12kV Current Transformer (Indoor type)

12kV Current Transformer

12 kV Current Transformer (Indoor Type) is a medium-voltage instrument transformer designed for 12 kV power systems (typically used on 11 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 commonly installed inside metal-clad switchgear, ring main units (RMUs), motor control centers (MCCs), and indoor substations. They are usually insulated with epoxy resin using vacuum casting technology, providing excellent dielectric strength, mechanical durability, and resistance to moisture.

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

Construction
A typical 12 kV indoor current transformer consists of:
Primary Conductor
-Busbar or cable passing through the transformer window.
-In wound-type CTs, the primary consists of one or more turns of conductor.
Magnetic Core
-High-permeability silicon steel or nanocrystalline core.
-Designed for low excitation current and minimal losses.
Secondary Windings
-Metering winding for accurate current measurement.
-Protection winding for relay operation during fault conditions.
-Multiple secondary cores can be incorporated in one unit.
Epoxy Resin Insulation
-Vacuum-cast epoxy resin provides insulation and mechanical strength.
-Flame-retardant, moisture-resistant, and maintenance-free.
Mounting Base
-Suitable for installation in switchgear panels or indoor substations.
-Compact design minimizes required installation space.
Secondary Terminal Block
-Clearly marked terminals for wiring.
-Includes grounding point and short-circuiting provisions for maintenance.

Typical Technical Specifications
·Rated Voltage: 12 kV
·Highest System Voltage: 12 kV
·Nominal System Voltage: 11 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: Epoxy resin cast
·Installation: Indoor

Working Principle
1.The primary conductor carries the system current through or around the magnetic core.
2.The primary current generates a magnetic flux proportional to the line current.
3.The magnetic flux induces a proportional current in the secondary winding according to the transformer ratio.
4.The standardized secondary current (1 A or 5 A) is supplied to meters, protection relays, and monitoring devices.
5.The transformer provides electrical isolation while maintaining high measurement accuracy.

Advantages
·Compact design suitable for indoor switchgear.
·High measurement and protection accuracy.
·Maintenance-free epoxy resin insulation.
·Excellent resistance to humidity, dust, and aging.
·High short-circuit withstand capability.
·Easy installation and replacement.
·Long service life, typically exceeding 25 years.
·Safe and environmentally friendly, with no insulating oil or SF6 gas.

Typical Applications
·11 kV distribution substations
·Metal-clad switchgear
·Ring Main Units (RMUs)
·Motor Control Centers (MCCs)
·Industrial power distribution systems
·Commercial buildings
·Renewable energy distribution systems
·Data centers and critical power facilities

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: 12 kV
·System Voltage: 11 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: 5–15 VA
·Installation: Indoor switchgear

Key Features
·Vacuum-cast epoxy resin insulation provides excellent dielectric and mechanical performance.
·Compact dimensions for installation inside medium-voltage switchgear.
·Available in window type, bar-primary type, and wound-primary type configurations.
·High resistance to partial discharge, moisture, vibration, and thermal aging.
·Suitable for continuous operation under demanding industrial and utility conditions.
·Fully compliant with modern IEC and IEEE standards.

A 12 kV Current Transformer (Indoor Type) is a compact, maintenance-free instrument transformer specifically designed for 11 kV indoor distribution systems. With epoxy resin insulation, high measurement accuracy, robust short-circuit performance, and reliable long-term operation, it is widely used in switchgear, substations, industrial facilities, and commercial power distribution applications.