ABSTRACT
A number of cases in electric circuits of complex configuration with several power sources, neither maximal current protections nor directional protections (power direction relays), can provide selectivity of tripping a short circuit (SC). Why? Let us consider a simple example of a ring circuit with a two-way supply (Figure 13.1), provided with directional current protection (with each set of relays Rel containing current relays and power direction relays). The direction of the relay action and the directions of the power flows in the circuit when an SC occurs, are indicated by the arrows. As it can be seen in Figure 13.1a, an SC (SC1) creates conditions for picking up of sets of relays (Rel1, Rel2 and Rel3). Rel4 does not pick up because the direction of the power flow at that point of its installation does not agree with the standard installation of the power direction relay (from the bus bar to the line). For selective deenergization of only Line 1, on which the SC occurs, relays Rel2 and
Rel1 must pick up before relay Rel3 does. That is: t2 < t3. But if an SC (SC2) occurs on the other line (Line 2), for selective deenergization, the relays Rel3 and Rel4 must pick up before relay Rel2, that is, t2 > t3. Therefore, we can see that it is impossible to meet such opposing requirement with the help of current and directional protections. In such cases, so-called ‘‘distance protection’’ is used. Distance protection is that in which the time delay varies according to the distance to the point at which the SC has occurred (Figure 13.2). If each of the relays installed along the line have time delays depending on impedance
(distance), the relay which picks up first will always be the one that is nearest to the point of short circuiting. This is the main purpose of distance protection. In circuits with twoway supply, distance protection is directional (that is it responds only to one direction of the power flow). An example of a distance protection scheme having coordinated characteristics of
circuit breaker protection on a system with two-end power supply is seen in Figure 13.3. The time-delay characteristics of the odd-numbered relays 1, 3, and 5, operating on the fault-power flow in the left-to-right direction (denoted by the arrows at the circuit breakers) are represented on the top side of the axis, with those of the evennumbered protections below the axis. In the diagram, the shaded portions represent the tripping times of the distance protections on the corresponding lines. Thus, for example, when a fault appears on line BC (at point SC.), relays 3 and 4 operate with the minimum time delay tl. If for one or another reason, these protections fail to operate, circuit breakers 1 and 6 of the preceding circuit sections from the power supply are tripped, with the time delay t2 and t3. This is how back-up of the protection on the adjoining circuits is achieved.
