Traction power substations take commercial alternating current power from the local utility company and convert it into the direct current required by the LRVs. The optimal locations for the traction power substations are determined using a computer model that simulates proposed LRT operations along an accurate geometrical and geographical depiction of the planned route.
The model will include not only the horizontal and vertical alignment of the track, but also the achievable design speed so as to determine the power demand of the LRT system during peak periods. Therefore, in the early stages of any light rail transit project, track and traction power designers must interface to integrate the traction power system into the overall system design.
The final selection of substation sites is an iterative process with repeated simulations to confirm the capability of the traction power system to sustain peak-hour operations.
The sequence of events to develop substation sites is as follows:
The traction power designer, using the simulation program, selects theoretically ideal TPSS positions along the route, taking into account the distribution system’s voltage drop and the lowest voltage acceptable to the vehicle without degrading performance. The normal, single contingency criterion for determining traction power system sufficiency is to test the system with alternate substations out of operation and verify whether an acceptable level of LRT operations can be sustained.
The designer discusses these proposed locations with the local power utility to determine any impacts of the proposed power demand on their network. The utility then evaluates the availability of power circuits and the potential impacts on its other customers.
An agreement is eventually reached, if necessary, by moving the substation to enable it to be supplied from lightly loaded power circuits or by building spur cables to the substation location. It is also important, for reliability, that the power company avoid supplying two adjacent substations from the same circuit.
It is not always possible to position the traction power substations in the optimal location, particularly in urban areas where available sites may be limited by many issues, including political realities.
After an agreement is reached with the power company, the traction power designer can finalize the substation design. While the TPSS can be a constructed building into which equipment is installed, most substations for new and renovated light rail systems are modular, factory assembled units that are delivered to the site complete. They are erected on a prepared base that incorporates an extensive grounding network below the concrete. These modular units are more economical than constructed buildings. Depending on the neighborhood where they are sited, modular TPSS units are sometimes screened by landscaping or architectural walls.
Substations are located along the route as close to the tracks as possible within the constraints of available real estate. However, the final placement must also consider interfaces and underground cable duct routes for the power distribution supply and return systems, access roadways, and security requirements. The impact of this construction on trackwork design is limited to the interfaces with the supply and return power distribution system.
The electrical sectionalization of the distribution system usually takes place at the substation for all travel directions. Placement of a substation at, or near, a crossover is often desired to sectionalize electrical supply for each travel direction and to optimize the operational flexibility of the track system.
