In power survivability is understood as property of object to resist to indignations, not supposing their cascade development with mass infringement of a consumers’ feed.

           Reserve automatic inclusion is one of the basic ways of maintenance of survivability of unit of loading. Restoration of electro supply of unit of loading should take place not only at damage of the basic power supply or disappearance of a pressure on the feeding side, but also in cases when switching-off of the power supply occurs owing to false or not selective work of relay protection, spontaneous switching-off of the switch, erroneous actions of the attendants at carrying out repaired or adjusted works, and also at damage of isolation to the circuits of an operative current leading switching-off of the switch.

           High technical economic efficiency and simplicity of schemes ÀÂÐ have caused their wide application in power supply systems and networks of the industrial enterprises.

           Figure 1 - Main reduce substations (MRS)

           MRS fails at concurrence in space and time of three casual events: short circuit on trunks 6-10 êV; refusal in operation of the introduction switch 10 êV; refusal in operation of the introduction switch 220-110 êV.

           We consider that current protection on the switching device «4» is not sensitive to currents ÊZ on trunks of substation 10 êV.

           Survivability of substation is defined by means of system of the linear differential equations of the kind:

,

           Survivability of an electro power system depends on its structure, a configuration, reliability of an electric equipment, means of relay protection and anti emergency automatics, and also from qualification of the attendants, a stock of stability, a reserve of active capacity, etc.

           As parameter of survivability of the scheme of electro supply of the industrial enterprise frequency of occurrence of system chained failures with various depth of infringement of electro supply can serve.

           The formula for definition of frequency of occurrence of chained failures which can occur at ÊZ in a protected element of a network and refusal in operation of some protective switching devices through which emergency current, has passed looks like:

,            (1)

                     - parameter of a stream of refusals in operation ³-that protective switching device. Here the index s specifies that the stream of refusals in operation ³-that protective switching device is considered;

                     - an interval of time between routine inspections of system of switching-off ³-that protective switching device together with its relay protection or systems automatic inclusion of a reserve;

                     m - number of protective switching devices through which emergency current has passed, thus action of their basic relay protection necessarily or number of section switching devices with MRS, given up in inclusion;

                     n - number of units of the electric equipment receiving the electric power from the given unit of loading.

           The formula (1) is fair at performance of the condition: intervals of time between occurrences ÊZ in an element of a network and intervals of time between refusals in operation of protective switching devices do not contradict exponential to functions of distribution of probability with parameters accordingly , and besides the condition is observed:

.

           In the event that terms of preventive maintenance of systems of switching-off of protective switching devices will be identical (i.e. , ) then the formula (1) will become:

.            (2)

           The probability of occurrence of cascade failure Q(t) during time t can be defined as follows:

.            (3)

           Thus, using the theory homogeneous Markovskyy processes the system of the linear differential equations for an estimation of survivability of schemes MRS has been received.