WO2023094975A1

WO2023094975A1 - A dc traction power supply method and system

Info

Publication number: WO2023094975A1
Application number: PCT/IB2022/061246
Authority: WO
Inventors: Danuta DMOWSKA-ANDRZEJUK; Michał STRUKOWICZ
Original assignee: Dmowska Andrzejuk Danuta; Strukowicz Michal
Priority date: 2021-11-23
Filing date: 2022-11-22
Publication date: 2023-06-01
Also published as: PL439601A1

Abstract

The object of the invention is a direct current electric traction power supply system comprising a power supply object (9) comprising a main busbar (7) and a negative busbar (8), a power supply converter (4) connected in series with a protection switch (3) and connected between the main busbar (7) and the negative busbar (8), and a control system, wherein the power supply converter (4) is connected to a power supply source (5, 6), wherein it additionally comprises a battery pack (1) connected in series with a protection switch (2) and connected between the main busbar (7) and the negative busbar (8), in parallel with respect to the power supply converter (4) and the protection switch (3) connected in series. The object of the invention is also a direct current electric traction power supply method realized in the direct current electric traction power supply system.

Description

A DC traction power supply method and system

The present invention relates to a direct current electric traction power supply method and a direct current electric traction power supply system applicable in the powering of devices supplying electric energy to vehicles propelled by electric motors in transport systems such as railway, tramway or trolleybus.

One of important power supply objects in electric traction systems is the traction substation, in which the electric energy supplied to the substation is converted to electric energy adjusted to a particular electric traction power supply system. Therefore, the task of the traction substation is to convert the received energy into an electric energy having a characteristic (for example voltage and current levels) adequate to the desired application. In direct current traction transport systems based on traction networks such as railway lines or tramway lines, traction substations are in the form of transformer rectifier systems in which the output electric energy is adjusted to the requirements of a particular system, including to the direct voltage of 3 kV DC and 600 kV DC, respectively. In such traction substations, the supply energy, most typically the three-phase 15 kV or 110 kV alternating current is converted into direct current required for a particular traction system. Except their basic purpose, which is to provide traction system power supply having required performance parameters, traction substations are also used to power traction system peripheral devices, as well as to receive energy from recuperation, for example from the regenerative braking of traction vehicles.

State-of-the-art knows solutions in which battery packs are used as elements allowing a reduction of the contracted power and of the energy consumption in the DC traction (recuperation). Solutions known in the art connect the battery pack with the main busbar of the 3 kV DC substation through a two-way DC/DC converter which is responsible for preserving the parameters of the cooperation between the pack and the 3 kV DC traction network. During the charging and discharging of the battery pack, losses occur which are a natural consequence of these processes and influence the efficiency of the battery pack.

In state-of-the-art solutions, the output of the converter system must meet a multitude of requirements regarding e.g. resistance to voltage interference from the grid (for example as a result of potential overvoltage), harmonic interferences, etc.

Polish patent PL229395B1 discloses a method and a system for storing electric energy for the purposes of direct current railway tractions. In the method for storing electric energy in battery packs for use in railway line direct current tractions supplied with a nominal voltage of 0.6-3 kV DC, the generated voltage is lower than the voltage on the railway rail, and energy is drawn from the traction network, or alternatively from the power grid, for generating direct voltage having a potential lower than the potential of the railway rail, the voltage being supplied to the charging system of the lithium battery pack which is energy storage for the traction network during high load periods, and the charging current is controlled by modifying the resistance in the charging system. On the other hand, in the system for storing electric energy for use in railway line direct current tractions, the traction network, for example +3 kV, is connected through a circuit- breaker/disconnector system with the positive pole of the lithium battery pack, and its negative pole is connected through a diode and a transistor with the railway rail, and also through a diode and a variable resistor with the lower-voltage busbar (-U), which is connected to a device generating direct voltage (-U) and connected to the railway rail.

Polish patent PL230710B1 discloses a method for assisting the powering of the direct current electric traction network and the system for assisting the powering of the direct current electric traction network. The method comprises drawing electric energy from this traction network in the low-load periods in order to store it in an energy storage unit and in supplying the stored electric energy to the traction network in the periods of increased electricity demand, while constantly monitoring the load on the traction network and the energy storage state of charge, wherein the energy storage is charged with lower traction voltage. The traction network is assisted with the energy stored in the energy storage unit by increasing the voltage of the energy storage unit above the traction voltage, the measured values of voltage and/or current in the traction network are compared with their preset values, and simultaneously the measured energy storage state of charge is compared with the preset value of its state of charge, thus using both comparison results to create a signal switching the energy storage charging state into the traction network powering state. Moreover, both on the side of the energy storage and on the side of the traction network, signals are generated which disconnect the systems on the side of the energy storage from the traction network. Additionally, a system for assisting the powering of the direct current electric traction network, in which the energy storage is connected to the poles of the traction network through an in-series power supply unit, wherein the power supply unit is a bi-directional DC-DC converter, the main output of the energy storage is connected with the left input-output of the converter, the measurement output of the energy storage is connected with the first input of the converter control unit, the output of the system for measuring load on the traction network is connected to the second input of the converter control unit, and the right inputoutput of the converter is connected to the traction network through a circuitbreaker unit and preferably through a filtering and overvoltage protection system.

The technical problem of the present invention is to provide such a direct current electric traction power supply method and system which, after meeting basic requirements for power supply objects, e.g. traction substations, by providing the voltage and current across the operating range of the traction substation, provides resistance to interference, especially to short-term current-voltage loads, and which allows the reduction of operating losses and as a result the reduction of the electric power contracted for a particular power supply object such as a traction substation. Moreover, it is also desirable to provide a direct current electric traction power supply system having reduced peak power requirements for the components of the traction system, as well as reduced complexity level of the system. In addition, it is also desirable to ensure the possibility of using and facilitate the usage of energy recuperation systems, as well as to simplify the systems with the use of renewable energy sources and to provide an additional effect of maintaining the correct operation of the traction system ensuring the proper short-circuit parameters of the power supply.

In one aspect, the present invention provides a direct current electric traction power supply system comprising a power supply object comprising a main busbar and a negative busbar, a power supply converter connected in series with a protection switch and connected between the main busbar and the negative busbar, and a control system, wherein the power supply converter is connected to a power supply source, characterized in that it additionally comprises a battery pack connected in series with a protection switch and connected between the main busbar and the negative busbar, in parallel with respect to the power supply converter and the protection switch connected in series.

Preferably, the power supply converter is a DC-DC converter, and the power supply source is a DC power supply source or the power supply converter is an AC- DC converter, and the power supply source is an AC power supply source.

Preferably, the electric traction is powered with a nominal voltage in the range from 0.6 kV DC to 3 kV DC.

Also preferably, the battery pack is made of lithium cells. Preferably, the protection switch is a contactor, a DC high-speed circuit breaker or a disconnector connected in series with a fuse.

In another aspect, the present invention provides a direct current electric traction power supply method characterized in that in the direct current electric traction power supply system as defined in the first aspect of the invention power is supplied from the power supply source to the power supply converter, where the power supply voltage is converted into the traction network power supply voltage and delivered to the main busbar and to the negative busbar, wherein the main busbar and the negative busbar are simultaneously supplied with voltage from the battery pack connected in series with the power supply converter.

Preferably, the control system keeps the battery pack charged to a state of charge in the range from 40% to 80%.

Also preferably, after the control system detects that voltage in the battery pack is lower than the threshold state of charge value of 70%, the battery pack is charged by the power supply converter.

Also preferably, in a situation when the traction network supplies energy from recuperation to the main busbar and to the negative busbar while the battery pack is kept at the state of charge in the range from 40% to 80%, the energy is delivered through the power supply converter to the power supply network.

A solution for supplying power to direct current traction with the use of battery packs connected directly to the main busbar of the power supply object, e.g. the traction substation, is related to the character of loads occurring in the direct current traction, the loads being characterized by a low value of the mean load current, e.g. from 100 A to 300 A (consumed power, e.g. from 300 kW to 1 MW) and short-duration loads on the traction substation (the power supply object) with the current reaching e.g. up to 2000-3000 A (power, e.g. from 6 MW to 9 MW). High load durations are typically single minutes. Between the high-load peaks, there are interruptions having durations from several to more than a dozen minutes or more. The voltage on the traction network is typically 3300 V ±300 V. The power supply standards for the 3 kV DC traction are met within the voltage from 2.9 kV to 3.9 kV.

At the currents and voltages occurring in the traction network, the use of a battery pack connected directly to the 3 kV DC busbar of the traction substation as a power supply element is justified by the properties of the cells, i.e. except their capacity, also by a very good resistance to short-duration loads. The currentvoltage characteristics of the cells correspond to the character of loads and voltages occurring in the DC traction network.

The voltage in the DC electric traction power supply system of the present invention is stabilized, and the power required (contracted) to ensure the mobility of rail vehicles is limited.

The voltage characteristic of a typical lithium cell, such as a lithium-ferro- phosphate (LFP) cell is between 2.9 V at discharge and 4 V at charge (in such case 1000 cells in series provide a 4 kV voltage). A cell of this type can be safely loaded with currents being a multiple of the 1C rate, i.e. up to 3C current (a battery with a capacity of 1000 Ah allows the substation to be loaded for 20 minutes with a power of 9 MW).

These properties of the battery pack are of key importance for its use in the DC electric traction power supply system of the present invention, as they meet the requirements of supplying power to direct current traction across both the voltage and current (load) operating conditions of traction substations operating in the railway traction system.

In this solution for powering railway traffic, the amount of energy supplied from the outside must be similar (slightly greater) than the mean power of a particular traction substation. The energy supplied from the outside is used to maintain the battery packs in such a state of charge which allows them to accept the energy from or deliver it to the traction network at any time. The battery pack is charged within the range from 40% to 80%, depending on the local parameters of the traction network and on the maximum capacity of the pack.

In the traction substation, the supplied alternating current energy is converted by the converter(s) into direct current and voltage in such a manner that the output voltage on the converter is simultaneously the voltage on the battery packs and vice versa. The battery packs and the converters are connected in parallel to each other. The power supply converters operate as current sources. The current flowing from the converter is used to charge the battery packs (i.e. to maintain their state of charge at a set level).

In justified cases, when the load on the traction substation increases above the allowed current of the power supply converter, the deficit energy (current) is drawn directly from the battery pack. In this case the energy (power) supplied from the battery packs is summed with the energy (power) supplied to the traction network from the power supply converters, thus limiting the power drawn from the power supply network and providing a DC power surplus available for the power supply object, e.g. the traction substation. This process takes place without increasing the peak power drawn from the AC system above the contracted power. The value of the current supplied to the traction network by the power supply converter is limited by its rated power and the energy can be drawn by the traction network to this limit.

The use of battery packs directly connected (through the necessary protection means) to the main 3 kV DC busbar of the traction substation allows:

• the reduction of the contracted power for the traction substation on the alternating current side,

• the stabilization of the power drawn by the traction substation,

• the reduction of voltage fluctuations in the DC traction network, • the reduction of the power of the traction substation components, especially of the converters and transformers, as the power of the components is limited to the mean power of the traction substation and can be increased as needed by adding another converter and, if needed, another battery pack.,

• the reduction of the protection-related currents of the AC traction substation,

• the simplification of the recuperation system in the DC network (e.g. for the recuperation of the energy of the breaking rail vehicle),

• the reduction of the harmonics generated by the traction substation on the DC traction network by directly connecting it to the battery pack, which functions as a capacity having filtering properties,

• the increase of the traction system reliability by ensuring mobility without AC power supply.

In addition, the electric traction can be powered with the use of renewable sources of energy, such as photovoltaic panels, by connecting them through appropriate DC-DC converters, to the battery packs. Moreover, the different voltage levels between the positive busbars of the adjacent power supply objects allow a small heating current between the objects, resulting in an additional functionality of preventing icing on the traction conductors. In addition, the use of AC-DC converters as power supply converters in the DC electric traction power supply system of the present invention allows the traction substation to return energy to the AC power supply network.

Importantly, the connection of the battery pack directly (i.e. in parallel) to the traction network eliminates numerous disadvantageous phenomena present in the currently used solutions and allows the reduction of losses when using battery packs, as the power supply converter(s) does not participate in the process of discharging the battery pack and recuperation. Importantly, in the DC electric traction power supply system of the present invention recuperation can be realized automatically, as the battery packs are kept at a defined state of charge (SOC) which allows recuperation. The proper SOC level of the battery packs is maintained by the control system and by the power supply converters programmed adequately to the demand on the local network at a particular time.

The solution according to the present invention has been shown in the embodiments below and illustrated in the drawing, in which Fig. 1 is a schematic diagram of the direct current electric traction power supply system according to the first embodiment of the invention, and Fig. 2 is a schematic diagram of the direct current electric traction power supply system according to the second embodiment of the invention.

Example 1

The first embodiment of the direct current electric traction power supply of the present invention has been shown in a schematic view in Fig. 1. As shown in Fig. 1, the DC electric traction power supply system comprises a power supply object 9 being in this embodiment a traction substation in which there are located the remaining components of the system according to the invention. In the power supply object 9 there is a main busbar 7 (i.e. the positive 3 kV direct current busbar) and a negative busbar 8, which are the power supply bars for the traction system. The DC electric traction power supply system additionally comprises a power supply converter 4, which in this embodiment is in the form of a DC-DC converter. The type of the employed power supply converter 4 is not a limitation to the scope of the present invention and depends on the type of the power supply source for the power supply object 9. In this embodiment, the power supply 9 is powered from a power supply source 5 being a source of DC voltage having a power ensuring the coverage of approx. 120% of the mean traction energy consumption for a particular power supply object 9, wherein the power supply source 5 is connected to the power supply converter 4. The power supply converter 4 is connected in series to a protection switch 3, and the serial system of the power supply converter4 and the protection switch 3 is connected between the main busbar 7 and the negative busbar 8 of the power supply object 9. The purpose of the protection switch 3 is to disconnect the power supply converter 4 from the main busbar 7 and can be realized by a disconnector connected in series with a fuse having nominal currents higher than the rated current of the power supply converter 4, however this fact is not a limitation to the scope of the present invention and other protection switches 3, such as contactors, can be used in alternative embodiments on condition that appropriate rated currents are ensured.

The power supply object 9 is moreover provided with a battery pack 1, which is connected in series with the corresponding protection switch 2, and the system of the serially connected battery pack 1 and the protection switch 2 is connected between the main busbar 7 and the negative busbar 8 of the power supply object 9, parallel to the serial system of the power supply converter 4 and the protection switch 3. The purpose of the protection switch 2 is to limit the current of the battery pack 1 to the allowable values resulting from its parameters and can be realized by an appropriate direct current high-speed circuit breaker.

In this embodiment, the battery pack 1 is a system of approx. 1000 lithium-ferro- phosphate (LFP) cells connected in series, which together provide a battery pack 1 voltage of approx. 4000 V. The type of the battery pack 1 is in this case not a limitation to the scope of the present invention and in alternative embodiments it is possible to use a battery pack 1 in the form of lithium cells, other than lithium- ferro-phosphate cells, on condition that the necessary operating parameters, such as maximum voltage and maximum current of the battery pack 1, are ensured.

In addition, the power supply object 9 is provided with a control system (not shown in the figures), which is connected with the power supply converter 4 and whose purpose is to control the operation of the power supply converter 4 for maintaining a defined SOC level of 40%-80% in the battery pack 1, depending on the local load characteristic and on the capacity of the battery pack 1. The PLC SCADA system is an example of a control system possible for use in this invention. Generally, the control system monitors on an ongoing basis the value of the SOC level in the battery pack 1 and in the case when this parameter drops below a defined threshold value it switches on the power supply converter 4 which charges the battery pack 1 up to a defined minimum SOC value. As a result, the battery pack 1 and the power supply 4 provide power and energy necessary to maintain railway traffic on a particular section.

The DC electric traction power supply method according to one embodiment of the invention is realized inter alia in the embodiment of the system for supplying power to direct current electric traction described above. In this method, power is supplied by the power supply source 5 to the power supply converter 4 and the control system is used to monitor the state of charge of the battery pack 1, and in the case when the state of charge drops below a defined threshold SOC value of 70%, the power supply converter 4 is switched on for charging the battery pack 1 up to an assumed SOC of 80%, wherein the main busbar 7 of the power supply object 9 maintains direct voltage within 2.9-3.9 kV, resulting however from the state of charge of the battery pack 1 cells, which is the case at the parallel connection of the power supply converter 4 and the battery pack 1.

Example 2

The second embodiment of the DC electric traction power supply system of the present invention has been shown in a schematic view in Fig. 2. Generally, the structure of the DC electric traction power supply system according to this embodiment is substantially similar to the structure of the DC electric traction power supply system shown in example 1, and therefore similar elements of the structure will not be repeated for clarity of this disclosure.

Unlike in the first embodiment, the DC electric traction power supply system according to this embodiment comprises a power supply converter 4 which is an AC-DC converter. As a consequence, the DC power supply source has been replaced with an AC power supply source 6 which is an alternating current power source and which is connected with an AC-DC type power supply converter 4.

Moreover, unlike in the first embodiment, the DC electric traction power supply system is a system for supplying power to direct current tramway traction with a nominal voltage of 600 V and therefore all components of the system are devices adjusted to this type of voltage ranges.

Additionally, unlike in the DC electric traction power supply system shown in example 1, in this embodiment in the case when the busbars 7, 8 are supplied with voltage from the traction network, for example due to the recuperation of the energy of the breaking rail vehicle, and when the battery pack 1 reaches the fully charged state, the control system switches the AC-DC power supply converter 4 to returning the excess energy to the AC grid, to which the power supply source 6 is connected.

List of reference numerals:

1 - battery pack

2 - protection switch

3 - protection switch

4 - power supply converter 5 - DC power supply source

6 - AC power supply source

7 - main busbar

8 - negative busbar 9 - power supply object

Claims

Claims A direct current electric traction power supply system comprising a power supply object (9) comprising a main busbar (7) and a negative busbar (8), a power supply converter (4) connected in series with a protection switch (3) and connected between the main busbar (7) and the negative busbar (8), and a control system, wherein the power supply converter (4) is connected to a power supply source (5, 6), characterized in that it additionally comprises a battery pack (1) connected in series with a protection switch

(2) and connected between the main busbar (7) and the negative busbar (8), in parallel with respect to the power supply converter (4) and the protection switch

(3) connected in series. The direct current electric traction power supply system according to claim 1, characterized in that the power supply converter

(4) is a DC-DC converter and the power supply source

(5) is a DC power supply source or the power supply converter (4) is an AC-DC converter and the power supply source (6) is an AC power supply source. The direct current electric traction power supply system according to claim 1 or 2, characterized in that the electric traction is powered with a nominal voltage in the range from 0.6 kV DC to 3 kV DC. The direct current electric traction power supply system according to any of claims 1 - 3, characterized in that the battery pack (1) is made of lithium cells. The direct current electric traction power supply system according to any of claims 1 - 4, characterized in that the protection switch (2, 3) is a contactor, a DC high-speed circuit breaker or a disconnector connected in series with a fuse. 6. A direct current electric traction power supply method characterized in that in the direct current electric traction power supply system as defined in any of claims 1 - 5, power is supplied from the power supply source (5,

6) to the power supply converter (4), where the power supply voltage is converted into the traction network power supply voltage and delivered to the main busbar (7) and to the negative busbar (8), wherein the main busbar (7) and the negative busbar (8) are simultaneously supplied with voltage from the battery pack (1) connected in series with the power supply converter (4).

7. The method according to claim 6, characterized in that the control system keeps the battery pack (1) charged to a state of charge in the range from 40% to 80%.

8. The method according to claim 7, characterized in that after the control system detects that voltage in the battery pack (1) is lower than the threshold state of charge value of 70%, the battery pack (1) is charged by the power supply converter (4).

9. The method according to any of claims 6 - 8, characterized in that, in a situation when the traction network supplies energy from recuperation to the main busbar (7) and to the negative busbar (8) while the battery pack (1) is kept at the state of charge in the range from 40% to 80%, the energy is delivered through the power supply converter (4) to the power supply network.