Technical Specifications of a Furnace Transformer

1. Electrical Ratings

• Power Capacity: Typically ranges from 1 MVA to 120 MVA, depending on furnace size and application.
• Primary Voltage: Up to 33 kV; common values include 6.6 kV, 11 kV, and 22 kV.
• Secondary Voltage: Low voltage outputs, generally between 100 V to 1,000 V, to accommodate high current requirements.
• Frequency: Standard operation at 50 Hz or 60 Hz.
• Phase Configuration: Available in both single-phase and three-phase designs.
• Vector Group: Commonly Dyn11, facilitating phase shift and grounding considerations.

2. Current and Impedance

• Secondary Current: Designed to handle extremely high currents, often in the range of 10 kA to 60 kA, depending on furnace requirements.
• Impedance Voltage: Typically between 4.5% and 21%, balancing voltage regulation and fault current limitations.

3. Cooling Methods

• Cooling Types:
• ONAN: Oil Natural Air Natural
• ONAF: Oil Natural Air Forced
• OFAF: Oil Forced Air Forced
• OFWF: Oil Forced Water Forced

The choice depends on the transformer's size, duty cycle, and installation environment.

4. Tap Changer Options

• On-Load Tap Changer (OLTC): Allows voltage adjustments under load conditions, essential for processes requiring voltage flexibility.
• Off-Circuit Tap Changer (OCTC): Voltage adjustments made when the transformer is de-energized; suitable for applications with stable voltage requirements.

5. Construction Features

• Winding Material: High-conductivity copper is standard for both primary and secondary windings.
• Core Material: Constructed from cold-rolled grain-oriented (CRGO) silicon steel to minimize core losses.
• Insulation Class: Typically Class A or Class F, depending on thermal requirements.
• Enclosure: Designed for indoor or outdoor installation, with appropriate ingress protection ratings.

6. Protection and Monitoring

• Standard Protections:
• Buchholz Relay: Detects gas accumulation due to internal faults.
• Oil Temperature Indicator (OTI) and Winding Temperature Indicator (WTI): Monitor operating temperatures.
• Pressure Relief Device: Releases excess pressure to prevent tank rupture.
• Magnetic Oil Level Gauge (MOG): Indicates oil levels within the conservator.
• Surge Arresters: Protect against voltage transients.

7. Applicable Standards

• IS 2026: General requirements for power transformers.
• IS 12977: Specific to arc furnace transformers.
• IEC 60076: International standard for power transformers.
• IEEE C57.12.00: Standard for liquid-immersed distribution, power, and regulating transformers.

8. Applications

Furnace transformers are integral to various industrial processes, including:

• Electric Arc Furnaces (EAF): Used in steelmaking and foundries.
• Induction Furnaces: For melting ferrous and non-ferrous metals.
• Submerged Arc Furnaces: Employed in the production of ferroalloys.
• Ladle Furnaces: Used for refining molten steel.