CT magnetization curves |
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· 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
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
I_{L}
=maximum secondary current in amperes; 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 (cm^{2}) B_{max} =flux density (lines/cm^{2}) |
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