MMC type photovoltaic grid-connected device suitable for high-power traction power supply system and control method

Abstract

The application discloses a traction power supply system MMC type photovoltaic grid-connected device and a control method suitable for high power, and relates to the technical field of traction power supply systems. The traction power supply system MMC type photovoltaic grid-connected device comprises a main circuit, a photovoltaic array, a DC / DC converter and a traction power supply system; wherein the main circuit comprises a single-phase MMC converter I, a single-phase MMC converter II and a DC support capacitor; one end of the AC side of the single-phase MMC converter I is connected with an alpha-phase power supply arm of the traction power supply system; one end of the AC side of the single-phase MMC converter II is connected with a beta-phase power supply arm of the traction power supply system; the other ends of the AC sides of the single-phase MMC converter I and the single-phase MMC converter II are connected with each other and grounded through a steel rail; the DC sides of the single-phase MMC converter I and the single-phase MMC converter II are connected in parallel with the DC support capacitor, thereby forming a back-to-back structure; and the photovoltaic array is connected to the DC support capacitor through the DC / DC converter.

Description

Technical Field

[0001] This invention relates to the field of traction power supply technology, and specifically to an MMC type photovoltaic grid-connected device and control method suitable for high-power traction power supply systems. Background Technology

[0002] With the increasing depletion of traditional energy sources worldwide, the energy crisis has become one of the greatest challenges facing humanity. Photovoltaic power generation, with its advantages of being noiseless, pollution-free, and easy to maintain, is a promising alternative to traditional energy sources. As global attention to photovoltaic power generation increases, photovoltaic electricity is gradually shifting from a supplementary energy source to a replacement. my country's railway system is undergoing rapid development, and energy conservation, emission reduction, and the utilization of renewable energy in the railway sector are receiving increasing attention. The growing maturity and widespread application of photovoltaic power generation technology provide new ideas for the development of my country's railways.

[0003] Organically combining traction power supply systems with photovoltaic power generation systems reduces the dependence of traction power supply systems on the main power system and has good development prospects. Commonly used traction transformers in traction power supply systems include single-phase traction transformers, V-connected traction transformers, YNd11 connected traction transformers, Scott connected traction transformers, and impedance matching balancing transformers. Of these, except for the first type, the other four convert the three-phase voltage in the power system into a two-phase voltage to supply power to single-phase loads on two power supply arms, thus enabling single-phase photovoltaic power generation.

[0004] Currently, there is considerable research on grid-connected photovoltaic (PV) power generation systems both domestically and internationally, but only a few studies have explored the comprehensive compensation performance of grid-connected PV systems and their application in railway systems. Existing power converters in traction power supply systems cannot achieve comprehensive reactive power and negative sequence compensation, thus failing to meet the power quality requirements when PV power generation systems are connected to the grid through the traction power supply system. Summary of the Invention

[0005] The purpose of this invention is to provide an MMC-type photovoltaic grid-connected device suitable for high-power traction power supply systems, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, this invention provides a MMC-type photovoltaic grid-connected device suitable for high-power traction power supply systems, comprising a main circuit, a photovoltaic array, a DC / DC converter, and a traction power supply system. The main circuit includes a single-phase MMC converter I, a single-phase MMC converter II, and a DC support capacitor. One end of the AC side of the single-phase MMC converter I is connected to the α-phase power supply arm of the traction power supply system, and one end of the AC side of the single-phase MMC converter II is connected to the β-phase power supply arm of the traction power supply system. The other ends of the AC sides of the single-phase MMC converter I and single-phase MMC converter II are interconnected and grounded via a rail. The DC sides of the single-phase MMC converter I and single-phase MMC converter II are connected in parallel with a DC support capacitor, forming a back-to-back structure. The photovoltaic array is connected to the DC support capacitor via the DC / DC converter.

[0007] In a preferred embodiment, the photovoltaic array includes multiple photovoltaic modules connected in series and parallel. The photovoltaic array is connected to the input terminal of the DC / DC converter, and the DC support capacitor is connected to the output terminal of the DC / DC converter.

[0008] In a preferred embodiment, the MMC type photovoltaic grid-connected device suitable for high-power traction power supply systems is connected to the three-phase power grid through a traction transformer, and the traction transformer is one of the following: a Vv-connected traction transformer, a YNd11-connected traction transformer, a Scott-connected traction transformer, or an impedance matching and balancing transformer.

[0009] In a preferred embodiment, single-phase MMC converter I3 and single-phase MMC converter II4 constitute a four-arm structure, which is divided into four phases: T, U, V, and W. Each arm includes four sub-module circuits, so that the photovoltaic array power level can reach the megawatt level.

[0010] This invention also provides a control method for MMC-type photovoltaic grid-connected devices suitable for high-power traction power supply systems, characterized by comprising the following steps:

[0011] S1. Real-time acquisition of various operating statuses of the main circuit, DC / DC converter, and traction power supply system, including the DC side voltage u in the traction power supply system. dc α-phase power supply arm voltage RMS value U α β-phase power supply arm voltage RMS value U β α-phase power supply arm voltage synchronization signal, α-phase power supply arm load current i αL and the real-time output power P of the photovoltaic array PV ;

[0012] S2, The desired DC voltage value u dc * With real-time acquired DC side voltage u dcThe difference is calculated to obtain the DC voltage deviation. After the DC voltage deviation passes through the PI controller and low-pass filter, the corresponding converter reference current is obtained to characterize the influence of DC voltage on the converter output current.

[0013] S3. Use single-phase MMC converter I and single-phase MMC converter II to transfer active current between the two traction power supply arms and output reactive current to the traction power supply system α and β power supply arms respectively to achieve reactive and negative sequence compensation functions.

[0014] S4. The load current i of the traction power supply system α and β power supply arms αL i βL Expressed in Fourier series form, without considering harmonic components:

[0015]

[0016] Among them, I αLp I βLp I αLq I βLq The active and reactive current amplitudes of the α and β power supply arms.

[0017] S5. Define the active component of the load current I. m It is half the sum of the active current amplitudes of phases α and β.

[0018]

[0019] The load current i in phasor form of the α and β power supply arms is... αL i βL After multiplying each component by its respective phase voltage synchronization signal and summing the results, the active component of the load current I is obtained after low-pass filtering. m ;

[0020] S6. Utilize the active component I of the load current obtained in step S5. m Calculate the reference value i of the compensation current for the α power supply arm. α * ;

[0021] S7. Calculate the compensation current reference signal i′ for photovoltaic access. αc * ;

[0022] S8. Multiply the converter reference current corresponding to the DC voltage deviation in step S2 with the α-phase synchronization signal, and then multiply it with the compensation current reference signal i′ of the photovoltaic access in step S7. αc * Adding them together, we obtain the converter compensation current reference value i. αc * At this point, the calculation of the compensation current reference signal is complete;

[0023] S9. The compensation current reference signal i from step S8 is... αc * Subtract the output current i of the α-phase converter αc After passing through the hysteresis control loop, the initial modulation wave of the α-phase converter is obtained;

[0024] S10. Perform voltage equalization and voltage regulation control on each phase of single-phase MMC converter I and single-phase MMC converter II respectively. Finally, superimpose the output of voltage regulation control and the output of voltage equalization control onto the original control modulation wave to obtain the final MMC converter modulation wave.

[0025] In a preferred embodiment, in step S6, the active component I of the load current obtained in step S5 is utilized. m Calculate the reference value i of the compensation current for the α power supply arm. α * This includes: the active component I of the load current obtained in step S5. m Multiplying the α-phase voltage synchronization signal yields the active current i. pα Reactive current i qα Phase lead u α 90°, amplitude I qα The calculation is as follows:

[0026]

[0027] The active current component I m Multiply by tan30°, then multiply by the corresponding synchronization signal, and add i. pα That is, the reference value i of the compensation current of the α power supply arm is obtained. α * .

[0028] In a preferred embodiment, in step S7, the compensation current reference signal i′ for photovoltaic access is calculated. αc * Includes: taking the active power output P of the photovoltaic system. PV α-phase power supply arm voltage RMS value U α β-phase power supply arm voltage RMS value U β The active current component of the photovoltaic grid connection is calculated. Multiply by the corresponding synchronization signal and superimposed on the α power supply arm compensation current reference value i α * Above, subtract the α-phase load current i αL The compensation current reference signal i′ for photovoltaic access is obtained. αc * .

[0029] In a preferred embodiment, step S10 includes: taking the average value of the capacitor voltage of all sub-modules of each phase bridge arm of the MMC converter, comparing it with the command value as the actual value of the voltage outer loop control, and taking the average value of the current of the upper and lower bridge arms in each phase as the actual value of the current inner loop.

[0030] In a preferred embodiment, step S10 includes voltage regulation control: using the current of the upper and lower bridge arms in each phase of the MMC converter to achieve voltage regulation control. When the bridge arm current is greater than 0, the charging time of the capacitor is reduced, and vice versa.

[0031] In a preferred embodiment, the modulation strategy of the MMC converter includes: based on the SPWM modulation of the two-level converter, the modulation wave remains unchanged, the triangular carrier is expanded into multiple channels, which have the same frequency and amplitude but different phases. The modulation wave is compared with all the triangular carriers, and then the PWM pulses generated by the comparison are superimposed with the same number of triangular carriers to form a multi-level waveform.

[0032] Compared with the prior art, the beneficial effects of the present invention are:

[0033] Chinese patent CN106786742A discloses an integrated conversion device and control method for a photovoltaic traction power supply system. The primary sides of two single-phase transformers are connected to the busbars and rails of two power supply arms, respectively, while the secondary sides are connected to two single-phase four-quadrant converters, forming a back-to-back structure. A photovoltaic array is connected to a DC / DC converter. The system monitors the busbar voltage, feeder current, and photovoltaic output power of the two power supply arms in real time, dynamically allocating the photovoltaic active power output through the two four-quadrant converters. It calculates the current amplitude and phase of the two four-quadrant converters, ensuring that the phasor sum of the feeder current and the four-quadrant converter current in both power supply arms is equal, and that the phase of the combined current is identical to the voltage phase of the power system. This allows the current of the two four-quadrant converters to track a given value, achieving comprehensive reactive power and negative sequence compensation for the traction power supply system. This patent is based on the structure of a railway power regulator (RPC) and integrates photovoltaics into the traction power supply system. However, the structure of this patent has a problem: it is not suitable for high-power applications. With the increasing installed capacity of photovoltaics today, this structure can only achieve megawatt-level photovoltaic integration through cascading. Moreover, the compensation control strategy proposed in this patent is a very basic one. The photovoltaic power is evenly distributed between the two traction power supply arms, but the traction load is obviously asymmetrical. Therefore, the effect on improving power quality is generally limited.

[0034] Based on this, this invention proposes an MMC-type photovoltaic grid-connected device suitable for high-power traction power supply systems. A four-arm MMC structure is introduced into the back-to-back structure, making it suitable for high-power applications and facilitating capacity expansion, reducing hardware costs, and enabling photovoltaic power generation and grid power supply to cooperate. When photovoltaic power generation is sufficient to supply traction power, surplus power can be fed into the grid; when photovoltaic power generation is insufficient to supply traction power, the grid provides corresponding supplementary power. The MMC-type photovoltaic grid-connected device in this invention's traction power supply system has only one DC support capacitor, and two single-phase MMC converters share a single DC bus. The DC support capacitor maintains the DC-side voltage, enabling the converter to output a stable, sinusoidal single-phase current to ensure the realization of photovoltaic grid connection and compensation.

[0035] The control method of this invention enables the converter to perform inverter functions, realizing photovoltaic access to the traction power supply system. When the loads of the two traction power supply arms are different, by collecting information such as load current in real time, the current output by the converter to the two traction power supply arms is dynamically adjusted to achieve dynamic distribution of photovoltaic output power between the two power supply arms. According to different traction load states, by detecting information such as the bus voltage and load current of the power supply arms in the traction power supply system in real time, and through real-time calculation, this control method can realize the transfer of active and reactive power between the two single-phase MMC converters, thereby realizing the comprehensive compensation of reactive power and negative sequence in the traction power supply system and realizing the function of railway power regulator (RPC). The control method of this invention comprehensively considers the dynamic distribution of photovoltaic power and takes into account the compensation effect. Moreover, the modulation of the MMC converter adopts carrier phase-shift modulation, which can suppress harmonics of specific orders, completing the harmonic suppression function not found in the original structure. Attached Figure Description

[0036] Figure 1 A schematic diagram of the existing photovoltaic grid-connected device for traction power supply system;

[0037] Figure 2 This is a schematic diagram of the structure of the MMC type photovoltaic grid-connected device for the traction power supply system of the present invention;

[0038] Figure 3 This is a schematic diagram of the control method for the MMC type photovoltaic grid-connected device in the traction power supply system of the present invention.

[0039] Figure 4 This is a schematic diagram of the carrier phase-shift modulation strategy used in this invention;

[0040] Figure 5 This is a schematic diagram of the voltage equalization control strategy adopted by the T phase of the four-arm MMC converter in this invention.

[0041] Figure 6 This is a schematic diagram of the voltage regulation control strategy adopted by the T phase of the four-arm MMC converter in this invention.

[0042] Explanation of reference numerals in the attached figures:

[0043] 1-Single-phase transformer I, 2-Single-phase transformer II, 3-Single-phase MMC converter I, 4-Single-phase MMC converter II, 5-DC support capacitor, 6-Photovoltaic array, 7-DC / DC converter; 8-Three-phase power grid, 9-Traction transformer, 10-Traction power supply system α-phase power supply arm, 11-Connecting traction power supply system β-phase power supply arm; 12-Railway. Detailed Implementation

[0044] The technical solutions in the embodiments of the present invention will be clearly and completely described below. All other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of the present invention.

[0045] like Figure 2 As shown, the preferred embodiment of the present invention, a photovoltaic grid-connected device suitable for high-power traction power supply systems using an MMC type, includes a main circuit, a photovoltaic array 6, a DC / DC converter 7, and a traction power supply system. The main circuit includes a single-phase MMC converter I3, a single-phase MMC converter II4, and a DC support capacitor 5. One end of the AC side of the single-phase MMC converter I3 is connected to the α-phase power supply arm 10 of the traction power supply system, and one end of the AC side of the single-phase MMC converter II4 is connected to the β-phase power supply arm 11 of the traction power supply system. The other ends of the AC sides of the single-phase MMC converter I3 and single-phase MMC converter II4 are interconnected and grounded via a rail 12. The DC support capacitor 5 is connected in parallel on the DC sides of the single-phase MMC converter I3 and single-phase MMC converter II4, forming a back-to-back structure. The photovoltaic array 6 is connected to the DC support capacitor 5 through the DC / DC converter 7. The function of the DC / DC converter 7 is to raise and stabilize the output voltage of the photovoltaic array, bringing it to the DC side voltage level of the photovoltaic grid-connected device, and it also needs to have MPPT (Multi-Phase Power Transmission Test) function to ensure that the photovoltaic array can efficiently output photovoltaic power.

[0046] Furthermore, the photovoltaic array 6 includes multiple photovoltaic modules connected in series and parallel. The photovoltaic array 6 is connected to the input terminal of the DC / DC converter 7, and the DC support capacitor 5 is connected to the output terminal of the DC / DC converter 7. This invention uses a converter with an MMC structure, which can be expanded by adding MMC sub-modules, so the power level of the photovoltaic array can reach the megawatt level.

[0047] Furthermore, the MMC type photovoltaic grid-connected device suitable for high-power traction power supply systems is connected to the three-phase power grid 8 through the traction transformer 9, and the traction transformer 9 is one of the following: Vv connection traction transformer, YNd11 connection traction transformer, Scott connection traction transformer, or impedance matching balance transformer.

[0048] Furthermore, the single-phase MMC converter I3 and the single-phase MMC converter II4 form a four-arm structure, consisting of four phases: T, U, V, and W. The two MMC arms connecting the α phase are the T and U phases, and the two MMC arms connecting the β phase are the V and W phases. Each arm includes 2n half-bridge sub-modules (SMs). The sub-modules have the same structure and are cascaded with each other. The capacity can be expanded according to the actual application to enable the photovoltaic array power level to reach the megawatt level.

[0049] The MMC structure is highly symmetrical, with each phase's upper and lower bridge arms operating on the same principle. Each bridge arm contains n sub-modules and one bridge arm inductor L. The bridge arm inductor L is used to control the circulating current of the MMC system and to suppress overcurrent and overcurrent rise rate caused by short circuits, thereby preventing damage to the MMC sub-modules. In this invention, the AC side of the single-phase MMC converter I3 is connected to the α-phase power supply arm 10 of the traction power supply system via a filter inductor Ls, and the AC side of the single-phase MMC converter II4 is connected to the β-phase power supply arm 11 of the traction power supply system via a filter inductor Ls. The basic sub-module structures are either full-bridge or half-bridge. The sub-module shown in Figure 1 is a half-bridge structure, and the sub-modules are controlled by trigger pulses U and D. The more sub-modules on each bridge arm of the MMC, the more voltage levels the AC side outputs, resulting in an AC side output that is closer to a sine wave. The highly modular structure of the MMC greatly facilitates the expansion of power levels according to requirements, and the identical sub-module construction facilitates mass production. It has a common DC bus and is suitable for high-voltage, high-power power conversion and transmission.

[0050] Example 2

[0051] This invention also provides a control method (taking α phase as an example) for MMC type photovoltaic grid-connected devices suitable for high-power traction power supply systems, comprising the following steps:

[0052] S1. Real-time acquisition of various operating statuses of the main circuit, DC / DC converter, and traction power supply system, including the DC side voltage u in the traction power supply system. dc α-phase power supply arm voltage RMS value U α α-phase power supply arm voltage synchronization signal, α-phase power supply arm load current i αL and the real-time output power P of the photovoltaic array PV Parameters such as these.

[0053] S2. The control method of this invention is based on voltage and current dual closed-loop control, u dc As the DC-side supporting capacitor voltage, its level represents the absorption or release of capacitor energy, that is, the amount of active power that the two converters need to absorb or release. The expected DC voltage value u... dc * With real-time acquired DC side voltage u dcThe difference is calculated to obtain the DC voltage deviation. After the DC voltage deviation passes through a PI controller and a low-pass filter (used to eliminate the influence of the second harmonic voltage), the corresponding converter reference current is obtained to characterize the influence of DC voltage on the converter output current.

[0054] S3. Single-phase MMC converter I and single-phase MMC converter II are used to transfer active current between the two traction power supply arms and output reactive current to the traction power supply arms α and β respectively to achieve reactive and negative sequence compensation. The purpose of compensation is to ensure that the current phase difference between the two power supply arms is 120° and that the voltage and current of the two power supply arms are in phase.

[0055] S4. The load current i of the traction power supply system α and β power supply arms αL i βL Expressed in Fourier series form, without considering harmonic components:

[0056]

[0057] Among them, I αLp I βLp I αLq I βLq The active and reactive current amplitudes of the α and β power supply arms.

[0058] Step S5: Define the active component I of the load current. m It is half the sum of the active current amplitudes of phases α and β.

[0059]

[0060] The load current i in phasor form of the α and β power supply arms is... αL i βL After multiplying each component by its respective phase voltage synchronization signal and summing the results, the active component of the load current I is obtained after low-pass filtering. m .

[0061] S6. Utilize the active component I of the load current obtained in step S5. m Calculate the reference value i of the compensation current for the α power supply arm. α * .

[0062] Specifically, in step S6, the active component I of the load current obtained in step S5 is used. m Calculate the reference value i of the compensation current for the α power supply arm. α * This includes: the active component I of the load current obtained in step S5. m Multiplying the α-phase voltage synchronization signal yields the active current i. pα Reactive current i qα Phase lead uα 90°, amplitude I qα The calculation is as follows:

[0063]

[0064] The active current component I m Multiply by tan30°, then multiply by the corresponding synchronization signal, and add i. pα That is, the reference value i of the compensation current of the α power supply arm is obtained. α * .

[0065] S7. Calculate the compensation current reference signal i′ for photovoltaic access. αc * .

[0066] Specifically, in step S7, the compensation current reference signal i′ for photovoltaic access is calculated. αc * This includes the fact that the integration of photovoltaic (PV) power will cause the converter output current to include both the original compensation current and the active power component introduced by the PV integration. For the traction power supply system, it is necessary to ensure that the original compensation performance is not affected; therefore, the reference value i of the α power supply arm compensation current is maintained. α * Unchanged. Take the photovoltaic output active power P. PV α-phase power supply arm voltage RMS value U α β-phase power supply arm voltage RMS value U β The active current component of the photovoltaic grid connection is calculated. Multiply by the corresponding synchronization signal and superimposed on the α power supply arm compensation current reference value i α * Above, subtract the α-phase load current i αL The compensation current reference signal i′ for photovoltaic access is obtained. αc * .

[0067] S8. Multiply the converter reference current corresponding to the DC voltage deviation in step S2 with the α-phase synchronization signal, and then multiply it with the compensation current reference signal i′ of the photovoltaic access in step S7. αc * Adding them together, we obtain the converter compensation current reference value i. αc * At this point, the calculation of the compensation current reference signal is complete.

[0068] S9. The compensation current reference signal i from step S8 is... αc * Subtract the output current i of the α-phase converter αc After passing through the hysteresis control loop, the initial modulation wave of the α-phase converter is obtained;

[0069] During operation, the connection and bypass times of each submodule in the S10 and MMC converters are not always identical, leading to capacitor imbalance and unstable DC voltage. Therefore, voltage equalization and regulation control are required for each phase of both single-phase MMC converter I and single-phase MMC converter II. Finally, the outputs of the voltage regulation control and the voltage equalization control are superimposed onto the original control modulation wave to obtain the final MMC converter modulation wave.

[0070] Furthermore, in step S10, the voltage equalization control principle of phase T in phase α is as follows: Figure 5 As shown, the average capacitor voltage of all submodules in each phase arm of the MMC converter is taken as the actual value of the voltage outer loop control and compared with the command value. The average current of the upper and lower arms in each phase is taken as the actual value of the current inner loop. The voltage equalization control of the remaining phases is similar.

[0071] Furthermore, in step S10, the voltage equalization control principle of phase T in phase α is as follows: Figure 6 As shown, voltage regulation control is achieved by using the current of the upper and lower bridge arms in each phase of the MMC converter. When the bridge arm current is greater than 0, the charging time of the capacitor is reduced, and vice versa.

[0072] The MMC modulation strategy of this invention employs carrier phase-shift modulation (CPS-PWM). Its principle is as follows: based on the SPWM modulation of a two-level converter, the modulating wave remains unchanged, while the triangular carriers are expanded into multiple channels, each with the same frequency and amplitude but different phases. The modulating wave is compared with all the triangular carriers, and the resulting PWM pulses, equal in number to the number of triangular carriers, are superimposed to form a multi-level waveform. This allows for high equivalent switching frequency at a relatively low switching frequency, provides relatively good output waveform performance, and effectively reduces harmonics.

[0073] Specifically, such as Figure 4 As shown, CPS-PWM uses N sub-modules on one arm of the MMC bridge as the modulation target. Let the modulation waves of the upper and lower arms be u, respectively. pr u nr The initial phase difference is π; the triangular carriers are u pci u nci (i = 1, 2, ..., N), with a period of T c Let the initial phase of the first carrier wave in the upper arm be... The initial phase of each carrier wave in the upper arm is then... The i-th carrier of the lower bridge arm is out of phase with the i-th carrier of the upper bridge arm. (The last part, "u", appears to be a typo and can be omitted.) pr with u pci The comparison is performed, and the result is used as the switching trigger pulse U in the i-th submodule of the upper bridge arm. U is inverted (considering the dead time) and used as the switching trigger pulse D. The definitions of U and D are as follows: Figure 1The sub-module circuit is shown in the diagram. By adding the results of directly comparing the modulated wave with each carrier wave, an N+1 level can be output; the trigger pulses of the switching transistors of each sub-module in the lower bridge arm are handled similarly. When CPS-PWM is applied to MMC, each sub-module in the upper and lower bridge arms bears 1 / N of the bridge arm voltage. Taking one bridge arm of a four-module MMC as an example, the phases of the four triangular carrier waves differ by T / 4 sequentially.

[0074] The entire control method of the present invention can realize photovoltaic access to the traction power supply system, as well as grid-side reactive power, negative sequence, and harmonic comprehensive compensation.

[0075] The present invention relates to an MMC-type photovoltaic grid-connected device and control method for a traction power supply system, using a four-arm MMC converter as an example. The control strategy fully considers the impact of DC voltage and photovoltaic access on power quality compensation. The structure and control method involved in this invention are universal; even if the number of sub-modules changes, they can be flexibly modified based on the structure and control method mentioned in this invention. Furthermore, the sub-modules in the MMC structure have identical constructions, facilitating mass production without customization, and eliminating the need for high-power transformers, thus controlling costs. The power rating of the MMC converter depends on the number of sub-modules, making it suitable for high-voltage, high-power power conversion and transmission. In today's era of ever-increasing photovoltaic installed capacity, the MMC converter is fully capable of serving as a grid-connected device in a megawatt-level photovoltaic traction power supply system. The MMC converter has a common DC bus, which is stabilized through voltage regulation and equalization control. Even under harsh conditions involving photovoltaic arrays, traction power supply systems, and power grids, it can ensure a stable DC bus voltage, guaranteeing power transmission. This feature is particularly suitable for the Sichuan-Tibet Railway.

[0076] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A MMC-type photovoltaic grid-connected device suitable for high-power traction power supply systems, characterized in that: It includes the main circuit, photovoltaic array (6), DC / DC converter (7) and traction power supply system; The main circuit includes a single-phase MMC converter I (3), a single-phase MMC converter II (4), and a DC support capacitor (5). One end of the AC side of the single-phase MMC converter I (3) is connected to the α-phase power supply arm (10) of the traction power supply system, and one end of the AC side of the single-phase MMC converter II (4) is connected to the β-phase power supply arm (11) of the traction power supply system. The other end of the AC side of the single-phase MMC converter I (3) and the single-phase MMC converter II (4) are connected to each other and grounded through the rail. The DC support capacitor (5) is connected in parallel with the DC side of the single-phase MMC converter I (3) and the single-phase MMC converter II (4) to form a back-to-back structure. The photovoltaic array (6) is connected to the DC support capacitor (5) via a DC / DC converter (7). The single-phase MMC converter I (3) and single-phase MMC converter II (4) constitute a four-bridge arm structure, which is divided into four phases: T, U, V, and W. Each bridge arm includes four sub-module circuits. The two MMC bridge arms connected to the α phase are the T phase and the U phase, and the two MMC bridge arms connected to the β phase are the V phase and the W phase. Each bridge arm includes 2n half-bridge sub-modules with the same structure and cascaded with each other. Each bridge arm includes a bridge arm inductor L. The AC side of the single-phase MMC converter I is connected to the α phase power supply arm of the traction power supply system through the filter inductor Ls, and the AC side of the single-phase MMC converter II is connected to the β phase power supply arm of the traction power supply system through the filter inductor Ls.

2. The MMC-type photovoltaic grid-connected device for high-power traction power supply systems according to claim 1, characterized in that: The photovoltaic array (6) includes multiple photovoltaic modules connected in series and parallel. The photovoltaic array (6) is connected to the input terminal of the DC / DC converter (7), and the DC support capacitor (5) is connected to the output terminal of the DC / DC converter (7).

3. The MMC-type photovoltaic grid-connected device for high-power traction power supply systems according to claim 2, characterized in that: The MMC type photovoltaic grid-connected device suitable for high-power traction power supply system is connected to the three-phase power grid (8) through a traction transformer (9), and the traction transformer (9) is one of the following: Vv connection traction transformer, YNd11 connection traction transformer, Scott connection traction transformer or impedance matching balance transformer.

4. A control method for MMC-type photovoltaic grid-connected devices suitable for high-power traction power supply systems, characterized in that: Includes the following steps: S1. Real-time acquisition of various operating statuses of the main circuit, DC / DC converter, and traction power supply system, including the DC side voltage in the traction power supply system. u dc α-phase power supply arm voltage RMS value U α β-phase power supply arm voltage RMS value U β α-phase power supply arm voltage synchronization signal, α-phase power supply arm load current i αL and the real-time output power of the photovoltaic array P PV ; S2, Desired DC voltage value u dc * With real-time acquired DC side voltage u dc The difference is calculated to obtain the DC voltage deviation. After the DC voltage deviation passes through the PI controller and low-pass filter, the corresponding converter reference current is obtained to characterize the influence of DC voltage on the converter output current. S3. Use single-phase MMC converter I and single-phase MMC converter II to transfer active current between the two traction power supply arms and output reactive current to the traction power supply system α and β power supply arms respectively to achieve reactive and negative sequence compensation functions. S4. The load current of the α and β power supply arms of the traction power supply system. i αL , i βL Expressed in Fourier series form, without considering harmonic components: ; in, I αLp , I βLp , I αLq , I βLq The active and reactive current amplitudes of the α and β power supply arms; S5. Define the active component of the load current. I m It is half the sum of the active current amplitudes of phases α and β. ， The load current in phasor form for the α and β power supply arms i αL , i βL The active component of the load current is obtained by multiplying each component by its respective phase voltage synchronization signal and then summing the results, followed by low-pass filtering. I m ; S6. Utilize the active component of the load current obtained in step S5. I m Calculate the reference value of the compensation current for the α power supply arm. i α * ; S7. Calculate the compensation current reference signal for photovoltaic access. ; S8. Multiply the converter reference current corresponding to the DC voltage deviation in step S2 with the α-phase synchronization signal, and then combine this multiplication with the compensation current reference signal for photovoltaic access in step S7. Add them together to obtain the reference value of the converter compensation current. i αc * At this point, the calculation of the compensation current reference signal is complete; S9. The compensation current reference signal from step S8 i αc * Subtract the output current of the α-phase converter i αc After passing through the hysteresis control loop, the initial modulation wave of the α-phase converter is obtained; S10. Perform voltage equalization and voltage regulation control on each phase of single-phase MMC converter I and single-phase MMC converter II respectively. Finally, superimpose the output of voltage regulation control and the output of voltage equalization control onto the original control modulation wave to obtain the final MMC converter modulation wave. In step S10, the voltage equalization control includes: taking the average value of the capacitor voltage of all sub-modules of each phase bridge arm of the MMC converter, comparing it with the command value as the actual value of the voltage outer loop control, and using the average current of the upper and lower bridge arms in each phase as the actual value of the current inner loop; the voltage regulation control includes: using the current of the upper and lower bridge arms in each phase of the MMC converter to achieve voltage regulation control. When the bridge arm current is greater than 0, the charging time of the capacitor is reduced, and vice versa; the modulation strategy of the MMC converter includes: based on the SPWM modulation of the two-level converter, the modulation wave remains unchanged, the triangular carrier is expanded into multiple channels, with the same frequency and amplitude but different phases, the modulation wave is compared with all the triangular carriers, and then the PWM pulses generated by the comparison are superimposed with the same number of triangular carriers to form a multi-level waveform.

5. The control method for MMC-type photovoltaic grid-connected devices applicable to high-power traction power supply systems as described in claim 4, characterized in that: In step S6, the active component of the load current obtained in step S5 is used. I m Calculate the reference value of the compensation current for the α power supply arm. i α * This includes: the active component of the load current obtained in step S5. I m Multiplying the α-phase voltage synchronization signal yields the active current. i pα Reactive current i qα Phase advance u α 90°, amplitude I qα The calculation is as follows: ； Active current component I m Multiply by tan30°, then multiply by the corresponding synchronization signal, and add... i pα That is, the reference value of the compensation current of the α power supply arm is obtained. i α * .

6. The control method for MMC-type photovoltaic grid-connected devices suitable for high-power traction power supply systems as described in claim 5, characterized in that: In step S7, the compensation current reference signal for photovoltaic access is calculated. Includes: obtaining the active power output of photovoltaic power P PV α-phase power supply arm voltage RMS value U α β-phase power supply arm voltage RMS value U β The active current component of the photovoltaic grid connection is calculated. Multiply by the corresponding synchronization signal and superimposed on the α power supply arm compensation current reference value i α * Above, subtract the α-phase load current. i αL The compensation current reference signal for photovoltaic access is obtained. .

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