Introduction about auto-reclosing

Faults on overhead lines fall into one of three categories:

• transient
• semi-permanent
• 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.

Figure 1:  Single-shot auto-reclose scheme operation for a transient fault

Figure 2:  Operation of single-shot auto-reclose scheme on a permanent fault

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 1 and 2. Figure 1 shows a successful reclosure in the event of a transient fault, and Figure 2 shows an unsuccessful reclosure followed by lockout of the circuit breaker if the fault is permanent.

Reference: Siemens, "Network Protection & Automation Guide".