how is auto high speed reclosing of CB functioned
Faults on overhead lines fall into one of three categories: a. transient b. semi-permanent c. permanent 80-90% of faults on any overhead line network are transient in nature. The remaining 10%-20% of faults are either semi-permanent or permanent. Transient faults are commonly caused by lightning and temporary contact with foreign objects. The immediate tripping of one or more circuit breakers clears the fault. Subsequent re-energisation of the line is usually successful. A small tree branch falling on the line could cause a semi-permanent fault. The cause of the fault would not be removed by the immediate tripping of the circuit, but could be burnt away during a time-delayed trip. HV overhead lines in forest areas are prone to this type of fault. Permanent faults, such as broken conductors, and faults on underground cable sections, must be located and repaired before the supply can be restored. Use of an auto-reclose scheme to re-energise the line after a fault trip permits successful re-energisation of the line. Sufficient time must be allowed after tripping for the fault arc to de-energise prior to reclosing otherwise the arc will re-strike. Such schemes have been the cause of a substantial improvement in continuity of supply. A further benefit, particularly to EHV systems, is the maintenance of system stability and synchronism. A typical single-shot auto-reclose scheme is shown in Figures 14.1 and 14.2. Figure 14.1 shows a successful reclosure in the event of a transient fault, and Figure 14.2 an unsuccessful reclosure followed by lockout of the circuit breaker if the fault is permanent. 14.2 APPLICATION OF AUTO-RECLOSING The most important parameters of an auto-reclose scheme are: 1. dead time 2. reclaim time 3. single or multi-shot These parameters are influenced by: a. type of protection b. type of switchgear c. possible stability problems d. effects on the various types of consumer loads • 14 • Auto-Reclosing Network Protection & Automation Guide The weighting given to the above factors is different for HV distribution networks and EHV transmission systems and therefore it is convenient to discuss them under separate headings. Sections 14.3 and 14.4 cover the application of auto-reclosing to HV distribution networks while Sections 14.5-14.9 cover EHV schemes. The rapid expansion in the use of auto-reclosing has led to the existence of a variety of different control schemes. The various features in common use are discussed in Section 14.10. The related subject of auto-closing, that is, the automatic closing of normally open circuit breakers, is dealt with in Section 14.11. 14.3 AUTO-RECLOSING ON HV DISTRIBUTION NETWORKS On HV distribution networks, auto-reclosing is applied mainly to radial feeders where problems of system stability do not arise, and the main advantages to be derived from its use can be summarised as follows: a. reduction to a minimum of the interruptions of supply to the consumer b. instantaneous fault clearance can be introduced, with the accompanying benefits of shorter fault duration, less fault damage, and fewer permanent faults As 80% of overhead line faults are transient, elimination of loss of supply from this cause by the introduction of auto-reclosing gives obvious benefits through: a. improved supply continuity b. reduction of substation visits
The rapid expansion in the use of auto-reclosing has led to the existence of a variety of different control schemes.