CT magnetization curves Figure 8a CT magnetization curves Figure 8b CT magnetization curves

·        a Defining the knee point in a CT excitation curve according to  European standards

·        b Typical excitation curves for a multi ratio class C CT (From IEEE Standard C57.13-1978; reproduced by permission of the IEEE).

Table 4.3 Standard burdens for protection
CTs with 5
Α secondary current

 Designation Resistance (Ω) Inductance (mH) Impedance (Ω) Volt-amps (at 5 A) Power factor B-1 0.5 2.3 1.0 25 0.5 B-2 1.0 4.6 2.0 50 0.5 B-4 2.0 9.2 4.0 100 0.5 B-8 4.0 18.4 8.0 200 0.5

The errors do not exceed acceptable limits. These factors can be assessed from:

·   formulae;

·   CT magnetization curves;

·   CT classes of accuracy.

The first two methods provide precise facts for the selection of the CT. The third only provides a qualitative estimation. The secondary voltage Ε in Figure 4.6U has to be determined for all three methods. If the impedance of the magnetic circuit, Xm is high, this can be removed from the equivalent circuit with little error' giving Es=Vs and thus:

Vs=IL (ZL+ZC+ZB)    (1)

Where

Vs = r.m.s. voltage induced in the secondary winding

IL       =maximum secondary current in amperes;
this can be determined by dividing the maximum
Fault current on the system by the transformer
turns ratio selected

ZB = external impedance connected

ZL = impedance of the secondary winding

ZC =impedance of the connecting wiring

Use of the formula

This method utilizes the fundamental transformer equation:

Vs = 4.44.f. Α. N. Bmax.10-8 V      (2)

Where

f    =frequency in Hz,

Α    =cross-sectional area of core (cm2)

Ν   =number of turns

Bmax =flux density (lines/cm2)

Next