Diagram of a Transit Traction Power System
In the United States and around the world, new electrically propelled subway rail trains and light rail vehicles (LRVs) use AC propulsion with induction traction motors and variable voltage as well as variable frequency drives. Benefits of these AC trains over older trains include higher performance, reduced starting energy use, regenerated braking energy, lower maintenance and a broadened range of operating conditions. However, transit operators have found that without an energy storage system available to capture the regenerated power, AC trains and earlier DC chopper trains have not fully delivered their potential economic benefits. Therefore, transit operators are highly interested in finding an energy storage solution to capture the full energy savings benefit of regenerative braking. They know that such a solution also reduces the peak-power usage that incurs high peak power demand charges.
A WESS can also mitigate voltage sag and drop problems that most U.S. transit agencies experience in several sections of their existing tracks. Voltage sag results from physical limitations of the traction power supply, including substation location, rating and loads. Older traction power infrastructure can become limited by the addition of newer fleets or rail cars with higher power demand as well as by increased ridership, which either reduces headway times or adds cars to the existing trains. The current solution for this problem is the addition of a new traction power substation. “Transit operators have limited capital budgets for traction power improvements to resolve these problems,” notes Krishna. “Fortunately, an effective energy storage system can resolve voltage sag problems by supporting the grid voltage during a sag occurrence, yet can also switch to its energy-saving mode during other periods of the day. The double benefit of a WESS in these situations is a clear example of improving our rail systems with the future in mind.” Figure 2 shows a line diagram of a wayside energy storage system (WESS).
Reduction of Total Energy Consumption
Each time a train brakes and stops, it generates energy that often goes unused due to the lack of an effective energy storage device in the system. The result is wasted energy, typically converted into heat and dissipated into the air. If we consider the amount of energy wasted per station over the next 20 years and the environmental effects of using utility power where regenerated energy could have been used, there is no question that energy storage must be a key component of every rail transit system.
To reduce total energy consumption, the WESS must capture and efficiently enable reuse of all of the energy regenerated by trains during braking. These savings directly reduce a transit operator’s electric costs. In the upcoming demonstration project in L.A., testing is expected to show that the addition of the WESS will reduce energy consumption by up to 48 percent, a significant number which has a major impact on utility costs and environmental advantages. The proposal submitted to the FTA by the LACMTA detailed the direct impact on cost to the agency by analyzing the current utility billing format. By taking this energy recycling concept and applying it to each of the 200 stations in the LACMTA lines, we can see that the savings in energy costs and improvement in energy availability are critical for a growing and sustainable modern transportation system.