Railway mass transit systems like subways play a fundamental role in the concept of sustainable cities. In these systems, the amount of passengers strongly fluctuates along the day. Hence, in order to provide a proper service without incurring disproportionate energy consumption, operation at different traffic densities is required.

The majority of underground systems are DC-electrified. Standard DC voltages in railway systems are low for historical and safety reasons. In the rush hours, the large number of trains demanding power of the system may lead to overloaded substations and voltage dips. This problem is partially mitigated by means of substation-transformer tap regulation, which allows operators to increase the no-load voltage.

High no-load voltage has a beneficial effect at all traffic-density scenarios in terms of transmission losses. However, at the same time it effectively reduces the system’s capacity to absorb regenerated energy, which may lead to inefficient energy consumption figures during off-peak hours.

In this paper, the sensitivity of system energy consumption to no-load voltage has been analyzed. Several traffic-density scenarios in a case-study system are explored. As a result, a scheduled no-load voltage scheme is proposed for the operation of the system. This operation strategy improves energy efficiency without incurring a high investment cost. The only costs related to this proposed method are the costs of wear-and-tear in tap-changers. In case there are devices such as energy storage systems installed in the system, there would be additional operation costs related to a simultaneous update of the voltage limits for their operation.

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