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Oil insulated Combined Instrument Transformers

Oil-insulated combined instrument transformers are used to convert high transmission line voltages and currents (voltage up to 145kV and current up to 5000 A) to standardized low and easily measurable values, which are used for metering, protection and control of the high voltage system. As such, the need for accurate and reliable voltage and current transformation is essential.

The reliability and security of Trench oil-insulated instrument transformers is based on over 100 years of innovation with oil-insulated units operating under a wide range of environmental conditions. Oil-insulated combined instrument transformers also ensure suitable electrical insulation between high voltage and low voltage measuring equipment.

Our oil-insulated combined 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.

Modern production and testing facilities guarentee not only short delivery times but also a reliable operation for many years.


Reduce by %

the required footprint

for instrument transformers by combining Trench CTs and VTs into one product.

Thousands of oil-immersed combined transformers installed and in service for more than 50 years all over the world and in all environmental conditions are the best guarantee of the quality and reliability of our products.


  • Portfolio available from 72 to 145 kV
  • Meet all lEC and ANSI metering and protection classes, including special core classes TPS, TPX, TPV and TPZ ones
  • Rated primary Voltage up to 145 kV; rated primary current up to 5000A
  • Rated Secondary Voltages: 100 V, 100:1’3 V, 100:3 V, 110 V, 110:1’3 V, …
    Rated secondary current of 1A, 2A or 5A
  • Primary and/or secondary reconnection available. Rated short circuit withstand up to 200 kA peak
  • Suitable also for Une Oischarge use
  • Fully type tested according to international standards
  • The key oil impregnated paper insulation has been optimized after lang experience based an famous Heafely-Thrench original group
  • Special tests are also available to meet specific customer requirements.

Product Design

  • Oil/paper internal insulation
  • External insulation with porcelain or composite insulator
  • External parts made of aluminum or stainless materials
  • CT core/windings located an the top of the equipment VT core/winding located an the bottom of the equipment
  • Bar-type primary conductor allows higher short circuit current and avoids large voltage drop across the primary winding
  • Available to withstand:
    • Effects of internal arc [according to IEC 61869 Standard]
    • Multichopped Test [according to IEC 61869 Standard]
    • Heavy seismic solicitations, according to IEC 62271, IEEE 693 and specific customers’ requests [i.e. Chile ETG A.0.20]
  • Hermetically sealed. A suitable lnox steel bellows located an the top of the Combined Transformer 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 numbers of components
  • Partial discharge free design and process
  • Optimized Drying and Soaking Oil Process allows easily 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 to ISO 9001, ISO 14001 and OHSAS 18001 standards

Mechanical and electrical data

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