The most important function of an electric traction system is to keep the traction unit fed with the energy that it needs. Current collection from the third rail, used in the metro underground system has the problem of insulation and quality of the current collection. The practice of power collection from overhead wire by means of the pantograph is mostly used.
The simplest form of overhead trolley wire system consists of a single solid wire. But, this arrangement has the drawback of sag. So, this system needs less distance between supports. Hence, the number of supports increases, and speed will be limited to 30kmph above which the pantograph may lose contact. This arrangement is used only in tramways.
The type of conductor used is 25kV traction consists of a stranded catenary from which trolley-wire or contact wire is suspended by means of droppers. The layout and its main components of overhead equipment are shown below. In this system, even though the catenary wire has sagged, droppers ensure that the solid contact wire is almost horizontal with respect to the track.
Design of Overhead Equipment in Electric Traction :
The electric traction overhead equipment is one of the most expensive items in electric traction. Simplicity and standardization of equipment must be kept in mind to keep the cost low. The design aspects of overhead equipment (OHE) are discussed below.
Composition of OHE :
The term overhead equipment (OHE) mainly indicates catenary and contact wire. Their cross-sections are determined mainly based on the following factors.
- Load current.
- Distance between feeding points.
- Temperature-rise.
- Voltage-drop.
- Lifetime of OHE.
In practice, a current density of 4A/sq.mm for copper is considered. An overload of 50% can be allowed for 3 minutes. For a maximum load current of 600A, copper with a cross-sectional area of about 150sqmm has been used for OHE. To increase the tensile strength of OHE, bronze or cadmium copper can be used. But they have low conductivity. The maximum tensile strength of various materials used for OHE are,
- Hard drawn copper (42kg/Sq.mm).
- Cadmium copper (62kg/Sq.mm).
- Annealed copper (25kg/Sq.mm).
Contact wire is supported from catenary wire by means of 5mm diameter droppers spaced at not more than 9 meters apart. Saving of copper in OHE can be achieved by using ac electrification instead of dc.
Height of Contact Wire :
The minimum height of the contact wire depends on the height of the train and the clearances required for operating system voltage. Normally, a minimum height is used only under bridges and in tunnels. For large spans, a small downward curvature of the contact wire is provided. It avoids upward curving at low temperatures if the contact wire is kept horizontal. This is desired from the current collection point of view.
Normally, the minimum mid-span height of contact wire above rail level is 5.5m. The height of contact wire at suspension points is maintained at a higher level depending upon the type of equipment and temperature range of operation. The height of the contact wire under various conditions of operation is given below.
| Condition of Operation | Minimum Height of Contact Wire Above Rail in Meters |
|---|---|
| At the suspension point | 5.55 m |
| Regulated OHE for a temperature range of 4°C to 65°C | 5.75 m |
| Regulated OHE for a temperature range of 16°C to 65°C | 5.65 m |
| Broad gauge | 4.8 m |
| Meter gauge | 4.02 m |
| Electric loco sheds and over locomotive inspection pits | 5.8 m |
| At level crossing both for broad and meter gauge | 5.5 m |
Contact Wire Gradient :
Whenever OHE passes under a bridge or tunnel, the height of the contact wire must be reduced in some cases. On the other hand, the height of OHE is increased at level crossings. This increase and decrease should be done gradually depending upon the speed or else the pantograph may lose contact or exert excess pressure on the contact wire.
As the maximum running speed increases, the gradient with which the change in height of the contact wire should decrease. If the maximum running speed is more than 100kmph, a maximum gradient (slope) of 3mm per meter is allowed. If the maximum running speed is less than 100kmph, a maximum gradient (slope) of 4mm per meter is allowed.
For sidings (electro-diesel locomotives operate as an electric locomotive on electrified lines but have an on-board diesel engine for non-electrified sections or sidings) maximum slope of 10mm per meter is allowed. The junction of these slopes with the horizontal at the point of support is made by the slopes of less than half of the above slopes.
Encumbrances :
The axial distance between the catenary and contact wire is called encumbrance. This is normally maintained at 1.4m except at turnouts, overlaps (insulated and uninsulated), and near overline structures.
Span Length :
The span length is the distance between two supports of the overhead conductors in a traction system. The maximum span length is limited by the maximum wind conditions. If the span length is too high, the contact wire may get blown off from the pantograph. The standard values of wind pressure in various areas are,
- Coastal area (88 kg/Sq.m).
- Areas nearer to the coastal area (88 kg/Sq.m).
- Interior area (74 kg/Sq.m).
A maximum span length of 72m is adopted on straight tracks for wind pressure of 88kg/Sq.m and 67.5m for wind pressure of 98kg/Sq.m. On curves, the span length is reduced to keep the contact wire within the working width of the pantograph. Under maximum wind conditions, to guard against blowing away, a contact wire is located centrally over the track at mid-span for maximum span.
Span lengths have to be standardized in multiples of 4.5m varying from 27m to 72m. This facilitates the erection of OHE with standard dropper lengths and avoids mistakes in the erection.
But, the difference in span lengths between two consecutive spans should not be more than 18m to avoid an unbalanced load on masts. A larger span length is economical but, a reduced span length decreases the elasticity of OHE and improves current collection.