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Oil Insulated Current Transformers

Oil-insulated current transformer are used to convert high transmission line currents (up to 5000 A) to standardized low and easily measurable values, which will be used for metering, protection and control of the high voltage system. As such, the need for accurate and reliable current transformation is essential.

The reliability and security of Trench oil-insulated instrument transformers is based on 100 years of innovation with oil-insulated units operating under a wide range of environmental conditions. Oil-insulated current transformers also ensure suitable electrical insulation between high voltage and low voltage measuring equipment.
Our oil-immersed current transformers are compliant to the latest international safety standard requirements in terms of explosion-proof, insulating material multiple chopped ageing, seismic qualification and extreme temperature conditions.
The production in different Trench locations across the globe with modern manufacturing and testing capabilities ensures a long life time and reliability.


As early as

production started

in Italy for oil instrument transformers – making Trench a real pioneer in this field.

Thousands of oil-immersed current transformers installed and in service for more than 100 years all over the world and in all environmental conditions are the best guarantee of the quality and reliability of our products.


  • Fully covering the range from 72 to 550 kV
  • Fully meeting all lEC and ANSI metering and protection classes comprising TPS, TPX, TPY and TPZ ones
  • Rated primary current up to 5000 A
  • Rated secondary current of 1A, 2A or 5A
  • Primary and/or secondary reconnection available. Rated short circuit withstand up to 200 kA peak
  • Fully type tested according to International Standards
  • Key oil impregnated paper insulation has been optimized after lang experience based on famous Haefely-Trench original design

Product Design

  • Oil/paper internal insulation
  • External insulation with porcelain or composite insulator
  • External part made by aluminum or stainless materials
  • Cores/windings located an the top of CT [head type]
  • Bar-type primary conductor allows higher short circuit current and avoids large voltage drop across the primary winding
  • Available to withstand:
    • Internal arc effects [according to IEC 61B69 Standards]
    • Multichopped Test [according to IEC 61869 Standards]
    • Heavy seismic stress according to IEC 6221, IEEE 693 and specific Customer request [ i.e. Chile ETG A.0.20 ]
  • Hermetically sealed. A suitable lnox steel bellows allows the oil volume variation and keep the internal pressure equal to ambient one in every service conditions

Product Process

  • Optimized Design to Manufacturing
  • Lean manufacturing concept applied to the whole supply chain
  • Modular design allows the use of reduced number of components
  • Partial discharge free design and process
  • Optimized Drying and Soaking Oil Process easily allow the use of special Oils in case of specific Customer’s needs. (Low ambient temperature applications or special Oil requirement for various reasons)
  • Maintenance free during a lang lifetime of more than 30 years
  • Trench Management System has been certified according to ISO ISO 9001, ISO 14001 and OHSAS 18001 Standards

Mechanical and electrical data

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