Page 498 - Industrial Power Engineering and Applications Handbook

15/472 Industrial Power Engineering and Applications Handbook

(a) Single-phase HT CT
11 kVCT 33 kV CT

Figure 15.15(a) Typical outdoor-type oil-filled bar primary or
wound primary HV CTs (Courtesy: Kappa Electricals)

(b) Three-phase LT CT
Figure 15.14 Typical indoor-type bar primary epoxy resin
cast CTs (Courtesy: Kappa Electricals)

circuit (the impedance of all the devices connected to it),
expressed in ohms and power-factor or volt-amperes, at
the rated secondary current. The CTs will be selected
nearest to the computed total VA burden in the circuit. A
CT with a higher VA burden than connected will have a
slightly higher error besides size. A slightly less VA rating
than the connected may normally be permissible, subject
to confirmation by the manufacturer. Typical values
of VA burdens at the rated current for the devices that
a measuring CT may have to usually feed are as
follows:

Instruments and measuring devices*
Moving iron ammeters 1.5 to 5 VA Figure 15.1 5(b) 400 kV bar primary outdoor current transformer
Recording ammeters 2 to 10 VA (Courtesy: BHEL)
Current coils of watt-meters 5 VA
Recording watt-meters 5 VA
kWh and kVAr meters 5 VA
Thermal demand ammeters 3 VA *These values are typically for moving iron instruments and devices.
Thermal maximum demand ammeters 4 to 8 VA For electronic instruments and devices they would be of the order
Power factor (p.f.) meters 5 VA of 0.1 to 0.5 VA.