400 KVA Transformer Technical Specifications

Technical Specifications for 

400 kVA, 11 kV/433V 

Oil-Immersed Distribution Transformer

1. General Requirements

• Type: Double copper wound, three phase, oil immersed, ONAN cooled

• Rating: 400 kVA

• Primary Voltage: 11 kV

• Secondary Voltage: 433 V

• Frequency: 50 Hz ± 5%

• Supply Voltage Variation: ±10%

• Vector Group: Dyn11

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2. Electrical Parameters

• Impedance (at 75 °C): 5%

• No-load losses (max): 0.7 kW

• Full-load losses (max): 5.1 kW

• Short circuit withstand: 3 sec at 5% impedance

• Harmonic suppression: 3rd, 5th, 7th harmonics + high-frequency disturbances

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3. Thermal Limits

• Oil temperature rise: ≤ 35 °C over ambient (40 °C)

• Winding temperature rise: ≤ 40 °C over ambient (40 °C)

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4. Insulation & Withstand Levels

• Power frequency withstand (1 min):

  • 3 kV (433 V side)

  • 28 kV (11 kV side)

• Lightning impulse withstand: 75 kVp (11 kV side)

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5. Tap Changer

• HT side off-load tapping: ±5% in steps of 2.5%

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6. Design Clearances

• For 11 kV system:

  • Phase–Phase: 180 mm

  • Phase–Earth: 120 mm

• For 433 V system:

  • Phase–Phase: 25 mm

  • Phase–Earth: 25 mm

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7. Transformer Tank

• Sealed type with corrugated fins, oil-filled or nitrogen-cushioned

• MOC: Mild steel plate (low carbon)

• Features:

  • Double welded joints, stress-relieved

  • Leak-proof, capable of vacuum oil filling

  • Corrugated fins ≥ 1.2 mm CRCA sheets

  • Adequate stiffeners to prevent deformation

  • Skid base with lifting lugs

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8. Transformer Oil

• Class 1 new mineral insulating oil (naphthalene base, inhibitor added)

• PCB-free, conforming to IS standards

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9. Windings

• Material: Electrolytic copper

• Current density: ≤ 3 A/mm²

• Insulation: Class A, oil-compatible, inert

• Mechanical features: shrinkage treatment, rigid supports, transposed conductors, uniform insulation

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10. Bushings & Terminations

• HV Side: Epoxy cast, 11 kV, 630 A, interface type ‘C’ (EN50180/EN50181)

• LV Side: Indoor epoxy cast resin, 1 kV class, creepage 31 mm/kV

• Neutral Bushing: Integral copper stem & palm (100×12 mm)

• Creepage distance: ≥31 mm/kV (min 50% protected)

• Short-time thermal current: 26.3 kA / 3 sec

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11. Current Transformers (CTs)

• Location: All three phases + neutral on LV side

• Type: Resin cast ring type, outdoor

• Ratio: 1000/5 A

• Accuracy class: 5P10

• Burden: 5 VA

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12. Noise Level

• Should conform to NEMA TR-1 / IEC 551 limits with accessories running.

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13. Core

• Material: High-grade, non-ageing, CRGO silicon steel

• Features:

  • Low loss, high permeability

  • Braced against short-circuit stresses

  • Rigid clamping, minimal air gaps

  • Sandblasted & stress-relieved supports

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14. Name Plate Details

Shall include:

• Manufacturer, serial no., year of manufacture

• Standard compliance (IS/IEC)

• Rated voltage, current, frequency, power, cooling method

• Losses (no-load, load)

• Vector group

• Temperature rise (oil & winding)

• Weights (core, winding, total, oil volume)

• Tapping range

• Impedance (%)

• Purchaser’s name

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15. Reference Standards & Tests

• IS / IEC standards compliance

• Type Tests: Short circuit, impulse, temperature rise, loss measurement, Tan Delta test (see details)

• Routine Tests: Ratio, polarity, resistance, insulation, oil test, pressure test, etc.

• Special Tests: Harmonic suppression, noise level, oil filtration (process reference)

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