Combined Current and Voltage Transformer:

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What is a combined current and voltage transformer?

A combined current and voltage transformer is also known as a combined instrument transformer (CIT) contains a current transformer and an inductive voltage transformer within the same body.

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 (like low values up to 110v and 1A), which are used for metering, protection, and control of the high voltage system.

These are predominantly used where the need for accurate and reliable voltage and current transformation is essential.

Combined current and voltage transformers in a substation
Combined current and voltage transformers

Application of Combined current and voltage transformer:

Combined transformers are suitable for use in substations where space or installation costs make using independent Transformers difficult.

  • Ideal for installation at metering points due to their very high accuracy class, both in current and voltage. Used for Revenue metering and Check or indication metering
  • Suitable for the discharge of high-voltage lines and capacitor banks. 
  • Excellent frequency response; ideal for monitoring power quality and measuring harmonics.

Construction of Combined current and voltage transformer:

As said above, A CIT accommodates the current transformer (CT) and voltage transformer in a single unit/ housing. The constructional feature of CIT is the active part of the voltage transformer situated at the bottom and the current transformer situated at top of the porcelain housing.

Combined Instrument Transformers are available either in oil or SF6 dielectric systems.

Here we consider an Oil-insulated combined instrument transformer as shown in the figure.

Construction of Combined current and voltage transformer
Parts of Combined current and voltage transformer
  • The HV terminals of both the transformers are at the top and the LV terminals are at the bottom. The center pipe of the CT bushing carries a secondary terminal (low potential) connected to the cable box and the VT bushing carries high voltage cables connected to the high voltage transmission line.
  • CT core/windings are located on the top of the equipment VT core/winding are located on the bottom of the equipment
  • Bar-type primary conductor allows higher short circuit current and avoids large voltage drop across the primary winding
  • Oil insulated Combined Instrument Transformers provide suitable electrical insulation between high voltage and low voltage measuring equipment.
  • Oil-paper internal insulation. High-grade, PCB-free mineral oil is used.
  • External insulation with porcelain or silicone composite insulator
  • External parts made of aluminum or stainless materials
  • The hermetically sealed housing protects the oil-paper insulation against atmospheric influences. A suitable lnox steel bellows located on the top of the Combined Transformer allows the oil volume variation and keep the internal pressure equal to ambient one in every service condition.
  • The oil level is indicated by a mechanical system in the window of the bellows cover.

Advantages of using Combined Instrument Transformers:

The location of both CT and VT modules in one housing offers reductions in the total cost of ownership of a substation due to:

  • A lower number of transformers in a bay.
  • Lower material costs due to a reduced number of supports and fewer primary connections
  • Lower transportation costs with one unit instead of two.
  • Less space is needed with just one footprint.
  • Lower cost of civil works.
  • Lower installation effort as only one unit has to be installed instead of two.

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