145kV Current Transformer (Top-core type)

145kV Current Transformer

145 kV Current Transformer (Top-core Type) is a high-voltage outdoor instrument transformer designed for 145 kV systems, typically used on 132 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 configuration, 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 insulation to ground. This design offers high measurement accuracy, excellent insulation coordination, and reliable long-term operation in outdoor substations.

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

Construction
A typical 145 kV top-core current transformer consists of:
Primary Conductor
-Carries the full transmission 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 excitation current and minimal core losses.
Secondary Windings
-Metering windings for precision measurement.
-Protection windings for relay operation during fault conditions.
-Multiple independent secondary cores can be provided.
Top-Core Assembly
-Houses the magnetic core and secondary windings.
-Hermetically sealed against moisture and environmental contamination.
Insulation System
-Oil-paper insulation for conventional air-insulated substations (AIS).
-SF6 gas insulation for gas-insulated substations (GIS).
-Composite insulation available for severe pollution environments.
Porcelain or Composite Insulator
-Provides external insulation and mechanical support.
-Designed to withstand environmental and seismic stresses.
Oil Expansion System
-Conservator or stainless-steel bellows to compensate for oil volume variations.
Terminal Box
-Contains secondary terminals, grounding facilities, and test links.

Typical Technical Specifications
·Rated Voltage: 145 kV
·Highest System Voltage: 145 kV
·Nominal System Voltage: 132 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 rated transformation ratio.
4.The standardized secondary current (1 A or 5 A) is supplied to metering instruments and protection relays.
5.Complete electrical isolation between the primary and secondary circuits ensures safe operation of measuring and protection equipment.

Advantages
·High metering and protection accuracy.
·Excellent dielectric performance for high-voltage systems.
·Compact and robust top-core construction.
·High thermal and dynamic short-circuit withstand capability.
·Multiple independent secondary cores for simultaneous metering and protection.
·Reliable operation in harsh outdoor environments.
·Low maintenance requirements with a service life typically exceeding 30 years.
·Compatible with both AIS and GIS installations.

Typical Applications
·132 kV transmission substations
·Utility transmission networks
·Industrial high-voltage substations
·Renewable energy grid interconnections
·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: 145 kV
·System Voltage: 132 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 enhances insulation coordination and simplifies maintenance.
·Available with porcelain or silicone rubber composite insulators.
·Supports multiple metering and protection cores within a single unit.
·Designed to withstand high fault currents, seismic loads, UV exposure, and heavy pollution.
·Fully compliant with modern IEC and IEEE standards for high-voltage instrument transformers.

A 145 kV Current Transformer (Top-core Type) is a high-performance instrument transformer specifically developed for 132 kV transmission systems. Its top-core construction provides superior insulation, precise current transformation, robust fault withstand capability, and long-term operational reliability, making it well suited for utility, industrial, and renewable energy substations.