Electric traction system

Abstract

The electric traction system 1 includes a traction converter module 2 including a step-down transformer 35 having a primary winding 36 operably connected to an AC power source 40 and a secondary winding 37 inductively coupled to the primary winding 36, first and second input terminals 4 and 6 operably connected to the secondary winding 37, and a plurality of AC / AC power converters 11. Each of the plurality of AC / AC power converters 11 includes first and second input nodes 3, 5 configured to receive AC power and an output node 9 configured to supply AC power. The first and second input nodes 3, 5 of the plurality of AC / AC power converters 11 are electrically connected in series between the first and second input terminals 4 and 6. The electric traction system 1 includes at least one electric motor 25 configured to be driven by the traction converter module 2.

Description

Technical Field

[0001] The present disclosure relates to an electric traction system. More particularly, and without limitation, the present disclosure relates to an electric traction system that receives alternating current (AC) power and drives an electric motor load. Such electric traction systems are suitable for use in various power electronics applications, such as rail transport systems.

Background Art

[0002] An electric traction system converts electrical energy into mechanical energy by driving an electric motor using electrical energy, thereby generating a traction force that causes the propulsion of an electric machine. Typical examples of electric machines are vehicles (e.g., locomotives, electric or hydrogen vehicles, elevators, or electric multiple units). The electric motor may also be referred to as a traction motor.

[0003] The electric traction system may use either direct current (DC) or AC power. Generally, electric locomotives and long-distance high-speed trains (e.g., electric multiple unit trains) employ a traction system supplied with AC power. AC power is typically supplied by a power grid via an overhead line and received by a locomotive or train via a pantograph.

[0004] In recent years, the electrification of rail transportation systems has continued to progress. Currently, due to the continuous improvement of Si-based insulated gate bipolar transistors (IGBTs) with high voltage ratings of 3300 - 6500V, traction converters based on Si-IGBT devices are widely used in the electric traction systems of electric locomotives and high-speed power-distributed trains. However, there is a continuing increase in the demand for traction systems with high reliability, low cost, high efficiency, light weight, and miniaturization. However, the development of Si-based IGBTs on which existing electric traction systems are based has been slowing down. Therefore, the room for further improvement of electric traction systems is relatively limited.

[0005] FIG. 1 schematically shows a conventional electric traction system 100 commonly used in electric locomotives and high-speed trains. The conventional traction system 100 includes a transformer 135, a traction converter 102, and an electric motor 125. The transformer 135 has a primary winding 136 connected to the power grid 140 and a secondary winding 137. The traction converter 102 has single-phase AC input terminals 104, 106 electrically connected to the two ends of the secondary winding 137 via a precharge circuit 130, and a three-phase AC output node 109 for driving the traction motor 125. The traction motor 125 is generally a three-phase asynchronous motor or a three-phase permanent magnet synchronous motor. The traction converter 102 includes a rectifier 113 that converts the received single-phase AC power into DC power, a DC link capacitor 114 that stores the DC power, and a power inverter 115 that converts the DC power into three-phase AC power.

[0006] In one embodiment, the power grid 140 supplies 25 kV AC power, and the transformer 140 steps down the 25 kV supplied by the grid to 950 V. The AC voltage with a rating of 950 V across the secondary winding 137 corresponds to an intermediate DC voltage with a rating of approximately 1800 V across the DC link capacitor 114. In this voltage scheme, the power semiconductor devices used in the rectifier 113 and the inverter 115 of the traction converter 102 are generally Si-based IGBTs with a rating of 3300 V.

[0007] In a further embodiment, the power grid 140 still supplies 25 kV AC power, but the transformer 140 steps down the voltage to approximately 1900 V. The AC voltage with a rating of 1900 V corresponds to an intermediate DC voltage with a rating of approximately 3600 V across the DC link capacitor 114. In this voltage scheme, the power semiconductor devices used in the rectifier 113 and the inverter 115 of the traction converter 102 are generally Si-based IGBTs with a rating of 6600 V. A few traction converters for rail vehicles also use 4500 V Si-based IGBTs.

Summary of the Invention

Problems to be Solved by the Invention

[0008] High efficiency, lightweight, and miniaturization have always been the main goals in the development of traction systems for rail transit applications. Furthermore, it is also desirable to reduce the cost of such traction systems.

[0009] Traction converters using Si-based IGBTs with a rating of 3300V to 6500V are well-developed and have limited room for weight reduction, size reduction, and cost reduction. In particular, Si-based IGBTs with a rating of 3300V to 6500V typically have large packages, high costs, relatively low reliability, relatively high losses, and low operating frequencies. Furthermore, the circuit topology of the conventional traction converter 102 lacks flexibility, making further system optimization difficult. Additionally, the traction converter 102, particularly the power inverter 115, outputs all of the power required by the traction motor 125. Therefore, the traction converter 102 is subject to high output power requirements, which in turn require the use of power semiconductor devices with high power ratings.

[0010] Latest silicon carbide (SiC)-based power semiconductor devices with a rated voltage of 3300V or higher can replace Si-based IGBTs with the same rated voltage. Using SiC-based devices inside the rectifier 113 and the power inverter 115 can reduce losses and improve efficiency by moderately increasing the switching frequency, contributing to the weight reduction and size reduction of the traction converter 102. However, the cost of SiC-based devices is very high, approximately ten times that of Si-based IGBTs with the same rated voltage, and the reliability of such high-voltage SiC-based devices has not yet been verified. Therefore, using SiC devices in the traction converter 102 is still in the prototype and experimental stages.

[0011] An object of the present disclosure is to provide an electric traction system that, among other things, brings improvements to known traction systems.

Means for Solving the Problems

[0012] According to a first aspect of the present disclosure, an electric traction system is provided, The electric traction system is, A step-down transformer having a primary winding operably connected to an AC power source and a secondary winding inductively coupled to the primary winding, A traction converter module comprising a first input terminal and a second input terminal operably connected to the secondary winding, and a plurality of AC / AC power converters, Each of the plurality of AC / AC power converters includes a first and a second input node configured to receive AC power, and an output node configured to supply AC power. The first and second input nodes of the plurality of AC / AC power converters are electrically connected in series between the first input terminal and the second input terminal. The electric traction system includes at least one electric motor configured to be driven by the traction converter module.

[0013] By electrically connecting the first and second input nodes of the plurality of AC / AC power converters in series between the first input terminal and the second input terminal, the input sides of the plurality of AC / AC power converters both share the AC voltage output by the secondary winding of the transformer. As a result, each of the AC / AC power converters receives a fraction of the magnitude of the AC voltage between its first and second input nodes. Accordingly, the AC / AC power converter can use a power semiconductor device having a reduced rated voltage.

[0014] In the case of a step-down transformer, the secondary winding has fewer turns than the primary winding, thereby reducing the voltage of the output power stream. The use of a step-down transformer further reduces the AC voltage between the first and second input nodes of each AC / AC power converter, thereby enabling the AC / AC power converter to use a power semiconductor device having an even more reduced rated voltage.

[0015] Power semiconductor devices with lower rated voltages typically have smaller package dimensions, lower prices, and higher maturity levels than power semiconductor devices with higher rated voltages. Furthermore, low-voltage power semiconductor devices provide lower switching losses and higher efficiency than high-voltage power semiconductor devices. Additionally, low-voltage power semiconductor devices ease the cooling and heat exchange requirements, thereby enabling the traction system to have reduced weight, volume, and cost.

[0016] Accordingly, the electric traction system of the present disclosure has higher efficiency, reduced weight and volume, and reduced cost compared to conventional electric traction systems.

[0017] Furthermore, the circuit topology of the traction converter module has great flexibility for expansion and reconfiguration. In particular, compared to conventional electric traction systems, it is much easier to change the number of AC / AC power converters connected in series between the first and second input terminals and / or to change the number of electric motors driven by the traction converter module of the traction system according to the present disclosure.

[0018] The electric motor may be referred to as a traction motor (which generates a traction force that causes the propulsion of the electric machine). It will also be understood that the electric motor is an AC motor.

[0019] The expression "the first and second input nodes of a plurality of AC / AC power converters are electrically connected in series between the first input terminal and the second input terminal" means that the second input node of the power converter is electrically connected to the first input node of the power converter adjacent to the subsequent stage and / or the first input node of the power converter is electrically connected to the second input node of the power converter adjacent to the preceding stage.

[0020] The expression "at least one electric motor configured to be driven by a traction converter module" means that at least one of the AC / AC power converters is configured to supply AC power to at least one electric motor to drive the at least one electric motor.

[0021] As used in this disclosure, the terms "operatively connected" or "operatively connect" mean that there may be one or more intervening elements connected between the connected elements.

[0022] The expression "primary winding for operatively connecting to an AC power source" means that the AC power source may not be part of the electric traction system.

[0023] It will be appreciated that a plurality of AC / AC power converters includes two or more AC / AC power converters.

[0024] Each of the plurality of AC / AC power converters may be configured to receive single-phase AC power at an input node.

[0025] One or more of the plurality of AC / AC power converters may be configured to supply more than three-phase AC power at an output node.

[0026] The at least one electric motor may include a polyphase electric motor.

[0027] A polyphase electric motor is a single motor that includes three or more phases, and it will be recognized that the polyphase electric motor can be driven by one or more of a plurality of AC / AC power converters. Compared to conventional three-phase electric motors, polyphase electric motors have greater fault tolerance because they provide phase redundancy and can operate during phase open faults. Therefore, the use of polyphase electric motors improves the reliability of electric traction systems. Furthermore, polyphase electric motors achieve higher torque density, reduced amplitude and increased frequency of torque ripple, higher efficiency, lower DC link current harmonics, and better noise and vibration characteristics compared to conventional three-phase motors. Still further, polyphase electric motors can be controlled with greater degrees of freedom than conventional three-phase electric motors, thereby enabling the polyphase electric motor to achieve greater regulation of torque and shaft voltage.

[0028] The plurality of AC / AC power converters may include a first AC / AC power converter and a second AC / AC power converter. The output nodes of the first and second AC / AC power converters may be configured to supply AC power to the polyphase electric motor to drive the polyphase electric motor.

[0029] As an advantage, by driving the polyphase electric motor together by the first and second AC / AC power converters, each of the first and second AC / AC power converters supplies a fraction of the total power required by the polyphase electric motor. Therefore, the required power rating of each AC / AC power converter and the required power rating of the semiconductor devices used therein can be reduced.

[0030] It will be appreciated that the plurality of AC / AC power converters may include one or more additional power converters in addition to the first and second AC / AC power converters. The terms "first" and "second" are used merely for the convenience of description to simply label the AC / AC power converters and are also understood not to imply any limitation to the sequence or position of the converters within the traction converter module. The first AC / AC power converter may or may not be directly adjacent to the second AC / AC power converter.

[0031] The first and second AC / AC power converters may have the same circuit topology. Advantageously, the same circuit topology enables the first and second AC / AC power converters to achieve power matching by supplying equal amounts of power to the polyphase electric motor.

[0032] The polyphase electric motor may comprise a first set of stator windings and a second set of stator windings. The output node of the first AC / AC power converter may be electrically connected to the first set of stator windings, and the output node of the second AC / AC power converter may be electrically connected to the second set of stator windings.

[0033] As used in this disclosure, the term "electrically connected" means that one or more intervening elements (e.g., electrical contacts) may be connected between the elements being connected.

[0034] It will be appreciated that the polyphase electric motor may include one or more additional sets of stator windings.

[0035] The first set of stator windings and the second set of stator windings may be electrically isolated from each other. Advantageously, the electrical insulation between the sets of stator windings improves the reliability of the system.

[0036] The first AC / AC power converter may be configured to output a first number of phases of AC power at its output node. The first number of phases may be the same as the number of phases of the first set of stator windings.

[0037] The first number of phases may be three or more phases.

[0038] The number of output nodes of the first AC / AC power converter may be the same as the first number of phases.

[0039] Alternatively, the number of output nodes of the first AC / AC power converter may be twice the first number of phases. This arrangement can be dangerous when driving open - ended stator windings.

[0040] The first set of stator windings may be connected in a Y or Δ configuration. Alternatively, the first set of stator windings may be open - ended stator windings that require power supply from both ends.

[0041] The second AC / AC power converter and the second set of stator windings may have similar characteristics as those described above for the first AC / AC power converter and the first set of stator windings.

[0042] The plurality of AC / AC power converters may have the same circuit topology. Advantageously, the same circuit topology enables the plurality of AC / AC power converters to equally share the voltage of the AC power source.

[0043] At least one of the plurality of AC / AC power converters may include a rectifier, a DC - link capacitor, and a power inverter. At least one of the plurality of AC / AC power converters may also be referred to as an indirect - type power converter or an AC - DC - AC power converter.

[0044] Each of the rectifier and the power inverter may include at least one power semiconductor device.

[0045] At least one power semiconductor device may include one or more of a Si-based power semiconductor device, a SiC-based power semiconductor device, and a GaN-based power semiconductor device.

[0046] The power inverter may include a plurality of inverter legs connected between two ends of a DC link capacitor. The plurality of inverter legs may provide output nodes of each AC / AC power converter.

[0047] Each inverter leg may include at least one power semiconductor device.

[0048] The electric traction system may further include a controller configured to control the on and off states of the at least one power semiconductor device so as to convert the AC power received at the input node of each AC / AC power converter into AC power at its output node during the traction mode of the traction system.

[0049] The controller may be further configured to control the on and off states of the power semiconductor devices of each AC / AC power converter so as to convert the mechanical energy of at least one electric motor into electrical energy in the secondary winding during the braking mode of the traction system. The electrical energy may be returned to the primary winding and further to the AC power supply.

[0050] At least one of the plurality of AC / AC power converters may further include a bypass switch connected between the first and second input nodes of each AC / AC power converter.

[0051] At least one of the plurality of AC / AC power converters may be configured such that each converter is brought into an operating state when the bypass switch is in the OFF state (i.e., open), and each converter is brought into a non-operating state when the bypass switch is in the ON state (i.e., closed).

[0052] The plurality of AC / AC power converters may further include a standby AC / AC power converter. The standby AC / AC power converter may include a bypass switch connected between its first and second input nodes.

[0053] The polyphase electric motor may include a set of standby stator windings, and the output nodes of the standby AC / AC power converter may be electrically connected to the set of standby stator windings.

[0054] The standby AC / AC power converter may be configured to replace a failed one of the plurality of AC / AC power converters.

[0055] When no failure occurs in the plurality of AC / AC power converters, the bypass switch of the standby AC / AC power converter is turned on (i.e., closed), and the bypass switches of the other power converters are turned off (i.e., open). When a failure occurs, the bypass switch of the failed AC / AC power converter is switched to the on state, and the bypass switch of the standby AC / AC power converter is switched to the off state. The traction converter module may be configured as such.

[0056] The controller may be configured to control the on and off states of the bypass switch.

[0057] The electric traction system may further include a precharge circuit electrically connected between the secondary winding and the traction converter module. The precharge circuit may be configured to charge the DC link capacitor before the normal operation of the traction converter module.

[0058] The precharge circuit may include a charging resistor and may be configured to charge the DC link capacitor via the charging resistor.

[0059] The pre-charge circuit may further include a first switch and a second switch. The second switch may be connected in series with the charging resistor. The first switch may be connected in parallel with the second switch and the charging resistor.

[0060] The transformer may include a line frequency transformer.

[0061] It will be understood that the line frequency transformer is a transformer that operates at a line frequency (which may also be referred to as a utility frequency, for example 50 Hz or 60 Hz).

[0062] According to a second aspect of the present disclosure, there is provided an electric machine comprising the electric traction system according to the first aspect.

[0063] The electric machine may include a vehicle. The vehicle may be selected from the group consisting of an electric locomotive, a train (railway car), and a multiple unit train.

[0064] Alternatively, the electric machine may include an industrial device.

[0065] According to a third aspect of the present disclosure, there is provided a power electronics system, the power electronics system comprising an AC power source and the electric traction system according to the first aspect, and the primary winding is operably connected to the AC power source.

[0066] According to a fourth aspect of the present disclosure, there is provided a rail transport system, the rail transport system comprising an AC power source and a vehicle comprising the electric traction system according to the first aspect, and the primary winding of the electric traction system is operably connected to the AC power source.

[0067] Where appropriate, any of the optional features described above with respect to one of the aspects of the present disclosure may be applied to another one of the aspects of the present disclosure.

Brief Description of the Drawings

[0068]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Mode for Carrying Out the Invention

[0069] In order to enable a more thorough understanding of the present disclosure, in this application, a number of embodiments of the present disclosure will be described with reference to the accompanying drawings only as examples.

[0070] In the drawings, like parts are denoted by the same reference numerals.

[0071] It will be recognized that the drawings are for illustrative purposes only and are not shown to scale.

[0072] Detailed Description of the Preferred Embodiment Figure 2 schematically shows a circuit diagram of an electric traction system 1 (hereinafter also referred to as "traction system") according to a first embodiment of the present disclosure. The traction system 1 uses a traction converter module 2 instead of the traction converter 102 used in the conventional traction system 100. The traction converter module 2 converts single-phase AC power into polyphase AC power in order to drive the electric motor 25. The traction system 1 further includes a step-down transformer 35 having a primary winding 36 that is electrically connected to an AC power source 40 during use and a secondary winding 37 having fewer turns than the primary winding 36. Therefore, the transformer 35 steps down the high voltage supplied by the AC power source 40 to a lower voltage. The AC power source 40 may be a power grid. The AC power supplied by the power grid typically has a frequency of 50 Hz or 60 Hz, that is, a line frequency or a utility frequency. Accordingly, the step-down transformer 35 may be a line frequency transformer, which means that it operates at the line frequency (i.e., 50 Hz or 60 Hz). It will be appreciated that the AC power source 40 may take different forms depending on a particular application of the traction system 1 and that the transformer 35 may operate at different frequencies. Apart from stepping down, the transformer 35 obtains power or supply power to the AC power source 40 and provides electrical insulation between the traction converter module 2 and the AC power source 40.

[0073] The traction converter module 2 includes a first input terminal 4 and a second input terminal 6, which are electrically connected to two ends of the secondary winding 37 respectively. The precharge circuit 30 is electrically connected in series between the first input terminal 4 and one end of the secondary winding 37. Referring to FIG. 2, the traction converter module 2 includes four AC / AC power converters (hereinafter referred to as "AC / AC converters") 111,... 114 (collectively referred to as 11), each of which is similar to the traction converter 102. Each AC / AC converter 11 i (i = 1,..., 4) receives single-phase AC power at a first input node 3i and a second input node 5 i and a switch 7 connected between the input nodes i and three output nodes 9i for supplying three-phase AC power to a drive motor load.

[0074] The first and second input nodes 3, 5 of the AC / AC converter 11 are electrically connected in series between the first input terminal 4 and the second input terminal 6. In particular, the second input node 5 i of the AC / AC converter 11 i (i = 2 or 3) is electrically connected to the first input node 3 i+1 of the subsequent AC / AC converter 11 i+1 and the first input node 3 i of the AC / AC converter 11 i is electrically connected to the second input node 5 i-1 of the preceding AC / AC converter 11 i-1 . The AC / AC converter 111 in front of the array of AC / AC converters 11 has its first input node 31 electrically connected to the first input terminal 4. The AC / AC converter 114 behind the array of AC / AC converters 11 has its second input node 54 electrically connected to the second input terminal 6.

[0075] The switch 7 functions to bypass the switch and can be used to put the corresponding AC / AC converter 11 into an operating state or a non-operating state. When the switch 7 i (for example, 74 in FIG. 2) is closed (i.e., in the ON state), the first and second input nodes 3 i and 5 i of the corresponding AC / AC converter 11 i are electrically short-circuited to each other. As a result, the AC / AC converter 11 i does not receive AC power and is thus put into a non-operating state. Conversely, an open (i.e., OFF) switch 7 i (for example, 71, 72, or 73 in FIG. 2) enables the corresponding AC / AC converter 11 iEnable it to function as an active converter as usual. In one embodiment, the AC / AC converter 114 is a standby converter having a bypass switch 74 that is normally closed, and the remaining three AC / AC converters 111-113 are active converters having bypass switches that are normally open.

[0076] As shown in FIG. 2, each of the AC / AC converters 11 includes a rectifier 13, a DC link capacitor 14, and a power inverter 15. The rectifier 13 includes four power semiconductor devices (shown as switches) T1 to T4. When devices T1 and T4 are turned on, it may be possible to charge the DC link capacitor 14 with the positive half-wave of the input AC wave, while when devices T2 and T3 are turned on, it may be possible to charge the DC link capacitor 14 with the negative half-wave of the input AC. The rectifier 13 is illustrated as a single-phase two-level full-bridge four-quadrant rectifier, but the rectifier 13 may take any suitable form not limited to the embodiment shown in FIG. 2. The power inverter 15 includes three inverter legs that each provide an output node. The three inverter legs have the same structure. For simplicity, the following description describes only the structure of the first inverter leg. The first inverter leg includes two power semiconductor devices T5 and T6 connected between two ends (i.e., V DD , V SS ) of the DC link capacitor 14, and its output node is between the two devices T5 and T6. By controlling the power semiconductor devices of the three inverter legs to turn on and off at different times, the AC power supplied at the three output nodes 9 has three different phases. Further, the power inverter 15 is illustrated as a two-level three-phase full-bridge inverter, but the inverter 15 may be implemented using different circuit topologies such as, for example, three-level, multi-level, half-bridge, etc. Further, the number of phases of the inverter 15 may be different from three by adjusting the number of inverter legs. FIG. 2 shows that a single power semiconductor device, on the one hand, each input node 3 i , 5i and each output node 9 i and, on the other hand, the voltage node V DD , V SS It is shown that it is provided between. This is merely for the sake of clarity of the concept, and more than one power semiconductor device may be provided, and each input / output node and the voltage node V DD , V SS It will be understood that they may be electrically connected in series or in parallel between each of and.

[0077] Referring to the traction system 1 of FIG. 2, an active AC / AC converter 11 having an open switch 7 shares the AC voltage received between the first and second input terminals 4, 6. Each active AC / AC converter 11 i The voltage drop across the input nodes 3 i , 5 i is merely a fraction of the AC voltage received between the terminals 4, 6. Compared with the conventional converter 102, each of the AC / AC converters 11 can be configured by using a power semiconductor device having a much lower rated voltage. This also expands the selection of power semiconductor devices beyond Si-based IGBTs and SiC-based devices. For example, a Si-based metal oxide semiconductor field effect transistor (MOSFET) and a gallium nitride (GaN)-based MOSFET may be used to configure the AC / AC converter 11.

[0078] In one embodiment, the AC power supply 40 supplies 25 kV of AC power, and the transformer 35 steps down the high voltage supplied by the AC power supply 40 to an AC voltage having a rating of 950 V across the secondary winding 37. When the standby AC / AC converter 114 is bypassed, the rated AC input voltage of each active AC / AC converter 111 - 113 is 316.7 V, which corresponds to an intermediate DC voltage having a rating of approximately 750 V across the DC link capacitor 14. In this voltage scheme, the power semiconductor devices used in the active AC / AC converters 111 - 113 are not devices having a rating of 3300 V used in the conventional traction system 100, but are generally devices having a rating of 1200 V. Examples of power devices having a rating of 1200 V include Si-based IGBTs, Si-based MOSFETs, SiC-based MOSFETs, GaN-based MOSFETs, or other semiconductor-based power devices.

[0079] When only two active AC / AC converters electrically connected in series between the input terminals 4 and 6 are present, the rated AC input voltage of each active AC / AC converter becomes 475 V, which corresponds to an intermediate DC voltage having a rating of approximately 1050 V across the DC link capacitor 14. In this voltage scheme, the power semiconductor devices used in the active AC / AC converter are generally devices having a rating of 1700 V, in contrast to the Si-based IGBTs having a rating of 3300 V used in the conventional traction system 100. Examples of power devices having a rating of 1700 V include Si-based IGBTs, Si-based MOSFETs, SiC-based MOSFETs, GaN-based MOSFETs, or other semiconductor-based power devices.

[0080] Power semiconductor devices with lower rated voltages typically have smaller package dimensions, lower prices, and higher maturity than power semiconductor devices with higher rated voltages. Furthermore, low-voltage power semiconductor devices provide lower switching losses and higher efficiency than high-voltage power semiconductor devices. Additionally, low-voltage power semiconductor devices relax the cooling and heat exchange requirements, thereby enabling the traction system 1 to have reduced weight, volume, and cost.

[0081] Since the traction system 1 does not require an AC power source with a lower rating but enables the use of power semiconductor devices with lower ratings by improving the circuit structure, the traction system 1 can use the same AC power source as the conventional traction system 100. Therefore, the traction system 1 may directly replace the existing traction system 100 in rail transport applications.

[0082] The traction system 1 further includes a controller 20. The controller 20 controls the on / off switching of the bypass switch 7 using the signal line 21. The controller 20 also controls the on / off switching of the power semiconductor devices in each AC / AC converter 11 using the signal line 22. As a result, the functions of each AC / AC converter 11 may be independently controlled by the controller 20. The switch 7 may be implemented as a gate-controlled power switch, such as a MOSFET or IGBT, or as a current controller power switch, such as a thyristor. The controller 20 may include a control device (e.g., a processor, a programmable logic circuit, and / or an application-specific integrated circuit (ASIC), etc.) and a driver circuit that converts a small current control signal output by the control device into a larger current control signal. Although FIG. 2 shows that the controller 20 is part of the traction system 1, it will be recognized that, alternatively, the controller 20 may be an external component of the traction system 1.

[0083] The precharge circuit 30 is electrically connected in series between the end of the secondary winding 37 and the first input terminal 4. The precharge circuit 30 includes a first switch 33 and a second switch 31 connected in series to the precharge resistor 32. The first switch 33 is connected in parallel with the second switch 31 and the resistor 32. Before the normal operation of the traction converter module 2, the second switch 31 is closed while the first switch 33 remains open. In this way, the DC link capacitor 14 of the AC / AC converter 11 can be charged via the precharge resistor 32. Once the precharge of the DC link capacitor 14 is completed, the precharge resistor 32 is bypassed by closing the first switch 33 and opening the second switch 31. The voltage drop across the first switch 33 can be ignored. Therefore, the magnitude of the AC voltage across the first and second input terminals 4, 6 is substantially the same as the magnitude of the AC voltage supplied by the secondary winding 37. The precharge of the DC link capacitor 14 is useful for preventing an excessive inrush current at system startup that could damage the DC link capacitor 14 of the AC / AC converter 11 and the power semiconductor devices (e.g., T1 - T4).

[0084] The traction system 1 also includes an electric motor 25, which is an AC motor. The stator winding of the motor 25 is electrically connected to the output node 9 of the AC / AC converter 11. As a result, the traction converter module 2 drives the motor 25 by supplying AC power to the motor 25. The motor 25 typically generates a traction force that causes the propulsion of an electric machine (e.g., a vehicle, an industrial machine, etc.) and is therefore sometimes referred to as a traction motor. The motor 25 may take the form of an induction motor or a permanent magnet synchronous motor.

[0085] Regarding the number of phases, the electric motor 25 is a polyphase electric motor. A polyphase motor generally has more than three phases (e.g., 5 to 12 phases). In a conventional three-phase motor (such as motor 125), if one of the phases is lost, the rotating magnetic field within the motor will also disappear, and the motor will stop operating. Compared to a conventional three-phase motor, the polyphase motor 25 has greater fault tolerance because it provides phase redundancy and can operate during a phase open fault. Therefore, the use of the polyphase motor 25 enables the traction system 1 to have higher reliability. Further, the polyphase motor 25 achieves higher torque density, reduced amplitude and increased frequency of torque ripple, higher efficiency, lower DC link current harmonics, and better noise and vibration characteristics compared to a conventional three-phase motor. Additionally, the polyphase electric motor 25 can be controlled with greater degrees of freedom than a conventional three-phase electric motor, thereby enabling the motor 25 to achieve greater regulation of torque and shaft voltage.

[0086] The circuit topology of the traction converter module 2 is particularly suitable for driving the polyphase electric motor 25. When the AC / AC converter 114 is a standby converter having a normally closed bypass switch 74, three active AC / AC converters 111 to 113 drive the motor 25 together. This can be achieved by electrically connecting the output nodes 91, 92, 93 of each active AC / AC converter to each set (for example, three or more phases) of the stator windings of the motor 25 such that each active AC / AC converter drives each set of the stator windings. Each active AC / AC power converter outputs AC power of a predetermined number of phases at its output node. Preferably, the number of phases (for example, three or more) output by the AC / AC converter is the same as the number of phases of the set of stator windings driven thereby. Accordingly, each of the AC / AC converters 111 to 113 supplies a fraction of the total power required by the motor 25. By giving the plurality of AC / AC converters driving the motor 25 the same circuit topology, each of the AC / AC converters supplies an equal amount of power (for example, one third of the total power required by the motor 25) to each set of the stator windings of the motor 25, and thus achieves power matching with each other.

[0087] The plurality of sets of stator windings in the motor 25 may be electrically insulated from each other, for example, by having neutral points separated from each other. The electrical insulation between the sets of stator windings is useful for improving the reliability of the system. Alternatively, the sets of stator windings in the motor 25 may share the same neutral point. The plurality of sets of stator windings may be independently controlled, and thus may allow a high degree of control freedom to optimize the torque and shaft voltage of the motor 25.

[0088] As described above, the AC / AC converter 114 may be a standby redundant converter that is normally in a non-operating state. The polyphase electric motor 25 may also be configured to have redundancy. Referring to FIG. 2, the motor 25 includes a set of standby stator windings electrically connected to the output node 94 of the standby AC / AC converter 114. During normal operation, the bypass switch 74 of the AC / AC converter 114 is kept closed by the controller 20 so as to put the standby AC / AC converter 114 in a non-operating state. On the other hand, the switches 71 to 73 of the AC / AC converters 111 to 113 are kept open by the controller 20. In this way, the AC / AC converters 111 to 113 drive the motor 25 together. If a failure occurs in one of the AC / AC converters 111 to 113 or in the stator windings driven thereby, the controller 20 closes the switch 7 j (j = 1, 2, or 3) to open the switch 74 of the standby AC / AC converter 114 so as to put the standby AC / AC converter 114 in an operating state. Accordingly, the standby AC / AC converter 114 drives the motor 25 using the other two active inverters. This redundancy mechanism provided by the polyphase electric motor 25 and the traction converter module 2 enables the traction system 1 to continue to function when a failure occurs in the stator windings of the motor 25 or in the AC / AC converter 11, thereby significantly improving the reliability of the traction system 1.

[0089] In one embodiment, the motor 25 is designed as a 12-phase motor but operates as a 9-phase motor, with three phases being reserve phases. The motor 25 may have a rated power of 600 kW. Under normal operation, each of the active AC / AC converters 111 - 113 supplies 200 kW of power to the motor 25. In particular, each rectifier 13 rectifies the AC input power received by each active AC / AC converter at a power factor of about 1.0 to achieve a DC link voltage of 750 V. The DC link voltage is then converted by each inverter 15 into three-phase AC power having a variable frequency and a variable fundamental quantity to drive the set of three-phase stator windings of the motor 25. In this process, the power is converted into mechanical power. This operating mode of the traction system 1 is sometimes called the traction mode.

[0090] The traction system 1 may also have a braking mode in which the mechanical power of the motor 25 is converted into electrical power in a controlled manner. In particular, the motor 25 functions as a generator that generates AC power, and the generated AC power is fed back to node 9 of the traction converter module 2. The AC / AC converter 11 of the traction converter module 2 is controlled by the controller 20 to convert the AC power received at node 9 into AC power for output at nodes 3, 5. During this process, the inverter 15 functions as a rectifier, while the rectifier 13 functions as an inverter. The AC power output by nodes 3, 5 is then transferred to the secondary winding 37 and returned to the AC power source 40 through the primary winding at a power factor of about -1.0.

[0091] It will be understood that during the traction mode and the braking mode, power flows in the reverse direction through the AC / AC converter 11. The controller 20 controls the operating mode of the AC / AC converter 11 and also controls the direction of power flow through the AC / AC converter 11. Each of the AC / AC converters 11 independently controls the power flow to / from each set of stator windings electrically connected to the converter.

[0092] Although not shown in FIG. 2, electrical contacts may be connected between one or more output nodes 9 of at least one AC / AC converter 11 and one or more respective stator windings of the motor 25. Electrical contacts are electrical circuit components commonly found in electrical switches, relays, connectors, and circuit breakers. Each contact is a single conductive material, typically metal. When a pair of electrical contacts touch, they can pass current and electrically connect the corresponding output node 9 and stator winding.

[0093] FIG. 2 shows that the traction converter module 2 includes four AC / AC converters 11, which are electrically connected in series on their input sides, but this is for illustrative purposes only and is not meant to imply any limitation on the number of AC / AC converters 11. In fact, having at least two AC / AC converters 11 connected in series on the input side will allow the traction converter module 2 to achieve the advantages described above.

[0094] FIG. 2 shows that the traction system of FIG. 2 includes a single motor 25, but it will be recognized that one or more additional motors may be used. One or more additional motors may be driven by one or more extra AC / AC converters of the traction converter module 2. One or more additional motors may include a conventional three-phase motor driven by a single AC / AC converter, or may include a polyphase electric motor similar to the motor 25, driven by one or more AC / AC converters.

[0095] It will also be understood that the bypass switch 7 may be omitted so that all of the AC / AC converters 11 become active AC / AC converters.

[0096] It is preferable that the plurality of active AC / AC converters 11 in the traction converter module 2 are identical to each other (i.e., have the same circuit topology together with the same device parameters) so as to equally share the AC voltage between the first input terminals 4 and 6 and output the same AC current at the output node. However, it will be recognized that this configuration is not essential.

[0097] The AC / AC converter 11 shown in FIG. 2 is an AC-DC-AC converter that converts input AC power into intermediate DC power, which is then converted into AC power for output. It will be recognized that the AC / AC converter 11 may have any suitable circuit topology as long as it adjusts one or more of the phase, frequency, and magnitude of the input AC power when generating the output AC power. For example, the AC / AC converter 11 may directly convert the input AC power into the output AC power without the need for any conversion to intermediate DC power. Alternatively, the AC / AC converter 11 may perform more than one AC-DC-AC conversion.

[0098] The traction system 1 of FIG. 2 has flexible scalability, for example, by changing the number of AC / AC converters connected in series between the input terminals 4 and 6 and / or by changing the number of motors driven by the traction converter module. FIG. 3 schematically shows a circuit diagram of an electric traction system 1A according to a second embodiment of the present disclosure. Components of the traction system 1A that are the same as those of the traction system 1 are identified using the same reference numerals. Components of the traction system 1A that correspond to those of the traction system 1 but are different are labeled using the same numerals with the letter "A" added for distinction. The features and advantages described above with reference to the first embodiment are generally applicable to the second embodiment.

[0099] The traction system 1A is different from the traction system 1 in that the traction converter module 2 drives two polyphase motors 251 and 252, each of which is a six-phase motor (e.g., a dual three-phase motor). The AC / AC converters 111 and 112 drive the motor 251 together. The AC / AC converters 113 and 114 drive the motor 252 together. In the traction system 1A, all of the AC / AC converters are active converters. Therefore, the bypass switch is removed from FIG. 3.

[0100] The examples provided by FIGS. 2 and 3 include one or more polyphase motors 25 each driven by at least two AC / AC converters, although it will be recognized that one or more of the polyphase motors may alternatively be driven by a single AC / AC converter that outputs multiple phases of AC power. The traction converter module 2 may be used to drive one or more conventional three-phase motors, and it will also be recognized that each three-phase motor may be driven by a single AC / AC converter.

[0101] In the examples provided by FIGS. 2 and 3, each AC / AC converter has three output nodes that each supply one phase of AC power to the corresponding phase of the traction motor to drive the set of three-phase stator windings of the traction motor. This configuration may be suitable when the motor windings are connected in a star (i.e., Y) or Δ configuration. If the traction motor has open-ended stator windings, an AC / AC converter 11A or 11B as shown in FIGS. 4 and 5 may be used to replace each of the AC / AC converters 11 of FIGS. 2 and 3.

[0102] As shown in FIG. 4, the AC / AC converter 11A includes a first input node 3, a second input node 5, a rectifier 13, a DC link capacitor 14, and a power inverter 15A. The AC / AC converter 11A is different from the converter 11 in the configuration of the power inverter 15A. More specifically, the power inverter 15A includes three pairs of inverter legs having the same structure. For the sake of simplicity, the following description will only describe the structure of the first pair of inverter legs. The first pair of inverter legs includes two power semiconductor devices G 3T and G 3B connected in series between two ends of the DC link capacitor 14, and an output node 9u-l between the devices G 3T and G 3B and two further power semiconductor devices G 6T and G 6B connected in series between two ends of the DC link capacitor 14, and an output node 9u-r between the devices G 6T and G 6B . The stator winding is connected between a pair of output nodes 9u-l and 9u-r, which supplies AC power to both ends of the stator winding. FIG. 4 shows a set of three-phase stator windings 26. To drive the three-phase stator windings 26, three pairs of inverter legs are used, resulting in three pairs of output nodes.

[0103] The power inverter 15A may be regarded as a combination of two power inverters 15A-L and 15A-R connected to the opposite side of the stator winding 26. The two power inverters 15A-L and 15A-R share the same DC link capacitor 14. Each of the inverters is a two-level three-phase full-bridge power inverter similar to the power inverter 15 in FIG. 2.

[0104] Referring to FIG. 5, the AC / AC converter 11B is different from the converter 11 in the configuration of the power inverter 15B. Similar to the power inverter 15A, the power inverter 15B may be regarded as a combination of two power inverters 15BA-L and 15B-R arranged on the opposite side of the stator winding 26. However, the three inverter legs of the inverter 15B-R are connected to the two ends of the floating capacitor 17 instead of the DC link capacitor 14.

[0105] The traction systems 1, 1A of the present disclosure may be part of an electric machine. Typical examples of electric machines include vehicles (e.g., electric locomotives or motorized multiple units) and industrial devices. It will be understood that the AC power source 40 may not be part of the traction systems 1, 1A or the electric machine.

[0106] The traction systems 1, 1A of the present disclosure are particularly suitable for use in rail transport applications, but they may also be used in any power electronics traction system that drives an AC electric motor load using an AC power source.

[0107] Terms such as "having", "including", "containing", "comprising", etc. are open-ended, and these terms indicate the presence of the stated structure, component, or feature, but do not preclude the presence of additional components or features. The phrases "one" and "the" are intended to include both the singular and the plural, unless the context clearly indicates otherwise.

[0108] Although the present disclosure has been described with respect to the preferred embodiments described above, it should be understood that these embodiments are merely illustrative and that the claims are not limited to these embodiments. Those skilled in the art can make changes and substitutions related to the present disclosure that are intended to be within the scope of the appended claims. Each feature disclosed and illustrated in this specification may be incorporated into the present disclosure alone or in any suitable combination with any other feature disclosed or illustrated in this application.

Claims

1. An electric traction system, wherein the electric traction system comprises: A step-down transformer including a primary winding operably connected to an AC power source and a secondary winding inductively coupled to the primary winding; A traction converter module including a first input terminal and a second input terminal operably connected to the secondary winding, and a plurality of AC / AC power converters; Each of the plurality of AC / AC power converters includes a first and a second input node configured to receive AC power, and an output node configured to supply AC power; The first and second input nodes of the plurality of AC / AC power converters are electrically connected in series between the first input terminal and the second input terminal; The electric traction system further comprises: At least one electric motor configured to be driven by the traction converter module; A controller; The at least one electric motor includes a multi-phase electric motor having more than three phases, and the multi-phase electric motor includes a first set of stator windings, a second set of stator windings, and a set of auxiliary stator windings; The plurality of AC / AC power converters include a first AC / AC power converter, a second AC / AC power converter, and an auxiliary AC / AC power converter; The output node of the first AC / AC power converter is electrically connected to the first set of stator windings, the output node of the second AC / AC power converter is electrically connected to the second set of stator windings, and the output node of the auxiliary AC / AC power converter is electrically connected to the set of auxiliary stator windings; The first AC / AC power converter includes a first bypass switch connected between the first and second input nodes of the first AC / AC power converter, the second AC / AC power converter includes a second bypass switch connected between the first and second input nodes of the second AC / AC power converter, and the auxiliary AC / AC power converter includes a third bypass switch connected between the first and second input nodes of the auxiliary AC / AC power converter. When a failure occurs in one of the first and second sets of the stator windings, the controller is configured to switch one of the first and second bypass switches from an off state to an on state to deactivate one of the first and second AC / AC power converters, and to switch the third bypass switch from an on state to an off state to activate the standby AC / AC power converter. An electric traction system. **Claim 2** The first and second AC / AC power converters have the same circuit topology. The electric traction system according to claim 1. **Claim 3** The first set of the stator windings and the second set of the stator windings are electrically isolated from each other. The electric traction system according to claim 1 or 2. **Claim 4** The first AC / AC power converter is configured to output a first number of phases of AC power at its output node. The first number of phases is the same as the number of phases of the first set of the stator windings. The electric traction system according to any one of claims 1 to 3. **Claim 5** At least one of the plurality of AC / AC power converters includes a rectifier, a DC link capacitor, and a power inverter. The electric traction system according to any one of claims 1 to 4. **Claim 6** Each of the rectifier and the power inverter includes at least one power semiconductor device. The electric traction system according to claim 5. **Claim 7** The power inverter includes a plurality of inverter legs connected between two ends of the DC link capacitor. The plurality of inverter legs provide the output nodes of the respective AC / AC power converters. The electric traction system according to claim 5 or 6. **Claim 8** The controller is configured to control the on and off states of the at least one power semiconductor device to convert the AC power received at the input node of each AC / AC power converter into AC power at its output node during the traction mode of the electric traction system. The electric traction system according to claim 6. **Claim 9** The controller is further configured to control the on and off states of the power semiconductor devices of each of the AC / AC power converters so as to convert the mechanical energy of the at least one electric motor into electrical energy in the secondary winding during a braking mode of the electric traction system. The electric traction system according to claim 8.

10. Further comprising a pre-charge circuit electrically connected between the secondary winding and the traction converter module, The pre-charge circuit is configured to charge the DC link capacitor before normal operation of the traction converter module. The electric traction system according to any one of claims 5 to 9.

11. The step-down transformer comprises a line frequency transformer. The electric traction system according to any one of claims 1 to 10.

12. An electric machine comprising the electric traction system according to any one of claims 1 to 11.

13. The electric machine includes a vehicle. The electric machine according to claim 12.

14. An AC power supply, Comprising the electric traction system according to any one of claims 1 to 11, and The primary winding is operably connected to the AC power supply. A power electronics system.

15. An AC power supply, Comprising a vehicle comprising the electric traction system according to any one of claims 1 to 11, and The primary winding of the electric traction system is operably connected to the AC power supply. A rail transportation system.

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