In order to understand the rationale for distance relaying on transmission lines, it is useful to recognize the limitations of simple overcurrent (50/51) protection. Consider this single-line diagram of a transmission line bringing power from a set of bus-connected generators to a substation at some remote distance. For simplicity’s sake, only one protective relay is shown in this diagram, and that is for breaker “F” feeding the transmission line from the generator bus:
The purpose of the overcurrent relay tripping breaker “F” is to protect the transmission line and associated equipment from damage due to overcurrent in the event of a fault along that line, and so the relay must be set appropriately for the task. The amount of fault current this relay will see depends on several factors, one of them being the location of the fault along the transmission line. If we imagine a fault occurring on the line near breaker “F,” the fault current will be relatively high because it is close to the generator bus and therefore experiences little transmission line impedance to limit current. Conversely, if we imagine a fault farther out on the transmission line (closer to breaker “G”), the amount of current caused by the fault will be less, even for the exact same type
of fault, simply due to the added series impedance of the transmission line’s length (if I = V/Z and Z increases while line voltage V remains the same, I must decrease). Any similar fault further downstream of the generators – such as a fault in one of the transformers in the substation – will draw even less current through breaker “F” than a similar fault on the transmission line for the same reason of greater series impedance.
An important concept in protective relaying is that of protection zones. Protective relays exist to protect the power system from damage due to faults, and they do so by tripping circuit breakers to interrupt the flow of power to a fault. However, in the interest55 of maintaining power to customers it is best for protective relays to only trip those breakers necessary to clear the fault, and no more. Thus, protective relays are designed to trip specific breakers to protect limited “zones” within the system. In this next single-line diagram, we show the same system with rectangular zones overlaid on the system components:
This zone diagram makes it clear that breaker “F” and its associated overcurrent relay should only act to protect the transmission line from fault-induced damage. If a fault happens to occur within one of the transformer zones within the substation, we would prefer that fault be cleared by the protective relays and breakers for that transformer alone (i.e. either breakers “H” and “J” or breakers “I” and “K” depending on which transformer faults), in order that power be maintained in the rest of the system. This means the overcurrent relay controlling breaker “F” needs to be sensitive to faults within the transmission line zone, but insensitive to faults lying outside of the transmission line zone. If the 50/51 relay happened to overreach its zone and trip breaker “F” because of current sensed from a fault in one of the substation transformers, it would unnecessarily cut power to the entire substation, with a loss of power to all loads fed by that substation.
At first, the problem of overreaching may seem simple to solve: just calculate the maximum fault current in the transmission line due to any worst-case fault outside of that zone, and be sure to set the overcurrent relay so that it will only trip at some current greater than that amount, or set it so it will trip after a longer time delay than the substation relay(s) will trip, to give the substation relays a chance to clear the fault first. The weakness of this approach is that fault location is not the only factor influencing fault current magnitude. Another important variable is the number of generators in service at the time of the fault. If one or more of the generators happens to go off-line, it reduces the generator bus’s ability to supply current to a fault. Another way of saying this is that the power source’s impedance changes with the number of generators on-line. This means any given fault downstream of breaker “F” will cause less fault current than it would if all generators were on-line.
This causes a problem for the “reach” of the overcurrent relay controlling breaker “F.” With reduced current capacity from the generator bus, the same relay setting that worked well to protect the transmission line zone will now be too high for faults lying toward the far end of that line. In other words, the overcurrent relay may underreach and fail to trip breaker “F” because the amount of fault current for a transmission line fault is now less than what the relay has been set to protect against, and all because we happen to have fewer generators on-line to supply power. The impedance of the transmission line and fault may be precisely the same as before, but the overcurrent relay will not trip because the circuit’s total impedance has changed due to fewer generators being on-line.
We see that the location of a fault within a long-distance power distribution system cannot be reliably detected by sensing current alone. In order to provide more consistent and reliable zone protection for the transmission line, we need a form of protection better able to discriminate fault location. One such method is to measure the impedance of the protected zone, based on current and voltage measurements at the entry point of power into that zone. This is the fundamental concept of distance protection: calculating the impedance of just the protected zone, and acting to trip breakers feeding power to that zone if the impedance suggests a fault within the boundaries of that zone.