Regulation for Electrical Crossing of Railway Tracks PDF

REGULATION FOR ELECTRICAL CROSSING OF RAILWAY TRACKS 9.0 Provision of the regulation for power line crossing of Railway Track, 1987 are given below. 9.0.1 Power line crossing up to and including 11kv require to be cabled and crossed un- derground 9.0.2 Power Line Crossings up to and...

REGULATION FOR ELECTRICAL CROSSING OF RAILWAY TRACKS 9.0 Provision of the regulation for power line crossing of Railway Track, 1987 are given below. 9.0.1 Power line crossing up to and including 11kv require to be cabled and crossed un- derground 9.0.2 Power Line Crossings up to and including 33kv is recommended to be croosed through underground cables. 9.0.3 For overhead crossing of power lines, the minimum height above rail level to the lowest conductor including the guard wire shall be as given below. Voltage(kv) Minimum height above Rail level(m) above 11 to 66 14.10 above 66 to 132 14.60 above 132 to 220 15.40 above 220 to 400 17.90 above 400 to 500 19.30 above 500 to 800 23.40 9.0.4 The angle of overhead crossing shall be perpendicular to the track. In exceptional cases it may be permitted with a skew, the angle of deviation not exceeding 30 degree. In case it is likely to be more than this value the case should be referred to the Electrical Inspector to the government (Chief Electrical Engineer of the open Line Railway). 9.0.5 In view of high cost of modifications to Power Line Crossing and also difficulty in getting shut down of important transmission line crossings of tracks contemplated to be electrified Electrical inspectors to Government may approve crossings with lower heights provided the clearance between lowest conductor of the power line to the highest traction conductor under most adverse ambient temperature condition is as indicated below. Voltage(kv) Minimum clearance(m) 0 to 66 4.44 above 66 to 110 4.75 above 110 to 132 5.05 220 6.58 400 9.71 500 11.45 800 16.67 If power line crossing has guard wire minimum clearance from the guard wire to the highest traction conductonr should be 2 m 156 9.1 For other provision the Regulation should be refered to.Modification to Electrical over- head Distribution system at station and yards where 25kv ,ac 50Hz single phase traction is to be introduced. 9.1.1 It is essential for an electrical overhead distribution system that. a) There is adequate physical clearance to the 25 kv, system so that maintenance personel do not come within 2m of nearest point at a potential of 25kv. b) Elrctro-magnetically induced (EMI) voltage in the distribution system from adjacent 25kv traction current remains within safe limit, and c) Electro-statically induced discharge current due to Capacitive Coupling between 25 kv system and the power distribution is within safe limit. 9.1.2 If the above safety condition are not met with ,the overhead distribution system shall be modified to bring the value within safe limit. 9.1.3 The safe limit for : a) Electro-magnetically induced voltage are: 1) For installations to which only trained and authorised personnel have access 60V 2) For installation to which public or untrained staff have access 25v b) Electro-statically induced discharge current shall not exceed 8mA 9.1.4 Neutral of a 3 phase, 4 wire earthed system should be earthed at only one point,that is at the substation 9.1.5. In long metallic structures such as fencing or an overbridge,the electro-magnetically induced voltage shall not exceed 25 volts. 9.1.6. Formula adopted for Electro-magnetically induced voltage. Electro-magnetically induced voltage ‘E’ is given by the expression: E = 2π.Kr.Kc.M.I.L. where, E is in volts f is frequency of power supply=50Hz kr is coefficient dependent upon type of return circuit and has been empirically established by SNCF the values are given below Value of Kr Rails per track Section having available for Double Single traction current track track 2 0.39 0.56 1 0.53 0.69 157 Kc is cable screening factor and it is 1 for bare conductor and 0.8 for lead sheathed underground cable (figures adopted from SNCF) M is mutual inductance in Henry and is a function of average distance between the conductors and the earth resistivity. I is current in ampere in OHE L is length of parallelism in metre 9.1.7. The length of parallelism of an overhead conductor is measured as projected on the alignment of the OHE. The separation distance, which determines the value of M for a given value of earth resistivity is the average of the length of offsets from the alignment of OHE to the overhead conductor. 9.1.8. Table 9.1 gives the voltage rise on an overhead distribution line due to induction from traction current. The table, obtained empirically by SNCF is adopted for use on Indian Railways. This table is applicable for following conditions : a) The Section is double track and all the four rails are available for traction current. b) The Current in OHE is 600 A c) The earth conductivity is 8 x 10 -2 S/m. The value of earth conductivity generally encountered is expected to bse higher than this value, as such the values in the table should be safe for majority of the conditions. 9.1.9. To obtain the voltage rise on an overhead conductor having different environment, the values given in the Table 9.1 requires to be modified. For example, to obtain the voltage rise on an overhead distribution line from an OHE on single track, with only one rail available for traction return current, the value obtained for the corresponding length of parallelism has to be modified to allow for : a) Single track section. This will have a maximum current of 300 A. The figure obtained should be divided by 2, i.e. 300/600. b) Only one rail is available for traction return current, hence the value of Kr adopted for double track, double rail section, being 0.39 is no longer applicable. The value Kr applicable for single track, single rail section is 0.6 9. The result obtained after modification as given in (a) above should be further modified by multiplying it with a factor of 0.69/0.39. c) When booster transformer and return conductors are used the value of voltage rise are reduced further. As a rough approximation the value obtained rom the table in paragraph 8 above should be divided by 2.3 for a booster transformer spacing of 2.66 km and 1.7 for a transformer spacing of 4 km. 9.1.10. A Worked Example : See Fig.9.01 below giving the plan of an overhead line running along a double track section proposed to be electrified at 25 kV. 158 Fig 9.01 The considerations are : a) In the above diagram the project length of the overhead line A, B, C, D, E on electrified, route is a, b, c, d, e. a to e totals, say 0.8 km. Then this is the length of parallelism. b) From the diagram average of this offsets at a, b, c, d and e are taken. This is found to be, say 16 m. This is the mean distance between the two lines. c) Although there are four rails, if the section is near a station, there may be likelihood of track circuits being introduced in future. To be on the safe side only two rails are taken for return circuit. On the above basis interpolating the readings of the table, the induced voltage comes to 50 V. If all four rails were available for traction currents the voltage rise expected would be 36 V. Both the values are within safe limits. 9.1.11. If it is found that the induced voltage due to electromagnetic induction is higher than the permissible limit, one or several of the means listed below may be adepted to bring the voltage down to safe limits a) Shift the point of feed from the end to the center of the line so as to reduce the parallelism b) Sectionalize the distribution line and feed short length separately. c) Provide separate 25 KV/240 auxilary transformers for different areas with short distribution lines d) Sectionalize the line through isolating transformers e) Shift the distribution line farther away from the electrified track. f) Cable the overhead line in lead sheathed or aluminum sheathed cables. 9.1.12. To assess the electro-statically induced current Table 9.2 may be made use This table has been obtained from SNGF. In case the discharge current is found to be above safe limits, the separation distance may be increased so as to bring the value to a safe limit. It is generally found that the electrostatic induction presents little problem. -------------------------------------------- 159