24kV Current Transformer (Indoor type)

24kV Current Transformer

24 kV Current Transformer (Indoor Type) is a medium-voltage instrument transformer designed for 24 kV power systems, commonly used on 22 kV and 20 kV distribution networks. It accurately 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 primary and secondary circuits.

Indoor current transformers are typically installed in metal-clad switchgear, ring main units (RMUs), motor control centers (MCCs), and indoor substations. They are generally manufactured using vacuum-cast epoxy resin insulation, which provides excellent dielectric strength, mechanical durability, and resistance to moisture and contamination.

Main Functions
·Measures line current with high accuracy.
·Supplies current signals to protection relays.
·Provides electrical isolation between medium-voltage and low-voltage circuits.
·Supports energy metering, SCADA, and power quality monitoring.
·Detects overloads, short circuits, and earth faults.

Construction
A typical 24 kV indoor current transformer consists of:
Primary Conductor
-Busbar or cable passing through the CT window.
-Wound-primary versions are available for lower current applications.
Magnetic Core
-Manufactured from high-permeability silicon steel or nanocrystalline material.
-Designed for low excitation current and minimal core losses.
Secondary Windings
-Metering winding for precision energy measurement.
-Protection winding for relay operation during fault conditions.
-Multiple independent secondary cores available.
Epoxy Resin Insulation
-Vacuum-cast epoxy resin encapsulates the active components.
-Provides excellent dielectric strength and mechanical protection.
-Maintenance-free and flame-retardant.
Mounting Base
-Compact design for installation inside switchgear panels.
-Suitable for horizontal or vertical mounting depending on equipment design.
Secondary Terminal Block
-Clearly identified secondary terminals.
-Includes grounding point and short-circuiting provisions for safe maintenance.

Typical Technical Specifications
·Rated Voltage: 24 kV
·Highest System Voltage: 24 kV
·Nominal System Voltage: 22 kV
·Frequency: 50/60 Hz
·Primary Current: 5–3000 A
·Secondary Current: 1 A or 5 A
·Accuracy Class: 0.2, 0.2S, 0.5, 1.0, 5P10, 5P20, 10P10, PX
·Rated Burden: 2.5–30 VA
·Short-Time Thermal Current: Up to 31.5 kA (1 s)
·Dynamic Current: Up to 80 kA peak
·Insulation Type: Vacuum-cast epoxy resin
·Installation: Indoor

Working Principle
1.The primary conductor carries the system current through or around the magnetic core.
2.The primary current produces a magnetic flux proportional to the line current.
3.The magnetic flux induces a proportional secondary current according to the CT ratio.
4.The standardized secondary current (1 A or 5 A) is supplied to meters, protection relays, and monitoring systems.
5.The transformer provides electrical isolation while maintaining accurate current transformation.

Advantages
·Compact construction for indoor switchgear.
·High metering and protection accuracy.
·Maintenance-free epoxy resin insulation.
·Excellent resistance to moisture, dust, and aging.
·High thermal and dynamic short-circuit withstand capability.
·Easy installation and replacement.
·Long service life, typically exceeding 25 years.
·Environmentally friendly with no insulating oil or SF6 gas.

Typical Applications
·22 kV distribution substations
·20 kV utility distribution networks
·Metal-clad switchgear
·Ring Main Units (RMUs)
·Industrial power distribution systems
·Commercial buildings
·Renewable energy distribution systems
·Data centers and critical infrastructure

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: 24 kV
·System Voltage: 20–22 kV
·Current Ratio: 50/5 A, 100/5 A, 200/5 A, 400/5 A, 800/1 A, 2000/1 A
·Metering Accuracy: Class 0.2S
·Protection Accuracy: 5P20 or PX
·Rated Burden: 5–20 VA
·Installation: Indoor switchgear

Key Features
·Vacuum-cast epoxy resin insulation ensures excellent dielectric and mechanical performance.
·Available in window type, bar-primary type, and wound-primary type configurations.
·Compact design for installation inside medium-voltage switchgear and RMUs.
·High resistance to partial discharge, vibration, moisture, and thermal aging.
·Supports multiple metering and protection cores within a single unit.
·Fully compliant with modern IEC and IEEE standards for medium-voltage instrument transformers.

A 24 kV Current Transformer (Indoor Type) is a compact, maintenance-free instrument transformer designed for 20 kV and 22 kV indoor distribution systems. Its epoxy resin insulation, high measurement accuracy, excellent short-circuit withstand capability, and reliable long-term performance make it an ideal choice for utility substations, industrial facilities, commercial buildings, and renewable energy installations.