Researchers from Tongji University in Shanghai and Sichuan Normal University in Chengdu have developed a new method for stabilizing metro power supply via superconducting energy storage devices. This system makes it possible to offset voltage fluctuations that occur during simultaneous braking and acceleration of trains in real time, reducing energy losses and enabling the reuse of braking energy.
Constant fluctuations in network load constitute the biggest challenge in metro operation. When a train starts moving, it suddenly draws electricity, causing a voltage drop. While braking, however, it returns some of the energy to the grid, causing a surge in voltage. During rush hour, when dozens of trains start and stop nearly simultaneously, these fluctuations become particularly severe. If the voltage exceeds the permissible limit, the energy recovery system is disabled and the braking energy gets dissipated as heat. Over time, these fluctuations create the risk of overloads, accelerate equipment wear and require additional infrastructure maintenance costs from municipal services.
To eliminate these fluctuations, the Chinese engineers have proposed using a superconducting energy storage device, which, unlike a battery, stores energy not in chemical compounds but in the form of a magnetic field. This storage device consists of a high-temperature superconductor coil cooled to a state where current circulates without resistance. This allows energy to be stored and released almost instantly and without any losses. When the grid voltage increases, the storage device immediately absorbs the excess energy, and when the voltage goes down, the device releases it back. This system acts as a shock absorber for the power grid, maintaining the voltage at a stable level regardless of train operation.
In order to verify their data, the researchers created a computer model of the metro power supply system in MATLAB/Simulink on the basis of a standard 1,500-volt traction network. The simulation reproduced typical scenarios: simultaneous acceleration, braking and movement of multiple trains. Without the energy storage device, voltage fluctuated between 1,400 volts and 1,600 volts, remaining virtually constant with the device connected. The system responded to changes within milliseconds; the higher the current in the coil would get, the more precise the smoothing would become.
The researchers then conducted a test to see how the technology would work in a large transfer hub where several metro lines converge. They modeled a typical metro station with three lines connected to a common 1,500-volt DC substation, a configuration common in many metropolitan areas. Since it is too costly to install a separate energy storage device on each line, a common system with a single superconducting module was proposed. The controller distributed energy among the lines according to priorities: if voltage fluctuations were higher on one line, energy would be sent there first to offset the difference. This made it possible to maintain the voltage within acceptable limits even during simultaneous disturbances on all three lines.
The simulation showed that the use of a superconducting storage device reduces the amplitude of voltage fluctuations by more than 20 times. This improves the quality of power supply, reduces the load on equipment and increases the share of energy returned to the system during braking. The study’s authors believe that superconducting storage devices could become part of the smart energy infrastructure of urban transport, serving as an instantaneous buffer that balances the energy flow.
