The term auto-reclosing in transmission lines refers to the automatic reclosing of breaker contacts after momentary isolation of the system from the fault. Generally, the faults on the transmission lines are categorized into three types namely transient, semi-permanent and permanent faults.
It has been found that about 70% to 80% of the faults occurring on the overhead transmission lines are temporary faults (transient and semi-permanent faults) which are of a short duration of time. Thus by implementing an automatic reclosing mechanism for breakers, the longer duration of outages due to temporary faults can be overcome. Successful auto-reclosing can enhance stability margins and overall system stability.
Suppose if the fault persists even after the automatic reclosing of the breaker, the breaker again initiates the tripping of the line. The tripping and reclosing operations are performed for predetermined attempts and if the fault is temporary or semi-permanent it will be cleared in these attempts.
Though it is advantageous to use an auto-reclosing scheme, it adversely affects the system stability when the auto-reclosing is done in the case of permanent faults. For permanent faults in the transmission lines, the auto-reclosing of breaker contacts will be stopped after the mentioned predetermined attempts and the contacts remain open until there is a manual reclosing which is done when the permanent fault is cleared.
Types of Auto-reclosing Schemes Employed in Transmission Lines :
The auto-reclosing schemes in transmission lines are classified according to the number of tripping and reclosing operations. The different types of auto-reclosing schemes employed in transmission lines are,
Single Shot Auto-reclosing :
Single-shot auto-reclosing refers to the auto-reclosing of a circuit breaker, only once. The reclosure is made as soon as possible so that no drift occurs in phase angle between the voltages at the two ends of the open line.
Generally, this type of auto-reclosing scheme is employed in the case of EHV transmission lines where there is a possibility of occurrence of either temporary or permanent faults but not semipermanent faults. A temporary fault on the EHV line will disappear within one reclosure operation.
However, there is no need for further reclosure operations for permanent faults. The reason for the absence of semipermanent fault (falling of tree branch on line conductors) is the height at which the EHV transmission system is installed.
In case of protection of EHV lines at both ends, both the circuit breakers must be tripped and reclosed simultaneously in order to ensure that the line remains de-energized for a specified duration.
Multi-Shot Auto-reclosing :
Multi-shot auto-reclosing refers to the auto-reclosing of a circuit breaker more than once within a predetermined reclosing sequence. The reclosing can be provided up to three times. The first reclosure is provided as soon as possible.
If the supply is not fully restored then second and third reclosures are provided after few seconds and 1 or 2 minutes respectively. Even then, if the fault persists then it is a permanent fault and further reclosure will be provided manually only after thorough detection and repair by the staff.
This type of auto-reclosing scheme is employed with the lines where semipermanent faults can occur i.e., for lines with voltages ≤ 33 kV. Because semipermanent faults can be cleared only after 2 or 3 reclosures.
In this auto-reclosing scheme, as several tripping and reclosing operations take place, the maintenance of the circuit breaker increases. So, the vacuum circuit breakers which are almost maintenance-free, are most suitable for this scheme.
Single-phase Auto-reclosing :
Single-phase or single-pole auto-reclosing refers to the auto-reclosing of one pole of a circuit breaker using a designed single-pole trip for single-phase to ground faults. The design of the circuit breaker is such that each pole will have its own tripping and closing mechanisms.
For the implementation of this auto-reclosing scheme, a complex relaying scheme has to be provided which can detect the faulty phase. During multi-phase faults, the relaying scheme trips all three phases, and auto-reclosing will be done for all three phases.
Due to the complexity in the design of the circuit breaker and relaying scheme, this is the most expensive auto-reclosing scheme. Also, this scheme suffers from the problem of large deionization time. It is due to the fact that when only the faulty phase is isolated, the capacitive coupling between the healthy phases and the faulty phase tends to maintain the arc. Hence, deionization takes a long time.
This type of auto-reclosing scheme is provided for the interconnectors where two different power systems are being interconnected with a single tie line. Because, during a single line to ground fault if the only faulty phase is tripped, then the synchronizing power can still flow through the two healthy phases. Moreover, sudden tripping of all three phases results in the deviation of the phase angle of both the power system voltages.
Three-phase Auto-reclosing :
Three-phase auto-reclosing refers to the auto-reclosing of all three phases at a time. For the implementation of this auto-reclosing scheme, a simple relaying scheme is required. Since the relaying scheme need not detect the faulty phase, instead, it should simply detect the occurrence of fault and trip all three phases. So, the design of the circuit breaker is also quite simple as it is not required to have individual tripping and closing mechanism for each pole.
Hence, the cost of this auto-reclosing scheme is less when compared with that of a single-phase auto-reclosing scheme. Moreover, it has an additional advantage of having less deionization time since all three phases are tripped simultaneously.
Delayed Auto-reclosing :
Delayed auto-reclosing refers to the auto-reclosing of a circuit breaker after an intentional time delay. As the time delay will be longer compared to high-speed auto-reclosing, the problems associated with the deionization and power swings are absolutely absent. In this scheme, all three phases are tripped and reclosed simultaneously.
This scheme of auto-reclosing is employed for the interconnectors where two different power systems are being interconnected with a number of tie lines. Because, in such systems, there will not be any chances of losing synchronism or deviation of the phase angle of both the power system voltages.
