Page 382 - Industrial Power Engineering and Applications Handbook

Switchgear and controlgear assemblies 13/357
where suffixes 1 and 2 refer to the original and the new those by asymmetry, i.e. Id, as dynamic. The cumulative
base values respectively. effect of this is electrodynamic and is quite significant.
It requires adequate care while designing a current-canying
I, . rated current or supporting system. Switching devices and other current-
Fault current I,, = ].e.
short-circuit unit impedance carrying components and their mounting structures, such
(13.5) as the busbar systems in a PCC, MCC or a bus duct etc.
must withstand such stresses during a short-circuit. It is
and fault MVA = baseMVA etc. (13.6) therefore of vital importance to take account of this
~
LP asymmetry and to determine this to form an important
design parameter for switching devices and all current-
Fault levels of HT systems carrying systems.
The peak value of a fault current will depend upon the
We illustrate a typical powerhouse generation and content of the d.c. component. The d.c. component will
transmission system layout in Figure 13.21, and reproduce depend upon the p.f. of the faulty circuit and the instant
in Table 13.10 the typical fault levels of different at which the short-circuit commences on the current wave.
transmission and distribution networks in practice for (Refer to Figure 13.27, illustrating the variation in
different voltage systems. asymmetry with the p.f. of the faulty circuit. For ease of
We also provide a brief reference to a protective scheme, application, it is represented as a certain multiple of the
usually adopted in a large power-generating station as in r.m.s. value of the symmetrical fault current Isc.)
Section 16.8.2. The content of asymmetry may decay quickly and
may exist in the system for just three or four cycles
6 Duration of fault from the commencement of the fault, depending upon
We have mentioned two systems, 1 second and 3 second. the time constant, z, of the circuit. The time constant, 7,
A choice of any of them would depend upon the location is the measurement of the rate of decay of the d.c.
and the application of the equipment and criticality of component, and is the ratio of the system reactance, L,
the installation. Generally speaking, it is only the one- to the system resistance R, i.e. L/R. A large L/R will
second system that is in practice. The three-second system indicate a high time constant and a slow rate of decay
may sometimes be used for low fault level networks, and vice-versa, as illustrated in Figure 13.22. The
where a. I,,, would fall within the capability of the asymmetry is therefore measured by the peak of the
available interrupting devices and at reasonable cost. first major loop of the fault current (which may occur in
any of the phases, as it has occurred in phase Y in the
7 Rated momentary peak value of the fault current oscillogram shown in Figure 13.23). The subsequent
loops will be smaller and less severe and thus the
A fault current on a power system is normally asym- significance of the first loop. This is referred to as the
metrical as discussed next, and is composed of a momentary peak value of the short-circuit current for
symmetrical a.c. component ISC(r,m,s,) and an asymmetrical the most severe fault conditions, such as at extremely
sub-transient d.c. component Id, (Figure 14.5). The forces low p.f.s (RlX, being very low) when the recovery voltage
arising out of Zsc are referred to as electromagnetic and may be the maximum and the fault current the highest.
These values are given in Table 13.11 in the form of
likely multiplying factors for different symmetrical r.m.s.
Table 13.10 Typical fault levels for an integrated transmission values of fault currents, Zsc, according to IEC 60439-1
and distribution network
for LT and IEC 60694 for HT systems and are drawn in
Figure 13.22. These values are almost the same as
Nominal Highest Symmetrical Minimum provided by ANSI-C-37120C as well as in Table 28.1.
system system voltage interrupting momentary
voltage current current peak for The exact values of this factor may be estimated by
rating (r.m.s.) dynamic rating creating a short-circuit condition and obtaining an
kV(r.m.s.) kV (r.m.s.) kA Is, kA(PW oscillogram. For details refer to Figure 13.23.
~
765 800 40 IO0 Notes
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400 420 40 100 The rated momentary peak value of the fault current, lM, will
220 245 3 1.5140 791100 relate to the dynamic rating of an equipment. It is also known
as the making current of a switching device and defines its
132 2513 1 62.5177.5 capability to make on fault.
33 25 62.5 The peak value of the asymmetry is considered to determine
the electrodynamic stresses to design the mechanical system
15/24kV* - -*- -*- and the supporting structure for the current-canying components.
11 12 40 IO0 A breaker will not trip instantly when a fault occurs, but only
6.6 7.2 40 100 after a few cycles, depending upon the actuating time of the
3.3 3.6 40 100 protective relays and the breaker’s own operating time. It will
0.415 0.44 43/50 90.01105 therefore generally trip only during the transient state of the
fault. The breaking capacity of an interrupting device, unlike
*Since this represents the generator voltage, therefore the fault its making capacity, is therefore defined by the peak value of
level will be governed by the generator and the generator transformer the transient state fault current, Le. by Is, (Table 13.9).
as indicated in Table 13.8. Conventionally it is termed the r.m.s. value of the fault current.