Current Tranformers (Part-3 Finish)

Note:

General

CTs should be specified as follows:

RATIO: input / output current ratio

VA: total burden including pilot wires.

CLASS: Accuracy required for operation

DIMENSIONS: maximum & minimum limits

Metering CTs In general, the following applies:
CLASS • 0.1 or 0.2 for precision measurements
• 0.5 for high grade kilowatt hour meters for commercial grade kilowatt hour meters
• 3 for general industrial measurements
• 3 or 5 for approximate measurements
BURDEN (depending on pilot lead length) • Moving iron ammeter 1-2VA
• Moving coil rectifier ammeter 1-2.5VA
• Electrodynamic instrument 2.5-5VA
• Maximum demand ammeter 3-6VA
• Recording ammeter or transducer 1-2.5VA

Protection CTs In addition to the general specification required for CT design, protection CT’s require an Accuracy Limit Factor (ALF). This is the multiple of rated current up to which the CT will operate while complying with the accuracy class requirements.
In general the following applies:
• Instantaneous overcurrent relays & trip coils - 2.5VA Class 10P5
• Thermal inverse time relays - 7.5VA Class 10P10
• Low consumption Relay - 2.5VA Class 10P10
• Inverse definite min. time relays (IDMT) overcurrent - 15VA Class 10P10/15
• IDMT Earth fault relays with approximate time grading - 15VA Class 10P10
• IDMT Earth fault relays with phase fault stability or accurate time grading required - 15VA Class 5P10

Class X CTs Class X CTs are special CTs used mainly in balanced protection systems (including restricted earth fault) where the system is sensitively dependent on CT accuracy. Further to the general CT specifications, the manufacturer needs to know:
• Vkp - Voltage knee point
• Io - Maximum magnetising current at Vkp
• Rs - Maximum resistance of the secondary winding