Figure 3.5 also shows why the TDOC relay cannot be used without additional help. The closer the fault is to the source, the greater the fault current magnitude, yet the longer the tripping time. The addition of an instantaneous over current relay makes this system of protection viable. If an instantaneous relay can be set to ‘‘see’’ almost up to, but not including , the next bus, all of the fault clearing times can be lowered as shown in Fig . 3.6. In order to properly apply the instantaneous over current relay, there must be a substantial reduction in short circuit current as the fault moves from the relay toward the far end of the line. However, there still must be enough of a difference in the fault current between the near and far end faults to allow a setting for the near end faults. This will prevent the relay from operating for faults beyond the end of the line and still provide high-speed protection for an appreciable portion of the line.
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| FIGURE 3.5 Coordination of TDOC relays. |
Since the instantaneous relay must not see beyond its own line section, the values for which it must be set are very much higher than even emergency loads. It is common to set an instantaneous relay about 125–130% above the maximum value that the relay will see under normal operating situations and about 90% of the minimum value for which the relay should operate.
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| FIGURE 3.6 Effect of instantaneous relays. |
