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Low-Frequency Heating
Intro
Low-Frequency Heating (LFH) is an advanced method for drying transformer insulation. Unlike traditional methods that heat from the outside, LFH generates heat directly inside the windings, making the process faster, more efficient, and reliable. It is widely used both in factories and on-site to restore and protect transformers.
Principle of Operation
Generate internal heat: Standard grid frequency (50/60 Hz) is converted to ~1 Hz using a frequency converter.
Apply current to windings: LFH current flows through HV windings while LV windings are short-circuited.
Inside-out heating: Current produces heat within the windings (IĀ²R losses), pushing out moisture from the innermost layers of the paper insulation.
Vacuum drying: A vacuum lowers the boiling point of water, allowing deep moisture extraction. This is critical because the electrical breakdown voltage of the insulation system is lower under vacuum, so a lower voltage is needed to prevent damage (Paschewn’s Law).
Combine with hot oil: Windings are heated from inside while hot oil treats the outside for uniform drying, resulting in a more uniform and comprehensive drying process.
Continuous monitoring: Temperatures are tracked to ensure safety and avoid insulation damage by winding resistance method and temperature controller.
Advantages of LFH Drying
ā” High efficiency & speed ā 30ā50% faster than conventional methods.
š On-site application ā eliminates need to transport large transformers.
š” Energy savings ā requires lower applied voltage and less reactive power.
š Improved insulation quality ā deep drying, moisture below 1%, this improves insulations dielectric strength and hence extends transformer life.
š„ Automated control ā real-time monitoring is being planned to be implemented in future.