Outdoor Oil Cooled Current Transformer

1.Service Voltage :
System Voltage is which CT is to be installed e.g. 11 kV, 22 kV, 33 kV. etc.

2.Installation :
Whether OUTDOOR or INDOOR

3.Atmospheric Conditions :
Such as condition of Pollution, Altitude Ambient Temperature etc.

4.Insulation Level :
If insulation level other than associated with service voltage is required, it should be specifically mentioned

5.Rated primary current :
Specify rated primary current / currents (if required more than one value) Also indicate if different primary current is required for different cores.

6.Continuous Primary current :
Max. primary current that can be withstood continuously by current transformer e.g. 120% of primary current.

7.Rated secondary current :
Whether 1 Amp or 5 Amps

8.Short Time Current & its Duration :
Specify fault current of the system in which CT is to be installed along with its duration. It is most important to specify realistic value of S.T.C. as at lower primary current, higher S.T.C. value necessitates bulky & costlier design. Also specify dynamic current if other than 2.5 times S.T.C. is required.

9.No. of cores Their Burdens Accuracy :
Basis of application, No. of cores, their burdens and accuracy class should be specified. It is advisable to specify minimum required Burden for metering core as unnecessary high burden will necessitate for bulky and costlier design. Specified accuracy is guaranteed for 100% to 25% of rated burden only. Current transformer offers minimum error if 75% to 60% burden is connected to secondary. Therefore, ideally rated burden higher than 1.5 time actual burden should be specified.

Also, it is important to specify correct burden in context of Instrument Security Factor (I.S.F.) The I.S.F. indicates the over current as multiple of rated current at which the metering core will saturate, thus limiting the secondary current flowing through meter and protect it from damage. If actual burden connected is half of the rated burden, the actual I.S.F. will increase two-fold of its rated value.

10. Knee point voltage, Secondary resistance and excitation current :
For differential protection. R.E.F. Protection, Bus Bar Protection. C.T. with accuracy class PS is required. The Knee Point Voltage. Secondary resistance and excitation current should be specified for this core. It is always better to specify Formula for Knee Point Voltage related to relay used for the protection. This will help designerto optimise the design.