A modular multilevel DC / DC converter and its application system

Abstract

This application discloses a modular multilevel DC / DC converter and its application system. The scheme includes an M-phase upper bridge arm, an M-phase lower bridge arm, an M-phase harmonic filtering module, and a controller. The m-phase upper bridge arm is connected to both the m-phase lower bridge arm and the m-phase harmonic filtering module. The m-phase harmonic filtering module is composed of a first inductor and N full-bridge sub-modules connected in series. The N full-bridge sub-modules in the m-phase harmonic filtering module filter out harmonic components from their own output current and, under the control of the controller, output a second DC voltage component and the fundamental frequency component of the second AC voltage component. The sum of the fundamental frequency components in all second AC voltage components is 0. Therefore, this scheme extracts the second DC voltage component generated by the lower bridge arm through the harmonic filtering module composed of full-bridge sub-modules, thereby achieving DC voltage conversion, replacing the function of a passive large inductor, and reducing footprint and energy loss.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of DC / DC converter, in particular to a modular multilevel DC / DC converter and application system. BACKGROUND

[0002] In recent years, with the rapid development of high-voltage direct current transmission, the direct current grid technology has been widely concerned. The direct current grid not only can realize large-scale new energy access in different regions, but also can realize direct current interconnection between different power grids, so as to realize efficient configuration of electric energy. However, due to the different voltage levels of the direct current transmission lines that have been put into operation, how to realize the interconnection of direct current lines with different voltage levels, the existing technology adopts a modular multilevel DC / DC converter. The current modular multilevel DC / DC converter includes M-phase upper bridge arms, M-phase lower bridge arms and a passive large inductor. The mth phase upper bridge arm and the mth phase lower bridge arm are composed of half-bridge sub-modules, 1≤m≤M. The mth phase high-voltage side of the modular multilevel DC / DC converter is composed of the mth phase upper bridge arm and the mth phase lower bridge arm in series. The passive large inductor is used for filtering the direct current voltage component generated by the mth phase lower bridge arm at the connection of the mth phase upper bridge arm and the mth phase lower bridge arm, so as to realize the conversion of the direct current voltage. The passive large inductor increases the floor area and the loss of the system to which the modular multilevel DC / DC converter is applied. SUMMARY

[0003] The purpose of the present application is to provide a modular multilevel DC / DC converter and application system. The second direct current voltage component generated by the lower bridge arm is extracted by the harmonic filter module composed of full-bridge sub-modules, so as to realize the conversion of the direct current voltage, replace the function of the passive large inductor, and reduce the floor area and energy loss.

[0004] To solve the above technical problems, the present application provides a modular multilevel DC / DC converter, which comprises M-phase upper bridge arms, M-phase lower bridge arms, M-phase harmonic filter modules and a controller. The mth phase upper bridge arm is connected to the mth phase lower bridge arm and the mth phase harmonic filter module, respectively. The mth phase upper bridge arm and the mth phase lower bridge arm are connected in series to form the mth phase high-voltage side of the modular multilevel DC / DC converter. The M-phase high-voltage sides are connected in parallel to form a total high-voltage side. The mth phase lower bridge arm and the mth phase harmonic filter module are connected in series to form the mth phase low-voltage side of the modular multilevel DC / DC converter. The M-phase low-voltage sides are connected in parallel to form a total low-voltage side. Each phase harmonic filter module is composed of a first inductor and N full-bridge sub-modules connected in series, M≥2, 1≤m≤M, N≥1.

[0005] The mth phase upper bridge arm is used to generate a first direct current voltage component and a first alternating current voltage component under the control of the controller.

[0006] The lower bridge arm of the mth phase is used to generate a second DC voltage component and a second AC voltage component under the control of the controller;

[0007] The N full-bridge sub-modules in the harmonic filtering module of the mth phase are used to filter harmonic components in the output current and output fundamental frequency components in the second DC voltage component and the second AC voltage component under the control of the controller, and the sum of all fundamental frequency components in the second AC voltage component is 0.

[0008] Preferably, the upper bridge arm of each phase is composed of a second inductor and N full-bridge sub-modules connected in series.

[0009] When the control mode of the harmonic filtering module and the upper bridge arm of the mth phase is exchanged by the controller, the total high voltage side becomes the new total low voltage side, and the total low voltage side becomes the new total high voltage side.

[0010] Preferably, the upper bridge arm of each phase is composed of a second inductor and N full-bridge sub-modules connected in series.

[0011] When the upper bridge arm of each phase generates a negative voltage, the total high voltage side becomes the new total low voltage side, and the total low voltage side becomes the new total high voltage side.

[0012] Preferably, the lower bridge arm of each phase is composed of a third inductor and N half-bridge sub-modules connected in series.

[0013] Preferably, the controller is further used to control the AC current in the upper bridge arm of each phase to be the minimum value, and control the AC current in the lower bridge arm of each phase to be the minimum value.

[0014] Preferably, the controller is further used to control the N full-bridge sub-modules in the harmonic filtering module of each phase to enter a lockout state until the DC voltage at the output end of the total low voltage side recovers to a preset low voltage value when a short circuit fault occurs at the output end of the total low voltage side.

[0015] Preferably, the upper bridge arm of each phase is composed of a second inductor and N full-bridge sub-modules connected in series.

[0016] The controller is further used to control the N full-bridge sub-modules in the harmonic filtering module of each phase to enter a lockout state and control the N full-bridge sub-modules in the upper bridge arm of each phase to enter a lockout state until the DC voltage at the output end of the total low voltage side recovers to a preset low voltage value when a short circuit fault occurs at the output end of the total low voltage side.

[0017] Preferably, the controller is further configured to control N full-bridge sub-modules in the harmonic filtering module of each phase to enter a negative input state until the DC voltage of the input end of the total high-voltage side recovers to a preset high-voltage value when a short-circuit fault occurs at the input end of the total high-voltage side.

[0018] Preferably, the upper bridge arm of each phase is composed of a second inductor and N full-bridge sub-modules connected in series, and the controller is further configured to control N full-bridge sub-modules in the harmonic filtering module of each phase to enter a negative input state and control N full-bridge sub-modules in the upper bridge arm of each phase to enter a negative input state until the DC voltage of the input end of the total high-voltage side recovers to a preset high-voltage value when a short-circuit fault occurs at the input end of the total high-voltage side.

[0019] To solve the above technical problems, the application further provides an application system comprising the modular multi-level DC / DC converter.

[0020] The application provides a modular multi-level DC / DC converter and an application system. The scheme comprises M-phase upper bridge arms, M-phase lower bridge arms, M-phase harmonic filtering modules and a controller. The mth upper bridge arm is connected to the mth lower bridge arm and the mth harmonic filtering module. The mth harmonic filtering module is composed of a first inductor and N full-bridge sub-modules connected in series. The N full-bridge sub-modules in the mth harmonic filtering module are used to filter harmonic components in the output current and output the fundamental frequency components in the second DC voltage component and the second AC voltage component under the control of the controller. The sum of the fundamental frequency components in all second AC voltage components is 0. Thus, the second DC voltage component generated by the lower bridge arm is extracted by the harmonic filtering module composed of full-bridge sub-modules, thereby realizing DC voltage conversion and replacing the function of a large passive inductor, reducing the floor area and energy loss. BRIEF DESCRIPTION OF DRAWINGS

[0021] To more clearly illustrate the technical solutions in the embodiments of the application, the following will briefly introduce the drawings needed in the prior art and the embodiments. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without any creative effort based on these drawings.

[0022] Figure 1 A structural schematic diagram of a modular multi-level DC / DC converter provided by the application;

[0023] Figure 2 A structural schematic diagram of another modular multi-level DC / DC converter provided by the application;

[0024] Figure 3A simplified voltage source equivalent circuit diagram of a single-phase modular multilevel DC / DC converter provided in this application;

[0025] Figure 4 A schematic diagram of a controller provided in this application;

[0026] Figure 5 A schematic diagram of a short-circuit fault provided in this application;

[0027] Figure 6 A schematic diagram of another short-circuit fault provided in this application. Detailed Implementation

[0028] The core of this application is to provide a modular multilevel DC / DC converter and its application system. This solution extracts the second DC voltage component generated by the lower bridge arm through a harmonic filtering module composed of full-bridge sub-modules, thereby realizing DC voltage conversion, replacing the function of passive large inductors, and reducing the footprint and energy loss.

[0029] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0030] Figure 1 This is a schematic diagram of a modular multilevel DC / DC converter provided in this application. Figure 1 Taking M=3 and N=2 as an example, it includes an M-phase upper bridge arm 1, an M-phase lower bridge arm 2, an M-phase harmonic filtering module 3, and a controller. The m-th phase upper bridge arm 1 is connected to the m-th phase lower bridge arm 2 and the m-th phase harmonic filtering module 3 respectively. The m-th phase upper bridge arm 1 and the m-th phase lower bridge arm 2 are connected in series to form the m-th phase high voltage side of the modular multilevel DC / DC converter. The M-phase high voltage side is connected in parallel to form the total high voltage side. The m-th phase lower bridge arm 2 and the m-th phase harmonic filtering module 3 are connected in series to form the m-th phase low voltage side of the modular multilevel DC / DC converter. The M-phase low voltage side is connected in parallel to form the total low voltage side. Each phase harmonic filtering module 3 is composed of a first inductor L1 and N full-bridge sub-modules U1 connected in series in sequence. M≥2, 1≤m≤M, N≥1;

[0031] The upper bridge arm 1 of the m-th phase is used to generate the first DC voltage component and the first AC voltage component under the control of the controller;

[0032] The lower bridge arm 2 of the m-th phase is used to generate the second DC voltage component and the second AC voltage component under the control of the controller;

[0033] The N full-bridge sub-modules U1 in the m-th phase harmonic filtering module 3 are used to filter out the harmonic components in their own output current and output the second DC voltage component and the fundamental frequency component in the second AC voltage component under the control of the controller. The sum of the fundamental frequency components in all the second AC voltage components is 0.

[0034] In recent years, with the rapid development of high-voltage direct current (HVDC) transmission, DC grid technology has received widespread attention. DC grids offer advantages such as flexible control and smaller footprint. They not only enable large-scale integration of new energy sources into different regions but also facilitate DC interconnection between different grids, achieving efficient power allocation. However, due to the different voltage levels of existing DC transmission lines, interconnecting these lines at different voltage levels remains a challenge in DC grid construction. Current technologies utilize modular multilevel DC / DC converters to achieve bidirectional energy flow in high-voltage, high-power DC / DC converters.

[0035] In the prior art, modular multilevel DC / DC converters have advantages such as easy expansion, bidirectional energy flow, and suitability for high-voltage and high-power applications. They include an M-phase upper bridge arm, an M-phase lower bridge arm, and a passive large inductor. The m-th phase upper bridge arm and the m-th phase lower bridge arm are formed by half-bridge submodules, where 1≤m≤M. The m-th phase high-voltage side of the modular multilevel DC / DC converter is formed by connecting the m-th phase upper bridge arm and the m-th phase lower bridge arm in series. At the connection between the m-th phase upper bridge arm and the m-th phase lower bridge arm, a passive large inductor is used to filter and output the DC voltage component generated by the m-th phase lower bridge arm, thus realizing the DC voltage conversion. However, this passive large inductor increases the footprint and the losses of the system in which the modular multilevel DC / DC converter is applied, and it is also difficult to manufacture.

[0036] In this application, the M-phase harmonic filtering module 3 replaces the passive large inductor for filtering. Although this increases the number of switching devices in the full-bridge submodule U1 of the M-phase harmonic filtering module 3 and leads to an increase in switching losses, the overall losses are less than those of the topology using a high-power isolation transformer and a large filter inductor.

[0037] Specifically, each phase upper bridge arm 1 can be composed of N half-bridge submodules or N full-bridge submodules; each phase lower bridge arm can be composed of N half-bridge submodules or N full-bridge submodules; among which, Figure 2Taking a configuration where each phase upper arm 1 consists of N full-bridge submodules U1 and each phase lower arm 2 consists of N half-bridge submodules U2 (M=3, N=2) as an example, the controller can control the operating state of the full-bridge submodule U1 by controlling the frequency and duty cycle of the voltage at the control terminals of the IGBTs in the full-bridge submodule U1. When in the active operating state, the voltage of a single full-bridge submodule U1 can be Vc; when in the deactivated operating state, the voltage of a single full-bridge submodule U1 is 0. The voltage of each phase upper arm 1, constructed using a modular multilevel structure, is the sum of the voltages of all full-bridge submodules U1 on each phase upper arm 1. The controller can control the opening and closing of each IGBT by sending a modulation signal to the control terminal of each IGBT, thereby controlling the operating state of the full-bridge submodule U1. Ultimately, this generates an ideal voltage waveform on each phase upper bridge arm 1. The voltage on each phase upper bridge arm 1 then generates the sum of the first DC voltage component and the first AC voltage component, approximating a stepped sine wave, for example, approximately 2500 + 2500sinwt. The controller can control the half-bridge submodule U2 in the same way. Furthermore, the modulation signal can be set to a sine wave containing both DC and AC components.

[0038] It should also be noted that, with Figure 2 For example, if the total low-voltage side outputs a DC voltage, and each phase lower arm 2 only has a second DC voltage component, the voltage of the capacitors in the full-bridge submodule U1 and half-bridge submodule U2 on the total high-voltage side cannot be maintained and may continuously increase or decrease. In this case, the total high-voltage side cannot operate continuously. Therefore, when the first AC voltage component generated by the upper arm 1 of phase m and the second AC voltage component generated by the lower arm 2 of phase m increase and decrease respectively, energy exchange can be achieved between the upper arm 1 and the lower arm 2 of phase m through AC voltage, maintaining the continuous operation of the total high-voltage side. Furthermore, the DC voltage in the total high-voltage side is used to drive the DC current, allowing power to flow bidirectionally between the total high-voltage side and the total low-voltage side. The AC voltage is used to drive the AC circulating current to exchange the active power between the upper arm 1 and the lower arm 2 of phase m, maintaining the energy balance between the upper arm 1 and the lower arm 2 of phase m. Under steady-state conditions, the voltage of the capacitors in the full-bridge submodule U1 and the half-bridge submodule U2 is balanced, and the sum of all DC and AC power is zero.

[0039] Alternating current is introduced into each phase upper arm and each phase lower arm to achieve energy exchange between the arms. At the same time, due to the complexity of the control strategy, a PI closed-loop control strategy can be adopted to achieve power balance control between M-phase upper arm 1 and M-phase lower arm 2, thereby improving stability. The reference values ​​of each voltage are obtained to achieve output current control of the modular multilevel DC / DC converter, thereby achieving DC voltage or power control on the total low voltage side.

[0040] More importantly, the capacitors in the N full-bridge submodules U1 of the m-th phase harmonic filtering module 3 filter out the harmonic components in their own output current, thus leaving only the fundamental frequency component in their own output second AC voltage component. Simultaneously, the controller can use modulation signals to control the operating state of the N full-bridge submodules U1 in each phase harmonic filtering module 3, ensuring that they all generate only AC voltage components. These AC voltage components are waveforms adjusted from the fundamental frequency components of each second AC voltage component, achieving a sum of zero for the fundamental frequency components in each second AC voltage component. Ultimately, only DC voltage is output from the total low-voltage side. Here, the waveforms of the fundamental frequency components in the M second AC voltage components can be adjusted to waveforms that differ in phase by 2π / M angles. M must be greater than or equal to 2 to be added together to eliminate the fundamental frequency component.

[0041] When each phase harmonic filtering module 3 is limited to being composed of a first inductor L1 and N full-bridge sub-modules U1 connected in series, a short-circuit fault occurs at the output terminal of the total low-voltage side. The controller needs to lock all IGBTs into a latched state. In this latched state, the capacitors in all IGBTs charge, and the fault current can flow through the capacitors in the N full-bridge sub-modules U1 of each phase harmonic filtering module 3 to be absorbed, thus supporting the voltage on the total low-voltage side. At this time, the N full-bridge sub-modules U1 of each phase harmonic filtering module 3 act as DC circuit breakers. The short-circuit fault is blocked. When a short-circuit fault occurs at the input of the main high-voltage side, the controller needs to put all IGBTs into a negative-energized state. In the negative-energized state, the capacitors in all IGBTs are charged, and the fault current can flow through the capacitors in the N full-bridge sub-modules U1 of each phase harmonic filtering module 3 to be absorbed, thus supporting the voltage of the main high-voltage side. At this time, the N full-bridge sub-modules U1 of each phase harmonic filtering module 3 act as DC circuit breakers, thereby blocking the short-circuit fault and ultimately enabling the converter to have bidirectional DC fault ride-through capability. In addition, the inductors in the path through which the fault current flows can suppress the fault current and play a protective role. In the prior art, when a short-circuit fault occurs at the output of the main low-voltage side, the controller controls the N half-bridge sub-modules in the upper bridge arm of each phase to enter a blocked state. In the blocked state, the capacitors in all IGBTs are charged to absorb the fault current, thus having DC fault ride-through capability on the main low-voltage side; however, when a short-circuit fault occurs at the input of the main high-voltage side, the fault current cannot flow through the capacitors in the charging state, and therefore does not have DC fault ride-through capability on the main high-voltage side.

[0042] In summary, this application provides a modular multilevel DC / DC converter, which includes an M-phase upper bridge arm 1, an M-phase lower bridge arm 2, an M-phase harmonic filtering module 3, and a controller. The m-th phase upper bridge arm 1 is connected to the m-th phase lower bridge arm 2 and the m-th phase harmonic filtering module 3, respectively. The m-th phase harmonic filtering module 3 is composed of a first inductor L1 and N full-bridge sub-modules U1 connected in series. The N full-bridge sub-modules U1 in the m-th phase harmonic filtering module 3 are used to filter out the harmonic components in their own output current and output the second DC voltage component and the fundamental frequency component of the second AC voltage component under the control of the controller. The sum of the fundamental frequency components in all the second AC voltage components is 0. Therefore, this solution extracts the second DC voltage component generated by the lower bridge arm 2 through the harmonic filtering module 3 composed of the full-bridge submodule U1, thereby realizing DC voltage conversion, replacing the function of the passive large inductor, reducing the footprint and making it suitable for applications with small footprints such as offshore wind farms and oil drilling platforms, and reducing energy loss, avoiding problems related to cooling and insulation design.

[0043] Based on the above embodiments:

[0044] Please refer to Figure 2 , Figure 2 A schematic diagram of another modular multilevel DC / DC converter provided in this application.

[0045] In a preferred embodiment, each phase upper bridge arm 1 is composed of a second inductor L2 and N full-bridge sub-modules U1 connected in series.

[0046] When the controller switches the control modes of the M-phase harmonic filtering module 3 and the M-phase upper bridge arm 1, the total high voltage side becomes the new total low voltage side, and the total low voltage side becomes the new total high voltage side.

[0047] In this embodiment, when each phase upper bridge arm 1 is composed of a second inductor L2 and N full-bridge sub-modules U1 connected in series, specifically, the M-phase upper bridge arm 1 and the M-phase harmonic filtering module 3 have a symmetrical structure. If the controller changes the control mode for the M-phase harmonic filtering module 3 and the M-phase upper bridge arm 1, the voltage generation modulation strategy will also be changed, and their functions will also be changed. The original high voltage becomes low voltage, and the original low voltage becomes high voltage.

[0048] In summary, the symmetrical structure of the M-phase harmonic filtering module 3 and the M-phase upper bridge arm 1 enables it to achieve boost function, increases the flexibility of the modular multilevel DC / DC converter, and makes it suitable for high-ratio conversion applications.

[0049] In a preferred embodiment, each phase upper bridge arm 1 is composed of a second inductor L2 and N full-bridge sub-modules U1 connected in series.

[0050] When a negative voltage is generated in each phase upper bridge arm 1, the total high voltage side becomes the new total low voltage side, and the total low voltage side becomes the new total high voltage side.

[0051] Considering that the upper arm of the M phase in the existing technology is composed of half-bridge sub-modules, and the half-bridge sub-modules do not have the ability to output negative levels, the voltage output on the total low voltage side cannot be greater than that on the total high voltage side, so the modular multilevel DC / DC converter does not have the ability to boost voltage.

[0052] Therefore, in this embodiment, the upper bridge arm 1 of the M phase is composed of a full-bridge submodule U1. The negative input state of the full-bridge submodule U1 makes its output negative level, so that the voltage output of the total low voltage side is greater than that of the total high voltage side, enabling the modular multilevel DC / DC converter to have a boost capability and be suitable for high-ratio conversion applications.

[0053] In a preferred embodiment, each phase lower bridge arm 2 is composed of a third inductor L3 and N half-bridge sub-modules U2 connected in series.

[0054] In this embodiment, the lower bridge arm 2 of each phase plays a relatively minor role. It can be composed entirely of half-bridge sub-modules or entirely of full-bridge sub-modules. However, considering that while a full-bridge sub-module can generate both positive and negative voltage levels, it requires twice the number of semiconductor devices as a half-bridge module. The additional semiconductor devices would introduce additional losses and increase costs. Therefore, when there are no further functional requirements, [the following is omitted as the original text is incomplete and requires further context]. Figure 2 For example, each phase lower bridge arm 2 can be composed of a third inductor L3 and N half-bridge sub-modules U2 connected in series.

[0055] In a preferred embodiment, the controller is also used to control the AC current in the upper arm of each phase to a minimum value and to control the AC current in the lower arm of each phase to a minimum value.

[0056] The controller can minimize the AC current in the upper arm of each phase and minimize the AC current in the lower arm of each phase, that is, minimize it as much as possible while maintaining the normal operation of the modular multilevel DC / DC converter. This can reduce the loss of the modular multilevel DC / DC converter, and at the same time reduce the current stress of the switching devices in the full-bridge submodule U1 and half-bridge submodule U2, thereby reducing costs.

[0057] In addition, such as Figure 3 As shown, Figure 2 The simplified voltage source equivalent circuit of the single-phase modular multilevel DC / DC converter is presented. The semiconductor switching devices in the full-bridge submodule U1 and half-bridge submodule U2 are ideal switching devices, and the higher harmonic components are ignored. Based on this, a mathematical model of the single-phase modular multilevel DC / DC converter is established. The positive direction of power is defined as flowing from the high-voltage side (single-phase, i.e., the total high-voltage side) to the low-voltage side (single-phase, i.e., the total low-voltage side).

[0058] The specific principle is as follows: V is defined as follows: dc1 V is the DC voltage on the high-voltage side. dc2 For the low-voltage side DC voltage, the voltage turns ratio of a modular multilevel DC / DC converter is defined as: D = V dc2 / V dc1 Transmission power is defined as: P = V dc2 I dc2 I dc2 The current is the DC current on the low-voltage side; the voltage source equivalent circuit is decomposed into DC and AC parts, and the total voltage generated by upper bridge arm 1 is expressed as: The first DC voltage component of the upper bridge arm 1, The first AC voltage component (e.g., a sine wave) of the upper bridge arm 1 is represented as: The total voltage generated by the lower bridge arm 2 is expressed as: The second DC voltage component of the lower bridge arm 2 is the DC voltage on the low-voltage side. This refers to the second AC voltage component (e.g., a sine wave) of the lower bridge arm 2. The phase angle difference between the first AC voltage component of the upper bridge arm 1 and the second AC voltage component of the lower bridge arm 2 is represented as: The total voltage of the harmonic filtering module 3 is represented as: β represents the phase angle difference between the first AC voltage component of the upper bridge arm 1 and the AC voltage component of the harmonic filtering module 3, which is the AC voltage component (e.g., a sine wave) generated by the harmonic filtering module 3.

[0059] The total current i generated by upper bridge arm 1 p The total current i generated by the lower bridge arm 2 n The total current i generated by harmonic filtering module 3 o They are represented as follows:

[0060]

[0061]

[0062]

[0063] in For the DC current generated by upper bridge arm 1, The AC current generated by upper bridge arm 1, M is the number of phases, X L2 Let X be the inductive reactance of the second inductor L2 in each phase upper bridge arm 1, and also the inductive reactance of the third inductor L3 in each phase lower bridge arm 2. L1 The inductive reactance of the first inductor L1 in each phase harmonic filtering module 3; For the DC current generated by the lower bridge arm 2, The alternating current generated by the lower bridge arm 2; The DC current generated by harmonic filtering module 3 This is the AC current generated by the harmonic filtering module 3.

[0064] Different For different transmission powers P, both upper bridge arm 1 and lower bridge arm 2 have DC voltage and DC current components. Therefore, the DC power flowing through upper bridge arm 1 and lower bridge arm 2 has the same absolute value but opposite direction, which can be expressed as:

[0065] in, For the DC power of upper bridge arm 1, This represents the DC power of the lower bridge arm 2.

[0066] AC power of upper arm 1 AC power of lower arm 2 AC power of harmonic filtering module 3 They are represented as follows:

[0067]

[0068]

[0069]

[0070] To ensure the normal operation of the modular multilevel DC / DC converter and maintain the voltage balance of the capacitors in each full-bridge submodule U1 and half-bridge submodule U2 under steady-state conditions, so that upper bridge arm 1 and lower bridge arm 2 do not consume power (i.e., non-zero DC power transmission requires non-zero AC power to maintain capacitor voltage balance), the bridge arm power of upper bridge arm 1 and lower bridge arm 2 must satisfy the following:

[0071]

[0072] The power flowing through the harmonic filtering module 3 is P / M, but since there is no DC voltage component, the harmonic filtering module 3 does not affect the AC power exchange between the upper and lower bridge arms 2, thus maintaining the balance of AC active power.

[0073] Since the upper bridge arm 1 and lower bridge arm 2 on the high-voltage side need to exchange constant power, the AC voltage should be maximized to reduce the AC current, i.e.:

[0074]

[0075]

[0076]

[0077] in, The maximum value of the first AC voltage component of the upper bridge arm 1. This represents the maximum value of the second AC voltage component of the lower bridge arm 2. This is the maximum value of the AC voltage of harmonic filtering module 3; and when the modular multilevel DC / DC converter is working normally, the full-bridge submodule U1 operates according to the properties of the half-bridge submodule U2, so it is necessary to ensure that the voltage is non-negative.

[0078] In response to Figure 2 and Figure 3 For example, in the topology of the modular multilevel DC / DC converter, the controller adopts a general control scheme, such as... Figure 4 As shown, the controller includes an output current controller and a power balance controller. The purpose of controlling the output current is to control the DC voltage or power on the low-voltage side of the DC grid, while the purpose of power balance control is to stabilize the capacitor voltages of the full-bridge submodule U1 and the half-bridge submodule U2. Reference values ​​for each voltage are obtained through PI closed-loop control, and the turn-on signals for the full-bridge submodule U1 and the half-bridge submodule U2 are obtained through triangular carrier comparison. A sorting algorithm is used to balance the capacitor voltages in the full-bridge submodule U1 and the half-bridge submodule U2, ultimately achieving precise control of the current and capacitor voltage on the low-voltage side.

[0079] Specifically, the power balance controller uses a filter and a PI controller to filter the error between the capacitor voltage and nominal value of the submodule of upper bridge arm 1 (e.g., full-bridge submodule U1) and the submodule of lower bridge arm 2 (e.g., half-bridge submodule U2) and generate a phase angle reference value Φ for the AC voltage component of lower bridge arm 2. ref The amplitude of the AC current component is adjusted accordingly to achieve balanced power control. The output current controller controls the transmitted power and the current on the total low-voltage side. When the total low-voltage side is active, the difference between the current on the total low-voltage side and the rated current (power) is used by the PI controller to generate the rated DC component values ​​of the voltages of upper bridge arm 1 and lower bridge arm 2, thereby changing the transmitted DC power. When the total low-voltage side is passive, the primary goal is to stabilize the DC voltage on the passive side, which requires inputting the difference between the voltage on the total low-voltage side and the rated voltage into the PI controller. Finally, to minimize the AC circulating current, reference values ​​for the amplitude of the AC component of the voltages of upper bridge arm 1 and lower bridge arm 2 are obtained using a formula. The complete reference values ​​for the voltages of upper bridge arm 1 and lower bridge arm 2 are then obtained, and a preliminary switching signal is generated based on the comparison with the triangular carrier wave, followed by the sorting of capacitor voltages in the submodule. The sorting algorithm selects the submodule with the smallest capacitor voltage to charge first and the submodule with the largest capacitor voltage to discharge first, based on the direction of the current, to maintain the balance of the capacitor voltage of the submodules, and generates the final turn-on signal for each submodule, which is then sent to the power device of the submodule.

[0080] Please refer to Figure 5 and Figure 6, Figure 5 This application provides a schematic diagram of a short-circuit fault structure. Figure 6 A schematic diagram of another short-circuit fault provided in this application.

[0081] In a preferred embodiment, the controller is also used to control the N full-bridge sub-modules U1 in each phase harmonic filtering module 3 to enter a locked state when a short-circuit fault occurs at the output terminal of the total low voltage side until the DC voltage at the output terminal of the total low voltage side is restored to the preset low voltage value.

[0082] In this embodiment, the configuration of each phase upper bridge arm 1 is not yet defined. When the output terminal V of the total low voltage side... dc2 When a short circuit fault occurs, you can refer to Figure 5 As shown by the dashed line, at this time, the controller controls the N full-bridge sub-modules U1 in each phase harmonic filtering module 3 to enter the lockout state. In the lockout state, the capacitors in the full-bridge sub-modules U1 are charged, which can absorb the fault current and reduce the fault current to zero, so as to support the voltage of the total low voltage side. At this time, the N full-bridge sub-modules U1 in each phase harmonic filtering module 3 act as DC circuit breakers, thereby blocking the short-circuit fault and having the DC fault ride-through capability of the total low voltage side.

[0083] In a preferred embodiment, each phase upper bridge arm 1 is composed of a second inductor L2 and N full-bridge sub-modules U1 connected in series.

[0084] The controller is also used to control N full-bridge sub-modules U1 in each phase harmonic filtering module 3 to enter the lockout state and control N full-bridge sub-modules U1 in each phase upper bridge arm 1 to enter the lockout state when a short-circuit fault occurs at the output terminal of the total low voltage side until the DC voltage at the output terminal of the total low voltage side is restored to the preset low voltage value.

[0085] In this embodiment, the configuration of each phase upper bridge arm 1 is defined, when the output terminal V of the total low voltage side... dc2 When a short circuit fault occurs, you can refer to Figure 5 As shown by the dashed line, at this time, the controller controls the N full-bridge submodules U1 in each phase harmonic filtering module 3 to enter the blocking state, and at the same time controls the N full-bridge submodules U1 in each phase upper bridge arm 1 to enter the blocking state. The fault current flows through the capacitors of the full-bridge submodules U1 in each phase upper bridge arm 1 and the full-bridge submodules U1 in each phase harmonic filtering module 3. At this time, the capacitors of each phase upper bridge arm 1 and the full-bridge submodules U1 in each phase harmonic filtering module 3 are charged, which accelerates the absorption of the fault current, thereby reducing the fault current to zero and quickly supporting the voltage of the total low voltage side. At this time, each phase upper bridge arm 1 and the full-bridge submodules U1 in each phase harmonic filtering module 3 act as DC circuit breakers, thereby blocking the short-circuit fault and having a better DC fault ride-through capability of the total low voltage side.

[0086] In existing technology, at the output terminal V on the total low-voltage side... dc2 When a short circuit fault occurs, you can refer to Figure 6 As shown by the dashed line, the controller controls N half-bridge sub-modules in each phase upper bridge arm to enter the locked state. In the locked state, the capacitors in all half-bridge sub-modules are charged to absorb fault current and have the DC fault ride-through capability of the total low voltage side. However, compared with this embodiment, the absorption of fault current is slower.

[0087] In a preferred embodiment, the controller is also used to control the N full-bridge sub-modules U1 in each phase harmonic filtering module 3 to enter a negative input state when a short-circuit fault occurs at the input terminal of the total high voltage side until the DC voltage at the input terminal of the total high voltage side is restored to the preset high voltage value.

[0088] In this embodiment, the configuration of each phase upper bridge arm 1 is not yet defined. When the input terminal V of the total high voltage side... dc1 When a short circuit fault occurs, you can refer to Figure 5 As shown by the solid line, at this time, the controller only needs to block the modulation signals of the IGBTs in all full-bridge submodules U1, so that all full-bridge submodules U1 enter the negative input state. In the negative input state, the capacitors in the full-bridge submodules U1 are charged, which can absorb the fault current and reduce the fault current to zero, thereby supporting the voltage on the total high-voltage side. At this time, the N full-bridge submodules U1 in each phase harmonic filtering module 3 act as DC circuit breakers, thereby blocking short-circuit faults and possessing DC fault ride-through capability on the total high-voltage side. Combined with the aforementioned DC fault ride-through capability on the total low-voltage side, the modular multilevel DC / DC converter ultimately has bidirectional DC fault ride-through capability.

[0089] Furthermore, since DC circuit breakers have not yet been commercially available, modular multilevel DC / DC converters with bidirectional DC fault ride-through capability are a better choice, which can simplify the DC power grid.

[0090] In existing technologies, at the input terminal V on the total high-voltage side... dc1 When a short circuit fault occurs, you can refer to Figure 6 As shown by the solid line in the diagram, the capacitor of the half-bridge submodule in each phase upper bridge arm will be short-circuited by the diode. At this time, the fault current will not flow through the capacitor that is charging, and will not be absorbed. Therefore, it cannot support the voltage of the total high voltage side and does not have the DC fault ride-through capability of the total high voltage side, thus reducing reliability.

[0091] In a preferred embodiment, each phase upper bridge arm 1 is composed of a second inductor L2 and N full-bridge sub-modules U1 connected in series. The controller is also used to control the N full-bridge sub-modules U1 in each phase harmonic filtering module 3 to enter the negative input state when a short-circuit fault occurs at the input terminal of the total high voltage side, and to control the N full-bridge sub-modules U1 in each phase upper bridge arm 1 to enter the negative input state until the DC voltage at the input terminal of the total high voltage side is restored to the preset high voltage value.

[0092] In this embodiment, the configuration of each phase upper bridge arm 1 is defined, when the input terminal V on the total high voltage side... dc1 When a short circuit fault occurs, you can refer to Figure 5 As shown by the solid line, at this time, the controller controls the N full-bridge submodules U1 in each phase harmonic filtering module 3 to enter the negative input state, and simultaneously controls the N full-bridge submodules U1 in each phase upper bridge arm 1 to enter the negative input state. The fault current flows through the capacitors of the full-bridge submodules U1 in each phase harmonic filtering module 3 and the full-bridge submodules U1 in each phase upper bridge arm 1. At this time, the capacitors of each phase upper bridge arm 1 and each phase harmonic filtering module 3 full-bridge submodule U1 are charged, accelerating the absorption of the fault current, thereby reducing the fault current to zero and quickly supporting the voltage on the total high voltage side. At this time, each phase upper bridge arm 1 and each phase harmonic filtering module 3 full-bridge submodule U1 act as a DC circuit breaker, thereby blocking the short-circuit fault and possessing better DC fault ride-through capability on the total high voltage side. Combined with the aforementioned DC fault ride-through capability on the total low voltage side, the modular multilevel DC / DC converter ultimately has bidirectional DC fault ride-through capability, resulting in higher practicality and reliability.

[0093] This application provides an application system including a modular multilevel DC / DC converter.

[0094] For a description of the modular multilevel DC / DC converter in the application system provided in this application, please refer to the above embodiments; further details will not be repeated here.

[0095] It should be noted that, in this specification, the terms "comprising," "including," or any other variations thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0096] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A modular multilevel DC / DC converter, characterized in that, The module includes an upper bridge arm of an M phase, a lower bridge arm of the M phase, a harmonic filtering module of the M phase, and a controller, the upper bridge arm of the m phase is connected with the lower bridge arm of the m phase and the harmonic filtering module of the m phase, the upper bridge arm of the m phase and the lower bridge arm of the m phase are connected in series to form a high-voltage side of the m phase of the modular multilevel DC / DC converter, the high-voltage sides of the M phases are connected in parallel to form a total high-voltage side, the lower bridge arm of the m phase and the harmonic filtering module of the m phase are connected in series to form a low-voltage side of the m phase of the modular multilevel DC / DC converter, the low-voltage sides of the M phases are connected in parallel to form a total low-voltage side, the harmonic filtering module of each phase is composed of a first inductor and N full-bridge sub-modules connected in series, M≥2, 1≤m≤M, N≥1; The upper bridge arm of the m phase is used for generating a first DC voltage component and a first AC voltage component under the control of the controller. The lower bridge arm of the m phase is used for generating a second DC voltage component and a second AC voltage component under the control of the controller. The N full-bridge sub-modules in the harmonic filtering module of the m phase are used for filtering harmonic components in the output current and outputting fundamental frequency components in the second DC voltage component and the second AC voltage component under the control of the controller, and the sum of the fundamental frequency components in all the second AC voltage components is 0. The controller is further used for controlling the N full-bridge sub-modules in the harmonic filtering module of each phase to enter a lockout state until the DC voltage of the output end of the total low-voltage side recovers to a preset low-voltage value when a short-circuit fault occurs at the output end of the total low-voltage side. The controller is further used for controlling the N full-bridge sub-modules in the harmonic filtering module of each phase to enter a negative input state until the DC voltage of the input end of the total high-voltage side recovers to a preset high-voltage value when a short-circuit fault occurs at the input end of the total high-voltage side.

2. The modular multilevel DC / DC converter of claim 1, wherein, The upper bridge arm of each phase is composed of a second inductor and N full-bridge sub-modules connected in series; When the controller exchanges the control modes of the harmonic filtering modules of the M phases and the upper bridge arms of the M phases, the total high-voltage side is a new total low-voltage side, and the total low-voltage side is a new total high-voltage side.

3. The modular multilevel DC / DC converter of claim 1, wherein, The upper bridge arm of each phase is composed of a second inductor and N full-bridge sub-modules connected in series; When the upper bridge arm of each phase generates a negative voltage, the total high-voltage side is a new total low-voltage side, and the total low-voltage side is a new total high-voltage side.

4. The modular multilevel DC / DC converter of claim 1, wherein, The lower bridge arm of each phase is composed of a third inductor and N half-bridge sub-modules connected in series.

5. The modular multilevel DC / DC converter of claim 1, wherein, The controller is further used for controlling the AC current in the upper bridge arm of each phase to be a minimum value and controlling the AC current in the lower bridge arm of each phase to be a minimum value.

6. The modular multilevel DC / DC converter of claim 1, wherein, The upper bridge arm of each phase is composed of a second inductor and N full-bridge sub-modules connected in series; The controller is further used for controlling the N full-bridge sub-modules in the upper bridge arm of each phase to enter a lockout state until the DC voltage of the output end of the total low-voltage side recovers to a preset low-voltage value when a short-circuit fault occurs at the output end of the total low-voltage side.

7. The modular multilevel DC / DC converter according to any of the claims 1 to 6, characterized in that, The upper bridge arm of each phase is composed of a second inductor and N full-bridge sub-modules connected in series, and the controller is further configured to control N full-bridge sub-modules in the upper bridge arm of each phase to enter a negative input state until the DC voltage of the input end of the total high-voltage side recovers to a preset high-voltage value when a short-circuit fault occurs at the input end of the total high-voltage side.

8. An application system characterized by comprising: A modular multilevel DC / DC converter comprising a modular multilevel DC / DC converter as claimed in any one of claims 1 to 7.

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