Electric Traction Supply System on Indian Railways

Electric Traction Supply System on Indian Railways Nothern Inidan Allahabad

Railways,

1.0 Introduction Indian Railways is the largest government institutional costumer and purchaser from various Electricity boards and other Electric Supply Authorities. Out of total RKM 19000 RKM is electrified which carries 60% of total freight as well as passenger traffic across India. Annually, approximately 30 billion units of electricity is consumed by Railways, out of which 10.4 billion units are used for electric traction purpose. Railway is paying approximate Rs. 5000 crores every year on account of traction energy charges which constitutes about 20% of total revenue budget of Railways. 2.0

Bar diagram

3.0 Traction Power Supply Scheme Indian Railway has adopted 25 KV industrial frequency ( 50 Hz) A.C supply system for traction purposes. The power supplies are derived from 220 KV / 132 KV 3 phase transmission system from the various grids.

The typical schematic of power supply arrangement is shown in fig. 3.1 The basic arrangement constitutes incoming supply to Railway traction sub station at a voltage level of 220 KV / 132 KV , which normally feeds power along the track for 35-40 Km. Adjacent traction sub station are fed from different phases in rotation in order to balance the 3 phase load in its entirety. Neutral sections are provided in between two adjacent sub station to prevent the bridging of different phases while passing the electric locomotive. Level of voltage is reduced to 25 KV for the end use of locomotives by 21.6 MVA signal phase power transformers placed at traction sub stations which are located at every 30-35 Kms distance along the track. 4.0 Locomotive Power Schematic General arrangement of power circuit of electric locomotive is shown in fig 4.1

1

25 KV input voltage, fed to the locomotive through over head wires running parallel to track, is transformed to low voltage through loco transformer equipped with on-load tap changing facility. This alternating voltage is converted into D.C. voltage by the rectifier and smoothing reactor circuitry before being fed to set of six D.C. series traction motors placed at the underframe of a conventional loco motive. However, in modern 3 Phase locomotives, the rectified D.C. output is fed to the 3 Phase induction motors through 3 Phase invertors. 5.0 Details of TSS 2 Phase (R-Y) incoming 220/132 KV supply from Power supply authorities is fed to traction transformer through double pole isolators and circuit breakers with relevant protection scheme. Out put voltage at 25 KV is fed to overhead wire running along the track through single pole circuit breaker and remote controlled interrupters. Overhead lines are separated at periodic intervals through sectioning post(SP) and sub sectioning post (SSP) for isolation of faulty section through remote control. Sub sector are further sub divided into different elementary section through manual isolators in order to reduce the faulty

section to the bare minimum and carry out smooth train operation the advancement healthy section. 6.0 Protection Scheme A number of protection equipments are provided at traction sub station as well in the locomotive for protection of traction equipments. Following protective device are provided at traction substation:i) Differential protection ii) Restricted earth fault protection iii) Buchholz relay iv) Oil temperature indicator v) Winding temperature indicator The following further protective relays are provided for protection of overhead line against any fault i) Mho relay for distance protection ii) Wrong coupling relays between two sub station iii) Over current relays for fault closure to sub station. iv) Panto flash over protection With the emerging digital technology, old electromechanically and solid-state type relays are being replaced by microprocessor based numeric

2

station, based on Pentium IV processor, Main server & stand by server and window based communication processor.

relays in a phased manner. These relays are having parallelogram characteristic instead of circular characterstices with feature of inbuilt fault locator.

The data from Remote Control Center is communicated through 600/1200 BPS, FSK, modem and sent to remote terminal unit via communication media which interact with various controlled and protective equipments provided at the controlled posts.

7.0 Traction Power Control Supervisory control and data acquisition system ( SCADA) is in place for operation and control of traction power supply. All the switching operations required for maintenance as well as segregation of fault is being performed from Remote Control Center through SCADA.

8.0 Power of Electric

8.1 The term power quality is used to describe the extent of variation of the voltage, current and frequency on the power system . The variation of voltage and current can either be in terms of magnitude or waveform shape/ distortion.

Remote Control System constitutes standard SCADA software and remote terminal units provided at TSS/SP/SSP. SCADA system is so designed that RTU at various switching posts shall be able to communicate as standard communication media, which may be optical fiber cable , chord cable or through microwave, VHF radio patch. • The typical schematic block diagram of complete SCADA is shown at fig 7.1 . The communication protocol is employed for present SCADA system has been standardized and based on IEC- 870. Remote control center are provided with operator works

Quality aspects Traction

8.2 Power Quality Standards While IEEE 519, IEC 610003-2/4 describes current distortion limits at PCC for different power system voltages & fault levels and for different equipments respectively, IEEE 1159 describes recommended practices for monitoring of power quality. Performance standards like EN 50160 describe limits of voltage un balance, harmonic

3

distortion that have fixed for utilities.

been

8.3 Effects of harmonics in the system • Form factor deterioration, i.e. ratio between the RMS and the Average value of the rectified traction voltage. Increase of form factor means reduction in ability of loco to absorb power from system. • Harmonics are also useful reactive energy cause poor PF. • Increase in and losses.

line

non and

current

• Limitations of signaling track circuits. • Interference to telecommunication. • Limitations of harmonic injections into public utility. 8.4 Controlling Harmonic currents Shunt filters are tuned series L-C circuits. On IR presently HT capacitor with 13% reactor acts as harmonic filter. • Harmonic compensation or injection using voltage source converters or active filters. • Phase multiplication. The basic 3- Phase converter is a six pulse unit. Phase shifting transformers are

used to combine 6 – bridges to construct pulse converters.

pulse 12/24

8.5 Main features of Traction Load Traction load is inductive in nature with a poor power factor in the range of 0.7 to 0.8. Because of presence of nonlinear components like thyristors, power-diodes etc, locomotive is nonlinear and prominent source of generation of odd current harmonics ( 16-20%) THD). Moreover, wide variation of load in a very short duration leads to voltage flickers and fluctuations (swell and sags in power quality terminology) and also results into poor voltage regulation. Although conventional locomotives do not have elaborate provision for containing harmonics, advanced locomotives which are equipped with three phase technology do have extensive circuits for harmonic filtrations as well as voltage regulation. 8.6 Effort taken by Indian Railways for improving power quality Compensation of reactive power in order to improve power factor is being done by series parallel combination of LC circuits. • Compensation or elimination of harmonics in the system, for improvement

4

in voltage form factor and reduction of line losses is achieved by using passive filters (LC circuits) at RTSS. • Reduction in voltage imbalance in 3 phase supply system of Grid is being achieved by balancing the traction load over a bigger geographical arena by alternatively changing the pair of incoming phases taken from the grid for supplying successive RTSS.

• Self commutated VAR generators: The fast switching power electric devices are used to act either as voltage or current fed inverter to inject required amount of leading / lagging reactive KVAR into the system.

• Monitoring of short duration voltage disturbances i.e. sag, swell and flickers is not being done by the railways, at present. 8.7 Compensation of reactive power to improve power factor Reactive power compensation Methods of improve PF • Using HT capacitors with 13% detuned reactors. It also act as passive filters for filtering of harmonics. • Static VAR compensators ( SVC ) : It uses switching devices to control capacitive or inductive energy into the system. Available SVC configurations are Thyristor switched capacitors ( TSC). Thyristor controlled reactors with passive filters ( TCR).

5