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Current Transformers

Stemar's works are fully equipped to manufacture and test current transformers to any specified standard with Primary Currents up to 20000 amps and secondary currents up to 10amps.

Basic Current Transformer Theory.
 

Types of Current Transformers

Current Transformers are divided into 2 major categories:

  • MEASUREMENT: Intended for use with indicating or integrating meters
  • PROTECTION: Intended for use with protection devices

WARNING!!! The use of metering class CTs with protection devices is strongly undesirable as the characteristics are DIFFERENT!!!

Basic Design Concepts

Fundamentally all Current Transformers follow the same basic principle:

  • AT s in primary= ATs in secondary +ATs error
  • That is: Current x primary turn or turns fundamentally equals:Current x Turns in Secondary
  • Due to excitation current and internal voltage drop due to internal resistance we do not have a perfect vector transposition between primary and secondary currents. This is due to: Ratio or CURRENT ERROR and Phase angle shift or PHASE ANGLE ERROR
  • The more accurate the reflection in the secondary of the primary current the smaller the current and phase angle errors.

These errors are dependant on:

  • Excitation ampere turns
  • Type of magnetic material used
  • Configuration of the transformer; in general the smaller the ID and OD the more accurate the transformer for the same excitation ATS
  • The above basic principles are fundamental to both metering and protection CTs.

Construction

Current Transformers can be constructed in a number of different ways:

  • WOUND PRIMARY: with a fixed primary with a number of turns around the core;the winding is fully insulated and form an integral part of the assembly
  • BAR PRIMARY: this has a fixed insulated primary conductor passing through the core and essentially the primary conductor is equal to 1 turn
  • BUSHING TYPE:this has no primary winding;it is for use with a fully insulated conductor acting as the primary winding.
  • WINDOW TYPE:this has no primary winding but it normally has insulation in the window so that conductors can be passed through

Applicable Standards

With the event of a global market Current Transformers are manufactured to a number of different standards but in Australia we predominately work with the following:

  • Australian Standards AS 1675, 60044-1:2003 60044-1:2007
  • IEC standards 185, 60044-1 & 60044-6
  • ANSI standard C57-13

The approach taken in specifying CTs by each of these standards is different

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