9 THEORY OF OPERATION
Figure 9–7: VECTOR RELATIONSHIPS IN A
2-TERMINAL FAULTED LINE (B-TO-G)
Actually, only two simple faults were investigated. It is obvious that different results would have been obtained for these
same kind of faults if the relative magnitudes of load current, positive sequence fault current, and zero sequence fault cur-
rent had been assumed differently. Also, for the values of currents assumed, different results would obtain for other types of
faults. In addition, if different combinations and weighting factors of the sequence components had been investigated still
different answers would have resulted. In the proper selection of sequence components and weighting factors for Mixed
Excitation phase comparison, the following points must be considered:
Whatever combination and weighting factors are employed, the application rules should be simple enough to make the
As a corollary to the above point, the fewest number of sequence components should be used.
The effects of load current must be minimized. Thus, negative and/or zero sequence components should be weighted
over the positive sequence components.
The limits of application should be broad enough to render the scheme useful as a protection tool.
In line with the considerations stipulated above, the best overall results using mixed excitation would be attained by using
I_2 – KI_1, where K is a constant that is adjustable within limits. While it is likely that the inclusion of zero sequence excita-
tion would be helpful for one case or another, it is not generally employed because the problem of evaluating the overall
performance of the scheme would be magnified considerably. This is true mainly because the current distribution in the
Figure 9–8: VECTOR RELATIONSHIPS IN A
L60 Line Phase Comparison System
2-TERMINAL FAULTED LINE (A-TO-G)