Power system protection refers to the complex of the automatic devices intended for a rapid definition and disconnection from the electric power system the damaged elements of this electric power system in emergency situations with the purpose of ensuring normal functionality of the system.
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The functioning of the power system protection devices is organized on the principle of continuous estimation of the technical state of the separate controlled elements of the electric power systems. Power system protection carries out continuous control of the state of all elements of the electric power system and reacts in the case the system’s elements are damages or the system is working in an abnormal mode. In the case of damages, power system protection is to define the damaged elements and disconnect it from the system, operating the special power switches, intended for breaking of damaging currents (short circuit).
Reliability is the main property, characterizing ability of the power system protection to operate correctly and smoothly in all modes of the controlled object at all types of damages and abnormal modes this protection is intended, and not to react at normal functionality, as well as at such damages and violations of the normal mode, at that the reaction of this defence is not intended. In other words, reliability is such a property of the power system protection that ensures the system functioning in any external environments. Basic reliability indicators are time of faultless work and intensity of failure.
One of the most popular type of power system protection is a protection based on the principle of current control. Current protection of maximal type is on at exceeding the controlled current of certain level (response level). Current protection of minimum type is on at reduction of current to the certain level (response level). Depending on how selectivity is provided with subsequent (from the source of feed) protection, overcurrent protection and current cut-off are distinguished.
In radial (open) networks with the voltage class of 6-10 kV and higher the most widespread variant of protection from three-phase and line-to-line short circuits is two-stage protection, including overcurrent and current cut-off protections.
For realization of overcurrent protection, time dependant relays are used in a number of cases, and for current cut-off protection, it is always time independent relays. Thus, the protection can be with on two separate relays, or with one relay combining both stages, and also on the base of digital multi-stage relays.
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