Incremento dei tempi di attesa alla richiusura degli interruttori MT per il miglioramento della continuità del servizio

The large deployment of network automation with reclosing practices has been the usual solution adopted by distributors to achieve the goal of providing a distribution network with high performances in terms of number and duration of interruptions. In particular, the usual practice adopted by distributor when managing faults in distribution circuits consists in clearing the faults by the immediate tripping of one or more circuit breakers, and re-energize the lines after the automatic reclosing time. In distribution networks the reclosing time of a circuit breaker is typically 300 milliseconds, that is the shortest permissible time in which the circuit breaker need to reclose. Various studies have shown that up to as 90% of faults on most distribution overhead lines are transient (e.g. insulator flashover, contact with foreign objects like animals and trees, etc.) and transient faults can be cleared by momentarily de-energizing the line, in order to allow the fault to extinguish, autoreclosing can then restore service to the line. Autoreclosing success rates vary depending on several technical and environmental (temperature, pressure, humidity, etc.) parameters, but the majority of faults can be successfully cleared by the proper use of tripping and autoreclosing. In the paper the authors evaluate the possible benefits of adding an external time delay to circuit breaker reclosing procedure. The goal of this additional delay is to be reasonably sure that the arc at the fault location will not reignite at the reclosing instant creating a second fault, and, then, obtain significant benefits on continuity of service. In order to deeply investigate the problem, an arc model is first developed by using the Electromagnetic Transient Program EMTP-RV, and, then, the fault model is incorporated at different locations of a 20kV Medium Voltage (MV) network with real size characteristics. Faults in insulated and resonant grounding networks of different characteristics (OHL lines and cables) are investigated. The findings of the study are that an increased reclosing time may have a significant effect in increasing the number of self-extinguish faults, especially in transient double-phase faults in metal enclosed electrical installations. The strategy of extending the switching reclosing time may permit reducing the number of transient faults that reignite after reclosing, permitting to significantly reduce the outage time due to faults and providing a higher level of service continuity to customers.