Low Tension Current Transformers are used for measurement and/or protective
applications in 660V Class. In Measuring Current Transformers (Refer TABLE-
1) main concern is the accuracy of the secondary current over a wide operating
range. Typically the errors are guaranteed to be within specified limits. The nature
of errors are current (Ratio) error and phase displacement. Metering current
transformers are specified by (1) a burden and (2) accuracy class.
Protective Current transformers (Refer TABLE-II) are specified by (1) a burden
(2) accuracy class and (3) accuracy limiting factor. In other words the errors of the
protective current transformers at the rated full loaded current as well as at a current
given by the accuracy limit factor times the full load current are specified.
There exists a class of protective current transformers for special purpose application
known as class PS CTs. These are mainly intended for use in applications where the
required characteristics of the current transformers cannot be conveniently expressed
in terms used for class 5P, 10P and 15P current transformers. The knee point voltage,
exciting current and secondary winding resistance are so dependent on the protective
gear involved. The manufacturers of protection gear stipulates the requirements in
terms of minimum knee point voltage and maximum exciting current. The current
transformers of class PS type are of low reactance type since the core is made up
of jointless ring type (spirally wound toroidal type) cores and secondary turns are
substantially evenly distributed along the whole length of the magnetic circuit. PS
class current transformers are characterised by
(1) Normal ratio
(2) Knee point voltage (KPV)
(3) Exciting current at KPV or a stated percentage
(4) Secondary resistance at 750 C.
CONSTRUCTIONAL FEATURES
Current Transformer can be of Ring type, Rectangular type, or Resin moulded
type. High quality cores are used for the construction. Computer aided designs are
used for all ratings to improve quality and to satisfy customer requirements
A. Ring Type & Rectangular Type
Low Tension Current Transformers are tape insulated, toroidal type. They are available
in both ring type and rectangular type as per consumer requirement. High quality
core is used in the Current Transformers and the manufacturing is under strict
supervision and high quality control.
B. Matching / Interposing Current Transformer
1) Interposing CT: CTs are either Resin cast or framed construction. They are
accurate and suitable for the required matching ratio as specified by the customer.
2) Universal Matching CT: Suitable for matching any ratio. Nine tappings are
provided and it can be used to match any ratio.
C. Resin cast Low Tension CTs
Low Tension Resin cast CTs with terminals : ARI, AR2, AR3
Low Tension Resin cast CT with bar primary : AR4
Low Tension Resin cast CTs window type : AR-5, AR-6.
Given below are standard sizes of LTCTs for metering & Protection. Details of special CTs will be furnished on request
