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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.

Substation

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.

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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:

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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.

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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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