400 KVA Transformer Technical Specifications

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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

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

3. Thermal Limits

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

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

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)

5. Tap Changer

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

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

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

8. Transformer Oil

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

  • PCB-free, conforming to IS standards

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

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

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

12. Noise Level

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

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

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

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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