72.5kV Current Transformer (Top-core type)

72.5kV Current Transformer

72.5 kV Current Transformer (Top-core Type) is a high-voltage outdoor instrument transformer designed for 72.5 kV systems (commonly used on 66 kV transmission and sub-transmission networks). It accurately transforms 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 low-voltage secondary equipment.

In the top-core type configuration, the magnetic core and secondary windings are housed in the upper head of the current transformer. The primary conductor passes through this upper assembly, while the porcelain or composite insulator supports the active part and provides the required insulation to ground. This design offers excellent electrical performance, compact construction, and high reliability for outdoor substations.

Main Functions
·Accurately measures line current.
·Supplies current signals for protective relays.
·Provides electrical isolation between primary and secondary circuits.
·Supports revenue metering and system monitoring.
·Detects overloads, short circuits, and earth faults.

Construction
A typical 72.5 kV top-core current transformer consists of:
Primary Conductor
-Carries the full system current.
-Passes through the magnetic core in the top head.
Magnetic Core
-Constructed from high-permeability silicon steel or nanocrystalline material.
-Designed for low losses and high measurement accuracy.
Secondary Windings
-Metering windings for precise energy measurement.
-Protection windings for relay operation during fault conditions.
-Multiple secondary cores can be provided.
Top-Core Assembly
-Contains the magnetic core and secondary windings.
-Sealed to protect against moisture and contaminants.
Insulation System
-Oil-paper insulation (most common).
-SF6 gas insulation for GIS applications.
Composite insulation available for polluted environments.
-Porcelain or Composite Insulator
-Provides external insulation and mechanical support.
Oil Expansion System
-Conservator or bellows-type expansion device for oil-filled units.
Terminal Box
-Contains secondary terminals, grounding connections, and test links.

Typical Technical Specifications
·Rated Voltage: 72.5 kV
·Highest System Voltage: 72.5 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 current generates magnetic flux within the core.
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 instruments and protective relays.
5.The transformer maintains accurate current transformation while providing galvanic isolation between the high-voltage and low-voltage circuits.

Advantages
·High measurement and protection accuracy.
·Compact top-core design with efficient insulation.
·Excellent short-circuit withstand capability.
·Reliable performance in outdoor environments.
·Multiple metering and protection cores available.
·Low maintenance requirements.
·Long operational life, typically exceeding 30 years.
·Suitable for both conventional substations and GIS installations.

Typical Applications
·66 kV transmission substations
·Utility sub-transmission networks
·Industrial power systems
·Renewable energy grid connections
·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: 72.5 kV
·System Voltage: 66 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

A 72.5 kV Current Transformer (Top-core Type) is a dependable high-voltage instrument transformer widely used in 66 kV transmission and sub-transmission systems. Its top-core construction provides excellent insulation performance, accurate current measurement, robust fault withstand capability, and long-term reliability for utility and industrial substations.