Flexible live-transfer phase-separation system based on common DC capacitor multi-level converter

Abstract

The present invention discloses a flexible live phase-splitting system based on a common DC side capacitor type multi-level converter, wherein the electronic power converter has n converter bridge arms to define n converter bridge arm AC terminals, and the electronic power converter is configured as follows: the n converter bridge arms can be divided into a first converter bridge arm group and a second converter bridge arm group according to their functions, and the first converter bridge arm group and the second converter bridge arm group can be electrically coupled through a DC side capacitor, wherein: when the number of converter bridge arms included in the first converter bridge arm group is two or three, the first converter bridge arm group can be electrically connected to the traction substation via a first single-phase transformer and a second single-phase transformer. Compared with the existing solutions, the present application has a smaller total number of components, a lower hardware cost, and no hot backup loss in the system after optimizing the specific circuit structure and the number of levels.

Description

Technical Field

[0001] The present invention relates to the field of electrified railway over-phase technology and power quality management, and in particular to an AC electrified railway live over-phase and negative sequence and reactive power compensation system based on a high-power power electronic converter. Background Art

[0002] my country's AC electrified railways mainly use single-phase power frequency phase-changing power supply. In order to avoid interphase short circuit, a section of overhead wire with electrical segments at both ends is set at the exit of the traction substation and the partition booth, that is, electrical phase separation. Electrical phase separation is mainly divided into two types: device type and joint type, and the joint type is the most common. The joint type electrical phase separation consists of a neutral zone and joints at both ends of the anchor section. Since the neutral zone is not electrified, special operations are required for the on-board equipment or ground devices when the locomotive passes. At present, my country generally adopts the on-board power-off automatic phase separation scheme, and the train relies on inertia to slide through the neutral zone. Since the neutral zone is not powered, this scheme causes serious loss of traction and speed loss of the train, which seriously restricts the development of high-speed heavy-load railways. In order to ensure that the train does not lose traction in the neutral zone and that the train continues to be powered in the neutral zone, a variety of "live automatic phase separation system" schemes have been proposed one after another. For example, the ground live automatic phase separation system based on mechanical switches can greatly shorten the power-off time when the train passes the phase separation. However, due to the short life of mechanical switches, high failure rate, and inability to accurately control the action time, this solution still has problems such as long power outage time, overvoltage, and overcurrent.

[0003] For example, in response to the many defects of the ground automatic phase-shifting scheme based on mechanical switches, relevant institutions have carried out research on the ground live automatic phase-shifting system that uses high-voltage thyristor valve groups to replace mechanical switches in recent years. This scheme uses the natural shutdown characteristics of the thyristor current zero-crossing to avoid overvoltage and effectively suppress arcs, and can accurately control the opening time, thereby further reducing the power-off time when the train passes the phase. However, due to the different voltage phases of the two adjacent power supply arms, this scheme still cannot achieve uninterrupted power-off. In this scheme, there is a short power-off time when the voltage switches from one power supply arm to another. Although it has no effect on AC-DC electric locomotives such as the Shaoshan series in my country, it will cause the protection of AC-DC-AC electric locomotives mainly in the Harmony series.

[0004] In order to solve the various shortcomings of the existing on-board automatic power-off phase-splitting devices and the ground automatic phase-splitting devices based on mechanical switches or electronic switches, the live phase-splitting scheme based on back-to-back power electronic converters has gradually attracted attention. Figure 1As shown. After the train enters the neutral zone, the converter draws power through the power supply arm on one side, and the voltage output to the contact network in the neutral zone gradually shifts the phase from the voltage of the power supply arm on one side to the voltage of the power supply arm on the other side. This scheme can enable the train to pass through the phase-splitting area smoothly without power outage. When the train passes through the electrical phase-splitting area, there is no need for the on-board main circuit breaker to operate, no cut-off overvoltage, no train main transformer excitation surge current, and no traction and speed loss. The back-to-back power electronic converter of the energized phase-splitting device in this scheme requires four phase bridge arms composed of power electronic switches, but the actual topology of the power electronic converter is not clearly given. In addition, the time when the locomotive passes through the neutral zone is very short, so the device does not work most of the time, which greatly reduces the utilization rate of the device.

[0005] Existing power electronic converters can be divided into two-level and multi-level structures. The basic two-level converter is limited by the withstand voltage of existing switching devices and is not suitable for high voltage applications. However, if the withstand voltage is improved by connecting switching devices in series, there will be a problem of dynamic voltage imbalance between the switching devices in series, and a complex hardware voltage equalization circuit needs to be added. In addition, the output harmonic characteristics of the two-level converter are poor and the electromagnetic noise is large, which limits its application. The multi-level converter solves these problems well and has the following advantages: 1) It breaks through the voltage level limitation of existing power semiconductor devices, avoids the dynamic voltage equalization problem caused by direct series connection of devices, and is suitable for high-voltage and large-capacity applications; 2) The equivalent switching frequency is low, which reduces the switching loss of the device; 3) The output level steps are many, the harmonic characteristics are good, and the volume of the filter is reduced; 4) The output dv / dt is small, the switching noise is small, and the electromagnetic interference of the system is effectively reduced. Therefore, the introduction of the multi-level converter topology is more conducive to the realization of the function of the flexible live over-phase system.

[0006] In existing solutions, modular multilevel converters (MMC) are widely used as power electronic converters. Figure 2It is a flexible live phase-splitting system based on MMC. The three-phase MMC structure is adopted in this scheme, and the output ends are respectively connected to the power supply arms on both sides and the neutral zone. In addition to realizing the phase-splitting function, the power quality management function can also be performed during the period when there is no locomotive passing through the neutral zone, thereby improving the utilization rate of the device. However, MMC adopts a multi-phase distributed capacitor structure, and the module contains a large number of suspended capacitors. The DC side midpoint cannot be directly led out, and an additional set of DC side capacitor bridge arms needs to be added. In addition, the proportion of time when there are locomotives in the neutral zone is quite low, so most of the time the bridge arm connected to the neutral zone does not play a role. However, the bridge arm submodule needs to be in a hot backup state at all times to keep the voltage of the submodule capacitor stable, which will cause unnecessary losses. It can be seen that the topologies adopted in the existing schemes have inherent disadvantages, so it is urgent to introduce new multi-level converter topologies in the flexible live phase-splitting system.

[0007] Figure 3 This is a simplified schematic diagram of a multi-level converter with a common DC side capacitor. The DC bus of the converter consists of multiple cascade capacitors. Assuming that the rated values ​​of the voltages of each capacitor are the same, the DC bus is divided into multiple levels, and each node represents a different level. The path between the DC bus node and the AC terminal (referred to as the path) is composed of a fully controlled switching device or a diode, which is equivalent to a single-pole multi-throw switch in principle. The differences between various types of multi-level converters with a common DC side capacitor are mainly reflected in the different composition structures of the path. By controlling the on and off of the device, the AC port can be clamped to different DC side voltage levels. Figure 4 Two typical common DC side capacitor type multilevel converters are shown. Figure 4 (a) is a traditional diode clamped multilevel converter. Figure 4 (b) is a new type of V-clamped multilevel converter. Figure 5 by Figure 4 Taking the topology in (a) as an example, it shows the connection method of sharing DC link capacitors among multiple phases.

[0008] According to the topology of the shared DC side capacitor type multi-level converter, it is not difficult to find that this type of topology has the following characteristics: 1) It does not require multiple independent DC power supplies and has a simple structure; 2) It does not contain floating capacitors, and the pre-charging of the shared DC side capacitor type multi-level converter is simpler, and the switch control is easy to implement; 3) Each phase shares the DC side capacitor, and the number of capacitors required in the multi-phase system is small, and the overall control complexity is low. It can be seen that the shared DC side capacitor type topology makes up for the shortcomings of MMC very well, and therefore has certain application value in the field of electrified railways.

[0009] This application is preferred Figure 4 The V-clamped multilevel converter in (b) is compared with the traditional common DC side capacitor type multilevel converter, e.g. Figure 4(a) shows the diode-clamped multilevel converter. The total number of components of the V-clamped multilevel converter is significantly reduced, the circuit topology is relatively simple, and the switching devices do not have the problem of dynamic voltage balancing. Compared with the modular multilevel converter, the V-clamped multilevel converter has certain advantages in the total number of components when the level is thirteen or less, and does not require a bridge arm inductor.

[0010] However, the common DC side capacitor type multi-level topology also has its inherent defects. If no voltage balancing measures are taken, some capacitors will be continuously charged or discharged, and their capacitor voltage will seriously deviate from the rated voltage value. This will eventually cause the number of levels of the AC output voltage to degrade. This shortcoming seriously limits the practical application of this type of converter, so it is necessary to select an appropriate voltage balancing method.

[0011] In summary, the present application aims to provide a new type of flexible live phase-transfer system based on a common DC side capacitor-type multi-level converter, so as to ensure that the train passes through the phase-transfer smoothly, without power outages and speed loss during use, and to achieve power quality management throughout the entire train travel process, and compensate for the negative sequence current and reactive current on the grid side. Summary of the invention

[0012] One of the technical problems to be solved by the present invention is the voltage balancing problem of the DC side capacitor. Determine a suitable capacitor voltage balancing strategy, such as zero-sequence voltage injection method, auxiliary voltage balancing circuit method, carrier overlap modulation method, and design the system structure according to the voltage balancing method, including the number of bridge arms of the converter, the connection method, the configuration method of the DC side capacitor midpoint, etc. Make the system structure match the voltage balancing strategy, so as to achieve capacitor voltage balancing at a lower cost.

[0013] The second technical problem to be solved by the present invention is to migrate the existing mature control strategy and timing logic to this system on the basis of the capacitor voltage balancing that has been achieved, and make appropriate modifications to adapt it to the flexible live phase-transition system based on the common DC side capacitor type multi-level converter. Ensure that the system can achieve the following functions: 1) By controlling the converter bridge arm connected to the neutral zone, the power supply source of the train is smoothly transitioned from the first power supply arm to the second power supply arm, so that the train can pass the phase without power outage and at full power. 2) When the train passes the phase or does not enter the neutral zone, the reactive current and negative sequence current to be compensated are obtained by collecting and calculating the grid-side current, and the converter is controlled to inject the corresponding current into the traction network, thereby eliminating the negative sequence and reactive current at the connection point between the traction substation and the grid as much as possible.

[0014] The object of the present invention is achieved through the following technical solutions: A flexible live phase-transition system based on a common DC side capacitor type multi-level converter, comprising a main circuit located between a traction substation and a neutral zone, wherein the main circuit comprises an electronic power converter, at least three single-phase transformers, a plurality of filter inductors and filter capacitors, wherein the electronic power converter has n converter bridge arms to define n converter bridge arm AC terminals, and the electronic power converter is configured as follows: the n converter bridge arms can be divided into a first converter bridge arm group and a second converter bridge arm group according to their functions, wherein the first converter bridge arm group The first converter arm group and the second converter arm group can be electrically coupled through a DC side capacitor, wherein: when the first converter arm group includes two or three converter arms, the first converter arm group can be electrically connected to the traction substation via a first single-phase transformer and a second single-phase transformer; when the second converter arm group includes one or two converter arms, the second converter arm group can be electrically connected to the neutral zone via a third single-phase transformer; the converter arm and the single-phase transformer can be electrically connected through a filter inductor.

[0015] Preferably, when the power electronic converter has four converter bridge arms to define the converter first bridge arm AC terminal, the converter second bridge arm AC terminal, the converter third bridge arm AC terminal and the converter fourth bridge arm AC terminal, the first terminal of the high-voltage side winding of the first single-phase transformer is connected to the first power supply arm, the first terminal of the high-voltage side winding of the second single-phase transformer is connected to the second power supply arm, the first terminal of the low-voltage side winding of the first single-phase transformer is connected to the converter first bridge arm AC terminal via the first filter inductor, the first terminal of the low-voltage side winding of the second single-phase transformer is connected to the converter third bridge arm AC terminal via the third filter inductor, the first terminal of the low-voltage side winding of the third single-phase transformer is connected to the converter fourth bridge arm AC terminal via the fourth filter inductor, the filter capacitor is connected between the first terminal and the second terminal of the low-voltage side winding of the third single-phase transformer, the first terminal of the high-voltage side winding of the third single-phase transformer is connected to the neutral zone, and the second terminal of the high-voltage side winding of the third single-phase transformer is connected to the rail.

[0016] Preferably, the second terminal of the low-voltage side winding of the first single-phase transformer and the second terminal of the low-voltage side winding of the second single-phase transformer are simultaneously connected to the second bridge arm AC terminal of the converter via the second filter inductor, and the second terminal of the low-voltage side winding of the third single-phase transformer is connected to the midpoint of the DC side capacitor.

[0017] Preferably, the power electronic converter can have three converter bridge arms to define a first bridge arm AC terminal of the converter, a third bridge arm AC terminal of the converter and a fourth bridge arm AC terminal of the converter, wherein the second terminal of the low-voltage side winding of the first single-phase transformer and the second terminal of the low-voltage side winding of the second single-phase transformer are grounded at the same time, and the second terminal of the low-voltage side winding of the third single-phase transformer and the midpoint of the DC side capacitor are grounded at the same time.

[0018] Preferably, the power electronic converter can have 5 converter bridge arms to further define the fifth bridge arm AC terminal of the converter, wherein the second terminal of the low-voltage side winding of the first single-phase transformer and the second terminal of the low-voltage side winding of the second single-phase transformer are simultaneously connected to the second bridge arm AC terminal via the second filter inductor, and the second terminal of the low-voltage side winding of the third transformer is connected to the fifth bridge arm AC terminal of the converter.

[0019] Preferably, the DC side capacitor comprises at least two capacitors connected in series, and the arm switches of the electronic power converter are defined by fully controlled devices including insulated gate bipolar transistors or integrated gate-commutated thyristors.

[0020] Preferably, when the first converter bridge arm group has three converter bridge arms, the DC side capacitor voltage is voltage balanced according to the following steps: calculate the value range of the zero-sequence voltage component, calculate the corresponding cost function for different zero-sequence components, select a zero-sequence component that can make the cost function approach the minimum value, superimpose it on the original reference voltage, and obtain the final reference voltage.

[0021] Preferably, the value range of the zero-sequence voltage component is determined as follows: the first reference voltage of the converter bridge arm and the second reference voltage after adding the zero-sequence voltage component in the previous cycle are determined according to the control target, and the maximum value v of the zero-sequence voltage is obtained in a way to avoid overmodulation. z_max With the minimum value v z_min To determine the value range of the zero-sequence voltage component.

[0022] Preferably, the cost function F can be expressed by the formula In other words, ΔU Cj =i Cj T s / C dc , ΔU Cj After a switching cycle T s The change in the voltage of each capacitor after dc is the capacitance of the DC side capacitor, j represents 1 to N L An integer between -1, N L is the number of converter levels, i Cj is the capacitor current.

[0023] Preferably, before the train enters the first anchor section joint from the first power supply arm, the flexible live phase-splitting system is only in the power quality management working state; when the train is about to enter the first anchor section joint from the first power supply arm, the electronic power converter adjusts the output voltage so that the neutral zone contact network voltage is the same as the first power supply arm voltage, and power quality management is performed at the same time; when the train enters the first anchor section joint from the first power supply arm, the electronic power converter adjusts the output current, and the current supplied by the neutral zone contact network is gradually increased to the train load current, causing the current provided by the first power supply arm contact network to gradually decrease to 0, and power quality management is performed at the same time; after the relevant pantograph of the train completely enters the neutral zone, the converter adjusts the output voltage, The neutral zone contact network voltage is gradually transferred from the first power supply arm voltage to the second power supply arm voltage, and power quality management is performed at the same time; before the train travels to the second anchor section joint in the neutral zone, the electronic power converter has made the neutral zone contact network voltage the same as the second power supply arm voltage, and power quality management is performed at the same time; when the train enters the second anchor section joint from the first power supply arm, the electronic power converter adjusts the output current, and the current supplied by the neutral zone contact network is gradually reduced to 0, so that the current provided by the second power supply arm contact network to the train is gradually increased to the train load current, and power quality management is performed at the same time; after the train leaves the second anchor section joint and enters the second power supply arm, the flexible live phase separation system is only in the power quality management working state. Power quality management refers to the use of the first converter bridge arm group to inject a certain current into the power supply arm to achieve the function of eliminating the reactive power and negative sequence current components of the traction power supply system in the traction power supply system.

[0024] The present invention has the following advantages:

[0025] (1) The present invention can completely realize the phase transition without power outage. When the locomotive passes through the neutral zone, the contact network voltage in the neutral zone is gradually and smoothly transferred from the power supply arm on one side to the power supply arm on the other side, ensuring that the train continues to receive current and passes through the phase transition smoothly, effectively avoiding abnormal situations such as traction loss and undervoltage protection when the train passes through the phase transition; the present invention also has the function of power quality management, which can compensate for the negative sequence current and reactive current of the traction network.

[0026] (2) Compared with the flexible live phase separation scheme using MMC, the common DC side capacitor type multi-level converter adopted in the present invention can reduce the total number of components by optimizing the specific circuit structure and the number of levels, thereby reducing the hardware cost and significantly reducing the number of required capacitors. The control method is simpler, and the bridge arm connected to the neutral zone does not require hot backup, thereby reducing the loss of the system.

[0027] (3) In the proposed specific system structure, the voltage balance problem of the DC side capacitor can be well solved through control, which maximizes the advantages of this type of topology. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the structure of a live over-phase system based on power electronic converter;

[0029] Figure 2 It is a schematic diagram of the structure of a flexible live phase-by-phase system based on modular multi-level converter;

[0030] Figure 3 It is the schematic diagram of the common DC side capacitor type multi-level converter;

[0031] Figure 4 These are two topology examples of common DC capacitor multilevel converters. Figure 4 (a) is a diode clamped multilevel converter. Figure 4 (b) is a V-clamped multilevel converter;

[0032] Figure 5 It is a connection method for sharing DC side capacitors among multiple phases in a multi-level converter topology;

[0033] Figure 6 It is a schematic diagram of a first embodiment of the structure of a flexible live-transition phase-splitting system based on a common DC side capacitor type multi-level converter according to the present invention;

[0034] Figure 7 It is a marked diagram of the structure of the flexible live-transition phase separation system embodiment 1 based on the common DC side capacitor type multi-level converter of the present invention;

[0035] Figure 8 This is a rectifier side control block diagram of the flexible live-transition phase-splitting system structure embodiment 1 of the present invention based on the common DC side capacitor type multi-level converter; wherein Figure 8 (a) is a control block diagram of the first and third bridge arms (10, 12), Figure 8 (b) is a control block diagram of the second bridge arm (11), Figure 8 (c) is the block diagram for reactive power and negative sequence current calculation;

[0036] Fig. 9 The inverter side control block diagram of the first embodiment of the flexible live-transition phase separation system structure based on the common DC side capacitor type multi-level converter of the present invention; wherein Fig. 9 (a) is the current control block diagram, Fig. 9 (b) is the voltage control block diagram.

[0037] Fig.10 The simulation results of the first embodiment of the flexible live-transition phase separation system structure based on the common DC side capacitor type multi-level converter of the present invention are shown in FIG. Fig.10 (a) is the DC side capacitor voltage waveform, Fig.10 (b) is the voltage waveform in the neutral zone during the phase shift process. Fig.10(c) is the current waveform of the three-phase power system. Fig.10 (d) is the current transfer waveform;

[0038] Fig.11 It is a structural schematic diagram of Embodiment 2 of the flexible live-transition phase-splitting system based on a common DC side capacitor type multi-level converter of the present invention;

[0039] Fig.12 (a) is a schematic diagram of the main circuit structure of Embodiment 3 of the flexible live-transfer phase-splitting system based on a common DC side capacitor-type multi-level converter according to the present invention; Fig.12 (b) is a schematic diagram of the auxiliary voltage balancing circuit structure of the third embodiment of the flexible live phase-splitting system based on the common DC side capacitor type multi-level converter of the present invention;

[0040] Fig.13 Schematic diagram of phase voltage of the carrier overlap PWM voltage balancing method used in the fourth embodiment of the flexible live phase-splitting system based on the common DC side capacitor type multi-level converter of the present invention, wherein Fig.13 (a) is the phase voltage when the reference voltage is greater than or equal to zero, Fig.13 (b) is the phase voltage when the reference voltage is less than zero.

[0041] In the figure, 1: the first power supply arm; 2: the second power supply arm; 3: the neutral zone; 4: the rail; 5: the first anchor section joint; 6: the second anchor section joint; 7: the train; 8: the traction substation; 9: the DC side capacitor; 10: the first bridge arm of the converter; 11: the second bridge arm of the converter; 12: the third bridge arm of the converter; 13: the fourth bridge arm of the converter; 14: the first single-phase transformer; 15: the second single-phase transformer; 16: the first filter inductor; 17: the second filter inductor; 18: the third filter inductor; 19: the fourth filter inductor; 20: the filter capacitor; 21: the third single-phase transformer; 22: the midpoint of the DC side capacitor; 23: the first terminal of the high-voltage side winding of the first single-phase transformer; 24: the second terminal of the high-voltage side winding of the first single-phase transformer; 25: the first terminal of the low-voltage side winding of the first single-phase transformer; 26: the low-voltage side winding of the first single-phase transformer 27: the second terminal of the high-voltage side winding of the second single-phase transformer; 28: the first terminal of the high-voltage side winding of the second single-phase transformer; 29: the second terminal of the low-voltage side winding of the second single-phase transformer; 30: the first terminal of the low-voltage side winding of the second single-phase transformer; 31: the second terminal of the low-voltage side winding of the third single-phase transformer; 32: the first terminal of the low-voltage side winding of the third single-phase transformer; 33: the second terminal of the high-voltage side winding of the third single-phase transformer; 34: the first terminal of the high-voltage side winding of the third single-phase transformer; 35: the AC terminal of the first bridge arm of the converter; 36: the AC terminal of the second bridge arm of the converter; 37: the AC terminal of the third bridge arm of the converter; 38: the AC terminal of the fourth bridge arm of the converter; 39: the AC terminal of the fifth bridge arm of the converter; 40: the fifth bridge arm of the converter; 41: the resonant branch of the auxiliary voltage-equalizing circuit; 42: the switch network of the auxiliary voltage-equalizing circuit. DETAILED DESCRIPTION

[0042] The present invention is further described below in conjunction with the accompanying drawings, and the protection scope of the present invention is not limited to the following:

[0043] like Figures 6 to 13As shown, the present application provides a flexible live phase-splitting system based on a common DC side capacitor type multi-level converter, including a main circuit located between a traction substation 8 and a neutral zone 3, the main circuit including an electronic power converter, at least three single-phase transformers, a plurality of filter inductors and filter capacitors 20, the electronic power converter having n converter bridge arms to define n converter bridge arm AC terminals, the electronic power converter is configured as follows: the n converter bridge arms can be divided into a first converter bridge arm group and a second converter bridge arm group according to a set number ratio, the first converter bridge arm group and the The second converter bridge arm group can be electrically coupled through the DC side capacitor 9, wherein: when the first converter bridge arm group includes two or three converter bridge arms, the first converter bridge arm group can be electrically connected to the traction substation via the first single-phase transformer 14 and the second single-phase transformer 15; when the second converter bridge arm group includes one or two converter bridge arms, the second converter bridge arm group can be electrically connected to the neutral zone via the third single-phase transformer 21; the converter bridge arm and the single-phase transformer can be electrically connected through a filter inductor.

[0044] Preferably, when the power electronic converter has four converter bridge arms to define the converter first bridge arm AC terminal 35, the converter first bridge arm AC terminal 36, the converter third bridge arm AC terminal 37 and the converter fourth bridge arm AC terminal 38, the first terminal 23 of the high-voltage side winding of the first single-phase transformer is connected to the first power supply arm 1, the first terminal 28 of the high-voltage side winding of the second single-phase transformer is connected to the second power supply arm 2, the first terminal 25 of the low-voltage side winding of the first single-phase transformer is connected to the first bridge arm AC terminal 35 via the first filter inductor 16, the first terminal 30 of the low-voltage side winding of the second single-phase transformer is connected to the third bridge arm AC terminal 37 via the third filter inductor 18, the first terminal 32 of the low-voltage side winding of the third single-phase transformer is connected to the converter fourth bridge arm AC terminal 38 via the fourth filter inductor 19, a filter capacitor 20 is connected between the first terminal 32 and the second terminal 31 of the low-voltage side winding of the third single-phase transformer, the first terminal 34 of the high-voltage side winding of the third single-phase transformer is connected to the neutral zone 3, and the second terminal 33 of the high-voltage side winding of the third single-phase transformer is connected to the rail 4.

[0045] Preferably, the second terminal 26 of the low-voltage side winding of the first single-phase transformer and the second terminal 29 of the low-voltage side winding of the second single-phase transformer are simultaneously connected to the second bridge arm AC terminal 36 via the second filter inductor 17, and the second terminal 31 of the low-voltage side winding of the third single-phase transformer is connected to the midpoint 22 of the DC side capacitor 9.

[0046] Preferably, the present application adopts a voltage balancing strategy based on injection of zero-sequence voltage components. The advantage of this method is that it is based on the principle of the nearest three vectors and does not require additional switching times. The disadvantage is that this method cannot achieve capacitor voltage balance under high modulation coefficient and high power factor conditions on the converter side when applied to a single-ended three-phase system. However, the flexible energized over-phase system of the present application generally operates under conditions with low power factor on the converter side, which does not belong to the above conditions. Therefore, introducing the voltage balancing strategy based on injection of zero-sequence voltage components into the over-phase system can easily achieve voltage balance and maximize the advantages of the multi-level converter with common DC side capacitors. Specifically, the basic principle of the voltage balancing strategy based on injection of zero-sequence voltage components is described as follows: the key factor that determines whether the DC side capacitor voltage is balanced or not is the node current of the DC bus, that is, Figure 3 Flow through P x The current at each point. Only when the average value of each node current is 0 within a certain period can the voltage balance of the capacitor be achieved. Therefore, the fundamental goal of voltage balancing control is to change the node current. When the output phase voltage of the three-phase converter increases or decreases by the same zero-sequence voltage at the same time, the output line voltage and line current are not affected. Different zero-sequence voltages will change the action time of each switching state in each modulation cycle, thereby changing the current flowing through each DC bus node, thereby achieving the purpose of controlling the DC side capacitor voltage. The steps of the voltage balancing algorithm based on zero-sequence voltage component injection are as follows: (1) Zero-sequence voltage range calculation: Determine the reference voltage v of the first bridge arm 10, the second bridge arm 11 and the third bridge arm 12 of the converter according to the control target. a_ref 、v b_ref and v c_ref , and the reference voltage v after adding the zero-sequence voltage component in the previous cycle az_last 、v bz_last and v cz_last , the value range is between -1 and 1. Calculate the zero sequence voltage v z Maximum value v z_max and the minimum value v z_min , v z The value of should meet the restriction condition: To ensure that the converter works in linear modulation mode, the modulation wave of each phase after the zero-sequence voltage is injected should be between -1 and 1. (2) Calculation of the specific value of zero-sequence voltage: After determining the range of zero-sequence voltage, divide the range into (n z -1) copies, so that n z A discrete zero-sequence voltage. The expression is v z =v z_min +k z (v z_max -v z_min ) / (n z -1), where k z =0,1,2,…,n z-1. The three-phase reference voltage after adding the zero-sequence voltage is: v xz =v x_ref +v z , where x = a, b, c. (3) Calculation of node current and capacitor current: Taking one phase of the converter bridge arm as an example, let the number of converter levels be N L , in order to facilitate calculation, the reference voltage v xz The value range is changed from [-1,1] to [0,N L -1],v xz_pu =(N L -1)(v xz +1) / 2. v xz_pu It can be rewritten as the sum of the integer part and the decimal part: xz_pu =v xI +v xD , where v xI is the integer part, v xD is the decimal part. The node duty cycle is defined as follows: the ratio of the conduction time of the path between each DC bus node and the AC terminal (i.e. the time of outputting the level of the node) to the switching cycle in one switching cycle. The duty cycle of each node can be calculated as: Where n represents the DC node number, 0≤n≤N L -2 and is an integer. The current expression of each node is: i Pn =d an i a +d bn i b +d cn i c , where i x It is defined as the output current of the three-phase bridge arm AC terminal, and the direction of the current flowing out of the node is defined as the positive direction. The capacitor current expression is: Where 1≤i≤N L -2 and is an integer, the zero-sequence current i can be calculated according to the above formula Ci+1 =i Ci +i Pi (4) Cost function calculation: For each zero-sequence voltage value, calculate the corresponding cost function value. s Perform a zero-sequence voltage calculation. The capacitance of each DC side capacitor is C dc After T s , the change in each capacitor voltage is: ΔU Cj =i Cj T s / C dc , where 1≤j≤N L -1 and is an integer. The cost function is: The physical meaning of F is the derivative of the total energy of the capacitor. Traverse all zero-sequence voltages to complete the calculation of capacitor current and cost function. When the cost function takes the minimum value, the corresponding zero-sequence voltage is most conducive to capacitor voltage balancing. After calculating the final reference voltage, the traditional carrier stacking PWM method is used to control all switching devices.

[0047] Example 1

[0048] like Figure 6 As shown, the present application provides a flexible live phase-splitting system based on a common DC side capacitor type multi-level converter, including a first bridge arm 10 of the converter, a second bridge arm 11 of the converter, a third bridge arm 12 of the converter, a fourth bridge arm 13 of the converter, a DC side capacitor 9, a first single-phase transformer 14, a second single-phase transformer 15, and a third single-phase transformer 21. The first terminal 23 of the high-voltage side winding of the first single-phase transformer is connected to the first power supply arm 1, and the second terminal 24 of the high-voltage side winding of the first single-phase transformer is grounded; the first terminal 28 of the high-voltage side winding of the second single-phase transformer is connected to the second power supply arm 2, and the second terminal 27 of the high-voltage side winding of the second single-phase transformer is grounded. The first terminal 25 of the low-voltage winding of the first single-phase transformer is connected to the first bridge arm AC terminal 35 of the converter via the first filter inductor 16, the first terminal 30 of the low-voltage winding of the second single-phase transformer is connected to the third bridge arm AC terminal 37 of the converter via the third filter inductor 18, and the second terminal 26 of the low-voltage winding of the first single-phase transformer is connected to the second terminal 29 of the low-voltage winding of the second single-phase transformer, and then connected to the second bridge arm AC terminal 36 of the converter via the second filter inductor 17. The first terminal 32 of the low-voltage winding of the third single-phase transformer 21 is connected to the fourth bridge arm AC terminal 38 of the converter via the fourth filter inductor 19, and the filter capacitor 20 is connected between the first terminal 32 of the low-voltage winding of the third single-phase transformer and the second terminal 31 of the low-voltage winding of the third single-phase transformer. The first terminal 34 of the high-voltage winding of the third single-phase transformer is connected to the neutral zone 3, and the second terminal 33 of the high-voltage winding of the third single-phase transformer is connected to the rail 4. The second terminal 31 of the low-voltage winding of the third single-phase transformer needs to be connected to the midpoint 22 of the DC side capacitor. The DC side capacitor 9 is an energy storage element including at least two capacitors connected in series, and the arm switches of the four-arm converter are composed of fully controlled switching devices such as IGBT or IGCT.

[0049] In order to facilitate the explanation of the system control scheme, Figure 7 Some components, nodes and variables are marked in the figure. Assume that the traction power supply station uses Scott transformer. The first power supply arm 1 and the second power supply arm 2 are named as α-phase and β-phase power supply arms, i α 、i β is the traction substation current, and the current flowing from the traction substation to the transformer is defined as the positive direction; i cα 、i cβThe current of the power supply arm is defined as the positive direction from the traction substation to the contact network; the first bridge arm of the converter to the fourth bridge arm of the converter are named bridge arm a, bridge arm b, bridge arm c, and bridge arm d, respectively, where bridge arms a, b, and c belong to the first converter bridge arm group, also known as the rectifier side; bridge arm d belongs to the second converter bridge arm group, also known as the inverter side. The corresponding first filter inductor to the fourth filter inductor are named L a , L b , L c , L d , the inductor currents are i La 、i Lb 、i Lc 、i Ld , where i La 、i Lb 、i Lc The positive direction is defined as flowing into bridge arm a, bridge arm b, and bridge arm c, i Ld The positive direction is defined as flowing out of the d bridge arm. The first single-phase transformer 14 and the second single-phase transformer 15 are named T1 and T2, and the high-voltage side input voltage is u α1 and u β1 , the low voltage side output voltage is u α2 and u β2 The third single-phase transformer 21 is named T3, and its low-voltage side voltage is u Cf , the voltage on the high voltage side is u nz , the current on the high voltage side is i nz The transformation ratio between the high voltage side and the low voltage side of transformers T1 and T2 is k d :1, the transformation ratio between the high voltage side and the low voltage side of transformer T3 is k u :1. The midpoint of the DC side capacitor 9 is defined as N, and the node where the second terminal 26 of the low voltage side winding of the first single-phase transformer is connected to the second terminal 29 of the low voltage side winding of the second single-phase transformer is defined as O. The positive sequence active component of the current of the α-phase and β-phase power supply arm is i αac and i βac , the positive sequence reactive component is i αre and i βre , the negative sequence component is i α- and i β- , the fundamental component is i α1 and i β1 , the operator delayed by 90° is q. The reference voltages of the bridge arms of each converter are u a_ref 、u b_ref 、u c_ref 、u d_ref , with V dc / 2 is the base value. After standardizing the three-phase sinusoidal modulation wave, the reference voltage v of each converter bridge arm is obtained. a_ref 、v b_ref 、v c_ref 、vd_ref The reference values ​​of AC reference current (inductor current) are i a_ref 、i b_ref 、i c_ref 、i d_ref The a-bridge arm and the c-bridge arm adopt a double closed-loop control strategy with a DC voltage outer loop and an AC current inner loop. The control block diagram is shown in Figure 8 (a) shows. The collected voltage u α2 and u β2 , and the phase is detected by the phase-locked loop as the phase of the positive-sequence active current component; the reference value of the DC side voltage is subtracted from the actual value, and the amplitude of the positive-sequence active current component is obtained after the proportional integral link. The positive-sequence active current component i is generated using the phase and amplitude αac and i βac This component is used to maintain the DC side voltage stability.

[0050] Can be used Figure 8 (c) Calculate the positive sequence reactive power and negative sequence current components. The specific calculation method is as follows: Collect the current supply arm current i α 、i β , through the second order generalized integrator (SOGI), the current fundamental component i is obtained α1 、i β1 The component qi that is delayed by 90° α1 、qi β1 The negative sequence current component calculation formula is:

[0051] The positive sequence reactive current is calculated by converting the current i α1 The component qi that is delayed by 90° α1 As the α and β axis components, Park transformation is performed respectively; only the q axis component is subjected to Park inverse transformation, and the obtained α axis component is the positive sequence reactive current i αre The same is true for the β-phase power supply arm current. According to the internal model principle, a proportional resonance link is added after the positive-sequence reactive current and the negative-sequence current to obtain the actual compensation amount, which can achieve steady-state tracking without static error. The positive-sequence active current component minus the positive-sequence reactive current compensation amount and the negative-sequence current compensation amount, multiplied by the transformation ratio k d Perform current conversion to obtain the inductor current reference value i a_ref and i c_ref The error between the inductor current reference value and the actual value is passed through the quasi-PR controller to obtain the inductor voltage drop u La and u Lc . Add voltage feedforward to get the reference voltage of bridge arm a and bridge arm c:

[0052] The control block diagram of bridge arm b is as follows Figure 8 As shown in (b), the inductor current reference value is: i b_ref =-i a_ref -i c_ref The error between the reference value and the actual value of the inductor current is passed through the quasi-PR controller to obtain the inductor voltage drop u Lb And the reference voltage of bridge arm b: u b_ref =-u Lb The control target and control mode of the D bridge arm need to be switched according to the working mode of the system. The timing logic of each working mode is consistent with the timing logic of the flexible live-transition phase system based on the common DC side capacitor type multi-level converter. There are two specific control methods: current control and voltage control. The control block diagram is shown in the figure. Fig. 9 The current control block diagram is shown in Fig. 9 As shown in (a), the neutral zone reference current i is obtained according to the control requirements. nz_ref , reference current and filter capacitor current i Cf After adding, with the inductor current i Ld The inductor voltage drop reference value is obtained by subtracting the inductor voltage drop reference value and filtering capacitor voltage u Cf The sum of the bridge arm reference voltage u d_ref The voltage control block diagram is as follows: Fig. 9 As shown in (b), the neutral zone reference voltage u is obtained according to the control requirements. nz_ref , reference voltage and actual filter capacitor voltage u Cf The capacitor current reference value is obtained by subtracting the actual value of the capacitor current from the reference value of the capacitor current. Cf The difference is obtained by the quasi-PR controller to obtain the inductor voltage drop reference value, and the result is consistent with the filter capacitor voltage u Cf The sum of the bridge arm reference voltage u d_ref .

[0053] In order to verify the feasibility of the flexible live phase-splitting system based on the common DC side capacitor type multi-level converter of the present invention, the simulation verification was carried out using Simulink. The voltage of the three-phase power system is 220kV, and the traction power supply station uses a 220kV:27.5kV×2 Scott transformer. The transformation ratio of the high-voltage side and the low-voltage side of transformers T1, T2 and T3 is 27.5kV:4kV, the four filter inductors are 1mH, the DC side capacitor is 40mF, the filter capacitor is 100uF, and the DC side voltage reference value is 12.6kV, that is, each level is 1.8kV, and the load capacity is 10MW. Fig.10 (a) shows the voltage balance of the DC side capacitors. The capacitor voltage balance problem is well solved by using the method based on zero-sequence voltage component injection. Fig.10(b) is the neutral zone voltage waveform during the phase shift process. The neutral zone voltage gradually transitions from the α phase to the β phase, achieving smooth phase switching. Fig.10 (c) is the current in the three-phase power system, in which the reactive power and negative sequence current components have been basically eliminated. Fig.10 (d) is the current transfer waveform of the whole process. The source of the locomotive current is transferred from the α-phase power supply arm to the neutral zone, and then to the β-phase power supply arm.

[0054] Example 2

[0055] This embodiment is a further improvement on the embodiment 1, and the repeated contents will not be repeated here.

[0056] like Fig.11 As shown, the power electronic converter can have five converter bridge arms to further define the converter fifth bridge arm AC terminal 39. The difference between this embodiment and embodiment 1 is that this embodiment has five converter bridge arms, namely, in addition to the converter first bridge arm 10, the converter second bridge arm 11, the converter third bridge arm 12 and the converter fourth bridge arm 13, it also has a converter fifth bridge arm 40, the first single-phase transformer low-voltage side winding second terminal 26 and the second single-phase transformer low-voltage side winding second terminal 29 are simultaneously connected to the second bridge arm AC terminal 36 via the second filter inductor 17, and the third transformer low-voltage side winding second terminal 31 is connected to the converter fifth bridge arm AC terminal 39.

[0057] Example 3

[0058] This embodiment is a further improvement on the embodiment 1, and the repeated contents will not be repeated here.

[0059] like Fig.12 As shown in (a), the power electronic converter can have three converter bridge arms to omit the converter second bridge arm AC terminal 36. The difference between this embodiment and embodiment 1 is that this embodiment has three converter bridge arms, the first single-phase transformer low-voltage side winding second terminal 26 and the second single-phase transformer low-voltage side winding second terminal 29 are grounded at the same time, and the third single-phase transformer low-voltage side winding second terminal 31 and the midpoint 22 of the DC side capacitor 9 are grounded at the same time. After omitting the converter second bridge arm 11, the system does not meet the three-phase three-wire conditions, and the voltage balancing method based on zero-sequence component injection can no longer be used. Here, a voltage balancing circuit can be added to the DC side capacitor 9. An auxiliary voltage balancing circuit based on a resonant switched capacitor converter is shown in FIG. Fig.12(b). For each DC side capacitor, there is an auxiliary voltage equalizing circuit switch network 42 connected in parallel at both ends, and the midpoints of the half-bridges corresponding to the two adjacent capacitors are respectively connected to the two ends of the same group of auxiliary voltage equalizing circuit resonant branches 41. Its control method is also relatively simple. The upper and lower tubes of all half-bridges adopt complementary control with a duty cycle of 50%, and the switching frequency is set to the resonant frequency of the resonant branch. In addition to this type of voltage equalizing circuit, there are also voltage equalizing circuits based on switching capacitors or Buck-boost available for use. The principles are similar and will not be repeated here.

[0060] Example 4

[0061] This embodiment is a further improvement on the basis of the embodiment 3, and the repeated contents will not be repeated here.

[0062] Preferably, after omitting the second bridge arm 11 of the converter, the system does not meet the conditions of the three-phase three-wire system, and the voltage balancing method based on zero-sequence component injection can no longer be used. In this embodiment, a software voltage balancing control method based on carrier overlap PWM is adopted. Compared with embodiment 3, Fig.12 The hardware circuit shown in (b) is as follows: The number of levels of the common DC side capacitor type multi-level converter is N L , each level of voltage is E, that is, it can output 0, E, 2E...(N L -2)E, (N L -1) E to N L level, the reference voltage is v ref , the value is between -1 and 1, and is divided into two cases:

[0063] When v ref <0, all levels except the highest level can be output; when v ref ≥0, all levels except the lowest level can be output. The phase voltage output in one switching cycle is as follows Fig.13 As shown, the level duty cycle is defined as follows: the ratio of the time of outputting each level to the switching cycle in a switching cycle. The duty cycle of each level is:

[0064] Where k represents 0 to N L Integer between -1, d k Represents the duty cycle of the kth level. According to the above rules, the corresponding level is output, and the voltage balance of the capacitor can be achieved.

[0065] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A flexible live phase transfer system based on a common DC side capacitor type multi-level converter, comprising a main circuit located between a traction substation (8) and a neutral zone (3), characterized in that: The main circuit includes an electronic power converter, at least three single-phase transformers, a plurality of filter inductors and filter capacitors, the electronic power converter having four converter bridge arms to define four converter bridge arm AC terminals, and the electronic power converter is configured as follows: The converter bridge arm can be divided into a first converter bridge arm group and a second converter bridge arm group according to function, and the first converter bridge arm group and the second converter bridge arm group can be electrically coupled via a DC side capacitor (9), wherein: The first converter bridge arm group can be electrically connected to the traction substation via a first single-phase transformer (14) and a second single-phase transformer (15); The second converter bridge arm group can be electrically connected to the neutral zone via a third single-phase transformer (21); The converter bridge arm and the single-phase transformer can be electrically connected via a filter inductor; In the case where the electronic power converter has four converter bridge arms to define a converter first bridge arm AC terminal (35), a converter second bridge arm AC terminal (36), a converter third bridge arm AC terminal (37) and a converter fourth bridge arm AC terminal (38), the first terminal (23) of the high-voltage side winding of the first single-phase transformer is connected to the first power supply arm (1), the first terminal (28) of the high-voltage side winding of the second single-phase transformer is connected to the second power supply arm (2), the first terminal (25) of the low-voltage side winding of the first single-phase transformer is connected to the converter first bridge arm AC terminal (35) via a first filter inductor (16), and the first terminal (28) of the high-voltage side winding of the second single-phase transformer is connected to the second power supply arm (2). The first terminal (30) of the low-voltage side winding of the second single-phase transformer is connected to the third bridge arm AC terminal (37) of the converter via the third filter inductor (18); the first terminal (32) of the low-voltage side winding of the third single-phase transformer is connected to the fourth bridge arm AC terminal (38) of the converter via the fourth filter inductor (19); the filter capacitor (20) is connected between the first terminal (32) and the second terminal (31) of the low-voltage side winding of the third single-phase transformer; the first terminal (34) of the high-voltage side winding of the third single-phase transformer is connected to the neutral zone (3); and the second terminal (33) of the high-voltage side winding of the third single-phase transformer is connected to the rail (4); The second terminal (26) of the low-voltage side winding of the first single-phase transformer and the second terminal (29) of the low-voltage side winding of the second single-phase transformer are simultaneously connected to the second bridge arm AC terminal (36) of the converter via the second filter inductor (17), and the second terminal (31) of the low-voltage side winding of the third single-phase transformer is connected to the midpoint (22) of the DC side capacitor (9); Before the train (7) enters the first anchor section joint (5) from the first power supply arm (1), the flexible live phase separation system is only in a power quality management working state; When the train (7) is about to enter the first anchor section joint (5) from the first power supply arm (1), the electronic power converter adjusts the output voltage so that the contact network voltage in the neutral zone (3) is the same as the voltage of the first power supply arm (1), and power quality management is performed at the same time; When the train (7) enters the first anchor section joint (5) from the first power supply arm (1), the electronic power converter adjusts the output current, gradually increasing the current supplied by the neutral zone contact network to the train load current, causing the current supplied by the first power supply arm contact network to gradually decrease to 0, while simultaneously performing power quality management; After the relevant pantograph of the train (7) completely enters the neutral zone, the converter adjusts the output voltage so that the contact network voltage in the neutral zone (3) gradually transitions from the voltage of the first power supply arm (1) to the voltage of the second power supply arm (2), while power quality management is performed at the same time; Before the train (7) travels in the neutral zone (3) to the second anchor section joint (6), the electronic power converter has made the contact network voltage in the neutral zone (3) the same as the voltage of the second power supply arm (2), and simultaneously performs power quality management; When the train (7) enters the second anchor section joint (6) from the first power supply arm (1), the electronic power converter adjusts the output current, and the current supplied by the neutral zone contact network gradually decreases to 0, so that the current supplied to the train by the second power supply arm contact network gradually increases to the train load current, and power quality management is performed at the same time; After the train (7) leaves the second anchor section joint (6) and enters the second power supply arm (2), the flexible live phase separation system is only in a power quality management working state.

2. The flexible live-phase transfer system based on a common DC side capacitor type multi-level converter according to claim 1 is characterized in that: The electronic power converter can have three converter bridge arms to define a converter first bridge arm AC terminal (35), a converter third bridge arm AC terminal (37) and a converter fourth bridge arm AC terminal (38), wherein the first single-phase transformer low-voltage side winding second terminal (26) and the second single-phase transformer low-voltage side winding second terminal (29) are simultaneously grounded, and the third single-phase transformer low-voltage side winding second terminal (31) and the midpoint (22) of the DC side capacitor (9) are simultaneously grounded.

3. The flexible live-phase transfer system based on a common DC side capacitor type multi-level converter according to claim 1 is characterized in that: The electronic power converter can have five converter bridge arms to further define a converter fifth bridge arm AC terminal (39), wherein the first single-phase transformer low-voltage side winding second terminal (26) and the second single-phase transformer low-voltage side winding second terminal (29) are simultaneously connected to the second bridge arm AC terminal (36) via a second filter inductor (17), and the third transformer low-voltage side winding second terminal (31) is connected to the converter fifth bridge arm AC terminal (39).

4. The flexible live-phase transfer system based on a common DC side capacitor type multi-level converter according to claim 1 is characterized in that: The DC side capacitor (9) comprises at least two capacitors connected in series, and the bridge arm switch of the electronic power converter is defined by a fully controlled device including an insulated gate bipolar transistor or an integrated gate commutated thyristor.

5. The flexible live-phase transfer system based on a common DC side capacitor type multi-level converter according to claim 1 is characterized in that: When the first converter bridge arm group has three converter bridge arms, the voltage of the DC side capacitor (9) is voltage-balanced according to the following steps: calculating the value range of the zero-sequence voltage component, calculating the corresponding cost function for different zero-sequence components, selecting a zero-sequence component that can make the cost function approach the minimum value, and superimposing it on the original reference voltage to obtain a final reference voltage.

6. The flexible live-phase transfer system based on a common DC side capacitor type multi-level converter according to claim 5 is characterized in that: The value range of the zero-sequence voltage component is determined as follows: the first reference voltage of the converter bridge arm and the second reference voltage after adding the zero-sequence voltage component in the previous cycle are determined according to the control target, and the maximum value of the zero-sequence voltage is obtained in a way to avoid overmodulation. v z_max With minimum v z_min To determine the value range of the zero-sequence voltage component.

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