical strength.

2. They must have good thermal stability and withstand instantaneous overheating during a short circuit.

3. They should be compact, lightweight, have good sealing performance, strong versatility, and be easy to maintain.

External Structure of Bushings:

The external components of a bushing include a terminal board, lead connector, rain cover, oil gauge, oil plug, oil reservoir, upper porcelain sleeve, lower shield, lifting ring, oil valve, nameplate, vent plug, connecting bushing, lower porcelain sleeve, and equalizing ball.

Internal Structure:

1. The main insulation is composed of a multi-layer cylindrical capacitor core made of oil-impregnated cable paper and aluminum foil equalizing electrodes, with porcelain acting as the external insulation and a container for the transformer oil.

2. The bushing is a fully sealed structure, with its internal transformer oil forming an independent system unaffected by atmospheric conditions.

3. The overall connection of the bushing uses a strong spring mechanical fastening, ensuring both sealing and compensation for length changes due to temperature variations.

The oil reservoir at the top of the bushing is used to adjust for changes in oil volume due to temperature variations, preventing significant pressure changes. The oil gauge on the oil reservoir monitors the oil level during operation. The equalizing ball at the tail end improves the electric field distribution, reducing the insulation distance between the tail end of the bushing and grounded parts or coils.

The small bushing on the end screen of the oil-paper capacitor-type bushing is used for capacitance and dissipation factor tests and partial discharge tests of the transformer. During normal operation, this small bushing should be reliably grounded. Care must be taken to prevent rotation or pulling out of the small bushing rod when disassembling the end screen's small bushing to avoid lead disconnection or damage to the copper foil on the electrode plate.

Arrangement of Three-Phase Transformer Bushings:

From the high-voltage bushing side of the transformer, the sequence from left to right is labeled as follows:

High voltage: O, A, B, C

Medium voltage: Om, Am, Bm, Cm

Low voltage: O, a, b, c

Classification of Bushings by Insulation Material and Structure:

Bushings can be classified into three types:

1. Single Insulation Bushings: This category includes both pure porcelain and resin bushings.

2. Composite Insulation Bushings: This category is further divided into oil-filled, gel-filled, and gas-filled bushings.

3. Capacitor Bushings: This category includes oil-paper capacitor bushings and resin-paper capacitor bushings.

Oil-Paper Capacitor Bushings:

Based on the current-carrying structure, oil-paper capacitor bushings can be classified into cable-through and conduit current-carrying types. Among these, conduit current-carrying types can be further divided into direct and rod-through types based on the connection method of the oil-side terminal and the bushing. Cable-through and direct conduit current-carrying bushings are widely used in power systems, while rod-through oil-paper capacitor bushings are less common.

The capacitor core of capacitor bushings is formed by tightly wrapping a layer of insulation, consisting of cable paper with a thickness of 0.08-0.12 mm, around a hollow conductive copper tube. Subsequently, a layer of aluminum foil with a thickness of 0.01 mm or 0.007 mm is wrapped around this insulation layer as a capacitor shield. This process is repeated alternately with cable paper and aluminum foil until the required number of layers and thickness is achieved.

This creates a multi-layer series capacitor circuit where the conductive tube has the highest potential, and the outermost aluminum foil is grounded (ground shield). According to the principle of series capacitor voltage division, the voltage between the conductive tube and ground equals the sum of the voltages between each capacitor shield. The voltage between capacitor shields is inversely proportional to their capacitance, ensuring that the total voltage is evenly distributed across the entire insulation of the capacitor core. This results in bushings that are compact and lightweight.

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