Commonly used Voltages in Traction Power Systems

Following are the commonly used voltages in Traction power systems:

1. 25 kV AC
2. 2 x 25 kV AC
3. 15 kV AC
4. 1500V DC
5. 750 V DC.

In general power systems, the allowable voltage variations will be +/- 6 % but in the Traction power system the voltage variation limits with respect to each voltage will be based on EN 50163. This standard also specifies the limits for each traction system voltages and following are the criteria which are commonly used in the traction system for the voltage variations.

• Nominal voltage- desired system voltage
• Lowest permanent voltage- This is the minimum permitted voltage in which the trains can be operated.
• Lowest non-permanent Voltage- This voltage can be allowed only for short duration of time.( I,e 2 minutes)
• Highest permanent Voltage- This is the maximum permitted voltage in which the trains can be operated.
• Highest non-permanent voltage- This voltage can be allowed only for short duration of time.( I,e 5 minutes)

Based on the above terminologies along with the specified limits the traction power supply system is carefully designed for suitable operation of Rolling stocks.

In addition, the number of substations and the maximum length of the feeding system will be based on the above criteria’s.

Selection of Traction power supply voltages (I,e AC or DC system) will be based on the below factors:

1. Length of the Line- If the electrification needs to be done for lengthy lines, 25 kV AC system will be always preferred because of the less voltage drops and intern reduces the number of traction power substations, Hence for the main lines and freight lines 1×25 kV and 2 x25 kV AC are commonly used.
2. Based on the area of Installation-  If the installation  is inside the city areas, the track will be laid either on via duct or inside the tunnels and hence DC systems are preferred with 3^rdRail systems.

If the installation is on the ground level, Third rail systems are not preferred due to the personal safety concerns and hence 25 kV systems will be used.

3. Based on Land availability- Though the 25 kV systems requires less number of substations but the land required for the traction power supply substations will be higher but whereas for the DC systems the substations shall be installed in the passenger stations itself. Hence if the system is installed on City areas DC systems are preferred and for rural areas 25 kV systems are preferred based on the cost of the land.
4. Based on speed and Headway- If the high speed trains with the lesser headway are operated then 25kV systems are preferred because there are the limitations in DC system equipment’s due to high load currents.
5. Capital costs- Generally the DC systems substation equipment cost will be higher because of rectifiers. However based on the cost of foot print area and number of substation requirements the investment cost comparison needs to be done in the feasibility studies to decide the Nominal voltage ratings of the traction systems.Following are the locations in India, where AC tractions system is used.
   1. Delhi Metro
   2. Mumbai Metro
   3. Chennai Metro
   4. Lucknow Metro
   5. Pune Metro
   6. Nagpur metro
   7. Jaipur Metro

   In addition to above, all main lines and freight corridor projects uses the AC traction systems.

   Following are the locations in India, where DC tractions system is used.

   1. Bangalore Metro
   2. Kochi Metro
   3. Pune Metro
   4. Agra metro
   5. Kanpur Metro