Modular multi-level direct-current transformer topology applicable to multi-voltage-class direct-current power grid, and control method therefor

Abstract

A modular multi-level direct-current transformer topology applicable to a multi-voltage-class direct-current power grid, and a control method therefor. The topology comprises a bridge arm inductor, an upper bridge arm, a lower bridge arm and a filter inductor, wherein one end of the bridge arm inductor is connected to a positive electrode of a high-voltage side, and the other end thereof 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 separately connected to a negative electrode of the high-voltage side and a negative electrode of a low-voltage side; the other end of the filter inductor is connected to a positive electrode of the low-voltage side; and the upper bridge arm and the lower bridge arm each comprise a sub-module string, the sub-module string comprising several sub-modules, which are sequentially connected in series, and the upper bridge arm and the lower bridge arm regulate respective bridge arm voltages by means of controlling the number of inserted sub-modules, so as to realize direct-current voltage conversion between different voltage classes. In the present application, the use of upper and lower sets of chained sub-modules reduces the use of complex assemblies, thereby reducing the cost and the control difficulty, and also improving the safety, reliability and efficiency of a system.

Description

A modular multilevel DC transformer topology and control method suitable for multi-voltage DC power grids

[0001] This application claims priority to Chinese Patent Application No. 2024114936241, filed on October 24, 2024, entitled "A Modular Multilevel DC Transformer Topology and Control Method Applicable to DC Grids with Multiple Voltage Levels", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application belongs to the field of power transmission and distribution technology, specifically relating to a modular multilevel DC transformer topology and control method suitable for multi-voltage DC power grids. Background Technology

[0003] Modular multilevel DC transformer technology is crucial for multi-voltage DC grids, effectively improving energy conversion efficiency, optimizing power quality, and reducing construction costs. With the widespread use of renewable energy, this technology has become key to improving energy conversion efficiency and achieving grid interconnection. Modular multilevel DC transformers can handle high voltages, facilitate standardized production and maintenance, and have excellent scalability, helping to reduce R&D time and operating costs. Furthermore, these transformers are suitable for connecting different DC distribution networks, and their modular design supports multi-port operation, facilitating integration with distributed energy and energy storage systems.

[0004] Modular multilevel DC transformers (MMCs) are primarily implemented through fundamental frequency modulation. This method involves only one sampling and control cycle within a single high-frequency period, unlike traditional power-frequency MMCs. By adjusting the phase difference of the half-bridge module's drive signal, an asymmetric stepped waveform can be generated, and the transformer's output can be adjusted according to the output voltage. This method simplifies the design of magnetic components and achieves zero-current turn-off of the low-voltage side switching transistors, thereby reducing current stress. Other research has proposed a quasi-two-level modulation method, which can modulate arbitrary waveforms on the AC side and is suitable for high-voltage DC transmission. However, its application is limited in medium-voltage DC distribution networks due to the smaller number of levels.

[0005] The application of modular multilevel DC transformer technology in multi-voltage DC power grids, through its unique modulation and control methods, achieves efficient energy conversion and optimizes the grid structure, providing crucial technical support for future high-voltage DC grid interconnection and large-scale renewable energy consumption. However, this technology also has some drawbacks, primarily including complex circuit structure, the need for numerous independent DC power supplies, a large number of components, high cost, and low circuit safety and reliability. The complexity of the circuit structure necessitates the use of numerous clamping diodes or high-voltage clamping capacitors, increasing circuit complexity and control difficulty, while also reducing circuit safety and reliability. Furthermore, to achieve the modular multilevel structure, multiple independent DC power supplies are required, which not only increases cost but may also limit the portability and efficiency of the device. The complexity of the circuit structure and the use of numerous independent components may reduce the overall system safety and reliability. Summary of the Invention

[0006] In view of this, the present invention provides a modular multilevel DC transformer topology and control method applicable to multi-voltage DC power grids, aiming to optimize the structure of the modular multilevel DC transformer so that it can more effectively convert and control electrical energy between the high-voltage side and the low-voltage side, while reducing costs and improving system safety.

[0007] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0008] In a first aspect, the present invention provides a modular multilevel DC transformer topology suitable for multi-voltage DC power grids, comprising:

[0009] Bridge arm inductor, upper bridge arm, lower bridge arm, and filter inductor;

[0010] One end of the bridge arm inductor is connected to the positive terminal of the high voltage side, and the other end of the bridge arm inductor is connected to one end of the upper bridge arm.

[0011] 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;

[0012] The other end of the lower bridge arm is connected to the negative terminal on the high-voltage side and the negative terminal on the low-voltage side, respectively.

[0013] The other end of the filter inductor is connected to the positive terminal of the low-voltage side;

[0014] Both the upper and lower bridge arms include submodule strings, which consist of several submodules connected in series. The upper and lower bridge arms are used to adjust the voltage of their respective bridge arms by controlling the number of submodules engaged.

[0015] Furthermore, the upper and lower bridge arms are used to control the activation of the submodule according to the first control signal to adjust the output voltage to the target value; the first control signal is generated based on the difference between the low-voltage side voltage and the reference voltage.

[0016] Furthermore, the upper and lower bridge arms are also used to perform shift ratio control according to the second control signal to maintain the energy balance between the upper and lower bridge arms; 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.

[0017] Furthermore, the steady-state value of the shift ratio is calculated as follows:

[0018] In the formula, d s For the comparison, d1 and d2 are the duty cycles of the upper and lower bridge arms, respectively, P is the total transmitted power, and L is the duty cycle of the lower bridge arm. a U is the inductance value of the bridge arm. dc1 Where is the high-voltage side voltage, and T is the switching period.

[0019] Furthermore, the submodule adopts a half-bridge submodule, and each half-bridge submodule includes two IGBTs. The trigger pulse control signals of the two IGBTs are complementary. When one IGBT is turned on, the corresponding submodule is activated; when the other IGBT is turned on, the corresponding submodule is deactivated.

[0020] Furthermore, the upper and lower bridge arms are also used to adjust the voltage and shift ratio according to the first and second control signals to maintain the stability of the submodule capacitor voltage.

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

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

[0023] In a second aspect, the present invention provides a control method for a modular multilevel DC transformer topology suitable for multi-voltage-level DC power grids, applied to the modular multilevel DC transformer topology for multi-voltage-level DC power grids as described in the first aspect, comprising:

[0024] A first control signal is generated based on the difference between the low-voltage side voltage and the reference voltage. The switching of the sub-modules of the upper and lower bridge arms is controlled based on the first control signal to change the voltage of each bridge arm and adjust the output voltage to the target value.

[0025] A second control signal is generated based on the difference between the average value of the capacitor voltages of all sub-modules in the upper bridge arm and the average value of the capacitor voltages of all sub-modules in the lower bridge arm. The phase shift ratio for phase shift control between the upper and lower bridge arms is determined based on the second control signal.

[0026] The voltage and shift ratio of the upper and lower bridge arms are adjusted according to the first and second control signals to maintain the stability of the submodule capacitor voltage.

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

[0028] In the formula, d s For the comparison, d1 and d2 are the duty cycles of the upper and lower bridge arms, respectively, P is the total transmitted power, and L is the duty cycle of the lower bridge arm. a U is the inductance value of the bridge arm. dc1 Where is the high-voltage side voltage, and T is the switching period.

[0029] This invention provides a modular multilevel DC transformer topology suitable for multi-voltage DC power grids, including 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 positive terminal of the high-voltage side, 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 both the negative terminal of the high-voltage side and the negative terminal of the low-voltage side. The other end of the filter inductor is connected to the positive terminal of the low-voltage side. Both the upper and lower bridge arms include a sub-module string, which consists of several sub-modules connected in series. The upper and lower bridge arms adjust their respective bridge arm voltages by controlling the number of sub-modules connected, thereby achieving DC voltage conversion between different voltage levels. This invention reduces the use of complex components, lowers costs and control difficulty, and improves the safety, reliability, and efficiency of the system by using two sets of chained sub-modules.

[0030] The present invention also provides a control method for a modular multilevel DC transformer topology applicable to multi-voltage DC power grids. This method has similar effects to the topology when implemented, and will not be described in detail here. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 is a topology diagram of Buck-MMDCT provided in an embodiment of the present invention;

[0033] Figure 2 is an equivalent circuit diagram of Buck-MMDCT provided in an embodiment of the present invention;

[0034] Figure 3 is an exploded view of the AC / DC circuit of the Buck-MMDCT provided in the embodiment of the present invention;

[0035] Figure 4 is a two-level modulation waveform diagram of Buck-MMDCT provided in an embodiment of the present invention;

[0036] Figure 5 is a control block diagram of Buck-MMDCT provided in an embodiment of the present invention. Detailed Implementation

[0037] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0038] The main disadvantages of modular multilevel 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.

[0039] Modular multilevel DC transformers (MMC-DC), as an advanced DC transformer technology, offer advantages such as high scalability, high power ratings, and input / output electrical isolation. However, this technology also has some drawbacks, primarily including:

[0040] The circuit structure is complex: it requires the use of a large number of clamping diodes or high-voltage clamping capacitors, which increases the complexity and control difficulty of the circuit, and also reduces the safety and reliability of the circuit.

[0041] Multiple independent DC power supplies are required: To achieve the modular multilevel 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.

[0042] 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.

[0043] To address the aforementioned issues, this invention provides a modular multilevel DC transformer topology and control method suitable for multi-voltage DC power grids. By optimizing the structure of the modular multilevel DC transformer, it enables more efficient conversion and control of electrical energy between the high-voltage and low-voltage sides, while reducing costs and improving system safety.

[0044] The following is a detailed description of an embodiment of a modular multilevel DC transformer topology applicable to multi-voltage DC power grids according to the present invention.

[0045] This invention provides a modular multilevel DC transformer topology suitable for multi-voltage DC power grids, comprising: a bridge arm inductor, an upper bridge arm, a lower bridge arm, and a filter inductor.

[0046] A bridge arm inductor is an inductor 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.

[0047] The upper and lower bridge arms constitute the main part of the DC transformer, and they regulate the voltage through series-connected submodules. Each upper and lower bridge arm includes a submodule string, which consists of several submodules connected in series. 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, while the other end of the lower bridge arm is connected to both the high-voltage side negative terminal and the low-voltage side negative terminal.

[0048] Submodules are the basic units that make up the bridge arms. They can be half-bridge or full-bridge structures, and each submodule contains switching devices and corresponding diodes. 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 bridge arm increases, the voltage of the upper bridge arm increases accordingly; conversely, the voltage of the lower bridge arm decreases. In this way, DC voltage conversion between the high-voltage side and the low-voltage side can be achieved.

[0049] The filter inductor is located between the lower bridge arm and the positive terminal of the low-voltage side. Its main function is to further smooth the current, reduce high-frequency components in the current, and improve power quality. The other end of the filter inductor is connected to the positive terminal of the low-voltage side.

[0050] The control strategies for the aforementioned modular multilevel DC transformers typically include modulation and voltage equalization control strategies. The modulation strategy generates appropriate switching signals to control the switching states of the submodules, achieving the required output voltage and current. The voltage equalization control strategy maintains the voltage balance of the submodule capacitors, preventing excessive voltage deviation. Its working principle involves controlling the number of submodules engaged in the upper and lower bridge arms (composed of multiple series-connected submodules) to adjust the bridge arm voltage, thereby achieving 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.

[0051] This embodiment provides a modular multilevel DC transformer topology suitable for DC power grids of multiple voltage levels. By using two sets of chain-like sub-modules, the use of complex components is reduced, thereby lowering costs and control difficulty, while improving the safety, reliability and efficiency of the system.

[0052] In one embodiment, when the modular multilevel DC transformer is working, a voltage feedback control strategy is used to accurately 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 and lower bridge arms, thereby changing the voltage of their respective bridge arms to adjust the output voltage to the target value.

[0053] It should be noted that one of the control strategies for modular multilevel DC transformers (MMCs) is implemented using a closed-loop control system based on voltage feedback. This control strategy relies on a feedback loop where 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 deviation of the output voltage and is the amount that needs to be adjusted. Based on this difference, the control system generates a first control signal. This control signal is the basis for adjusting the switching of sub-modules, and its purpose is to correct the deviation of the output voltage. The first control signal is used to control the switching of sub-modules in the upper and lower bridge arms.

[0054] In one embodiment, when the modular multilevel DC transformer is operating, a phase-shifting control strategy is adopted to maintain energy balance between the upper and lower bridge arms. In the phase-shifting control strategy, a second control signal is generated based on the difference between the average value of the capacitor voltages of all sub-modules of the upper bridge arm and the average value of the capacitor voltages of all sub-modules of the lower bridge arm. The second control signal is used to determine the phase-shifting ratio for performing phase-shifting control between the upper and lower bridge arms.

[0055] It's important to note that in MMC (Multi-Module Control), both the upper and lower bridge arms contain a series of sub-modules connected in series. The capacitor voltages of these sub-modules need to be kept balanced to ensure stable transformer operation. The phase-shift control strategy controls the energy flow between the two bridge arms by adjusting the switching angles (i.e., phase shift angles) of the sub-modules in both arms, thereby maintaining energy balance. In this strategy, the control system monitors the average capacitor voltages of all sub-modules in both the upper and lower bridge arms. The difference between these two averages is used to generate a second control signal. This signal is the basis for adjusting the phase shift angle to ensure energy balance between the upper and lower bridge arms; specifically, the second control signal determines the phase shift ratio for the phase-shift control between the upper and lower bridge arms. The phase shift ratio is the phase difference between the switching signals of the upper and lower bridge arms. By adjusting this phase difference, the power flow between the two bridge arms can be controlled, thereby regulating the sub-module capacitor voltages to maintain balance.

[0056] In a further embodiment, the steady-state value of the shift ratio is calculated as follows:

[0057] In the formula, d sFor the comparison, d1 and d2 are the duty cycles of the upper and lower bridge arms, respectively, P is the total transmitted power, and L is the duty cycle of the lower bridge arm. a U is the inductance value of the bridge arm. dc1 Where is the high-voltage side voltage, and T is the switching period.

[0058] In one embodiment, the submodule is a half-bridge submodule, and each half-bridge submodule includes two IGBTs. The trigger pulse control signals of the two IGBTs are complementary. When one IGBT is turned on, the corresponding submodule is activated; when the other IGBT is turned on, the corresponding submodule is deactivated.

[0059] It should be noted that each half-bridge submodule mainly consists of two insulated-gate bipolar transistors (IGBTs) T1 and T2, an energy storage capacitor C, and related anti-parallel diodes D1 and D2. IGBT T1 is connected between the positive terminal of the capacitor and the upper bridge arm, and IGBT T2 is connected between the negative terminal of the capacitor and the lower bridge arm. The anti-parallel diodes D1 and D2 are connected in reverse parallel with IGBTs T1 and T2, respectively, to allow current to flow when the IGBTs are turned off.

[0060] When a submodule needs to be put into operation, the control signal turns on IGBT T1 while keeping T2 off. At this time, the capacitor C within the submodule is connected to the bridge arm circuit, and its voltage 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 then out to the low-voltage side, or vice versa, depending on the overall circuit operating mode. If the submodule needs to be removed from the bridge arm, the control signal turns on IGBT T2 while keeping T1 off. This disconnects capacitor C from the main circuit, preventing it from contributing to the bridge arm voltage. Simultaneously, the conduction of T2 provides a path for capacitor C to discharge or charge through T2 and its anti-parallel diode D2, thus maintaining a stable capacitor voltage.

[0061] Complementary trigger pulse control signals mean that for two IGBTs T1 and T2 within the same submodule, their drive signals are mutually exclusive—that is, only one IGBT can be on at any given time, while the other must be off. This complementary control strategy ensures that the submodule can safely and reliably switch between on and off states, avoiding faults such as short circuits.

[0062] In one embodiment, when the modular multilevel DC transformer is working, a submodule voltage equalization control strategy is adopted to achieve voltage equalization of the submodules. In the submodule voltage equalization control strategy, the voltage and shift ratio of the upper and lower bridge arms are adjusted according to the first control signal and the second control signal to maintain the stability of the submodule capacitor voltage.

[0063] It's important to note that in MMC, each submodule contains a storage capacitor. Voltage imbalances among these capacitors can lead to premature failure of some submodules or a degradation in 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 is used to adjust the operating phase difference (i.e., the phase shift) between the upper and lower bridge arms, which helps optimize output waveform quality and reduce harmonic content.

[0064] When performing voltage equalization control, a voltage sorting method can be used. This involves monitoring the voltage value on the capacitor of each submodule in real time and sorting the submodules according to their values. Then, the submodules to be put into operation are selected according to certain rules, such as always prioritizing those submodules with lower voltages.

[0065] In one embodiment, the system further includes a PI controller. Both the first and second control signals are generated by the corresponding PI controllers and are used to regulate the voltage and shift ratio of the upper and lower bridge arms. A PI controller is a common feedback controller that combines proportional and integral control. Proportional control can quickly respond to system deviations, while integral control can eliminate steady-state errors, improving system stability and accuracy.

[0066] In one embodiment, the modular multilevel DC transformer employs stepped-wave quasi-two-level modulation. In this modulation strategy, multiple stepped levels can be generated in the output voltage by controlling the switching action of the submodules. The combination of these levels approximates an ideal two-level waveform (i.e., high and low levels), but with a greater number of levels, thereby reducing voltage fluctuations and harmonic content.

[0067] Based on the above embodiments, the following presents a design example of a modular multilevel DC transformer, the Buck-MMDCT, whose topology is shown in Figure 1. The chain of submodules connected to the positive terminal of the high-voltage side is considered the upper bridge arm, and the chain of submodules connected to the negative terminal of the high-voltage side is considered the lower bridge arm. Each 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.

[0068] During the operation of the Buck-MMDCT, the number of sub-modules in operation can be controlled by controlling the trigger pulses of the control sub-modules, thereby controlling the bridge arm voltage. Ideally, the upper and lower bridge arms in the topology can be equivalently represented as controlled voltage sources to obtain the equivalent circuit diagram of the Buck-MMDCT, as shown in Figure 2.

[0069] In Figure 2, U dc1 U is the high-voltage side voltage of the converter. dc2 This is the low-side voltage of the converter. p Voltage is applied to the upper bridge arm, where U p,dc u p,ac These represent the DC and AC components of the upper bridge arm voltage; correspondingly, u n Apply voltage to the lower bridge arm, where U n,dc u n,ac These represent the DC and AC components of the lower bridge arm voltage. i1, i2, and i3 are the high-voltage side, low-voltage side, and lower bridge arm current, respectively, with reference directions indicated by the arrows in Figure 2. L a For the bridge arm inductance, L f This is a filter inductor.

[0070] Applying Kirchhoff's voltage and current laws to the equivalent circuit diagram, we obtain: i2=i1+i3 (2)

[0071] 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. Thus: U p =U dc1 -U dc2 (3) U n =U dc2 (4) I dc3 =I dc2 -I dc1 (5)

[0072] In the formula I dc1 I dc2 and I dc3 These are the DC components of the current on the high-voltage side, low-voltage side, and lower bridge arm, respectively.

[0073] If the power transferred by the converter is P, and all losses are ignored, then I dc1 I dc2 and I dc3 It can be represented as:

[0074] Based on the foregoing analysis, the DC components of the active power absorbed by the upper and lower bridge arms can be obtained from equations (2) and (3):

[0075] Equation (7) indicates that in steady state, the DC components of the 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, resulting in a bridge arm energy balance problem. To address this, an energy transfer path should be established between the upper and lower bridge arms to relocate excess energy from either the upper or lower bridge arm, thus maintaining energy balance between the upper and lower bridge arms. Referring to the modulation mode of the Dual Active Bridge (DAB), a common-mode AC current is injected between the upper and lower bridge arms of the Buck-MMDCT through phase shifting. The equivalent circuit diagram is decomposed into a DC loop and an AC loop, as shown in Figure 3. Therefore, according to the phase-shift converter control principle, the upper and lower bridge arms can transfer energy through the AC loop, and the transferred AC power P... ac satisfy:

[0076] 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.

[0077] The Buck-MMDCT employs a stepped-wave quasi-two-level modulation method, where adjacent sub-modules are only engaged or disengaged after a time interval Td. Therefore, there is a delay when the bridge arm switches between high and low levels. However, considering that the effect of the stepped-wave edge on modulation is negligible when the converter is under heavy load, the voltage waveforms of the upper and lower bridge arms are treated as square waves for simplified analysis. The two-level modulation waveform of the Buck-MMDCT is shown in Figure 4.

[0078] When a common-mode AC current is induced between the upper and lower bridge arms by generating an AC voltage of the same frequency through phase shifting, the upper and lower bridge arms must satisfy the condition that the algebraic sum of the DC energy and AC energy absorbed within one switching cycle is zero. Therefore, the phase shift d can be calculated. s The steady-state value.

[0079] When a Buck-MMDCT employs stepped-wave quasi-two-level modulation, its basic operating principle is similar to that of a traditional Buck DC-DC converter, and the duty cycle is consistent. Therefore, closed-loop control can refer to the control strategy of a Buck converter, performing closed-loop control on the output voltage. By comparing the output voltage value with a given reference value, the difference is used as the duty cycle setpoint via a PI controller. (The shift ratio d...) s The steady-state value is coupled with the duty cycle. When performing capacitor voltage equalization control on all submodules in a certain bridge arm, it is also necessary to adjust the shift ratio d of the upper and lower bridge arms. s To ensure energy balance between the bridge arms and maintain a constant average capacitor voltage across all submodules. When the bridge arm shifts relative to d... sWhen the given value is greater than the theoretical value of equation (9), the submodule capacitor of the upper bridge arm releases energy in one cycle, while the submodule capacitor of the lower bridge arm absorbs energy in one cycle. Therefore, the DC voltage of the upper bridge arm will decrease, and the DC voltage of the lower bridge arm will increase, resulting in an energy imbalance problem between the bridge arms. Similarly, when the bridge arm shift ratio is greater than d... s When the given value is less than the theoretical value of equation (9), the submodule capacitor of the upper bridge arm absorbs energy in one cycle, while the submodule capacitor of the lower bridge arm releases energy in one cycle. Therefore, the DC voltage of the upper bridge arm will rise, and the DC voltage of the lower bridge arm will fall. Therefore, in order to control the energy of the upper and lower bridge arms, the difference between the average voltage of all submodule capacitors in the upper bridge arm and the average voltage of all submodule capacitors in the lower bridge arm can be selected as the given value of the shift ratio via a low-pass filter and a PI regulator.

[0080] This invention also provides a control method for a modular multilevel DC transformer topology suitable for multi-voltage DC power grids, applied to the modular multilevel DC transformer topology for multi-voltage DC power grids as described in the foregoing embodiments, including:

[0081] Voltage feedback control strategy, phase shift control strategy, and submodule voltage equalization control strategy;

[0082] The voltage feedback control strategy is used to generate a first control signal 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 and lower bridge arms, thereby changing the voltage of their respective bridge arms to adjust the output voltage to the target value.

[0083] The phase-shift control strategy generates a second control signal based on the difference between the average value of the capacitor voltages of all sub-modules in the upper arm and the average value of the capacitor voltages of all sub-modules in the lower arm. The second control signal is used to determine the phase-shift ratio for phase-shift control between the upper and lower arms. The sub-module voltage equalization control strategy is used to adjust the voltages and phase-shift ratios of the upper and lower arms based on the first and second control signals and to maintain the stability of the sub-module capacitor voltages.

[0084] The control strategy of the Buck-MMDCT is shown in Figure 5. Voltage feedback control is part of the closed-loop control, used to maintain the stability of the output voltage. The output voltage u... dc2 With a given reference voltage U ref The comparison results in a difference that is processed by a PI controller to generate a control signal d for adjusting the duty cycle. Phase-shift control enables energy flow control between the two bridge arms, maintaining energy balance. The average capacitor voltage (u) of all submodules in both upper and lower bridge arms... p_avg and u n_avg The difference between the two values ​​is processed by a PI controller to generate a shift ratio d. sThe control signal. In submodule voltage equalization control, the capacitor voltages of the submodules are equalized by sorting to determine their switching order. By controlling the switching of the submodules, the capacitor voltage balance of the submodules is maintained. Simultaneously, when performing capacitor voltage equalization control on a submodule of a certain bridge arm, the shift ratio d of the upper and lower bridge arms needs to be adjusted. s This is to ensure the energy balance of the bridge arm.

[0085] Furthermore, the steady-state value of the shift ratio is calculated as follows:

[0086] In the formula, d s For the comparison, d1 and d2 are the duty cycles of the upper and lower bridge arms, respectively, P is the total transmitted power, and L is the duty cycle of the lower bridge arm. a U is the inductance value of the bridge arm. dc1 Where is the high-voltage side voltage, and T is the switching period.

[0087] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A modular multilevel DC transformer topology suitable for multi-voltage DC power grids, 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 terminal 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 high-voltage side negative electrode and the low-voltage side negative electrode respectively. The other end of the filter inductor is connected to the positive terminal of the low-voltage side; Both the upper bridge arm and the lower bridge arm include a sub-module string, which includes several sub-modules 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 sub-modules engaged.

2. The modular multilevel DC transformer topology suitable for multi-voltage DC power grids according to claim 1, characterized in that, The upper bridge arm and the lower bridge arm are used to control the activation of the submodule according to a first control signal to adjust the output voltage to a target value; the first control signal is generated based on the difference between the low-voltage side voltage and the reference voltage.

3. The modular multilevel DC transformer topology suitable for multi-voltage DC power grids according to claim 2, characterized in that, The upper bridge arm and the lower bridge arm are also used to perform shift ratio control according to a second control signal to maintain the 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.

4. The modular multilevel DC transformer topology suitable for multi-voltage DC power grids according to claim 3, characterized in that, The steady-state value of the shift ratio is calculated as follows: In the formula, d s For the shift ratio, d1 and d2 are the duty cycles of the upper bridge arm and the lower bridge arm, respectively, P is the total transmitted power, and L is the duty cycle of the lower bridge arm. a U is the inductance value of the bridge arm. dc1 Where is the high-voltage side voltage, and T is the switching period.

5. The modular multilevel DC transformer topology suitable for multi-voltage DC power grids according to claim 3, characterized in that, The submodule is a half-bridge submodule, and each half-bridge submodule includes two IGBTs. The trigger pulse control signals of the two IGBTs are complementary. When one IGBT is turned on, the corresponding submodule is activated; when the other IGBT is turned on, the corresponding submodule is deactivated.

6. The modular multilevel DC transformer topology suitable for multi-voltage DC power grids according to claim 5, characterized in that, The upper bridge arm and the lower bridge arm are also used to adjust the voltage and shift ratio according to the first control signal and the second control signal to maintain the stability of the submodule capacitor voltage.

7. The modular multilevel DC transformer topology suitable for multi-voltage DC power grids 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 multilevel DC transformer topology suitable for multi-voltage DC power grids according to claim 1, characterized in that, The modular multilevel DC transformer adopts stepped wave quasi-two-level modulation.

9. A control method for a modular multilevel DC transformer topology applicable to multi-voltage DC power grids, characterized in that, The modular multilevel DC transformer topology applicable to multi-voltage DC power grids as described in any one of claims 1-8 includes: A first control signal is generated based on the difference between the low-voltage side voltage and the reference voltage. The switching of the sub-modules of the upper and lower bridge arms is controlled based on the first control signal to change the voltage of each bridge arm and adjust the output voltage to the target value. A second control signal is generated based on the difference between the average value of the capacitor voltages of all sub-modules in the upper bridge arm and the average value of the capacitor voltages of all sub-modules in the lower bridge arm. The phase shift ratio for performing phase shift control between the upper bridge arm and the lower bridge arm is determined based on 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 maintain the stability of the submodule capacitor voltage.

10. The control method for a modular multilevel DC transformer topology applicable to multi-voltage DC power grids according to claim 9, characterized in that, The steady-state value of the shift ratio is calculated as follows: In the formula, d s For the shift ratio, d1 and d2 are the duty cycles of the upper bridge arm and the lower bridge arm, respectively, P is the total transmitted power, and L is the duty cycle of the lower bridge arm. a U is the inductance value of the bridge arm. dc1 Where is the high-voltage side voltage, and T is the switching period.

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