Auto Reclosing Scheme of Transmission System:
Auto Reclosing Systems are series connected network and which is used to reduce the operating Cost and improves the reliability of the network. The extra high voltage (EHV) transmission lines are used to transmit bulk amount of electricity around severer thousands of MW (Mega Watts). Hence, it should not get interrupted at any cost. Even though the fault on these overhead lines are Natural. Since the of power flow through these lines should not be interrupted for a long time due to temporary or permanent fault in the lines. Here, the temporary faults such as falling trees on the transmission lines, lightning, birds etc. and they get cleared automatically, and they do not need any fault rectification process. In case, permanent faults such as broken conductors, failure of insulators, they cannot be restored in short time. In that time the auto reclose does not help us. Watch the video for better understanding.
During manual operation, operator reset the relays, and close the circuit breaker. But, If the fault is transient, the line holds after the second attempt of closing the circuit breaker, but if the fault still in the line, our protection system again trips the circuit and it is declared as permanent fault. However, during temporary fault, the significant delay occurs due to manual operation for reclosing the circuit. Generally, extra high voltage transmission lines (EHV) carries bulk amount of power, if any delayed operation causes big loss of system in the view of cost and stability. To avoid such delayed operation (manual switching), the auto reclosing scheme in the extra high voltage transmission systems has been introduced. we can avoid the unwanted delay due to human operation.
Reclosers address these faults by further dividing up the network into smaller sections (sectionalizer) as shown in the figure. Reclosers are programmed to automate the reset process and allow a rougher approach to service restoration. The result is increased availability of supply.
The Main Purpose of Auto Reclosing System:
- Reduce the interruptions of supply to the consumer.
- Improved supply continuity.
- Reduction of substation visits.
The faults in electrical transmission system in three ways,
- Transient Fault: Very short time of faults (Temporary faults) such as lighting fall on the transmission line causes over voltage and This over voltage will be suppressed completely by various equipment in very short period. Afterwards it clears automatically. Transient faults occur Almost 80 to 90% in the overhead transmission lines.
- Semi-Permanent Fault: The faults extend one or more arc strike, such as trees touches the ground terminal with live phase conductor, cause arc between the conductor and ground. The arc will be developed for few mill seconds until the tree get burnt out. Afterwards it clears automatically. Such fault occurs 5 to 8%.
- Permanent Fault: Broken conductor, Insulator failure, or any electrical equipment failure causes permanent fault in the transmission line. It cannot be restored until replacing or repairing the materials.
Recovery time of First two faults can be reduced by Auto-recloser or auto-reclosing scheme. Auto reclosing scheme consist of high speed operating contacts with Solid Dielectric Insulation with vacuum interrupters for current interruption, arc quenching and advanced current and voltage sensing devices. In auto-reclosing scheme if the fault is not cleared at first attempt, there will be double or triple shot of reclosing until the fault is cleared. If the fault still persists, this scheme permanently opens the circuit breaker. A prescribed time delay may be imposed on the auto-reclosing system to permit the semi-permanent fault to remove from the circuit.
Factors affecting Auto Reclosing Scheme:
The factors that influence the choice of dead time reclaim time, and the numbers of shots. Several factors affect the selection of system dead time as follows:
- System stability and synchronism.
- Type of load.
- CB characteristics.
- Fault path de-ionization time.
- Protection reset time.
For rapid reclosing type it is not required to check synchronism while reclosing however for delay reclosing synchronism should be checked before reclosing. For this purpose, synchronous relays are employed
Auto-recloser Video Explanation: