A control method for reverse starting and grid connection of a medium voltage DC voltage converter

Abstract

The present invention discloses a control method for reverse starting and grid connection of a medium-voltage DC converter. To address the limited research on reverse starting of cascaded medium- and low-voltage DC converters in the prior art, which makes it difficult for existing control methods to address DC converter startup failures caused by startup shocks, the present invention specifically designs the startup method for redundant modules during the reverse starting process of the DC converter. This ensures a smooth transition of submodule capacitor voltages and loop currents during the startup process while achieving precise control of the high-voltage side voltage, thereby reducing the risk of DC converter startup failures caused by startup shocks.

Description

Technical Field

[0001] The present invention relates to the technical field of control of a medium voltage direct current (DC) voltage converter, and in particular to a method for controlling reverse starting and grid connection of a medium voltage DC voltage converter. Background Art

[0002] A typical cascaded bidirectional DC / DC converter includes a high-voltage side startup circuit, an ISOP converter, and a low-voltage side startup circuit. The ISOP converter is a cascaded sub-module high-voltage side series and low-voltage side parallel conversion circuit. Each sub-module is an isolated resonant converter or phase-shift converter.

[0003] The medium-voltage DC voltage converter described in this paper converts medium- and low-voltage DC voltages. As a voltage converter with bidirectional power flow, it must be capable of forward starting, meaning starting from the high-voltage side to the low-voltage side. After forward starting, the DC voltage converter controls the DC voltage or power on the low-voltage side. As a common starting method, forward starting has become increasingly mature in engineering applications.

[0004] For applications where renewable energy DC is integrated into a medium-voltage DC grid and a low-voltage DC generation system is needed to support the medium-voltage DC grid, the DC voltage converter must be capable of reverse starting to support the DC bus voltage on the medium-voltage side. Existing literature on reverse starting of cascaded medium- and low-voltage DC converters is very limited. As renewable energy capacity continues to increase, there is a pressing need for DC voltage converters to reverse start renewable energy to support the voltage of the medium-voltage DC system.

[0005] The reverse start-up of the cascaded medium and low voltage DC voltage converters puts forward certain requirements for the slow start-up of the low-voltage side capacitor voltage and the slow start-up of the high-voltage side capacitor voltage of the resonant sub-module or the phase-shifted sub-module. In addition, in order to improve reliability, the general sub-module cascaded DC voltage converter is equipped with certain redundant modules. During the reverse start-up process of the DC voltage converter, the starting method of the redundant module needs to be specially designed to ensure the smooth transition of the sub-module capacitor voltage and loop current during the startup process while achieving precise control of the high-voltage side voltage, so as to reduce the risk of startup failure of the DC voltage converter due to startup impact.

[0006] For example, a "two-stage bidirectional DC / DC converter and its reverse start-up control method" disclosed in Chinese patent literature, with publication number CN109450241B, includes a voltage-regulated front-stage DC / DC converter and an isolated rear-stage resonant converter, wherein the rear-stage resonant converter is composed of at least an inverter / rectifier unit, a resonant conversion unit, and a rectifier / inverter unit connected in cascade. The reverse start-up control method includes: upon receiving a reverse start command, keeping the front-stage DC / DC converter and the inverter / rectifier unit in a shutdown state, and controlling the rectifier / inverter unit to start slowly; at the end of the rectifier / inverter unit slow start, controlling the front-stage DC / DC converter to start slowly to establish a high-voltage bus voltage; after the high-voltage bus voltage is established, controlling the inverter / rectifier unit to start, causing the two-stage bidirectional DC / DC converter to enter a reverse operating state. This solution can avoid the backflow of intermediate bus energy during reverse start-up, but the slow start method will slow down the startup speed to a certain extent, so this solution has certain drawbacks. Summary of the Invention

[0007] The present invention mainly solves the problem that the reverse start-up of cascaded medium and low voltage DC converters in the prior art is limited, which makes it difficult for existing control methods to solve the problem of DC voltage converter startup failure caused by startup impact; and provides a control method for reverse start-up and grid connection of a medium voltage DC voltage converter. During the reverse start-up process of the DC voltage converter, a special design needs to be made for the startup mode of the redundant module to ensure a smooth transition of the capacitor voltage and loop current of the sub-module during the startup process while achieving precise control of the high-voltage side voltage, so as to reduce the risk of DC voltage converter startup failure caused by startup impact.

[0008] The above technical problems of the present invention are mainly solved by the following technical solutions:

[0009] The present invention includes a control method for reverse starting and grid connection of a medium-voltage DC voltage converter. The medium-voltage DC voltage converter is a cascaded medium- and low-voltage DC conversion device, including a high-voltage side starting circuit, an ISOP conversion circuit, and a low-voltage side starting circuit. The reverse starting and grid connection process involves gradually energizing the medium-voltage DC voltage converter from the low-voltage side to the high-voltage side until the high-voltage side of the medium-voltage DC voltage converter is connected to the high-voltage bus.

[0010] A control method for reverse starting and grid connection of a medium voltage DC voltage converter, a control process for reverse starting and grid connection, mainly includes:

[0011] a. Charge the low-voltage side capacitor of the submodule through the slow-start circuit of the low-voltage startup circuit;

[0012] b. When the difference between the DC capacitor voltage and the low-voltage DC bus voltage is within the allowable deviation range, close the bypass switch to bypass the slow-start resistor;

[0013] c. The inverter / rectifier unit of the rear stage of the submodule is unlocked and operates in the inverter working state, and the rectifier / inverter unit of the front stage of the submodule operates in the uncontrolled rectification working state;

[0014] d. After the inverter / rectifier unit of the rear stage of the submodule is unlocked, the rectifier / inverter unit of the front stage of the submodule is not unlocked. By controlling the unlocking pulse length of the rear stage inverter / rectifier unit, the voltage hysteresis control of the front stage rectifier / inverter unit is realized, and the output voltage of the front stage module is controlled within the allowable range (the submodule operates in a no-load working state);

[0015] e. When the high-voltage side voltage of the front stage of the submodule is within the allowable deviation range and the secondary control board of the front stage of the submodule is operating normally, unlock the front stage switch tube of the submodule and perform stable closed-loop control on the high-voltage side voltage of the front stage of the submodule. The control target is UN_REF / (NM);

[0016] f. When the average voltage on the high-voltage side of the front stage of the submodule is controlled at the set target value, the redundant module is removed and the other modules are put into the high-voltage side main circuit. The normal modules support the total output voltage of the high-voltage side. The voltage closed-loop control is switched to control the output voltage of the high-voltage side port, and the control target is UN_REF.

[0017] g. By connecting the slow-start circuit, the DC voltage converter is electrically connected to the high-voltage side DC bus;

[0018] h. Close the bypass switch to bypass the slow-start resistor; if there is no other voltage control source on the high-voltage DC bus, the control process ends after the voltage stabilizes; if there is other voltage control source on the high-voltage side DC bus, the DC voltage converter will receive instructions from the upper controller to switch to power control mode, or maintain the original voltage droop control mode, receive instructions from the upper controller on the voltage reference value or droop coefficient adjustment amount, and the entire control process ends after the voltage stabilizes.

[0019] Preferably, the cascaded medium- and low-voltage DC converter comprises a high-voltage side startup circuit, an ISOP conversion circuit, and a low-voltage side startup circuit; the ISOP conversion circuit is a modular cascade conversion circuit, wherein the high-voltage side comprises sub-modules (N modules, N greater than 2) connected in series, and the low-voltage side comprises sub-modules (N modules, N greater than 2) connected in parallel; the ISOP sub-module converter is a series structure of a half-bridge converter and an isolated resonant converter or a phase-shift converter, wherein the resonant converter or the phase-shift converter is composed of at least an inverter / rectifier unit, a passive inductor-capacitor network, and a rectifier / inverter unit connected in sequence in cascade.

[0020] Preferably, the ISOP conversion circuit is classified into two types of modules according to the working status of the modules. The first type of module is a module directly connected in series with the high-voltage main circuit, and the second type of module is a module that is bypassed and in a hot standby state. The second type of module is defined as a redundant module (the number of redundant modules is M, which is generally greater than 8% of the total number).

[0021] Preferably, in step c of the reverse startup control process, the inverter / rectifier unit of the submodule's rear stage is unlocked, the rectifier / inverter unit of the submodule's front stage is locked to operate only in rectification mode, and the submodule's isolation transformer is reversely excited. The voltage control method of the rectifier / inverter unit of the submodule's front stage is a hysteresis control method. After the inverter / rectifier unit of the module's rear stage is unlocked, the switching tube operates in a 50% duty cycle mode, the inverter voltage is a voltage rectangular wave with a 50% duty cycle, and the switching frequency is 10 kHz. The hysteresis period is generally above 100 milliseconds. When the voltage of the current stage unit exceeds Ulim_up, the pulse of the module's rear stage unit is locked at the end of the current switching cycle, and the voltage begins to decrease. When the voltage of the current stage unit falls below Ulim_down, the pulse of the module's rear stage unit is unlocked at the end of the current switching cycle. The hysteresis control mode is exited until the front stage unit is unlocked, and the rear stage unit remains unlocked.

[0022] As a preferred embodiment, in the fth step of the reverse start and grid connection control process, the high-voltage side voltage reference value UN_REF of the DC voltage converter continuously tracks the voltage UH of the high-voltage side DC bus, so that no current shock will be generated when the high-voltage DC bus is electrically connected in the next step, and the high-voltage side voltage control mode of the DC voltage converter is the voltage droop control mode.

[0023] Preferably, in step f of the reverse startup and grid-connected control process, the redundant module exits, that is, the lower tube of the half-bridge circuit of the sub-module is turned on and the upper tube is locked; the isolated resonant converter or phase-shift converter is in an unlocked state, and its control target is to control the capacitor voltage on the high-voltage side of the sub-module to the average value of the voltages of all sub-modules.

[0024] Preferably, in step h of the reverse start-up and grid-connected control process, when the high-voltage DC bus has other voltage control sources, the DC voltage converter can be connected to the DC bus and, according to the instructions of the upper controller, switch to the power control mode; or maintain the voltage droop control mode and coordinate with other voltage control sources to control the bus voltage. The DC voltage converter receives the voltage reference value or droop coefficient adjustment amount instructions from the upper controller to achieve output power regulation.

[0025] The beneficial effects of the present invention are:

[0026] 1. This solution rapidly charges the low-voltage side capacitor through a low-voltage side slow-start circuit. 2. This solution prevents the high-voltage side capacitor voltage from recirculating back into the low-voltage side capacitor by first unlocking the full-bridge switches on the low-voltage side of the DAB circuit. After the high-voltage side capacitor voltage stabilizes, the high-voltage side full-bridge switches are unlocked to stabilize the high-voltage side capacitor voltage. 3. This solution deactivates the redundant module and activates the normal module by unlocking the pre-stage half-bridge circuit of the submodule. After the half-bridge is unlocked, the voltage control loop switches to a closed-loop control of the port voltage, thereby achieving high-voltage side port voltage control. This enables the converter to slowly charge the high-voltage side DC bus and carry loads. 4. During operation, the control method of this solution automatically operates the DC voltage converter in steps, resulting in a smooth and disturbance-free process. This optimizes the entire reverse startup process, achieves a bumpless startup of the DC voltage converter, and reduces the risk of startup failure. This can enhance the ability of renewable energy sources such as photovoltaics to support medium-voltage system voltage through reverse startup of the DC voltage converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a topology diagram of the cascaded medium and low voltage DC voltage converter of the present invention;

[0028] Figure 2 This is a flow chart of the reverse start-up of the cascaded medium and low voltage DC voltage converter of the present invention;

[0029] Figure 3 It is a schematic diagram of the principle of the redundant module of the cascaded medium and low voltage DC voltage converter of the present invention being shut down and the normal module being put into operation;

[0030] Figure 4 This is the voltage control closed-loop switching logic diagram after the front-stage half-bridge circuit of the ISOP circuit submodule of the present invention is unlocked;

[0031] Figure 5 This is a control block diagram of the high-side voltage droop strategy for reverse starting of the cascaded medium- and low-voltage DC voltage converter of the present invention;

[0032] Figure 6 This is a schematic diagram of hysteresis control for controlling the capacitor voltage of the front-stage rectifier / inverter unit by controlling the rear-stage inverter / rectifier unit when the front-stage rectifier / inverter unit of the ISOP circuit submodule of the present invention is not unlocked. DETAILED DESCRIPTION

[0033] The technical solution of the present invention will be further specifically described below through embodiments and in conjunction with the accompanying drawings.

[0034] Example:

[0035] A method for controlling reverse starting and grid connection of a medium voltage DC voltage converter according to this embodiment is as follows: Figure 1As shown, it includes a high-voltage starting circuit, an ISOP converter, and a low-voltage starting circuit. The ISOP converter is a modular cascade circuit topology that uses a sub-module input series and output parallel connection mode. Figure 1 The topology diagram of the typical submodule is also shown in the figure. The topology diagram of the typical ISOP circuit with high-side half-bridge circuit is shown in the attached figure. Figure 3 .

[0036] A method for controlling reverse startup and grid connection of a medium-voltage DC converter. The medium-voltage DC converter is a cascaded medium- and low-voltage DC converter device, comprising a high-voltage side startup circuit, an ISOP conversion circuit, and a low-voltage side startup circuit. The reverse startup and grid connection process involves gradually energizing the medium-voltage DC converter from the low-voltage side to the high-voltage side until the high-voltage side of the medium-voltage DC converter is connected to the high-voltage bus.

[0037] A control method for reverse starting and grid connection of a medium voltage DC voltage converter, a control process for reverse starting and grid connection, Figure 2 This is a flow chart of the reverse start-up of the DC voltage converter. The control process mainly includes:

[0038] a. Charge the low-voltage side capacitor of the submodule through the slow-start circuit of the low-voltage startup circuit;

[0039] b. When the difference between the DC capacitor voltage and the low-voltage DC bus voltage is within the allowable deviation range, close the bypass switch to bypass the slow-start resistor;

[0040] c. The inverter / rectifier unit of the rear stage of the submodule is unlocked and operates in the inverter working state, and the rectifier / inverter unit of the front stage of the submodule operates in the uncontrolled rectification working state;

[0041] d. After the inverter / rectifier unit of the rear stage of the submodule is unlocked, the rectifier / inverter unit of the front stage of the submodule is not unlocked. By controlling the unlocking pulse length of the rear stage inverter / rectifier unit, the voltage hysteresis control of the front stage rectifier / inverter unit is realized, and the output voltage of the front stage module is controlled within the allowable range (the submodule operates in a no-load working state);

[0042] e. When the high-voltage side voltage of the front stage of the submodule is within the allowable deviation range and the secondary control board of the front stage of the submodule is operating normally, unlock the front stage switch tube of the submodule and perform stable closed-loop control on the high-voltage side voltage of the front stage of the submodule. The control target is UN_REF / (NM);

[0043] f. When the average voltage on the high-voltage side of the front stage of the submodule is controlled at the set target value, the redundant module is withdrawn and the other modules are put into the high-voltage side main circuit. The normal input module supports the total output voltage of the high-voltage side; the voltage closed-loop control is switched to control the output voltage of the high-voltage side port, and the control target is UN_REF. The logic block diagram of the control target switching is shown in the attached figure. Figure 4 ;

[0044] g. By connecting the slow-start circuit, the DC voltage converter is electrically connected to the high-voltage side DC bus;

[0045] h. Close the bypass switch to bypass the slow-start resistor. If there is no other voltage control source on the high-voltage DC bus, the control process ends after the voltage stabilizes. If there is another voltage control source on the high-voltage side DC bus, the DC voltage converter will receive the instruction from the upper controller to switch to the power control mode, or maintain the original voltage droop control mode (the control block diagram of the droop strategy is shown in the attached figure). Figure 5 ), receives the voltage reference value or droop coefficient adjustment instruction from the upper controller, and the entire control process ends after the voltage stabilizes.

[0046] A cascaded medium- and low-voltage DC converter device includes a high-voltage side startup circuit, an ISOP conversion circuit, and a low-voltage side startup circuit. The ISOP conversion circuit is a modular cascade conversion circuit, with submodules (N modules, N greater than 2) connected in series on the high-voltage side and submodules (N modules, N greater than 2) connected in parallel on the low-voltage side. The ISOP submodule converter is a series structure of a half-bridge converter and an isolated resonant converter or phase-shift converter. The resonant converter or phase-shift converter is composed of at least an inverter / rectifier unit, a passive inductor-capacitor network, and a rectifier / inverter unit connected in sequence.

[0047] In this embodiment, reverse starting is a starting method that starts from the low-voltage starting circuit and gradually starts to the high-voltage starting circuit. The ultimate control target is to stabilize the voltage on the high-voltage DC bus side of the high-voltage starting circuit. The high-voltage side capacitor and the low-voltage side capacitor of the submodule of the ISOP circuit are both equipped with measuring elements to collect voltage. The input side and output side of the high-voltage starting circuit are both equipped with measuring elements to collect voltage. The voltage measuring elements are specifically shown in Figure 1 .

[0048] The ISOP conversion circuit is classified into two types of modules according to the working status of the module. The first type of module is the module directly connected in series with the high-voltage main circuit, and the second type of module is the module that is bypassed and in hot standby status. The second type of module is defined as a redundant module (the number of redundant modules is M, generally greater than 8% of the total).

[0049] In step c of the reverse start control process, the inverter / rectifier unit of the submodule's rear stage is unlocked, the rectifier / inverter unit of the submodule's front stage is locked and only works in the rectifier mode, and the isolation transformer of the submodule is reversely excited. The voltage control method of the rectifier / inverter unit of the submodule's front stage is the hysteresis control method. The hysteresis control method is shown in the figure below. Figure 6As shown, after the inverter / rectifier unit of the module's rear stage is unlocked, the switch tube operates in a 50% duty cycle mode, the inverter voltage is a voltage rectangular wave with a duty cycle of 50%, and the switching frequency is 10kHz.

[0050] Hysteresis control method, the schematic diagram of hysteresis control is as follows Figure 6 As shown, the period of the hysteresis loop is generally more than 100 ms. When the voltage of the current unit is higher than Ulim_up, the pulse of the module's subsequent unit is locked at the end of the current switching cycle, and the voltage begins to decrease. When the voltage of the current unit is lower than Ulim_down, the pulse of the module's subsequent unit is unlocked at the end of the current switching cycle. The hysteresis control mode is exited after the previous unit is unlocked, and the subsequent unit is in a state of being unlocked.

[0051] In step f of the reverse start and grid connection control process, the high-side voltage reference value UN_REF of the DC voltage converter continuously tracks the voltage UH of the high-side DC bus. This ensures that no current surge occurs during the next step of electrical connection to the high-voltage DC bus. The high-side voltage control mode of the DC voltage converter is the voltage droop control mode.

[0052] In step f of the reverse start-up and grid-connection control process, the redundant module exits, i.e., the lower tube of the half-bridge circuit of the sub-module is turned on and the upper tube is locked; the isolated resonant converter or phase-shift converter is in the unlocked state, and its control target is to control the capacitor voltage on the high-voltage side of the sub-module to the average voltage of all sub-modules. Figure 3 The figure shows the switching state of the switch tube after the redundant module of the high-voltage side half-bridge conversion circuit is exited, including the operating state of the two modules. The redundant module is in a no-load state, and other normally operating modules support the DC voltage of the output port.

[0053] In step h of the reverse start and grid-connection control process, when the high-voltage DC bus has other voltage control sources, the DC voltage converter can switch to power control mode according to the instructions of the upper controller after being connected to the DC bus; or maintain voltage droop control mode and coordinate with other voltage control sources to control the bus voltage. The DC voltage converter receives the voltage reference value or droop coefficient adjustment value instructions from the upper controller to adjust the output power.

[0054] In step c of the reverse startup control process, the inverter / rectifier unit at the rear stage of the submodule is unlocked, and the isolation transformer of the submodule is reversely excited and energized, thus achieving a reverse charged shock to the isolation transformer.

[0055] In step f of the reverse start control process, the redundant module exits, that is, the lower tube of the half-bridge circuit of the sub-module is turned on and the upper tube is locked; the isolated resonant converter or phase-shift converter is in the unlocked state, and its control target is to control the capacitor voltage on the high-voltage side of the sub-module to the average voltage of all sub-modules.

[0056] The above description is only a preferred embodiment of the present invention, which is used to illustrate the technical solution of the present invention rather than to limit it. It should be pointed out that modifying the technical solutions described in the aforementioned embodiments or replacing some of the technical features therein by equivalents does not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

[0057] It should be understood that the embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope limited by the appended claims of the application.

Claims

1. A method for controlling reverse startup and grid connection of a medium voltage DC voltage converter, comprising a medium voltage DC voltage converter, characterized in that: The medium voltage DC voltage converter is a cascade medium and low voltage DC conversion device, including a high voltage side starting circuit, an ISOP conversion circuit and a low voltage side starting circuit; The reverse start and grid connection control method specifically includes: starting the medium voltage DC voltage converter from the low voltage side to the high voltage side of the converter until the high voltage side of the medium voltage DC voltage converter is connected to the high voltage bus; The control method for reverse starting and grid connection of the medium voltage DC voltage converter mainly includes the following steps: a. Charge the low-voltage side capacitor of the submodule through the slow-start circuit of the low-voltage startup circuit; b. When the difference between the DC capacitor voltage and the low-voltage DC bus voltage is within the allowable deviation range, close the bypass switch to bypass the slow-start resistor; c. The inverter / rectifier unit of the rear stage of the submodule is unlocked and operates in the inverter working state, and the rectifier / inverter unit of the front stage of the submodule operates in the uncontrolled rectification working state; d. After the inverter / rectifier unit of the rear stage of the submodule is unlocked, the rectifier / inverter unit of the front stage of the submodule is not unlocked. By controlling the unlocking pulse length of the inverter / rectifier unit of the rear stage, the voltage hysteresis control of the front stage rectifier / inverter unit is realized, and the output voltage of the front stage module is controlled within the allowable deviation value range. The submodule works in a no-load working state; e. When the high-side voltage of the front-stage submodule is within the allowable deviation range and the secondary control board of the front-stage submodule is operating normally, unlock the front-stage switch tube of the submodule and perform stable closed-loop control on the high-side voltage of the front-stage submodule. The control target is UN_REF / (NM), where UN_REF is the overall target voltage of the high-side port, N is the total number of submodules, and M is the number of redundant modules. f. When the average voltage on the high-voltage side of the front stage of the submodule is controlled at the set target value, the redundant module is removed and the other modules are put into the high-voltage side main circuit. The normal modules support the total output voltage of the high-voltage side. The voltage closed-loop control is switched to control the output voltage of the high-voltage side port, and the control target is UN_REF. g. By connecting the slow-start circuit, the DC voltage converter is electrically connected to the high-voltage side DC bus; h. Close the bypass switch to bypass the slow-start resistor; if there is no other voltage control source on the high-voltage DC bus, the control process ends after the voltage stabilizes; if there is other voltage control source on the high-voltage side DC bus, the DC voltage converter will receive instructions from the upper controller to switch to power control mode, or maintain the original voltage droop control mode, receive instructions from the upper controller on the voltage reference value or droop coefficient adjustment amount, and the entire control process ends after the voltage stabilizes.

2. A method for controlling reverse startup and grid connection of a medium voltage DC voltage converter according to claim 1, characterized in that: The cascaded medium- and low-voltage DC converter includes a high-voltage side startup circuit, an ISOP conversion circuit, and a low-voltage side startup circuit. The ISOP conversion circuit is a modular cascade conversion circuit, with submodules connected in series on the high-voltage side and submodules connected in parallel on the low-voltage side. The submodule converters of the ISOP conversion circuit are a series structure of a half-bridge converter and an isolated resonant converter or a phase-shift converter, wherein the resonant converter or the phase-shift converter is composed of an inverter / rectifier unit, a passive inductor-capacitor network, a rectifier / inverter unit, and a configurable structure connected in sequence in cascade.

3. A method for controlling reverse startup and grid connection of a medium voltage DC voltage converter according to claim 2, characterized in that: According to the working status of the module, the ISOP conversion circuit is classified into two modules, including a first seed module and a second seed module. The first seed module is a module directly connected in series with the high-voltage main circuit, and the second seed module is a module that is bypassed and in a hot standby state. The second seed module is defined as a redundant module.

4. The method for controlling reverse startup and grid connection of a medium voltage DC voltage converter according to claim 1, characterized in that: In step c of the reverse startup control process, the inverter / rectifier unit of the submodule's rear stage is unlocked, the rectifier / inverter unit of the submodule's front stage is locked and operates only in rectification mode, and the isolation transformer of the submodule is reversely excited and energized. The voltage control method of the rectifier / inverter unit of the submodule's front stage is hysteresis control. After the inverter / rectifier unit of the module's rear stage is unlocked, the switch tube operates in 50% duty cycle mode. The inverter voltage is a voltage rectangular wave with a duty cycle of 50%, and the switching frequency is 10kHz. For the hysteresis control method, the hysteresis period is more than 100 ms. When the voltage of the current unit is higher than the maximum limit Ulim_up of the front-stage voltage of the submodule allowed, the pulse of the rear-stage unit of the module is locked at the end of the current switching cycle, and the voltage begins to decrease. When the voltage of the current unit is lower than the minimum limit Ulim_down of the front-stage voltage of the submodule allowed, the pulse of the rear-stage unit of the module is unlocked at the end of the current switching cycle. After the front-stage unit is unlocked, the hysteresis control mode is exited, and the rear-stage unit is in a state of being unlocked.

5. The method for controlling reverse startup and grid connection of a medium voltage DC voltage converter according to claim 1, characterized in that: The high-side voltage reference value UN_REF of the medium-voltage DC voltage converter continuously tracks the voltage UH of the high-side DC bus, so that no current shock is generated when the medium-voltage DC voltage converter is electrically connected to the high-voltage DC bus in the next step. The high-side voltage control mode of the medium-voltage DC voltage converter is the voltage droop control mode.

6. A method for controlling reverse startup and grid connection of a medium voltage DC voltage converter according to claim 1, 4 or 5, characterized in that: In step f of the reverse start-up and grid-connection control process, the redundant module exits, i.e., the lower tube of the half-bridge circuit of the sub-module is turned on and the upper tube is locked; the isolated resonant converter or phase-shift converter is in the unlocked state, and its control target is to control the capacitor voltage on the high-voltage side of the sub-module to the average voltage of all sub-modules.

7. A method for controlling reverse startup and grid connection of a medium voltage DC voltage converter according to claim 6, characterized in that: In step h of the reverse start and grid connection control process, when the high-voltage DC bus has other voltage control sources, the medium-voltage DC voltage converter is connected to the DC bus and, according to the instructions of the upper controller, switches to power control mode; or maintains voltage droop control mode and coordinates with other voltage control sources to control the bus voltage. The medium-voltage DC voltage converter receives the voltage reference value or droop coefficient adjustment value instructions from the upper controller to adjust the output power.

Images