145 kV Current Transformer (Hair-pin Type) is a high-voltage outdoor instrument transformer used in 132 kV and 145 kV air-insulated substations (AIS). It accurately converts high primary currents into standardized secondary currents (1 A or 5 A) for protective relays, energy metering, fault recording, and substation automation. The hair-pin type features an integral U-shaped primary conductor that forms a single-turn primary winding, providing high mechanical strength, excellent insulation performance, and reliable operation.
Working Principle
The 145 kV hair-pin type current transformer operates on the principle of electromagnetic induction.
When alternating current flows through the U-shaped primary conductor, an alternating magnetic flux is produced in the magnetic core. This changing flux induces a proportional current in the secondary winding while maintaining electrical isolation between the primary and secondary circuits.
Construction
A typical 145 kV hair-pin type current transformer consists of:
·Integral U-shaped copper or aluminum primary conductor
·High-permeability toroidal magnetic core
·Multiple secondary windings for metering and protection
·Oil-paper insulation system (most common)
·Porcelain or composite external insulator
·Oil expansion chamber or stainless-steel bellows
·Secondary terminal box
·Grounding terminal
·Galvanized steel support base
·Stainless steel nameplate
Features
·Single-turn hair-pin primary conductor.
·Excellent measurement accuracy.
·Multiple independent secondary cores.
·High insulation performance for 145 kV systems.
·High thermal and dynamic short-circuit withstand capability.
·Weather-resistant outdoor construction.
·Low partial discharge characteristics.
·Long service life with minimal maintenance.
Typical Ratings
·Highest system voltage: 145 kV
·Rated frequency: 50 or 60 Hz
·Primary current: 200–4000 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, TPX, TPY, TPZ
·Rated burden: 10–60 VA
·Lightning impulse withstand: Typically 650 kV
·Power-frequency withstand: Typically 275 kV (1 min)
·Installation: Outdoor AIS
Advantages
·High accuracy for both protection and revenue metering.
·Excellent insulation coordination for 132/145 kV systems.
·Robust single-turn primary conductor provides high mechanical strength.
·Multiple secondary cores support separate metering and protection functions.
·High resistance to thermal and mechanical stresses during short circuits.
·Reliable long-term performance under outdoor environmental conditions.
Limitations
·Oil-filled units require periodic inspection of oil condition, seals, and insulation integrity.
·Larger and heavier than medium-voltage resin-cast CTs.
·Requires adequate structural support during installation.
·The secondary winding must never be open-circuited while the primary is energized, because dangerous voltages can be induced across the secondary terminals.
Applications
145 kV hair-pin type current transformers are widely used in:
·132 kV and 145 kV transmission substations
·Transmission line bays
·Power transformer protection
·Busbar differential protection
·Circuit breaker bays
·Revenue and utility metering
·Industrial high-voltage substations
·Renewable energy grid substations
·Switching stations
Typical Secondary Core Arrangement
Core 1 Revenue metering 0.2S
Core 2 Operational metering 0.5
Core 3 Overcurrent and earth-fault protection 5P20
Core 4 Busbar or transformer differential protection PX / TPX / TPY / TPZ
Typical Uses
The 145 kV Hair-pin Type Current Transformer is widely deployed in 132 kV and 145 kV air-insulated substations for:
·Current measurement for supervisory control and data acquisition (SCADA)
·Revenue and utility energy metering
·Overcurrent and earth-fault protection
·Transformer and busbar differential protection
·Fault recording and disturbance monitoring
·Grid monitoring and substation automation
Its integral single-turn primary conductor, high dielectric strength, multiple secondary cores, and excellent measurement accuracy make it a dependable solution for modern high-voltage transmission systems where reliability, safety, and precision are critical.