MMC-based port-isolated type phase power supply system and control method thereof

Abstract

The application discloses a port isolation type in-phase power supply system based on MMC and a control method thereof, which comprises a three-phase high-voltage DC bus at a power grid side, a traction Scott transformer for traction of an electric locomotive, a three-phase MMC-STATCOM reactive power compensation device, and a matching Scott transformer for connecting the traction Scott transformer and the three-phase MMC-STATCOM reactive power compensation device; the primary side of the traction Scott transformer is connected with the three-phase high-voltage DC bus, and the secondary side is connected with the corresponding secondary side of the matching Scott transformer; the primary side of the matching Scott transformer is connected with the three-phase MMC-STATCOM reactive power compensation device; the alpha phase and the beta phase of the secondary side of the traction Scott transformer are connected in series and connected to a traction bus to supply power to the electric locomotive. The system can not only realize three-end isolation of the power grid system, the traction system and the compensation system and ensure safety, but also form a three-phase symmetrical voltage with the same phase as the power grid voltage at the alternating current side of the three-phase MMC-STATCOM reactive power compensation device, and has a small capacity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrified railway traction power supply system, and particularly relates to an electrified railway in-phase power supply system based on a Scott transformer. BACKGROUND

[0002] At present, single-phase power frequency alternating current AC-DC-AC type electric locomotives are widely used in high-speed railways, and the harmonic content is small, and the power factor is close to 1, so the negative sequence problem is the main power quality problem. Since the traction power of the AC-DC-AC type electric locomotive is large, and with the increase of the number of high-speed railways, the power quality problem brought by the electric locomotive to the power system is increasingly serious, and needs to be solved urgently. In order to reduce the influence of the negative sequence problem, the electric phase separation power supply scheme of rotating phase sequence and phase separation is widely used in high-speed railways. However, the electric phase separation brings problems such as speed drop of the electric locomotive, overvoltage, and limited construction site, which has certain safety hazards to the normal operation of the electric locomotive. The existing automatic phase-over technology mainly includes ground switch automatic switching phase-over, vehicle-mounted automatic phase-over, and column automatic phase-over. However, the automatic phase-over technology cannot completely solve the hazards brought by the electric phase separation.

[0003] In order to solve the negative sequence and electric phase separation problems, an in-phase power supply system is proposed. Theory and practice show that the in-phase power supply technology can cancel the electric phase separation at the outlet of the substation, and can effectively solve the negative sequence problem. The in-phase power supply system includes an in-phase power supply system based on a compensation device and a through-type in-phase power supply system. At present, the in-phase power supply system based on the compensation device has been put into use, which mainly consists of a traction transformer, a matching transformer and a compensation device. The compensation device includes a passive compensation device, an active compensation device and a hybrid compensation device combining passive and active compensation devices. The passive compensation device cannot dynamically adjust the reactive power, has poor flexibility and cannot effectively suppress harmonics; the hybrid compensation device can reduce the active compensation capacity, but the active compensation capacity is still large, and the control method is complex. Therefore, the active compensation device with strong applicability, strong dynamic regulation capability and significant compensation effect has become a research hotspot. However, the active compensation device has the problems of large capacity and high cost, which is not conducive to popularization. SUMMARY

[0004] In view of the above problems, the present application provides an in-phase power supply system based on MMC and a control method thereof. The system not only realizes three-end isolation of the power grid system, the traction system and the compensation system, and ensures safety, but also forms three-phase symmetrical voltage with the same phase as the grid voltage at the three-phase MMC AC side, and has small capacity.

[0005] The purpose of the present application is achieved by the following technical solutions:

[0006] The application discloses a port isolation type same-phase power supply system based on MMC, which comprises a three-phase high-voltage DC bus on the power grid side, a traction Scott transformer TT for traction of an electric locomotive, a three-phase MMC-STATCOM reactive power compensation device, and a matching Scott transformer MT for connecting the traction Scott transformer TT and the three-phase MMC-STATCOM reactive power compensation device.

[0007] The primary side of the traction Scott transformer TT is connected with the three-phase high-voltage DC bus, and the secondary side is connected with the corresponding secondary side of the matching Scott transformer MT; the primary side of the matching Scott transformer MT is connected with the three-phase MMC-STATCOM reactive power compensation device; the alpha phase and the beta phase of the secondary side of the traction Scott transformer TT are connected in series and connected to a traction bus to supply power to the electric locomotive.

[0008] As a further improvement of the application, the three terminals a, b and c of the primary side of the traction Scott transformer TT are connected with the A phase, the B phase and the C phase of the three-phase high-voltage DC bus respectively; the four terminals α1, n1, n2 and β1 of the secondary side of the traction Scott transformer TT are connected with the four terminals α2, m1, m2 and β2 of the secondary side of the matching Scott transformer MT respectively, wherein n1 is connected with n2 and m1 is connected with m2; the three terminals u, v and w of the primary side of the matching Scott transformer MT are connected with the U phase, the V phase and the W phase of the three-phase MMC-STATCOM reactive power compensation device respectively; the terminals α1 and β1 of the secondary side of the traction Scott transformer TT are led out and connected to the traction bus to supply power to the electric locomotive.

[0009] As a further improvement of the application, each bridge arm of the three-phase MMC-STATCOM reactive power compensation device has n sub-modules, and the sub-modules adopt a half-bridge structure.

[0010] A control method of a port isolation type same-phase power supply system based on MMC, comprising:

[0011] 1) a positive and negative sequence separation method using double synchronous coordinate system decoupling is used, a decoupling operation of a double-frequency component is introduced to eliminate the double-frequency component, so that the dq axis direct current components of the positive and negative sequence currents are obtained respectively;

[0012] 2) the positive sequence component of the three-phase MMC-STATCOM reactive power compensation device is subjected to synchronous rotating coordinate transformation, the d and q axes are subjected to decoupling control, and the positive sequence fundamental voltage quantity of the expected output on the alternating current side is obtained; the control method of the negative sequence component is the same as that of the positive sequence, and current control is performed;

[0013] 3) capacitor voltage sharing control is performed, and specifically includes:

[0014] ①MMC in the running process has two states of charging or discharging, by changing the duty cycle of each sub-module switch driving signal, to change its charging and discharging time, control the sub-module capacitor voltage to balance;

[0015] ②Introducing interphase capacitor voltage balance control, comparing the average value of all sub-module capacitor voltages with the average value of each phase sub-module capacitor voltage in the voltage outer loop control, generating each phase circulating current instruction value;

[0016] 4) Using the positive and negative sequence separation method based on double synchronous coordinate system decoupling to control the circulating current suppression.

[0017] As a further improvement of the application, the positive and negative sequence separation method based on double synchronous coordinate system decoupling specifically comprises:

[0018] The three-phase asymmetric current is subjected to positive sequence fundamental frequency rotating coordinate transformation:

[0019]

[0020] That is, the positive sequence component is transformed into a direct current component, and the negative sequence component is transformed into a double frequency component.

[0021] The three-phase asymmetric current is subjected to negative sequence fundamental frequency rotating coordinate transformation:

[0022]

[0023] That is, the negative sequence component is transformed into a direct current component, and the positive sequence component is transformed into a double frequency component.

[0024] As a further improvement of the application, the feedforward decoupling control based on synchronous rotating coordinate transformation specifically comprises:

[0025] For the positive sequence component, according to Kirchhoff's law, it is obtained:

[0026]

[0027] In the formula,

[0028]

[0029] Synchronous rotating coordinate transformation is performed on formula (12), and the following formula is obtained:

[0030]

[0031] Decoupling control is performed on the d and q axes, and

[0032]

[0033] The positive sequence fundamental frequency voltage quantity of the expected output of the alternating current side is obtained from formula (14) and formula (15):

[0034]

[0035] The control method for the negative sequence component is the same as the positive sequence.

[0036] As a further improvement of the application, the charging and discharging time of each sub-module is changed by changing the duty ratio of the switch driving signal of each sub-module, so as to control the sub-module capacitor voltage to balance, specifically comprising:

[0037] When i ap *u error >0, increase the on-duty ratio of the sub-module, and prolong the charging or discharging time; when i ap *u error <0, reduce the on-duty ratio of the sub-module, and shorten the charging or discharging time.

[0038] As a further improvement of the application, the positive and negative sequence separation method based on double synchronous coordinate system decoupling is used for the control of circulating current suppression, specifically comprising:

[0039] In the three-phase positive and negative sequence network, the mathematical model of circulating current is:

[0040]

[0041] In the formula, i zx is the internal three-phase circulating current of the MMC, and u zx is the internal three-phase unbalanced voltage drop of the MMC;

[0042] Synchronous rotating coordinate transformation is performed on the formula (17) to obtain:

[0043]

[0044] The d and q axes are decoupled and controlled.

[0045] Compared with the prior art, the application has the following beneficial effects:

[0046] The application provides a MMC-based port-isolated in-phase power supply system, which comprises a three-phase high-voltage DC bus at the power grid side, a traction Scott transformer three-phase MMC-STATCOM reactive power compensation device for traction of electric locomotives, and a matching Scott transformer for connecting the traction Scott transformer and the three-phase MMC-STATCOM reactive power compensation device for traction of electric locomotives; the system is connected in complete symmetry by the traction Scott transformer and the matching Scott transformer, which not only realizes three-end isolation of the power grid system, the traction system and the compensation system, ensures safety, but also forms three-phase symmetrical voltage with the same phase as the power grid voltage at the AC side of the three-phase MMC-STATCOM reactive power compensation device, and has small capacity. The application can be used in the in-phase power supply system of electrified railways, can solve the negative sequence problem and the electric phase separation problem in the existing railway power supply system, has small capacity of the converter of the compensation system, saves cost, and is conducive to popularization and application of the in-phase power supply system. BRIEF DESCRIPTION OF DRAWINGS

[0047] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the application, and for those skilled in the art, other drawings can also be obtained without creative labor based on these drawings.

[0048] Figure 1 The typical topology structure of the in-phase power supply system is shown in the figure.

[0049] Figure 2 The topology structure of the three-phase MMC-STATCOM reactive power compensation device is shown in the figure.

[0050] Figure 3 The in-phase power supply system based on MMC is shown in the figure.

[0051] Figure 4 The topology structure of the MMC-PFC converter is shown in the figure.

[0052] Figure 5 The extraction of the fundamental active current effective value of the load current is shown in the figure.

[0053] Figure 6 The equivalent circuit diagram of the STATCOM is shown in the figure.

[0054] Figure 7 The working phasor diagram of the STATCOM is shown in the figure.

[0055] Figure 8 The implementation block diagram of the positive and negative sequence separation method based on double synchronous coordinate system decoupling is shown in the figure.

[0056] Figure 9The equivalent circuit diagram of positive sequence component of one phase of three-phase MMC-STATCOM reactive power compensation device;

[0057] Figure 10 The positive and negative sequence component current control block diagram;

[0058] Figure 11 The schematic diagram of charging or discharging of MMC sub-module in operation process;

[0059] Figure 12 The MMC sub-module capacitor voltage equalization control block diagram;

[0060] Figure 13 The three-phase MMC inter-phase capacitor voltage equalization control block diagram;

[0061] Figure 14 The control block diagram of three-phase MMC internal circulating current suppression in the system;

[0062] Figure 15 The grid voltage and grid current waveform when the railway power supply system does not access three-phase MMC-STATCOM reactive power compensation device for negative sequence compensation;

[0063] Figure 16 The simulation waveform when the system accesses three-phase MMC-STATCOM reactive power compensation device for negative sequence compensation. DETAILED DESCRIPTION

[0064] In order to make the purpose and technical scheme of the present application more clear and convenient to understand. The present application is further described in detail below in combination with the drawings and examples. The specific examples described herein are only used to explain the present application, and are not used to limit the present application.

[0065] In the description of the present application, it needs to be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the device or element indicated must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second" are only for description purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined as "first", "second" can be explicitly or implicitly included one or more of the features. In the description of the present application, unless otherwise specified, the meaning of "a plurality of" is two or more. In the description of the present application, it needs to be explained that, unless otherwise specified and limited, the terms "mounting", "connection", "connection" should be understood in a broad sense, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium, or it can be the communication between the two elements inside. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0066] The technical solutions of the present application will be described clearly and completely in combination with the drawings and specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of the present application.

[0067] The present application provides an electrified railway same-phase power supply system based on Scott transformer and modular multilevel converter MMC. The system comprises a three-phase high-voltage DC bus at the grid side, a traction Scott transformer TT for traction of electric locomotives, a matching Scott transformer MT for connecting the traction system and the compensation device, and a three-phase MMC-STATCOM reactive power compensation device.

[0068] The primary side of the traction Scott transformer TT is connected with the three-phase high-voltage DC bus, and the secondary side is connected with the corresponding secondary side of the matching Scott transformer MT; the primary side of the matching Scott transformer MT is connected with the three-phase MMC-STATCOM reactive power compensation device. The α phase and the β phase of the secondary side of the traction Scott transformer TT are connected in series and connected to the traction bus to supply power to the electric locomotive.

[0069] Figure 1The typical topology of the same phase power supply system is shown in the figure. The three terminals of the primary side of the traction Scott transformer TT are a, b and c, and the four terminals of the secondary side are α1, n1, n2 and β1. The three terminals of the primary side of the matching Scott transformer MT are u, v and w, and the four terminals of the secondary side are α2, m1, m2 and β2. The connection mode is as follows:

[0070] The three terminals a, b and c of the primary side of the traction Scott transformer TT are connected with the three-phase high-voltage DC bus A phase, B phase and C phase respectively. The four terminals α1, n1, n2 and β1 of the secondary side of the traction Scott transformer TT are connected with the four terminals α2, m1, m2 and β2 of the secondary side of the matching Scott transformer MT respectively, wherein n1 is connected with n2, and m1 is connected with m2, so that the three-phase voltage u phase, v phase and w phase which are completely same in phase and symmetrical with the A phase, B phase and C phase of the three-phase power grid can be obtained at the primary side of the matching Scott transformer MT. The three terminals u, v and w of the primary side of the matching Scott transformer MT are connected with the U phase, V phase and W phase of the three-phase MMC-STATCOM reactive power compensation device respectively. The terminals α1 and β1 of the secondary side of the traction Scott transformer TT are led out and connected to the traction bus to supply power to the electric locomotive.

[0071] Figure 2 The topology structure of the three-phase MMC-STATCOM reactive power compensation device is shown in the figure. Each bridge arm has n sub-modules, and the sub-modules adopt a half-bridge structure.

[0072] Figure 3 The same phase power supply system based on MMC is shown in the figure.

[0073] Figure 4 The topology structure of the MMC-PFC converter is shown in the figure. Table 1 shows the comparison of the capacity of the MMC converter in the present application and the same phase power supply system based on MMC. As shown in Table 1, the capacity of the MMC converter in the present application can be the smallest. At the same time, Figure 3 The same phase power supply system based on MMC is shown in the figure. The capacity of the MMC converter in the present application is the same as that in the present application, but Figure 3 The same phase power supply system shown in (a) cannot realize the isolation of the traction system port and the compensation system port. The present application can realize the three-end isolation of the power grid system, the traction system and the compensation system, and ensures the safety of the same phase power supply system.

[0074] Table 1

[0075]

[0076] The basic principle and control method of the present application are as follows:

[0077] The extraction of the three-phase MMC-STATCOM reactive power compensation device AC output side command current under the same phase power supply topology, the establishment of the equivalent circuit of three-phase MMC-STATCOM reactive power compensation device, the control principle is introduced, and a kind of closed loop control strategy.

[0078] (1) three-phase MMC AC side command current extraction

[0079] In high-speed railway, the compensation of negative sequence current is the main purpose of the same phase power supply system compensation. The total negative sequence current caused by the traction transformer secondary side port current in three-phase power grid is:

[0080]

[0081] Among them, k m is the ratio of the secondary side voltage of the traction transformer to the primary side line voltage; I m is the effective value of the traction transformer secondary side port current; Ψ m is the angle of the traction transformer secondary side port voltage lagging behind the primary side A phase voltage; is the power factor angle of the traction transformer secondary side port.

[0082] For traction Scott transformer, when its secondary side port current satisfies I α = I β , the negative sequence of the grid current can be completely compensated.

[0083] Let the traction Scott transformer (TT) secondary side α phase, β phase and load voltage be:

[0084]

[0085] Then the desired secondary side port current is:

[0086]

[0087] Let the load current be:

[0088]

[0089] Among them, is the load power factor angle, i h is the harmonic component, I L is the current effective value.

[0090] According to the power conservation, the energy provided by the power supply is equal to the energy consumed by the load, which can be obtained:

[0091]

[0092] Among them, I Lpis the effective value of the active current of the load,

[0093] The desired secondary port current of the Scott transformer (MT) is:

[0094]

[0095] According to the characteristics of the Scott transformer, the desired primary current of the Scott compensation transformer (MT) can be obtained, i.e. the three-phase MMC-STATCOM reactive power compensation device AC output side command current is:

[0096]

[0097] Among them, the extraction of the fundamental active current effective value of the load current uses a 90° phase angle offset scheme of the second-order generalized integrator (SOGI) to generate two-phase orthogonal signals containing only the fundamental wave, and after a series of operations, the twice frequency components are filtered out to obtain The block diagram is shown in Figure 5 .

[0098] (2) STATCOM working principle

[0099] STATCOM can be regarded as an AC voltage source with controllable amplitude and phase at the same frequency as the grid. When considering losses, the equivalent circuit and working phasor diagram of STATCOM are shown in Figure 6 、 Figure 7 .

[0100] The phasor diagram shown in Figure 5 is analyzed, and from the geometric relationship we can get:

[0101]

[0102] In the formula, δ is the phase difference between and , is the impedance angle of the connected reactor.

[0103] Therefore, the effective values of the reactive current and active current absorbed by STATCOM from the grid at steady state are:

[0104]

[0105] As can be seen from equation (9), by controlling the values of δ and , the output current of STATCOM can be controlled to follow the command current value, thereby achieving the purpose of compensating the grid current.

[0106] (3) MMC closed-loop control

[0107] For the control of three-phase MMC-STATCOM reactive power compensation device, the system adopts feedforward decoupling control strategy based on synchronous rotating coordinate transformation.

[0108] 1) Positive and negative sequence separation based on double synchronous coordinate system decoupling

[0109] The three-phase MMC-STATCOM reactive power compensation device AC output side command current is three-phase asymmetric current. If the three-phase asymmetric current is directly subjected to positive sequence fundamental frequency rotating coordinate transformation:

[0110]

[0111] That is, the positive sequence component is transformed into a direct current component, and the negative sequence component is transformed into a double frequency component.

[0112] Similarly, the three-phase asymmetric current is subjected to negative sequence fundamental frequency rotating coordinate transformation:

[0113]

[0114] That is, the negative sequence component is transformed into a direct current component, and the positive sequence component is transformed into a double frequency component.

[0115] Because the frequency band of the low-pass filter is narrow, if the double frequency component is filtered out using a low-pass filter, the dynamic performance of the control system will be affected. Therefore, the double coordinate system decoupling sequence separation method is adopted, and the double frequency component is eliminated by introducing decoupling operation of the double frequency component, so as to obtain the dq axis direct current components of the positive and negative sequence currents respectively. The specific implementation block diagram is shown in Figure 8 .

[0116] 2) Current control

[0117] For the positive sequence component of one phase of the three-phase MMC-STATCOM reactive power compensation device, the equivalent circuit is shown in Figure 9 .

[0118] According to Kirchhoff's law, the following can be obtained: Figure 6 Figure 9

[0119]

[0120] In the formula,

[0121]

[0122] Synchronous rotating coordinate transformation is performed on equation (12) to obtain:

[0123]

[0124] Decoupling control is performed on the d and q axes, and ​​

[0125]

[0126] The positive sequence fundamental voltage quantity of the AC side desired output is obtained from formula (14) and formula (15):

[0127]

[0128] For the negative sequence component, the control method is the same as the positive sequence. Thus, the control block diagram thereof is shown as Figure 10 .

[0129] 3) Capacitor voltage balancing control

[0130] ① Sub-module capacitor voltage balancing control

[0131] In the normal operation of the MMC, it is very important to ensure that the capacitor voltage of each sub-module is constant. The MMC sub-module has two states of charging or discharging in the operation process, as shown in Figure 11 , we can change the charging and discharging time of each sub-module by changing the duty cycle of the switch driving signal of each sub-module, so as to control the sub-module capacitor voltage to balance.

[0132] The control block diagram is shown as Figure 12 .

[0133] When i ap *u error >0, increase the on-duty cycle of the sub-module, and prolong the charging or discharging time; when i ap *u error <0, reduce the on-duty cycle of the sub-module, and shorten the charging or discharging time.

[0134] ② Inter-phase capacitor voltage balancing control

[0135] In the normal operation of the MMC, the problem of unbalanced sub-module capacitor voltage among three phases will occur. Therefore, the inter-phase capacitor voltage balancing control is introduced. In order to ensure that the additional power for balancing among three phases does not change the total power of three phases, the average value of all sub-module capacitor voltages is compared with the average value of each phase sub-module capacitor voltage in the voltage outer loop control, and the circulating current command value of each phase is generated, and the control block diagram is shown as Figure 13 .

[0136] 4) Circulating current suppression

[0137] The energy flow between the AC side and the DC side of the MMC will generate a double-frequency power fluctuation in the MMC, which will cause a double-frequency fluctuation of the capacitor voltage of the sub-module. The double-frequency fluctuation voltage of the capacitor will generate an additional double-frequency excitation voltage after modulation, which acts on the bridge arm inductance and generates a double-frequency circulating current in the MMC. The double-frequency circulating current will distort the bridge arm current and occupy the capacity of the switching device, increasing the loss, so it is necessary to suppress the circulating current in the MMC.

[0138] The three-phase unbalanced current output by the three-phase AC side of the three-phase MMC-STATCOM reactive power compensation device includes positive sequence current components and negative sequence current components, which generate double-frequency negative sequence circulating current and positive sequence circulating current in the MMC respectively, so the positive and negative sequence components of the circulating current are extracted by using the positive and negative sequence separation method based on double synchronous coordinate system decoupling, and the double-frequency circulating current is suppressed by positive and negative sequence independent control.

[0139] According to Figure 9 , the mathematical model of the circulating current in the three-phase positive and negative sequence network is:

[0140]

[0141] In the formula, i zx is the three-phase circulating current in the MMC, u zx is the three-phase unbalanced voltage drop in the MMC.

[0142] Synchronous rotating coordinate transformation is performed on formula (17) to obtain

[0143]

[0144] Decoupling control is performed on the d and q axes to obtain the control block diagram of the circulating current suppression in the system as shown in Figure 14 .

[0145] (4) Simulation verification

[0146] Table 2 shows the specific parameters of the Matlab / Simulink simulation model based on the system.

[0147] Table 2

[0148]

[0149] Figure 15 The grid voltage and grid current waveforms when the three-phase MMC-STATCOM reactive power compensation device is not connected to the railway power supply system for negative sequence compensation. In order to facilitate observation, the grid voltage is 1 / 100 of the original grid voltage when measured.

[0150] Figure 16The simulation waveforms when the three-phase MMC-STATCOM reactive power compensation device is accessed to the system for negative sequence compensation. Figure 16 (a) The upper graph is the actual current waveform of the three-phase MMC-STATCOM reactive power compensation device on the AC side, Figure 16 (a) The lower graph is the command current waveform of the three-phase MMC-STATCOM reactive power compensation device on the AC side, Figure 16 (b) is the grid voltage and current waveform. Figure 16 (a) It can be seen that under the above control method, the actual current of the three-phase MMC-STATCOM reactive power compensation device on the AC side can completely follow the command current, and has good control effect. Figure 16 (b) and Figure 15 It can be seen from the comparison that after the three-phase MMC-STATCOM reactive power compensation device is accessed to the railway power supply system for compensation, the large amount of negative sequence and reactive current formed on the grid side due to the traction locomotive is effectively suppressed, thereby realizing the symmetry of the three-phase current on the grid side, and the grid voltage and current are in phase.

[0151] The above is only the preferred embodiment of the present application, and does not limit the present application, and any simple modification, change and equivalent structural change according to the technical essence of the present application to the above embodiment are still within the protection scope of the technical solution of the present application.

[0152] The above embodiments are only used to illustrate the technical solutions of the present application and not to limit the present application, although the present application is described in detail with reference to the above embodiments, the ordinary skilled in the art can still modify or equivalently replace the specific implementation scheme of the present application, and these any modification or equivalent replacement without departing from the spirit and scope of the present application is within the protection scope of the claims of the present application.

Claims

1. A control method of an MMC-based port-isolated phase- balanced power supply system, characterized by, The port-isolated same-phase power supply system comprises a three-phase high-voltage DC bus on the power grid side, a traction Scott transformer TT for traction of electric locomotives, a three-phase MMC-STATCOM reactive power compensation device, and a matching Scott transformer MT for connecting the traction Scott transformer TT and the three-phase MMC-STATCOM reactive power compensation device; The primary side of the traction Scott transformer TT is connected with the three-phase high-voltage DC bus, and the secondary side is connected with the corresponding phase of the secondary side of the matching Scott transformer MT; the primary side of the matching Scott transformer MT is connected with the three-phase MMC-STATCOM reactive power compensation device; the α phase and the β phase of the secondary side of the traction Scott transformer TT are connected in series and connected to a traction bus to supply power to electric locomotives; The control method of the port-isolated same-phase power supply system based on MMC comprises the following steps: 1) a positive and negative sequence separation method based on double synchronous coordinate system decoupling is used, and a decoupling operation of a double-frequency component is introduced to eliminate the double-frequency component, so as to obtain the dq-axis direct current components of the positive and negative sequence currents respectively; 2) the positive sequence component of the three-phase MMC-STATCOM reactive power compensation device is subjected to synchronous rotating coordinate transformation, and the d and q axes are subjected to decoupling control to obtain the positive sequence fundamental voltage quantity of the expected output on the alternating current side; the control method of the negative sequence component is the same as that of the positive sequence, and current control is performed; 3) capacitor voltage balancing control is performed, specifically including: ① there are two states of charging and discharging in the operation of the MMC, the charging and discharging time of each sub-module is changed by changing the duty cycle of the switch driving signal of each sub-module, and the capacitor voltage of the sub-module is controlled to balance; ② an inter-phase capacitor voltage balancing control is introduced, and in the voltage outer loop control, the average value of all sub-module capacitor voltages is compared with the average value of the capacitor voltage of each phase sub-module to generate a current command value of each phase; 4) the control of circulating current suppression is performed by using the positive and negative sequence separation method based on double synchronous coordinate system decoupling.

2. The control method of the MMC-based port-isolated split-phase power supply system according to claim 1, characterized in that, The positive and negative sequence separation method based on double synchronous coordinate system decoupling specifically includes: positive sequence fundamental frequency rotating coordinate transformation is performed on the three-phase asymmetric current: (10) that is, the positive sequence component is transformed into a direct current component, and the negative sequence component is transformed into a double-frequency component; negative sequence fundamental frequency rotating coordinate transformation is performed on the three-phase asymmetric current: (11) that is, the negative sequence component is transformed into a direct current component, and the positive sequence component is transformed into a double-frequency component.

3. The control method of the MMC-based port-isolation type phase- balanced power supply system according to claim 1, characterized in that, The feedforward decoupling control based on synchronous rotating coordinate transformation specifically includes: for the positive sequence component of one phase of the three-phase MMC-STATCOM reactive power compensation device, according to Kirchhoff's law, the following equation is obtained: (12) in the equation, (13) synchronous rotating coordinate transformation is performed on equation (12) to obtain: (14) d and q axes are subjected to decoupling control, and (15) the positive sequence fundamental voltage quantity of the expected output on the alternating current side is obtained from equation (14) and equation (15): (16) the control method of the negative sequence component is the same as that of the positive sequence.

4. The control method of a MMC-based port-isolated split-phase power supply system according to claim 1, wherein, the charging and discharging time of each sub-module is changed by changing the duty cycle of the switch driving signal of each sub-module, so as to control the capacitor voltage of the sub-module to balance, specifically including: When i ap *u error >0, increase the conduction duty ratio of the sub-module, prolong the charging or discharging time; when i ap *u error <0, reduce the conduction duty ratio of the sub-module, shorten the charging or discharging time.

5. The control method of a MMC-based port-isolated split-phase power supply system according to claim 1, wherein, The control of the circulation suppression is performed by using a positive and negative sequence separation method based on double synchronous coordinate system decoupling, and specifically includes the following steps: In the three-phase positive and negative sequence network, the mathematical model of the circulation is: (17) In the formula, is the three-phase circulating current inside the MMC, is the three-phase unbalanced voltage drop inside the MMC; Synchronous rotating coordinate transformation is performed on the equation (17) to obtain: (18) The d and q axes are decoupled and controlled.

6. The control method of a MMC-based port-isolated split-phase power supply system according to claim 1, wherein, The three terminals a, b and c of the primary side of the traction Scott transformer TT are connected with the three-phase high-voltage DC bus A phase, B phase and C phase respectively; the four terminals α1, n1, n2 and β1 of the secondary side of the traction Scott transformer TT are connected with the four terminals α2, m1, m2 and β2 of the secondary side of the matching Scott transformer MT respectively, wherein n1 is connected with n2 and m1 is connected with m2; the three terminals u, v and w of the primary side of the matching Scott transformer MT are connected with the U phase, V phase and W phase of the three-phase MMC-STATCOM reactive power compensation device respectively; the terminals α1 and β1 of the secondary side of the traction Scott transformer TT are led out and connected to the traction bus to supply power to the electric locomotive.

7. The control method of a MMC-based port-isolated split-phase power supply system according to claim 1, wherein, Each bridge arm of the three-phase MMC-STATCOM reactive power compensation device has n sub-modules, and the sub-modules adopt a half-bridge structure.

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