Conversion Systems for Braking Energy Recovery in 3 kVDC Railway Lines

Marco Barra Caracciolo, Matteo Berrera Ferrovienord SpA

Milano, Italy

Morris Brenna, Dario Zaninelli Politecnico di Milano – Department of Energy

Milano, Italy

Abstract—This paper deals with the energy saving given by the recovery of the braking energy coming from the trains in railway transportation systems. In particular, the paper is focused on the discussion about the conversion systems that are needed to achieve the power flow control from the braking trains to the station load through the energy exchange with the storage devices. These converters are actually realized to form a full-scale prototype applied to a regional 3 kVDC railway line.

Keywords—Energy saving, supercapacitors, railway transportation systems, electronic power converters

I. INTRODUCTION

Nowadays the energy efficiency is an important issue in industrial, tertiary and domestic applications, for both the environmental and economic concerns. This topic becomes more important for those sectors that employ great amount of energy in their processes.

Transportation systems are one of the areas in which there are many opportunities for energy saving obtaining at the same time a significant operating cost reduction. In particular, electric railway systems are immediately eligible for the energy efficiency improvement, since they use electric energy that can be generated through renewable sources or that can be stored for a better exploitation.

These are the reasons why the above topics are deeply considered in Horizon 2020 European Framework Program for Research and Innovation.

A way to save energy in electrified transportation systems is the recovery of the braking energy coming from the trains equipped with electric regenerative braking.

At present, this energy is mainly dissipated by dedicated rheostats, while a small amount is delivered to other trains in accelerating phase. In fact, various statistical analyses show that there is a low probability of having contemporarily one braking and another accelerating train on the same line.

A solution for the braking energy saving is its recovery in storage devices installed near the railway line. Since the braking energy presents high power peaks for short time, one of the best solution is the installation of supercapacitor banks. In fact, the supercapacitors have all the characteristics needed to this purpose in terms of maximum power and number of charge-discharge cycles. Nevertheless, due to their poor energy density,

the space needed for their installation suggests the application in fixed power substations.

Another feature is that the supercapacitors, as the mostly common storage systems, cannot be connected directly to the railway supply electric line. In particular, for heavy railway systems, as the Italian regional network supplied at 3 kVDC, the high voltage value of the catenary require a dedicated DC/DC step-down converter to adapt the system voltage to the storage voltage. However, a single conversion stage is not enough due to the high voltage variations at the capacitor terminals during the charge and discharge phases. To this purpose, an innovative solution has been studied in order to achieve all the requirements.

The proposal is a conversion system based on different DC/DC and DC/AC converters that are able to manage the energy coming from the trains and share it among various elements as storage systems and station loads.

The aim of this paper is the analysis of the various converters applied to the proposed system for the braking energy recovery. After that, a prototype of the entire system will described and discussed.

II. POWER CONVERTERS

This paragraph deals with the various power electronic converters that form the braking energy recovery system.

The basic electric scheme of the proposed system is reported in Figure 1, where it is possible to identify the following converters:

Unidirectional DC/DC step-down converter 3000/750 VDC

Bidirectional DC/DC converter for supercapacitor charge/discharge

DC/AC converter 750 VDC/400 VAC 50 Hz 3~

A. Unidirectional DC/DC step-down converter

This converter provides the conversion from the high voltage value of the contact line to an intermediate stabilized voltage that is suitable for both the charge/discharge converter of the storage system and the DC/AC inverter that supply the station loads. Its main features are:

constant voltage ratio

unidirectional power flow (intrinsic or imposed)

galvanic insulation

Fig. 1. Electrc basic scheme of the proposed braking energy recovery system.

The latter characteristic is achieved using a high frequency transformer in a flyback configuration or through a H-bridges.

Due to the high power application, the experimental prototype has been realized with two conversion stages based on H-IGBT bridges (Figure 2).

The first stage converts the input DC voltage into a 4 kHz AC voltage that supply the high frequency transformer.

The second stage realizes the high frequency AC into DC at 750 V.

The high frequency transformer is used to change the voltage ratio and to provide the galvanic insulation from input to output. The latter function is very important to protect the power converters and storage systems in case of fault in the contact line.

This system is intrinsically bidirectional, but in this application the power flow is kept unidirectional from high to medium voltage section. In this way the energy flow is well known to make easier the implementation of the protection system.

The DC/DC step-down converter is the core of this project due to its characteristics in terms of power and voltage ratings. In fact, this converter has been developed for this particular case starting from those applied on board light electric rail vehicles with some technical adjustments.

The first condition to respect regards the input voltage, that for a traditional heavy railway in Italy can reach 4 kVDC during the regular operation. This has meant that the IGBTs used in lower voltage applications up to 1.5 kVDC, as for light trams and subway vehicles, were replaced with modules that are able to withstand higher voltages. Therefore, all the power circuit including IGBT drives were deeply revised and redesigned.

Another important characteristic is its overloadability. Indeed, all the appliances that belong to traction chain, from fixed supply systems to the motor drives, are subjected to many power peaks due to the continuous acceleration and braking of the trains. This means that it is not useful to size all these components for their maximum power, otherwise an unnecessary oversize is obtained. Hence, they are designed for a lower rated power but choosing materials and components that are able to be overloaded for a short period (typically the duration of acceleration and braking). So, during the redesign phase of the converter, all the power components have been chosen to fulfill this requirement.

Besides, a particular attention has been paid to the grounding of the various parts. This is another issue typical of the traction systems, which can be different from other cases as in industrial and residential applications. In fact, in electric traction systems it is possible to identify at least two different earthing systems:

High frequency transformer

From catenary

Positive busbar (3000 VDC)

Negative busbar (rail)

750 VDC busbar

V+

V–

Fig. 2. Power electrc scheme of the 3000/750 VDC converter with high frequency transformer.

structure earth: is the interconnection of the metallic parts that belong to structure including the reinforcing rods of the structures;

traction earth: is constituted by the metallic rail of the track that are used as return conductor of the traction current.

In normal operating conditions, these two different earths are insulated each other in order to avoid the corrosion of the metallic parts caused by the stray currents. This leads to a potential difference between the two systems that depends by the working condition of the system, e.g. by the current absorbed by trains. Usually, the system is sized in order to keep low this potential difference for safety reasons. Whenever it tends to rise above 50 V a suitable device connect the earthing systems together to make equipotential all the conducting parts.

Nevertheless, a potential difference of a few volts between the two earths is physiologic and does not cause any safety problem or hazard risks for the human beings. But a few volts of the potential difference can cause malfunctioning of the electronic appliances, as they usually work at 5 V or 12 V.

Since the metallic frame of the converter is connected to structure earth and the input negative conductor is connected to the rails (that is the traction earth), all the electronic controller and communication systems have been designed in order to overcome this problem.

Another particularity of this converter is the high frequency transformer that is used to adapt the voltage ratio and to provide the metallic insulation between primary and secondary sides. It has been designed and realized in order to minimize the leakage parameters such as series resistance and inductance of the windings. In fact, at higher frequencies the series impedance rises quickly also because of the skin effect, increasing the voltage drop and the voltage spikes on the IGBT modules during their commutations. Moreover, the high frequency increases the power losses in the magnetic core. All these problems have been overcome by choosing proper materials and studying a suitable geometry of the magnetic core and an appropriate layout of the primary and secondary windings.

Sometimes all these measures are not enough to ensure that a high frequency transformer can work properly, especially in high power applications as in this case.

Therefore, the whole converter has been sized in order to realize the zero-current switching (ZCS) through a resonant circuit whose resonance frequency is the same of the switching frequency.

This working regime is kept for every operating condition since there is not any regulation of the transformer ratio, so the soft switching is always realized in both H-bridges.

As a consequence, it is possible to minimize the switching losses and increase the overall efficiency of the converter. This is the reason why a air forced cooling system has been applied instead of a stronger liquid cooling system based on water and glycol mixture.

The high voltage section is then completed with all protection devices, in particular, maximum current, minimum voltage and over-temperature relays.

Even if the system here presented is a prototype, a very careful attention has been paid to safety of the whole system. In fact, the prototype is installed in a real substation where many people can access for regulation and maintenance matter.

This is the reason why all the appliances have been installed in their own boxes or segregated areas equipped with disconnectors and earthing systems to prevent and avoid accidental access to the live parts.

The prototype converter is shown in Figure 3.

Fig. 3. 3000/750 VDC converter with high frequency transformer.

B. Bidirectional DC/DC converter for supercapacitor charge/discharge

This converter regulates the charge and discharge of the storage system adapting the output DC voltage in function of its state of charge (SOC). A promising technology to store energy with a high power rate is constituted by supercapacitor modules (Figure 4)

Fig. 4. Supercapacitor module.

The charge is controlled by imposing the output current, whose reference comes from the control logic of the whole system.

The charge/discharge converter is sized to exchange energy with a set of supercapacitor for a power rating of 50 kW. The advantage of this configuration is its modularity that allow to increase the maximum power or the rated energy capacity by connecting in parallel the desired number of modules. This is possible because all the modules are regulated by the same master controller, so all of them behave as many current generators connected in parallel. In fact, the voltage of 750 VDC busbar is kept quite constant by the 3000/750 step-down DC/DC converter.

Since the energy stored in the supercapacitors is a quadratic function of the voltage applied to their terminals, it is possible to control their charge or discharge by controlling the charging current and the charging duration as described by the equation (1)

⇒ ⇒

⇒

(1)

The maximum energy stored inside the supercapacitors depends by the maximum voltage that can be applied to their terminals, so they have to be protected against overvoltages.

Instead, the minimum working voltage of the supercapacitors has to be equal to ½ its rated value in order to preserve them against a fast aging. Therefore, the useful amount of energy that can be exchanged by the storage system is equal to ¾ the maximum one.

In this specific case, the supercapacitor bank is sized for a rated voltage equal to 600 V, so the voltage variation at the supercapacitor terminals is in the range of 300 – 600 V.

The low voltage rating and the small voltage ratio form input to output of the charge/discharge DC/DC converter does not require the application of a high frequency transformer, therefore this converter is constituted by a bidirectional chopper employing only one IGBT power module.

The power converter and the supercapacitors are placed inside a dedicated box with all the necessary protection devices, such as maximum current, minimum voltage and over-temperature relays. Besides, there are disconnectors to guarantee the safety during maintenance operations.

These systems are reported in Figure 5.

Fig. 5. Supercapacitors box.

The charge and discharge control has been implemented through a dedicated LabView that, after the acquisition of the catenary voltage signal, it is able to recognize the braking phase of the trains.

The application of this controller is shown in Figure 6 (a), while Figure 6 (b) represents its graphic interface.

(a)

(b)

Fig. 6. Control logic implementation (a) and graphic interface (b).

The power box is equipped with an emergency rheostat that is needed to discharge the supercapacitors before the maintenance operations. This measure protects the people against electric shocks because it forbid the access to the storage system box if the voltage on the supercapacitors is higher than 20 V.

C. DC/AC three-phase inverter

The DC/AC three-phase inverter in used to convert the busbar voltage at 750 VDC of the DC link into the 400 VAC usually employed in the common industrial distribution system. Its purpose is to supply the many electric loads installed in the railway stations using the braking energy coming from the various trains and recovered in the supercapacitors.

In particular, in the prototype system presented in this paper, the DC/AC inverter supply the lighting system, based on LED lamps, of the platforms in a recent underground station as shown in figure 5.

This DC/AC converter is based on the output stage used in uninterruptable power supply (UPS) devices, so that it includes all the control and protection function required for this application.

The DC/AC converter is installed inside a station room that is about 2 km far from the electric conversion substation (ESS) where the other devices are placed. Therefore, it had been needed to stretch the 750 VDC link by installing a dedicated DC line along the railway.

Fig. 5. Platform LED lamps supplied by the DC/AC inverter.

In order to guarantee the continuity of the power supply, in case of failure of the power converter a static switch commutate the station loads from the inverter to the public distribution grid.

III. CONCLUSIONS

The energy saving that can be achieved through the recovery of the braking energy coming from electric trains is a way to

increase the efficiency of the railway transportation systems and to reduce the operation costs.

For this aim, the paper presented a prototype of an energy recovery system based on supercapacitors as storage device. The prototype is constituted by different power converters that realize different functions in order to control the power flow from the braking trains to the station loads and exchanging energy with the storage system.

In particular it is possible to identify three main functions: the DC/DC conversion from the catenary to an intermediate DC link, the charge/discharge of the supercapacitors and the DC/AC conversion to supply the station loads.

Each converter has it is own specific characteristics in terms of power and voltage ratings, overloadability and protection system, that have been explained in detail in the paper.

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