Present transformers of Stub Protection
used do not transform their principal currents so accurately beneath transient situations as to get a quick time after a short circuit happens.This really is particularly correct when the shortcircuit existing is offset. Beneath such circumstances, supposedly identical existing transformers may not have identical secondary currents, owing to slight differences in magnetic properties or to their obtaining diverse amounts of residual magnetism, and the distinction present may possibly be higher, the bigger the magnitude of short-circuit present.
Even though the short-circuit existing to an external fault just isn't offset, the CT secondary currents may differ owing to differences in the CT kinds or loadings, specifically in power-transformer protection. Since the percentage-differential relay has a increasing pickup characteristic as the magnitude of by way of present increases, the relay is restrained against operating improperly.Figure 20 shows the comparison of a simple overcurrent relay with a percentage-differential relay under such conditions. An overcurrent relay having the same minimum pickup as a percentage-differential relay would operate undesirably when the differential present barely exceeded the value X, whereas there would be no tendency for the percentage-differential relay to operate.
Distance Relay Power
relays are often instantaneous or higher speed. Time delay just isn't necessary for selectivity since the percentage-differential characteristic and also other supplementary functions to become described later make these relays practically immune towards the effects of transients when the relays are properly applied. The adjustments offered with some percentage-differential relay will probably be described in connection with their application.A number of other kinds of differential-relay arrangements may be mentioned.
A single of those uses a directional relay. Yet another has added restraint obtained from harmonics and also the d-c component of the differential current. One more type makes use of an overvoltage relay instead of an overcurrent relay within the differential circuit. Specific present transformers may be used getting small or no iron in their magnetic circuit to avoid errors in transformation triggered by the d-c component of offset existing waves. All these types are extensions of Current Differential Relay Protection
which have been described, and they'll be treated later in connection with their specific applications.There has been great activity within the development of the differential relay due to the fact this type of relay is inherently the most selective of all the conventional kinds. However, every form of system element presents unique issues that have as a result far created it not possible to devise a differential-relaying equipment obtaining universal application.