2.1.1. Heavy-Rail Catenary Supplied Vehicles

This category mainly includes transport locomotives such as high-speed short-distance passenger and freight trains [28]. In general, the traction system of these vehicles is fed from an overhead line (primary power supply) through a pantograph. The voltage level varies depending on the country, generally 1.5–3 kV DC (short-haul train) or single phase 25–50 Hz (short and long-distance trains). Figure 6 shows a typical block diagram of a common heavy-rail catenary supplied traction drive without any ESS.

**Figure 6.** Simplified electrical diagram of the traction drive of a heavy-rail catenary supplied vehicles.

There are several researchers that describe the use of SCs in this type of vehicle. Dutta, O. [29] proposed a mathematical optimization methodology and a model of a stationary storage systemfor a DC rail transportation application, New York City Transit (NYCT). In this study different technologies are compared, including batteries, flywheels and SCs working independently or together. The results of the optimization process are based on the percentage of energy savings because of regenerative braking, voltage regulation, reduction in peak demand, estimated payback period and system re-siliency. The paper concludes that the cheapest storage system to operate autonomously are SCs. On the other hand, it highlights that regarding the the resilience of the system, a hybrid system made up of batteries and SCs is the economic option as long as the percentage of regenerative braking energy recovery is greater than 30%.

Khodaparastan, M. [30] compares two storage technologies (SCs and high-speed flywheels), from an economic point of view, to take advantage of the braking energy of a continuously powered train (650 V rectified voltage from the utility grid AC 13 kV). Two case studies are presented depending on the objective to be achieved: reduce the demand peak or stabilize the supply voltage. A cost analysis of both technologies is also performed for both purposes. It is concluded that the flywheel is the most suitable technology from the economical point of view. However, due to their technical characteristics, both technologies are appropriate for the purpose described.

Chen, J. [31] also proposedthe use of SCs to take advantage of the regenerative braking of a high-speed railway powered by a 27 kV AC catenary. The storage system is connected through a bidirectional DC/DC converter to the intermediate DC stage of a back-to-back converter, whose input and output are connected to two different points on the main power line of the trains. A state machine logic is proposed with four states (charge, discharge, transfer and standby mode) and transitions between them depending on the power line charge level and the maximum state of charge (SoC) of the ESS. One of the back-to-back converters acts as the master converter and the other converter and the DC/DC of the SCs act as slaves. The proposed control coordinates the operation of several converters to stabilize the DC bus voltage, improve the power supplyquality in high-speed railways and take advantage of braking energy. The cost savings will depend on the policy of power utility companies for the returned regenerative braking energy.

Zhong, Z. [32] proposes SCs as an on-board storage system to absorb braking energy and completely replace the brake resitor (see Figure 7). Despite the weight that this implies, it is justified that the storage system is on board and not stationary in order to take advantage of the whole braking energy and to be able to completely replace the on-board brake resistor. A hierarchical optimization energy management strategy is proposed based on an additional power stage in series, connected between the inverter of the traction motor and the main supply voltage (DC). The storage system would be connected to the DC stage through a bidirectional converter. The three benefits extracted from this configuration are:


Xu, C. [33] presented a study related to the use of SCs to extend the service life of the pantographs that feed high-speed trains. The arcing phenomenon due to the irregular movements of the trains and the intermittent line pantograph disconnection reduces its useful life. Morreover, it can even damage onboard equipment and measuring elements. In this study, the SCs based ESS absorbs the inductive energy and reduces the surges in the power line. The strategy to manage the system energy between the storage system, the train and the main power system is analyzed and validated by simulations to compensate the voltage fluctuations and take advantage of braking energy.

**Figure 7.** Topology of onboard-wayside supercapacitor hybrid energy storage system extracted adapted from [32].

#### 2.1.2. Heavy-Rail Diesel–Electric Vehicles

This section analyzes the applications of SCs in Heavy-Rail Diesel—Electric Vehicles. These types of vehicles run on fossil fuel and are commonly used in North America and some European countries [34]. The traction scheme of this type of vehicle is composed of an internal combustion engine (ICE) group (gas engine, petrol engine, diesel engine, or gas turbines) coupled to an electric generator that, through two power converters, feed the traction motors. They have also an energy dissipation system to evacuate the braking energy (DC chopper). A typical electrical diagram of this type of vehicle without any ESS is shown Figure 8.

**Figure 8.** Simplified electrical diagram of the traction drive of diesel-electric vehicles with permanent magnet synchronous generator (PMSG).

Da Silva Moraes, C.G. [35] proposed a hybrid storage system (lithium-ion batteries and SCs) whereby a percentage of the braking energy is used to power auxiliary equipment, while the ICE group remains the main power supply for the traction motor. Thids would reduce the cost and volume of the diesel group. SCs are used as buffers to absorb rapid power fluctuations in a multiport configuration in which each storage system consists of a DC/DC converter.

#### 2.1.3. Light-Rail Rapid Transit Vehicles

Light vehicles powered by a catenary constitute a type of transport that is normally used for moving passengers in urban environment [36]. The locomotive or towing vehicle is powered by an overhead line through a pantograph. The catenary in turn is fed by a power supply substation. The supply voltage level varies from one country to another, stablishing in most of them in the range of 1.5 kV or 3 kV. Figure 9 shows the most common electrical scheme used by this type of vehicle without any ESS. A three-phase inverter is usually used to power the traction motors from the catenary. On the other hand, it also has a DC/DC converter (DC chopper) to evacuate the braking energy.

**Figure 9.** Simplified electrical diagram of the traction drive of light-railupplied vehicles.

In the literature, there are several papers that study the SCs introduction in the feeding scheme of this type of vehicle. Zhang, X. [37] studied the existing problem arises due to the overcurrents suffered by the SCs that feed some light urban transport vehicles during charging from the catenary. This problem is more serious when, due to power needs, it is necessary to connect several chargers in parallel. These overcurrents also cause accelerated aging of the SCs. The authors propose a protocol to coordinate the load of the different storage systems to reduce the current overshoot without hardly increasing the load time.

The study proposed in [38] shows a power management control of wayside lithiumion capacitor to improve the efficiency of a light railway vehicle. The particularity of this paper is it concludes the SC technology that best suits the present application are Li-ion capacitor and not EDLC due to its higher energy density. The energy stored during regenerative braking is used again during the acceleration section. The energy management strategy is based on monitoring the SCs SoC and the current vehicle speed. Other important conclusions drawn from the study are that the peak power of the power supply is reduced by up to 46% and the maximum energy saved represents 30% of the energy consumed by a system without an ESS.

Zhu, F. [39] suggests a hierarchical control of a stationary supercapacitor-based energy storage system to save energy by taking advantage of braking power. The main power supply for the vehicle is a 750 V or 1500 V voltage catenary. The ESS is connected to this DC bus through a DC/DC converter. A real case study is described and analyzed in which the proposed control strategy is applied with a maximum energy saving of 12%.

Additionally, although no references related to real applications have been found, it is worth mentioning the possibility to use energy storage for ultrafast rail vehicles, mag-netic levitated trains, and the hyperloop, resulting in an especially convenient option in the second one, since the use of a power supply just for the acceleration leads to the use of short-term energy storage [40].

## *2.2. Electric Drive for Road Vehicles*
