Protection schemes can be divided into two major groupings:
a) Unit schemes
b) Non-unit schemes
In other words, it is possible to design protection systems that respond only to fault conditions occurring within a clearly defined zone. This type of protection system is known as unit protection. Certain types of unit protection are known by specific names, e.g. Restricted Earth Fault and Differential Protection. Unit protection can be applied throughout a power system and, since it does not involve time grading, is relatively fast in operation. The speed of response is substantially independent of fault severity.
Unit protection usually involves comparison of quantities at the boundaries of the protected zone as defined by the locations of the current transformers. This comparison may be achieved by direct hard-wired connections or may be achieved via a communications link. However certain protection systems derive their restricted property from the configuration of the power system and may be classed as unit protection, e.g. Earth Fault Protection applied to the high voltage delta winding of a power transformer. Whichever method is used, it must be kept in mind that selectivity is not merely a matter of relay design. It also depends on the correct coordination of current transformers and relays with a suitable choice of relay settings, taking into account the possible range of such variables as fault currents, maximum load current, system impedances and other related factors, where appropriate.
b) Non-unit Protection Scheme: The non-unit schemes, while also intended to protect specific areas, have no fixed boundaries. As well as protecting their own designated areas, the protective zones can overlap into other areas. While this can be very beneficial for backup purposes, there can be a tendency for too great an area to be isolated if a fault is detected by different non unit schemes.
The most simple of these schemes measures current and incorporates an inverse time characteristic into the protection operation to allow protection nearer to the fault to operate first.
The non unit type protection system includes following schemes:
a) Time graded over-current protection