Phase-locked power supply system based on multifunctional balancing transformer and control method thereof

Abstract

The application discloses a same-phase power supply system based on a multifunctional balancing transformer and a control method thereof. The system comprises a three-phase high-voltage DC bus at a power grid side, a Y-D multifunctional balancing transformer for power locomotive traction and power quality compensation, and a three-phase MMC active compensation device. The primary side of the Y-D multifunctional balancing transformer is star-connected, and the secondary side is delta-connected. Three terminals A, B and C of the primary side are connected with three-phase power grid high-voltage busbars A phase, B phase and C phase respectively. Three terminals u, v and w of the secondary side are connected with three terminals b, a and c of the three-phase MMC active compensation device respectively. Two ports alpha phase and beta phase of the secondary side are connected in series and connected to a traction bus to supply power to a power locomotive. The system can realize three-end isolation of the power grid system, the traction system and the compensation system without adding an additional isolation transformer, ensures the safety of the system, and effectively reduces the capacity and loss of the same-phase power supply system.

Description

Technical Field

[0001] This invention relates to the field of electrified railway traction power supply system technology, and in particular to a non-phase power supply system based on a multi-functional balancing transformer and its control method. Background Technology

[0002] In recent years, high-speed rail has developed rapidly, further promoting high-quality development of railways. At the same time, with the increasing number of high-speed railways, the problems brought about by electrification are becoming increasingly serious. How to ensure the long-term, efficient, and safe operation of electrified railways and reduce their impact on the power system have become urgent problems to be solved in current electrification railway research.

[0003] Currently, high-speed railways widely use single-phase AC-DC-AC electric locomotives with low harmonic content and a power factor close to 1. Therefore, negative sequence issues are their primary power quality problem. To mitigate the impact of negative sequence issues, high-speed railways widely employ phase-sequence rotation and phase-separation power supply schemes. However, phase separation can lead to problems such as locomotive speed reduction, overvoltage, and limitations in construction site selection, posing certain safety hazards to the normal operation of locomotives. Existing automatic phase-separation technologies mainly include automatic phase-separation switching via ground switches, automatic phase-separation on-board units, and automatic phase-separation on poles. However, automatic phase-separation technologies cannot completely eliminate the hazards caused by phase separation.

[0004] To address the issues of negative sequence and phase separation, in-phase power supply systems have been proposed. Theory and practice have shown that in-phase power supply technology can eliminate phase separation at substation outlets and effectively solve the negative sequence problem. In-phase power supply systems include in-phase power supply systems based on compensation devices and through-type in-phase power supply systems. Currently, the main type in-phase power supply system in use is the compensation-based system, which primarily consists of a traction transformer, a matching transformer, and compensation devices. These compensation devices include passive compensation devices, active compensation devices, and hybrid compensation devices combining passive and active compensation. Passive compensation devices cannot dynamically adjust reactive power, have poor flexibility, and cannot effectively suppress harmonics; while hybrid compensation devices can reduce the active compensation capacity, the active compensation capacity remains large, and the control method is complex. Therefore, active compensation devices, with their strong applicability, dynamic adjustment capability, and significant compensation effect, have become a current research hotspot. However, active compensation devices suffer from large capacity and high cost, hindering widespread adoption. Currently, traction transformers used in in-phase power supply systems can be divided into balanced transformers and unbalanced transformers. Balanced transformers mainly include Scott transformers and impedance matching balanced transformers, while unbalanced transformers mainly include VV transformers and YNd11 transformers. Selecting a suitable traction transformer is also crucial for solving problems in electrified railways. Furthermore, to ensure the safety of the railway power supply system, it is generally required that the power grid system, traction system, and compensation system of the in-phase power supply system achieve three-terminal isolation. Currently, the main method to achieve three-terminal isolation is to add additional isolation transformers between the unisolated terminals. This not only increases transformer investment and floor space but also increases system capacity and losses. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a non-phase power supply system based on a multi-functional balancing transformer and its control method. This system achieves three-terminal isolation between the power grid system, traction system, and compensation system without requiring an additional isolation transformer between the traction system and the compensation system, ensuring the safety of the non-phase power supply system. Furthermore, by designing the tap position of the YD-MFBT secondary winding to match the voltage level that the compensation device can withstand, and with both the compensation device and the transformer having relatively small capacities, the capacity and losses of the non-phase power supply system are effectively reduced.

[0006] The objective of this invention is achieved through the following technical solutions:

[0007] A non-phase power supply system based on a multi-functional balancing transformer includes a three-phase high-voltage DC bus on the grid side, a YD multi-functional balancing transformer YD-MFBT for electric locomotive traction and power quality compensation, and a three-phase MMC active compensation device MMC-APC.

[0008] The YD-MFBT primary side has three terminals A, B, and C, and the secondary side has five terminals α, β, u, v, and w. The MMC-APC AC side has three terminals a, b, and c.

[0009] The YD-MFBT primary side is a star connection, the secondary side is a delta connection, and an extension arm N is added to each end of the middle b phase. b1 N b2 Taps u and w are respectively led out from the middle of phase a and phase c. Tap u divides phase a into two winding segments N. a1 N a2 The tap w divides phase c into two winding segments N. c1 N c2 Tap v is drawn from the connection point between phase a and phase c. Taps u, v, and w output three-phase voltages for connecting MMC-APC.

[0010] The three terminals A, B, and C on the primary side are connected to phases A, B, and C of the three-phase high-voltage busbar of the three-phase power grid, respectively; the three terminals u, v, and w on the secondary side are connected to the three terminals b, a, and c on the AC side of the MMC-APC, respectively; the two ports on the secondary side, phases α and β, are connected in series to the traction busbar to supply power to the electric locomotive.

[0011] As a further improvement of the present invention, each phase of the MMC-APC consists of upper and lower bridge arms, and each bridge arm is composed of n identical sub-modules cascaded together, with the sub-modules adopting a half-bridge structure. The upper and lower bridge arms are connected by the bridge arm inductor L. B Series, R B The equivalent resistance of the bridge arm; the sub-modules connected in series in each phase and the bridge arm inductance L B They are identical and symmetrical.

[0012] As a further improvement of the present invention, the number of turns and impedance in the YD-MFBT satisfy the following relationship:

[0013]

[0014] Where, N k and Z k Let A, B, C be the number of turns and the impedance of winding k, respectively, where k = A, B, C; a, b, c; a1, a2; b1, b2; c1, c2.

[0015] As a further improvement of the present invention, the number of turns and impedance in the YD-MFBT satisfy x = 0.366, λ = 2.732, and γ = 1.577y.

[0016] A control method for a non-phase power supply system based on a multi-functional balancing transformer includes:

[0017] Two-phase load current Divided into symmetrical active components Harmonic components and reactive components MMC-APC AC side compensation current The three-phase current, harmonic components, and reactive power generated on the primary side of the YD-MFBT The sum of the three-phase currents generated on the primary side of the YD-MFBT is zero, which makes the power factor of the primary side current of the YD-MFBT 1 and the three phases symmetrical.

[0018] The control methods specifically include:

[0019] Based on Kirchhoff's laws and the characteristics of YD-MFBT, the basic current conversion relationship is as follows:

[0020]

[0021] in, This refers to the primary current of the YD-MFBT, i.e., the grid-side current. The load currents at the α and β ports of the YD-MFBT secondary side; The compensation currents for the u and w taps on the secondary side of the YD-MFBT, and the current for the v tap.

[0022]

[0023] Two-phase load current Divided into symmetrical active components Harmonic components and reactive components Two parts:

[0024]

[0025] Among them, the symmetrical active component A symmetrical three-phase current with a power factor of 1 is generated on the primary side of the YD-MFBT; this enables the compensation current to... satisfy:

[0026]

[0027] This can make the grid-side current Includes only the positive sequence fundamental component

[0028] Calculated from equations (3) and (5):

[0029]

[0030] Define two-phase load current The effective value of the fundamental active component is I pα I pβ Then its symmetrical active component The effective value satisfies I plα =Iplβ =0.5(I pα +I pβ ),and Phase and α, β port voltages They correspond to the same thing;

[0031] This yields the two-phase load current. Harmonic components and reactive components for:

[0032]

[0033] Then, from equations (6) and (7), we can obtain the compensation currents for the taps u, v, and w on the secondary side of the YD-MFBT.

[0034] Compared with the prior art, the multi-port power router of the present invention has the following advantages:

[0035] This invention provides a system that connects to the compensation system via a tap drawn from the secondary winding of the YD-MFBT. The compensation current is directly injected into the YD-MFBT winding. Therefore, it eliminates the need for an additional isolation transformer between the traction and compensation systems to achieve three-terminal isolation between the power grid, traction, and compensation systems, ensuring the safety of the in-phase power supply system. Furthermore, by designing the tap position of the YD-MFBT secondary winding to match the voltage level that the compensation device can withstand, and with both the compensation device and the transformer having relatively small capacities, the capacity and losses of the in-phase power supply system are effectively reduced. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1 This is the in-phase power supply system topology based on a multifunctional balancing transformer proposed in this invention.

[0038] Figure 2 This is the topology of MMC-APC.

[0039] Figure 3 Several port-isolated in-phase power supply systems based on MMC are described.

[0040] Figure 4 This is the topology of MMC-PFC.

[0041] Figure 5This is the phasor diagram of the secondary voltage of a YD-MFBT.

[0042] Figure 6 The control block diagram for extracting the effective value of the fundamental active current of the load current.

[0043] Figure 7 This is a block diagram of the positive and negative order decoupling operation unit.

[0044] Figure 8 The block diagram shows the decoupling implementation of the dual synchronous coordinate system method.

[0045] Figure 9 This is the equivalent circuit diagram of one phase of the MMC-APC.

[0046] Figure 10 The block diagram for AC side current control of MMC-APC is shown below; (a) positive sequence current control, (b) negative sequence current control.

[0047] Figure 11 The diagram illustrates the charging and discharging states of the MMC submodule during operation: (a) charging state; (b) discharging state.

[0048] Figure 12 This is a block diagram of the capacitor voltage balancing control for the MMC-APC submodule.

[0049] Figure 13 This is a block diagram of the MMC-APC phase-to-phase capacitor voltage balancing control.

[0050] Figure 14 The block diagram for MMC-APC circulating current suppression control is shown; (a) positive sequence circulating current suppression, (b) negative sequence circulating current suppression.

[0051] Figure 15 Simulation results of the present invention: (a) load voltage and current waveforms, (b) grid voltage and current waveforms when the system proposed in this invention is not connected to MMC-APC for compensation, (c) grid voltage and current waveforms when the system proposed in this invention is connected to MMC-APC for compensation, (d) actual current and command current waveforms on the AC side of MMC-APC, and (e) capacitor voltage waveform of MMC-APC submodule. Detailed Implementation

[0052] To make the objectives and technical solutions of this invention clearer and easier to understand, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0053] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more. In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0054] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of the present invention.

[0055] This invention proposes a port-isolated in-phase power supply system based on a YD-MFBT (YD Multifunction Balance Transformer) and an MMC-APC (Modular Multilevel Converter-Active Power Compensator). The system connects to the compensation system via taps drawn from the secondary winding of the YD-MFBT, with the compensation current directly injected into the YD-MFBT winding. Therefore, it eliminates the need for an additional isolation transformer between the traction and compensation systems, achieving three-terminal isolation between the power grid, traction, and compensation systems, thus ensuring the safety of the in-phase power supply system. Furthermore, the tap positions of the YD-MFBT secondary winding can be designed to match the voltage level that the compensation device can withstand. Simultaneously, both the compensation device and the transformer have relatively small capacities, effectively reducing the capacity and losses of the in-phase power supply system.

[0056] The topology of the embodiments of the present invention will be described below.

[0057] (1) Topology

[0058] This invention provides a port-isolated in-phase power supply system for electrified railways based on a YD multi-functional balancing transformer (YD-MFBT) and a modular multilevel converter active compensation device (MMC-APC). The system includes a three-phase high-voltage DC bus on the grid side, a YD multi-functional balancing transformer (YD-MFBT) for electric locomotive traction and power quality compensation, and a three-phase MMC active compensation device (MMC-APC).

[0059] Figure 1 This is a typical topology for this in-phase power supply system. The primary side of the YD-MFBT is star-connected (neutral point o can be directly grounded), and the secondary side is delta-connected, with an extension arm N added to each end of the middle phase (phase b). b1 N b2 Taps u and w are respectively led out from the middle of phase a and phase c. Tap u divides phase a into two winding segments N. a1 N a2 The tap w divides phase c into two winding segments N. c1 N c2 By changing the turns ratio of the two windings, the output voltages of taps u and w can be altered, thereby matching the voltage level that the MMC-APC can withstand. Tap v is drawn from the connection point between phase a and phase c, and taps u, v, and w output three-phase voltages, used to connect the compensation device MMC-APC in the same-phase power supply system.

[0060] The primary side of the YD-MFBT is connected to the high-voltage busbar of the three-phase power grid, while the two ports of the secondary side, α-phase and β-phase, are connected in series to the traction busbar to supply power to the electric locomotive. The three-phase taps u, v, and w from the secondary side are connected to the MMC-APC.

[0061] The YD-MFBT primary side has three terminals A, B, and C, and the secondary side has five terminals α, β, u, v, and w. The MMC-APC AC side has three terminals a, b, and c. Their connection methods are as follows:

[0062] The three terminals A, B, and C of the YD-MFBT primary side are connected to phases A, B, and C of the three-phase high-voltage busbar of the power grid, respectively. The three terminals u, v, and w of the YD-MFBT secondary side are connected to the three terminals b, a, and c of the MMC-APC AC side, respectively, to provide three-phase AC voltage to the MMC-APC. The two terminals α and β of the YD-MFBT secondary side are connected to the traction busbar to supply power to the electric locomotive.

[0063] Each phase of the MMC-APC consists of two bridge arms, upper and lower, for a total of six bridge arms. Each bridge arm is composed of n identical sub-modules cascaded together, and the sub-modules adopt a half-bridge structure. The upper and lower bridge arms are connected by the bridge arm inductor L. B Series, R B This represents the equivalent resistance of the bridge arm. The sub-modules connected in series in each phase and the bridge arm inductance L... B They are all exactly the same and have a high degree of symmetry.

[0064] Figure 2 The topology of MMC-APC is shown, where each bridge arm has n sub-modules, and the sub-modules adopt a half-bridge structure.

[0065] The specific structure is as follows: Each phase of the MMC-APC consists of two bridge arms, upper and lower. Each bridge arm is composed of n identical sub-modules cascaded together, and the sub-modules adopt a half-bridge structure. The upper and lower bridge arms are connected by the bridge arm inductor L. B Series, R B The equivalent resistance of the bridge arm; the sub-modules connected in series in each phase and the bridge arm inductance L B They are identical and symmetrical.

[0066] (2) Advantages Analysis

[0067] Figure 1 The present invention provides a port-isolated in-phase power supply system based on MMC-APC and YD-MFBT, which includes one transformer YD-MFBT (MMC-APC + 1YD-MFBT). Figure 3 Several in-phase power supply systems based on MMC are described.

[0068] Figure 3 (a) shows a port-isolated in-phase power supply system based on MMC-APC and Scott transformers. The system contains two Scott transformers: a traction Scott transformer TT for locomotive traction and an isolation Scott transformer MT (MMC-APC+2Scott) for isolation. Figure 3 (b) is a non-phase power supply system based on MMC-APC and Vv transformer, which includes one Vv transformer TT (MMC-APC+1Vv) for locomotive traction. Figure 3 (c) is a non-phase power supply system based on MMC-APC and Scott transformer, which includes one Scott transformer TT (MMC-APC+1Scott) for locomotive traction. Figure 3 (d) is a non-phase power supply system based on MMC power flow controller (MMC-PFC) and Vv transformer. The system includes one Vv transformer TT (MMC-PFC+1Vv) for locomotive traction. Figure 3(e) is a non-phase power supply system based on MMC-PFC and Scott transformer, which includes one Scott transformer TT (MMC-PFC+1Scott) for locomotive traction. Figure 4 This is the topology of MMC-PFC. Table 1 shows the invention and... Figure 3 A comparison of several in-phase power supply systems based on MMC is presented. For simplicity, an example with a load power factor of 0.6 and an MMC voltage modulation ratio of 1 is used.

[0069] The advantages of the in-phase power supply system based on a multifunctional balancing transformer of this invention compared with other MMC in-phase power supply systems are as follows:

[0070] 1) Other MMC in-phase power supply systems specifically include:

[0071] ① A port-isolated in-phase power supply system based on MMC-APC and Scott transformers, which includes two Scott transformers: a traction Scott transformer for locomotive traction and an isolation Scott transformer for isolation.

[0072] ② A phase-to-phase power supply system based on MMC-APC and Vv transformer, which includes one Vv transformer for locomotive traction.

[0073] ③ A co-phase power supply system based on MMC-APC and Scott transformer, which includes one Scott transformer for locomotive traction.

[0074] ④ A non-phase power supply system based on MMC power flow controller and Vv transformer, which includes one Vv transformer for locomotive traction.

[0075] ⑤ A co-phase power supply system based on MMC power flow controller and Scott transformer, which includes one Scott transformer for locomotive traction.

[0076] 2) Taking a load power factor of 0.6 and an MMC voltage modulation ratio of 1 as an example, this invention achieves minimum capacity for both the MMC and system transformer. Specifically, the MMC capacity can be reduced by up to 30.5%, the system transformer capacity by up to 67.0%, and the total capacity of the MMC and system transformer by up to 29.5%. Furthermore, without adding an isolation transformer, three-terminal isolation of the power grid system, traction system, and compensation system can be achieved. In addition, in the design of the YD-MFBT, the MMC port voltage can be changed by selecting appropriate u and w tap positions, thereby matching the voltage level that the MMC can withstand.

[0077] In this invention, the compensation current of the MMC-APC is directly injected into the YD-MFBT winding. Therefore, there is no need to add an additional isolation transformer between the traction system and the compensation system; three-terminal isolation between the power grid system, the traction system, and the compensation system can be achieved solely through the YD-MFBT. Furthermore, the winding taps u and w at the connection between the YD-MFBT and the MMC-APC are variable. Therefore, when designing the YD-MFBT, the appropriate tap position can be selected by matching the voltage level that the MMC-APC can withstand.

[0078] Comparison of the present invention with Figure 3 The MMC in-phase power supply system shown in Figure (a) can achieve three-terminal isolation in both systems, and the MMC converter capacities are basically the same. However, since the present invention can eliminate the isolation transformer, it is more efficient than other systems. Figure 3 The system shown in (a) has a 67.0% reduction in transformer capacity and an 8.5% reduction in the total capacity of the MMC and system transformer. This invention is compared to... Figure 3 The MMC in-phase power supply system shown in Figure (b) has the same MMC converter capacity in both systems, but the transformer capacity of this invention is smaller compared to... Figure 3 The system shown in (b) has a 13.4% reduction in transformer capacity and a 0.72% reduction in the total capacity of the MMC and system transformers. Furthermore, Figure 3 The system shown in (b) cannot achieve isolation between the traction system port and the compensation system port.

[0079] As shown in Table 1, the MMC capacity and system transformer capacity of this invention can be minimized. Specifically, the MMC capacity can be reduced by up to 30.5%, the system transformer capacity by up to 67.0%, and the total capacity of the MMC and system transformer by up to 29.5%. Furthermore, without adding an isolation transformer, three-terminal isolation of the power grid system, traction system, and compensation system can be achieved. In addition, the YD-MFBT in this invention allows for adjustment of the MMC port voltage by selecting appropriate u and w tap positions, thereby matching the voltage level that the MMC can withstand. In contrast, in existing MMC in-phase power supply technologies, the MMC port voltage is a fixed value and cannot be changed. Therefore, compared with existing MMC in-phase power supply technologies, the compensation converter port voltage of the topology proposed in this invention can be designed according to the voltage level that the converter can withstand, and achieves three-terminal isolation while maintaining minimum converter and transformer capacity and without adding an additional isolation transformer, ensuring the safety of the in-phase power supply system.

[0080] Table 1

[0081]

[0082] Based on the above topology, the present invention also explains the basic principles and control methods of the above topology.

[0083] This section introduces the compensation principle of the in-phase power supply system under the in-phase power supply topology of the present invention and the extraction of the command current on the AC output side of MMC-APC, analyzes its control principle, and introduces a closed-loop control strategy.

[0084] (1) System Compensation Principle

[0085] according to Figure 1 The relationship between the number of turns and impedance of a YD-MFBT is defined as follows:

[0086]

[0087] Where, N k and Z k These represent the number of turns and impedance of winding k (k = A, B, C; a, b, c; a1, a2; b1, b2; c1, c2). Based on the characteristics of the YD-MFBT, to meet the balance condition, the relationship between the number of turns and impedance of the YD-MFBT should satisfy x = 0.366, λ = 2.732, and γ = 1.577y. x = 0.366 ensures that the voltages at ports α and β on the secondary side of the YD-MFBT are perpendicular; λ = 2.732 and γ = 1.577y ensure that regardless of the values ​​of the load current and compensation current at the secondary side ports of the YD-MFBT, the three-phase current on the primary side of the YD-MFBT does not contain zero-sequence current, therefore the neutral point on the primary side of the YD-MFBT can be directly grounded. When designing the YD-MFBT, changing the value of y can change the output voltage of taps u, v, and w, thereby matching the voltage level that the MMC-APC can withstand.

[0088] A compensation principle for a non-phase power supply system based on a multi-functional balancing transformer is used to convert the two-phase load current... Divided into symmetrical active components and "harmonics + reactive power" components Two parts enable AC side compensation current of the MMC-APC. The three-phase current generated on the primary side of the YD-MFBT and the "harmonic + reactive power" components When the sum of the three-phase currents generated on the primary side of the YD-MFBT is zero, the compensation purpose of the in-phase power supply system can be achieved, making the power factor of the YD-MFBT primary side current 1 and the three phases symmetrical. Specifically, this includes:

[0089] right Figure 1 Based on Kirchhoff's laws and the characteristics of YD-MFBT, the basic current conversion relationship of the system proposed in this invention is as follows:

[0090]

[0091] in, This refers to the primary current of the YD-MFBT, i.e., the grid-side current. The load currents at the α and β ports of the YD-MFBT secondary side; The compensation currents for the u and w taps on the secondary side of the YD-MFBT, and the current for the v tap.

[0092]

[0093] Two-phase load current Divided into symmetrical active components and "harmonics + reactive power" components Two parts:

[0094]

[0095] Among them, the symmetrical active component A symmetrical three-phase current with a power factor of 1 is generated on the primary side of the YD-MFBT. Therefore, as long as the compensation current is made... satisfy:

[0096]

[0097] This can make the grid-side current Includes only the positive sequence fundamental component To achieve the purpose of compensation for the same-phase power supply system.

[0098] From equations (3) and (5), we can calculate:

[0099]

[0100] Define two-phase load current The effective value of the fundamental active component is I pα I pβ Then its symmetrical active component The effective value satisfies I plα =I plβ =0.5(I pα +I pβ ),and Phase and α, β port voltages They correspond to the same thing.

[0101] From this, the two-phase load current can be obtained. The "harmonic + reactive" components for:

[0102]

[0103] Then, from equations (6) and (7), the compensation currents for the u, v, and w taps on the secondary side of the YD-MFBT can be obtained.

[0104] (2) Extraction of AC command current from MMC-APC

[0105] according to Figure 1 The three-phase voltage on the grid side is defined as:

[0106]

[0107] The phasor diagram of the secondary voltage of the YD-MFBT is as follows: Figure 5 As shown.

[0108] Depend on Figure 5 Therefore, the voltages at ports α and β are... and feeder voltage for:

[0109]

[0110] Define the effective value of the feeder traction voltage as U2, i.e., U αβ =U2, then Equation (8) can be rewritten as:

[0111]

[0112] Define the load power factor angle as θ, and the load current of the two power supply arms can be expressed as:

[0113]

[0114] Its symmetrical active component for:

[0115]

[0116] From equations (6), (7), (10), and (11), the compensation currents for the u, v, and w taps on the secondary side of the YD-MFBT can be obtained. That is, the AC side command current of MMC-APC.

[0117] In summary, extracting the RMS value of the fundamental active component of the load current is crucial for extracting the command current on the AC side of the MMC-APC. The RMS value of the fundamental active component of the load current, I, is defined as follows: Lp =I L cosθ, for I Lp The extraction was performed using a 90° phase shift scheme with a second-order generalized integrator (SOGI) to generate two orthogonal signals containing only the fundamental frequency. These signals were then multiplied by the unit sine and unit cosine signals of the load voltage, summed, and the second harmonic component was filtered out to obtain the final signal. Its block diagram is as follows Figure 6 As shown.

[0118] (3) MMC-APC control strategy

[0119] In the in-phase power supply system proposed in this invention, the AC current on the MMC-APC side is a three-phase unbalanced current, containing positive-sequence, negative-sequence, and zero-sequence components. Since a three-phase three-wire system does not have zero-sequence current, the zero-sequence component is ignored. After positive and negative sequence separation, the positive and negative sequence components of the AC current can be controlled independently.

[0120] 1) Detection of positive and negative order components

[0121] If the positive-sequence and negative-sequence components rotate at frequencies of ω and -ω respectively, then the current space vector in the two-phase α and β stationary coordinate systems can be expressed as:

[0122]

[0123] In the formula: i P i N These represent the magnitudes of the positive and negative order components. The initial phase angles are for the positive and negative sequence components.

[0124] Considering two rotating coordinate systems of positive and negative order, with rotation angles of ω and -ω respectively, the component expressions of the current space vector in the positive and negative order rotating coordinate systems are as follows:

[0125]

[0126] It can be seen that in the positive-sequence rotating coordinate system, the positive-sequence component is transformed into a DC component, while the negative-sequence component is transformed into a second harmonic component; in the negative-sequence rotating coordinate system, the negative-sequence component is transformed into a DC component, while the positive-sequence component is transformed into a second harmonic component.

[0127] Because low-pass filters have a narrow bandwidth, using them to filter out second harmonic components would negatively impact the dynamic performance of the control system. Therefore, a dual-coordinate decoupling and sequence-based method is employed, using... Figure 7 The decoupling unit shown uses a low-pass filter to remove high-frequency oscillations, thereby obtaining the dq-axis DC components of the positive and negative sequence currents respectively. The block diagram for the decoupling implementation of the dual synchronous coordinate system method is shown below. Figure 8 As shown.

[0128] For one phase of the MMC-APC, considering only the positive sequence component, its equivalent circuit is as follows: Figure 9 As shown.

[0129] From Kirchhoff's laws:

[0130]

[0131] In the formula,

[0132] Performing a synchronous rotational coordinate transformation on equation (12) yields:

[0133]

[0134] Decouple the d and q axes and make them

[0135]

[0136] From equations (15) and (16), the desired positive-sequence fundamental voltage of the AC side output is obtained as follows:

[0137]

[0138] For the negative sequence component, the control method is the same as for the positive sequence. Thus, the MMC-APC current control block diagram is obtained as follows: Figure 10 As shown.

[0139] 2) Capacitor voltage equalization control

[0140] ① Submodule capacitor voltage equalization control

[0141] Due to differences in switching device losses and DC capacitor losses among each submodule, the MMC-APC exhibits submodule capacitor voltage imbalance during normal operation. Excessively high submodule capacitor voltages can jeopardize the safety of capacitors and power semiconductor devices; therefore, ensuring a constant capacitor voltage in each submodule is crucial.

[0142] MMC has two states during operation: charging and discharging. Figure 11 As shown, the charging and discharging time of each submodule can be changed by altering the duty cycle of its switch drive signal, thereby balancing the voltage of the submodule capacitors.

[0143] The block diagram of the MMC-APC submodule capacitor voltage balancing control is as follows: Figure 12 As shown.

[0144] In the diagram, i xp Let x be the current in the x-phase bridge arm (x = a, b, c), u dcref The reference value for the DC capacitor voltage of the submodule is u. dcn Let u be the capacitor voltage of the nth submodule of the bridge arm. cx0 The original sinusoidal modulation wave of the submodule, u cxn This is the sinusoidal modulated wave of the nth submodule after the submodule capacitor voltage equalization control.

[0145] When sign(i) xp )*u errorn When the value is greater than 0, increase the duty cycle of the submodule to extend the charging or discharging time; when sign(i xp )*u errorn When the duty cycle is less than 0, the duty cycle of the submodule is reduced to shorten the charging or discharging time.

[0146] ② Interphase capacitor voltage equalization control

[0147] During normal operation of the MMC, in addition to the equalization of the capacitor voltages of the submodules, there will be an imbalance of capacitor voltages between the three phases. To address this, an interphase capacitor voltage equalization control strategy has been introduced.

[0148] To ensure that the additional power required to balance the three phases does not change the total three-phase power, the average value of the total submodule capacitor voltage is used in the outer voltage loop control. Average voltage of capacitors in each phase submodule In comparison, the deviation value, after passing through the PI controller, results in an additional phase circulating current. An excitation voltage Δu is generated on the bridge arm reactance. cx This ensures that the voltage across the phase capacitors is balanced. The excitation voltage Δu acting on the reactance... cx Generated by the inner current loop estimation. The MMC-APC interphase capacitor voltage equalization control block diagram is as follows: Figure 13 As shown.

[0149] 3) Circulation suppression

[0150] The energy flow between the AC and DC sides of the MMC-APC generates second-harmonic power fluctuations within the MMC-APC. These second-harmonic power fluctuations cause second-harmonic fluctuations in the submodule capacitor voltage. The modulated second-harmonic voltage fluctuations of the submodule capacitors generate an additional second-harmonic excitation voltage. This voltage acts on the bridge arm inductor, inducing a second-harmonic circulating current within the MMC-APC. This second-harmonic circulating current distorts the bridge arm current, consumes switching device capacity, and increases losses. Therefore, it is necessary to suppress the circulating current within the MMC-APC.

[0151] The three-phase unbalanced current on the AC side of the MMC-APC of this invention includes positive-sequence current components and negative-sequence current components, which generate double-frequency negative-sequence circulating current and positive-sequence circulating current inside the MMC-APC, respectively. Therefore, the positive and negative sequence separation method based on dual synchronous coordinate system decoupling is used to extract the positive and negative sequence components of the circulating current, and the double-frequency circulating current is suppressed by independent control of positive and negative sequences.

[0152] Depend on Figure 9 In a three-phase positive and negative sequence network, the mathematical model of the circulation is:

[0153]

[0154] In the formula, i zx For the internal x-phase circulating current of MMC-APC, u zx The unbalanced pressure drop of phase x inside the MMC-APC (x=a,b,c).

[0155] Performing a synchronous rotational coordinate transformation on equation (18) yields:

[0156]

[0157] By decoupling the d and q axes, the control block diagram for circulating current suppression can be obtained as follows: Figure 14 As shown.

[0158] Based on the above topology and control method, the simulation verification is described below.

[0159] Table 2 shows the specific parameters of the Matlab / Simulink simulation model built based on the in-phase power supply system proposed in this invention.

[0160] Table 2

[0161]

[0162]

[0163] Figure 15 In the middle (a), the load voltage and current waveforms are shown. Figure 15 (b) shows the grid voltage and current waveforms when the system proposed in this invention is not connected to MMC-APC for compensation. Figure 15 (c) The grid voltage and current waveforms when the system proposed in this invention is connected to MMC-APC for compensation. Figure 15 The waveforms of the actual current and command current on the AC side of the MMC-APC are shown in (d). Figure 15 Image (e) shows the capacitor voltage waveform of the MMC-APC submodule. Figure 15 (b) and Figure 15 A comparison in (c) shows that, after the proposed in-phase power supply system is connected to the MMC-APC for compensation, the large amount of negative sequence and reactive current generated on the grid side due to the traction locomotive is effectively suppressed, thereby achieving symmetrical three-phase currents on the grid side and ensuring that the grid current and voltage are in phase. Figure 15 (d) and Figure 15 (e) The comparison shows that, under the above control method, the actual current on the AC side of the MMC-APC can completely follow its command current, and the DC side capacitor voltage of the submodule is balanced with small fluctuations, demonstrating good control performance. Simulation results verify the correctness and effectiveness of the proposed scheme.

[0164] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent structural changes made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

[0165] The above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still make modifications or equivalent substitutions to the specific implementation schemes of the present invention, and these modifications or equivalent substitutions do not depart from the spirit and scope of the present invention, and are all within the protection scope of the claims of the present invention.

Claims

1. A phase-balanced power supply system based on a multi-functional balun, characterized by, The three-phase high-voltage bus including a power grid side, Y-D multi-functional balance transformer YD-MFBT for power locomotive traction and power quality compensation and three-phase MMC active compensation device MMC-APC; The three terminals of the primary side of the YD-MFBT are A, B and C, the five terminals of the secondary side are alpha, beta, u, v and w, and the three terminals of the AC side of the MMC-APC are a, b and c; YD-MFBT primary side is star connection, secondary side is triangle connection, and two extension arms N are added at both ends of the middle b phase b1 , N b2 , the taps u and w are drawn out from the middle of the a phase and the c phase, the tap u divides the a phase into two segment windings N a1 , N a2 , the tap w divides the c phase into two segment windings N c1 , N c2 ; the tap v is drawn out from the connection of the a phase and the c phase, and the three-phase voltage output by the taps u, v and w is used for connecting the MMC-APC; The three terminals A, B and C of the primary side are connected with the three-phase power grid high-voltage bus A phase, B phase and C phase respectively; the three terminals u, v and w of the secondary side are connected with the three terminals b, a and c of the AC side of the MMC-APC respectively; the two ports alpha phase and beta phase of the secondary side are connected in series and connected to the traction bus to supply power to the power locomotive; Each phase of the MMC-APC consists of two bridge arms, upper and lower. Each bridge arm is composed of n identical sub-modules cascaded together, and the sub-modules adopt a half-bridge structure; the upper and lower bridge arms are connected by the bridge arm inductor L. B Series, R B The equivalent resistance of the bridge arm; the sub-modules connected in series in each phase and the bridge arm inductance L B They are identical and symmetrical; The number of turns and impedance in the YD-MFBT satisfy the following relationship: where Nk k and Zk k are the number of turns and the impedance of winding k, respectively, k = A, B, C; a, b, c; a1, a2; b1, b2; c1, c2; The number of turns and impedance in the YD-MFBT satisfy x=0.366, λ=2.732 and γ=1.577y.

2. A control method of the phase-balanced power supply system based on the multifunctional balun as claimed in claim 1, characterized by, The YD-MFBT includes: Two-phase load current Divided into symmetrical active components Harmonic components and reactive components MMC-APC AC side compensation current The three-phase current, harmonic components, and reactive power generated on the primary side of the YD-MFBT The sum of the three-phase currents generated on the primary side of the YD-MFBT is zero, which makes the power factor of the primary side current of the YD-MFBT 1 and the three phases symmetrical.

3. The control method according to claim 2, characterized by, The YD-MFBT specifically includes: According to the Kirchhoff law and the characteristics of the YD-MFBT, the basic current conversion relationship is: wherein, YD-MFBT primary current, i.e. grid side current; YD-MFBT secondary side a, b port load current; YD-MFBT secondary side u, w tap compensation current, v tap current The two-phase load current is divided into a symmetrical active component a harmonic component and a reactive component in two parts: wherein the symmetrical active component The YD-MFBT primary side generates symmetrical three-phase current with power factor of 1; makes compensation current satisfies: to enable grid side current contains only positive sequence fundamental components According to the formula (3) and (5), it is calculated that Definition of the fundamental active component of the two-phase load current I pα , I pβ , the effective value of the symmetrical active component I plα = I plβ = 0.5(I pα + I pβ ), and the phase and the α, β port voltages respectively correspond to the same; The two-phase load current thus has a harmonic component and a reactive component of: YD-MFBT secondary side u, v, w tap compensation current is obtained from formula (6), (7)

Images