72.5 kV Current Transformer (Hair-pin Type) is a high-voltage instrument transformer used in transmission and sub-transmission substations to accurately measure high primary currents and provide isolated secondary currents for protection, metering, monitoring, and control. The hair-pin type incorporates a single U-shaped (hair-pin) primary conductor that passes through the magnetic core, offering a simple, robust, and reliable design.
Working Principle
A 72.5 kV hair-pin type current transformer operates according to the principle of electromagnetic induction.
When alternating current flows through the integral hair-pin primary conductor, it generates an alternating magnetic flux in the transformer core. This flux induces a proportional current in the secondary winding.
Construction
A typical 72.5 kV hair-pin current transformer includes:
·Integral U-shaped copper or aluminum primary conductor
·High-permeability toroidal magnetic core
·One or more secondary windings
·Porcelain or composite hollow insulator
·Oil-paper or SF6 insulation system (depending on manufacturer)
·Expansion chamber or oil conservator (for oil-filled designs)
·Secondary terminal box
·Grounding terminal
·Mounting pedestal
·Stainless steel nameplate
Features
·Integral single-turn hair-pin primary.
·Excellent measurement accuracy.
·Suitable for both metering and protection.
·Multiple secondary cores available for different functions.
·High thermal and dynamic short-circuit withstand capability.
·Outdoor weather-resistant construction.
·Low partial discharge levels.
·Long operating life with minimal maintenance.
Typical Ratings
·Highest system voltage: 72.5 kV
·Rated frequency: 50 or 60 Hz
·Primary current: 100–4000 A
·Secondary current: 1 A or 5 A
·Metering accuracy class: 0.1, 0.2, 0.2S, 0.5, 0.5S
·Protection accuracy class: 5P10, 5P20, 10P10, PX, TPY
·Rated burden: 10–60 VA
·Lightning impulse withstand: Typically 325 kV
·Power-frequency withstand: Typically 140 kV (1 min)
·Installation: Outdoor AIS
Advantages
·High accuracy over a wide current range.
·Compact primary conductor design.
·Excellent mechanical strength.
·Reliable insulation for 72.5 kV systems.
·High short-circuit withstand capability.
·Multiple secondary cores support separate metering and protection circuits.
·Long service life under outdoor operating conditions.
Limitations
·Cannot be serviced while energized.
·Larger and heavier than medium-voltage resin-cast CTs.
·Oil-filled units require periodic inspection for oil condition and seals.
·The secondary winding must never be open-circuited while the primary is energized, as hazardous voltages can be induced.
Applications
A 72.5 kV hair-pin type current transformer is commonly used in:
·66 kV and 72.5 kV transmission substations
·Utility switching stations
·Power transformer bays
·Transmission line feeder bays
·Busbar protection schemes
·Circuit breaker protection
·Revenue metering
·Industrial high-voltage substations
·Renewable energy grid interconnections
A 72.5 kV Hair-pin Type Current Transformer is a dependable instrument transformer for 66/72.5 kV outdoor air-insulated substations (AIS). Its integral single-turn primary conductor provides high mechanical strength, excellent accuracy, and reliable performance for protection relays, energy metering, fault recording, and supervisory control systems under demanding transmission network conditions.