123kV Current Transformer (Hair-pin type)

123kV Current Transformer

123 kV Current Transformer (Hair-pin Type) is a high-voltage instrument transformer used in 110 kV and 123 kV air-insulated substations (AIS) to accurately transform high primary currents into standardized secondary currents (typically 1 A or 5 A) for protection, metering, control, and monitoring. The hair-pin type uses an integral U-shaped primary conductor that forms a single-turn primary winding, providing a robust, compact, and highly reliable design.

Working Principle

The 123 kV hair-pin type current transformer operates on the principle of electromagnetic induction.

When alternating current flows through the U-shaped primary conductor, it creates an alternating magnetic flux in the transformer core. This magnetic flux induces a proportional current in the secondary winding.

Construction
A typical 123 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 protection and metering
·Porcelain or composite hollow insulator
·Oil-paper insulation system (most common) or SF6 insulation (depending on design)
·Oil expansion chamber or bellows (for oil-filled units)
·Secondary terminal box with terminal blocks
·Grounding terminal
·Galvanized steel mounting base
·Stainless steel nameplate

Features
·Integral single-turn hair-pin primary conductor.
·High measurement accuracy over a wide current range.
·Multiple independent secondary cores for metering and protection.
·Excellent insulation performance for 123 kV systems.
·High thermal and dynamic short-circuit withstand capability.
·Outdoor weather-resistant construction.
·Low partial discharge characteristics.
·Long operating life with minimal maintenance.

Typical Ratings
·Highest system voltage: 123 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
·Rated burden: 10–60 VA
·Lightning impulse withstand: Typically 550 kV
·Power-frequency withstand: Typically 230 kV (1 min)
·Installation: Outdoor AIS

Advantages
·Excellent accuracy for protection and revenue metering.
·High insulation reliability for 110/123 kV networks.
·Compact single-turn primary construction.
·High mechanical strength and seismic capability.
·Multiple secondary cores allow independent protection and metering circuits.
·Long service life with low maintenance requirements.

Limitations
·Larger and heavier than medium-voltage resin-cast CTs.
·Oil-filled designs require periodic inspection of oil level, seals, and insulation condition.
·Installation requires adequate support due to the height and weight of the unit.
·The secondary winding must never be open-circuited while the primary is energized, as dangerous voltages can develop across the secondary terminals.

Applications
A 123 kV hair-pin type current transformer is widely used in:
·110 kV and 123 kV transmission substations
·Incoming and outgoing transmission line bays
·Power transformer protection
·Busbar protection schemes
·Circuit breaker bays
·Revenue metering systems
·Industrial high-voltage substations
·Wind and solar farm grid substations
·Utility switching stations

Typical Secondary Core Arrangement
A 123 kV hair-pin CT often includes multiple secondary cores, for example:
·Core 1 Revenue metering 0.2S
·Core 2 Plant metering 0.5
·Core 3 Overcurrent and earth-fault protection 5P20
·Core 4 Differential or busbar protection PX / TPX / TPY

The 123 kV Hair-pin Type Current Transformer is a key component in 110/123 kV air-insulated substations, providing precise current measurement, galvanic isolation, and dependable signals for protective relays, energy meters, and substation automation systems. Its integral single-turn primary conductor, high insulation performance, and multiple secondary cores make it well suited for modern transmission networks requiring high reliability and measurement accuracy.