Surge impedance

Note Since the surge impedance of a circuit is normally very high, as noted in Section 17.8, it is the voltage transient that is the cause of concern in the above case than the current transient. 

Figure 17.7(a) Surge impedance of rotating machines as a function of inpuf MVA/VUne kV (from Pretorious and Eriksson, 1982)... 

Figure 17.7(b) Typical value rating of equipmenf of surge impedances for HT mofors and transformers... 

Figure 17.7 (c) Surge impedance of HT motors as a function of output power... 

Figure 17.8 Likely surge voltages that may be developed by different types of circuit breakers as a function of circuit surge impedance...

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... 

Circuit interruption is a transient condition, as it constitutes abrupt changes in the circuit parameters L and C, which also alter the characteristics of the transient wave and its behaviour. The characteristics of a transient wave depends upon the circuit s surge impedance, which, in turn, depends upon the circuit parameters L and C ... 

The value of L and C of a machine will determine its surge impedaiice = -jUC and surge frequency /, = LC). A low surge impedance will help to... 

A C-R suppressor also helps to reduce the surge impedance, of the circuit and thus limits the amplitude... 

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. 

The behaviour of a transient wave at a junction of two conductors, such as at junction J in Figure 18.8, is similar to that of water, when it passes through one large-diameter pipe to another of a smaller diameter. Some of the water will flow ahead and the remainder will backflow at the junction. Similarly, a transient wave will also reflect in part or in full at a junction between two conductors of different surge impedances, depending upon the surge impedance of the circuit ahead of the junction. This would give rise to two types of waves, i.e. 

If Zsi = surge impedance of the incoming circuit Zs = surge impedance of the outgoing circuit (Fiaure 18.8(a))... 

If the surge impedance of the transformer is considered as 4000 n, then the voltage of the refracted wave... 

Approx, surge impedance from graphs of Figure 17.7(b) by extrapolation (obtain accurate value from the manufacturer)... 

Zi = surge impedance of Ihe eqtiipmeni lo be protected Z, = surge impedance of the arrester it I ... 

Z, = surge impedance of the affected line T = travelling time of the switching surge in us II = number of consecutive discharges... 

V Z./C is the surge impedance, Z, L and C the circuit constants of the interrupting circuit, as discussed in (Section 17.6.4). C represents the dielectric capacitance between the parting contacts of the interrupter. must be prevented, as far as practicable, from reaching dangerous levels with the use of surge arresters. 

At the instant of switching the surge impedance, Z, (equation (23.12)), and the natural frequency of the switched circuit, i.e. the transient frequency, /i, (equation (2.2.14)). determines the amount of inrush current. 

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