Multi-port power electronic transformer based on MMC and control method thereof

Abstract

The invention provides an MMC-based multi-port power electronic transformer, which is characterized by comprising a medium-voltage stage, an isolation stage and a low-voltage stage, a three-phase six-bridge arm MMC structure adopted by the medium-voltage stage is a Si-SiC hybrid topological structure, each phase unit is composed of an upper bridge arm and a lower bridge arm, and each phase unit is composed of an upper bridge arm and a lower bridge arm. Each of the upper bridge arm and the lower bridge arm is formed by connecting N full-bridge sub-modules in series; power bidirectional flow between a medium-voltage alternating current port and a medium-voltage direct current port, between the medium-voltage direct current port and a low-voltage direct current port, and between the low-voltage direct current port and an energy storage module can be realized; one-way power flow among ports such as a low-voltage direct-current port to a low-voltage alternating-current port, a photovoltaic power generation port to a middle-low-voltage direct-current port and a wind power generation port to a low-voltage direct-current port is achieved, and the direct-current side short-circuit instantaneous current suppression capability is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic converters, and in particular to an MMC-based multi-port power electronic transformer and a control method thereof. Background Art

[0002] Power electronic transformers, as innovative power conversion devices, are gaining increasing penetration in AC / DC hybrid distribution systems. They not only efficiently integrate renewable energy sources such as photovoltaics and wind power, but also enhance grid stability and reliability, balancing supply and demand by rapidly responding to grid fluctuations. MMC-based power electronic transformers significantly improve power quality by reducing harmonics, enhancing voltage stability, and reducing power losses. Furthermore, they offer high flexibility and scalability, enabling grid designers to adapt grid configurations to meet growing energy demands. When used in conjunction with energy storage systems such as batteries or supercapacitors, power electronic transformers play a key role in energy storage and release, optimizing grid operation. Integrating these transformers with smart grid technologies enables more advanced power management, including demand response, distributed generation, and automated control. The multi-port power conversion capability of power electronic transformers enables them to adapt to different voltage levels and types of power, meeting the complex requirements of AC / DC hybrid grids. Their modular design further reduces costs and improves system reliability, making power electronic transformers a key component in building smart, efficient, and sustainable power systems. However, existing power electronic transformers still have disadvantages such as low power density, large system loss and a small number of ports. Summary of the Invention

[0003] In view of this, an object of the present invention is to provide a multi-port power electronic transformer based on MMC and a control method thereof, so as to at least solve the above problems.

[0004] The technical solution adopted in the present invention is as follows: The first aspect of the present application provides a multi-port power electronic transformer based on MMC, including a medium voltage stage, an isolation stage and a low voltage stage. The three-phase six-bridge arm MMC structure adopted by the medium voltage stage is a Si-SiC hybrid topology structure, wherein each phase unit is composed of an upper bridge arm and a lower bridge arm respectively, and the upper bridge arm and the lower bridge arm are both composed of N full-bridge sub-modules connected in series, and the full-bridge sub-modules include a first full-bridge sub-module and a second full-bridge sub-module. The first full-bridge sub-module of the upper bridge arm and the lower bridge arm both use the first full-bridge sub-module, and the remaining full-bridge sub-modules all use the second full-bridge sub-module. The isolation stage includes a symmetrical upper bridge arm and lower bridge arm, and the upper bridge arm and the lower bridge arm of the isolation stage are both controlled by a carrier phase-shift modulation strategy. The low voltage stage is a three-phase four-bridge arm inverter.

[0005] Furthermore, the upper bridge arm and the lower bridge arm of the isolation stage are both composed of M+1 high-frequency isolation sub-modules.

[0006] Furthermore, the isolation stage adopts an ISOP topology and is directly connected in parallel to the medium voltage DC bus. The medium voltage stage is connected in series to the medium voltage DC bus, and the low voltage stage is connected in parallel to the medium voltage DC bus.

[0007] A second aspect of the present application provides a multi-port power electronic transformer control method based on MMC, including a medium voltage level control method, an isolation level control method, and a low voltage level control method; The medium voltage level control method includes a rectification mode control method, and the rectification mode control method comprises the following steps: Step 11: The actual value of DC bus voltage Subtract reference value After inputting into the PI controller, the current d-axis reference value is obtained ; Step 12: Input the three-phase voltage of the grid into the phase-locked loop controller to obtain the grid frequency , Phase and angles , Step 13: Perform Park transformation on the three-phase voltage to obtain the d-axis voltage and q-axis voltage ; Perform park transformation on the three-phase current to obtain the d-axis current and q-axis current ; Step 14: - and - Input the PI controller separately, and then introduce the disturbance 、 and 、 To eliminate the d and q axis coupling effects, obtain the d and q axis system control reference voltages and ,Will and Perform Park inverse transformation to obtain the three-phase control reference voltage 、 and ; Step 15: Set the actual value of DC bus voltage Subtract 、 and Get the reference voltage of the A, B and C three-phase upper bridge arm of the rectifier side MMC, and convert the actual value of the DC bus voltage Add separately 、 and Obtain the reference voltage of the A, B and C three-phase lower bridge arms of the inverter side MMC; Step 16: Using Functions for Reference Voltage To find out the NLM control strategy and the number of second full-bridge sub-modules that need to be put into operation in PWM mode, sort the capacitor voltages of all second full-bridge sub-modules in the bridge arm, and combine the direction of the bridge arm current, the sorting results of the capacitor voltages of the full-bridge sub-modules, and the number of full-bridge sub-modules that need to be put into operation to determine the switching state of the full-bridge sub-module for industrial frequency switching in the bridge arm.

[0008] Furthermore, the medium voltage level control method further includes an inverter mode control method, and the inverter mode control method includes the following steps: Step 21: Input the three-phase voltage of the inverter side grid into the phase-locked loop controller to obtain the operating frequency of the inverter side grid 、 and ; Perform Park transformation on the three-phase voltage input to the inverter side grid to obtain and , the inverter side grid input three-phase current is Park transformed to obtain and ; Step 22: - and - Input PI controller, and then introduce disturbance 、 and 、 To eliminate the d and q axis coupling effects, obtain the d and q axis system control reference voltages and ,Will and Perform Park inverse transformation to obtain the three-phase control reference voltage 、 and ; Step 23: Set the actual DC bus voltage Subtract 、 and Get the reference voltage of the U, V and W three-phase upper bridge arm of the inverter side MMC, and convert the actual value of the DC bus voltage Add separately 、 and Obtain the reference voltage of the U, V and W three-phase lower bridge arms of the inverter side MMC; Step 24: Using Functions for Reference Voltage Calculate the NLM control strategy and the number of second full-bridge sub-modules that need to be put into operation in PWM mode, sort the capacitor voltages of all second full-bridge sub-modules in the bridge arm, and determine the switching state of the full-bridge sub-modules switching at the industrial frequency in the bridge arm based on the direction of the bridge arm current, the sorting results of the capacitor voltages of the full-bridge sub-modules, and the number of full-bridge sub-modules that need to be put into operation.

[0009] Furthermore, the isolation level control method includes a control method for a single high-frequency isolation submodule of the isolation level, and the control method for a single high-frequency isolation submodule of the isolation level is specifically as follows: The high-frequency isolator module low-voltage side H-bridge adopts the working principle based on the phase-shifted full-bridge, specifically: the phase difference between the diagonal power switch tubes of the low-voltage side H-bridge is the phase shift angle. The upper and lower tubes of each bridge arm of the H-bridge on the low-voltage side are complementary turned on, and the duty cycle of each switch tube is 0.5; the upper and lower tubes of each bridge arm of the H-bridge on the medium-voltage side of the high-frequency isolation submodule are complementary turned on, and the duty cycle of each switch tube is 0.5.

[0010] Furthermore, the isolation level control method also includes a modulation strategy for the isolation level, specifically: in steady state, the voltage on the medium voltage side of the high-frequency isolation transformer is , the voltage on the low voltage side is .

[0011] Furthermore, the low-voltage level control method includes the following steps: Step 31: Convert the three-phase voltage of the low voltage level Input the phase-locked loop controller to get the phase angle ; Step 32: Convert the three-phase current of the low voltage stage and phase angle Perform park transformation to obtain the orthogonal component of the rotation and

[0012] Step 33: As the reference value of the PI controller, As the feedback value, after PI control, the active voltage is obtained by subtracting the voltage drop on the inductor. ; Step 34: As the reference value of the PI controller, As the feedback value, after PI control, the voltage drop on the inductor is added to obtain the reactive voltage. ; Step 35: and After the park inverse transformation, the three-phase voltage control reference instruction of the three-phase full-bridge converter is obtained.

[0013] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a multi-port power electronic transformer based on MMC and a control method thereof, which can realize bidirectional power flow between ports such as medium-voltage AC ports and medium-voltage DC ports, medium-voltage DC ports and low-voltage DC ports, and low-voltage DC ports and energy storage modules, and realize unidirectional power flow between ports such as low-voltage DC ports to low-voltage AC ports, photovoltaic power generation to medium- and low-voltage DC ports, and wind power generation to low-voltage DC ports, and has the ability to suppress transient current of DC-side short circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only preferred embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0015] Figure 1 This is a structural diagram of a multi-port power electronic transformer based on MMC proposed by the present invention; Figure 2 This is a control block diagram of the medium voltage stage of the present invention using Si-SiC hybrid MMC working in rectification mode; Figure 3 This is a control flow chart for realizing capacitor voltage balance of Si-IGBT submodule in the present invention; Figure 4 This is a control block diagram of the medium voltage stage of the present invention using Si-SiC hybrid MMC working in inverter mode; Figure 5 This is the key waveform of the isolation-level high-frequency isolation submodule of the present invention working in the phase-shift control mode; Figure 6 This is a modulation strategy diagram of the upper bridge arm and the lower bridge arm of the isolation stage of the present invention working in carrier phase shift; Figure 7 This is a control block diagram of the grid-connected inversion of the low-voltage DC-AC converter of the present invention; Figure 8 This is an equivalent circuit diagram when a short circuit occurs on the medium voltage DC side of the present invention at a certain moment. DETAILED DESCRIPTION

[0016] The principles and features of the present invention are described below with reference to the accompanying drawings. The enumerated embodiments are only used to explain the present invention and are not used to limit the scope of the present invention.

[0017] Reference Figure 1An embodiment of the present invention provides a multi-port power electronic transformer based on MMC, including a medium voltage stage, an isolation stage and a low voltage stage. The three-phase six-bridge arm MMC structure adopted by the medium voltage stage is a Si-SiC hybrid topology structure, wherein each phase unit is composed of an upper bridge arm and a lower bridge arm respectively, and the upper bridge arm and the lower bridge arm are both composed of N full-bridge sub-modules connected in series, and the full-bridge sub-modules include a first full-bridge sub-module and a second full-bridge sub-module. The first full-bridge sub-module of the upper bridge arm and the lower bridge arm both use the first full-bridge sub-module, and the remaining full-bridge sub-modules all use the second full-bridge sub-module. The isolation stage includes a symmetrical upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm of the isolation stage are both controlled by a carrier phase-shift modulation strategy. The low voltage stage is a three-phase four-bridge arm inverter.

[0018] The upper bridge arm and the lower bridge arm of the isolation stage are both composed of M+1 high-frequency isolation sub-modules.

[0019] The isolation stage adopts an ISOP topology and is directly connected in parallel to the medium voltage DC bus. The medium voltage stage is connected in series to the medium voltage DC bus, and the low voltage stage is connected in parallel to the medium voltage DC bus.

[0020] For example, the power electronic transformer can achieve bidirectional power flow between medium-voltage AC ports and medium-voltage DC ports, medium-voltage DC ports and low-voltage DC ports, and low-voltage DC ports and energy storage modules, and can achieve unidirectional power flow between low-voltage DC ports and low-voltage AC ports, photovoltaic power generation to medium- and low-voltage DC ports, and wind power generation to low-voltage DC ports. The medium-voltage side of the isolation-level submodule does not use capacitors, and the three-phase six-leg MMC uses full-bridge submodules. This power electronic transformer has the ability to suppress transient currents during DC-side short circuits. The first full-bridge submodule uses SiC-MOSFET power devices, and the second full-bridge submodule uses Si-IGBT power devices. The low-voltage DC bus receives power from photovoltaic and wind power generation and configures energy storage.

[0021] A second aspect of the present application provides a multi-port power electronic transformer control method based on MMC, including a medium voltage level control method, an isolation level control method, and a low voltage level control method; The medium voltage level control method includes a rectification mode control method, and the rectification mode control method comprises the following steps: Step 11: The actual value of DC bus voltage Subtract reference value After inputting into the PI controller, the current d-axis reference value is obtained ; Step 12: Input the three-phase voltage of the grid into the phase-locked loop controller to obtain the grid frequency , Phase and angles , Step 13: Perform Park transformation on the three-phase voltage to obtain the d-axis voltage and q-axis voltage ; Perform park transformation on the three-phase current to obtain the d-axis current and q-axis current ; Step 14: - and - Input the PI controller separately, and then introduce the disturbance 、 and 、 To eliminate the d and q axis coupling effects, obtain the d and q axis system control reference voltages and ,Will and Perform Park inverse transformation to obtain the three-phase control reference voltage 、 and ; Step 15: Set the actual value of DC bus voltage Subtract 、 and Get the reference voltage of the A, B and C three-phase upper bridge arm of the rectifier side MMC, and convert the actual value of the DC bus voltage Add separately 、 and Obtain the reference voltage of the A, B and C three-phase lower bridge arms of the inverter side MMC; Step 16: Using Functions for Reference Voltage To find out the NLM control strategy and the number of second full-bridge sub-modules that need to be put into operation in PWM mode, sort the capacitor voltages of all second full-bridge sub-modules in the bridge arm, and combine the direction of the bridge arm current, the sorting results of the capacitor voltages of the full-bridge sub-modules, and the number of full-bridge sub-modules that need to be put into operation to determine the switching state of the full-bridge sub-module for industrial frequency switching in the bridge arm.

[0022] The medium voltage level control method further includes an inverter mode control method, and the inverter mode control method includes the following steps: Step 21: Input the three-phase voltage of the inverter side grid into the phase-locked loop controller to obtain the operating frequency of the inverter side grid 、 and ; Perform Park transformation on the three-phase voltage input to the inverter side grid to obtain and , the inverter side grid input three-phase current is Park transformed to obtain and ; Step 22: - and - Input PI controller, and then introduce disturbance 、 and 、 To eliminate the d and q axis coupling effects, obtain the d and q axis system control reference voltages and ,Will and Perform Park inverse transformation to obtain the three-phase control reference voltage 、 and ; Step 23: Set the actual DC bus voltage Subtract 、 and Get the reference voltage of the U, V and W three-phase upper bridge arm of the inverter side MMC, and convert the actual value of the DC bus voltage Add separately 、 and Obtain the reference voltage of the U, V and W three-phase lower bridge arms of the inverter side MMC; Step 24: Using Functions for Reference Voltage Calculate the NLM control strategy and the number of second full-bridge sub-modules that need to be put into operation in PWM mode, sort the capacitor voltages of all second full-bridge sub-modules in the bridge arm, and determine the switching state of the full-bridge sub-modules switching at the industrial frequency in the bridge arm based on the direction of the bridge arm current, the sorting results of the capacitor voltages of the full-bridge sub-modules, and the number of full-bridge sub-modules that need to be put into operation.

[0023] The isolation level control method includes a method for controlling a single high-frequency isolation submodule at the isolation level. The method for controlling a single high-frequency isolation submodule at the isolation level is specifically as follows: The high-frequency isolator module low-voltage side H-bridge adopts the working principle based on the phase-shifted full-bridge, specifically: the phase difference between the diagonal power switch tubes of the low-voltage side H-bridge is the phase shift angle. The upper and lower tubes of each bridge arm of the H-bridge on the low-voltage side are complementary turned on, and the duty cycle of each switch tube is 0.5; the upper and lower tubes of each bridge arm of the H-bridge on the medium-voltage side of the high-frequency isolation submodule are complementary turned on, and the duty cycle of each switch tube is 0.5.

[0024] The isolation level control method also includes a modulation strategy for the isolation level, specifically: in steady state, the voltage on the medium voltage side of the high-frequency isolation transformer is , the voltage on the low voltage side is .

[0025] The low-voltage level control method comprises the following steps: Step 31: Convert the three-phase voltage of the low voltage level Input the phase-locked loop controller to get the phase angle ; Step 32: Convert the three-phase current of the low voltage stage and phase angle Perform park transformation to obtain the orthogonal component of the rotation and

[0026] Step 33: As the reference value of the PI controller, As the feedback value, after PI control, the active voltage is obtained by subtracting the voltage drop on the inductor. ; Step 34: As the reference value of the PI controller, As the feedback value, after PI control, the voltage drop on the inductor is added to obtain the reactive voltage. ; Step 35: and After the park inverse transformation, the three-phase voltage control reference instruction of the three-phase full-bridge converter is obtained.

[0027] Figure 2 The figure shows the control block diagram of the medium voltage stage using Si-SiC hybrid MMC circuit topology and working under rectification condition. The three-phase voltage of the large power grid is obtained after the phase-locked loop. , The angle used for coordinate transformation. The three-phase voltage is transformed by Park to obtain the d-axis and q-axis components of the voltage. and The three-phase current is transformed by Park to obtain the d-axis and q-axis components of the current and , the Park transformation matrix used is:

[0028] The transformation equation is:

[0029] The actual DC bus voltage and the reference voltage are given to the PI controller to obtain the d-axis current reference value. The theoretical derivation is as follows:

[0030] Where: is the proportional coefficient of the PI control loop of the DC bus voltage on the rectifier side, is the integral coefficient of the PI control loop of the DC bus voltage on the rectifier side.

[0031] isd_ref and isd are used as the reference value and feedback value of the PI controller respectively, and we can get:

[0032] Where: is the proportional coefficient of the PI control loop of the three-phase current on the rectifier side, is the integral coefficient of the PI control loop of the three-phase current on the rectifier side.

[0033] and As the reference value and feedback value of the PI controller respectively, we can get:

[0034] Considering filter coupling and voltage feedforward control, the control references for the system-level d-axis and q-axis are:

[0035] The system level d Axis and q The control reference of the axis is subjected to Park inverse transformation to obtain the system-level three-phase voltage control reference value, which is derived as follows:

[0036] The transformation matrix is:

[0037] The DC bus voltage is subtracted from the system-level three-phase voltage reference value to obtain the reference voltage waveform of each upper bridge arm, and the DC bus voltage is added to the system-level three-phase voltage reference value to obtain the reference voltage waveform of each lower bridge arm:

[0038] The reference voltage of each bridge arm is given to the SU-PWM modulation control strategy module, which generates the drive signal of the sub-module according to the above control method. The specific process is as follows: Taking the lower bridge arm of phase A as an example for analysis, in the sub-module unified pulse width modulation strategy (SU-PWM), in order to balance the capacitor voltage of the PWM modulation sub-module, the number of sub-modules nw required at each moment and the voltage VPWMw of the first sub-module of the bridge arm need to be selected according to the working conditions; there are two selection modes: the calculation formula for mode 1 and the calculation formula for mode 2 are shown below. The number of low-frequency sub-modules put into use in mode 1 is calculated using the flooring function floor(), and the number of low-frequency sub-modules put into use in mode 2 is calculated using the ceiling function ceil(). Among them, the PWM modulation signal of the SiC MOSFET sub-module is determined by the voltage of the SiCMOSFET sub-module. With triangular carrier By comparison, its working principle is shown below, and the working status of the SiCMOSFET full-bridge sub-module is summarized in Table 1.

[0039]

[0040]

[0041] Where: represents the ceiling function, and Indicates the results calculated in the above two modes respectively A Number of lower bridge arm submodules put into use .

[0042] The following relationship exists between the four variables in the above two equations (10) and (11):

[0043] like Figure 3 As shown in FIG, a flowchart of the specific implementation of the low-frequency switching submodule capacitor voltage balance control method working in two modes is listed. For example, if the bridge arm current changes and is greater than 0, i arm The direction is greater than 0, and the SiC MOSFET submodule capacitor voltage Greater than the submodule reference voltage , then choose mode 2 to calculate The submodules with lower voltage are in operation and the remaining submodules are bypassed. On the contrary, if the SiC MOSFET submodule capacitor voltage is Less than the submodule reference voltage , then select mode 1 to calculate The submodules with lower voltage are put into operation and the remaining submodules are bypassed. The high-frequency submodule using SiC MOSFET devices achieves self-capacitor voltage balance through its PWM control method. Its operating status is summarized in Table 1. The remaining submodules using Si IGBT devices also achieve submodule capacitor voltage balance through a sorting and selection algorithm and operating in Modes 1 and 2 described above. In addition, based on the switching advantages of these devices, the switching frequency is concentrated on the SiC MOSFET devices. The Si IGBTs switch at a low frequency and only provide the required output voltage level. This control strategy further reduces the system's switching losses and effectively increases the system's switching frequency.

[0044] Table 1 Working status of SiC MOSFET full-bridge submodule

[0045] Figure 4 The control block diagram of the medium voltage stage using Si-SiC hybrid MMC circuit topology and working under grid-connected inverter conditions is shown. The three-phase voltage of the large power grid is obtained after the phase-locked loop θ g , θ g The three-phase voltage is transformed by Park to obtain the voltage d Axis and q Axis component u gd and u gq The three-phase current is transformed by Park to obtain the current d Axis and q Axis component i gd and i gq The transformation is derived as follows:

[0046] i gd_ref and i gd As the reference value and feedback value of the PI controller respectively, we can get:

[0047] Where: K p2 is the proportional coefficient of the inverter side three-phase current PI control loop, K i2 is the integral coefficient of the PI control loop of the three-phase current on the inverter side.

[0048] i gq_ref and igq As the reference value and feedback value of the PI controller respectively, we can get:

[0049] Considering filter coupling and voltage feedforward control, the system level d Axis and q The control reference of the axis is:

[0050] The system level d Axis and q The control reference of the axis is subjected to Park inverse transformation to obtain the system-level three-phase voltage control reference value. The transformation is derived as follows:

[0051] The DC bus voltage is subtracted from the system-level three-phase voltage reference value to obtain the reference voltage waveform of each upper bridge arm, and the DC bus voltage is added to the system-level three-phase voltage reference value to obtain the reference voltage waveform of each lower bridge arm:

[0052] The reference voltage of each bridge arm is given to the SU-PWM modulation control strategy module, which generates the drive signal of the submodule according to the above control method. The method of generating the submodule drive signal by the SU-PWM modulation control strategy module is the same as that on the rectifier side and will not be repeated here.

[0053] Figure 5 It is the key waveform of the isolation-level high-frequency isolation submodule working in the phase-shift control mode.

[0054] Figure 6 This is a modulation strategy diagram of the isolation-level bridge arm phase-leading high-frequency isolation submodule working in carrier phase shift control.

[0055] Figure 7 This is the control block diagram of a low-voltage DC-AC converter operating in grid-connected inverter mode. The specific derivation process is as follows: The three-phase voltage of the 0.4kV large power grid is obtained after the phase-locked loop ω t , ω t The angle used for coordinate transformation. The three-phase current is transformed by Park to obtain the current d Axis and q Axis component i d and i q , the transformation is derived as:

[0056] id_ref and i d As the reference value and feedback value of the PI controller respectively, we can get:

[0057] Where: K pi_1 is the proportional coefficient of the three-phase current PI controller of the DC-AC three-phase full-bridge inverter, K ii_1 is the integral coefficient of the three-phase current PI controller of the DC-AC three-phase full-bridge inverter. i q_ref and i q As the reference value and feedback value of the PI controller respectively, we can get:

[0058] Considering filter coupling and voltage feedforward control, the system level d Axis and q The control reference of the axis is:

[0059] The system level d Axis and q The control reference of the axis is subjected to Park inverse transformation to obtain the system-level three-phase voltage control reference value. The transformation is derived as follows:

[0060] The three-phase voltage reference value obtained from the above formula is given to the SPWM controller and compared with the triangular carrier to generate the driving signal of the switching power device. The driving signal is given to the corresponding switching power device to control it to turn on or off.

[0061] Figure 8 This is the equivalent circuit diagram when a short circuit occurs at a certain moment on the medium voltage DC port. Based on the circuit topology and working principle of the isolation stage, the use of centralized capacitors in the medium voltage DC bus is avoided. Therefore, after a short circuit occurs on the medium voltage DC port, no high transient discharge current from the centralized capacitors will occur. When a short circuit occurs on the medium voltage DC bus port, the system immediately blocks the pulses of all power switching devices after detecting the fault. Figure 8 The figure shows the fault current path diagram of the power electronic transformer (PET) proposed in the present invention. Assuming that the power grid is v A >0, v B <0, v C<0. Since the medium-voltage three-phase six-bridge-arm MMC adopts a full-bridge submodule circuit topology, the output voltage of the full-bridge submodule is opposite to the grid voltage. The voltage of the upper bridge arm of phase A, phase B, and phase C can be expressed as:

[0062] in, V armA 、 V armB and V armB are the bridge arm voltages of phases A, B and C respectively, n Represents the number of bridge arm submodules, V C Represents the reference voltage of the submodule, V C =V dc / n .

[0063] To limit the current in the phase A and phase B paths shown in the figure, the following relationship should be satisfied:

[0064] in, V m is the phase voltage peak value.

[0065] To limit the current in the B-phase and C-phase paths shown in the figure, the following relationship should be satisfied:

[0066] Furthermore, we can get:

[0067] in, m is the modulation ratio of MMC.

[0068] According to the above relationship, the power electronic transformer (PET) proposed in the present invention can prevent DC bus short-circuit faults. The electric energy stored in the MMC bridge arm inductance will be absorbed by the capacitor of the full-bridge sub-module. Therefore, the short-circuit fault current is well limited and gradually decays to zero.

[0069] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A multi-port power electronic transformer based on MMC, characterized in that: It includes a medium voltage stage, an isolation stage and a low voltage stage. The three-phase six-bridge arm MMC structure adopted by the medium voltage stage is a Si-SiC hybrid topology structure, wherein each phase unit is composed of an upper bridge arm and a lower bridge arm respectively, and the upper bridge arm and the lower bridge arm are both composed of N full-bridge sub-modules connected in series. The full-bridge sub-module includes a first full-bridge sub-module and a second full-bridge sub-module. The first full-bridge sub-module of the upper bridge arm and the lower bridge arm both use the first full-bridge sub-module, and the remaining full-bridge sub-modules all use the second full-bridge sub-module. The isolation stage includes a symmetrical upper bridge arm and lower bridge arm. The upper bridge arm and the lower bridge arm of the isolation stage are both controlled by a carrier phase-shift modulation strategy. The low voltage stage is a three-phase four-bridge arm inverter.

2. The multi-port power electronic transformer based on MMC according to claim 1, characterized in that: The upper bridge arm and the lower bridge arm of the isolation stage are both composed of M+1 high-frequency isolation sub-modules.

3. The multi-port power electronic transformer based on MMC according to claim 2, characterized in that: The isolation stage adopts an ISOP topology and is directly connected in parallel to the medium voltage DC bus. The medium voltage stage is connected in series to the medium voltage DC bus, and the low voltage stage is connected in parallel to the medium voltage DC bus.

4. A multi-port power electronic transformer control method based on MMC, characterized in that: Applied to the power electronic transformer according to any one of claims 1 to 3, the method comprises a medium voltage level control method, an isolation level control method and a low voltage level control method; The medium voltage level control method includes a rectification mode control method, and the rectification mode control method comprises the following steps: Step 11: The actual value of DC bus voltage Subtract reference value After inputting into the PI controller, the current d-axis reference value is obtained ; Step 12: Input the three-phase voltage of the grid into the phase-locked loop controller to obtain the grid frequency , Phase and angles ; Step 13: Perform Park transformation on the three-phase voltage to obtain the d-axis voltage and q-axis voltage ; Perform park transformation on the three-phase current to obtain the d-axis current and q-axis current ; Step 14: - and - Input the PI controller separately, and then introduce the disturbance 、 and 、 To eliminate the d and q axis coupling effects, obtain the d and q axis system control reference voltages and ,Will and Perform Park inverse transformation to obtain the three-phase control reference voltage 、 and ; Step 15: Set the actual value of DC bus voltage Subtract 、 and Get the reference voltage of the three-phase upper bridge arm of A, B and C of the rectifier side MMC, and convert the actual value of the DC bus voltage Add separately 、 and Obtain the reference voltage of the A, B and C three-phase lower bridge arms of the inverter side MMC; Step 16: Using Functions for Reference Voltage To find out the NLM control strategy and the number of second full-bridge sub-modules that need to be put into operation in PWM mode, sort the capacitor voltages of all second full-bridge sub-modules in the bridge arm, and combine the direction of the bridge arm current, the sorting results of the capacitor voltages of the full-bridge sub-modules, and the number of full-bridge sub-modules that need to be put into operation to determine the switching state of the full-bridge sub-module for industrial frequency switching in the bridge arm.

5. The MMC-based multi-port power electronic transformer control method according to claim 4, characterized in that: The medium voltage level control method further includes an inverter mode control method, and the inverter mode control method includes the following steps: Step 21: Input the three-phase voltage of the inverter side grid into the phase-locked loop controller to obtain the operating frequency of the inverter side grid 、 and ; Perform Park transformation on the three-phase voltage input to the inverter side grid to obtain and , the inverter side grid input three-phase current is Park transformed to obtain and ; Step 22: - and - Input PI controller, and then introduce disturbance 、 and 、 To eliminate the d and q axis coupling effects, obtain the d and q axis system control reference voltages and ,Will and Perform Park inverse transformation to obtain the three-phase control reference voltage 、 and ; Step 23: Set the actual DC bus voltage Subtract 、 and Get the reference voltage of the U, V and W three-phase upper bridge arm of the inverter side MMC, and convert the actual value of the DC bus voltage Add separately 、 and Obtain the reference voltage of the U, V and W three-phase lower bridge arms of the inverter side MMC; Step 24: Using Functions for Reference Voltage Calculate the NLM control strategy and the number of second full-bridge sub-modules that need to be put into operation in PWM mode, sort the capacitor voltages of all second full-bridge sub-modules in the bridge arm, and determine the switching state of the full-bridge sub-modules switching at the industrial frequency in the bridge arm based on the direction of the bridge arm current, the sorting results of the capacitor voltages of the full-bridge sub-modules, and the number of full-bridge sub-modules that need to be put into operation.

6. The MMC-based multi-port power electronic transformer control method according to claim 4, characterized in that: The isolation level control method includes a control method for a single high-frequency isolation submodule of the isolation level. The control method for a single high-frequency isolation submodule of the isolation level is specifically as follows: The high-frequency isolator module low-voltage side H-bridge adopts the working principle based on the phase-shifted full-bridge, specifically: the phase difference between the diagonal power switch tubes of the low-voltage side H-bridge is the phase shift angle. The upper and lower tubes of each bridge arm of the H-bridge on the low-voltage side are complementary turned on, and the duty cycle of each switch tube is 0.5; the upper and lower tubes of each bridge arm of the H-bridge on the medium-voltage side of the high-frequency isolation submodule are complementary turned on, and the duty cycle of each switch tube is 0.

5.

7. The MMC-based multi-port power electronic transformer control method according to claim 4, characterized in that: The isolation level control method also includes a modulation strategy for the isolation level, specifically: in steady state, the voltage on the medium voltage side of the high-frequency isolation transformer is , the voltage on the low voltage side is .

8. The MMC-based multi-port power electronic transformer control method according to claim 4, characterized in that: The low-voltage level control method comprises the following steps: Step 31: Convert the three-phase voltage of the low voltage level Input the phase-locked loop controller to get the phase angle ; Step 32: Convert the three-phase current of the low voltage stage and phase angle Perform park transformation to obtain the orthogonal component of the rotation and Step 33: As the reference value of the PI controller, As the feedback value, after PI control, the active voltage is obtained by subtracting the voltage drop on the inductor. ; Step 34: As the reference value of the PI controller, As the feedback value, after PI control, the voltage drop on the inductor is added to obtain the reactive voltage. ; Step 35: and After the park inverse transformation, the three-phase voltage control reference instruction of the three-phase full-bridge converter is obtained.

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