Supply of continuous electricity is the main moto of power generation, transmission and distribution. However, from our previous discussions, it is known to us that a continuous supply of power is an ideal case. Auto recloser is therefore a relay that functions to reclose the contacts of a circuit breaker once it has tripped because of a certain electrical fault in the system.
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Electrical substations, be they generating, transmission substations or distribution substations have many equipment that are prone to failure. The long power transmission lines that carry the power from remote generating plants to urban or suburban load centers are also prone to faults.
We have learned that the faults in the electrical system are of three types:
ELECTRICAL FAULTS
TRANSIENT FAULTS
Those faults are cleared by immediate tripping of circuit breakers. These faults do not reoccur when the lines are re-energized. For example, a lightning strike often creates a line to line over the current fault but upon re-energization, the line stands as previously without issue.
SEMI-PERMANENT FAULTS
These type of faults however requires some time interval to disappear before the line is charged again. For example, a tree’s branch touching a live conductor creates an earth fault. These faults may sometimes get cleared upon a few re-energization of the line as the arc burns it up.
PERMANENT FAULTS
These types of faults are rare and require human intervention to locate and fix the fault and get the line re-energized again. For example, a broken conductor touching the ground. It has to be repaired and then the line could be charged.
WHY IS AUTO-RECLOSER USED?
Since maximum faults in an electrical system are found to be transient in nature, it is beneficial for the utilities to have a scheme that can re-close a circuit after the fault has been intervened and isolated by the circuit breaker.
ADVANTAGES OF AUTO RECLOSERS:
- Auto reclosing helps minimize the outage time.
- It improves the reliability of the system.
- It improves the system’s stability.
- In some cases, it reduces fault damage and hence the maintenance time.
TYPES OF AUTO RECLOSING SCHEMES:
There are various kinds of auto reclosing schemes that have been developed since time. The following are the classifications.
BASED ON PHASE:
THREE-PHASE AUTO RECLOSING:
When the fault is intervened all three phases of a circuit breaker trip and on command of the auto-recloser all three phases are closed again. This type of scheme causes an intermediate drift apart of two systems and hence interchange of synchronizing power can take place during the dead time.
SINGLE PHASE AUTO RECLOSING:
In this scheme, only the faulty phase is opened and reclosed only for single line-to-ground faults. It does not cause an interruption in the interchanging synchronizing power between two systems through the other two healthy phases.
BASED ON ATTEMPTS TO RECLOSE
SINGLE SHOT AUTO RECLOSER:
In this scheme, the circuit breaker recloses only once for a single fault before the auto recloser goes on lockout. It is a system that is invariably used in all EHV line circuit breakers.
MULTI-SHOT AUTO RECLOSER
In this scheme, more than one shot of reclosing attempts is made before the circuit breaker goes into a lockout. This scheme is mostly used in distribution lines or lines that go through forest areas. Multi-shot reclosing helps burn off the fault-causing element. However, even on the distribution side, it is not advisable to go more than three shots of auto-reclosing.
Repeated attempts of reclosing with high fault levels harm the electrical equipment. Multi-shot auto reclosing also depends on the circuit breaker’s ability to go for quick successive trip close operation. As the percentage of semi-permanent faults that could be burnt off is moderate, lines through forest areas and those accessible to trees, multi-shot reclosing is followed.
BASED ON SPEED
HIGH-SPEED AUTO RECLOSING:
This helps in the fast restoration of the supply but should be applied by considering the following factors.
- System disturbance time can be tolerated without the loss of system stability
- And characteristics protection scheme of the circuit breaker.
LOW SPEED OR DELAYED RECLOSING:
This scheme is suitable for highly interconnected systems where the loss of a single line is unlikely to cause two sections of the system to drift apart and lose synchronism.
IMPORTANT PARAMETERS OF AUTO RECLOSERS
DEAD TIME:
It is the time between the auto-reclosing Scheme being energized and the operation of its contacts that energizes the Circuit Breaker closing circuit. In other words, the time between the fault arc being extinguished and the circuit breaker’s contacts remaking.
RECLAIM TIME:
The time following a successful closing operation is measured from the instant the auto-reclosing relay closing contacts are made which must elapse before the auto-reclosing relay initiates another reclosing attempt. In other words, it may be said to be the time between 1st and 2nd auto recloser.
CHOICES OF EHV SYSTEM:
CHOICE OF DEAD TIME:
- The lower limit is decided by deionizing of circuit breaker.
- The upper limit is decided by transient stability and synchronism.
- Longer transmission lines require longer dead time.
- The dead time for a high-speed auto-reclosing scheme with the EHV System is 0.3 to 1.2 sec.
CHOICE OF RECLAIM TIME:
This should not be set to such a low value that the operating cycle of the Breaker is exceeded when two faults incident occurs close together. The reclaim time will be in the range of 10 seconds to 30 seconds, depending on the breaker opening and closing mechanisms.
CHOICE OF ZONE:
The auto recloser should normally be kept in Zone 1. Since the S-L-G fault is a Zone-1 fault and the S-L-G fault only auto recloser comes into the picture. In other zones, the auto recloser is generally blocked.
