INTRODUCTION ON OVERHEAD EQUIPMENT (OHE)

INTRODUCTION ON RAILWAY ELECTRIFICATION (RE)

???????Railway Electrification is generally is known as RE all over the world

???????The earliest form of RE was in 19th Century When Electric Trams were introduced in cities in Europe and America

???????The earliest form of Electric Trams utilized 600 Volt DC or in some cases 750 Volt DC

???????It was found that electric trams were more economical form of transportation

???????This system of hauling the trams was replicated on railway lines on sub-urban sections

???????Once it was found that the sub-urban railway electrification is successful and economical, it was decided to extend it to inter-city or main lines

???????Once it was decided in principle to introduce RE on intercity and main lines, the evolution of RE for better forms took place over?a period of time

???????The system which was earlier on 750 Volt DC was upgraded to 1500 Volt DC, subsequently it was further upgraded to 3000 Volt DC

???????Then it was realized AC traction would be the most technically and Economically the most viable option, the same was upgraded to 25kV AC

???????Presently world over 2x25kV system is also in commercial operations

Thus, the systems presently working in the world are as under:

???????750 Volt DC on suburban lines

???????1500 Volt DC on suburban lines

???????3000 Volt DC on suburban lines

???????25kV AC on main lines

???????2x25kV AC on freight corridors

???????750 Volt DC in the form of Third Rail/OHE in Metros

Systems on Indian Railways

???????Indian railways initially had 1500 Volt DC Electric traction was introduced in 1930s in suburban section of Bombay & Madras

???????Subsequently it was extended on main line from Bombay to Pune & Kalyan .

???????All the above systems were working on 1500 Volt DC

???????In the year 1955, French National Railways (SNCF) perfected the system of Electrification on 25kV AC

The major advantages of 25kV AC.

???????Lower running cost of locomotives and multiple units.

???????Higher power-to-weight ratio, resulting in

???????Faster acceleration

???????Higher practical limit of power

???????Higher limit of speed

???????Less noise pollution

???????Lower power loss at higher altitudes

???????Lack of dependence on crude oil as fuel

???????Less environmental pollution

Indian railways decided in 1958 to adopt 25kV AC electrification as their main form of RE, The same 25kV AC as adapted from SNCF designs is continuing in the country as on date

???????In India presently following systems of RE are in operations

???????Indian Railways – 25kV AC & 2x25kV AC

???????Dedicated Freight Corridor – 2x25kV AC

???????Metros – 750 Volt DC, 1500 Volt DC & 25kV AC

?Railway electrification as it exists in India today can be split into 4 parts.

???????Traction sub-stations

???????Switching stations (SP/SSP)

???????Overhead Equipment (OHE)

???????SCADA

OVERHEAD EQUIPMENT (OHE)

The electrical conductors over the track together with their associated fittings, insulators and other attachments by means of which they are suspended and registered in position is called Over Head Equipment.

The electrical conductors over the track for the purpose of supplying power to the pantograph of the electrical locomotive.

Types of OHE’s are:

???????Conventional type OHE (Which comprising of 65 mm2 Cadmium Copper catenary and 107 mm2 copper contact wire)

??????Composite type OHE (Which comprising of aluminium alloy catenary and 107 mm2 copper contact wire)

??????Tramway type OHE (Which comprising of 107 mm2 copper contact wire)

?OVERHEAD ELECTRIFICATION DESIGN

List of Drawings for the Overhead Electrification design are as follows,

??General Power Supply Diagram

??Sectioning Diagram

??Pegging Plan

??Survey

??Layout Plan

??Cross Section Drawing

??Structural Erection Drawing

??Profile Drawings

??As Erected Drawings

GENERAL POWER SUPPLY DIAGRAM

General Power supply Diagram shows the basic scheme of switching sector and Sub-section wise.

SECTIONING DIAGRAM

The Sectioning Diagram shows the location of every insulated overlap, section insulator, Isolator and the detailed plan of switching & sectioning in the schematic form.

PEGGING PLAN

Pegging plans are the plans of the railway tracks to be electrified, indicating the position of the structure to support the Over Head wire system (OHE).

LAY OUT PLAN (LOP)

After the finally corrected pegging plan OHE layout plan is prepared. Layout plan shows not only every OHE structure but also the actual run of catenary/contact wires, stagger of contact wire, and number of brackets on each structure, the point in which OHE is terminated, anti creep points and all out of run wires and complete sectioning details as per the sectioning diagram.??

CROSS SECTION DRAWING (CSD)

Cross Section Drawings are prepared for each structures, indicating the cross section of formation, height and nature of bank, whether new or old, nature of soil, type of foundation block, type and size of structure, reverse deflection to be provided.

STRUCTURAL ERECTION DRAWINGS (SED)

This drawing gives complete data relating to every OHE structure on one sheet of paper and form the basis for every subsequent operation during erection, then during final checking, and subsequently for adjusting the OHE to the designed configuration during operation and maintenance.

PROFILE DRAWINGS:

These drawings shows the profile of the catenary and contact wire under every over line structure, the gradient and height of contact wire under the structure and on both side and also the encumbrance on the structure, until normal values are restored.

AS ERECTED DRAWINGS:

During the course of construction, there could be several changes, modifications or improvements which may be decided upon. A careful note should be made of every such change and action to withdraw the superseded drawings and to be canceled and destroyed.

FOUNDATION

Are provided to hold the OHE structures.

Foundations are designed on the basis of FBM code, type and bearing capacity of soil/shoulder width and the extent of projection above ground level. Selection of the type and size of foundation is done from the volume chart

STEEL (MAST/PORTAL) ERECTION

A single vertical post embedded in the foundation or otherwise rigidly fixed in vertical position to support the overhead equipment with cantilever assembly. It may be rolled section or fabricated.

Type of Mast:

• 6x6 BFB
• ?8x6 RSJ
• ?“B” Series Mast(B 150, B 175, B 200, B250)
• ?SWS Masts(S1, S3, S5, S6, S7, S8) ?

PORTALS

On multiple track sections, where adequate track centres are not available and tracks cannot be slewed, portals are used. Each portal consists of two fabricated uprights and one fabricated boom consisting of with or without one central piece and two end pieces.

Three types of Portals have been standardized

a). “N” type - is used for clear spans of 10m - 20 m

b). “O” type - is used for clear spans of 20m -30 m

c). “R” type – is used for clear spans of 30 m -40 m

Where the upright of standard portals cannot be erected, due to limited track centres, ‘P' type portal may be used in place of 'N' type and 'G' type may be used in place of 'O' type. The width of upright-of these portals is 300 mm and 250 mm as against 450 mm of 'N' type and 550 mm of '0' type respectively. In exceptional cases, BFB uprights of 152 mm width may also be used with 'N' type portal boom.

TWO TRACK CANTILEVER (TTC)

In the yards and sidings when the mast cannot be erected near the track to be equipped, it may be erected spanning one or two tracks using a two-track cantilever.

TTC ‘s are having a boom length of 5.5m and 8.0mtr.

BRACKET ASSEMBLY

It is an insulated swivelling type structural member, comprising of different sizes of steel tubes, to support and to keep the overhead catenary system in position so as to facilitate current collection by the pantograph at all speed without infringing the structural members.

It consists of the following structural members:-

??Stay arm

??Bracket tube

??Register Arm

??Steady arm assembly

ANTICREEP

Anticreep is an arrangement provided for fixing the OHE at approximately midpoint of a regulated OHE wire.

WIRING OF CATENARY/CONTACT CONDUCTOR

Two Conductors are provided to to Run the electrical locomotive supported at bracket with the help of droppers .

COUNTER WEIGHT ERECTION

To provide required tension in OHE .

3 Pulley type

????????????Mechanical Advantage = 1:3

????????????Counter weight required = 1000 Kg+1000Kg / 3 = 666 Kg

Winch type

???????????????Mechanical Advantage = 1:5

???????????????Counter weight required = 1000Kg +1000Kg / 5 = 400 Kg

DROPPERING & CLIPPING

Supports Contact Wire , Takes partial load of the contact wire and hence always under Tension. When the pantograph is under speed, it pushes up the contact wire and hence the pre-sag momentarily vanishes by bringing in horizontal position so that under the condition the current collection is maximum achieved. During pushing up of the contact wire, since the dropper is made in looped form, the flexibility is achieved and hence the permanent distortion of droppers thus avoided.

AUTOMATIC TENSIONING DEVICE (ATD)

In order to improve current collection performance, it is essential to keep the tension in OHE conductors constant irrespective of expansion in higher temperatures and contraction at lower temperatures.

To achieve the same Auto tensioning devices or Regulating Equipments are being installed at termination of OHE conductors.

Types of Auto Tensioning Devices are:

a). Winch type Regulating Equipment (1:5 ratio)

b). Three pulley type Regulating Equipment (1:3 ratio)

CUT-IN-INSULATORS : ??

In an insulated overlap the two OHEs are made electrically separated by provision of cut-in-insulators in the?catenary and contact wire.

SECTION INSULATOR:

?A device installed in the contact wire for insulating two elementary electrical sections from each other while providing a continuous path for the pantograph without break of current.

JUMPERS:

?A jumper is a conductor or an arrangement of conductors, not in tension, used for electrical connection between two conductors or equipment’s.

????????Types of Jumpers are:

???????????????a). In span Jumpers?or “H” Jumpers

???????????????b).Turnout /Un-insulated overlap Jumpers or “G” Jumpers

???????????????c). Potential Equalizing Jumper of “F” Jumpers

OVERLAP

An arrangement of overhead equipment over a track where two sets of traction conductors are run parallel to each other for a shorter distance over span/spans providing a smooth passage of pantograph. verlap is the junction at one or two spans at which one tension lengths of contact wire system terminate and the next tension length take off.

Uninsulated Overlap

In an un insulated overlap the overlap portion of the two tension lengths of OHE are not electrically separated. i.e they are not insulated from each other, In the Uninsulated overlap the separation of the conductors is maintained at 200mm normally on tangent tracks.For Electrical continuity in UIOL a jumper is provided between both the conductors.

Insulated Overlap

In an insulated overlap the two OHEs are made electrically separated by provision of cut-in-insulators in the contact wire system, Insulated overlaps the two sets of conductors are separated by 500mm in air. The electrical continuity is provided by an Isolator /Interrupter etc.

BONDING & EARTHING

Bond is an electrical connection across a joint in or between adjacent lengths of rail.

The various types of bonds are:-

Continuity Bond: A rail bond used for maintaining continuity of the rail circuit at crossings and junctions.

Cross Bond: A rail bond used for connecting together two rails of a track or rails of adjacent tracks.

Impedance Bond: A special rail bond used to bridge an insulated rail joint in ac track circuited sections in areas equipped for electric traction.

?Rail Bond : An electrical connection across a joint between two adjacent lengths of rail as part of the track return

?Structure Bond: An electrical connection between the steel work of track structures, bridge or station building, to which the traction overhead equipment is attached and the track returns

ISOLATORS :

????????Function of isolator is to make and break?the Electrical continuity of an Insulated overlap

AUXILIARY TRANSFORMER OR L.T SUPPLY TRANSFORMER :

????????230 V single phase power supply required for operation of substation equipment e.g. circuit breakers, interruptors, etc. lighting of the station yard, tunnels and working of colour light signals, is obtained through 25 kV / 230 V, 100/10/5 Kva, 50.Hz. single phase LT supply transformer. It is rovided at substations feeding and switching posts,stations, block-huts and at other outdoor locations e.g. level crossings with gate signals.

SOME IMPORTANT DEFINITIONS

Chainage

The chainage of any point is its distance from the KM stone measured in metres.

Span

Span is the distance in meters between two adjacent structures supporting OHE.

Clear span

In case of portal structure, clear span is the distance between the inner faces of portal uprights.

Obligatory location

Structure for equipping turnouts and diamond crossings should be located as close as possible to the theoretical centre of turnout.

Track Separation

At the structure location for turnout and crossing with the slip should be between 150 mm and 750 mm depending upon the type of turnouts i.e 1 in 81/2 or 1in12.

Implantation

Implantation is the setting distance from the nearest face of traction mast to the centre line of the track.

Versine

Versine is the maximum offset in millimeters of the chord connecting the same edge of rail from two points just opposite to the two structure locations of a span.

Rail level

This should be the highest point of the track rails.

Super-elevation

Super-elevation is the difference in levels of the two rails of the same track on curvature.

Track Centre

Track Centre is the perpendicular distance between the center line of two adjacent track.

Stagger

The stagger of the catenary is the horizontal distance of the eye of the catenary suspension bracket from the vertical plane through the centre of the track.

Stagger of the contact wire is the horizontal distance of the contact wire from the vertical plane through the centre of pantograph pan at the contact surface.

(The Contact wire will be staggered in a Zig -Zag manner on either side of the track.)

Turnout

?Turnout is the portion of the secondary track branching off from main track.

Cross Over

?Cross Over is a small length of track connected between two main tracks to facilitate the diversion of traffic from one track to the other.

Diamond Crossing

?When two tracks cross each other at a certain angle it is called a diamond crossing.

Scissor Crossing

?Scissor Crossing is the crossing of two nearly opposite cross-overs between two main lines.

Regulated OHE

Regulated OHE is one in which the tension in the OHE conductors is kept constant at all temperatures by means of auto tensioning devices and counter-weights.

Un-Regulated OHE

In the Unregulated OHE the tension in the OHE conductors are not regulated by any mechanical means.

Step distance

?It is the difference in the vertical levels of the reference rail and the top of structure foundation.

Encumbrance

?Encumbrance is the vertical distance between the underside of the contact wire and the center of the catenary wire at the OHE supporting bracket assembly.

Height of contact wire

?The distance from rail level to the underside of contact wire.?