Insulated overlap in Railway over head Electrification

Insulated overlap in Railway over head Electrification

A simple polygonal type of overhead equipment comprises of a single 65 sq. mm. size Catenary wire of Cadmium Copper and a hard drawn grooved copper Contact wire of 107 sq. mm. size suspended from the Catenary by 5 mm diameter copper dropper wire spaced 9 meters apart.

The OHE is supported by swiveling type Cantilever bracket assembly. A tension of 1000 kg is given in each conductors example Catenary and Contact wire. This tension is  kept constant, automatically compensating the variations in conductor length due to change in temperature through the regulating equipment erected at the termination of conductors, also known as Automatic Tensioning Device.

The Catenary wire comprises of 19 strands of cadmium copper, each strand of 2.10 mm , with overall  of 10.5 mm having about 80% conductivity and 65 sq. mm cross-sectional area. The contact wire is a solid hard drawn grooved electrolytic.

copper of 12.24 mm and 107 sq. mm cross-sectional area. Total current carrying capacity of both wires is 600 Amps. The condemning size of contact wire is 8.25 mm.

The OHE span varies between 72 Meters and 27 Meters with a step of 4.5 M. The maximum span of 72 M is adopted on section having wind pressure of 112.5 kg/ M2 and 75 kg/M2 only. The span is  reduced on curvature depending upon the degree of curvature.

As a standard practice, an independent mast is used to support the OHE for each track to obtain mechanical independence Steel masts are of Four types i.e., BFB (Broad

Over Head Equipment's Flanged Beam), RSJ (Rolled Steel) and fabricated rectangular sectional mast of K and B type.

Portals are also used to serve multiple track section where space between two tracks to locate an independent mast is not adequate. There are three types of Portals in use ex-, N, O & R type. P,G and double BFB type uprights are used where track

separation is less. All Masts & Portals are galvanized before installation.

Traction Mast / Portals are embedded in the concrete foundation. There are different type of foundations which are used according to soil pressure and location. The five standard types of foundations mostly used are :

(a) Side Bearing (b) Side gravity (c) New Pure gravity

(d) Wet Black cotton soil (e) Dry Black Cotton soil

A pre sag of 100 mm is  provided on the contact wire for the maximum span of 72 meters and proportionately reduced for lesser spans. Regulated OHE with 100 mm pre sag is considered suitable for 160 Kmph speed operation.

The contact wire is staggered at support so that as the pantograph glides along the contact wire, the contact wire sweeps to and from across the bearing surface of the pantograph span up to a distance of 200 mm on either side of the center line of pan on tangent track and 300 mm on curved tracks towards the outer side. This ensures uniform wear of the steel strips of the pantograph.

The electrical clearance between live part, and earthed part i.e. fixed structures or moving load shall be maintained normally as large as possible. The minimum clearance under worst condition of temperature, winds etc. are given below:

(a) Minimum Vertical distance for

Long duration – 250 mm

Short duration – 200 mm

(b) Minimum horizontal distance for

Long duration – 250 mm

Short duration – 200 mm

The OHE conductors are terminated at intervals of 1.5 kilometers and suitably anchored. The changeover is made by overlapping the conductors, normally on 3 spans. The conductors height at support is so adjusted that the conductors are physically clear from any obstruction under all conditions as well as the pantograph glides over from one conductor to another smoothly without any spark. There are two types of overlap:

(a) Uninsulated Overlap

In this type of overlap the distance between two conductors is kept 200 mm and the conductors are permanently connected by jumpers to have electrical continuity.

(b) Insulated Overlap

In this case, the two OHE conductors are kept apart at a distance of 500 mm. The electrical continuity at the insulated overlap is bridged by Interrupters or Isolating Switches except at Neutral Section (SP).

In regulated OHE, to ensure uniform distribution of the mechanical tension in the OHE conductors, an anti creep point is  installed at the midpoint of the tension length of OHE conductor.

Section Insulators are provided to insulate the OHE of one track and another track, such as at turn outs & cross over, and to separate secondary tracks and sidings from the main line or other sidings.

When the pantograph of a locomotive passes from one track to another along a cross over, current collection changes from one OHE to another. The runners do have the overlap so that there may not be any sparking during change over.

Solid core Porcelain Insulators are used to support the OHE as Bracket and Stay arm Insulators 

CONTACT WIRE HEIGHT

1. i) Over Line Structure to permit C class ODC – 4.92 M
2. ii) Electric Loco Shed and Inspection Pits – 5.80 M

iii) Level Crossing – 5.50 M

1. iv) Unregulated OHE Temperature 4°C to 65° C – 5.75 M

Temperature 15°C to 65°C – 5.65 M

1. v) Regulated OHE with 50 mm Sag – 5.55 M
2. vi) Regulated OHE with 100 mm Sag – 5.60 M

vii) Height of the Rail Gauge at level crossing – 4.67 M

TRACTION SUB STATION (TSS):

Every TSS has 2 nos. traction transformers out of which one is working at a time and the second transformer is stand by. The capacity of each traction transformer is sufficient to feed its own feed zone and half of the adjoining feed zone.

Feeding Post (FP):

To feed 25 KV traction power to OHE.

The section of OHE normally fed by a traction transformer.

Feed Length:

The distance for which a traction transformer will feed power in emergent conditions i.e., the distance between two adjoining FPs.

Over Head equipment (OHE)

A system of conductors / equipment's carrying traction power from traction substation to electric locomotive.

Neutral Section (NS)

To separate OHE of two adjoining feed posts. A short neutral section (PTFE) type is provided opposite the Traction Sub Station to avoid the need of lowering the pantograph during extended feed conditions.

Sectioning Post (SP)

1. To facilitate the extension of traction power from one feed zone to half of the adjoining feed zone during emergency.
2. Parallel the UP and DN OHE in double the sections.

Sub-sectioning and paralleling post (SSP):

1. To sectionalize OHE.
2. To parallel the UP and DN OHE in double line sections.

Sector:

The section of the OHE between the FP & SP is called the sector.

Sub-Sector:

The section of the OHE between the FP & SSP, SSP & SP is called sub-sector. This is the shortest section of the OHE which can be isolated through Remote Control by the Traction Power Controller.

Elementary Section (ES):

This is the shortest section of the OHE which can be isolated manually for carrying out OHE maintenance work.