ELECTRIC TRACTION SYSTEM

There is provided an electric traction system 1, comprising: a traction converter module 2 comprising: a positive input terminal 4 and a negative input terminal 6 for operatively coupling to a DC power supply, and a plurality of power inverters 11, each of which comprises positive and negative input nodes 3, 5 configured to receive DC power and output nodes 9 configured to supply AC power, wherein the positive and negative input nodes 3, 5 of the plurality of power inverters 11 are electrically connected in series between the positive input terminal 4 and the negative input terminal 6; and at least one electric motor 8 configured to be driven by the traction converter module 2, the at least one electric motor 8 comprising a multi-phase electric motor 81.

TECHNICAL FIELD

The present disclosure relates to an electric traction system. More particularly, but not exclusively, the present disclosure relates to an electric traction system which receives direct current (DC) power and drives electric motor loads. Such an electric traction system is suitable for use in various power electronics applications, such as, urban rail transit applications.

BACKGROUND

An electric traction system converts electrical energy into mechanical energy by driving an electric motor using the electrical energy, thereby generating a traction force which causes the propulsion of an electric machine. A typical example of the electric machine is a vehicle (such as, a locomotive, an electric or hydrogen vehicle, an elevator or an electric multiple unit). The electric motor may also be referred to as a traction motor.

An electric traction system may use either a DC or an alternating current (AC) power supply. Generally speaking, urban rail transit applications (e.g., subways) adopt traction systems powered by a DC grid. The DC power may be supplied by either an overhead wire or a third rail in an urban rail transit application.

FIG.1schematically illustrates a prior electric traction system100used in urban rail transit applications. The prior traction system100comprises a DC-AC power converter circuit102which receives DC power at input nodes104,106and supplies AC power at output nodes109. The DC-AC power converter circuit102may also be referred to as a power inverter. In a typical example, the power inverter102is a three-phase full-bridge inverter, with either two output levels (two-level) or three output levels (three-level). The prior traction system100further comprises an electric motor108. The electric motor108is typically a three-phase AC motor, which may be an asynchronous motor or a permanent magnet synchronous motor in conventional rail transit applications. The stator windings of the motor108are electrically connected to the output nodes109of the power inverter102.

The power inverter102is constructed with power semiconductor devices and is responsible for inverting input DC power to three-phase AC power so as to drive the motor108. For urban rail transit applications, the DC grid voltage applied between the positive and negative input nodes104,106of the inverter102is normally rated at 1500V. In the event that the inverter102is constructed as a three-phase full-bridge two-level inverter, 3300V-rated silicon-based (Si-based) insulated gate bipolar transistors (IGBTs) are often used as the power semiconductor devices within the inverter102. In the event that the inverter102is constructed as a three-phase full-bridge three-level inverter, 1700V-rated Si-based IGBTs are often used as the power semiconductor devices within the inverter102. These two circuit topologies of the power inverter102are well developed and there is limited room for further improving the efficiency and reducing the costs of the power inverter102. Further, the power inverter102outputs all of the power required by the motor108. Therefore, the power inverter102is subject to high power output requirements, which in turn require the use of power semiconductor devices with high power ratings.

High-efficiency, lightweight, and miniaturization have always been the main targets for the development of traction systems in rail transit applications. In addition, it is also desirable to reduce the costs of such traction systems.

The latest 1700V or 3300V rated Silicon Carbide (SiC) based power semiconductor devices may be used to replace Si-based IGBTs with the same voltage ratings. Using SiC-based devices within the power inverter102may reduce loss, increase efficiency, and contribute to lightweight and miniaturization of the power inverter102through design optimization. However, costs of SiC-based devices are very high, and reliabilities of SiC-based devices at 1700V or higher voltage ratings are yet to be verified. Therefore, employing SiC devices within the power inverter102is still at prototyping and experimenting stages.

It is an object of the present disclosure, among others, to provide an electric traction system, which provides improvements over known traction systems.

SUMMARY

According to a first aspect of the present disclosure, there is provided an electric traction system, comprising:a traction converter module comprising: a positive input terminal and a negative input terminal for operatively coupling to a DC power supply, and a plurality of power inverters, each of which comprises positive and negative input nodes configured to receive DC power, and output nodes configured to supply AC power, wherein the positive and negative input nodes of the plurality of power inverters are electrically connected in series between the positive input terminal and the negative input terminal; andat least one electric motor configured to be driven by the traction converter module, the at least one electric motor comprising a multi-phase electric motor.

By electrically connecting the positive and negative input nodes of the plurality of power inverters in series between the positive input terminal and the negative input terminal, the input sides of the plurality of power inverters collectively share a DC voltage provided by the DC power supply. As a result, each of the power inverters receives a fraction of the DC voltage between its positive and negative input nodes. Consequently, each of the power inverters is allowed to use power semiconductor devices which have reduced voltage ratings. Power semiconductor devices with lower voltage ratings typically have smaller package dimensions, lower prices, and higher maturity than power semiconductor devices with higher voltage ratings. Further, low-voltage power semiconductor devices provide lower switching loss and higher efficiency than high-voltage power semiconductor devices. In addition, low-voltage power semiconductor devices relax cooling and heat exchange requirements, enabling the traction system to have reduced weight, volume and costs.

It would be appreciated that the multi-phase electric motor is a single motor which comprises more than three phases. As compared to a conventional three-phase electric motor, the multi-phase electric motor has a greater fault tolerance because it provides phase redundancy and can operate during phase open fault. Therefore, the use of the multi-phase electric motor improves the reliability of the electric traction system. Further, the multi-phase electric motor achieves higher torque density, reduced amplitude and increased frequency of torque pulsation, higher efficiency, lower DC link current harmonics as well as better noise and vibration characteristic, as compared to a conventional three-phase motor. Further still, the multi-phase electric motor can be controlled with a greater degree of freedom than a conventional three-phase electrical motor, thereby enabling the multi-phase electric motor to achieve greater regulations of torque and the shaft voltage.

Therefore, the electric traction system of the present disclosure has a higher efficiency, reduced weight and volume as well as reduced costs as compared to prior electric traction systems.

The electric motor may also be referred to as a traction motor (which generates a traction force causing the propulsion of an electric machine). It would also be understood that the electric motor is an AC motor.

The multi-phase electric motor may be driven by one or more of the plurality of power inverters.

With the expression “for operatively coupling to a DC power supply”, it is intended to mean that the DC power supply may not be a part of the electric traction system.

With the expression “the positive and negative input nodes of the plurality of power inverters are electrically connected in series between the positive input terminal and the negative input terminal”, it is meant that the negative input node of a power inverter is connected to the positive input node of a subsequent neighbouring power inverter, and/or the positive input node of the power inverter is connected to the negative input node of a precedent neighbouring power inverter.

The term “power inverter” may also be referred to as a DC-to-AC power converter. In other words, a power inverter converts DC power received at its input nodes to AC power for outputting at its output nodes.

The term “operatively coupled” or “operatively coupling” used in the present disclosure means that one or more intervening elements may be connected between the coupled elements.

The plurality of power inverters may comprise a first power inverter and a second power inverter, and the output nodes of the first and second power inverters may be configured to supply AC power to the multi-phase electric motor so as to drive the multi-phase electric motor.

By having the first and second power inverters to collectively drive the multi-phase electric motor, each of the first and second power inverters supplies a fraction of the total power required by the multi-phase electric motor. According, the required power rating of each power inverter as well as the required power ratings of the semiconductor devices used therein may be reduced.

It would be appreciated that the plurality of power inverters may include further power inverter(s) in addition to the first and second power inverters. It would further be appreciated that the terms “first” and “second” are simply used to label the power inverters for the ease of description, and do not imply any limitations to the sequences or locations of the inverters within the traction converter module. The first power inverter may or may not be immediately adjacent to the second power inverter.

The first and second power inverters may have identical circuit topologies. Advantageously, the identical circuit topologies allow the first and second power inverters to achieve power matching by supply an equal amount of power to the multi-phase electric motor.

The multi-phase electric motor may comprise a first set of stator windings and a second set of stator windings. The output nodes of the first power inverter may be electrically coupled to the first set of stator windings, and the output nodes of the second power inverter may be electrically coupled to the second set of stator windings.

The term “electrically coupled” used in the present disclosure means that one or more intervening elements (e.g., electrical contacts) may be connected between the coupled elements.

It would be appreciated that the multi-phase electric motor may include further set(s) of stator windings.

The first set of stator windings and the second set of stator windings may be electrically isolated from one another.

Advantageously, the electrical isolation between sets of stator windings improves system reliability.

A number of phases of the first power inverter may be identical to a number of phases of the first set of stator windings.

In other words, the first power inverter outputs M phases of AC power at its output nodes, and the first set of stator windings comprises M phases of stator windings. M may be an integer equal to or greater than three.

A number of the output nodes of the first power inverter may be identical to the number of phases of the first power inverter.

Alternatively, the number of the output nodes of the first power inverter may be two times the number of phases of the first power inverter. This arrangement may be adventurously for driving open-ended stator windings.

The first set of stator windings may be connected in a wye or delta configuration. Alternatively, the first set of stator windings may be open-ended stator windings which require power to be supplied from both ends.

The second power inverter and the second set of stator windings may have features similar to those described above for the first power inverter and the first set of stator windings.

The plurality of power inverters may have identical circuit topologies.

One or each of the first and second power inverters may be a two-level power inverter.

One or each of the first and second power inverters may be a three-level power inverter.

One or each of the first and second power inverters may be a multi-level power inverter.

One or each of the first and second power inverters may be a full-bridge power inverter.

One or each of the first and second power inverters may be a half-bridge power inverter.

One or more of the plurality of power inverters may comprise at least one power semiconductor device electrically connected between each of the positive and negative input nodes, on the one hand, and each of the output nodes, on the other hand.

The power semiconductor devices function as switches to selectively connect the output nodes to the input nodes.

One or more of the plurality of power inverters may comprise a DC link capacitor connected between the positive and negative input nodes of the respective power inverter.

One or more of the plurality of power inverters may comprise a plurality of inverter legs connected between the positive and negative input nodes of the respective power inverter. The plurality of inverter legs may provide the output nodes of the respective power inverter, respectively.

Each of the plurality of inverter legs may comprise at least one power semiconductor device.

The electric traction system may further comprise a controller which is configured to control on and off statuses of the power semiconductor devices of the respective power inverter so as to invert the DC power received at the input nodes to AC power at the output nodes during a traction mode of the traction system.

The controller may be further configured to control on and off statuses of the power semiconductor devices of the respective power inverter so as to convert mechanical energy of the at least one electric motor to electrical energy between the positive and negative input terminals of the traction converter module during a braking mode of the traction system.

The electrical energy may be charged back to the DC power supply.

At least one of the plurality of power inverters may further comprise a bypass switch connected between the positive and negative input nodes of the respective power inverter.

The at least one of the plurality of power inverters may be configured such that when the bypass switch is in an off state (i.e., open), the respective inverter is activated and when said bypass switch is in an on state (i.e., closed), the respective inverter is deactivated.

Optionally, each of the plurality of power inverters may comprise a bypass switch connected between the positive and negative input nodes of the respective power inverter.

The plurality of power inverters may further comprise a redundant power inverter, and the redundant power inverter comprises a bypass switch connected between its positive and negative input nodes.

The redundant power inverter may be configured to replace a faulty one of the plurality of power inverters.

The traction converter module may be configured such that when the plurality of power inverters are fault free, the bypass switch of the redundant power inverter are in an on state (i.e., closed), and the bypass switches of other power converters are in an off state (i.e., open), and that when fault occurs, the bypass switch of the faulty power inverter is switched to the on state and the bypass switch of the redundant power inverter is switched to the off state.

The multi-phase electric motor may comprise a redundant set of stator windings. The output nodes of the redundant power inverter may be electrically coupled to the redundant set of stator windings.

The controller may be configured to control on and off statuses of the bypass switch.

The electric traction system may further comprise an electronic filter electrically coupled to the positive input terminal, wherein the electronic filter is configured to attenuate high-frequency current signals receivable by the positive input terminal from the DC power supply.

The electronic filter may be for electrically coupling the positive input terminal to the DC power supply. The electronic filter may comprise an inductor.

The electric traction system may further comprise a pre-charge circuit electrically coupled to the positive input terminal. The pre-charge circuit is configured to charge the DC-link capacitor prior to a normal operation of the traction converter module.

The pre-charge circuit may be for electrically coupling the positive input terminal to the DC power supply.

According to a second aspect of the present disclosure, there is provided an electric machine comprising an electric traction system according to the first aspect.

The electric machine may comprise a vehicle. The vehicle may be selected from a group consisting of an electric locomotive, an electric or hydrogen vehicle, an elevator and an electric multiple unit.

Alternatively, the electric machine may comprise an industrial apparatus.

According to a third aspect of the present disclosure, there is provided a power electronics system, comprising a DC power supply and an electric traction system according to the first aspect, wherein the positive input terminal of the electric traction system is operatively coupled to the DC power supply.

According to a fourth aspect of the present disclosure, there is provided an urban rail transit system, comprising: a DC power supply and a vehicle comprising an electric traction system according to the first aspect, wherein the positive input terminal of the electric traction system is operatively coupled to the DC power supply.

The DC power supply may comprise a DC grid.

It would be understood that the urban rail transit system may comprise one or more of a tram system, a light rail system, a rapid transit system (e.g., metro, underground, and/or subway), a monorail system, a commuter rail system, funicular, cable car, and guided bus etc.

Where appropriate any of the optional features described above in relation to one of the aspects of the present disclosure may be applied to another one of the aspects of the disclosure.

In the figures, like parts are denoted by like reference numerals.

It will be appreciated that the drawings are for illustration purposes only and are not drawn to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG.2schematically illustrates a block diagram of an electric traction system1(referred to as “traction system” below) according to the present disclosure. The traction system1uses a traction converter module2to replace the power inverter102used in the prior traction system100. The traction converter module2converts DC power to AC power for driving electric motors81, . . .8Q. As shown inFIG.2, the traction converter module2includes a positive input terminal4and a negative input terminal6, which in use are electrically coupled to a DC power supply (e.g., a DC grid).

The traction converter module2further includes a plurality of power inverters111, . . .11N(which are collectively referred to as11), each of which is similar to the power inverter102. Each power inverter11i(i=1, . . . . N) includes a positive input node3iand a negative input node5iwhich receives DC power, a switch7iconnected between the input nodes, and output nodes9iwhich supply AC power for driving motor loads.

The positive and negative input nodes3,5of the power inverters11are electrically connected in series between the positive input terminal4and the negative input terminal6. In particular, the negative input node5iof a power inverter11iis electrically connected to the positive input node3i+1of a subsequent power inverter11i+1, and the positive input node3iof the power inverter11iis electrically connected to the negative input node5i−1of a precedent power inverter11i−1. The power inverter111at the front of the array of power inverters11has its positive input node31electrically connected to the positive input terminal4. The power inverter11Nat the rear of the array of power inverters11has its negative input node5Nelectrically connected to the negative input terminal6.

The switches7function as bypass switches, and can be used to activate or deactivate corresponding power inverters11. When a switch7i(e.g.,73inFIG.2) is closed (i.e., at the ON status), the positive and negative input nodes3iand5iof a corresponding power inverter11iare electrically shorted together. As a result, the power inverter11ireceives no DC power and thus is deactivated. Conversely, an open (i.e., OFF) switch7i(e.g.,71,72or7NinFIG.2) allows its corresponding power inverter11ito function normally.

With reference to the circuit ofFIG.2, the active power inverters11which have open switches7(e.g., all except113inFIG.2) collectively share a DC voltage received between the positive and negative input terminals4,6. The voltage drop across the input nodes31,5iof each active inverter11iis merely a fraction of the DC voltage received between the terminals4,6. As compared to the prior power inverter102, each of the power inverters11can be constructed by using power semiconductor devices with much lower voltage ratings (e.g., 1700V or lower). This also expands the choice of power semiconductor devices beyond Si-based IGBTs and SiC-based devices. For example, Si-based metal-oxide-semiconductor field effect transistors (MOSFETs) and Gallium nitride (GaN) based MOSFETs may be used to construct the power inverters11.

Power semiconductor devices with lower voltage ratings typically have smaller package dimensions, lower prices, and higher maturity than power semiconductor devices with higher voltage ratings. Further, low-voltage power semiconductor devices provide lower switching loss and higher efficiency than high-voltage power semiconductor devices. In addition, low-voltage power semiconductor devices relax cooling and heat exchange requirements, enabling the traction system1to have reduced weight, volume and costs.

Because the traction system1allows the use of lower rating power semiconductor devices by improving the circuit structure, rather than requiring a lower rating DC power supply, the traction system1may use the same DC power supply as the prior traction system100. Accordingly, the traction system1may directly replace existing traction system100in urban rail transit applications.

The traction system1further includes a controller10. The controller10controls the on/off switching of the bypass switches7using signal lines12. The controller10also controls the on/off switching of power semiconductor devices within each power inverter11using signal lines14. As a result, the functioning of each power inverter11can be independently controlled by the controller10. The switches7may be implemented as gate controlled power switches, e.g., MOSFETs or IGBTs, or current controller power switches, e.g., thyristors. The controller10may comprise a controlling unit (such as, a processor, a programmable logic device, and/or an application-specific integrated circuit (ASIC) etc.) as well as driver circuitry for transforming low-current control signals output by the controlling unit to higher-current control signals. WhileFIG.2shows that the controller10is part of the traction system1, it would be appreciated that the controller10may alternatively be an external component of the traction system1.

The traction system1also includes electric motors81, . . .8Q, which are AC motors. The stator windings of the electric motors81, . . .8Qare electrically coupled to the output nodes9of one or more of the power inverters11. As a result, the traction converter module2drives the electric motors81, . . .8Qby supplying AC power to the motors. The electric motors81, . . .8Qtypically generates a traction force causing the propulsion of an electric machine (e.g., a vehicle or an industrial machine etc.), and thus may be referred to as tractor motors. One or more of the electric motors81, . . .8Qmay be an asynchronous motor or a permanent magnet synchronous motor.

In the example ofFIG.2, the electric motor81is a multi-phase electric motor. A multi-phase motor generally has more than three phases (e.g., five to nine phases). For a conventional three-phase motor (such as the motor108), if one of the phases is lost, the rotatory field within the motor also disappears and the motor would stop working. As compared to conventional three-phase motors, the multi-phase motor81has a greater fault tolerance because it provides phase redundancy and can operate during phase open fault. Therefore, the use of the multi-phase motor81enables the traction system1to have a higher reliability. Further, the multi-phase motor81achieves higher torque density, reduced amplitude and increased frequency of torque pulsation, higher efficiency, lower DC link current harmonics as well as better noise and vibration characteristic, as compared to a conventional three-phase motor. Further, the multi-phase electric motor81can be controlled with a greater degree of freedom than a conventional three-phase electrical motor, thereby enabling the motor81to achieve greater regulations of torque and the shaft voltage. The electric motor81may be an asynchronous motor or a permanent magnet synchronous motor.

The circuit topology of the traction converter module2is particular suitable for driving a multi-phase electric motor such as the motor81. With reference toFIG.2, at least two of the power inverters11drive the motor81. For example, the output nodes91of the power inverter111drive a set (e.g., three phases) of the stator windings of the motor81, and the output nodes92of the power inverter112drive another set of the stator windings of the motor81. The motor81may have further stator windings driven by other inverter(s)11. Preferably, the number of phases (e.g., equal to or more than three) of a power inverter may be identical to the number of phases of the set of stator windings driven thereby. Therefore, each power inverter supplies a fraction of the total power required by the motor81. By making the power inverters (e.g.,111,112, etc.) driving the motor81to have identical circuit topologies, each of the power inverters supplies an equal amount of power (e.g., 50% of the total power required by the motor81) to a respective set of stator windings of the motor81and thus achieves power matching with respect to one another.

The sets of stator windings within the motor81may be electrically isolated from one another, by for example having separated neutral points. The electrical isolation between the sets of stator windings is useful for improving system reliability. Alternatively, the sets of stator windings within the motor81may share the same neutral points. In any event, the motor81has more than three phases. The multiple sets of stator windings can be independently controlled, and thus allow a higher degree of control freedom for optimising torque and shaft voltage of the motor81.

The traction system1is also configured with redundancy. With reference toFIG.2, the multi-phase electric motor81includes a redundant set of stator windings which are electrically coupled to the output nodes93of a redundant power inverter113. During normal operation, the bypass switch73of the power inverter113is kept closed by the controller10so as to deactivate the redundant power inverter113. Meanwhile, the switches71,72of the power inverters111,112are kept open by the controller10. As such, the power inverters111,112drive the motor81collectively. In the event that faults occur in the power inverter111or112or the stator windings driven thereby, the controller10closes the switch71or72of the faulty branch, and opens the switch73of the redundant power inverter113so as to activate the redundant power inverter113. Thus the redundant power inverter113drives the motor81with other active inverter(s). This redundancy mechanism provided by the multi-phase electric motor81and the traction converter module2allows the traction system1to continue functioning in the event of faults occurring in the motor81or the power inverters11, thereby significantly improving the reliability of the traction system1.

FIG.2shows that the traction system1includes a further motor8Q, which is driven by at least the output nodes9Nof the power inverter11N. The further motor8Qmay be a conventional three phase motor or a multi-phase electric motor similar to the motor81. It will however be appreciated that the further motor8Qmay be omitted such that the traction system1includes a single multi-phase electric motor81or multiple multi-phase electric motors81to8Q-1. The multiple multi-phase electric motors81-8Q-1may have identical motor topologies, and/or may be driven by groups of inverters having identical circuit topologies. WhileFIG.2shows that the multi-phase motor81is driven by at least two power inverters (e.g.,111,112), it would be understood that the multi-phase motor81may be driven by a single power inverter which outputs multiple phases of AC power.

Although it is not shown inFIG.2, electrical contacts may be connected between one or more output nodes9of at least one power inverter11and respective stator winding(s) of motor(s)8. An electrical contact is an electrical circuit component commonly found in electrical switches, relays, connectors and circuit breakers. Each contact is a piece of electrically conductive material, typically metal. When a pair of electrical contacts touch, they can pass an electrical current and allow the corresponding output node9and the stator winding to be electrically connected.

It will also be understood that the bypass switches7may be omitted such that all of the power inverters11are active power inverters.

While the power inverters11convert DC power received at the input nodes3,5to AC power at the output nodes9, the power inverters11may also perform an opposite function of AC-to-DC rectification, i.e., converting AC power received at the output nodes9to DC power at the input nodes3,5. The opposite function of rectification enables regenerative braking of the traction system1, and converts mechanical energy of the motor8back to electric energy. The electric energy can be stored at DC-link capacitors (described below in more detail) which are connected between the input nodes3,5of the power inverters11, and may be further returned to the DC power supply. The controller10controls the working modes of the power inverters11as well as the directions of power flow through the power inverters11.

WhileFIG.2shows that the traction converter module2includes more than four power inverters11which are electrically connected in series at their input sides, it would be understood that this is just for illustration and in no way imply any limitation to the number of power inverters11. Indeed, having at least two power inverters11connected in series at the input sides would allow the traction converter module2to achieve the advantages described above.

Each of the power inverters11may be implemented using various circuit topologies, e.g., a two-level, three-level or multi-level inverter, a full-bridge or half-bridge inverter etc. It would be understood that at least one power semiconductor device is electrically connected between each of the output nodes9and each of the input nodes3,5. The at least one power semiconductor device may be electrically connected in series or in parallel between the nodes.

It is preferable that the power inverters11within the traction converter module2are identical to one another (i.e., identical circuit topology with identical device parameters), so that the active power inverters11would equally share the DC voltage between the positive and input terminals4,6. However, it would be appreciated that this arrangement is not necessary.

Further, the power inverters11may be three-phase inverters which are ideal for driving sets of three-phase windings of the motor(s)8. However, it would be appreciated that the phases of each power inverter11may be more than three.

FIGS.3to6provide exemplary circuit diagrams of the traction system1and the power inverter11.

FIG.3shows the circuit diagram of an electric traction system1A which has been constructed based upon the block diagram ofFIG.2. In the example ofFIG.3, the traction converter module2includes two power inverters11A1and11A2(collectively referred to as11A), which drive a multi-phase motor8together. The input sides of the power inverters11A are electrically connected in series between the positive and negative input terminals4,6of the traction converter module2. In particular, the negative input node51of the first power inverter11A1is electrically connected to the positive input node32of the second power inverter11A2, and the positive input node31of the first power inverter11A1is electrically connected to the positive input terminal4. Meanwhile, the negative input node52of the second power inverter11A2is electrically connected to the negative input terminal6. For simplicity, the electric traction system1A does not include any bypass switch7or redundant power inverter. While it is not shown inFIG.3, it would be understood that the operations of power semiconductor devices within the power inverters11A are controlled by a controller (similar to the controller10ofFIG.2).

The two power inverters11A1and11A2have identical circuit topology and identical device parameters. Thus, the power inverters11A1and11A2equally share the DC voltage received by the positive and negative input terminals4,6. As shown inFIG.3, each of the power inverters11A is a two-level three-phase full-bridge inverter. For simplicity, the description below describes the structure of the power inverter11A1only. It would be understood that the description similarly applies to the power inverter11A2. The power inverter11A1comprises a DC-link capacitor231electrically connected between the positive and negative input nodes31,51, and three inverter legs each providing an output node. The three inverter legs have identical structures. The first inverter leg comprises two power semiconductor devices T1and T2connected between the positive and negative input nodes31,51, with its output node being between the two devices T1, T2. By controlling the power semiconductor devices of the three inverter legs to switch on and off at different times, the AC power provided at the three output nodes91of the power inverter11A1has three phases.

In an example, the multi-phase motor8is a dual three-phase motor (i.e., 6 phases in total), which includes two sets of three-phase stator windings. The two sets of three-phase stator windings are electrically isolated from one another, by for example having separated neutral points. The multi-phase motor8replaces the three-phase motor108of the prior traction system100, but maintains the same rated power. In an example, the multi-phase motor8has a rated power of 200 kW, and each set of three-phase stator windings is thus rated at 100 KW. The output nodes91of the power inverter11A1are electrically coupled to the first set of three-phase stator windings. The output nodes92of the power inverter11A2are electrically coupled to the second set of three-phase stator windings. The power inverters11A1and11A2therefore independently control power flow into/from the respective set of three-phase stator windings. Each of the power inverters11A1and11A2supplies a half of the total power required by the motor8.

With reference toFIG.3, the electric traction system1A has a positive terminal21. In use, a DC power supply (e.g., a DC grid) is connected between the positive terminal21and the negative input terminal6of the traction converter module2. A pre-charge circuit19and an electronic filter20(described below in more detail) are electrically connected in series between the positive terminal21and the positive input terminal4. In use, the voltage drop across the pre-charge circuit19and the electronic filter20would be negligible. Therefore, the DC voltage across the positive input terminal4and the negative input terminal6is substantially identical to the DC voltage provided by the DC power supply.

The DC grid may be approximately 1500V or adjusted according to actual requirements. In the event that the DC grid is rated at 1500V, the input of each power inverter would be rated and stabilized at 750V DC through control. The power semiconductor devices used within the power inverters11A may be 1700V rated devices rather than 3300V Si-based semiconductor power devices used in the prior traction system100. Examples of 1700V rated power devices include Si-based IGBTs, Si-based MOSFETs, SiC-based MOSFETs, GaN-based MOSFETs, or other semiconductor-based power devices.

The pre-charge circuit19includes a first switch17in series connection with a pre-charge resistor18, and a second switch19in parallel connection with the switch17and the resistor18. Prior to normal operation of the traction converter module2, the first switch17is closed while the second switch19is open. In this way, DC-link capacitors23of the power inverters11A are charged by the DC power supply through the pre-charge resistor18. Once pre-charge of the DC-link capacitors23is completed, the pre-charge resistor18is bypassed by closing the second switch19and opening the first switch17. Pre-charging the DC-link capacitors23is useful for preventing excessive inrush current at system start-up which may damage the DC-link capacitors23and the power semiconductor devices of the power inverters11A.

The electronic filter20comprises an inductor, and is useful for reducing high-frequency current contents from/to the DC grid. The high-frequency current contents may cause resonance within the traction system1A, and thus it is beneficial to filter out the high-frequency current contents. The pre-charge circuit19and the electronic filter20may also be applied within the traction system1ofFIG.2.

The traction system1A may operate at a traction mode and a braking mode. During the traction mode, the traction system1A draws power from the DC grid and the voltage of the DC grid is equally shared by the two power inverters11A in case of balanced loading. Two sets of three-phase AC voltages, with variable frequency and variable fundamental magnitude, are produced by the two power inverters11A to drive the six-phase traction motor8. In this process electrical power is converted to mechanical power. During the braking mode, mechanical power of the six-phase motor8is regenerated to electrical power in a controlled way. Through the two power inverters11A, the electrical power is rectified back to the two DC-link capacitors23in series then back to the DC grid via an L-C filter. The L-C filter is constructed by the DC-link capacitors23and the inductor20.

The traction system1A ofFIG.3is flexible for extension. For example, it may be extended such that more than two power inverters11A are connected in series between the positive and negative input terminals4,6, and that the multiple-phase motor8comprises more than two sets of three-phase windings. In an example, the traction converter module2comprises three power inverters11A with their input nodes electrically connected in series between the terminals4,6, and the three power inverters11A collectively drive a 9-phase (e.g., triple 3-phase) traction motor. Thus, each of the three power inverters11A is responsible for a third of the motor power. In the event that the DC grid is rated at 1500V, the input of each power inverter would be rated and stabilized at 500V DC through control. Thus, 1200V-rated power semiconductor devices are suitable for use within the power inverters11A. Examples of 1200V rated power devices include Si-based IGBTs, Si-based MOSFETs, SiC-based MOSFETs, GaN-based MOSFET, or other semiconductor-based power devices.

The power inverters11A ofFIG.3are two-level three-phase full-bridge inverters. It would be appreciated that they may be replaced by other types of power inverters.FIG.4illustrates a three-level three-phase neutral-point-clamped (NPC) full-bridge inverter11B. The number of levels of a power inverter indicate the number of output levels. A three-level power inverter means that the inverter can output three different voltage levels (e.g., 0, Vdd/2, Vdd) at its output nodes, with Vddbeing the voltage difference between the positive and negative input nodes.

As shown inFIG.4, the power inverter11B comprises a positive input node3, a negative input node5, and two DC-link capacitors24,25electrically connected in series between the positive and negative input nodes3,5. A node30is between the two capacitors24,25. Because the capacitors24,25have identical capacitance, the potential of the node30is centred between the potentials of the input nodes3,5. The power inverter11B further includes the three inverter legs which have identical structures and device parameters. For simplicity, the description below describes the structure of the first inverter leg only. It would be understood that the description similarly applies to the other two inverter legs. The first inverter leg comprises two power semiconductor devices T1and T2connected in series between the positive input node3and an output node U, two further power semiconductor devices T3and T4connected in series between the negative input node5and the output node U, a diode D1connected between the node30and a middle node between the devices T1and T2, and a diode D2connected between the node30and a middle node between the devices T3and T4. When the devices T1and T2are switched on, the output node U has a voltage level equal to that of the positive input node3. When the devices T3and T4are switched on, the output node U has a voltage level equal to that of the negative input node5. When the devices T2and T3are switched on, the output node U has a voltage level equal to that of the node30. By controlling the power semiconductor devices of the three inverter legs to switch on and off at different times, the AC power provided at the three output nodes9of the power inverter11B has three phases.

The power inverter11B ofFIG.4may be used to replace each of the power inverters11A1and11A2ofFIG.3. In the event that the DC grid is 1500V, the input of each power inverter11B would be rated and stabilized at 750V DC through control. Because there are always two power semiconductor devices (e.g., T1&T2, or T3&T4) connected in series between each of the input nodes3,5and an output node U, V or W, the power semiconductor devices used within the power inverters11B may be 900V rated devices rather than 1700V rated devices used in the power inverter11A or 3300V rated devices used in the prior traction system100. Examples of 900V rated power devices include Si-based IGBTs, Si-based MOSFETs, SiC-based MOSFETs, GaN-based MOSFETs, or other semiconductor-based power devices.

In the examples provided byFIGS.3and4, in order to drive a set of three-phase stator windings of a traction motor8, the respective power inverter has three output nodes each supplying one phase of AC power to a corresponding phase of the traction motor8. This arrangement may be suitable when the motor windings are connected in star (i.e., wye) or delta configuration. In the event that the traction motor has open-end stator windings, a power inverter11C or11D as shown inFIGS.5and6may be used to replace each of the power inverters11A1and11A2ofFIG.3.

As shown inFIG.5, the power inverter11C comprises a positive input node3, a negative input node5, and a DC-link capacitor23electrically connected between the positive and negative input nodes3,5. The power inverter11C further includes the three pairs of inverter legs which have identical structures. For simplicity, the description below describes the structure of the first pair of inverter legs only. The first pair of inverter legs comprises two power semiconductor devices T1and T2connected in series between the input nodes3,5, an output node9u-I between the devices T1and T2, two further power semiconductor devices T3and T4connected in series between the input nodes3,5, and an output node9u-rbetween the devices T3and T4. A stator winding is connected between the pair of output nodes9u-I,9u-r, which supply AC power to the stator winding at both ends.FIG.5shows a set of three-phase stator windings32. To drive the three-phase stator windings32, three pairs of inverter legs are employed, generating three pairs of output nodes.

The power inverter11C may also be considered as being a combination of two power inverters11C-L,11C-R connected at opposite sides of the stator windings32. The two power inverter11C-L,11C-R share the same DC-link capacitor23, the same input nodes3,5and the same DC power supply (not shown). Each of the inverters is a two-level three-phase full-bridge power inverter that is similar to the power inverter11A ofFIG.3.

Similar to the power inverter11C, the power inverter11D may also be considered as being a combination of two power inverters11D-L,11D-R arranged at opposite sides of the stator windings32. However, the DC power supply of the inverter11D-R is replaced by a capacitor34.

The traction systems1,1A of the present disclosure may be part of an electric machine. Typical examples of the electric machine include a vehicle (such as, an electric locomotive, an electric or hydrogen vehicle, an elevator or an electric multiple unit) and an industrial apparatus.

While the traction systems1,1A of the present disclosure are particularly suitable for use in urban rail transit applications, they can also be used in any power electronics traction system which uses a DC power supply to drive AC electric motor loads.

Urban rail transit is an all-encompassing term for various types of local rail systems providing passenger service within and around urban or suburban areas. An urban rail transit system typically comprises one or more of a tram system, a light rail system, a rapid transit system (e.g., metro, underground, and/or subway), a monorail system, a commuter rail system, funicular, cable car, guided bus etc.