36kV Current Transformer (Indoor type)

36kV Current Transformer

36 kV Current Transformer (Indoor Type) is a medium-voltage instrument transformer designed for 36 kV power systems, typically used on 33 kV distribution and sub-transmission 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 high-voltage primary circuit and the low-voltage secondary equipment.

Indoor current transformers are widely installed in metal-clad switchgear, switchboards, ring main units (RMUs), motor control centers (MCCs), and indoor substations. Most modern units are manufactured using vacuum-cast epoxy resin insulation, providing excellent dielectric strength, mechanical durability, moisture resistance, and maintenance-free operation.

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

Construction
A typical 36 kV indoor current transformer consists of:
Primary Conductor
-Busbar or cable passing through the transformer window.
-Wound-primary versions are available for low-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 accurate current measurement.
-Protection winding for relay operation during fault conditions.
-Multiple independent secondary cores may be provided.
Epoxy Resin Insulation
-Vacuum-cast epoxy resin encapsulates the active components.
-Provides excellent dielectric strength, mechanical support, and environmental protection.
-Flame-retardant and maintenance-free.
Mounting Base
-Compact design for installation inside switchgear cubicles.
-Suitable for horizontal or vertical mounting depending on equipment configuration.
Secondary Terminal Block
-Clearly identified secondary terminals.
-Includes grounding point and short-circuiting facilities for safe maintenance.

Typical Technical Specifications
·Rated Voltage: 36 kV
·Highest System Voltage: 40.5 kV
·Nominal System Voltage: 33 kV
·Frequency: 50/60 Hz
·Primary Current: 5–4000 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 generates a magnetic flux proportional to the line current.
3.The magnetic flux induces a proportional secondary current according to the CT transformation ratio.
4.The standardized secondary current (1 A or 5 A) is supplied to metering devices, protection relays, and monitoring equipment.
5.Electrical isolation between the primary and secondary circuits ensures safe operation of connected instrumentation.

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

Typical Applications
·33 kV distribution substations
·Metal-clad switchgear
·Ring Main Units (RMUs)
·Industrial power distribution systems
·Renewable energy collection substations
·Commercial and institutional facilities
·Mining and heavy industrial plants
·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: 36 kV
·Highest System Voltage: 40.5 kV
·System Voltage: 33 kV
·Current Ratio: 50/5 A, 100/5 A, 200/5 A, 400/5 A, 1000/1 A, 2000/1 A, 3000/1 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 provides superior dielectric and mechanical performance.
·Available in window type, bar-primary type, and wound-primary type configurations.
·Compact dimensions enable easy installation in medium-voltage switchgear.
·High resistance to partial discharge, vibration, humidity, and thermal cycling.
·Supports multiple metering and protection cores within a single transformer.
·Fully compliant with modern IEC and IEEE standards for medium-voltage instrument transformers.

A 36 kV Current Transformer (Indoor Type) is a reliable, compact, and maintenance-free instrument transformer specifically designed for 33 kV indoor distribution systems. With vacuum-cast epoxy resin insulation, high measurement accuracy, excellent short-circuit withstand capability, and long-term operational reliability, it is an ideal solution for utility substations, industrial facilities, renewable energy plants, and commercial power distribution networks.