292 HANDBOOK OF ELECTRICAL ENGINEERING
- Use a set of multiplying factors to modify the precalculated value of the rms symmetrical sub-
transient currentIf′′. Apply the factor at the given fault clearance time. (This factor functions in
a manner similar to the ‘doubling factor’ described in sub-sections 11.6 and 11.6.1.3.) Suitable
values of the factor are given in clause 12.2.1.3 of IEC60909, equation (47) and Figure 16 therein.
Whichever method is used it is not usually necessary to include the contribution of fault
current from induction motors, because such current will have decayed to almost zero at the fault
clearance time. If there are large motors connected to the main switchboard then their contribution
will be similar to a generator and should be included, see sub-section 7.2.7 and Reference 3 therein,
and sub-section 11.8.5.
11.8 Computer Programs for Calculating Fault Currents
Now that computers have become so widely available in both the office and in the home, it is
relatively easy to program the calculations described in the previous sub-sections. Radial system
equations are particularly easy to compute.
As a project moves into the detail design phase it acquires more precise data for all aspects
of the work. It is then possible to calculate the fault currents more accurately. However, it should
be noted that the tolerances on most of the data are seldom better than plus or minus 15%, and
so increasing the quantity of data will not necessarily improve the results significantly. During the
detail design phase the power system tends to be modified and additional switchboards added. It is
then necessary to calculate the fault currents at least at the busbars of each switchboard, and this can
become a laborious task if hand calculations are attempted.
There are many commercially available computer programs for calculating fault currents.
Some programs include other features such as load flow, harmonic penetration, transient stability,
motor starting and volt-drop calculations, since these features tend to use the same database. Usually
a program that calculates fault currents will have several special features for different types of
faults e.g.,
- Radial and meshed networks.
- Three-phase zero impedance fault.
- Three-phase non-zero impedance fault.
- Single-phase faults.
- Line-to-line faults.
- Line-to-line-to-ground faults.
These features are calculated with the aid of symmetrical component theory, see Reference 5
to 8. Apart from the simplest situations the solutions are too complicated and time consuming to
attempt by hand.
11.8.1 Calculation of fault current – rms and peak asymmetrical values
For most LV and all HV generators it is often acceptable to ignore the armature resistance as far as
calculating the magnitude of ‘first-cycle’ fault currents is concerned. It is usual to assume that the