The shortcomings of over-current relays graded by current and time led to the widespread use of distance protection. The distance between a relay and the fault is proportional to the ratio (voltage/current) measured by the distance relay. Relays responsive to impedance, admittance (mho), or reactance may be used.
Although a variety of time-distance characteristics are available for providing correct selectivity, the most popular one is the stepped characteristic shown in Figure 11.23. Here, A, B, C, and D are distance relays with directional properties and A and C only measure distance when the fault current flows in the indicated direction. Relay A trips its associated breaker instantaneously if a fault occurs within the first 80% of the length of feeder 1. For faults in the remaining 20% of feeder 1 and the initial 30% of feeder 2 (called the stage 2 zone), relay A initiates tripping after a short time delay. A further delay in relay A is introduced for faults further along feeder 2 (stage 3 zone). Relays B and D have similar characteristics when the fault current flows in the opposite direction.
Figure 11.23 Characteristic of three-stage distance protection
The selective properties of this scheme can be understood by considering a fault such as at F in feeder 2, when fault current flows from A to the fault. For this fault, relay A starts to operate, but before the tripping circuit can be completed, relay C trips its circuit breaker and the fault is cleared. Relay A then resets and feeder 1 remains in service. The time margin of selectivity provided is indicated by the vertical intercept between the two characteristics for relays A and C at the position F, less the circuit-breaker operating time.
It will be noted that the stage 1 zones are arranged to extend over only 80% of a feeder from each end. The main reason for this is because practical distance relays and their associated equipment have errors, and a margin of safety has to be allowed if incorrect tripping for faults which occur just inside the next feeder is to be avoided. Similarly, the stage 2 zone is extended well into the next feeder to ensure definite protection for that part of the feeder not covered by stage 1. The object of the stage 3 zone is to provide general back-up protection for the rest of the adjacent feeders.
The characteristics shown in Figure 11.23 require three basic features: namely, response to direction, response to impedance and timing. These features need not necessarily be provided by three separate relay elements, but they are fundamental to all distance protective systems. As far as the directional and measuring relays are concerned, the number required in any scheme is governed by the consideration that three-phase, phase-to-phase, phase-to-earth, and two-phase-to-earth faults must be catered for. For the relays to measure the same distance for all types of faults, the applied voltages and currents must be different. With electro-mechanical relays it was common practice, therefore, to provide two separate sets of relays, one set for phase faults and the other for earth faults, and either of these caters for three- phase faults and double-earth faults. Each set of relays was, in practice, usually further divided into three, since phase faults may concern any pair of phases, and, similarly, any phase can be faulted to earth. With digital relays, a pre-selection of relaying quantities allows just one processor to deal with all types of fault.