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Overhead Equipment Used in Electric Traction Systems (Insulated and Uninsulated Overlap)
In this article, we will take a look at insulated overlap and uninsulated overlap, which are overhead equipment used in electric traction systems.
What is an Overhead Equipment?
Overhead Equipment (OHE) in electric traction system are used for transmitting electrical energy to the electric locomotives, trolley buses, trams and other electric traction systems.
As in any electrical circuit, the series and parallel connections are made by twisting cables together, using junction box, soldering, brazing, etc. But, in case of overhead conductor wire (or catenary wire), the use of these equipment is not possible. Therefore, the arrangement employed for the same purpose in the overhead equipment is called overlap.
Thus, the arrangement in which two sets of contact wire run together in parallel for a short distance without any mechanical or electrical connectivity but provide smooth movement of the pantograph over it, is known as overlap in OHE.
The distance between the overlaps is restricted because of the following reasons −
In order to avoid the difficulties encountered in handling and transportation of conductor due to limited size of conductor and weight of the drum on which conductor is rolled.
Need of isolation of portions of OHE which are affected by the faults for effective operation and easy maintenance. Hence, it makes necessary that the overlaps are to be provided at lesser intervals.
When swivelling type of bracket assemblies are used, there is friction of components. This makes difficult to maintain uniform tension in the conductors if the length of the OHE between the anticreep and the regulating equipment is long.
Therefore, the length of the regulated OHE never exceeds 1.5 km.
Types of Overlap in OHE
There are two types of overlaps in OHE −
- Uninsulated Overlap
- Insulated Overlap
When OHE is made continuous, then long lengths of it would be affected wherever there is a fault or damage and the rectification of damage may need long time. Therefore, it is essential to divide the length of the elementary section into a large number of sub-elementary sections and each of sub-elementary sections is separated from its adjacent one by some mean which is known as uninsulated overlap.
Therefore, when two contact wires belonging to two adjacent sub-elementary sections are run parallel to each other for at least one span and electrically connected to each other either by means of jumper is known as an uninsulated overlap.
In the uninsulated overlap, the contact wires cannot be of indefinite length and the standard length of 1.6 km is used which is known as one span or one tension length. This standard length is designed by taking into consideration the tension, weight, thermal expansion, etc. Also, the gap provided between two contact wires, in parallel running portion is about 20 cm which is maintained by mechanical linkage of the conductors.
In 25 kV AC traction system, three span uninsulated overlap is normally provided on regulated OHE with swivelling type bracket assemblies as standard overlap.
Insulated overlaps are mainly used for providing isolating facilities required for the operation and maintenance of OHE. The insulated overlaps are also provided at feeding posts, subsectioning posts and booster stations. When two length of OHE belong to two adjacent elementary sections, then there will be insulated overlap span.
In case of insulated overlap, following measures are made which are not in the uninsulated overlap −
To avoid the accidental contact between the contact wires, the gap between them is kept more.
In an insulated overlap, the electrical continuity between two sections is maintained by means of jumpers in series with isolator or interruptor. With this arrangement, a section can be made dead and maintenance work can be carried out on the other equipment away from the overlap.
The anchoring portion of the catenary and contact wire is separated from the rest of the portion by means of insulators provided at a distance of 2.5 km from the supports and within the overlap span. The tail ends of the anchoring OHE are connected to the other nearby equipment to prevent it from attaining other potential.
Insulated overlaps are also used in the following traction system −
In DC traction systems, single span insulated overlap is used. The lateral distance between two equipment is kept as 0.3 meters for 1500 volts and 0.4 meters for 3000 volts. This separation is maintained by means of insulated rods clamped on to the wires.
In case of 25 kV AC traction system, the lateral separation between two equipment is of 0.5 meters and three span insulated overlap is normally provided with swivelling type of bracket assemblies.
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