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25kV Traction Power Supply System

By on August 31, 2013

25 kV Single Phase is the most preferred and universally accepted voltage standard for overhead traction system. Looking for any other voltage system, means every things has got to be developed namely traction power supply system, Over Head Equipment, vehicle traction system etc. In order to increase OHE voltage. 2×25 kV system is developed so that at vehicle level, the voltage remains 25 kV. Some Railways have preferred for 50 kV system but the same is limited to heavy haul dedicated rail system in which the locomotive also remains confined in that territory only. There are not many suppliers of this traction system. Dedicated freight Corridor project in India thought meant for heavy haul, but the locomotive has to work in DFC and IR territory, therefore, 2x25kV power supply system is chosen.

25 kV Traction power system was designed, developed and demonstrated all over world by SNCF. Indian while completing electrification of HWH-Bardwan section  of  Eastern Railway decided to adopt 25 kV system in collaboration with SNCF and converted 3000V DC just when it was completed  into 25 kV instead running it at 3000 V DC. The rolling stock procured from abroad was diverted for Mumbai Sub-urban system and used after conversion from 3000V DC into 1500 V DC.

132 kV/220 kV 3 phase supply is taken from State Utility and two phase (say U-V)are dropped at Traction Sub-station and for the purpose of load balancing V-W, VW and W-U phase are dropped at subsequent  TSSs. There are mainly five types of power supply arrangement so far prevailing over Indian Railway network. These developments are historical instead for any technical advantages.

  1.  Power is purchased and paid at 25 kV from supply utility. Supply utility is responsible for owning, installation, operation, maintenance of /100/132/220 kV transmission line, grid substation upto 25 kV outgoing terminal.  25 kV feeder circuit breaker is controlled through SCADA by Traction Power Controller. This arrangement was adopted when IR was not having the resource and experience of handling HV equipment. Such substations were called GSS.
  2. It is similar to 1 above except that 25 kV circuit breakers are owned, installed, operated and maintained by the Railways. In this arrangement of operation by Railways and maintenance by State Utility has ended.
  3.  IR observed that there is always delay in which the GSS (Grid Sub-station) were not getting ready and delaying the RE projects. Scheme was thus adopted to have power supply at 132/220 kV at TSS incoming and thereafter every thing under the control of IR. These substations were called TSS (Traction Substation).
  4.  For Tundla-Kanpur section of Allahabad Division, another scheme was adopted in which 132 kV transmission line is also owned by IR and 3 phase supply is taken at one point and two phase power is tapped at IR owned TSSs. This system benefited IR to save time on execution of transmission line work, complain from State Utility of power imbalance and helping IR of improving Load factor. This arrangement has helped IR at a  later stage to take power supply directly from NTPC at better tariff and distribute on its network.
  5. In Bhusawal Division of Central Railway, Scott connected transformer used to take care of the issue of power imbalance generally complaint by State Utility in which 3 phase supply connection is taken and converted in to two phases by Scott connection transformer. The TSSs are  owned by Railways.

Power supply arrangement will take a new shape when evacuation of power supply from IR owned captive power plant at Nabinagar is synchronized to the grid. This is the subject of discussion at different forums of IR. This will be discussed in a new chapter later on.

Reliability of Traction Power Supply

Reliability of traction power supply means to design redundancy in such a manner that at no instant there is discontinuity  of power supply to traction vehicle for more than a tripping time plus the time take for isolation and feed extension. This is ensured by having power supply system to consist Traction Substation, Feeding Post, Sectioning and paralleling Post (SP or also called Neutral Section) and Sub-sectioning and Paralleling post (SSP) and Elementary section. Redundancies are designed at all stages ensuring availability of power supply on account of failure of supply from source or any other power supply equipment.


Special features for ensuring continuity of supply are summarized as follows:

  1. 132/220 kV supply is taken through double circuit 3 phase transmission line or drawing two phases from the nearby grid substation. If the grid substation is nearby, it is better to avail three phases. Advantage is to facilitate its usage for general supply. When general supply load is high, then the same can also be arranged by installing 132/33kV transformer at TSS. Railway Board has issued instruction to this effect to improve reliability of power supply for general purpose and avoidance of use of Diesel Generator set for emergency loads. Some other important points in this regard which can be helpful in decision making are
    • It is observed that failure of 132kV feeder is rare therefore it is better to avoid expenditure for running duplicate feeder. This has been done in some cases
    • When there are two feeders, some of the Railways preferred for paralleling of two feeders to reduce transmission losses. This can be of use of the metering is at supply point. However there are instructions of Regulatory commission that the metering shall be provided at user point.
    • There are some utility which asks for high load factor, say 35% DVC, which is difficult to achieve. The load factor of Railway feeder is generally 25% and therefore Railways had to pay heavy penalty. By having general supply feeder also at TSS, load factor can be improved and penalty payment can be converted into purpose full gain.
  2. Two sets of 132kV busbars are provided. One is called main busbar and the other maintenance or transfer busbar. They are coupled through bus coupler circuit breaker.
  3. Isolators on both sides of circuit breakers and provision of transfer busbar and bus coupler enable maintenance of circuit breakers and busbars.
  4. Two numbers of Traction Transformer initially of 10/12.5 MVA and presently 30/32.5 MVA of 132-220/25 kV single phase  are provided at each Traction Substation. For cost cutting, there was a phase during which only one transformer was provided at alternate TSSs. One number transformer is a hot standbye. One leg of the transformer is solidly earthed and connected to rail which acts as a return conductor. In order to avoid unbalance, Scott connected transformer were also used in BSL division of Central Railway.
  5. An auxiliary transformer of  25kV/230V of 5/10/25 kVA rating is  provided to meet the auxiliary load of lighting, control, fannage etc. at the substation. During initial state single phase connection was also given for oil filtration plant which was discontinued later on and mobile oil filtration plants working on DG sets were preferred.
  6. In order to improve power factor, series condenser with reactor is introduced on the return conductor at the secondary side of the transformer. Dynamic compensation is also provided at few of the Sub-station to take care of over compensation. There are some state utilities who levy penalty for over compensation but in many it has been over ruled by Regulatory Commission. The cost of Dynamic compensation is high and its own power consumption is so high through out the day that it exceeds the benefit.
  7. Lightening arresters are provided at each control posts to protect every sub-section agaisnt voltages surges. Earth connection to lightening arrestor should have copper equipvalent area of 50 sq.mm.

Layout diagram of a typical traction substation is shown in Fig below.

Location of Traction Sub-station or Feeding Post

Following points are considered while selecting the site for TSS

  1. Norms for locating the TSS based capaicty of transformer and voltage regulation is 50-80 Km for 25 kV without booster transformer, 40-50 Km with booster transformer and 80-100 Km for 2×25 kV AT system.
  2. Voltage is maintained between 27.5 kV (Max.)
  3. TSS shall be approachable by all weather road and preferably near a full fledged Railway station for better connectivity by Road as well rail.
  4. Location should be free from water flooding and water logging.
  5. To avoid land acquisition
  6. There is a need to convert FP with neutral section. Such planning can be decided in advance.
Feeding and Sectioning Arrangements

Power generation and transmission is at three phase whereas electric traction requires power supply at single phase. It is not possible to avail only single phase therefore, each of the three phase system is dropped at subsequent substation  and segregated at 25 kV side OHE by a neutral section. The substations along the rail route are located about 50-60 Km and one neutral section in between. Details of the neutral section will be discussed in separate chapter. Loco pilot is required to switch off the locomotive circuit breaker while approaching the neutral section for which warning board is provided at 500 and 250 M before the opening and closing command for circuit breaker. Switching Off and On of circuit breaker is required to avoid bridging of live and dead wire and at entrance and dead wire with live wire at the exit. A typical layout of OHE feeding system is given in the figure below:



Sectioning and Paralleling Post

Sectioning and Paralleling post is provided at neutral section to extend feed of one zone into another zone during power supply failure of any zone. SP post consists of two paralleling interrupters to parallel OHE of UP and DN line, one on each side of neutral section. Two bridging interrupters are provided to bridge the neutral section after ensuring no power supply by no volt relay. Bridging interrupter is normally kept open except when an emergency feed is extended. During feed extension it is also necessary to warn the driver by a caution order to drop the pantograph while negotiating the insulated over lap.
OHE and feeding arrangement in between feeding post and neutral section is termed as Section.

Sub-sectioning and paralleling Post (SSP)

Each section is subdivided into two or three subsection by the provision of SSP every 10-15 Km. This facilitates isolation of faulty sections. Each SSP is provided with two numbers of interrupter of 600 A capacity and one interrupter. Interrupter is operated on load and is used for feed extension or isolation during failure.

Elementary Section

Each subsection of 10-15 Km is further subdivided into elementary section. Two adjacent elementary sections are separated by insulated overlap bridged by manually operated isolator. Isolators are generally located near a Railway stations  where station staff is trained to operate the isolator. Traction Power Controller issues direction therough a control order along with exchange of private number to operate a particular isolator.

Video of current collection


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There Are 29 Comments

  1. thomas sitashi says:

    lam aqualified power remote controller looking for employment having worked for the zimbabwe railways carrying out switching and controlling of 25 and 132kv on trackside feeder substations and track sides intermediate switching stations from dabuka to harare carrying out competing of ET 1 TO 11 forms for electrification maintanance staff to carry out their maintanance and dewirements lm looking for employment any were how can lfind employment

    • Mahesh Kumar Jain says:

      You are having a good experience. I will suggest to contact L&T- constructing western Dedicated freight corridor on 2×25 kV electric traction, Alsthom- construting Estern Dedicated freight corridor, Tata Project also doing some of the work of these corridors.Beside this you may approach AEiGS consultant to KMRCL for metro project, Siemens main contractor doing the sub-station work of KMRCL. You may also find out the contractor during traction work for DMRCL. There are good opportunities and search them out.

    • Forward your resume to Us

  2. SM NAIK says:

    Urgent requirement regarding the maximum height between the track and the contact wire and minimum height between track and contact wire under foot over bridge and Road over Bridge for 25kV Traction System in the DFCCIL western corridor.

    • Mahesh Kumar Jain says:

      I will suggest to search for the Standard Schedule of dimensions issued by Railway Board for WDFC and all information is available thereon.

  3. gaurav says:

    Why have metros and railways chosen 25 kV as standard power supply for traction system

    • Mahesh Kumar Jain says:

      SNCF/France was the first railway to introduce 50-Hz AC traction at 20 kV in 1951 and changed to 25 kV. The voltage system than adopted as the standard voltage for traction.
      For selection of the standard traction voltage, the following points must have been the likely reason
      1. Availability of technology of insulator, single phase transformer with large harmonics and mounted on the locomotive, mercury arc rectifier etc.
      2. A range which is neither too high nor too low was obvious. Higher voltage will require higher insulation level on OHE and difficult in inhabited area. 50 kV voltage is also adopted for heavy load train dedicated lines. Low voltage will have more transmission losses.
      3. There is nothing on record in public domain for choosing 25 kV as standard and why not 26 or 27 or 30 kV.

  4. Mythili says:

    I am not able to see the figures in this page only. kindly help

  5. m p singh says:

    Insulation Sleave for 25 KV Traction bond 40x6mm, Size- 45mmx50cm .qty-2200.00 Numbers

  6. Mirza says:

    What kind of small transformer in between the railway track

  7. Bharat says:

    “In order to avoid unbalance, Scott connected transformer were also used in BSL division of Central Railway.”
    Can you provide some more information that how and which Scott connection can avoid un-balancing.
    Also, why it is not using in every sub-station.

    • Pankaj says:

      Did u get a reply on this? Please share the same.

    • Mahesh Kumar Jain says:

      There is only one arrangement of Scott connection. In case, there is unbalanced in two output phases, it will get reflected on the primary side. Why IR decided to go for balancing the power supply by tapping one phase at each substation instead going for Scott connection may be based on the decision taken along with State Electricity Board and the best possible financial alternative.

  8. Om Prakash Srivastava says:


  9. RAJAN M says:


  10. मै अ श्रेणी विधुत ठेकेदार हु मै रेलवे के टेण्डर लेना चाहता हु कृपया कर टेण्डर लेने की प्रक्रिया प्रदान करें और 5kvaऔर 10kva व अन्य मटेरियल लेना है रेलवे के ऐपरूब बेंडर की जानकारी दी जावे


    Dear sir what is the main difference between circuit breaker and interrupt
    In TSS

  12. Peddi manoj kumar says:

    Sir in metro rail 280 kva booster transformer rated voltage is 0.559 kv on both sides and current on both sides is 500 amps and it is 1:1 ratio transformer,
    1 . what is the applied voltage to BT , BT primary is in series with contact wire(train) and BT secondary is with return conductor (it is connected with rail)
    2. Why in Name plate manufacturer mentioned 0.559 KV only on both sides?
    3. How it is nullifying ELECTRO MAGNETIC INTERFERENCE?
    4.What is subtractive polarity in transformer ?
    please explain

  13. Prashanth says:

    Please send the how calculate the bracket dimensions with implantation to [email protected]

  14. Brijesh says:

    25kv ohe substation all material schedule with weight

  15. Much of the topics were covered in this section and special Thanks to the contributors for their efforts

  16. Sanjay Singh says:

    Sir mujhe apne cv me railway traction substation se releted kuch points add krne h ,
    Vo points bta skte h Electrical Engineer ke liye

  17. Sanjay Singh says:

    Railway traction substation se releted point in Electrical field