Modular multilevel converter and method for controlling the same

Abstract

The application relates to the technical field of flexible direct current and discloses a modular multilevel converter and a control method thereof. The modular multilevel converter comprises at least one phase unit, each phase unit comprises a main converter unit, an auxiliary energy transfer unit and a switching unit, the main converter unit comprises an upper main bridge arm and a lower main bridge arm which are connected in series between a positive pole of a DC bus and a negative pole of the DC bus, the auxiliary energy transfer unit is controllably connected with the upper main bridge arm, the lower main bridge arm, the positive pole of the DC bus and the negative pole of the DC bus through the switching unit, and the auxiliary energy transfer unit is independent of the main converter unit. The application provides a shunt path for the over-threshold current of the upper main bridge arm or the lower main bridge arm through the auxiliary energy transfer unit, avoids that the main bridge arm power semiconductor device bears an overload current, improves the overload capacity of the converter without replacing the main bridge arm device, reduces the hardware cost, and guarantees the safe and stable operation of the system.

Description

Technical Field

[0001] This invention relates to the field of flexible DC technology, specifically to a modular multilevel converter and its control method. Background Technology

[0002] Since its invention, the Modular Multilevel Converter (MMC) has eliminated the dynamic voltage equalization problem of simultaneous switching of switching devices required in two-level high-voltage converters, because its switching devices withstand small voltage variations and can be switched on and off at different times. Furthermore, the modular structure of the MMC makes it easy to extend to higher levels, thus leading to its widespread application in high-voltage fields.

[0003] With the rapid development of the new energy industry, the demand for grid-supporting capabilities in MMCs is constantly increasing. This requires converters to not only have normal power transmission capabilities but also power support capabilities, during which the MMC is under overload. However, the bridge arms of an MMC consist of cascaded sub-modules, each containing a large number of IGBT devices. IGBT devices cannot withstand a certain degree of overload capacity like primary equipment such as transformers. Current technology addresses this issue by selecting IGBTs with higher current-carrying capacity. Summary of the Invention

[0004] In view of this, the present invention provides a modular multilevel converter and its control method to solve the problem that the main arm power semiconductor devices of the existing modular multilevel converter have insufficient current carrying capacity under overload conditions, resulting in the system being unable to effectively provide grid support.

[0005] In a first aspect, the present invention provides a modular multilevel converter, comprising at least one phase unit, each phase unit comprising: a main converter unit, an auxiliary energy transfer unit, and a switching unit, wherein the main converter unit comprises an upper main bridge arm and a lower main bridge arm connected in series between the positive and negative terminals of a DC bus, the connection point of the upper and lower main bridge arms serving as an AC terminal; the auxiliary energy transfer unit is controllably connected to the upper main bridge arm, the lower main bridge arm, the positive terminal of the DC bus, and the negative terminal of the DC bus via the switching unit, and the auxiliary energy transfer unit is independent of the main converter unit; when the current of the upper or lower main bridge arm exceeds a preset threshold, the switching unit controls the auxiliary energy transfer unit to connect forward between the upper main bridge arm and the positive terminal of the DC bus, or reverse between the lower main bridge arm and the negative terminal of the DC bus, so as to provide a shunt path for the over-threshold current of the upper or lower main bridge arm.

[0006] The modular multilevel converter provided by this invention, when the current in the upper or lower main bridge arm exceeds a preset threshold, controls the auxiliary energy transfer unit to connect forward between the upper main bridge arm and the positive terminal of the DC bus, or reverse between the lower main bridge arm and the negative terminal of the DC bus. This provides a shunt path for the over-threshold current, transferring it to the DC bus and thus preventing the main bridge arm power semiconductor devices from experiencing overload current. By adding an auxiliary energy transfer unit independent of the main converter unit, the overload capacity of the converter is improved without replacing the main bridge arm devices, reducing hardware costs. Simultaneously, the auxiliary energy transfer unit can be connected forward or reverse, handling overcurrent situations in the upper and lower bridge arms separately, resulting in a compact structure. Furthermore, the switching unit enables controllable connection between the auxiliary energy transfer unit and the main circuit, activating only during overload and incurring no additional losses during normal operation. Each phase unit is independently controlled and does not affect others, allowing for simultaneous handling of multi-phase overload events, making it suitable for various complex power grid conditions.

[0007] In one optional embodiment, the main converter unit further includes a first reactor and a second reactor, wherein the first reactor and the second reactor are connected in series and then connected between the upper main bridge arm and the lower main bridge arm, and the connection point of the first reactor and the second reactor serves as an AC terminal.

[0008] In one optional implementation, the upper main bridge arm and the lower main bridge arm are each composed of multiple cascaded power submodules, and the power submodules are half-bridge submodules, full-bridge submodules, or a hybrid structure of half-bridge and full-bridge submodules.

[0009] In one optional embodiment, the auxiliary energy transfer unit includes: multiple full-bridge sub-modules and a transfer reactor, wherein the first end of the multiple full-bridge sub-modules connected in series is controllably connected to the positive and negative terminals of the DC bus via a switching unit, and the second end of the multiple full-bridge sub-modules connected in series is connected to the first end of the transfer reactor; the second end of the transfer reactor is controllably connected to the upper main bridge arm and the lower main bridge arm via a switching unit.

[0010] In one optional embodiment, the switching unit includes a first switch, a second switch, a third switch, and a fourth switch, wherein the first switch is connected between the auxiliary energy transfer unit and the positive terminal of the DC bus; the second switch is connected between the auxiliary energy transfer unit and the negative terminal of the DC bus; the third switch is connected between the auxiliary energy transfer unit and the upper main bridge arm; and the fourth switch is connected between the auxiliary energy transfer unit and the lower main bridge arm. When the current of the upper main bridge arm exceeds a preset threshold, the first and third switches are turned on, and the second and fourth switches are turned off. When the current of the lower main bridge arm exceeds the preset threshold, the second and fourth switches are turned on, and the first and third switches are turned off.

[0011] Secondly, the present invention provides a control method for a modular multilevel converter, applied to a modular multilevel converter according to the first aspect above or any corresponding embodiment thereof. The method includes: real-time detection of the current in the upper main bridge arm and the lower main bridge arm of each phase unit; when the current of the upper main bridge arm of any phase unit exceeds a first preset threshold, controlling an auxiliary energy transfer unit to be connected in the forward direction between the upper main bridge arm and the positive terminal of the DC bus, and controlling the auxiliary energy transfer unit to output a positive voltage to divert the over-threshold current of the upper main bridge arm to the positive terminal of the DC bus; when the current of the lower main bridge arm of any phase unit exceeds a second preset threshold, controlling an auxiliary energy transfer unit to be connected in the reverse direction between the lower main bridge arm and the negative terminal of the DC bus, and controlling the auxiliary energy transfer unit to output a negative voltage to divert the over-threshold current of the lower main bridge arm to the negative terminal of the DC bus.

[0012] The control method for a modular multilevel converter provided by this invention detects the current in the upper and lower main bridge arms of each phase unit in real time. When the current exceeds a corresponding preset threshold, it controls the auxiliary energy transfer unit to connect in the forward or reverse direction, and simultaneously outputs a voltage of the corresponding polarity. This precisely diverts the over-threshold current to the positive or negative terminal of the DC bus, effectively limiting the main bridge arm current and protecting the power semiconductor devices from overload impact. This method offers rapid response and precise control, automatically adjusting the current diversion path according to the overcurrent direction. It improves the converter's overload capacity without replacing the main bridge arm devices, reducing system cost and modification difficulty. Furthermore, each phase unit is controlled independently and in parallel, without affecting each other, allowing for simultaneous handling of multi-phase overload events. This adapts to complex and changing grid conditions, ensuring the safe and stable operation of the flexible DC transmission system.

[0013] In one optional implementation, the process of controlling the output positive or negative voltage of the auxiliary energy transfer unit includes: real-time acquisition of the current of the overcurrent bridge arm and comparison with the corresponding preset threshold; and dynamic adjustment of the output voltage amplitude of the auxiliary energy transfer unit according to the comparison result, so that the current of the overcurrent bridge arm is limited to below the corresponding preset threshold.

[0014] In one optional implementation, the first preset threshold is a preset multiple of the peak rated current of the upper main bridge arm; when the specifications and heat dissipation conditions of the upper main bridge arm and the lower main bridge arm are the same, the second preset threshold is equal to the first preset threshold; when the specifications and heat dissipation conditions of the upper main bridge arm and the lower main bridge arm are different, the second preset threshold is determined according to the short-time safe current carrying capacity of the lower main bridge arm.

[0015] In one optional implementation, the method further includes: when the current of the overcurrent bridge arm drops to or below the corresponding preset threshold, controlling the output voltage of the auxiliary energy transfer unit to gradually decrease to zero, and then turning off the switching unit to allow the auxiliary energy transfer unit to exit operation.

[0016] In one optional implementation, before the output voltage of the auxiliary energy transfer unit is gradually reduced to zero, the method further includes: starting a delay timer after the current of the overcurrent bridge arm falls back to equal to or below the corresponding preset threshold, and then performing the output voltage reduction operation after the delay ends. Attached Figure Description

[0017] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0018] Figure 1 This is a first configuration diagram of a modular multilevel converter according to an embodiment of the present invention; Figure 2 (a) to (c) are specific circuit diagrams of the main bridge arm according to an embodiment of the present invention; Figure 3 This is a specific circuit diagram of a modular multilevel converter according to an embodiment of the present invention; Figure 4 This is a specific circuit diagram of the auxiliary energy transfer unit according to an embodiment of the present invention; Figure 5 (a) to (c) are specific circuit diagrams of each switch in the switching unit according to an embodiment of the present invention; Figure 6 This is a flowchart illustrating the control method of a modular multilevel converter according to an embodiment of the present invention; Figure 7 This is a flowchart illustrating the closed-loop regulation of the overcurrent bridge arm current according to an embodiment of the present invention. Figure 8 This is a detailed flowchart of the control method for a modular multilevel converter according to an embodiment of the present invention; Figure 9 This is a waveform diagram of the three-phase bridge arm current during the energy transfer process according to an embodiment of the present invention; Figure 10 This is an actual current diagram of the energy transfer bridge arm according to an embodiment of the present invention; Figure 11 This is an operational state diagram of the switch in the corresponding switch unit according to an embodiment of the present invention. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] It is understood that before using the technical solutions disclosed in the various embodiments of the present invention, users should be informed of the types, scope of use, and usage scenarios of the personal information involved in the present invention and their authorization should be obtained in accordance with relevant laws and regulations through appropriate means.

[0021] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0022] This embodiment provides a modular multilevel converter, such as... Figure 1 As shown, the system includes at least one phase unit (A~N). Each phase unit includes a main converter unit 1, an auxiliary energy transfer unit 2, and a switching unit 3. The main converter unit 1 includes an upper main bridge arm 11 and a lower main bridge arm 12 connected in series between the positive terminal Up+ and the negative terminal Up- of the DC bus. The connection point between the upper main bridge arm 11 and the lower main bridge arm 12 serves as the AC terminal (Ua~Un). The auxiliary energy transfer unit 2 is controllably connected to the upper main bridge arm 11, the lower main bridge arm 12, the positive terminal Up+ of the DC bus, and the negative terminal Up- of the DC bus via the switching unit 3. The auxiliary energy transfer unit 2 is independent of the main converter unit 1. The main converter unit 1 also includes a first reactor L1 and a second reactor L2. The first reactor L1 and the second reactor L2 are connected in series between the upper main bridge arm 11 and the lower main bridge arm 12. The connection point between the first reactor L1 and the second reactor L2 serves as the AC terminal (Ua~Un).

[0023] Figure 1 In the process, when the current of the upper main bridge arm 11 or the lower main bridge arm 12 exceeds the preset threshold, the switching unit 3 is used to control the auxiliary energy transfer unit 2 to be connected in the forward direction between the upper main bridge arm 11 and the positive terminal Up+ of the DC bus, or in the reverse direction between the lower main bridge arm 12 and the negative terminal Up- of the DC bus, so as to provide a shunt path for the over-threshold current of the upper main bridge arm 11 or the lower main bridge arm 12.

[0024] Specifically, Figure 1In normal operation, the switching unit 3 remains open, the auxiliary energy transfer unit 2 is isolated from the main circuit, and the current flows only through the upper main bridge arm 11 and the lower main bridge arm 12 of the main converter unit 1. The modular multilevel converter operates in conventional MMC mode.

[0025] Specifically, Figure 1 In the process, when the controller detects that the current of the upper main bridge arm 11 of any phase unit exceeds the first preset threshold, the modular multilevel converter enters the overload shunt mode: the switching unit 3 controls the auxiliary energy transfer unit 2 to connect in the positive direction between the upper main bridge arm 11 and the DC bus positive terminal Up+, and at the same time, the auxiliary energy transfer unit 2 outputs a positive voltage, forming a shunt path from the DC bus positive terminal Up+ through the auxiliary energy transfer unit 2 to the upper main bridge arm 11, transferring the over-threshold current of the upper main bridge arm 11 to the DC bus positive terminal Up+.

[0026] Specifically, Figure 1 In the process, when the controller detects that the current of the lower main bridge arm 12 of any phase unit exceeds the second preset threshold, the modular multilevel converter enters the overload shunt mode: the switching unit 3 controls the auxiliary energy transfer unit 2 to be connected in reverse between the lower main bridge arm 12 and the DC bus negative terminal Up-, and at the same time, the auxiliary energy transfer unit 2 outputs a negative voltage, forming a shunt path from the lower main bridge arm 12 through the auxiliary energy transfer unit 2 to the DC bus negative terminal Up-, thus transferring the over-threshold current of the lower main bridge arm 12 to the DC bus negative terminal Up-.

[0027] Specifically, Figure 1 During the current shunting process, the controller adjusts the output voltage amplitude of the auxiliary energy transfer unit 2 through closed-loop regulation, so that the current of the overcurrent bridge arm is precisely limited below the corresponding preset threshold. The control of each phase unit is executed independently and in parallel without affecting each other, and the modular multilevel converter can handle multi-phase overload events simultaneously.

[0028] The modular multilevel converter provided in this embodiment, when the current in the upper or lower main bridge arm exceeds a preset threshold, controls the auxiliary energy transfer unit to connect forward between the upper main bridge arm and the positive terminal of the DC bus, or reverse between the lower main bridge arm and the negative terminal of the DC bus. This provides a shunt path for the over-threshold current, transferring it to the DC bus and thus preventing the main bridge arm power semiconductor devices from experiencing overload current. By adding an auxiliary energy transfer unit independent of the main converter unit, the converter's overload capacity is improved without replacing the main bridge arm devices, reducing hardware costs. Simultaneously, the auxiliary energy transfer unit can be connected forward or reverse, handling overcurrent situations in the upper and lower bridge arms separately, resulting in a compact structure and high device utilization. Furthermore, the switching unit achieves controllable connection between the auxiliary energy transfer unit and the main circuit, activating only during overload and incurring no additional losses during normal operation. Each phase unit is independently controlled and does not affect others, allowing for simultaneous handling of multi-phase overload events, making it suitable for various complex power grid conditions.

[0029] In some alternative implementations, such as Figure 2 (a) ~ Figure 2 As shown in (c), the upper main bridge arm and the lower main bridge arm are each composed of multiple cascaded power sub-modules. The power sub-modules are half-bridge sub-modules, full-bridge sub-modules, or a hybrid structure of half-bridge and full-bridge sub-modules.

[0030] Specifically, such as Figure 2 As shown in (a), the half-bridge submodule includes two IGBT devices and their anti-parallel diodes, as well as a DC energy storage capacitor. The collector of the first IGBT is connected to the positive terminal of the capacitor, and the emitter of the first IGBT is connected to the collector of the second IGBT, serving as one output terminal of the half-bridge submodule. The emitter of the second IGBT is connected to the negative terminal of the capacitor, serving as the other output terminal of the half-bridge submodule. By controlling the on and off states of the two IGBTs, the half-bridge submodule can output either 0 or a positive capacitor voltage.

[0031] like Figure 2 As shown in (b), the full-bridge submodule includes four IGBT devices and their anti-parallel diodes, as well as a DC energy storage capacitor. Two IGBTs are connected in series to form one bridge arm, and the other two IGBTs are connected in series to form another bridge arm. The two bridge arms are connected in parallel across the capacitor. The two output terminals of the full-bridge submodule are connected to the midpoints of the first and second bridge arms, respectively. By controlling the on and off states of the four IGBTs, the full-bridge submodule can output three voltage levels: positive capacitor voltage, 0, and negative capacitor voltage.

[0032] like Figure 2As shown in (c), the main bridge arm can simultaneously contain both half-bridge and full-bridge submodules, which are cascaded in a certain ratio. For example, M half-bridge submodules and N full-bridge submodules can be connected in series in the bridge arm, where M and N are both positive integers, and M+N equals the total number of submodules in that bridge arm. The DC capacitor voltages of the two types of submodules can be designed to be the same or different. For example, the capacitor voltage of the full-bridge submodule can be set to half that of the half-bridge submodule to obtain a finer level output and improve the output voltage waveform quality.

[0033] Optionally, the upper or lower main bridge arm can independently select the submodule type and mixing ratio, which can be flexibly configured according to specific application scenarios to meet the needs of different voltage levels, power capacity and output waveform quality.

[0034] In some alternative implementations, such as Figure 3 As shown, Figure 3 It includes three phase units: phase A, phase B, and phase C. The three phases have the same structure. The following detailed explanation will take phase A as an example. The structure and control logic of phase B and phase C can be referred to the description of phase A, and will not be repeated here.

[0035] Figure 3 In this structure, the A-phase unit includes an upper main bridge arm A11, a lower main bridge arm A12, reactors LA1 and LA2, an A-phase auxiliary energy transfer unit ARM_A, a first switch DS_A1, a second switch DS_A2, a third switch DS_A3, and a fourth switch DS_A4. Specifically, the first switch DS_A1 is connected between ARM_A and the positive terminal Up+ of the DC bus; the second switch DS_A2 is connected between ARM_A and the negative terminal Up- of the DC bus; the third switch DS_A3 is connected between ARM_A and the upper main bridge arm A11; and the fourth switch DS_A4 is connected between ARM_A and the lower main bridge arm A12.

[0036] Specifically, such as Figure 4 As shown, taking phase A as an example, the phase A auxiliary energy transfer unit ARM_A includes: multiple full-bridge sub-modules and a transfer reactor. The first end of the multiple full-bridge sub-modules connected in series is controllably connected to the positive and negative terminals of the DC bus through a switching unit. The second end of the multiple full-bridge sub-modules connected in series is connected to the first end of the transfer reactor. The second end of the transfer reactor is controllably connected to the upper main bridge arm and the lower main bridge arm through a switching unit.

[0037] Specifically, in combination Figure 3 and Figure 4Multiple full-bridge submodules are connected in series, forming a series chain. One end of the chain is connected to the positive terminal Up+ of the DC bus via a first switch DS_A1, and to the negative terminal Up- of the DC bus via a second switch DS_A2. The other end of the chain is connected to one end of a transfer reactor. The other end of the transfer reactor is connected to the upper main bridge arm A11 via a third switch DS_A3, and to the lower main bridge arm A12 via a fourth switch DS_A4. The transfer reactor is used to limit the rate of change of the shunt current.

[0038] Specifically, Figure 3 In the process, the controller monitors the current of the upper main bridge arm A11 and the lower main bridge arm A12 in real time. When the controller detects that the current of the upper main bridge arm A11 exceeds the first preset threshold I_th1, the controller controls the first switch DS_A1 and the third switch DS_A3 to be turned on simultaneously, while controlling the second switch DS_A2 and the fourth switch DS_A4 to remain off. At this time, the first terminal of the auxiliary energy transfer unit ARM_A is connected to the upper main bridge arm A11 through the third switch DS_A3, and the second terminal of the auxiliary energy transfer unit ARM_A is connected to the positive terminal Up+ of the DC bus through the first switch DS_A1. The controller also controls the auxiliary energy transfer unit ARM_A to output a positive voltage, i.e. Figure 4 The voltage at the second terminal is positive relative to the first terminal. This forms a shunt path from the positive terminal Up+ of the DC bus through the first switch DS_A1, the auxiliary energy transfer unit ARM_A, and the third switch DS_A3 to the upper main bridge arm A11. The over-threshold current component in the upper main bridge arm A11 is transferred to the positive terminal Up+ of the DC bus through this shunt path, thereby limiting the actual current of the upper main bridge arm A11 to below the first preset threshold I_th1.

[0039] Specifically, Figure 3 and Figure 4 In the process, when the controller detects that the current in the lower main bridge arm A12 exceeds the second preset threshold I_th2, the controller controls the second switch DS_A2 and the fourth switch DS_A4 to be turned on simultaneously, while controlling the first switch DS_A1 and the third switch DS_A3 to remain off. At this time, the first terminal of the auxiliary energy transfer unit ARM_A is connected to the lower main bridge arm A12 through the fourth switch DS_A4, and the third terminal of the auxiliary energy transfer unit ARM_A is connected to the negative terminal Up- of the DC bus through the second switch DS_A2. The controller also controls the auxiliary energy transfer unit ARM_A to output a negative voltage, i.e. Figure 4The voltage at the first terminal is positive relative to the second and third terminals (or the voltage at the second and third terminals is negative relative to the first terminal). This forms a shunt path from the lower main bridge arm A12 through the fourth switch DS_A4, the auxiliary energy transfer unit ARM_A, the second switch DS_A2, to the negative terminal Up- of the DC bus. The over-threshold current component in the lower main bridge arm A12 is transferred to the negative terminal Up- of the DC bus through this shunt path, thereby limiting the actual current of the lower main bridge arm A12 to below the second preset threshold I_th2.

[0040] Specifically, Figure 3 and Figure 4 During the current shunting process, the controller continuously monitors the current of the overcurrent arm and compares it with a corresponding preset threshold. When the current of the overcurrent arm exceeds the preset threshold, the controller increases the output voltage amplitude of the auxiliary energy transfer unit ARM_A, thereby increasing the shunting current. Conversely, when the current of the overcurrent arm falls below the preset threshold, the controller decreases the output voltage amplitude of the auxiliary energy transfer unit ARM_A, thereby decreasing the shunting current. Through this continuous closed-loop regulation, the controller can precisely limit the current of the overcurrent arm below the preset threshold, achieving accurate current limiting.

[0041] Specifically, Figure 3 In this circuit, when the current in the overcurrent bridge arm drops to or below the corresponding preset threshold, the controller does not immediately turn off the switch. Instead, it initiates a delay exit time T_delay. This delay exit time can be set according to actual needs, for example, from 10ms to 100ms. The purpose of setting the delay exit time is to prevent the switch from repeatedly turning on and off when the current fluctuates frequently around the preset threshold, thereby avoiding overheating or damage to the switching devices. During the delay exit time, if the current in the overcurrent bridge arm exceeds the preset threshold again, the controller continues to maintain the shunt state and resets the delay timer. If the current in the overcurrent bridge arm remains below the preset threshold during the delay exit time, after the delay ends, the controller first gradually reduces the output voltage of the auxiliary energy transfer unit ARM_A to zero, so that the shunt current gradually decreases to zero. Then, it simultaneously turns off the first switch DS_A1, the second switch DS_A2, the third switch DS_A3, and the fourth switch DS_A4, so that the auxiliary energy transfer unit ARM_A completely exits operation, and the modular multilevel converter returns to normal operation.

[0042] It should be noted that the structure and control logic of phase B and phase C units are exactly the same as those of phase A unit. The control of each phase unit is executed independently and in parallel, without interference. When the upper main bridge arm of phase A experiences overcurrent, the lower main bridge arm of phase B may also experience overcurrent simultaneously. The controller will simultaneously activate the energy transfer branches of both phases to shunt the current to their respective overcurrent arms. This independent parallel control method enables the modular multilevel converter to flexibly handle various complex overload conditions.

[0043] Specifically, taking the first switch DS_A1 as an example, Figure 5 (a) ~ Figure 5 (c) is a detailed structural diagram of the first switch DS_A1: like Figure 5 As shown in (a), the first switch DS_A1 can adopt an IGBT series structure. This structure consists of multiple IGBT devices and their anti-parallel diodes connected in series. A diode is connected in reverse parallel between the collector and emitter of each IGBT to provide a reverse current path. The number of IGBTs N connected in series is determined according to the system voltage withstand requirements, and the specific calculation formula is as follows: N = ceil(1.1 × V_req / V_rated) Where V_req is the maximum voltage that the switch must withstand, V_rated is the rated voltage of a single IGBT device, and ceil indicates rounding up. With this structure, the gate drive circuit simultaneously sends turn-on or turn-off signals to all series-connected IGBTs, achieving synchronous operation of all IGBTs. To ensure dynamic voltage equalization among the IGBT devices during series operation, an RC snubber circuit can be connected in parallel across each IGBT, or an active voltage equalization control circuit can be used. Furthermore, the IGBT devices can be replaced with other fully controllable power semiconductor devices such as IGCTs.

[0044] like Figure 5 As shown in (b), the first switch DS_A1 can adopt a hybrid series structure of thyristor and IGBT. In this structure, the main current-carrying device is a thyristor, taking advantage of its high surge current withstand capability and low on-state voltage drop. A diode is connected in reverse parallel across each thyristor, and a low-voltage IGBT is connected in series as an auxiliary turn-off switch. When conducting, the IGBT is triggered first to establish current in the circuit, and then the thyristor is triggered to conduct. After the thyristor conducts, the voltage across the IGBT is clamped, and the IGBT can exit the conducting state. When turning off, the IGBT is turned off first, forcing the main current to transfer to the commutation circuit connected in parallel with the thyristor, so that the current flowing through the thyristor drops rapidly to zero, and the thyristor turns off naturally. This structure takes into account the low cost, high current-carrying capacity, and controllable turn-off capability of the thyristor.

[0045] like Figure 5 As shown in (c), the first switch DS_A1 can be a thyristor series structure or a mechanical circuit breaker. When using a thyristor series structure, each thyristor needs to be connected to a diode in reverse parallel, and the number of diodes in series is determined according to the voltage withstand requirements, with the calculation formula being the same as that for the IGBT series structure. When using a mechanical circuit breaker, the circuit is switched on and off by the opening and closing of mechanical contacts. This method has a simple structure and lower cost, but the switching speed is slower, making it suitable for operating conditions with long overload durations and low switching frequency.

[0046] It should be noted that in practical engineering applications, the choice can be made flexibly based on factors such as system voltage level, cost budget, and response speed requirements. For example, for applications requiring fast response, an IGBT series structure or a hybrid series structure of thyristors and IGBTs can be selected; for cost-sensitive applications with low switching frequency, a thyristor series structure or a mechanical circuit breaker can be selected.

[0047] This embodiment provides a control method for a modular multilevel converter, applicable to the modular multilevel converter described in the above embodiment or any corresponding implementation, such as... Figure 6 As shown, the method includes: Step S1: Real-time detection of the current in the upper and lower main bridge arms of each phase unit.

[0048] Specifically, refer to Figure 3 The controller collects the current signals of the upper and lower main bridge arms in each phase unit in real time at a fixed sampling frequency. Taking phase A as an example, the controller collects the instantaneous current i_A1 of the upper main bridge arm A11 and the instantaneous current i_A2 of the lower main bridge arm A12 in real time. The current direction is defined as follows: the current direction of the upper main bridge arm A11 is positive as it flows from the positive terminal of the DC bus to the AC side, and the current direction of the lower main bridge arm A12 is positive as it flows from the AC side to the negative terminal of the DC bus. Simultaneously, the controller also collects the current of each auxiliary energy transfer unit in real time for subsequent closed-loop regulation and control. The current detection methods for phases B and C are exactly the same as those for phase A, and will not be described further here.

[0049] Step S2: When the current of the upper main bridge arm of any phase unit exceeds the first preset threshold, control the auxiliary energy transfer unit to connect in the positive direction between the upper main bridge arm and the positive terminal of the DC bus, and control the auxiliary energy transfer unit to output a positive voltage so as to divert the over-threshold current of the upper main bridge arm to the positive terminal of the DC bus.

[0050] Step S3: When the current of the lower main bridge arm of any phase unit exceeds the second preset threshold, control the auxiliary energy transfer unit to connect in reverse between the lower main bridge arm and the negative terminal of the DC bus, and control the auxiliary energy transfer unit to output a negative voltage so as to divert the over-threshold current of the lower main bridge arm to the negative terminal of the DC bus.

[0051] Before the system is put into operation, two key current thresholds need to be set based on the short-time safe current carrying capacity of the power semiconductor devices (usually IGBTs) used in the main bridge arm: the first preset current threshold I_th1 is applied to the upper main bridge arm, and the second preset current threshold I_th2 is applied to the lower main bridge arm. The first preset threshold I_th1 is a preset multiple of the rated peak current of the upper main bridge arm, for example, 1.0 to 1.5 times the rated peak current, depending on the short-time overcurrent capacity allowed by the device. For example, if the rated peak current of the IGBT in the upper main bridge arm is 1500A and the maximum short-time allowable current is 1800A, then I_th1 can be set to 1600A, leaving a certain safety margin. When the specifications and heat dissipation conditions of the upper and lower main bridge arms are the same, the second preset threshold I_th2 is equal to the first preset threshold I_th1. When the specifications and heat dissipation conditions of the upper and lower main bridge arms are different, the second preset threshold I_th2 is determined according to the short-time safe current carrying capacity of the lower main bridge arm.

[0052] The process of controlling the output positive or negative voltage of the auxiliary energy transfer unit includes: real-time acquisition of the current of the overcurrent bridge arm and comparison with the corresponding preset threshold; and dynamic adjustment of the output voltage amplitude of the auxiliary energy transfer unit based on the comparison result, so that the current of the overcurrent bridge arm is limited to below the corresponding preset threshold.

[0053] Specifically, Figure 7 In this process, precise limitation of the overcurrent bridge arm current is achieved through closed-loop regulation, specifically including the following steps: (1) The controller collects the current of the overcurrent bridge arm in real time and compares the collected current value with the corresponding preset threshold to calculate the difference e(t). Taking the overcurrent of the upper bridge arm as an example, when the current i_A1 of the upper main bridge arm is greater than the first preset threshold I_th1, the difference e(t) = i_A1 - I_th1; when the current of the lower bridge arm is overcurrent, the difference e(t) = i_A2 - I_th2.

[0054] (2) The controller inputs the difference e(t) into the PI controller. The PI controller dynamically calculates the voltage reference value V_ref(t) based on the magnitude and trend of the difference. When the difference e(t) is positive and large, it indicates that the current of the overcurrent bridge arm exceeds the threshold by a large amount. The PI controller outputs a larger voltage reference value, increases the output voltage amplitude of the auxiliary energy transfer unit, thereby increasing the shunt current and causing the current of the overcurrent bridge arm to drop rapidly. When the difference e(t) gradually decreases, the PI controller correspondingly reduces the voltage reference value, reduces the output voltage amplitude of the auxiliary energy transfer unit, reduces the shunt current, and prevents the current of the overcurrent bridge arm from dropping excessively.

[0055] Through the continuous closed-loop control of real-time sampling, comparison, calculation, and adjustment, the output voltage amplitude of the auxiliary energy transfer unit is dynamically adjusted, so that the current of the overcurrent bridge arm is precisely limited to an allowable fluctuation range near the corresponding preset threshold, i.e., I_th±ΔI, thus achieving precise current limiting.

[0056] It should be noted that the control logic for overcurrent in the lower bridge arm is the same as that in the upper bridge arm. The only difference is that the polarity of the output voltage of the auxiliary energy transfer unit is set to negative, and the closed-loop regulation principle is exactly the same.

[0057] Specifically, Figure 3 Taking the upper main bridge arm A11 of phase A as an example, the controller continuously compares the current i_A1 of the upper main bridge arm A11 with the first preset threshold I_th1. When i_A1 is detected to be greater than I_th1, it is determined that the upper main bridge arm A11 is in an overload state and energy transfer needs to be initiated. The controller sends a turn-on pulse, simultaneously turning on the first switch DS_A1 and the third switch DS_A3, while keeping the second switch DS_A2 and the fourth switch DS_A4 off. DS_A1 connects the second terminal of the auxiliary energy transfer unit ARM_A to the positive terminal Up+ of the DC bus, and DS_A3 connects the first terminal of the auxiliary energy transfer unit ARM_A to the node between the upper main bridge arm A11 and the reactor LA1. DS_A1 and DS_A3 must be turned on simultaneously to ensure a complete shunt path; if only one is turned on, a current loop cannot be formed.

[0058] Synchronized with the switch's activation, the controller sends a modulation signal to the cascaded full-bridge submodule within the auxiliary energy transfer unit ARM_A, causing it to output a positive voltage, meaning the voltage at its second terminal (connected to the Up+ side) is positive relative to its first terminal (connected to the upper main bridge arm A11 side). The magnitude of this voltage is dynamically adjusted through closed-loop control, with the goal of clamping the actual current of the upper main bridge arm A11 near I_th1.

[0059] Under the positive voltage output of ARM_A, the total current i_A1 of the upper main bridge arm A11 is divided into two parts: one part continues to flow through the IGBT device inside the upper main bridge arm A11, and its value is controlled below I_th1; the other part is the over-threshold current component, which flows into the auxiliary energy transfer unit ARM_A through DS_A3, and then flows to the positive terminal Up+ of the DC bus through DS_A1, thereby diverting the over-threshold current to the positive terminal of the DC bus.

[0060] Specifically, Figure 3Taking the lower main bridge arm A12 of phase A as an example, the controller continuously compares the current i_A2 of the lower main bridge arm A12 with the second preset threshold I_th2. When i_A2 is detected to be greater than I_th2, it is determined that the lower main bridge arm A12 is in an overload state. The controller sends a turn-on pulse, simultaneously turning on the second switch DS_A2 and the fourth switch DS_A4, while keeping the first switch DS_A1 and the third switch DS_A3 off. DS_A2 connects the third terminal of the auxiliary energy transfer unit ARM_A to the negative terminal Up- of the DC bus, and DS_A4 connects the first terminal of the auxiliary energy transfer unit ARM_A to the node between the lower main bridge arm A12 and the reactor LA2.

[0061] Synchronized with the switch's activation, the controller controls the auxiliary energy transfer unit ARM_A to output a negative voltage, even if the voltage at its first terminal is positive relative to the second and third terminals (or the voltage at the second and third terminals is negative relative to the first terminal). The magnitude of this voltage is also dynamically adjusted through closed-loop control, with the goal of clamping the actual current of the lower main bridge arm A12 near I_th2.

[0062] Under the influence of the negative voltage output by ARM_A, the over-threshold current component of the lower main bridge arm A12 flows into the auxiliary energy transfer unit ARM_A through DS_A4, and then flows to the negative terminal Up- of the DC bus through DS_A2, thereby diverting the over-threshold current to the negative terminal of the DC bus.

[0063] Specifically, when the current in the overcurrent bridge arm drops to or below the corresponding preset threshold, the output voltage of the auxiliary energy transfer unit is gradually reduced to zero, and then the switching unit is turned off to deactivate the auxiliary energy transfer unit. Before the output voltage of the auxiliary energy transfer unit gradually decreases to zero, the process also includes: starting a delay timer after the current in the overcurrent bridge arm drops to or below the corresponding preset threshold, and then performing the output voltage reduction operation after the delay ends.

[0064] Specifically, refer to Figure 8 The purpose of setting this delay is to prevent the switching unit from repeatedly turning on and off when the current fluctuates frequently around the preset threshold, thereby avoiding overheating or damage to the switching device due to frequent operation. The delay time can be set according to the actual operating conditions, for example, from 10ms to 100ms.

[0065] During the delay period, the controller continues to monitor the current of the overcurrent bridge arm in real time. If the current of the overcurrent bridge arm exceeds the corresponding preset threshold again during the delay, the delay timer is immediately reset to zero, and the current shunt state is maintained. The timing restarts only when the current of the overcurrent bridge arm remains below the corresponding preset threshold at the end of the delay and never exceeds the threshold again, the overload condition is confirmed to be truly resolved, and then the subsequent exit operation is initiated.

[0066] After the delay ends, the controller first gradually reduces the output voltage of the auxiliary energy transfer unit to zero, causing the shunt current to gradually decrease. When the shunt current approaches zero, all switches in the switching unit are turned off, completely disengaging the auxiliary energy transfer unit and restoring the modular multilevel converter to normal operation.

[0067] It should be noted that the control logic of the upper and lower main bridge arms of phases B and C is exactly the same as that of phase A, controlling their respective switches and auxiliary energy transfer units, which will not be elaborated here.

[0068] For example, to verify the effectiveness of the modular multilevel converter with overload energy transfer branch and its control strategy proposed in this invention, this embodiment builds a simulation platform based on PSCAD / EMTDC as follows: Figure 3 The system model shown is illustrated, and the simulation parameters are shown in Table 1.

[0069] Table 1 Simulation parameters of the modular multilevel converter

[0070] In the simulation, the system is set to activate the overload energy transfer branch at 2.5 seconds to transfer the bridge arm current. The controller automatically activates the energy transfer branch according to the aforementioned control logic. The preset current threshold is set to 2.15kA.

[0071] Figure 9 The waveforms of the three-phase bridge arm currents during the energy transfer process are displayed. Figure 9 In the diagram, Ia1n is the upper arm current of phase A, Ia2 is the lower arm current of phase A, Ib1 is the upper arm current of phase B, Ib2 is the lower arm current of phase B, Ic1 is the upper arm current of phase C, and Ic2 is the lower arm current of phase C. Before 2.5 seconds, the system is in rated operating condition, and the currents of each arm exhibit a standard sinusoidal half-wave shape (MMC arm currents include both DC and AC components). At this time, all energy transfer switches are off, the output voltage of the energy transfer arms is zero, and the converter operates in conventional MMC mode.

[0072] At 2.5 seconds, the controller initiates detection and finds that multiple bridge arm current instantaneous values ​​exceed the threshold of 2.15kA. For example, the peak value of the bridge arm current i_A1 on phase A exceeds the corresponding current threshold of 2.15kA in the first cycle after 2.5 seconds. The controller responds immediately, activating the corresponding energy transfer switch and adjusting the output voltage of the energy transfer bridge arm according to the aforementioned control strategy. Figure 9It can be clearly seen that within half a cycle after approximately 2.5 seconds, the peak value of all bridge arm currents is rapidly clamped below 2.15kA. Taking the upper bridge arm of phase A as an example, its peak value is limited to 2.15kA, and the portion exceeding 2.15kA is flattened, exhibiting a waveform characteristic with a flat top. The current waveform after limitation still maintains a high sinusoidal degree, indicating that the current shunting effect of the energy transfer branch is smooth and stable, without introducing significant harmonic distortion. The bridge arm currents of phases B and C are also limited to within 2.15kA, proving that the three-phase control logic is independent and effective, and can simultaneously handle multi-phase overload events.

[0073] Figure 10 The actual current waveform of the A-phase auxiliary energy transfer unit ARM_A is shown. IrefA is the reference current generated internally by the controller, its value equal to the portion of the actual current in the upper main bridge arm A11 exceeding the threshold I_th, i.e., IrefA≈max(i_A1-2.15kA,0). I7 is the actual current flowing through the auxiliary energy transfer unit ARM_A, measured by a current sensor. Before 2.5s, no current flows through the auxiliary energy transfer unit. At 2.5s, i_A1 exceeds 2.15kA, and I_refA and I7 are basically consistent, indicating that the reference current generated by the controller matches the actual shunt current well, and the closed-loop control effect is ideal. Figure 11 The operation status of the corresponding energy transfer switch is shown, and the switching timing is completely consistent with the control logic.

[0074] The control method for the modular multilevel converter provided in this embodiment eliminates the need for the auxiliary energy transfer unit to withstand rated voltage. It only requires a transfer reactor to control current and voltage, reducing device selection costs and engineering implementation difficulty. By adjusting the output voltage amplitude of the auxiliary energy transfer unit in a closed loop, the current in the overcurrent bridge arm is precisely limited below a preset threshold, resulting in a smooth and stable current shunting process without introducing significant harmonic distortion. Setting a delayed exit time prevents repeated switching actions caused by frequent current fluctuations near the threshold, improving system stability and reliability. The control of each phase unit is executed independently and in parallel, without affecting each other, allowing for simultaneous handling of multi-phase overload events. This enables flexible responses to various complex grid conditions, making it particularly suitable for flexible DC transmission scenarios requiring grid support capabilities.

[0075] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A modular multilevel converter, characterized in that, It includes at least one phase unit, and each phase unit includes: a main converter unit, an auxiliary energy transfer unit, and a switching unit, wherein, The main converter unit includes an upper main bridge arm and a lower main bridge arm connected in series between the positive and negative terminals of the DC bus, and the connection point between the upper main bridge arm and the lower main bridge arm serves as an AC terminal. The auxiliary energy transfer unit is controllably connected to the upper main bridge arm, the lower main bridge arm, the positive terminal of the DC bus, and the negative terminal of the DC bus via the switching unit. The auxiliary energy transfer unit is independent of the main converter unit. When the current of the upper main bridge arm or the lower main bridge arm exceeds a preset threshold, the switching unit is used to control the auxiliary energy transfer unit to connect in the forward direction between the upper main bridge arm and the positive terminal of the DC bus, or to connect in the reverse direction between the lower main bridge arm and the negative terminal of the DC bus, so as to provide a shunt path for the over-threshold current of the upper main bridge arm or the lower main bridge arm.

2. The modular multilevel converter according to claim 1, characterized in that, The main converter unit further includes: a first reactor and a second reactor, wherein... After the first reactor and the second reactor are connected in series, they are connected in series between the upper main bridge arm and the lower main bridge arm, and the connection point of the first reactor and the second reactor serves as the AC terminal.

3. The modular multilevel converter according to claim 1, characterized in that, The upper main bridge arm and the lower main bridge arm are each composed of multiple cascaded power submodules, and the power submodules are half-bridge submodules, full-bridge submodules, or a hybrid structure of half-bridge and full-bridge submodules.

4. The modular multilevel converter according to claim 1, characterized in that, The auxiliary energy transfer unit includes: multiple full-bridge submodules and transfer reactors, wherein... The first end of the multiple full-bridge sub-modules connected in series is controllably connected to the positive and negative terminals of the DC bus via the switching unit, and the second end of the multiple full-bridge sub-modules connected in series is connected to the first end of the transfer reactor. The second end of the transfer reactor is controllably connected to the upper main bridge arm and the lower main bridge arm via the switching unit.

5. The modular multilevel converter according to claim 1, characterized in that, The switching unit includes: a first switch, a second switch, a third switch, and a fourth switch, wherein, The first switch is connected between the auxiliary energy transfer unit and the positive terminal of the DC bus; The second switch is connected between the auxiliary energy transfer unit and the negative terminal of the DC bus; The third switch is connected between the auxiliary energy transfer unit and the upper main bridge arm; The fourth switch is connected between the auxiliary energy transfer unit and the lower main bridge arm; When the current of the upper main bridge arm exceeds a preset threshold, the first switch and the third switch are turned on, and the second switch and the fourth switch are turned off. When the current in the lower main bridge arm exceeds a preset threshold, the second switch and the fourth switch are turned on, and the first switch and the third switch are turned off.

6. A control method for a modular multilevel converter, characterized in that, The method, applied to the modular multilevel converter according to any one of claims 1 to 5, comprises: Real-time monitoring of the current in the upper and lower main bridge arms of each phase unit; When the current of the upper main bridge arm of any phase unit exceeds the first preset threshold, the auxiliary energy transfer unit is controlled to be connected in the positive direction between the upper main bridge arm and the positive terminal of the DC bus, and the auxiliary energy transfer unit is controlled to output a positive voltage to divert the over-threshold current of the upper main bridge arm to the positive terminal of the DC bus. When the current of the lower main bridge arm of any phase unit exceeds the second preset threshold, the auxiliary energy transfer unit is controlled to be connected in reverse between the lower main bridge arm and the negative terminal of the DC bus, and the auxiliary energy transfer unit is controlled to output a negative voltage to divert the over-threshold current of the lower main bridge arm to the negative terminal of the DC bus.

7. The method according to claim 6, characterized in that, The process of controlling the output of the auxiliary energy transfer unit to produce a positive or negative voltage includes: The current of the overcurrent bridge arm is collected in real time and compared with the corresponding preset threshold. The output voltage amplitude of the auxiliary energy transfer unit is dynamically adjusted based on the comparison results, so that the current of the overcurrent bridge arm is limited to below the corresponding preset threshold.

8. The method according to claim 6, characterized in that, The first preset threshold is a preset multiple of the peak value of the rated current of the upper main bridge arm; When the specifications and heat dissipation conditions of the upper main bridge arm and the lower main bridge arm are the same, the second preset threshold is equal to the first preset threshold. When the specifications and heat dissipation conditions of the upper and lower main bridge arms are different, the second preset threshold is determined based on the short-term safe current carrying capacity of the lower main bridge arm.

9. The method according to claim 6, characterized in that, Also includes: When the current in the overcurrent bridge arm drops to or below the corresponding preset threshold, the output voltage of the auxiliary energy transfer unit gradually decreases to zero, and the switching unit is turned off, causing the auxiliary energy transfer unit to stop operating.

10. The method according to claim 9, characterized in that, Before the output voltage of the control-assisted energy transfer unit gradually decreases to zero, it also includes: When the current in the overcurrent bridge arm drops to or below the corresponding preset threshold, a delay timer is started, and the output voltage reduction operation is performed after the delay ends.

Images