Mechanical Strength of Paper Covered Wire

I. Introduction: “Mechanical armor” of paper wrapped wires

Paper Covered Wire (PCW) is a core winding material for oil-immersed transformers, oil-immersed reactors, and special motors, and is composed of copper/aluminum conductors + cable paper multilayer coating. In the long-term operation of the transformer, the paper-wrapped wire not only undertakes the function of electrical insulation , but also must withstand huge mechanical stresses –winding tension, compressive force, short-circuit electric power, thermal expansion force, vibration, shock.

Mechanical strength is the second largest core property after electrical insulation for paper wrapped wires.In the event of mechanical failure, the paper envelope will:
– The → breakdown voltage of the ❌ paper layer drops sharply
– ❌ Conductor deformation → inter-turn short-circuit
– → Partial discharge of broken ❌ insulation
– ❌ Lifetime reduction → Transformer early scrapping

In this paper, the 8 major dimensions of the mechanical strength of the paper wrapping line – tensile strength, elongation, bending performance, torsional performance, shear strength, compressive strength, vibration resistance, impact resistance – are systematically combed, and selection guidelines, quality control, and typical cases are given.

1.1 Why Mechanical Strength Matters

Fault Type	Proportion	Direct Cause
Winding deformation	25%	Short circuit electric power
Damaged insulation	30%	Mechanical damage
Turn Short	20%	Vibration/Shock
Life Reduction	15%	Aging/Mechanical Fatigue
Other	10%	–

Core conclusion : More than 50% of transformer failures are directly related to mechanical strength .

1.2 Top 5 sources of mechanical stress

Source	Size	Frequency
Winding tension	5-50 N/mm ²	Manufacturing process
Compression force	1-10 MPa	Manufacturing process
Short Circuit Electric Power	100-1,000 N/mm ²	Sudden
Thermal Expansion Force	0.5-5 MPa	Cycle
Vibration/Shock	1-50 m/s ²	Ongoing/Sudden

1.3 Core Concepts at a Glance

Terms	Definitions
Tensile Strength	Material’s ability to resist tensile fracture (MPa)
Elongation	Percentage increase in length at tensile fracture
Bending properties	Paperless layer breaks/wrinkles after bending
Torsion performance	Paperless layer loosening after torsion
Compressive strength	Ability to withstand axial pressure
Anti-vibration	No looseness/fatigue under vibration
Shock Resistance	No damage under impact
Short-circuit electric power	Electromagnetic force generated by short-circuit current
Modulus of Elasticity	Stress-Strain Ratio
Yield strength	Critical stress of permanent deformation

1.4 History of mechanical properties of paper wrapping lines

As the mainstream material of transformer windings, the mechanical properties of paper wrapped wires are continuously improved with the capacity, voltage level and design requirements of transformers:

• 1900-1950 : Low-voltage distribution transformer (< 1 kV), mainly covered with hemp rope/cotton yarn, low mechanical requirements
• 1950-1980 : Medium voltage power transformer (10-35 kV), cable paper cladding popular, tensile strength required 200 MPa
• 1980-2010 : High voltage power transformer (110-220 kV), high density cable paper + VPI impregnation, tensile 220 MPa
• 2010-2020 : Ultra High Voltage Transformer (500 kV), Nomex + Cable Paper Composite, mechanical requirements fully improved
• 2020 to present : UHV (1,000 kV UHV), smart transformer, new energy transformer, mechanical performance as the core indicator

Core Trends : Higher transformer capacity/voltage and more stringent mechanical performance requirements.

II. Structure and material foundation of paper wrapping

2.1 3 major components of a paper envelope

Section	Proportion	Role
Conductors (copper/aluminum)	70-90%	Conductive, subject to major mechanical stresses
Cable Paper	5-15%	Electrical Insulation, Mechanical Protection
Adhesive (optional)	0-2%	Fixed Paper Layer
Paint film (optional)	1-3%	Enhanced, auxiliary insulation

2.2 Key mechanical properties of conductor materials

Conductor	Tensile strength	Elongation	Flexibility	Resistivity
Soft copper (T2)	200-250 MPa	30-40%	Excellent	1.72 μΩ · cm
Semi-hard copper	250-300 MPa	15-25%	Good	1.74 μΩ · cm
Hard Copper	350-450 MPa	1-5%	Medium	1.78 μΩ · cm
Flexible Aluminum	70-100 MPa	20-30%	Excellent	2.83 μΩ · cm
Semi-hard aluminum	100-150 MPa	5-10%	Good	2.85 μΩ · cm
Hard Aluminum	150-200 MPa	1-3%	Medium	2.90 μΩ · cm

Core Conclusions :
– Soft copper/soft aluminum Elongation 20-40% → Suitable for large windings
– Hard copper/hard aluminum Tensile strength 350-450 MPa → Suitable for small, precision windings
– Paper wrapping usually uses soft copper as a conductor

2.2.1 Conductor Selection Principles

The following factors need to be balanced when choosing a conductor:

Factors to Consider	Soft Copper	Hard Copper	Soft Aluminum
Mechanical Strength	Medium	High	Low
Elongation	High	Low	High
Resistivity	Low	Low	High (1.64 ×)
Weight	Heavy	Heavy	Light (30%)
Cost	High	High	Low
Processability	Excellent	Poor	Excellent

Conclusion :
– Large/high voltage/short circuit resistant → soft copper
– Small/Precision Winding → Hard Copper
– Lightweight/economical → soft aluminium

2.3 Key mechanical properties of cable paper

Cable Paper Type	Thickness	Tensile Strength (Longitudinal)	Elongation	Tear Strength
Low density cable paper (DLK)	0.05-0.10 mm	6-8 kN/m	2-3%	0.3-0.5 N
Medium density cable paper (MK)	0.10-0.20 mm	8-12 kN/m	2-3%	0.4-0.6 N
High density cable paper (HDK)	0.20-0.50 mm	12-18 kN/m	2-4%	0.6-1.0 N
High Performance Cable Paper (HPK)	0.075-0.25 mm	14-20 kN/m	3-5%	0.8-1.5 N

2.4 Overall mechanical properties of the paper wrapper

Covered wire type	Tensile strength	Elongation	Bending radius	Main applications
Copper Covered Wire (Soft)	200-250 MPa	30-40%	1-3 × Diameter	Universal Transformer
Copper wrapped wire (semi-rigid)	250-300 MPa	15-25%	2-4 × diameter	Power transformer
Flat copper wrapped wire	200-300 MPa	20-35%	Thickness 2-3 ×	Large transformer
Circular aluminium wrapped wire	70-100 MPa	20-30%	1-2 × diameter	Small to medium transformer
Flat aluminium wrapped wire	80-120 MPa	15-25%	Thickness 2-3 ×	Distribution transformer

III. Tensile strength of paper wrapped wire

3.1 Definition of tensile strength

Tensile Strength (σb) : The maximum stress that the material can withstand during stretching, in MPa.

Test methods: GB/T 228, ASTM E8, ISO 6892.

3.2 Top 5 Factors Affecting Tensile Strength

Factor 1: conductor material
– Soft copper: 200-250 MPa
– Hard Copper: 350-450 MPa
– Soft Aluminum: 70-100 MPa

Factor 2: conductor cross-sectional area
– Small section (< 1 mm ²): susceptible to defects
– Large cross-section (> 10 mm ²): strength close to the intrinsic value of the material

Factor 3: Number of layers
– Floors 2-4: Paper layer contribution < 5%
– Floors 8-12: Paper layer contributes 10-15%

Factor 4: Packaging process
– Compactness: affects stress distribution
– Overlap: affects local strength

Factor 5: Temperature
– Room temperature: standard intensity
– High temperatures (> 100°C): 10-30% reduction in intensity
– Low temperature (< -40°C): 5-10% increase in strength and decrease in elongation

3.3 Test method for tensile strength

Standard	Method	Specimen
GB/T 228	Room Temperature Stretching	Standard Scale Sample
ASTM E8	Room Temperature Stretching	Round/Rectangular
ISO 6892	Room temperature/high temperature stretching	Standard ratios
IEC 60851	Enameled wire stretching	Straight segments
JIS C3003	Winding wire stretching	Circular wire

3.4 Typical tensile strength

Covered wire specifications	Conductor	Tensile strength	Elongation
1.0 mm copper wrapped wire	Soft copper	210-230 MPa	30-35%
2.0 mm copper wrapped wire	Soft copper	220-240 MPa	30-40%
3.0 mm copper wrapped wire	Soft copper	220-250 MPa	30-40%
5.0 mm copper wrapped wire	Soft copper	230-250 MPa	30-40%
2.0 × 5.0 copper wrapped wire	Soft copper	220-240 MPa	25-35%
3.0 × 8.0 copper wrapped wire	Soft copper	230-260 MPa	25-35%
2.0 mm Round Aluminum Covered Wire	Soft Aluminum	75-95 MPa	20-30%

3.5 Effect of unqualified tensile strength

• Wire interruption during ❌ winding
• ❌ Short circuit when conductor is stretched
• ❌ Long-term operational fatigue fracture
• ❌ Large transformer failure

3.6 Ways to increase tensile strength

Method 1: Use high-strength conductors
– Soft copper (200-250 MPa)→ High strength copper alloy (> 500 MPa)
– Suitable for scenarios with exceptionally large short-circuit currents

Method 2: Increase the number of layers
– Floors 2-4 Floors → 8-12
– Paper layer contributes 5-15% strength

Method 3: Improve the paper wrapping process
– Tightness: the paper layer is even and tight
– Overlap direction: consistent with the direction of force
– Impregnation: VPI impregnation enhances integrity

Method 4: Temperature control
– Decreased acceleration of high-temperature operation
– Controlling temperature rise < 65 K can slow down the intensity drop

IV. Bending performance of the paper envelope

4.1 Core indicators of bending performance

Metrics	Definitions	Criteria
Bending radius	Minimum bending radius that does not cause the layer to break	GB/T 40701
Number of bends	Number of repeated bends to fracture	GB/T 238
Winding performance	Broken paperless layer after tight winding	IEC 60851
Release angle	Release angle after bending	ASTM E290

4.2 Factors Affecting Bending Performance

Factor 1: conductor diameter
– Small diameter (< 1 mm): bending radius 0.5-1 × diameter
– Medium diameter (1-5 mm): bending radius 1-3 × diameter
– Large diameter (> 5 mm): bending radius 2-5 × diameter

Factor 2: conductor material
– Soft copper: excellent bending performance (elongation 30-40%)
– Hard copper: Poor bending performance (elongation 1-5%)
– Soft aluminum: excellent bending performance (elongation 20-30%)

Factor 3: Number of layers and thickness
– The thicker the paper layer, the worse the → bending performance
– Excellent bending performance when cable paper thickness < 0.075 mm

Factor 4: Bending direction
– Longitudinal bending: excellent bending performance (in the direction of cable paper fibers)
– lateral bending: poor bending performance (vertical fiber direction)

Factor 5: Temperature
– High temperature: poor bending performance (evident at > 100°C)
– Low temperature: poor bending performance

4.3 Bending performance test method

Testing	Methodology	Acceptance Criteria
Winding test	10 turns tightly wound on a round bar of specified diameter	No breakage or wrinkling of the paper layer
Repeated Bending	90° Repeated Bending to Fracture	Times > 10
Breakdown after bending	Test BDV after bending	BDV drop < 20%
Microscopy	Post-Bending Slice Microscopy	No Cracks in Paper Layer

4.4 Bending performance typical data

Envelope specifications	Minimum bending radius	Number of bends	Remarks
1.0 mm copper wrapped wire	1 mm	> 30	excellent
2.0 mm copper wrapped wire	2 mm	> 20	good
3.0 mm copper wrapped wire	3 mm	> 15	good
5.0 mm copper wrapped wire	5 mm	> 10	good
2.0 × 5.0 copper wrapped wire	Thickness direction 4 mm	> 10	Good
2.0 mm round aluminium wrapped wire	2 mm	> 20	good

4.5 Impact of unqualified bending performance

• ❌ The paper layer breaks when winding
• BDV drops sharply at ❌ bends
• ❌ Partial discharge onset
• ❌ Transformer insulation breakdown

4.6 Ways to improve bending performance

Method 1: Soft State Conductor
– Soft copper elongation 30-40% Excellent → bending performance
– Hard copper elongation 1-5% Poor → bending performance

Method 2: Using Thin Layers
– 0.05 mm paper with excellent → bending performance
– Poor 0.25 mm paper → bending performance

Method 3: Control the bending direction
– Bend in the direction of cable paper fibers
– Avoid bending in the direction of vertical fibers

Method 4: Bending Radius Control
– Round copper wrapped wire: bending radius ≥ 1-3 × diameter
– Flat copper wrapped wire: bending radius ≥ thickness 2-3 ×

Method 5: Temperature control
– Bending temperature: 20-80°C
– Avoid low temperature bending

V. Torsion and shear properties of paper wrapping

5.1 Torsion performance

Torsion refers to the stress state in which the paper wrapper rotates around the axis. Transformer windings end lead wires , transposition wires , spiral windings , etc. are subject to torsional forces.

Torsional performance indicators :
– Torsion angle: 360°/m
– Twisted appearance: paperless layer loose
– BDV after torsion: < 20% reduction

5.2 Torsion Test Method

Standard	Methodology	Acceptance Criteria
GB/T 239	Torsion Testing	Torsion to Fracture, Broken Paperless Layer
ASTM A938	Wire Twist	No visible damage after twisting 360°
IEC 60851-3	Enameled wire torsion	No wrinkling or loosening of the paper layer

5.3 Factors Affecting Torsional Performance

• The smaller the diameter of the conductor, the better the torsional performance
• The thicker the paper layer, the worse the torsional performance
• Excellent torsion performance of soft copper/soft aluminum
• Overlap direction affects torsional performance

5.4 Shearing performance

Shear refers to the force that the paper wrapper is subjected to in the axial vertical direction , mainly when the winding compression and short-circuit electric power are acting.

Shear strength :
– Copper wrapped wire: 150-200 MPa
– Aluminum wrapped wire: 50-100 MPa
– Paper layer contribution: 5-15%

5.5 Shear performance test method

Test	Method	Standard
Single shear	Cut perpendicular to the axis	GB/T 6400
Double shear	Fixed intermediate shear at both ends	ASTM B769
Punch Cutting	Punch Cutting	ISO 6892

VI. Compression strength and compression resistance of paper wrapping wires

6.1 Sources of compressive stress

In transformer manufacturing, windings need to be compressed to ensure tightness and heat dissipation. Excessive pressing force leads to permanent deformation of the paper wrapped wire and damage to the insulation .

Source of compressive force :
– Manufacturing process: Hydraulic press 1-10 MPa
– Short circuit power: 100-1,000 N/mm ²
– Thermal expansion: 0.5-5 MPa

6.2 Compressive strength test

Testing	Methodology	Acceptance Criteria
Radial compaction	Radial compression between two plates	No permanent deformation when the compression force is > 50 N/mm
Axial compression	Axial applied pressure	No bending when compression force > 100 N
BDV after compression	Test BDV after compression	Decrease < 20%

6.3 Typical values of compressive strength

Envelope Specifications	Compressive Strength	Permanent Deformation Threshold
2.0 mm copper wrapped wire	200-250 MPa	100-150 MPa
3.0 mm copper wrapped wire	200-240 MPa	100-150 MPa
5.0 mm copper wrapped wire	180-220 MPa	80-130 MPa
2.0 × 5.0 copper wrapped wire	180-230 MPa	90-140 MPa
2.0 mm round aluminium envelope	60-90 MPa	30-50 MPa

6.4 Effect of compaction on paper wrapping

• ✅ Moderate compression : enhanced winding strength, reduced noise, improved heat dissipation
• ❌ Excessive compression : permanent deformation, broken insulation, BDV drop

Suggested compaction process :
– Copper wrapped wire: Compression force 5-15 MPa
– Flat copper wrapped wire: compression force 3-10 MPa
– Circular aluminium paper wrapping: compression force 2-8 MPa
– Compaction time: 2-10 min
– Compaction temperature: room temperature or preheat 80-120°C

VII. Anti-vibration and impact properties of paper wrapped wires

7.1 Sources of vibration

Source	Frequency	Acceleration
Normal Operating Vibration	50/60 Hz	1-5 m/s ²
Transformer Excitation Vibration	100/120 Hz	5-20 m/s ²
Short Circuit Shock	–	50-200 m/s ²
Earthquakes	0.5-10 Hz	10-50 m/s ²
Transport Vibration	5-200 Hz	5-30 m/s ²

7.2 Anti-vibration test

Test	Method	Standard
Resonance Scanning	5-200 Hz Scanning	IEC 60068-2-6
Random Vibration	PSD Spectrum	IEC 60068-2-64
Sweep Cycles	10 + Cycles	ASTM D999
Post-Vibration BDV	Post-Vibration Test BDV	Decrease < 20%

7.3 Impact testing

Test	Method	Standard
Mechanical Shock	Half-Sine/Serrated Wave	IEC 60068-2-27
Free Fall	1-3 m Height	ASTM D5276
Post-Shock BDV	Post-Shock Test BDV	Decrease < 30%
Visual Inspection	Macroscopic/Microscopic	No damage

7.4 Factors Affecting Vibration Resistance

• The thicker the ✅ paper layer, the better the vibration resistance
• ✅ The thicker the conductor, the better the vibration resistance
• ✅ Soft copper is better than hard copper
• Improved anti-vibration performance after oil ✅ immersion
• ✅ Impregnation process (VPI) improves anti-vibration by 30-50%

7.5 Key Applications for Impact Resistance

Apply	Shock Acceleration	Frequency
Traction Transformer	50-100 m/s ²	Continuous
Mining Transformer	30-80 m/s ²	Continuous
Wind Power Transformers	10-30 m/s ²	Continuous
Marine Transformers	50-150 m/s ²	Sudden
Aeronautical Transformer	100-300 m/s ²	Burst
Seismic Transformer	50-200 m/s ²	Sudden (earthquake)

VIII. Short-circuit electrodynamic tolerance of paper-encapsulated wires

8.1 Sources of short-circuit electrical power

When the transformer is short-circuited , the winding current instantaneously increases by 10-20 times, generating a huge electromagnetic force :

F = B × I × L
Where:
F = Electrodynamic (N)
B = magnetic flux density (T)
I = short-circuit current (A)
L = conductor length (m)

Typical electric power :
– Distribution transformer short-circuit electric power: 500-5,000 N
– Power transformer short-circuit power: 5,000-100,000 N
– UHV transformer short-circuit power: > 100,000 N

8.2 3 Characteristics of Short Circuit Electric Power

Features	Values	Impacts
Sudden	Lasts 0.1-1 s	Shock Loads
Huge	100-1,000x normal	Vulnerable
Directionality	Radial + Axial	Complex Stresses

8.3 Effect of short-circuit electric power on paper wrapping

• Fracture of the ❌ conductor radially displaced → paper layer
• ❌ Conductor axial displacement → inter-turn short circuit
• The → insulation distance of the ❌ winding deformation decreases
• → Cumulative deformation of loose ❌ fasteners
• ❌ Multiple short-circuit → cumulative injuries

8.4 Anti-short circuit electrodynamic design

Design principles :
– Use soft copper (elongation 30-40%)
– Increase number of layers (2-4 layers → 4-8 layers)
– VPI impregnation (for added integrity)
– Optimize winding compression (reduce displacement space)
– Reinforced end insulation (anti-axial force)

8.5 Anti-short circuit electrodynamic test

Test	Method	Standard
Burst Short	Apply 10-25x rated current	IEC 60076-5
Number of Short Circuits	Usually 6-9	GB 1094.5
Post Short Test	Appearance, BDV, Temperature Rise	GB 1094.5
Short Circuit Impedance Change	< 2%	GB 1094.5

IX. Thermo-mechanical properties of paper-coated wires

9.1 Effect of thermal expansion on mechanical strength

When the temperature changes, the thermal expansion coefficient of copper, aluminum, and paper is different:

Materials	Coefficient of Thermal Expansion (10 °C/°F)
Copper	17
Aluminum	23
Cable Paper (Portrait)	5-10
Cable Paper (Landscape)	15-25
Transformer Oil	700-900

Thermo-mechanical stresses :
– Temperature change 100°C → Copper expansion 1.7%
– Shear stress between different materials
– Fatigue due to prolonged cycling

9.2 Effects of thermomechanical stress

• Loose ❌ paper layers
• ❌ Conductor displacement
• Broken ❌ end insulation
• ❌ Cumulative fatigue

9.3 Ways to improve thermo-mechanical performance

• ✅ Choose a material that matches the coefficient of thermal expansion
• ✅ Oil immersion reduces temperature gradient
• ✅ VPI impregnation enhances integrity
• ✅ Optimize winding structure to reduce constraints
• ✅ Control temperature rise < 65 K

X. Mechanical strength quality control of paper wrapping

10.1 Incoming Material Inspection Items

Project	Methodology	Acceptance Criteria	Frequency
Conductor Tensile Strength	Tensile Machines	Meets Specifications	Per Batch
Conductor Elongation	Tensile Machines	> 30% (soft copper)	Per batch
Conductor diameter	Micrometer	Tolerance ± 0.01 mm	Per batch
Cable Paper Thickness	Micrometer	Tolerance ± 0.005 mm	Per Batch
Tensile strength of cable paper	Tensile machine	> 8 kN/m	Per batch
Cable Tear Strength	Tear Gauge	> 0.4 N	Per Batch
Appearance	Visual Inspection	No Damage, No Pollution	100%

10.2 Process control

Process	Key Parameters	Detection Frequency
Winding	Tension 5-50 N/mm ²	Per Axis
Paper Covering	Compactness, Layers	Per Axis
Pressing	Pressure, time	Per set
Drying	Temperature, Vacuum	Per Tank
Oil Immersion	Vacuum, Pressure	Per Tank

10.3 Factory Inspection

Project	Methodology	Acceptance Criteria
Overall Tensile Strength	Tensile Machines	> 200 MPa
Post-Bending BDV	Post-Bending Test	Down < 20%
Appearance after bending	Microscope	No breakage
BDV after compression	Test after compression	Decrease < 15%
Immersion Adhesion	Rowing	100% Adhesion

10.4 Accelerated aging test

Test	Condition	Duration
Thermal Aging	130°C Oil Immersion	1,000 h
Vibration Aging	50 Hz, 10 m/s ²	1,000 h
Heat – Vibration Cycle	-40°C ↔ +130°C	100 cycles
Short Circuit Cycle	10x Rated Current	6-9 times

XI. Selection guide for paper wrapping

11.1 Top 5 principles for model selection

1. Large capacity?→ Soft copper + thick paper layer (8-12 layers)
2. High short-circuit current?→ Soft copper + VPI impregnation
3. Large vibration?→ Thick paper layer + oil impregnation + integral fastening
4. High bending requirements?→ Soft Copper + Thin Paper Layer
5. Economical?→ Soft Aluminum + Medium Paper Layer

11.2 Selection decision tree

Identify scenarios
    ↓
Capacity > 1 MVA?
  ├─ Is → the voltage > 35 kV?
  │         ├─ Is → Soft Copper + High Density Cable Paper 8-12 Layers + VPI
  │         └─ No Below → 35 kV?
  │                  ├─ Is → Soft Copper + Cable Paper 4-8 Layers
  │                  └─ No → Soft Copper + Cable Paper 2-4 Layers
  └─ Do you have → a miniature transformer?
            ├─ Is → Soft Copper/Aluminum + Cable Paper 2-4 Layers
            └─ No → Medium transformer → Soft copper + cable paper 4-8 layers

11.3 Mechanical strength requirements for different application scenarios

Application	Tensile strength	Bending radius	Vibration resistance	Shock resistance
Distribution Transformer	200-250 MPa	1-3 ×	Medium	Low
Power Transformer	200-250 MPa	1-3 ×	Medium	Medium
UHV Transformer	200-260 MPa	2-4 ×	Medium	Medium
Traction Transformer	220-260 MPa	1-3 ×	High	High
Mining Transformer	220-260 MPa	1-3 ×	High	High
Wind Power Transformer	220-260 MPa	1-3 ×	High	Medium
Marine Transformer	220-260 MPa	2-4 ×	High	High

11.4 Comparison of conductor specifications and mechanical properties

Conductor Specifications	Tensile Strength	Bending Radius	Applicable Scenarios
1.0 mm Round copper	210 MPa	1 mm	Small transformer
2.0 mm Round Copper	220 MPa	2 mm	Universal Transformer
3.0 mm Round Copper	230 MPa	3 mm	Distribution Transformer
5.0 mm Round Copper	240 MPa	5 mm	Power Transformer
2.0 x 5.0 copper flat	230 MPa	4 mm thickness	Medium transformer
3.0 × 8.0 copper flat	240 MPa	6 mm thickness	Large transformer
5.0 x 12.0 copper flat	245 MPa	10 mm thickness	Extra large transformer

XII. Typical Application Cases

12.1 Case 1: 110 kV oil-immersed power transformer

Application : A 110 kV oil-immersed power transformer

Specifications :
– Capacity: 50 MVA
– Voltage: 110 kV/35 kV
– Short circuit impedance: 10.5%
– Anti-short circuit requirements: 25 kA/3 s

Mechanical design :
– High voltage winding: soft copper 3.0 × 8.0 flat copper + 8 layers of cable paper + VPI impregnation
– Medium voltage winding: soft copper 2.0 × 5.0 flat copper + cable paper 6 layers
– Low voltage winding: soft copper 5.0 × 12.0 flat copper + cable paper 4 layers
– Overall VPI impregnation + vacuum oil immersion

Mechanical performance verification :
– Tensile strength: > 230 MPa
– Bending radius: < 6 mm
– Short circuit test: 6 times without deformation
– Vibration test: Pass IEC 60068-2-6

Running results :
– 15 years of operation
– Failure rate < 0.05%
– Design life of 40 years

12.2 Case 2: Traction transformer

Application : A high-speed rail traction transformer

Specifications :
– Capacity: 30 MVA
– Voltage: 220 kV/25 kV
– Anti-vibration: Strong
– Shock Resistance: High

Mechanical design :
– High voltage winding: Soft copper 2.0 mm round copper + 6 layers of cable paper + elastic glue + integral potting
– Low voltage winding: soft copper 5.0 × 10.0 flat copper + cable paper 4 layers
– Elastomer potting absorbs vibration

Mechanical performance verification :
– Anti-vibration: > 50 m/s ²
– Shock resistance: > 100 m/s ²
– Bending performance: Bending radius 2-3 × diameter

Running results :
– 8 years of operation
– Excellent anti-vibration and weather resistance
– Failure rate < 0.05%

12.3 Case 3: Wind Power Transformer

Application : An offshore wind power transformer

Specifications :
– Capacity: 5 MVA
– Voltage: 35 kV/0.69 kV
– Anti-vibration: continuous
– Corrosion Resistance: High

Mechanical design :
– High voltage winding: soft copper 2.0 mm round copper + cable paper 4 layers
– Low voltage winding: soft copper 3.0 × 6.0 flat copper + cable paper 2 layers
– Corrosion resistant housing
– Elastic cushion damping

Mechanical performance verification :
– Anti-vibration: > 30 m/s ²
– Corrosion resistance: Salt spray test > 1,000 h
– Bending performance: Bending radius 2-3 × diameter

Running results :
– 6 years of operation
– Harsh environment at sea is trouble-free
– Failure rate < 0.1%

12.4 Case 4: 220 kV oil-immersed power transformer (short-circuit electric power challenge)

Application : A 220 kV oil-immersed power transformer (large short-circuit current)

Specifications :
– Capacity: 180 MVA
– Voltage: 220 kV/110 kV/35 kV
– Short circuit current: 40 kA
– Short circuit impedance: 14%

Mechanical Design Challenge :
– Short circuit power: > 200,000 N
– Radial Force + Axial Force Composite

Mechanical design plan :
– Conductor: Soft copper 5.0 × 12.0 Flat copper (elongation 35%)
– Paper package: 10 layers of high density cable paper + rhombus dispensing (DDP)
– Overall: VPI Vacuum Pressure Impregnation
– Ends: Extra paper reinforcement
– Fastening: spring platen

Mechanical performance verification :
– Tensile strength: > 240 MPa
– Bending performance: Bending radius 8 mm
– Short circuit test: 9 times (more than 6 times) without deformation
– Short circuit impedance change: < 1.5%

Running results :
– 10 years of operation
– 3 external short circuits
– Excellent short-circuit resistance
– Failure rate < 0.02%

XIII. Future development of mechanical strength of paper wrapping

13.1 New conductor materials

High conductivity and high strength copper alloy :
– Cu-Cr alloy: tensile strength > 500 MPa
– Cu-Zr alloy: tensile strength > 450 MPa
– Cu-Ni-Si alloy: tensile strength > 600 MPa

Composite conductor :
– CCA: Tensile strength 100-150 MPa
– CCS: Tensile strength > 400 MPa
– High-strength aluminum alloy: tensile strength 200-300 MPa

13.2 New insulation materials

Polyimide (PI) film :
– Tensile strength > 200 MPa
– Temperature resistance 220°C
– Thickness 0.025-0.125 mm

Nomex :
– Tensile strength 50-80 kN/m
– Temperature resistance 220°C
– High tear strength

Polyester film (PET) :
– Tensile strength 150-200 MPa
– Thickness 0.012-0.35 mm
– Low cost

13.3 Intelligent monitoring

• Fiber optic sensors : monitoring strain
• Piezoelectric sensor : monitor vibration
• Strain gauge : monitor stress
• On-line monitoring system : predicting mechanical failure

13.4 New process

• Automatic winding : precise control of tension
• Robot compression : uniform pressure
• Online testing : Real-time feedback
• Digital twins : simulation optimization

XIV. 20 Glossary of Terms

Chinese	English	Abbreviations	Definitions
Tensile Strength	Tensile Strength	σb	Material’s ability to resist tensile fracture
Yield Strength	σs	Critical Stress of Permanent Deformation
Elongation	–	% increase in tensile breaking length
Bending Radius	Bending Radius	–	Minimum bending radius without fracture
Number of Bends	Bending Cycles	–	Number of Repeated Bends to Fracture
Twist	Torsion	–	Force rotating around the axis
Cut	Shear	–	Axial Vertical Stress
Compression Force	–	Axial or Radial Pressure
Short Circuit Electric Power	Short-Circuit Electromagnetic Force	–	Electromagnetic Force from Short Circuit Current
Vibration Resistance	–	Ability to resist vibration damage
Impact Resistance	–	Ability to Resist Impact Damage
Elastic Modulus	Elastic Modulus	E	Stress-Strain Ratio
Hardness	–	Resists local plastic deformation
Fatigue Life	Fatigue Life	–	Life under Cyclic Load
Winding Test	Mandrel Test	–	Winding Test on a Round Bar
Repeated Bending	Reverse Bending	–	90° Repeated Bending
Impregnation	–	Process for impregnating insulating lacquer
Vacuum Pressure Impregnation	Vacuum Pressure Impregnation	VPI	Vacuum + Pressure Impregnation Process
Oil Immersion	Oil Immersion	–	Immersion in Transformer Oil
Diamond Dispensing	Diamond Dotted	DDP	Surface Coating Paper

XVI. Summary and outlook

Mechanical strength is the second largest core property of paper wrapping after electrical insulation. In this paper, the mechanical strength of the paper wrapping line – tensile strength, elongation, bending performance, torsional performance, shear strength, compressive strength, vibration resistance, impact resistance – was combed from the 8 major dimensions system, and the selection guide, quality control, and typical cases were given.

Key conclusions

Scenarios	Recommended Envelopes	Priority Reasons
35-220 kV oil-immersed power distribution	Soft copper + cable paper 4-8 layers	Balanced mechanical/electrical/cost
110-500 kV oil-immersed power	Soft copper + cable paper 8-12 layers + VPI	Anti-short circuit, anti vibration
UHV Transformer	Soft Copper + Cable Paper 12-15 Layers + Nomex Composite	Extreme Mechanical Strength
Traction transformer	Soft copper + Elastomer + Overall potting	Anti-vibration, anti-impact
Wind Power Transformer	Soft Copper + Cable Paper + Corrosion Resistant Enclosure	Vibration Resistant, Corrosion Resistant
Mining Transformer	Soft Copper + Thick Paper Layer + Explosion Proof Housing	Shock Resistant, Explosion Proof

Future directions :
1. New conductor : High strength copper alloy (Cu-Cr 500 MPa, Cu-Zr 450 MPa)
2. New insulation : PI film (200 MPa), Nomex (80 kN/m)
3. Intelligent monitoring : fiber strain, piezoelectric vibration, online monitoring
4. Digital twin : Simulation optimizes mechanical design
5. Automation : robot winding, robot pressing

16.1 Selection Decision Tree

Identify scenarios
    ↓
Capacity > 1 MVA?
  ├─ Is → short-circuit current > 25 kA?
  │         ├─ Is → Soft Copper + High Density Cable Paper 8-12 Layers + VPI
  │         └─ No → Soft Copper + Cable Paper 4-8 Layers
  └─ Do you have → a miniature transformer?
            ├─ Is → Soft Copper/Aluminum + Cable Paper 2-4 Layers
            └─ No → Medium transformer → Soft copper + cable paper 4-8 layers

16.2 6 Tips for Action

1. Clarify the application scenario : Transformer type determines the mechanical strength level
2. Define short-circuit current : Decide on short-circuit resistant electrodynamic design
3. Define vibration/shock : Determine anti-vibration design
4. Specify conductor specifications : determine tensile strength
5. Clear process requirements : Decide VPI/oil immersion/potting
6. Define Economy : High Intensity vs. Economy Balance

LP Winding Wire is willing to work with global transformer, motor and electrical equipment manufacturers to provide a complete solution for high mechanical strength paper wrapping , contributing to the global energy transition and power development.