Momentary peaks current

From the oscillogram, shown in Figure 14.4 one can easily determine the average r.m.s. value of the short-circuit current, its duration and the momentary peak current. For easy evaluation, this oscillogram has been divided into ten equal parts (I to lO) and is redrawn in Figure 14.5 for more clarity. The short-circuit commences at point D and concludes at point A2, A A2 being the original zero axis. At the instant of short-circuit, the zero axis shifts to B A2- B) B is the initial d.c. component that decays to zero at A2 at the conclusion of the test. 

For the tap-offs, connecting a UAT through the main bus section between the generator and the generator transformer, however, as discussed above, the momentary peak current will depend upon the short-time rating of such tap-offs. The likely ratings are noted in Table 13.8. 

This design strategy could actually have succeeded. But there is a catch. Let us consider what happens if we just remove the input power. By allowing the duty cycle to go up to such a high level, the momentary peak currents actually increase much more now, as compared to a case where the Dmax is set lower. This has severe implications on the transformer, since its size is related to the saturation level. There are also some other subtle issues. For example, it can be shown that the dissipation in the zener clamp can... 

Rated momentary peak value of the fault current (making current for an interrupting device)... 

The rated momentary peak value of the fault current, /y, will relate to the dynamic rating of an equipment. It is also known as the making current of a switching device and defines its capability to make on fault. 

Table 13.11 Multiplying factors to obtain the momentary peak (maximum r.m.s. or dynamic) values of the short-circuit currents including the sub-transient d.c. component at different power factors (FI/Xl)...

Short-time rating and effect of momentary peak or dynamic currents 15/488... 

Rated momentary peak value of the fault current the same as assigned to the associated switchgear as in Tables 13.11 or 28.1. See also Section 13.4.1 (7). 

Table 28.1 Momentary peak (maximum r.m.s.) current ratings, asymmetrical, for switchgear and metal-enclosed bus systems, based on ANSI-C-37/20C...

Symmetrical Minimum intermitting momentary current current peak for... 

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