Provision of Effective Protection against Short Circuits in Internal Electrical Networks of Agro-Industrial Objects
The problem of low efficiency of protection from short circuits (SC) in the rural power-supply systems is caused, first of all, by the low level of SC currents. When an object of an agro-industrial complex is removed from the substation to 800 m, the SC-currents in the internal network with aluminum wires do not always reach even the triggering threshold of the electromagnetic releases (ER) of automatic circuit breakers with a rated current of l6 A. Therefore, it is very problematic to provide the maximum permissible protection triggering time (0.4 s) even with the usage of high sensitivity circuit breakers of BA series with the characteristic of type B. The situation is also exacerbated for internal networks due to the possibility of an electric arc impact to wiring, which often occurs during short-circuit. The electric arc can burn the wiring faster than the protection works, which is equivalent to its absence and uncontrolled flow of fire-hazardous processes. To increase the protection sensitivity and speed, an approach is suggested that involves changing the parameters of the electrical network in order to increase the SC-currents to a level at which the automatic circuit-breaker triggering zone is shifted to the sensitivity area of electromagnetic releases (ER). However, this action does not guarantee the increasing of short-circuit currents up to ER-threshold of the whole rated currents range of circuit breakers used. That is why it is additionally recommended to design the electrical protection system with an accounting of fire danger quantitative indicators. The most informative of these indicators are the network exposure factor for single-phase, two-phases and three-phases short circuits and an integral indicator of SC fire hazard. The exposure factor for each of the listed types of SC is determined by the ratio of the sections lengths sum of the network for which the triggering time of the protection is less than burning the wires time by an SC arc to the total length of all sections of the network. The integral indicator of fire hazard is numerically equal to the probability of fire at the object from single-phase, two-phases and three-phases SC during the year, assuming that the burnout of the wiring before the protection triggers necessarily leads to a fire.