1. Overview

Converter Duty Transformers are used to supply power to converters in industrial applications like electrolysis, traction, steel plants, cement plants, and HVDC transmission. Unlike conventional transformers, CDTs handle high harmonic content, unbalanced loading, and frequent switching stresses.

2. Key Applications

• Rectifier-fed drives (DC furnaces, traction)
• Inverter systems (AC drive systems)
• HVDC systems
• Variable Frequency Drives (VFDs)
• Electrochemical and metallurgical industries

3. Design Considerations

a. Electrical Parameters

• Voltage range: Typically from 11 kV to 400 kV on primary side; secondary voltages vary per converter design.
• Current rating: High current handling (can exceed 20 kA).
• Impedance: Designed to manage short-circuit currents and reduce harmonic effects.
• Phase shift: Multi-winding or phase-shifting transformers are used to reduce harmonic distortion (e.g., 12-pulse, 24-pulse configurations).

b. Harmonics and Waveform Distortion

• Special design to handle non-sinusoidal load currents.
• Windings are arranged to minimize harmonic propagation into the grid.

4. Construction Features

• Core: High-grade CRGO steel or amorphous metal; laminated to reduce eddy current losses.
• Windings: Copper with insulation suitable for high-frequency switching stresses.
• Tapping: Off-circuit or on-load tap changers (OLTC) for voltage regulation.
• Insulation Class: Often Class F or H, depending on thermal requirements.
• Cooling: ONAN, ONAF, OFAF, or OFWF as per duty cycle and power rating.
• Short-circuit strength: Designed to withstand mechanical stresses from DC fault currents.

5. Winding Configurations

• Primary: Usually star or delta.
• Secondary: Often delta with center-tap for 6-pulse rectifiers; multi-secondary for 12- or 24-pulse rectifiers.
• Inter-phase transformers (IPTs): Sometimes used to balance output currents.

6. Standards and Codes

• IEC 61378-1: Converter transformers – Transformers for industrial applications.
• IS 2026 (Indian Standard for Power Transformers)
• IEEE C57 series: Guidelines for transformer applications.
• IEC 60076: General transformer standards, with harmonics and loading considerations.

7. Special Features

• Electrostatic shielding between windings to suppress common-mode voltage.
• Magnetic shielding to reduce leakage flux.
• Enhanced cooling channels for high-duty cycles.
• Sometimes equipped with dry-type insulation in low to medium ratings.

8. Comparison with Conventional Transformers

9. Typical Industries Served

• HVDC transmission stations
• Aluminium and steel smelters
• Cement manufacturing
• Electric traction substations
• Data centers with power conversion units