1 Introduction
Paper covered wire is composite insulated winding wire formed by wrapping insulating paper on copper conductor or aluminum conductor, is the core insulation material for power transformer, distribution transformer, special transformer, oil immersed transformer. Paper covered wire by insulating paper type is divided into cable paper covered wire, telephone paper covered wire, high voltage cable paper covered wire, inter turn insulation paper covered wire, aromatic polyamide paper covered wire, diamond adhesive paper covered wire six major categories.
Paper covered wire electrical safety characteristic is the key technical index for transformer long term reliable operation. Based on IEC 60317, NEMA MW 1000-2018, GB/T 7673, JB/T 5268, GB/T 10580, IEC 60243, IEC 60156, ASTM D149 and other international and domestic standards, this article systematically describes the technical principle, parameter index, failure mechanism, detection method and development trend of paper covered wire electrical safety characteristic from six dimensions including paper covered wire overview, electrical safety fundamentals, electrical safety characteristics, transformer application, failure modes protection, detection standards and future trends.
2 Paper Covered Wire Overview
2.1 Paper Covered Wire Concept and Structure
Paper covered wire is composite insulated winding wire with insulating paper as external insulation material, copper conductor or aluminum conductor as internal conductor. Paper covered wire structure from inside to outside is conductor, inner varnish film or bare wire, insulating paper layer, outer insulating paper layer, impregnant layer.
Conductor, copper TU1 99.97%, copper TU2 99.95%, copper T2 99.90%, aluminum 1060 99.60%, aluminum 1050 99.50%, aluminum 1350 99.50%. Insulating paper layer, cable paper DLZ, telephone paper DH, high voltage cable paper GDL, inter turn insulation paper ZB, aromatic polyamide paper Nomex, diamond adhesive paper. Impregnant layer, 10, 25, 45 mineral oil, silicone oil, synthetic ester oil, vegetable oil.
2.2 Paper Covered Wire Product Type
Paper covered wire product type covers paper covered round copper wire, paper covered flat copper wire, paper covered round aluminum wire, paper covered flat aluminum wire, paper covered enameled round copper wire, paper covered enameled flat copper wire, paper covered enameled round aluminum wire, paper covered enameled flat aluminum wire eight major categories.
Paper covered round copper wire, conductor diameter 1.00 to 5.00 mm, paper layer thickness 0.10 to 0.80 mm, paper layer number 1 to 4 layers. Paper covered flat copper wire, width 1.00 to 12.50 mm, thickness 0.80 to 5.60 mm, paper layer thickness 0.20 to 1.20 mm, paper layer number 2 to 6 layers.
Paper covered enameled round copper wire, conductor diameter 0.50 to 4.00 mm, varnish film paper layer total thickness 0.20 to 1.00 mm. Paper covered enameled flat copper wire, width 1.00 to 12.50 mm, thickness 0.80 to 5.60 mm, varnish film paper layer total thickness 0.30 to 1.50 mm.
2.3 Paper Covered Wire Technical Standard
Paper covered wire technical standards cover NEMA MW 1000-2018 Aromatic Polyamide Paper Covered, NEMA MW 26-C, NEMA MW 33-C, NEMA MW 45-C, NEMA MW 46-C, IEC 60317 series, IEC 60851 enameled wire test method, IEC 60216 electrical insulation aging, IEC 60156 liquid insulation medium breakdown, GB/T 7673 paper covered winding wire, GB/T 10580 mineral oil impregnation, JB/T 5268 paper covered flat winding wire, ASTM D149 dielectric breakdown.
NEMA MW 1000-2018 divides aromatic polyamide paper covered wire into Class 200 polyester paper covered aluminum wire, Class 220 aromatic polyamide paper covered rectangular aluminum wire. GB/T 7673 divides paper covered wire by insulating paper into DLZ cable paper, DH telephone paper, GDL high voltage cable paper, ZB inter turn insulation paper, Nomex synthetic fiber paper five categories.
3 Electrical Safety Fundamentals
3.1 Electrical Insulation Principle
Electrical insulation principle is based on the ability of insulating material to prevent current flow. Electrical insulation strength measures the ability of insulating material to withstand electric field strength without breakdown, unit V/mm or kV/mm.
Insulation resistance, the resistance of insulating material to DC, unit MΩ·km. The higher the insulation resistance, the better the electrical safety characteristic. Dielectric loss, the energy loss of insulating material in AC electric field, expressed as tanδ, the lower the tanδ, the better the insulation performance.
Breakdown voltage, the critical voltage when insulating material is broken down, unit V or kV. The higher the breakdown voltage, the greater the electrical safety margin. Dielectric constant, the energy storage capacity of insulating material in electric field, relative dielectric constant ε_r between 2.0 to 7.0, affecting electric field distribution.
3.2 Insulation Material
Electrical insulation material covers insulating paper, varnish film, impregnant, composite insulation four major categories.
Insulating paper, cable paper DLZ thickness 0.10 to 0.17 mm, breakdown voltage 8000 to 12000 V/mm. Telephone paper DH thickness 0.05 to 0.08 mm, breakdown voltage 7000 to 10000 V/mm. High voltage cable paper GDL thickness 0.10 to 0.20 mm, breakdown voltage 12000 to 18000 V/mm. Inter turn insulation paper ZB thickness 0.05 to 0.10 mm, breakdown voltage 6000 to 9000 V/mm. Nomex synthetic fiber paper thickness 0.05 to 0.50 mm, breakdown voltage 18000 to 25000 V/mm, long term heat resistance 220℃.
Varnish film, polyester varnish film PEW thickness 0.02 to 0.10 mm, heat resistance 130℃, breakdown voltage 5000 to 8000 V/mm. Polyester imide varnish film EIW heat resistance 180℃. Polyamide imide varnish film AIW heat resistance 200℃. Polyimide varnish film PI heat resistance 240℃.
Impregnant, 10 transformer oil flash point 135℃, breakdown voltage greater than or equal to 35 kV. 25 transformer oil flash point 145℃, breakdown voltage greater than or equal to 40 kV. 45 transformer oil flash point 145℃, breakdown voltage greater than or equal to 35 kV. Silicone oil flash point 300℃, breakdown voltage greater than or equal to 35 kV. Synthetic ester oil Midel 7131 flash point 275℃, breakdown voltage greater than or equal to 35 kV.
3.3 Impregnation Process Essentials
Impregnation process is the key of electrical safety, adopts vacuum pressure impregnation VPI.
Vacuum pressure impregnation parameters, vacuum degree less than or equal to 50 Pa, pressure 0.2 to 0.5 MPa, impregnation time 8 to 24 hours, impregnation temperature 50 to 70℃, impregnant viscosity 0.5 to 5.0 Pa·s.
Vacuum pressure impregnation steps, pre drying 100 to 120℃, 24 to 72 hours, vacuum degassing 50 to 100 Pa, 2 to 4 hours, pressure impregnation 0.2 to 0.5 MPa, 8 to 24 hours, drainage, low pressure drying 80 to 100℃, 24 to 48 hours, high pressure drying 100 to 120℃, 24 to 48 hours.
Electrical safety key, vacuum degree ensures impregnant penetration fiber, pressure ensures impregnant filling void, temperature ensures impregnant fluidity. Time ensures impregnant full penetration.
4 Electrical Safety Characteristics
4.1 Breakdown Voltage
Breakdown voltage is the core index of electrical safety characteristic, breakdown mode covers electrical breakdown, thermal breakdown, partial discharge breakdown three categories.
Electrical breakdown, electric field strength exceeds critical value of insulating material then directly breakdown. Thermal breakdown, insulating material loss heating exceeds heat dissipation, long term accumulation leads to thermal collapse. Partial discharge breakdown, internal void, defect tip discharge long term accumulation leads to insulation destruction.
Breakdown voltage range, oil immersed cable paper covered flat copper wire 5000 to 30000 V, oil immersed high voltage cable paper covered round copper wire 5000 to 25000 V, oil immersed Nomex paper covered flat copper wire 8000 to 35000 V.
4.2 Dielectric Strength
Dielectric strength is breakdown voltage per unit thickness, unit kV/mm. Dielectric strength is related to insulating paper thickness, density, water content, oil immersion state.
Dry state dielectric strength, cable paper DLZ 8000 to 12000 V/mm, telephone paper DH 7000 to 10000 V/mm, high voltage cable paper GDL 12000 to 18000 V/mm, Nomex 410 18000 to 25000 V/mm.
Oil immersed dielectric strength, cable paper DLZ 15000 to 25000 V/mm, telephone paper DH 12000 to 18000 V/mm, high voltage cable paper GDL 25000 to 40000 V/mm, Nomex 410 30000 to 50000 V/mm.
Oil immersed dielectric strength improved 70% to 150% versus dry state, because impregnant fills fiber gap, eliminates void, breakdown voltage increases.
4.3 PDIV and Partial Discharge
PDIV Partial Discharge Inception Voltage is the key index for evaluating electrical safety characteristic.
PDIV test standard, IEC 61294 solid insulation PDIV test, ASTM D1868 partial discharge test, GB/T 7354 partial discharge measurement.
PDIV threshold, distribution transformer 5 to 10 kV class rated phase voltage 6 kV to 12 kV, PDIV greater than or equal to 2 times rated, i.e. 12 kV to 24 kV. 35 kV class PDIV greater than or equal to 70 kV. 110 kV class PDIV greater than or equal to 220 kV. 220 kV class PDIV greater than or equal to 440 kV. 500 kV class PDIV greater than or equal to 1000 kV.
Partial discharge quantity, oil immersed paper covered wire stable discharge quantity less than or equal to 10 pC, qualified. 10 to 100 pC, warning. Greater than or equal to 100 pC, alarm.
4.4 Electrical Safety Characteristic Parameters
Electrical safety characteristic parameters cover breakdown voltage, dielectric strength, PDIV, tanδ, insulation resistance five items.
Insulation resistance, dry state 1000 to 10000 MΩ·km, oil immersed 10000 to 100000 MΩ·km.
tanδ, dry state 0.005 to 0.010, oil immersed 0.001 to 0.003.
ε_r, cable paper 2.5 to 4.0, Nomex 1.6 to 2.5.
Safety factor, greater than or equal to 5 safe, greater than or equal to 3 qualified, less than 3 dangerous.
5 Transformer Application
5.1 Distribution Transformer
Distribution transformer application covers 10 kV, 20 kV, 35 kV three major categories. 10 kV distribution transformer capacity 100 to 2500 kVA, rated voltage 10/0.4 kV, high voltage winding paper covered enameled round copper wire, conductor diameter 0.50 to 2.50 mm, paper layer number 2 to 4 layers.
20 kV distribution transformer capacity 200 to 2500 kVA, rated voltage 20/0.4 kV, high voltage winding paper covered enameled round copper wire or flat copper wire, conductor diameter 0.80 to 3.00 mm, paper layer number 3 to 5 layers.
35 kV distribution transformer capacity 500 to 5000 kVA, rated voltage 35/10 kV, high voltage winding paper covered enameled flat copper wire, width 2.00 to 8.00 mm, thickness 1.00 to 3.15 mm, paper layer number 4 to 6 layers.
5.2 Power Transformer
Power transformer application covers 110 kV, 220 kV, 500 kV, 1000 kV four major categories.
110 kV power transformer capacity 50 to 200 MVA, rated voltage 110 kV, high voltage winding paper covered enameled flat copper wire, width 3.00 to 12.50 mm, thickness 1.50 to 5.60 mm, paper layer number 6 to 10 layers, impregnant 25 or 45 transformer oil.
220 kV power transformer capacity 100 to 500 MVA, rated voltage 220 kV, high voltage winding paper covered enameled flat copper wire, width 4.00 to 12.50 mm, thickness 2.00 to 5.60 mm, paper layer number 8 to 12 layers, adopt high voltage cable paper GDL with Nomex 410 composite insulation.
500 kV power transformer capacity 200 to 1000 MVA, rated voltage 500 kV, high voltage winding paper covered enameled flat copper wire, width 5.00 to 12.50 mm, thickness 2.50 to 5.60 mm, paper layer number 10 to 16 layers, adopt high voltage cable paper GDL, Nomex 410 composite insulation, VPI vacuum pressure impregnation.
1000 kV power transformer capacity 1000 to 4000 MVA, UHV transmission, high voltage winding special paper covered enameled flat copper wire, adopt Nomex 410 plus mica tape composite insulation, UHV PDIV greater than or equal to 2 times, partial discharge less than or equal to 10 pC.
5.3 Special Transformer
Special transformer application covers rectifier transformer, electric furnace transformer, traction transformer, mining transformer four major categories.
Rectifier transformer, used for electrolytic aluminum, electrolytic copper, electroplating, chemical rectification, capacity 100 to 50000 kVA, adopt paper covered enameled flat copper wire, VPI impregnation transformer oil.
Electric furnace transformer, used for arc furnace, induction furnace, resistance furnace, capacity 500 to 100000 kVA, adopt high temperature paper covered enameled flat copper wire, heat resistance 180 to 220℃, impregnate silicone oil or synthetic ester oil.
Traction transformer, used for electric locomotive, metro, EMU, adopt high frequency paper covered enameled flat copper wire, anti vibration greater than or equal to 10 g, impregnate 25 transformer oil.
Mining transformer, used for coal mine, metal mine, underground electrical equipment, adopt moisture proof explosion proof paper covered enameled wire, flame retardant impregnant, Class 220 heat resistance.
6 Failure Modes and Protection
6.1 Aging Failure
Paper covered wire aging failure is the main threat of electrical safety, including thermal aging, electrical aging, chemical aging, mechanical aging four categories.
Thermal aging, cellulose chain breaking. Life 105℃ 10 to 20 years, 120℃ 3 to 8 years.
Electrical aging, electric treeing under electric field, forming breakdown channel.
Chemical aging, oxidation, hydrolysis, acid erosion degradation.
Mechanical aging, vibration, thermal expansion, short circuit electric force causes insulating paper damage. Short circuit electric force 25 to 49 times rated, vibration 5 to 20 g.
6.2 Breakdown Failure
Breakdown failure includes instantaneous breakdown, accumulated breakdown two categories. Instantaneous breakdown, lightning strike, overvoltage, operation overvoltage causes insulation instantaneous breakdown. Accumulated breakdown, long term aging causes insulation weak point failure.
Breakdown failure cause, insulation defect such as void, impurity, damage. Electric field concentration such as tip, burr, edge. Water content too high, oil impregnant degradation, long term overload.
Breakdown protection, insulation coordination, overvoltage protection, arrester, online monitoring.
6.3 Partial Discharge Failure
Partial discharge failure is the main failure mode of oil immersed paper covered wire. Partial discharge occurs at internal void, impurity, conductor burr, oil gap of insulation.
Partial discharge failure mechanism, partial discharge produces ozone, nitrogen oxide, corrode cellulose. Partial discharge produces high speed electron, impact cellulose molecular chain. Partial discharge produces heat, accelerate thermal aging. Partial discharge long term accumulation, form discharge channel, finally breakdown.
Partial discharge protection, reduce electric field concentration, optimize insulation coordination, improve oil impregnation process, strengthen process cleanliness, adopt high quality insulating paper.
6.4 Protection Strategy
Electrical safety protection strategy covers material, process, design, monitoring, operation and maintenance five aspects.
Material, select DLZ, GDL, Nomex 410, acid value less than or equal to 0.03 mg KOH/g, water content less than or equal to 30 mg/kg.
Process, VPI less than or equal to 50 Pa, 0.2 to 0.5 MPa, 8 to 24 hours, 50 to 70℃.
Design, insulation thickness greater than or equal to 3 times, creepage greater than or equal to 25 mm, electric field uniform.
Monitoring, online partial discharge, oil chromatographic analysis DGA, temperature load vibration.
Operation maintenance, periodic inspection, periodic maintenance, periodic test, oil filter regeneration, long life design.
7 Detection Standards and Future Trends
7.1 Detection Standard
Detection standard covers appearance, dimension, electrical, mechanical, thermal five categories.
Appearance, smooth, no damage, wrapping neat, overlap rate 50% plus minus 10%.
Dimension, conductor diameter, varnish film, paper layer, insulation total thickness, tolerance plus minus 3%.
Electrical, insulation greater than or equal to 1000 MΩ·km, breakdown greater than or equal to 5000 V, tanδ less than or equal to 0.005, PDIV greater than or equal to 2 times, discharge less than or equal to 10 pC.
Mechanical, tensile, elongation, soft, wear resistance, adhesion.
Thermal, thermal shock, thermal aging, thermal life.
7.2 Future Trends
Future trends, composite insulation, high dielectric, intelligent monitoring, environmental material, UHV five directions.
Composite insulation, develop Nomex 410 and high voltage cable paper GDL composite, PDIV greater than or equal to 3 times, breakdown greater than or equal to 35000 V, heat resistance 220℃, voltage resistance greater than or equal to 1100 kV.
High dielectric, develop aluminum oxide nano modified paper covered wire, dielectric strength greater than or equal to 40000 V/mm, PDIV greater than or equal to 3 times, used for UHV 1000 kV transformer.
Intelligent monitoring, develop built in optical fiber sensor paper covered wire, real time monitoring temperature, partial discharge, vibration, strain, life warning, intelligent maintenance.
Environmental material, develop vegetable oil impregnant, bio based insulating paper, degradable insulating paper, recyclable paper covered wire, comply with RoHS, REACH.
UHV, develop plus minus 800 kV, plus minus 1100 kV DC converter transformer paper covered wire, DC PDIV greater than or equal to 3 times, space charge suppression, long term DC electric field stable operation.
8 Summary
Electrical safety characteristic is the key index of transformer reliable operation. Product system covers 8 categories, insulating paper 5 categories.
Characteristic covers breakdown 5000 to 30000 V, dielectric 8000 to 25000 V/mm, oil immersed improved 70% to 150%, PDIV greater than or equal to 2 times, tanδ less than or equal to 0.005.
Application covers 10 to 1000 kV, capacity 100 kVA to 4000 MVA.
Failure mode covers thermal, electrical, chemical, mechanical aging, breakdown, partial discharge. Protection strategy, material, process, design, monitoring, operation maintenance five aspects.
Future development includes Nomex plus GDL composite, nano modification, intelligent monitoring, environmental, UHV plus minus 800/1100 kV.
Reference standards: IEC 60317, NEMA MW 1000-2018, GB/T 7673, JB/T 5268, GB/T 10580, IEC 60243, IEC 60156, ASTM D149.
Contact Information: E-mail office@cnlpzz.com, WhatsApp 0086-19337889070, Zhengzhou LP Industry Co., Ltd.
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