The auto reclosing of power lines has become a generally accepted practice. Reports from different parts of the world show that in certain networks in regions subject to a high lightning intensity only about 5 percent of the faults are permanent.
Auto-reclosing, therefore, provides significant advantages. Outage times will be short compared with systems where station personnel have to turn our and re-energize the lines after a fault. Such emergency visits often entail long trips at inconvenient hours and in bad weather, which will have as consequence a long outage time.
Personnel can be saved if several substations can be unmanned. It is also possible to reduce the number of emergency visits to unmanned substations provided merely with alarm transmitters if they are equipped with an auto-reclosing system.
In other words, auto reclosing reduces operating costs and improves the reliability of service of the network.
In the case of transmission lines between network sections with their own generating facilities auto-reclosing is generally justified with respect to the network stability.
The choice of one or more reclosing shots, high-speed or delayed (low-speed) auto-reclosing, single-phase or three-phase auto reclosing depends on various factors.
Different Types of Faults
Depending upon the time, for which the faults exist on the system, are classified as follows:
- Transient fault
- Semi-permanent fault and
- Permanent Fault.
1. Transient Fault
The transient fault exists only for a short time. They can be removed faster still if the line is disconnected from the system momentarily so that the arc extinguishes.
After the arc is deionized, the line can be reclosed to restore normal service.
A characteristic feature of transient faults is that they either disappear after a short dead time or that they disappear quickly without any action being taken.
Lightning is the most usual cause of this type of fault. The overvoltages induced in the lines generally cause flashovers across the insulator strings. They are seldom extinguished on its own.
This means that the line must be tripped for de-ionization of the fault path and can then be re-energized, without the fault recurring.
Other transient faults may be caused by swinging conductors contacting each other due to high winds or the shedding of ice. Unfortunately, conductor swinging due to ice shedding has the characteristic that the conductors of different phases may contact each other several times in succession and the fault can then be interpreted as a permanent fault.
Birds, temporary contact with foreign objects such as trees, etc., can also cause transient faults.
It is found that about 80% of the faults are transient faults, 12% semi-permanent and 8% are permanent faults.
2. Semi-permanent Fault
If it is a semi-permanent fault, may be due to a twig falling on the power conductor or a bird spanning the power conductors.
Reclosing could have resorted with some delay so that the cause of the fault could be burnt away during a time delay trip and the line could be reclosed to restore normal service.
A fault of this type may be caused by, for example, a tree branch falling on the line. It is burnt up by the are when the line is re-energized.
A further 10 percent or so of all reclosures are consequently successful with the second shot. If a third shot is made, only a further 1 to 2 percent of the reclosures are successful. More than two shots are therefore generally rather meaningless and only subject the breaker to unnecessary wear.
3. Permanent Fault
However, for permanent fault reclosing does not help as it has to be attended and removed, and the line is to be taken out till the fault is cleared.
Permanent faults, such as a broken conductor, the collapse of a line tower, a tree leaning against the line, a fault in any cable included in the transmission, must first be traced, before the cause of the fault can be removed and the damage repaired.
This often entails time-consuming work and therefore imposes great demands on the ability of the network to cope with the load on the healthy section during this period.
Therefore, if the fault is not cleared after the first reclosure, a double or triple shot reclosing is desired. If the fault still persists, the line is taken out of service.
Single Phase and Three Phase Auto Reclosing
Auto Reclosing could be
- Single phase Autoreclosing or
- Three phase Autoreclosing
Single Phase Auto Reclosing
Single phase auto reclosing is resorted when a line to ground fault takes place and reclosed after a predetermined time.
For multiphase faults, all three phases are opened and reclosure is not attempted.
In single phase auto reclosing the power can still be fed through the healthy phases to the system and the system is less unstable as compared to 3-phase reclosing.
Three Phase Auto Reclosing
In case of three phase auto reclosing, all the three phases are opened independent of the type of fault, be it a single line to ground, or a line to line or 3-phase fault and are reclosed after a predetermined time.
Here during the opening period, no power can be transmitted and hence the system is liable to operate unstably.