1. Overview
Transformers are critical assets in power distribution systems, and their quality directly impacts grid stability and operational safety. This document outlines a comprehensive quality inspection framework based on international standards including IEC 60076, IEEE C57, and GB/T standards, tailored for both oil-immersed and dry-type distribution transformers.
2. Incoming Raw Material Inspection
2.1 Silicon Steel Sheet Inspection
Silicon steel is the core material for transformer magnetic circuits. Its quality directly impacts no-load losses and noise levels.
Thickness tolerance: ±0.02mm
Core loss test: P1.7/50 ≤ 1.10 W/kg (Grade 30Q130)
Magnetic induction: B8000 ≥ 1.88T
Coating insulation resistance: ≥ 50 Ω·cm²/sheet
Visual inspection: No rust, burrs, or cracks

2.2 Magnet Wire (Copper/Aluminum) Inspection
Conductor resistivity: ρ20 ≤ 0.017241 Ω·mm²/m (copper)
Insulation thickness: 0.12-0.20mm (depends on voltage class)
Film continuity: Pinholes ≤ 6 per 5m
Flexibility: No cracks at 1x diameter bend
Dielectric test: 2kV/1min without breakdown
2.3 Insulating Oil Inspection (Oil-Immersed Transformers)
Breakdown voltage: ≥ 35kV (new oil)
Dissipation factor: tanδ ≤ 0.5% (at 90°C)
Moisture content: ≤ 20mg/L
Acid value: ≤ 0.03 mg KOH/g
Interfacial tension: ≥ 40 mN/m
2.4 Epoxy Resin Inspection (Dry-Type Transformers)
Gel time: 150-300 seconds (at 180°C)
Glass transition temperature: ≥ 110°C
Dielectric strength: ≥ 20kV/mm
Flame resistance: Class F1 (IEC 60705)
Moisture resistance: Stable at ≥ 95% RH

3. In-Process Quality Control
3.1 Core Assembly Inspection
The core assembly is the magnetic circuit of the transformer. Its manufacturing quality directly impacts no-load performance and acoustic noise.
1.Stacking factor: ≥ 96%
2.Joint gap: ≤ 1.0mm
3.Core clamping force: 15-20 N/cm²
4.Core grounding: Single-point reliable grounding
5.Core loss test: Deviation ≤ 5% from calculated value
3.2 Winding Manufacturing Inspection
6.Winding dimensional accuracy: ±2mm
7.Turn-to-turn insulation: No damage or wrinkles
8.Welding quality: Resistance deviation ≤ 1%
9.Winding compression: Per process specification
10.Layer voltage test: 1.5x rated voltage/1min
3.3 Active Part Assembly Inspection
Insulation clearance: Compliant with GB/T 1094.3
Lead connections: Secure, contact resistance qualified
Insulation assembly: Clean, accurate positioning
Clamping system: Uniform pressure, no looseness

4. Final Inspection and Type Testing
4.1 Routine Tests (Performed on Every Unit)
Per IEC 60076-1 and GB/T 1094.1, every transformer must undergo the following tests before delivery:
11.Winding resistance measurement: Three-phase unbalance ≤ 2%
12.Voltage ratio measurement: Deviation ≤ 0.5%
13.Vector group verification: Dyn11, Yyn0, etc.
14.No-load loss and no-load current measurement
15.Load loss and impedance voltage measurement
16.Power frequency withstand test: 35kV class – 85kV/1min
17.Induced voltage test: 1.5x rated voltage
18.Insulating oil test (oil-immersed): Breakdown voltage, dissipation factor

4.2 Type Tests (Sampling or Special Requirements)
Temperature rise test: Top oil ≤ 55K (oil-immersed)
Lightning impulse test: Full/Chopped wave 75kV/170kV
Short-circuit withstand test: 2-second short-circuit current
Sound level: ≤ 55dB (10kV/400kVA)
Partial discharge: ≤ 10pC (dry-type transformers)
4.3 Special Tests (Per Customer Requirements)
Zero-sequence impedance measurement
Harmonic analysis test
Environmental adaptability test (high altitude, low temperature)
Seismic performance test
Noise spectrum analysis
5. Quality Documentation and Traceability
We maintain a comprehensive quality traceability system. Each transformer is assigned a unique serial number, and all inspection data is recorded in the quality file, including: material certificates, process inspection records, test reports, non-conformance handling records, and customer acceptance records. Best practices in quality management ensure product traceability and continuous improvement.
6. Inspection Differences: Oil-Immersed vs. Dry-Type
Reference: IEC 60076 Series, IEEE C57 Series, GB/T 1094 Series
Inspection Item Oil-Immersed Transformer Dry-Type Transformer
Insulating Medium Insulating oil: Breakdown voltage, dissipation factor, moisture Epoxy resin: Curing degree, partial discharge level
Sealing Test Tank sealing test: 0.05MPa/24h without leakage N/A; check casting integrity instead
Temperature Rise Test Measure top oil and winding temperature rise Measure winding temperature rise (no oil medium)
Fire Safety Oil chromatography, oil drainage/nitrogen injection Flame class F1, fire resistance time evaluation
Environmental Oil biodegradability, leak prevention Material recyclability, halogen-free require