123kV Current Transformer (Top-core type)

123kV Current Transformer

123 kV Current Transformer (Top-core Type) is a high-voltage outdoor instrument transformer designed for 123 kV systems (typically used on 110 kV transmission networks). It converts high primary current into a standardized secondary current (typically 1 A or 5 A) for metering, protection, monitoring, and control while providing complete electrical isolation between the high-voltage primary circuit and the secondary equipment.

In the top-core type design, the magnetic core and secondary windings are housed in the upper head of the transformer. The primary conductor passes through this top assembly, while the porcelain or composite insulator supports the active section and provides the required insulation to ground. This configuration delivers excellent insulation coordination, high measurement accuracy, and dependable long-term performance in outdoor substations.

Main Functions
·Measures transmission line current with high accuracy.
·Supplies current signals to protection relays.
·Provides galvanic isolation between primary and secondary circuits.
·Supports revenue metering, SCADA, and power quality monitoring.
·Enables fast detection of overloads, short circuits, and earth faults.

Construction
A typical 123 kV top-core current transformer consists of:
Primary Conductor
-Carries the full system current.
-Passes through the magnetic core located in the upper head.
Magnetic Core
-Manufactured from high-permeability silicon steel or nanocrystalline material.
-Optimized for low losses and high accuracy.
Secondary Windings
-Metering cores for precision measurement.
-Protection cores for relay operation under fault conditions.
-Multiple independent secondary cores available.
Top-Core Assembly
-Encloses the magnetic core and secondary windings.
-Hermetically sealed to prevent moisture ingress.
Insulation System
-Oil-paper insulation (most common for AIS).
-SF6 gas insulation for GIS applications.
-Composite insulation available for harsh or polluted environments.
Porcelain or Composite Insulator
-Provides external insulation and mechanical support.
-Designed for high mechanical and electrical strength.
Oil Expansion System
-Conservator or stainless-steel bellows to compensate for oil volume changes.
Terminal Box
-Contains secondary terminals, grounding points, and test links.

Typical Technical Specifications
·Rated Voltage: 123 kV
·Highest System Voltage: 123 kV
·Nominal System Voltage: 110 kV
·Frequency: 50/60 Hz
·Primary Current: 100–4000 A
·Secondary Current: 1 A or 5 A
·Accuracy Class: 0.1, 0.2, 0.2S, 0.5, 5P10, 5P20, 10P20, PX
·Rated Burden: 10–30 VA
·Short-Time Thermal Current: Up to 40 kA (1 s)
·Dynamic Current: Up to 100 kA peak
·Insulation Type: Oil-paper, SF6, or composite
·Installation: Outdoor AIS or GIS

Working Principle
1.The primary conductor carries the transmission current through the top-mounted magnetic core.
2.The primary current generates magnetic flux proportional to the line current.
3.The magnetic flux induces a proportional current in the secondary windings according to the transformer ratio.
4.The standardized secondary current (1 A or 5 A) is supplied to metering devices and protective relays.
5.Electrical isolation between the primary and secondary circuits ensures safe operation of control and protection equipment.

Advantages
·High metering and protection accuracy.
·Excellent insulation performance for high-voltage applications.
·Compact top-core construction.
·High thermal and dynamic short-circuit withstand capability.
·Multiple independent metering and protection cores.
·Reliable operation under severe environmental conditions.
·Minimal maintenance with long service life, typically exceeding 30 years.
·Suitable for both air-insulated substations (AIS) and gas-insulated substations (GIS).

Typical Applications
·110 kV transmission substations
·Utility transmission networks
·Industrial high-voltage substations
·Renewable energy grid interconnection
·Power generation switchyards
·Railway traction substations
·Gas-insulated substations (GIS)

Applicable Standards
·IEC 61869-1 – Instrument Transformers – General Requirements
·IEC 61869-2 – Current Transformers
·IEEE C57.13 – Instrument Transformers
·IEC 60044-1 – Legacy standard (superseded by IEC 61869-2)

Typical Ratings
·Voltage Class: 123 kV
·System Voltage: 110 kV
·Current Ratio: 200/1 A, 600/1 A, 1200/1 A, 2000/1 A, 3000/1 A
·Metering Accuracy: Class 0.2S
·Protection Accuracy: 5P20, 10P20, or PX
·Rated Burden: 15–30 VA
·Installation: Outdoor AIS / GIS

Key Features
·Top-core design improves insulation coordination and simplifies construction.
·Available with porcelain or silicone rubber composite insulators.
·Supports multiple secondary cores for simultaneous metering and protection.
·Designed to withstand severe weather, pollution, seismic loads, and high fault currents.
·Fully compliant with modern IEC and IEEE instrument transformer standards.

A 123 kV Current Transformer (Top-core Type) is a robust and highly accurate instrument transformer widely used in 110 kV transmission systems. Its top-core construction provides excellent insulation performance, dependable protection and metering accuracy, and long-term reliability, making it an ideal choice for utility, industrial, and renewable energy substations.