Paper Covered Wire in Power Plant Transformers

Paper-covered wire is the core insulated conductor for windings in oil-immersed transformers. Its structure consists of a conductor (copper or aluminum, round or flat wire) wrapped with multiple layers of insulating paper—such as cable paper, crepe paper, Dennison paper, or aromatic polyamide paper (Nomex)—and impregnated with insulating varnish. Power plant transformers—including main transformers, station service transformers, excitation transformers, and start-up/standby transformers—operate under demanding conditions: high voltage ratings (10.5 kV, 35 kV, 110 kV, 220 kV, 500 kV), large capacities (tens to over one thousand MVA), long service life (30–40 years), and harsh operational environments (oil temperature 95–105°C, short-circuit stress, overvoltage events). These requirements impose systematic technical demands on paper-covered wire regarding material selection, insulation performance, transposition capability, and mechanical properties.

This document addresses material selection, technical specifications, application scenarios, manufacturing processes, and failure modes of paper-covered wire for power plant transformers—providing technical reference for design, operation & maintenance, and procurement engineers.

I. Functional Role of Paper-Covered Wire in Power Plant Transformers

Paper-covered wire serves the following critical functions in power plant transformers:

1. Electrical Insulation Function

In oil-immersed transformers, paper-covered wire operates within a “turn-to-turn insulation + oil duct insulation” configuration. The insulating paper and transformer oil constitute a composite cellulose-oil insulation system, providing turn-to-turn, phase-to-phase, and ground insulation. The dielectric strength of insulating paper (typically 1.5–3 kV per layer, thickness 0.05–0.13 mm) synergizes with the dielectric properties of mineral oil to determine the winding’s voltage withstand rating.

2. Current Carrying and Current Collecting Function

The copper (or aluminum) conductor carries the operating current. In large power plant main transformers, low-voltage winding currents often reach several kiloamperes—for example:

110 kV / 50 MVA main transformer: LV-side current ≈ 260 A;
220 kV / 240 MVA main transformer: 35 kV-side current ≈ 3.9 kA.

This necessitates large conductor cross-sectional area, rational current density, and controllable temperature rise.

3. Mechanical Support and Short-Circuit Resistance Function

During system short circuits, main transformers experience enormous electromagnetic forces (proportional to the square of fault current). Paper-covered flat wire, after hot-press forming, forms a robust winding structure capable of withstanding axial and radial short-circuit forces—per GB 1094.5 and IEC 60076-5 short-circuit withstand requirements.

4. Heat Dissipation and Oil Duct Flow Function

Oil ducts (Oil Ducts) are maintained between paper layers of the wire, enabling transformer oil to remove heat from windings via convection. Oil duct width (typically 2–6 mm) is determined by insulating paper width and winding arrangement, directly influencing temperature rise—governed by GB 1094.2 and IEC 60076-2 temperature rise limits.

II. Key Performance Indicators of Paper-Covered Wire for Power Plant Transformers

2.1 Conductor Material and Geometry

Common conductors used in power plant transformers include:

Electrical Round Copper Wire** (TR round wire, diameters Φ 1.0–Φ 6.0 mm);
Electrical Flat Copper Wire** (TBR flat wire, thickness a = 0.80–5.60 mm, width b = 3.00–16.00 mm), compliant with GB/T 5584.2 and IEC 60317-0-2;
Aluminum conductors (LR round wire / LBR flat wire), applied in large transformers to reduce cost and weight, compliant with GB/T 5584.3.

Conductor requirements:

Purity ≥ 99.9% (copper) or ≥ 99.5% (aluminum), conforming to GB/T 3953 and ASTM B49;
Resistivity ≤ 0.01724 Ω·mm²/m (at 20°C for copper);
Surface must be smooth, free of burrs, oxide scale, pits, or other defects.

Surface treatment of the conductor significantly affects paper adhesion quality and insulation strength—copper conductors are commonly subjected to “roughening” or “passivation” treatments to enhance enamel coating bonding.

2.2 Insulating Paper Types and Thermal Classes

Common insulating paper types and their corresponding thermal classes used in power plant transformers (per ANSI/NEMA MW 1000-2018, GB/T 7672, and IEC 60317-0-2) are listed below:

Paper Type	Thermal Class	Thickness Range	Key Characteristics	Typical Application
Cable Paper (DLZ)	105°C (Class A)	0.05–0.13 mm	Cellulose-based, low-cost	Small- to medium-size oil-immersed transformers, LV windings
High-Voltage Cable Paper (GDLZ)	105°C (Class A)	0.05–0.13 mm	High-density, high dielectric strength	Windings rated ≥ 110 kV
Dennison Paper	105°C (Class A)	0.05–0.10 mm	High mechanical strength, low dielectric loss	Transposed conductors for ≥ 220 kV systems
Crepe Paper	105°C (Class A)	0.05–0.20 mm (after stretching)	High elasticity, cushioning effect	Lead wires, end-winding insulation
Aromatic Polyamide Paper (Nomex 410/414)	220°C (Class C)	0.05–0.76 mm	Exceptional high-temperature resistance, mechanical strength, flame retardancy	Class H dry-type transformers, high-temperature oil-immersed transformers

Key Performance Indicators of Insulating Paper:

Electrical strength ≥ 8 kV/mm (in air)
Tensile strength (MD ≥ 5 kN/m)
Elongation (MD ≥ 8%)
Moisture content ≤ 8% (as-manufactured)
Ash content ≤ 1%
pH value: 6.0–8.0 (neutral)
Dielectric constant (oil-impregnated state): ~3.5–4.0
Dielectric loss factor (tan δ) ≤ 0.005 (at 100 °C, 1 kHz)

2.3 Wrapping and Lapping Requirements

Wrapping method for paper-covered wire: Paper tape is helically wrapped with 30–60% overlap; total layers range from 2 to 10 (determined by voltage class). For high-voltage windings of 500 kV power plant main transformers, single paper-covered flat wire insulation thickness may reach 2.0–4.0 mm (total paper tape thickness), equivalent to 16–32 layers of 0.075 mm-thick paper.

Wrapping quality requirements:

Uniform paper tape tension (to prevent loose or tight wrapping);
Lapping positions must be staggered (no multi-layer lapping at the same axial location);
No conductive contaminants, fiber burrs, or air gaps permitted between paper layers;
Paper tape joints must be flush and free of wrinkles;
Post-wrapping wire diameter uniformity: ±0.05 mm.

2.4 Impregnation and Drying

After winding, paper-covered wire must undergo vacuum pressure impregnation (VPI) or atmospheric-pressure varnish impregnation:

Varnish viscosity (25 °C): 300–500 mPa·s;
Impregnation time: 30–60 min;
Vacuum level: ≤ 100 Pa;
Impregnation pressure: 0.2–0.6 MPa;
Curing temperature: 130–160 °C (Class B) or 150–180 °C (Class F/H).

Post-impregnation, paper-covered wire exhibits significantly improved insulation resistance, dielectric strength, moisture resistance, and mechanical strength. Impregnation quality shall comply with GB/T 11026 and IEC 60851-5.