Why Are Arcing Horns No Longer Used on Transformer Bushings?
Arcing horns were once widely used on high-voltage transformer bushings as a simple and economical means of protecting insulation of bushings and transformer windings, against lightning and switching surges. They provided an external flashover path, allowing surge energy to bypass the bushing & transformer insulation.
Today, arcing horns are rarely used on new transformer installations in view of modern insulation coordination practices and use of zinc oxide (ZnO) surge arresters which provide faster, safer, and more reliable protection.
What is an Arcing Horn?
An arcing horn consists of two metallic electrodes separated by a predetermined air gap and connected in parallel with the transformer bushing insulation.
Typically, one electrode is connected to the high-voltage terminal of the bushing, whereas the other electrode is connected to the grounded mounting flange. When a lightning or switching surge appears on HV terminal, flashover occurs between the electrodes of arcing horn. This provides an external discharge path, diverting the surge current to ground and thus protecting the bushing & transformer insulation.
Construction of the Arcing Horn
The most common design is a rod-to-rod configuration, consisting of:
One metallic rod connected to the high-voltage terminal of the transformer bushing.
One metallic rod connected to the grounded mounting flange.
A carefully designed variable air gap between the two electrodes, with one of the electrodes movable to adjust the air gap.
The gap is selected so that it flashes over at pre-decided voltage dependent on ambient air quality.
Why Modern Transformer Bushings Do Not Use Arcing Horns
Superior Protection with ZnO Surge Arresters: Modern gapless ZnO surge arresters limit transient overvoltages almost instantaneously without flashover, providing far superior insulation coordination than arcing horns.
Unpredictable Flashover Performance: The flashover voltage of an arcing horn is influenced by environmental conditions such as humidity, rain, pollution, altitude, air pressure, and electrode condition. Consequently, its protective performance is less predictable and less reliable.
Sparkover Voltage Changes Over Time: In addition to environmental conditions, electrode erosion and ageing gradually change the air-gap characteristics, making the sparkover voltage difficult to maintain throughout the service life.
Arcing Can Damage Equipment: Repeated flashovers can erode terminals, damage fittings, cause surface tracking, and shorten equipment life. Modern protection philosophy is to prevent flashovers rather than rely on them.
Electrode Erosion and Maintenance: Repeated flashovers erode the horn electrodes, alter the required air gap, and require periodic inspection and adjustment of the gap to ensure proper operation.
Risk of Sustained Arcing and Very Fast Transients (VFT): Following flashover, a power-frequency arc may persist until protective relays clear the fault. This sustained arcing generates significant thermal stress and produces Very Fast Transient (VFT) overvoltages with extremely steep wavefronts (typically less than 20 ns), imposing additional electrical stress on transformer windings and condenser-graded bushing insulation, potentially accelerating insulation degradation.
Electromagnetic Interference (EMI): Arcing generates electromagnetic interference and the electrode of arcing horn increases the EMI.
Disturbance of Electric Field Distribution: Modern condenser bushings are designed with carefully optimized electric field distribution. External arcing horns result in disturbing this field, creating localized electrical stress, making them unsuitable for HV and EHV applications.
Are Arcing Horns Completely Eliminated?
No. Arcing horns remain in some legacy installations, but modern transformer bushings rely on gapless ZnO surge arresters for superior, reliable, and maintenance-free surge protection.
Conclusion
Arcing horns were useful in earlier transformer protection, but modern insulation coordination and ready availability and use of gapless ZnO surge arresters have made them obsolete for new bushings.