The principal application of time delay over current relays (TDOC) is on a radial system where they provide both phase and ground protection. A basic complement of relays would be two phase and one ground relay. This arrangement will protect the line for all combinations of phase and ground faults using the minimum number of relays. Adding a third phase relay, however, provides complete backup protection, that is two relays for every type of fault, and is the preferred practice.
TDOC relays are usually used in industrial systems and on sub transmission lines that cannot justify more expensive protection such as distance or pilot relays.
There are two settings that must be applied to all TDOC relays: the pickup and the time delay.
THE PICKUP SETTING: is selected so that the relay will operate for all short circuits in the line section for which it is to provide protection. This will require margins above the maximum load current, usually twice the expected value, and below the minimum fault current, usually 1/3 the calculated phase to phase or phase to ground fault current. If possible, this setting should also provide backup for an adjacent line section or adjoining equipment.
THE TIME DELAY FUNCTION: is an independent parameter that is obtained in a variety of ways, either the setting of an induction disk lever or an external timer. The purpose of the time delay is to enable relays to coordinate with each other. Figure 3.4 shows the family of curves of a sing le TDOC model. The ordinate is time in milliseconds or seconds depending on the relay type; the abscissa is in multiples of pickup to normalize the curve for all fault current values. Figure 3.5 shows how TDOC relays on a radial line coordinate with each other.
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| FIGURE 3.4 Family of Time Difference Over current Relay time-current characteristics. |
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| FIGURE 3.5 Coordination of TDOC relays. |
