Example 7.2Figure 7.5 shows the dif

Example 7.2
Figure 7.5 shows the difference in the operating characteristics of the generator differential when
using an overcurrent relay as shown in Figure 7.1 or a constant percentage differential relay as
shown in Figure 7.3. Also shown is a typical plot of the error current due to CT unbalance caused
by different burdens or saturation. Both relays are set for the same pickup of 0.1 A. It is clear
that a through-current greater than 4.5 A will exceed 0.1 A in the operating coil and will trip the
overcurrent relay incorrectly, whereas the entire error current plot lies in the nonoperating region
of the differential relay, and there would be no tendency for the percentage differential relay to
operate.