Terminal impedance

FIGURE 5-6 Transmission line representation of infinite diffusion layer Warburg Impedance terminated in a load... 

Any main CT that is itndeiioaded will also add to the error in the measurement. Similarly, if provision is tnade in the primary of the summation CT to accommodate fntnre circuits but is not being utilized it tnust be left open, otherwise it will also add to the error. The impedance of the shorting terminals will add to the impedance of the circuit and will increase the total error. 

W = energy released iti kW-s or kj V, = prospective crest of the surge in kV Zj = surge impedance of the power system and the terminal equipment in Q... 

For instance, when lightning of, say, a nominal discharge current of 10 kA strikes a 400 kV (r.m.s.) overhead line, having a surge impedance of 350 Q, then two parallel waves will be produced each of amplitude 10 x 350/2 or 1750 kV which may be more than the impulse withstand level of the system and cause a flashover between the conductors and the ground, besides damaging the line insulators and the terminal equipment (Table 13.2). It is therefore imperative that the system is protected against such eventualities. 

Z( = zero phase sequence or residual impedance. This is measured between the three-phase terminals of a star winding shorted together and the neutral (Figure 20.9(b)) and is calculated by... 

The more recommended practice is to ground the neutral solidly or through an impedance, commensurate with the requirements of the protective scheme and the fault current limited to a desired level. The terminal equipment and the windings of all the machines may now be designed for a voltage corresponding to the relevant GFF. 

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