36kV Current Transformer (Top-core type)

36kV Current Transformer

36 kV Current Transformer (Top-core Type) is a medium-voltage outdoor instrument transformer designed for power systems with a rated voltage of 36 kV (typically used on 33 kV networks). It measures high primary current and converts it into a standardized secondary current (typically 1 A or 5 A) for metering, protection, and control while providing electrical isolation between the high-voltage system and secondary equipment.

In the top-core type design, the magnetic core and secondary windings are located in the upper head of the transformer. The primary conductor passes through the core at the top, while the lower section primarily serves as the insulated support. This configuration improves insulation performance, simplifies maintenance, and is widely used in outdoor substations.

Main Functions
·Measures line current accurately.
·Supplies current signals for protection relays.
·Provides isolation between high-voltage and low-voltage circuits.
·Supports revenue metering and system monitoring.
·Detects overloads, short circuits, and earth faults.

Construction
A typical 36 kV top-core current transformer consists of:

Primary Conductor
-Carries the full system current.
-Passes through the magnetic core located in the top head.
Magnetic Core
-High-permeability silicon steel or nanocrystalline material.
-Designed for high accuracy and low losses.
Secondary Windings
-Metering cores with high measurement accuracy.
-Protection cores with high accuracy under fault conditions.
-Multiple secondary windings available if required.
Top-Core Assembly
-Contains the magnetic core and secondary windings.
-Enclosed within an insulated housing for protection.
Insulation System
-Oil-paper insulation (most common).
-Epoxy resin insulation for indoor applications.
Composite insulation available for improved pollution resistance.
-Porcelain or Composite Insulator
-Provides electrical insulation and mechanical support.
Terminal Box
-Houses secondary terminals.
-Includes grounding and testing facilities.

Typical Technical Specifications
·Rated Voltage: 36 kV
·Highest System Voltage: 40.5 kV
·Frequency: 50/60 Hz
·Primary Current: 50–3000 A
·Secondary Current: 1 A or 5 A
·Accuracy Class: 0.2, 0.2S, 0.5, 5P10, 5P20, 10P10, PX
·Rated Burden: 5–30 VA
·Short-Time Thermal Current: Up to 31.5 kA (1 s)
·Dynamic Current : Up to 80 kA peak
·Insulation Type: Oil-paper, epoxy resin, or composite
·Installation: Outdoor or indoor

Working Principle
1.The primary conductor carries the system current through the top-mounted magnetic core.
2.The current creates a magnetic flux in the core.
3.The flux induces a proportional current in the secondary windings.
4.The secondary current (1 A or 5 A) is supplied to meters, protection relays, and monitoring devices.
5.The transformer maintains an accurate current ratio while isolating the secondary circuits from the high-voltage system.

Advantages
·High measurement accuracy.
·Compact and robust top-head construction.
·Excellent insulation performance.
·High short-circuit withstand capability.
·Multiple metering and protection cores available.
·Reliable operation in outdoor environments.
·Low maintenance requirements.
·Long service life, typically exceeding 25 years with proper maintenance.

Typical Applications
·33 kV distribution substations
·Utility distribution networks
·Industrial power distribution systems
·Renewable energy substations (wind and solar farms)
·Switchyards
·Ring main units (RMUs)
·Power generation facilities

Applicable Standards
·IEC 61869-1 – General requirements for instrument transformers
·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
·Current Ratio: 100/1 A, 300/1 A, 600/1 A, 1000/1 A, 2000/1 A
·Metering Accuracy: Class 0.2S
·Protection Accuracy: 5P20 or PX
·Rated Burden: 10–30 VA
·Installation: Outdoor or indoor

A 36 kV Current Transformer (Top-core Type) is a reliable and widely used instrument transformer for 33 kV medium-voltage networks, offering accurate current measurement, dependable protection performance, and robust insulation in a compact top-core design suitable for utility and industrial substations.