Question

Explain earthing in DC traction applications with a simple diagram. What are the hazards involved? Explain the mitigation strategies with reference to the standards.

Answer 1

Electric Traction System

It is the system where the propulsion force to drive the vehicle is obtained from electric drives. Hence it uses electrical energy for its locomotion.

How electricity is supplied? There are many ways. One such method is the DC Traction System. In this traction system, electrical motors are used that operate on DC supply to produce necessary pull to the vehicle.

Operating voltages of DC traction system are:600V, 750 V, 1500V and 3000V.

a. Earthing procedure in DC traction system:

Today most of the DC traction systems use a totally floating earth as their grounding strategy.

In a totally floating system, the potential difference between the rail and the ground needs to be restricted especially in safety critical places like passenger stations. This is achieved by the help of Rail Potential Control Devices (RPCD)

1. In RPCD, a control unit monitors the potential difference and the flowing current between the rail and the ground.
2. The switch is open under normal operating conditions (floating earth).
3. The switch is closed when a predefined threshold voltage (Vr) is exceeded, to allow current to flow through the ground and limit the rail voltage.
4. RPCD is in ON position when this happens.
5. The control unit opens the switch back to its normal position only after the current flowing through the circuit is below a given threshold (Ir) and a certain time limit is passed.
6. In the floating earth method, direct connection to earth is not possible and therefore a small resistance (RG) is assumed to exist between RPCD and the ground.

b. Hazards Involved

An obvious problem associated with "totally floating systems" is that the touch potentials can be dangerously high. In order to reduce the voltages on rails, several devices exist. These devices allow a direct connection to earth when a certain voltage threshold is exceeded.

The problem associated with the old DC traction system is the increase in stray currents, which results in energy loss, and high corrosion in surrounding utilities.

c. Mitigation strategies

To overcome the adverse effect of corrosion, "totally floating systems" method is employed.

RPCDs used in this method provides efficient mechanism to control the touch potentials.