Working condition judgment method suitable for alternating current traction network, PWM converter feedback control method and system

Abstract

The invention belongs to the technical field of large iron rail transit power supply, and particularly relates to a working condition judgment method suitable for an alternating current traction network and a PWM converter feedback control method and system. The method comprises the following steps: respectively determining a traction network voltage change value U1pu and an alternating current line voltage change value U2pu corresponding to the PWM converter in a current time window under the conditions that an alternating current single-phase side of the PWM converter is connected to a traction network corresponding to the PWM converter and an alternating current three-phase side is connected to an alternating current line for power supply and electric quantity feedback; the AC line comprises an adjacent AC traction network of the traction network; if the absolute value of U1pu-U2pu is less than or equal to a first set positive value, judging that the voltage fluctuation of the alternating-current line causes the voltage fluctuation of the traction network; if U1pu-U2pu is less than 0 and the absolute value is greater than a first set positive value and is less than a second set positive value at the same time, determining that the locomotive is started or passes to cause voltage fluctuation of the traction network; if U1pu-U2pu is larger than the second set positive value and smaller than the third set positive value, it is judged that locomotive braking causes traction network voltage fluctuation.

Description

Technical Field

[0001] This invention belongs to the field of power supply technology for large-scale railway transportation, specifically relating to a method for judging operating conditions and a feedback control method and system for PWM converters applicable to AC traction networks. Background Technology

[0002] In existing large-scale railway power supply systems, Scott transformers are commonly used as the power supply transformers for trains, converting three-phase (or single-phase) AC power into single-phase AC power. Scott transformers have advantages such as high efficiency, large capacity, and strong overload capacity. However, during downhill driving and deceleration braking, they generate a large amount of single-phase, impulsive regenerative energy, which is fed back into the power grid. In my country, over 2,000 traction substations feed back 8-10 billion kWh of regenerative energy annually, accounting for approximately 10% of the total railway electricity consumption. However, this large amount of regenerative energy not only pollutes the power quality of the grid but also causes abnormal voltage spikes at the end of the traction network, even leading to brake failure and breakdowns. For example, during locomotive startup or braking, the power imbalance between the M and T windings of the Scott transformer causes a drop in traction network voltage during startup and a rise during braking, resulting in significant voltage fluctuations in the traction network.

[0003] Currently, PWM converters can be used to maintain power balance between the M and T sections of the Scott transformer based on train operating conditions, achieving functions such as reactive power compensation and harmonic mitigation. The electricity generated during train braking can also be fed back to the AC power grid or the adjacent traction network power supply arm. For example, the principle of a comprehensive energy device and its control method for traction substations mentioned in Chinese invention patent application publication number CN109428331A is relevant. However, the feedback mechanism of the PWM converter causes fluctuations in the traction network voltage, making it difficult to accurately determine the train's operating status. This can easily lead to misjudgments by the PWM converter and the formation of power circulation currents with the Scott transformer, which is detrimental to the energy utilization of the track power supply system in large-scale railway systems. Summary of the Invention

[0004] The purpose of this invention is to provide a method for judging the operating conditions of AC traction networks, a PWM converter feedback control method and system, to solve the problems in the prior art where fluctuations in the traction network voltage caused by the PWM converter feedback mechanism make it difficult to accurately judge the train operating status, easily leading to PWM converter misjudgment and the formation of power circulating current with the Scott transformer.

[0005] To achieve the above objectives, the present invention provides a method for determining the operating conditions of an AC traction network, comprising: For cases where the AC single-phase side of the PWM converter is connected to the corresponding traction network, and the AC three-phase side is connected to an AC line used for power supply and power feedback, determine the voltage change value ΔU of the corresponding traction network for the PWM converter within the current time window. 1puand the AC line voltage variation value ΔU used for power supply and power feedback 2pu The AC lines used for power supply and power feedback include the adjacent AC traction networks of the traction network. If △U 1pu -△U 2pu If the absolute value is less than or equal to the first set positive value, it is determined that the voltage fluctuation of the AC line used for power supply and power feedback causes the voltage fluctuation of the traction network. If △U 1pu -△U 2pu If the value is less than 0 and the absolute value is greater than the first set positive value but less than the second set positive value, then it is determined that the locomotive traction start or passing by has caused voltage fluctuations in the traction network; the first set positive value is less than the second set positive value. If △U 1pu -△U 2pu If the value is greater than the second set positive value and less than the third set positive value, it is determined that the locomotive braking caused the traction network voltage fluctuation; the second set positive value is less than the third set positive value.

[0006] Furthermore, the first positive value is determined by calibrating based on the voltage fluctuation range of the power grid at the equipment site throughout the day; the second and third positive values ​​are determined by calibrating based on the converter's response speed or sensitivity at the site.

[0007] Furthermore, AC lines used for power supply and energy feedback also include AC power grids.

[0008] The above-described technical solution of this invention provides a new method for judging the operating conditions of an AC traction network. Its beneficial effects include: accurately judging the train's operating status by analyzing the voltage changes of the traction network connected to the PWM converter and the AC lines used for power feedback. It considers that the fluctuations in the local traction network caused by the AC lines used for power supply and power feedback are usually relatively small due to the indirect influence of the PWM converter, while the locomotive's influence on the local traction network is usually direct and therefore fluctuates more significantly. Accordingly, a criterion is designed to distinguish between these two types of operating conditions. It also considers that the traction network voltage corresponding to the PWM converter decreases due to locomotive traction or passing, and the traction network voltage decreases due to locomotive braking. The voltage of the AC lines used for power supply and power feedback is less affected by the locomotive; therefore, by distinguishing ΔU... 1pu -△U 2pu The conditions less than 0 and greater than 0 correspond to different intervals to distinguish between locomotive traction or passage and locomotive braking. This allows for more accurate processing of the train's operating conditions, and the criteria are simple and accurate.

[0009] The present invention also provides a working condition judgment system applicable to AC traction networks, including a processor, wherein the processor stores executable program instructions, which are executed to implement the above-described working condition judgment method applicable to AC traction networks.

[0010] The technical solution of the working condition judgment system for AC traction networks described above can achieve the same beneficial effects as the working condition judgment method for AC traction networks described above.

[0011] This invention also provides a PWM converter feedback control method suitable for AC traction networks, comprising: For cases where the AC single-phase side of the PWM converter is connected to the corresponding traction network, and the AC three-phase side is connected to an AC line used for power supply and power feedback, determine the voltage change value ΔU of the corresponding traction network for the PWM converter within the current time window. 1pu and the AC line voltage variation value ΔU used for power supply and power feedback 2pu The AC lines used for power supply and power feedback include the adjacent AC traction networks of the traction network. If △U 1pu -△U 2pu If the value is greater than 0 and less than or equal to the first set positive value, it is determined that the power grid voltage fluctuation caused the traction grid voltage fluctuation, and the starting voltage threshold is increased accordingly, and the PWM converter maintains the standby state. If △U 1pu -△U 2pu If the negative number is less than or equal to 0 and greater than the first set positive value, the starting voltage threshold will be reduced accordingly, and the PWM converter will remain in standby mode. If △U 1pu -△U 2pu If the value is greater than the second set positive value and less than the third set positive value, and the traction network voltage value corresponding to the PWM converter is greater than both the no-load voltage value of the traction network and the current starting voltage threshold, then the PWM converter is started to feed the energy of the traction network back to the AC line used for power supply and energy feedback; the first set positive value is less than the second set positive value, and the second set positive value is less than the third set positive value.

[0012] Furthermore, the first positive value is determined by calibrating based on the voltage fluctuation range of the power grid at the equipment site throughout the day; the second and third positive values ​​are determined by calibrating based on the converter's response speed or sensitivity at the site.

[0013] Furthermore, AC lines used for power supply and energy feedback also include AC power grids.

[0014] The above-described technical solution of this invention provides a novel PWM converter feedback control method suitable for AC traction networks. Its beneficial effects include: considering that the fluctuations in the local traction network caused by AC lines used for power supply and energy feedback are generally relatively small due to the indirect influence of the PWM converter, while the impact of locomotives on the local traction network is usually direct and therefore fluctuates significantly. Accordingly, a criterion is designed to distinguish between these two operating conditions. It also considers that locomotive traction or passing causes a decrease in the traction network voltage corresponding to the PWM converter, and locomotive braking causes a decrease in the traction network voltage corresponding to the PWM converter. Since the voltage of AC lines used for power supply and energy feedback is less affected by locomotives, the method distinguishes between different intervals corresponding to cases where ΔU1pu-ΔU2pu is less than 0 and greater than 0, thereby differentiating between locomotive traction or passing and locomotive braking. When the AC line voltage used for power supply and energy feedback fluctuates, the starting voltage threshold of the PWM converter is adaptively adjusted according to the sign of the fluctuation. This ensures that when the AC line voltage is high, the energy feedback threshold (i.e., the starting voltage threshold) is raised, thereby reducing energy feedback to that AC line; when the AC line voltage is low, the energy feedback threshold is lowered, thereby increasing energy feedback to that AC line. Based on the adaptively adjusted starting voltage threshold, energy feedback can be performed in a timely manner while taking into account the AC line's carrying capacity during normal locomotive braking. This avoids the PWM converter misjudging and forming a power circulating current with the Scott traction transformer, solving the stability and reliability issues in the engineering application of PWM converters. This allows for accurate control of the PWM rectifier's activation, achieving the goal of energy saving in the power supply system. The criteria are simple and accurate.

[0015] The present invention also provides a PWM converter feedback control system suitable for AC traction networks, including a processor, wherein the processor stores executable program instructions, which are executed to implement the above-described PWM converter feedback control method suitable for AC traction networks.

[0016] The technical solution of the PWM converter feedback control system for AC traction networks described above can achieve the same beneficial effects as the PWM converter feedback control method for AC traction networks described above. Attached Figure Description

[0017] Figure 1 This is a flowchart illustrating the PWM converter feedback control method applicable to AC traction networks in the implementation of the present invention. Figure 2 This is a schematic diagram illustrating the connection structure of the PWM converter in the embodiment of the PWM converter feedback control method applicable to AC traction networks according to the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0019] Implementation of PWM converter feedback control method applicable to AC traction network This embodiment presents a technical solution for a PWM converter feedback control method applicable to AC traction networks, the method comprising: Reference Figure 1 For cases where the AC single-phase side of a PWM converter is connected to the corresponding traction network, and the AC three-phase side is connected to an AC line used for power supply and power feedback, determine the voltage change value ΔU of the corresponding traction network for the PWM converter within the current time window. 1pu and the AC line voltage variation value ΔU used for power supply and power feedback 2pu The AC lines used for power supply and power feedback include the adjacent AC traction networks of the traction network. If △U 1pu -△U 2pu If the value is greater than 0 and less than or equal to the first set positive value δ1, it is determined that the power grid voltage fluctuation caused the traction grid voltage fluctuation, and the starting voltage threshold is increased accordingly, and the PWM converter maintains the standby state. If △U 1pu -△U 2pu If the value is less than or equal to 0 and greater than the opposite of the first set positive value δ1 (i.e., -δ1), then the starting voltage threshold is reduced accordingly in the opposite direction, and the PWM converter maintains the standby state. If △U 1pu -△U 2pu If the value is greater than the second set positive value δ2 and less than the third set positive value δ3, and the traction network voltage value corresponding to the PWM converter is greater than both the no-load voltage value of the traction network and the current start-up voltage threshold, then the PWM converter is started to feed the energy of the traction network back to the AC line used for power supply and energy feedback; the first set positive value δ1 is less than the second set positive value δ2, and the second set positive value δ2 is less than the third set positive value δ3.

[0020] Therefore, considering that the AC lines used for power supply and energy feedback, which are indirectly affected by the PWM converter, typically cause relatively small fluctuations in the local traction network, while the locomotive's impact on the local traction network is usually direct and therefore causes larger fluctuations, a criterion was designed to distinguish between these two operating conditions. It also considers that locomotive traction or passage causes a decrease in the traction network voltage corresponding to the PWM converter, and locomotive braking causes a decrease in the traction network voltage corresponding to the PWM converter. The voltage of the AC lines used for power supply and energy feedback is less affected by the locomotive; therefore, by distinguishing ΔU...1pu -△U 2pu Different intervals correspond to conditions less than 0 and greater than 0, distinguishing between locomotive traction or passage and locomotive braking. Furthermore, when the AC line voltage used for power supply and energy feedback fluctuates, the starting voltage threshold of the PWM converter is adaptively adjusted based on the sign of the fluctuation. This ensures that when the AC line voltage is high, the energy feedback threshold (starting voltage threshold) is raised, and when the AC line voltage is low, the energy feedback threshold is lowered, preventing circulating currents between the AC grid and the traction network. Based on this adaptively adjusted starting voltage threshold, energy feedback can be performed promptly while maintaining the AC line's carrying capacity during normal locomotive braking. This avoids the PWM converter misjudging and forming power circulating currents with the Scott traction transformer, solving the stability and reliability issues in PWM converter engineering applications. This allows for accurate control of the PWM rectifier's activation, achieving energy saving in the power supply system. The criteria are simple and accurate.

[0021] In this embodiment, the first positive value δ1 is determined by calibrating based on the voltage fluctuation range of the power grid at the equipment site throughout the day; the second positive value δ2 and the third positive value δ3 are determined by calibrating based on the response speed or sensitivity of the converter at the site.

[0022] Furthermore, in this embodiment, the AC lines used for power supply and power feedback also include the AC power grid.

[0023] Then refer to Figure 2 In one specific embodiment, the AC single-phase side of the PWM converter can be connected to the traction network corresponding to the PWM converter through a transformer or directly, while the AC three-phase side can be connected to the AC power grid (or adjacent traction network) through a transformer. The traction grid voltage and the AC grid (or adjacent traction grid) voltage are measured twice each within the time sliding window, and the corresponding traction grid voltage change value ΔU is calculated. 1pu The voltage change value ΔU of the AC power grid (or adjacent traction network) 2pu ; If 0 < △U 1pu -△U 2pu If ≤δ1, then the starting voltage threshold used to start the PWM converter is increased in the positive direction, and the PWM converter remains in standby state; If -δ1≤△U 1pu -△U 2pu If the value is ≤0, the starting voltage threshold will be reduced in the reverse direction, and the PWM converter will remain in standby mode.

[0024] If the voltage change value ΔU 1pu -△U 2pu >0, δ2<|△U 1pu -△U2pu If |≤δ3, the traction grid voltage is greater than the no-load voltage of the traction grid and the traction grid voltage is greater than the current starting voltage threshold, then the PWM converter is started to feed the energy of the traction grid back to the AC grid (or adjacent traction grid).

[0025] In practice, the initial value of the startup voltage threshold is a preset value, and it is adaptively adjusted based on the aforementioned positive increase and negative decrease scenarios. The condition that must be met for positively increasing the startup voltage threshold to start the PWM converter is (0 < ΔU). 1pu -△U 2pu ≤δ1) corresponds to the situation where voltage fluctuations in the AC lines used for power supply and energy feedback cause voltage fluctuations in the traction network. In this case, the voltage of the AC lines used for power supply and energy feedback is too high. Considering that feeding energy back to these AC lines will further increase their voltage, the starting voltage threshold for the PWM converter is increased in the positive direction to raise the threshold for energy feedback, thus minimizing unnecessary energy feedback and preventing excessively high voltage on the AC lines. Similarly, the condition that must be met to decrease the starting voltage threshold for the PWM converter in the reverse direction is (-δ1≤△U). 1pu -△U 2pu ≤0) corresponds to the situation where the voltage fluctuation of the AC line used for power supply and power feedback causes the voltage fluctuation of the traction network. However, at this time, the voltage of the AC line used for power supply and power feedback is low, which can accept more power feedback. Therefore, by reducing the starting voltage threshold of the PWM converter in reverse, the threshold for starting the PWM converter for power feedback is lowered, making the subsequent power feedback more timely.

[0026] Additionally, △U 1pu -△U 2pu Less than 0, and δ1 < |ΔU 1pu -△U 2pu In the case of |, no energy feedback or adjustment of the starting voltage threshold will be performed for the time being, and the situation where locomotive traction starting or passing causes voltage fluctuations in the traction network (i.e., ΔU) will be determined. 1pu -△U 2pu Less than 0, and δ1 < |ΔU 1pu -△U 2pu (In the case where |≤δ2) Energy feedback and adjustment of the starting voltage threshold are not considered.

[0027] Implementation of PWM converter feedback control system for AC traction network This embodiment provides a technical solution for a PWM converter feedback control system applicable to AC traction networks, including a processor containing executable program instructions. The executable program instructions are executed to implement the operating condition judgment method applicable to AC traction networks described above.

[0028] Since the working mode and principle of the PWM converter feedback control system applicable to AC traction grid in this embodiment have been described in detail in the above-described PWM converter feedback control embodiment applicable to AC traction grid, they will not be repeated here.

[0029] Implementation method for judging working conditions of AC traction networks This embodiment provides a technical solution for a method of judging the working condition of an AC traction network, the method comprising: For cases where the AC single-phase side of the PWM converter is connected to the corresponding traction network, and the AC three-phase side is connected to an AC line used for power supply and power feedback, determine the voltage change value ΔU of the corresponding traction network for the PWM converter within the current time window. 1pu and the AC line voltage variation value ΔU used for power supply and power feedback 2pu The AC lines used for power supply and power feedback include the adjacent AC traction networks of the traction network. If △U 1pu -△U 2pu If the absolute value is less than or equal to the first set positive value, it is determined that the voltage fluctuation of the AC line used for power supply and power feedback causes the voltage fluctuation of the traction network. If △U 1pu -△U 2pu If the value is less than 0 and the absolute value is greater than the first set positive value but less than the second set positive value, then it is determined that the locomotive traction start or passing by has caused voltage fluctuations in the traction network; the first set positive value is less than the second set positive value. If △U 1pu -△U 2pu If the value is greater than the second set positive value and less than the third set positive value, it is determined that the locomotive braking caused the traction network voltage fluctuation; the second set positive value is less than the third set positive value.

[0030] Therefore, the train's operating status can be accurately determined by observing the voltage changes in the traction network connected to the PWM converter and the AC lines used for power feedback. Considering that the fluctuations in the local traction network caused by the AC lines used for power supply and power feedback are usually relatively small due to the indirect influence of the PWM converter, while the locomotive's impact on the local traction network is usually direct and therefore larger, a criterion was designed to distinguish between these two operating conditions. It also considers that locomotive traction or passing causes a decrease in the traction network voltage corresponding to the PWM converter, and locomotive braking causes a decrease in the traction network voltage corresponding to the PWM converter. The voltage of the AC lines used for power supply and power feedback is less affected by the locomotive; therefore, by distinguishing ΔU... 1pu -△U 2pu The conditions less than 0 and greater than 0 correspond to different intervals to distinguish between locomotive traction or passage and locomotive braking. This allows for more accurate processing of the train's operating conditions, and the criteria are simple and accurate.

[0031] In this embodiment, the first positive value δ1 is determined by calibrating it according to the voltage fluctuation range of the power grid at the equipment site throughout the day; the second positive value δ2 and the third positive value δ3 are determined by calibrating them according to the response speed or sensitivity of the converter at the site.

[0032] Furthermore, in this embodiment, the AC line used for power feedback also includes an AC power grid.

[0033] In one specific implementation, the AC single-phase side of the PWM converter can be connected to the traction network corresponding to the PWM converter through a transformer or directly, while the AC three-phase side can be connected to the AC power grid (or adjacent traction network) through a transformer. The traction grid voltage and the AC grid (or adjacent traction grid) voltage are measured twice each within the time sliding window, and the corresponding traction grid voltage change value ΔU is calculated. 1pu The voltage change value ΔU of the AC power grid (or adjacent traction network) 2pu ; Compare the voltage change value ΔU in the traction network 1pu The voltage change value ΔU of the AC power grid (or adjacent traction network) 2pu Determine the system operating condition based on the preset conditions of the control system; where, if |△U 1pu -△U 2pu If |≤δ1, then it is determined that the voltage fluctuation of the traction network is caused by the voltage fluctuation of the AC line used for power supply and power feedback. If △U 1pu -△U 2pu <0, and δ1 <|△U 1pu -△U 2pu If |≤δ2, it is determined that the traction network voltage fluctuation is caused by the locomotive starting or passing through; If △U 1pu -△U 2pu >0, and δ2 < |ΔU 1pu -△U 2pu |≤δ3 (that is, δ2<△U) 1pu -△U 2pu If the value is less than or equal to δ3, then it is determined that the voltage fluctuation in the traction network is caused by the locomotive braking.

[0034] Implementation of a Condition Judgment System Applicable to AC Traction Networks This embodiment provides a technical solution for a working condition judgment system applicable to AC traction networks, including a processor. The processor stores executable program instructions, which are executed to implement the working condition judgment method applicable to AC traction networks described above.

[0035] Since the working mode and principle of the working condition judgment system applicable to AC traction network in this embodiment have been described in detail in the above-described working condition judgment method embodiment applicable to AC traction network, they will not be repeated here.

[0036] It should be understood that the above-described specific embodiments of the present invention are merely illustrative or explanatory of the principles of the present invention, and do not constitute a limitation thereof.

Claims

1. A method for determining the operating condition of an AC traction power network, characterized by Comprise: For the case that the PWM converter AC single-phase side is connected to the traction network corresponding to the PWM converter, and the AC three-phase side is connected to the AC line for power supply and electric quantity feedback, the traction network voltage change value ΔU 1pu corresponding to the PWM converter in the current time window and the AC line voltage change value ΔU 2pu for power supply and electric quantity feedback are determined respectively; the AC line for power supply and electric quantity feedback comprises a neighboring AC traction network of the traction network. If the absolute value of △U 1pu -△U 2pu is less than or equal to a first positive set value, it is determined that the AC line voltage fluctuation for power supply and power feedback causes traction network voltage fluctuation. If △U 1pu -△U 2pu is less than 0 and the absolute value is greater than a first positive set value and less than a second positive set value, it is determined that the locomotive is starting traction or passing through, causing traction network voltage fluctuations; the first positive set value is less than the second positive set value. if ΔU 1pu - ΔU 2pu greater than a second positive set value and less than a third positive set value, it is determined that the locomotive braking causes the traction network voltage to fluctuate; the second positive set value being less than the third positive set value.

2. The method for determining the operating condition of the AC traction power network according to claim 1, characterized in that The determination mode of the first set positive value is calibration according to the whole-day voltage fluctuation range of the on-site power grid; the determination mode of the second and third set positive values is calibration according to the response speed or sensitivity of the on-site converter.

3. The method for determining the operating condition of an AC traction power network according to claim 1 or 2, characterized in that The AC line for power supply and power feedback further comprises an AC power grid.

4. A working condition judging system for an alternating current traction network, comprising a processor, in which executable program instructions are stored, characterized in that, The executable program instructions are used to be executed to realize the working condition judgment method suitable for the AC traction network according to any one of claims 1-3.

5. A PWM converter feedback control method suitable for an alternating current traction network, characterized in that, Comprise: For the case that the PWM converter AC single-phase side is connected to the traction network corresponding to the PWM converter, and the AC three-phase side is connected to the AC line for power supply and electric quantity feedback, the traction network voltage change value ΔU corresponding to the PWM converter in the current time window is determined respectively 1pu and the AC line voltage change value ΔU for power supply and electric quantity feedback 2pu ; the AC line for power supply and electric quantity feedback comprises adjacent AC traction networks of the traction network; If △U 1pu -△U 2pu greater than 0 and less than or equal to the first set positive value, it is determined that the power supply network voltage fluctuation causes the traction network voltage fluctuation, the starting voltage threshold is increased in the corresponding positive direction, and the PWM converter maintains the standby state. if △U 1pu -△U 2pu the inverse of a value less than or equal to 0 and greater than a first set positive value, then the corresponding reverse reduction of the start-up voltage threshold, the PWM inverter maintains the standby state; If △U 1pu -△U 2pu If the PWM converter corresponding traction net voltage value is greater than the no-load voltage value of the traction net and the current starting voltage threshold value, start the PWM converter, and feed the energy of the traction net back to the AC line for power supply and energy feedback. The first positive value is less than the second positive value, and the second positive value is less than the third positive value.

6. The PWM converter feedback control method for an AC traction power network as claimed in claim 5, characterized in that, The determination mode of the first set positive value is calibration according to the whole-day voltage fluctuation range of the on-site power grid; the determination mode of the second and third set positive values is calibration according to the response speed or sensitivity of the on-site converter.

7. The PWM converter feedback control method for AC traction power network according to claim 5, characterized in that, The AC line for power supply and power feedback further comprises an AC power grid.

8. A PWM converter feedback control system suitable for use with an AC traction network, comprising a processor having executable program instructions stored therein, wherein: The executable program instructions are used to be executed to realize the PWM converter feedback control method suitable for the AC traction network according to any one of claims 5-7.

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