1. Insulation Structure Types and Classification
Power transformers typically adopt an oil-paper insulation structure, where the combination of oil and paper significantly enhances insulation strength, reduces the size of the insulation structure, and cuts costs. The insulation structures are mainly divided into three types: covering, insulation layer, and insulation barrier. In terms of insulation classification, it includes main insulation (such as insulation between high-voltage windings, winding-to-ground insulation) and longitudinal insulation (insulation between different points within the same winding and between lead-out wires).
2. Insulation Design Requirements and Principles
2.1 Basic Requirements
The coil insulation structure of transformers must meet fundamental requirements such as electrical strength (low partial discharge), thermal strength (unobstructed oil flow, no local overheating), and mechanical strength (strong short-circuit resistance) to ensure long-term safe and reliable operation of the transformer.
2.2 Design Principles
To achieve a reasonable insulation structure with cost-effectiveness and reliable safety, it is essential to use "field" analysis software (e.g., electric field, magnetic flux leakage field, temperature field, and electrodynamic analysis) to optimize the insulation structure and determine the minimum safe insulation distance and the most economical insulation arrangement. Conventional products can be fine-tuned based on mature reference structures, while special products without ready experience must be analyzed and verified using such software, particularly electric field analysis tools.
The design is based on the "thin paper tube, small oil gap" theory, aiming to reduce the electric field strength on the conductor surface and adjacent oil gaps, ensuring the withstand field strength is lower than the breakdown field strength, and adopting measures to minimize partial discharge and prevent surface creepage.
2.3 Main Insulation Arrangement
Main Airway Arrangement: The main airway is composed of oil gaps divided by baffles and 撑条 (spacers). Following the "thin paper tube, small oil gap" theory, the barrier paperboard is typically 2mm thick, and the oil gap between barriers is 6–10mm. The distance of the main airway and the number of barrier layers are related to the voltage insulation level of the windings and the voltage difference between adjacent windings.
End Insulation Arrangement: The surfaces of the coil and the electrostatic ring have the highest electric field strength and must be designed with small oil gaps. The edge of the electrostatic ring is generally chamfered with a 1/4 circle radius. For high-voltage coils, large oil gaps around the electrostatic ring are not allowed. When arranging angle rings, the R-angle should be perpendicular to the electric field lines as much as possible, and positive and negative angle rings should be staggered to increase the creepage distance.
Lead-Out Treatment: For high-voltage lead-outs, measures such as adding copper tubes, wrapping insulation, and installing lead-out angle rings or protective slots are used to reduce the surface electric field strength.
3. Components of Insulation
The coil insulation of transformers mainly consists of insulating paper, insulating pressboard, and other formed insulating components. The materials are generally made of sulfate fiber paper, and the insulating pressboard is typically T4 hardboard with densification treatment to enhance mechanical strength. Insulating components must have smooth edges with rounded corners, no burrs or cracks, and be free of moisture and foreign matter. They include paper tubes and barriers, spacers, cushion blocks, paper rings, end rings, electrostatic rings, angle rings, etc.
4. Coil Arrangement Types
4.1 Concentric Windings
High-voltage and low-voltage windings are nested on the same iron core, with the low-voltage winding usually inside and the high-voltage winding outside for easier insulation. For large-capacity transformers with high output current, the low-voltage winding may be placed outside. Concentric windings are widely used due to their simple structure and easy winding, and they can be divided into cylindrical, spiral, continuous, and interleaved types.
4.2 Overlapping Windings
High-voltage and low-voltage windings are alternately arranged on the iron core. This structure has complex insulation and heavy wrapping work but offers good mechanical performance, convenient lead-out arrangement, and low leakage reactance. It is generally used in furnace transformers with voltages of 35kV and below. Most transformers place the low-voltage winding inside the high-voltage winding for better insulation and easier tap adjustment of the high-voltage winding.
5. Coil Oil Duct Design
Coil oil ducts include radial and axial types. Layer-type coils have only axial oil ducts, while disc-type coils have both. Cooling oil flow patterns include vertical oil ducts, vertical and horizontal oil ducts, and forced oil circulation guided ducts. During design, ensure unobstructed oil flow, control the flow rate of the lower oil guide holes within 0.5m/s, and seal the openings in the barrier around the lead-outs to prevent oil leakage and charge accumulation in oil flow.
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