Modularized multi-level direct-current transformer topology suitable for multi-voltage-level direct-current power grid and control method

Abstract

The invention provides a modularized multi-level direct-current transformer topology suitable for a multi-voltage-level direct-current power grid and a control method. The topology comprises a bridge arm inductor, an upper bridge arm, a lower bridge arm and a filter inductor. One end of the bridge arm inductor is connected with the high-voltage side anode, and the other end is connected with one end of the upper bridge arm; the other end of the upper bridge arm is connected with one end of the lower bridge arm and one end of the filter inductor; the other end of the lower bridge arm is respectively connected with the high-voltage side cathode and the low-voltage side cathode; the other end of the filter inductor is connected with the low-voltage side anode; each of the upper bridge arm and the lower bridge arm comprises a sub-module string, each sub-module string comprises a plurality of sub-modules which are sequentially connected in series, and the voltage of each of the upper bridge arm and the lower bridge arm is adjusted by controlling the input number of the sub-modules, so that direct-current voltage conversion among different voltage levels is realized. Through the application of the upper and lower chain type sub-modules, the use of complex components is reduced, the cost and the control difficulty are reduced, and meanwhile, the safety, reliability and efficiency of the system are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of power transmission and distribution, and in particular relates to a modular multi-level DC transformer topology and a control method suitable for a multi-voltage DC grid. Background Art

[0002] Modular multilevel DC transformer technology is crucial for multi-voltage DC grids. It effectively improves energy conversion efficiency, optimizes power quality, and reduces construction costs. With the widespread use of renewable energy, this technology has become key to improving energy conversion efficiency and enabling grid interconnection. Modular multilevel DC transformers can handle high voltages, facilitate standardized production and maintenance, and offer excellent scalability, helping to reduce R&D time and operating costs. Furthermore, these transformers are suitable for connecting to diverse DC distribution networks. Their modular design supports multi-port operation, facilitating integration into distributed energy resources and energy storage systems.

[0003] Modular multilevel DC transformers are primarily implemented through fundamental frequency modulation. This method performs sampling and control only once within a high-frequency cycle, unlike traditional industrial frequency MMCs. By adjusting the phase difference of the drive signals of the half-bridge modules, an asymmetric staircase waveform can be generated, and the transformer output can be adjusted according to the output voltage. This method simplifies the design of magnetic components and achieves zero-current shutdown of the low-voltage side switches, thereby reducing current stress. Other research has proposed a quasi-two-level modulation method that can modulate arbitrary waveforms on the AC side. This method is suitable for high-voltage DC transmission, but its application in medium-voltage DC distribution networks is limited due to the limited number of levels.

[0004] The application of modular multi-level DC transformer technology in multi-voltage DC grids, through its unique modulation and control methods, achieves efficient energy conversion and optimizes the grid structure, providing important technical support for the interconnection of future high-voltage DC grids and the absorption of large-scale renewable energy. However, this technology also has some disadvantages, mainly including complex circuit structure, the need for a large number of independent DC power supplies, the use of many components, high cost, and low circuit safety and reliability. The complexity of the circuit structure requires the use of a large number of embedded diodes or high-voltage embedded capacitors, which increases the complexity and control difficulty of the circuit, while also reducing the safety and reliability of the circuit. In addition, in order to achieve a modular multi-level structure, multiple independent DC power supplies are required, which not only increases the cost but also may limit the portability and efficiency of the device. Due to the complexity of the circuit structure and the use of a large number of independent components, the safety and reliability of the entire system may be reduced. Summary of the Invention

[0005] In view of this, the present invention provides a modular multi-level DC transformer topology and control method suitable for multi-voltage DC grids, aiming to optimize the structure of the modular multi-level DC transformer so that it can more efficiently convert and control electrical energy between the high-voltage side and the low-voltage side, while reducing costs and improving system safety.

[0006] In order to achieve the above object, the technical solution provided by the present invention is as follows:

[0007] In a first aspect, the present invention provides a modular multi-level DC transformer topology applicable to a multi-voltage DC grid, comprising:

[0008] Bridge arm inductor, upper bridge arm, lower bridge arm and filter inductor;

[0009] One end of the bridge arm inductor is connected to the positive electrode of the high voltage side, and the other end of the bridge arm inductor is connected to one end of the upper bridge arm;

[0010] The other end of the upper bridge arm is connected to one end of the lower bridge arm and one end of the filter inductor;

[0011] The other end of the lower bridge arm is connected to the high-voltage side negative electrode and the low-voltage side negative electrode respectively;

[0012] The other end of the filter inductor is connected to the positive pole of the low-voltage side;

[0013] Both the upper bridge arm and the lower bridge arm include a submodule string, which includes several submodules connected in series. The upper bridge arm and the lower bridge arm are used to adjust the voltage of their respective bridge arms by controlling the number of submodules put into operation.

[0014] Furthermore, the upper bridge arm and the lower bridge arm are used to control the input of the submodule according to the first control signal to adjust the output voltage to the target value; the first control signal is generated according to the difference between the low-voltage side voltage and the reference voltage.

[0015] Furthermore, the upper bridge arm and the lower bridge arm are also used to perform phase shift control according to a second control signal to maintain energy balance between the upper bridge arm and the lower bridge arm; the second control signal is generated based on the difference between the average value of the capacitor voltage of all sub-modules of the upper bridge arm and the average value of the capacitor voltage of all sub-modules of the lower bridge arm.

[0016] Furthermore, the calculation formula for the steady-state value of the shift ratio is as follows:

[0017]

[0018] Where, For mobile comparison, and are the duty cycles of the upper and lower bridge arms, respectively. is the total power transmitted, is the bridge arm inductance, is the high-voltage side voltage, is the switching cycle.

[0019] Furthermore, the submodule adopts a half-bridge submodule, each half-bridge submodule includes two IGBTs, and the trigger pulse control signals of the two IGBTs are complementary. If one of the IGBTs is turned on, the corresponding submodule is put into operation, and if the other IGBT is turned on, the corresponding submodule is cut off.

[0020] Furthermore, the upper bridge arm and the lower bridge arm are further configured to adjust the voltage and the shift phase according to the first control signal and the second control signal to keep the submodule capacitor voltage stable.

[0021] Furthermore, it also includes: a PI controller, which is used to generate the first control signal and the second control signal.

[0022] Furthermore, the modular multi-level DC transformer adopts step wave quasi-two-level modulation.

[0023] In a second aspect, the present invention provides a control method for a modular multi-level DC transformer topology applicable to a multi-voltage DC grid, which is applied to the modular multi-level DC transformer topology applicable to a multi-voltage DC grid as in the first aspect, comprising:

[0024] Generate a first control signal according to the difference between the low-voltage side voltage and the reference voltage, and control the switching of the submodules of the upper bridge arm and the lower bridge arm according to the first control signal to change the voltage of each bridge arm and adjust the output voltage to a target value;

[0025] generating a second control signal according to a difference between an average value of capacitor voltages of all submodules of the upper bridge arm and an average value of capacitor voltages of all submodules of the lower bridge arm, and determining a phase shift value for performing phase shift control between the upper bridge arm and the lower bridge arm according to the second control signal;

[0026] The voltage and shift ratio of the upper bridge arm and the lower bridge arm are adjusted according to the first control signal and the second control signal to keep the submodule capacitor voltage stable.

[0027] Furthermore, the calculation formula for the steady-state value of the shift ratio is as follows:

[0028]

[0029] Where, For mobile comparison, and are the duty cycles of the upper and lower bridge arms, respectively. is the total power transmitted, is the bridge arm inductance, is the high-voltage side voltage, is the switching cycle.

[0030] The present invention provides a modular multi-level DC transformer topology suitable for a multi-voltage DC grid, comprising a bridge arm inductor, an upper bridge arm, a lower bridge arm, and a filter inductor; one end of the bridge arm inductor is connected to the high-voltage side positive electrode, and the other end is connected to one end of the upper bridge arm; the other end of the upper bridge arm is connected to one end of the lower bridge arm and one end of the filter inductor; the other end of the lower bridge arm is connected to the high-voltage side negative electrode and the low-voltage side negative electrode respectively; the other end of the filter inductor is connected to the low-voltage side positive electrode; the upper bridge arm and the lower bridge arm each include a submodule string, the submodule string includes a plurality of submodules connected in series, and the upper bridge arm and the lower bridge arm adjust the respective bridge arm voltages by controlling the number of submodules put into use, thereby realizing DC voltage conversion between different voltage levels. The present invention reduces the use of complex components through the application of two groups of chain submodules, reduces cost and control difficulty, and improves the safety, reliability and efficiency of the system.

[0031] The present invention also provides a control method for a modular multi-level DC transformer topology applicable to a multi-voltage DC grid. This method has similar effects to the topology when implemented and will not be described in detail here. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0033] Figure 1 Buck-MMDCT topology diagram provided by an embodiment of the present invention;

[0034] Figure 2 Buck-MMDCT equivalent circuit diagram provided by an embodiment of the present invention;

[0035] Figure 3 This is a decomposition diagram of the AC / DC circuit of the Buck-MMDCT provided in an embodiment of the present invention;

[0036] Figure 4 Buck-MMDCT two-level modulation waveform provided by an embodiment of the present invention;

[0037] Figure 5 This is a Buck-MMDCT control block diagram provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0038] In order to make the purposes, features, and advantages of the present invention more obvious and easy to understand, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0039] The main disadvantages of modular multi-level DC transformer topology technology include complex circuit structure, the need for a large number of independent DC power supplies, the use of many components, high cost, and low circuit safety and reliability.

[0040] As an advanced DC transformer technology, modular multilevel DC transformer (MMC-DC) offers advantages such as high scalability, high power rating, and electrical isolation between the input and output sides. However, this technology also has some disadvantages, including:

[0041] Complex circuit structure: A large number of clamping diodes or high-voltage clamping capacitors are required, which increases the complexity and control difficulty of the circuit, and also reduces the safety and reliability of the circuit.

[0042] ‌Requires a large number of independent DC power supplies‌: To achieve a modular multi-level structure, multiple independent DC power supplies are required, which not only increases the cost but may also limit the portability and efficiency of the device.

[0043] Low circuit safety and reliability: Due to the complexity of the circuit structure and the use of a large number of independent components, the safety and reliability of the entire system may be reduced.

[0044] To address the above issues, the present invention provides a modular multi-level DC transformer topology and control method suitable for multi-voltage DC grids. By optimizing the structure of the modular multi-level DC transformer, it can more efficiently convert and control electrical energy between the high-voltage side and the low-voltage side, while reducing costs and improving system safety.

[0045] An embodiment of a modular multi-level DC transformer topology applicable to a multi-voltage DC grid according to the present invention is described in detail below.

[0046] An embodiment of the present invention provides a modular multi-level DC transformer topology applicable to a multi-voltage DC grid, including: a bridge arm inductor, an upper bridge arm, a lower bridge arm, and a filter inductor.

[0047] The bridge arm inductor is an inductive component used to limit the rate of change of current, reduce current ripple, and improve current smoothness. In this topology, one end of the bridge arm inductor is connected to the positive terminal of the high-voltage side, and the other end is connected to one end of the upper bridge arm.

[0048] The upper and lower arms form the main components of the DC transformer, regulating voltage through series-connected submodules. Each arm consists of a submodule string, which consists of several submodules connected in series. The other end of the upper arm is connected to one end of the lower arm and one end of the filter inductor. The other end of the lower arm is connected to the negative terminal on the high-voltage side and the negative terminal on the low-voltage side.

[0049] Submodules are the basic units that make up the bridge arms. They can be half-bridge or full-bridge structures, and each submodule contains a switching device and corresponding diode. By controlling the number of these submodules in operation (i.e., turning them on or off), the total voltage of the bridge arm can be adjusted. When the number of submodules in the upper arm increases, the voltage of the upper arm increases accordingly; conversely, the voltage of the lower arm decreases. In this way, DC voltage conversion between the high-voltage side and the low-voltage side is achieved.

[0050] The filter inductor is located between the lower bridge arm and the positive terminal on the low-voltage side. Its main function is to further smooth the current, reduce the high-frequency components in the current, and improve the power quality. The other end of the filter inductor is connected to the positive terminal on the low-voltage side.

[0051] The control strategy for the aforementioned modular multilevel DC transformer typically includes a modulation strategy and a voltage-sharing control strategy. The modulation strategy generates appropriate switching signals to control the switching states of the submodules to achieve the desired output voltage and current. The voltage-sharing control strategy maintains balanced voltage across the submodule capacitors to prevent excessive voltage deviation. Its operating principle is to control the number of submodules in the upper and lower bridge arms, which consist of multiple series-connected submodules, and adjust the bridge arm voltage to achieve DC voltage conversion between the high-voltage and low-voltage sides. The bridge arm inductors and filter inductors smooth the current, reduce ripple and harmonics, and improve power quality.

[0052] This embodiment provides a modular multilevel DC transformer topology suitable for multi-voltage DC grids. By using two sets of upper and lower chain sub-modules, the use of complex components is reduced, the cost and control difficulty are lowered, and the safety, reliability and efficiency of the system are improved.

[0053] In one embodiment, when the modular multi-level DC transformer is operating, a voltage feedback control strategy is used to precisely control the output voltage. In the voltage feedback control strategy, a first control signal is generated based on the difference between the low-voltage side voltage and the reference voltage. The first control signal is used to control the switching of the sub-modules of the upper bridge arm and the lower bridge arm, thereby changing the voltage of each bridge arm to adjust the output voltage to the target value.

[0054] It should be noted that one of the control strategies for modular multilevel DC transformers (MMCs) is a closed-loop control system based on voltage feedback. This control strategy relies on a feedback loop in which the actual output voltage on the low-voltage side is monitored and compared with a preset reference voltage. The control system calculates the difference between the actual output voltage on the low-voltage side and the reference voltage. This difference reflects the output voltage deviation and is the amount that needs to be adjusted. Based on this difference, the control system generates a first control signal. This control signal serves as the basis for regulating the switching of submodules to correct the output voltage deviation. The first control signal is used to control the switching of submodules in the upper and lower bridge arms.

[0055] In one embodiment, when the modular multi-level DC transformer is operating, a phase-shift control strategy is adopted to maintain energy balance between the upper bridge arm and the lower bridge arm. In the phase-shift control strategy, a second control signal is generated based on the difference between the average value of the capacitor voltage of all sub-modules of the upper bridge arm and the average value of the capacitor voltage of all sub-modules of the lower bridge arm. The second control signal is used to determine the phase shift value for phase-shift control between the upper bridge arm and the lower bridge arm.

[0056] It should be noted that in an MMC, the upper and lower arms each consist of a series of submodules connected in series. The capacitor voltages of these submodules must be balanced to ensure stable operation of the transformer. The phase-shift control strategy controls the energy flow between the upper and lower arms by adjusting the switching angles (i.e., phase shift angles) of the submodules, thereby maintaining energy balance. In this phase-shift control strategy, the control system monitors the average capacitor voltages of all submodules in the upper and lower arms. The difference between these two averages is used to generate a second control signal. This signal serves as the basis for adjusting the phase-shift angle to ensure energy balance between the upper and lower arms. Specifically, the second control signal determines the phase shift ratio for phase-shift control between the upper and lower arms. The phase shift ratio is the phase difference between the switching signals of the upper and lower arms. By adjusting this phase difference, power flow between the two arms is controlled, thereby regulating the submodule capacitor voltages to maintain equilibrium.

[0057] In a further embodiment, the steady-state value of the shift ratio is calculated as follows:

[0058]

[0059] Where, For mobile comparison, and are the duty cycles of the upper and lower bridge arms, respectively. is the total power transmitted, is the bridge arm inductance, is the high-voltage side voltage, is the switching cycle.

[0060] In one embodiment, the submodule adopts a half-bridge submodule, each of which includes two IGBTs. The trigger pulse control signals of the two IGBTs are complementary. If one of the IGBTs is turned on, the corresponding submodule is put into operation. If the other IGBT is turned on, the corresponding submodule is cut off.

[0061] It should be noted that each half-bridge submodule primarily consists of two insulated gate bipolar transistors (IGBTs) T1 and T2, a storage capacitor C, and associated anti-parallel diodes D1 and D2. IGBT T1 is connected between the positive terminal of the capacitor and the upper bridge arm, while IGBT T2 is connected between the negative terminal of the capacitor and the lower bridge arm. Anti-parallel diodes D1 and D2 are connected in anti-parallel with IGBT T1 and T2, respectively, to allow current to flow when the IGBT is turned off.

[0062] When a submodule needs to be put into operation, a control signal turns on IGBT T1 while IGBT T2 remains off. At this point, capacitor C within the submodule is connected to the bridge arm circuit, and the voltage across it contributes to the effective voltage of the entire bridge arm. In this state, current can flow from the high-voltage side into capacitor C and out to the low-voltage side, or vice versa, depending on the overall circuit operating mode. If a submodule is to be removed from the bridge arm, a control signal turns on IGBT T2 while IGBT T1 remains off. This disconnects capacitor C from the main circuit, preventing it from contributing to the bridge arm voltage. At the same time, the conduction of T2 provides a path for capacitor C to discharge or charge through T2 and its antiparallel diode D2, thereby maintaining a stable capacitor voltage.

[0063] The complementary trigger pulse control signals mean that the drive signals for the two IGBTs T1 and T2 within the same submodule are mutually exclusive—that is, only one IGBT can be in the on state at any time; the other must be off. This complementary control strategy ensures that the submodule can switch safely and reliably between the on and off states, avoiding faults such as short circuits.

[0064] In one embodiment, when the modular multi-level DC transformer is operating, a sub-module voltage balancing control strategy is adopted to achieve voltage balancing of the sub-modules. In the sub-module voltage balancing control strategy, the voltage and shift ratio of the upper bridge arm and the lower bridge arm are adjusted according to the first control signal and the second control signal, and the sub-module capacitor voltage is kept stable.

[0065] It's important to note that in an MMC, each submodule contains an energy storage capacitor. Unbalanced voltages across these capacitors can lead to premature failure of certain submodules or degradation of overall system performance. Therefore, maintaining consistent capacitor voltages across all submodules is crucial for improving system reliability and efficiency. The first control signal is typically used to adjust the overall voltage level of the upper or lower bridge arm. The second control signal adjusts the operating phase difference (i.e., phase shift) between the upper and lower bridge arms, which helps optimize output waveform quality and reduce harmonic content.

[0066] When performing voltage balancing control, a voltage sorting method can be used. This involves monitoring the voltage on each submodule capacitor in real time and sorting the submodules by their magnitude. The submodules to be put into operation are then selected according to certain rules, such as always prioritizing those with lower voltages.

[0067] In one embodiment, the system further includes a PI controller, wherein the first control signal and the second control signal are generated by corresponding PI controllers to adjust the voltage and shift ratio of the upper and lower bridge arms. A PI controller is a common feedback controller that combines proportional control and integral control. Proportional control enables rapid response to system deviations, while integral control eliminates steady-state errors, improving system stability and accuracy.

[0068] In one embodiment, the modular multilevel DC transformer employs staircase-wave quasi-two-level modulation. This modulation strategy generates multiple staircase-like levels in the output voltage by controlling the switching of the submodules. The combination of these levels approximates an ideal two-level waveform (i.e., high and low), but with a greater number of levels, thereby reducing voltage fluctuations and harmonic content.

[0069] Based on the above embodiment, a design example of a modular multi-level DC transformer Buck-MMDCT is proposed below. Its topology is as follows: Figure 1 As shown in the figure, the chained submodule string connected to the positive terminal of the high-voltage side is considered the upper bridge arm, and the chained submodule string connected to the negative terminal of the high-voltage side is considered the lower bridge arm. The half-bridge submodule (HBSM) in the upper and lower bridge arms contains two corresponding sets of IGBTs and anti-parallel diodes. According to the topology of the half-bridge submodule, when the upper IGBT is turned on, the submodule capacitor is connected to the circuit; conversely, when the lower IGBT is turned on, the submodule capacitor is disconnected.

[0070] During the operation of Buck-MMDCT, the number of submodules put into operation can be controlled by controlling the trigger pulses of the submodules, thereby controlling the voltage of the bridge arm. In an ideal situation, the upper and lower bridge arms in the topology are equivalent to controlled voltage sources, and the equivalent circuit diagram of Buck-MMDCT can be obtained, as shown in Figure 2 shown.

[0071] exist Figure 2 middle, is the high-voltage side voltage of the converter, is the low-voltage side voltage of the converter. is the upper arm input voltage, where 、 are the DC and AC components of the upper arm voltage; correspondingly, is the input voltage of the lower bridge arm, where 、 are the DC and AC components of the lower arm voltage. 、 and They are the high voltage side, low voltage side and lower arm current, the reference direction is as follows Figure 2 Indicated by the arrow in the middle. is the bridge arm inductance, For the filter inductor.

[0072] Applying Kirchhoff's voltage-current law to the equivalent circuit diagram yields:

[0073] (1)

[0074] (2)

[0075] In steady state, the inductor voltage satisfies the volt-second balance principle. Therefore, in steady state, the sum of the DC components of the upper and lower bridge arm voltages is equal to the high-voltage side voltage, and the DC component of the lower bridge arm voltage is equal to the low-voltage side voltage. Therefore:

[0076] (3)

[0077] (4)

[0078] (5)

[0079] In the formula 、 and They are the DC components of the high voltage side, low voltage side and lower arm currents respectively.

[0080] If the power delivered by the converter is P and various losses are ignored, then 、 and It can be expressed as:

[0081] (6)

[0082] According to the above analysis, based on equations (2) and (3), the DC component of the active power absorbed by the upper and lower bridge arms can be obtained:

[0083] (7)

[0084] Equation (7) indicates that in steady state, the DC components of active power absorbed by the upper and lower bridge arms are equal in magnitude and opposite in direction. Therefore, the energy between the bridge arms cannot achieve self-balance, and there is a problem of energy balance in the bridge arms. To this end, an energy transmission path should be established between the upper and lower bridge arms to relocate the excess energy of the upper or lower bridge arm so that the energy of the upper and lower bridge arms remains balanced. You can refer to the modulation mode of the Dual Active Bridge (DAB) and inject common-mode AC current between the upper and lower bridge arms of the Buck-MMDCT by phase shifting, and decompose the equivalent circuit diagram into a DC circuit and an AC circuit, as shown in the following example: Figure 3 As shown. According to the control principle of the phase-shift converter, the upper and lower bridge arms can transfer energy through the AC circuit, and the transmitted AC power satisfy:

[0085] (8)

[0086] That is, the sum of AC power and DC power is 0, ensuring that the upper and lower bridge arm submodules of the converter can operate stably.

[0087] Buck-MMDCT uses a staircase wave quasi-two-level modulation method, with T intervals between adjacent submodules. d The time required for the bridge arm to be switched between high and low levels is called the time delay. However, considering that the converter is in a heavy load state, the effect of the step wave edge on the modulation effect can be ignored. To simplify the analysis, the upper and lower bridge arm voltage waveforms are considered as square waves. Buck-MMDCT two-level modulation waveform is as follows: Figure 4 shown.

[0088] When the upper and lower bridge arms generate the same frequency AC voltage through phase shifting, thereby inducing a common mode AC current between the bridge arms, the upper and lower bridge arms must meet the condition that the algebraic sum of the DC energy and AC energy absorbed in one switching cycle is 0, so the phase shift ratio d can be calculated. s steady-state value.

[0089] (9)

[0090] When Buck-MMDCT uses step wave quasi-two-level modulation, the basic working principle is similar to that of the traditional Buck DC-DC converter, and the duty cycle is consistent. Therefore, when performing closed-loop control, the control strategy of the Buck converter can be referred to. The output side voltage is closed-loop controlled by comparing the output side voltage value with the given reference value, and the difference is used as the given value of the duty cycle through the PI controller. s The steady-state value of is coupled with the duty cycle. When the capacitor voltage balance control is performed on all submodules in a bridge arm, the shift ratio of the upper and lower bridge arms also needs to be adjusted. s To ensure the energy balance of the bridge arm and to ensure that the average value of the capacitor voltage of all submodules remains constant. s When the given value of is greater than the theoretical value of formula (9), the upper arm submodule capacitor releases energy in one cycle, while the lower arm submodule capacitor absorbs energy in one cycle. Therefore, the DC voltage of the upper arm will drop, and the DC voltage of the lower arm will rise, resulting in the problem of energy imbalance in the bridge arm. Similarly, when the bridge arm shift is greater than d s When the given value of is less than the theoretical value of formula (9), the submodule capacitors of the upper bridge arm absorb energy in one cycle, while the submodule capacitors of the lower bridge arm release energy in one cycle. Therefore, the DC voltage of the upper bridge arm will increase, and the DC voltage of the lower bridge arm will decrease. Therefore, in order to control the energy of the upper and lower bridge arms, the difference between the average voltage of all submodule capacitors of the upper bridge arm and the average voltage of all submodule capacitors of the lower bridge arm can be selected as the given value of the shift phase through a low-pass filter and a PI regulator.

[0091] An embodiment of the present invention further provides a control method for a modular multi-level DC transformer topology applicable to a multi-voltage DC grid, which is applied to the modular multi-level DC transformer topology applicable to a multi-voltage DC grid as in the aforementioned embodiment, including:

[0092] Voltage feedback control strategy, phase shift control strategy and submodule voltage balancing control strategy;

[0093] The voltage feedback control strategy is used to generate a first control signal according to the difference between the low-voltage side voltage and the reference voltage. The first control signal is used to control the switching of the submodules of the upper bridge arm and the lower bridge arm, thereby changing the voltage of each bridge arm to adjust the output voltage to the target value.

[0094] The phase shift control strategy is used to generate a second control signal according to the difference between the average value of the capacitor voltages of all submodules of the upper bridge arm and the average value of the capacitor voltages of all submodules of the lower bridge arm. The second control signal is used to determine the phase shift ratio for phase shift control between the upper bridge arm and the lower bridge arm.

[0095] The submodule voltage balancing control strategy is used to adjust the voltage and shift ratio of the upper bridge arm and the lower bridge arm according to the first control signal and the second control signal and to keep the submodule capacitor voltage stable.

[0096] The control strategy of Buck-MMDCT is as follows Figure 5 As shown. Among them, voltage feedback control is part of the closed-loop control, which is used to maintain the stability of the output voltage. Output side voltage u dc2 With the given reference voltage U ref The difference is processed by the PI controller to generate a control signal d for adjusting the duty cycle. Phase shift control can achieve energy flow control between the two bridge arms and maintain energy balance. The average value of the capacitor voltage of all submodules in the upper and lower bridge arms (u p_avg and u n_avg ) is processed by the PI controller to generate a shift phase d s In the submodule voltage balancing control, the capacitor voltages of the submodules are balanced by sorting to determine their switching order. By controlling the switching of the submodules, the capacitor voltages of the submodules are kept balanced. At the same time, when the capacitor voltage balancing control is performed on the submodule of a certain bridge arm, the shift ratio of the upper and lower bridge arms needs to be adjusted. s To ensure the energy balance of the bridge arm.

[0097] Furthermore, the calculation formula of the steady-state value of the shift ratio is as follows:

[0098]

[0099] Where, For mobile comparison, and are the duty cycles of the upper and lower bridge arms, respectively. is the total power transmitted, is the bridge arm inductance, is the high-voltage side voltage, is the switching cycle.

[0100] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A modular multi-level DC transformer topology suitable for a multi-voltage DC grid, characterized in that: include: Bridge arm inductor, upper bridge arm, lower bridge arm and filter inductor; One end of the bridge arm inductor is connected to the positive electrode of the high voltage side, and the other end of the bridge arm inductor is connected to one end of the upper bridge arm; The other end of the upper bridge arm is connected to one end of the lower bridge arm and one end of the filter inductor; The other end of the lower bridge arm is connected to the negative electrode on the high voltage side and the negative electrode on the low voltage side respectively; The other end of the filter inductor is connected to the positive electrode of the low voltage side; The upper bridge arm and the lower bridge arm both include a submodule string, and the submodule string includes a plurality of submodules connected in series in sequence; the upper bridge arm and the lower bridge arm are used to adjust the voltage of the respective bridge arms by controlling the number of submodules put into operation.

2. The modular multi-level DC transformer topology applicable to a multi-voltage DC grid according to claim 1, characterized in that: The upper bridge arm and the lower bridge arm are used to control the input of the submodule according to a first control signal to adjust the output voltage to a target value; the first control signal is generated according to the difference between the low-voltage side voltage and the reference voltage.

3. The modular multi-level DC transformer topology applicable to a multi-voltage DC grid according to claim 2, characterized in that: The upper bridge arm and the lower bridge arm are also used to perform phase shift control according to a second control signal to maintain energy balance between the upper bridge arm and the lower bridge arm; the second control signal is generated according to the difference between the average value of the capacitor voltage of all sub-modules of the upper bridge arm and the average value of the capacitor voltage of all sub-modules of the lower bridge arm.

4. The modular multi-level DC transformer topology applicable to a multi-voltage DC grid according to claim 3, characterized in that: The calculation formula for the steady-state value of the shift ratio is as follows: In the formula, For the shift comparison, and are the duty cycles of the upper bridge arm and the lower bridge arm respectively, is the total power transmitted, is the bridge arm inductance, is the high voltage side voltage, is the switching cycle.

5. The modular multi-level DC transformer topology applicable to a multi-voltage DC grid according to claim 3, characterized in that: The submodules are half-bridge submodules, each of which includes two IGBTs. The trigger pulse control signals of the two IGBTs are complementary. If one of the IGBTs is turned on, the corresponding submodule is put into operation. If the other IGBT is turned on, the corresponding submodule is cut off.

6. The modular multi-level DC transformer topology applicable to a multi-voltage DC grid according to claim 5, characterized in that: The upper bridge arm and the lower bridge arm are further used to adjust the voltage and the shift phase according to the first control signal and the second control signal to keep the submodule capacitor voltage stable.

7. The modular multi-level DC transformer topology applicable to a multi-voltage DC grid according to claim 3, characterized in that: Also includes: A PI controller is used to generate the first control signal and the second control signal.

8. The modular multi-level DC transformer topology applicable to a multi-voltage DC grid according to claim 1, characterized in that: The modular multi-level DC transformer adopts step wave quasi-two-level modulation.

9. A control method for a modular multi-level DC transformer topology suitable for a multi-voltage DC grid, characterized in that: A modular multi-level DC transformer topology applicable to a multi-voltage DC grid as claimed in any one of claims 1 to 8, comprising: Generate a first control signal according to the difference between the low-voltage side voltage and the reference voltage, and control the switching of the submodules of the upper bridge arm and the lower bridge arm according to the first control signal to change the voltages of the respective bridge arms and adjust the output voltage to a target value; Generate a second control signal according to the difference between the average value of the capacitor voltages of all submodules of the upper bridge arm and the average value of the capacitor voltages of all submodules of the lower bridge arm, and determine the phase shift ratio for performing phase shift control between the upper bridge arm and the lower bridge arm according to the second control signal; The voltage and shift ratio of the upper bridge arm and the lower bridge arm are adjusted according to the first control signal and the second control signal to keep the submodule capacitor voltage stable.

10. The control method of modular multi-level DC transformer topology applicable to multi-voltage DC grid according to claim 9, characterized in that: The calculation formula for the steady-state value of the shift ratio is as follows: In the formula, For the shift comparison, and are the duty cycles of the upper bridge arm and the lower bridge arm respectively, is the total power transmitted, is the bridge arm inductance, is the high voltage side voltage, is the switching cycle.

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