A method for thermal equalization control between MMC half-bridge submodules

Abstract

The present application belongs to the technical field of modular multilevel converter, and particularly relates to a kind of MMC half-bridge sub-module inter-thermal balance control method.The present application takes voltage balance principle and junction temperature balance principle as core, by predicting the junction temperature trend of half-bridge sub-module switching device in next control cycle, the junction temperature of half-bridge sub-module switching device and capacitor voltage are combined to set weight factor C for half-bridge sub-module switching control, which can realize the junction temperature balance of switching device between different half-bridge sub-modules in the same bridge arm.By taking half-bridge sub-module as the research object, and considering the temperature of the four switching tubes in its topology, the thermal balance control of MMC under different operating modes can be realized.By embedding the junction temperature control variable into the voltage balance algorithm and ensuring the stable operation of the MMC system, the junction temperature balance effect of the switching devices between the half-bridge sub-modules can be effectively improved, which can effectively reduce the pressure of uneven thermal stress distribution between the half-bridge sub-modules and improve the operation reliability of the MMC system.

Description

Technical Field

[0001] This invention belongs to the technical field of modular multilevel converters, and particularly relates to a thermal balance control method between MMC half-bridge sub-modules. Background Technology

[0002] Unlike two-level and three-level converters, modular multilevel converters (MMCs) employ multiple cascaded half-bridge submodules, offering advantages such as high scalability, high output waveform quality, and low losses, thus becoming a research hotspot in the field of high-voltage direct current (HVDC) transmission. With the continuous increase in capacity and voltage levels, the number of half-bridge submodules and power semiconductor devices in high-voltage, high-capacity MMC systems is also rapidly increasing. Factors such as component aging, low switching frequency operation, and device manufacturing tolerances are leading to increasingly prominent thermal imbalances between MMC bridge arm half-bridge submodules, seriously jeopardizing system reliability.

[0003] To address the aforementioned issues, scholars both domestically and internationally are currently conducting research from both hardware and software perspectives. Due to the large number of half-bridge submodules in MMC systems, hardware-based temperature improvement strategies would significantly increase costs, and existing MMC systems lack the conditions for large-scale hardware upgrades. Furthermore, newly added hardware introduces new potential failure points, making system reliability assessment more difficult. Therefore, improving MMC system control strategies from a software perspective to achieve temperature management holds greater research potential. Existing research on MMC system thermal management from a software perspective mainly focuses on reducing device temperatures and balancing temperatures between devices. MMC system-level temperature control primarily reduces half-bridge submodule temperatures indirectly by decreasing valve group losses, neglecting the complexity of the MMC topology and control strategies, as well as the operating characteristics and parameter differences between different half-bridge submodules. Research on temperature control between half-bridge submodules mainly proposes corresponding control strategies by adjusting the switching frequencies of different half-bridge submodules and monitoring capacitor values ​​for feedback. However, this approach still suffers from problems such as complex control, the need for additional monitoring hardware, high costs, difficulty in application in high-voltage, high-capacity applications, and the difficulty in assessing switching frequencies, which is detrimental to cooling system design.

[0004] Therefore, how to improve the thermal balance problem between MMC half-bridge modules simply and effectively without adding extra control components has become an urgent problem to be solved. Summary of the Invention

[0005] To address the shortcomings of the existing technology, this invention provides a thermal balance control method for MMC half-bridge submodules, which can simply and effectively improve the thermal balance problem between MMC half-bridge submodules without adding additional control steps.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A method for thermal balance control between MMC half-bridge submodules, wherein the MMC includes multiple bridge arms, each bridge arm includes multiple half-bridge submodules with the same structure; each half-bridge submodule includes two switching devices connected in series, and a capacitor connected in parallel with the two switching devices, and the voltage of the half-bridge submodule is the voltage across the capacitor in the half-bridge submodule;

[0008] This method includes the following steps:

[0009] Step 1: Sample the temperature of each switching device in different half-bridge submodules on the same bridge arm, and collect the capacitor voltage of each half-bridge submodule on the bridge arm.

[0010] Step 2: Based on the sign of the bridge arm current and the charging and discharging data of each half-bridge submodule, analyze the temperature change trend of each switching device on the half-bridge submodule.

[0011] Step 3: Based on the principles of voltage balance and junction temperature balance, calculate the thermal balance factor considering the capacitor voltage of the half-bridge submodule and the temperature of each switching transistor; and determine the number N of half-bridge submodules to be rotated according to the capacitor voltage fluctuation requirements of the half-bridge submodule. b ;

[0012] Step 4: Divide the half-bridge sub-modules of the bridge arm into an engaged group and an unengaged group according to their engagement and disengagement status; sort the half-bridge sub-modules in the engaged group in ascending order of the thermal balance factor of their respective switching transistor temperatures; sort the half-bridge sub-modules in the disengaged group in ascending order of the thermal balance factor of their respective switching transistor temperatures.

[0013] Step 5: Record the number n of half-bridge submodules engaged in the current control cycle. onk The number of half-bridge submodules n put into operation in the previous control cycle onk-1 And calculate the input increment Δn of the half-bridge submodule in the current adjacent control cycle. ref =n onk -n onk-1 ;

[0014] Step 6: Acquire the real-time bridge arm current i of the MMC. arm According to the bridge arm current i arm With Δn ref The positive and negative values ​​are determined according to the preset switching strategy based on the voltage balance principle and the junction temperature balance principle, combined with the sorting of the input and output groups in step 4, to achieve thermal equilibrium control.

[0015] Preferably, each half-bridge submodule includes two IGBTs and two diode switches D, namely a first IGBT switch T1 and a second IGBT switch T2, and a first diode switch D1 and a second diode switch D2; wherein, the first diode switch D1 and the second diode switch D2 are connected in anti-parallel between the collector and emitter of the first IGBT switch T1 and the second IGBT switch T2, respectively, and each IGBT switch and a diode connected in parallel constitute a switching device; the emitter of the first IGBT switch T1 is connected to the collector of the second IGBT switch T2; the collector of the first IGBT switch T1 is connected to the positive terminal of the capacitor in the half-bridge submodule, and the emitter of the second IGBT switch T2 is connected to the negative terminal of the capacitor in the half-bridge submodule; the emitters of the first IGBT switch T1 and the second IGBT switch T2 serve as two connection terminals of the half-bridge submodule;

[0016] In step 3, the calculated thermal balance factors for the temperatures of each switch in the half-bridge submodule include the thermal balance factor Z corresponding to the first IGBT switch T1. T1 The thermal balance factor Z corresponding to the second IGBT switch T2 T2 The thermal balance factor Z corresponding to the first diode switch D1 D1 The thermal balance factor Z corresponding to the second diode switch D2 D2 ;

[0017] In step 4, the half-bridge submodules in the input group are respectively processed according to Z... T1 Z T2 Z D1 and Z D2 Sort in ascending order; divide the half-bridge submodules in the resection group according to Z... T1 Z T2 Z D1 and Z D2 Sort them in ascending order.

[0018] Preferably, in step 2, the temperature change trend of each switching device on the half-bridge submodule includes: bridge arm current i arm When the value is greater than 0 and the half-bridge submodule changes from the on state to the off state, the temperature of the second IGBT switch T2 increases; the bridge arm current i arm When the voltage is >0 and the half-bridge submodule changes from the off state to the on state, the temperature of the first diode switch D1 increases; the bridge arm current i arm When the voltage is less than 0 and the half-bridge submodule changes from the active state to the deactivated state, the temperature of the second diode switch D2 increases; the bridge arm current i arm When the temperature of the first IGBT switch T1 increases when the temperature of the half-bridge module changes from the off state to the on state and the temperature of the half-bridge module changes from the off state to the on state, the temperature of the half-bridge module T1 increases.

[0019] Preferably, in step 3, the formula for calculating the thermal balance factor considering the capacitor voltage of the half-bridge submodule and the temperature of each switching transistor is:

[0020]

[0021] In the formula, Z T1 This represents the capacitor voltage of the half-bridge submodule and the thermal balance factor of the first IGBT switch T1, Z. T2 This represents the capacitor voltage of the half-bridge submodule and the thermal balance factor of the second IGBT switch T2, Z. D1 This represents the capacitor voltage of the half-bridge submodule and the thermal balance factor of the first diode switch D1, Z. D2 The capacitor voltage of the half-bridge submodule represents the thermal balance factor of the second diode switch D2; C represents the weighting factor of temperature and voltage, and C = 0.5; u ci u represents the capacitor voltage of the i-th half-bridge submodule; cave T represents the average capacitor voltage of all half-bridge submodules in the bridge arm; iT1 T represents the temperature of the i-th half-bridge submodule T1; T1ave This represents the average temperature of all half-bridge submodules T1 within the bridge arm; T iT2 T represents the temperature of the i-th half-bridge submodule T2; T2ave This represents the average temperature of all half-bridge submodules T2 within the bridge arm; T iD1 T represents the temperature of the i-th half-bridge submodule D1; D1ave T represents the average temperature of all half-bridge submodules D1 within the bridge arm; iD2 T represents the temperature of the i-th half-bridge submodule D2. D2ave This represents the average temperature of all half-bridge submodules D2 within the bridge arm.

[0022] Preferably, in step 6, the preset switching strategy based on voltage balance and junction temperature balance principles includes:

[0023] If the bridge arm current i arm >0 and Δn ref If ≥0, then Δn is selected from the resection group. ref +N b Z D1 The smallest half-bridge submodule is set as the input; N is selected from the input group. b Z T2 The smallest half-bridge submodule is set to be cut off;

[0024] When the bridge arm current i arm >0 and Δn ref If the value is less than 0, then select |Δn| from the input group. ref |+Nb Z T2 The smallest half-bridge submodule is set as the cut; N is selected from the cut group. b Z D1 The smallest half-bridge submodule is set to input;

[0025] When the bridge arm current i arm <0 and Δn ref If ≥0, then Δn is selected from the resection group. ref +N b Z T1 The smallest half-bridge submodule is set as the input; N is selected from the input group. b Z D2 The smallest half-bridge submodule is set to be cut off;

[0026] When the bridge arm current i arm <0 and Δn ref If the value is less than 0, then select |Δn| from the input group. ref |+N b Z D2 The smallest half-bridge submodule is set as the cut; N is selected from the cut group. b Z T1 The smallest half-bridge submodule is set to input.

[0027] Preferably, the voltage balance principle is: bridge arm current i arm When the voltage is greater than 0, the half-bridge sub-module with the higher capacitor voltage is preferentially disconnected, and the half-bridge sub-module with the lower capacitor voltage is preferentially connected; the bridge arm current i arm When the voltage is less than 0, the half-bridge module with the higher capacitor voltage is given priority to be put into operation, and the half-bridge module with the lower capacitor voltage is given priority to be cut off.

[0028] Preferably, the junction temperature balance principle is: prioritize the use of switching devices whose temperature is about to rise, in order to balance the junction temperature differences of switching devices between different half-bridge sub-modules.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] 1. This invention is based on the principles of voltage balance and junction temperature balance. By predicting the junction temperature change trend of the switching devices in the half-bridge submodule during the next control cycle, it combines the junction temperature of the half-bridge submodule switching devices with the capacitor voltage and sets a weighting factor C for half-bridge submodule switching control, thereby achieving junction temperature equalization among the switching devices of different half-bridge submodules in the same bridge arm. By taking the half-bridge submodule as the research object and considering the temperature of the four switching transistors in its topology, thermal balance control of the MMC under different operating modes can be achieved. By embedding the junction temperature control variable into the voltage equalization algorithm and ensuring the stable operation of the MMC system, the junction temperature equalization effect of the switching devices among the half-bridge submodules can be effectively improved, which can effectively reduce the pressure of uneven thermal stress distribution among the half-bridge submodules and improve the operational reliability of the MMC system.

[0031] 2. This invention embeds junction temperature equalization into the capacitor voltage balance control of half-bridge submodules. Combined with existing fixed-rotation low-switching-frequency voltage equalization control methods, it can achieve junction temperature equalization control of switching devices in different half-bridge submodules within the same bridge arm of a high-voltage, high-capacity MMC-HVDC. This method achieves thermal equalization between bridge arm half-bridge submodules by adjusting the switching pulses of different half-bridge submodules. In other words, this method applies temperature equalization to voltage equalization. While existing technologies directly use the switching frequency and number of half-bridge submodules for voltage equalization, this method redetermines the specific half-bridge submodules to be switched on or off based on predicted junction temperature trends during each switching period and relevant thermal balance factors. This allows for temperature equalization simultaneously with voltage equalization. Furthermore, it does not add additional control steps, is simple to control, and easy to implement. While ensuring voltage equalization, it effectively improves the uneven distribution of thermal stress among MMC half-bridge submodules.

[0032] 3. This invention can predict the junction temperature change of the switching device corresponding to the half-bridge submodule in the next control cycle based on the direction of the bridge arm current. The optimal half-bridge submodule is selected by sorting the junction temperatures. By setting weight factors and combining them with the capacitor voltage of the half-bridge submodule, the half-bridge submodule that most urgently needs to change its switching state in the current cycle is determined. It has a certain degree of self-adaptive capability and takes into account both voltage and temperature, which greatly helps to improve the reliability of the system.

[0033] 4. This invention simultaneously considers the temperatures (T1, T2, D1, D2) of the four switching devices in the half-bridge submodule. Therefore, when the operating mode of the MMC system changes (rectification, inversion, STATCOM), thermal balance control between the half-bridge submodules can still be achieved based on the different values ​​of current and device temperature. It has wide applicability and can be effectively applied in multiple operating conditions and scenarios.

[0034] In summary, this invention can simply and effectively improve the thermal balance problem between MMC half-bridge sub-modules without adding additional control steps. Attached Figure Description

[0035] To make the objectives, technical solutions, and advantages of the invention clearer, the invention will now be described in further detail with reference to the accompanying drawings, wherein:

[0036] Figure 1 This is a flowchart of the present invention;

[0037] Figure 2 This is a diagram showing the topology and corresponding current flow path of the half-bridge submodule in the embodiment.

[0038] Figure 3 This is a schematic diagram of the switching strategy in the embodiment;

[0039] Figure 4 This is a waveform diagram of the junction temperature of the switching devices between the same bridge arm half-bridge submodules before and after thermal equalization in the MMC under inverter operation in the embodiment.

[0040] Figure 5 This is a waveform diagram of the junction temperature of the switching devices between the half-bridge submodules of the same bridge arm before and after thermal equalization in the MMC under rectification conditions in the embodiment. Detailed Implementation

[0041] The following detailed explanation illustrates the specific implementation methods:

[0042] Example:

[0043] like Figure 1 As shown in the figure, this embodiment discloses a thermal balance control method between MMC half-bridge submodules.

[0044] The MMC comprises multiple bridge arms, each of which includes multiple half-bridge submodules with the same structure. Each half-bridge submodule includes two switching devices connected in series and a capacitor connected in parallel with the two switching devices. The voltage across the capacitor in the half-bridge submodule is the voltage across the capacitor in the half-bridge submodule. The topology of the half-bridge submodule and the corresponding current flow path are as follows: Figure 2As shown, specifically, each half-bridge submodule includes two IGBTs and two diode switches D, namely a first IGBT switch T1 and a second IGBT switch T2, and a first diode switch D1 and a second diode switch D2. The first diode switch D1 and the second diode switch D2 are connected in anti-parallel between the collector and emitter of the first IGBT switch T1 and the second IGBT switch T2, respectively. Each IGBT switch and its parallel diode constitute a switching device. The emitter of the first IGBT switch T1 is connected to the collector of the second IGBT switch T2. The collector of the first IGBT switch T1 is connected to the positive terminal of a capacitor in the half-bridge submodule, and the emitter of the second IGBT switch T2 is connected to the negative terminal of a capacitor in the half-bridge submodule. The emitters of the first IGBT switch T1 and the second IGBT switch T2 serve as two connection terminals of the half-bridge submodule.

[0045] This method includes the following steps:

[0046] Step 1: Sample the temperature of each switching device in different half-bridge submodules on the same bridge arm, and collect the capacitor voltage of each half-bridge submodule on the bridge arm.

[0047] Step 2: Based on the sign of the bridge arm current and the charging and discharging data of each half-bridge submodule, analyze the temperature change trend of each switching device on the half-bridge submodule.

[0048] Specifically, the temperature variation trends of each switching device on the half-bridge submodule include: bridge arm current i arm When the value is greater than 0 and the half-bridge submodule changes from the on state to the off state, the temperature of the second IGBT switch T2 increases; the bridge arm current i arm When the voltage is >0 and the half-bridge submodule changes from the off state to the on state, the temperature of the first diode switch D1 increases; the bridge arm current i arm When the voltage is less than 0 and the half-bridge submodule changes from the active state to the deactivated state, the temperature of the second diode switch D2 increases; the bridge arm current i arm When the temperature of the first IGBT switch T1 increases when the temperature of the half-bridge module changes from the off state to the on state and the temperature of the half-bridge module changes from the off state to the on state, the temperature of the half-bridge module T1 increases.

[0049] Step 3: Based on the voltage balance principle and the junction temperature balance principle, calculate the thermal balance factor considering the capacitor voltage of the half-bridge submodule and the temperature of each switching transistor, including the thermal balance factor Z corresponding to the first IGBT switching transistor T1. T1 The thermal balance factor Z corresponding to the second IGBT switch T2 T2 The thermal balance factor Z corresponding to the first diode switch D1 D1 The thermal balance factor Z corresponding to the second diode switch D2 D2The number N of half-bridge submodules to be rotated is determined based on the capacitor voltage fluctuation requirements of the half-bridge submodule. b N b Determining the value of is a conventional technique in this field, and those skilled in the art can obtain it directly using existing methods, so it will not be elaborated here.

[0050] The voltage balance principle is as follows: bridge arm current i arm When the voltage is greater than 0, the half-bridge sub-module with the higher capacitor voltage is preferentially disconnected, and the half-bridge sub-module with the lower capacitor voltage is preferentially connected; the bridge arm current i arm When the voltage is less than 0, the half-bridge module with the higher capacitor voltage is prioritized for activation, and the half-bridge module with the lower capacitor voltage is prioritized for deactivation. The junction temperature balance principle is: prioritize the activation of switching devices whose temperature is about to rise, in order to balance the junction temperature differences of switching devices between different half-bridge modules.

[0051] The formula for calculating the thermal balance factor considering the capacitor voltage of the half-bridge submodule and the temperature of each switching transistor is as follows:

[0052]

[0053] In the formula, Z T1 This represents the capacitor voltage of the half-bridge submodule and the thermal balance factor of the first IGBT switch T1, Z. T2 This represents the capacitor voltage of the half-bridge submodule and the thermal balance factor of the second IGBT switch T2, Z. D1 This represents the capacitor voltage of the half-bridge submodule and the thermal balance factor of the first diode switch D1, Z. D2 The capacitor voltage of the half-bridge submodule represents the thermal balance factor of the second diode switch D2; C represents the weighting factor of temperature and voltage, and C = 0.5; u ci u represents the capacitor voltage of the i-th half-bridge submodule; cave T represents the average capacitor voltage of all half-bridge submodules in the bridge arm; iT1 T represents the temperature of the i-th half-bridge submodule T1; T1ave This represents the average temperature of all half-bridge submodules T1 within the bridge arm; T iT2 T represents the temperature of the i-th half-bridge submodule T2; T2ave This represents the average temperature of all half-bridge submodules T2 within the bridge arm; T iD1 T represents the temperature of the i-th half-bridge submodule D1; D1ave T represents the average temperature of all half-bridge submodules D1 within the bridge arm; iD2 T represents the temperature of the i-th half-bridge submodule D2. D2ave This represents the average temperature of all half-bridge submodules D2 within the bridge arm.

[0054] Step 4: Divide the half-bridge submodules of the bridge arm into an engaged group and a disengaged group according to their engagement and disengagement status; then, for the half-bridge submodules in the engaged group, classify them according to Z... T1 Z T2 Z D1 and Z D2 Sort in ascending order; divide the half-bridge submodules in the resection group according to Z... T1 Z T2 Z D1 and Z D2 Sort in ascending order;

[0055] Step 5: Record the number n of half-bridge submodules engaged in the current control cycle. onk The number of half-bridge submodules n put into operation in the previous control cycle onk-1 And calculate the input increment Δn of the half-bridge submodule in the current adjacent control cycle. ref =n onk -n onk-1 ;

[0056] Step 6: Acquire the real-time bridge arm current i of the MMC. arm According to the bridge arm current i arm With Δn ref The positive and negative values ​​are determined according to the preset switching strategy based on the voltage balance principle and the junction temperature balance principle, combined with the sorting of the input and output groups in step 4, to achieve thermal equilibrium control.

[0057] The preset switching strategy based on voltage balance and junction temperature balance principles is as follows: Figure 3 As shown, it includes:

[0058] If the bridge arm current i arm >0 and Δn ref If ≥0, then Δn is selected from the resection group. ref +N b Z D1 The smallest half-bridge submodule is set as the input; N is selected from the input group. b Z T2 The smallest half-bridge submodule is set to be cut off. When the bridge arm current i arm >0 indicates that MMC is in a charging state, and when Δn ref When ≥0, Δn is selected from the resection group. ref +N b Z D1 Setting the smallest module as the input indicates that the half-bridge submodule in operation must have the lowest voltage and the highest D1 temperature; N is selected from the input group. b Z T2 Setting the smallest module to be cut off indicates that the half-bridge sub-module to be cut off at this time must meet the requirements of the highest voltage and the highest temperature T2.

[0059] When the bridge arm current i arm >0 and Δn ref If the value is less than 0, then select |Δn| from the input group. ref |+N b Z T2 The smallest half-bridge submodule is set as the cut; N is selected from the cut group. b Z D1 The smallest half-bridge submodule is set to engage. When the bridge arm current i arm When Δn > 0, it indicates that the MMC is in a charging state, and when Δn ref When <0, select Δn from the input group. ref +N b Z T2 Setting the smallest module as the cut-off indicates that the half-bridge sub-module to be cut must meet the requirements of highest voltage and highest temperature T2; N is selected from the cut-off group. b Z D1 Setting the smallest module as the input means that the half-bridge sub-module input at this time must meet the requirements of the lowest voltage and the highest temperature of D1.

[0060] When the bridge arm current i arm <0 and Δn ref If ≥0, then Δn is selected from the resection group. ref +N b Z T1 The smallest half-bridge submodule is set as the input; N is selected from the input group. b Z D2 The smallest half-bridge submodule is set to be cut off. When the bridge arm current i arm When Δn < 0, it indicates that the MMC is in a discharge state, and when Δn ref When ≥0, Δn is selected from the resection group. ref +N b Z T1 Setting the smallest module as the input indicates that the half-bridge sub-module in operation must meet the requirements of highest voltage and highest temperature (T1); N is selected from the input group. b Z D2 The smallest module is set to be cut off, indicating that the half-bridge sub-module to be cut off at this time must meet the requirements of the lowest voltage and the highest temperature of D2.

[0061] When the bridge arm current i arm <0 and Δn ref If the value is less than 0, then select |Δn| from the input group. ref |+N b Z D2 The smallest half-bridge submodule is set as the cut; N is selected from the cut group. b Z T1 The smallest half-bridge submodule is set to engage. When the bridge arm current i armWhen Δn < 0, it indicates that the MMC is in a discharge state, and when Δn ref When <0, select Δn from the input group. ref +N b Z D2 The smallest module is set as the cut-off module, indicating that the cut-off half-bridge sub-module must meet the requirements of lowest capacitor voltage and highest D2 temperature; N is selected from the cut-off group. b Z T1 Setting the smallest module to be engaged indicates that the half-bridge sub-module with the highest capacitor voltage and highest temperature (T1) is engaged at this time.

[0062] To verify the effectiveness of this invention in improving the uneven thermal stress between half-bridge submodules, the inventors set differences in the capacitance values ​​of the half-bridge submodules to simulate capacitance deterioration in actual engineering. Through simulation comparison, the junction temperature equilibrium between the half-bridge submodules before and after using the proposed method was assessed. Simulation results are as follows: Figure 4 , Figure 5 As shown, where, Figure 4 The simulation comparison shows how this method improves the uneven thermal stress between half-bridge modules under inverter conditions. Figure 5 This image shows a simulation comparison of how the proposed method improves the thermal stress imbalance between half-bridge submodules under rectified conditions. Simulation results demonstrate that the thermal balance control proposed in this invention can significantly balance the thermal imbalance between half-bridge submodules, and is particularly effective under rectified conditions. Figure 5 ) and invert ( Figure 4 Under these conditions, it has a significant effect on improving the uneven thermal stress between half-bridge modules.

[0063] This invention, based on the principles of voltage balance and junction temperature balance, predicts the junction temperature change trend of the switching devices in the half-bridge submodule during the next control cycle. It then combines the junction temperature of the half-bridge submodule switching devices with the capacitor voltage, setting a weighting factor C for half-bridge submodule switching control. This achieves junction temperature equalization among the switching devices in different half-bridge submodules within the same bridge arm. By focusing on the half-bridge submodule and considering the temperatures of the four switching transistors in its topology, thermal equilibrium control of the MMC under different operating modes can be achieved. By embedding the junction temperature control variable into the voltage equalization algorithm and ensuring stable operation of the MMC system, the junction temperature equalization effect among the switching devices in the half-bridge submodules can be effectively improved. This effectively reduces the pressure of uneven thermal stress distribution among the half-bridge submodules and enhances the operational reliability of the MMC system.

[0064] Specifically, this invention embeds junction temperature equalization into the capacitor voltage balance control of half-bridge submodules. Combined with existing fixed-rotation low-switching-frequency voltage equalization control methods, it can achieve junction temperature equalization control of switching devices in different half-bridge submodules within the same bridge arm of a high-voltage, high-capacity MMC-HVDC. This method achieves thermal equalization between bridge arm half-bridge submodules by adjusting the switching pulses of different half-bridge submodules. In other words, this method applies temperature equalization to voltage equalization. While existing technologies directly use the switching frequency and number of half-bridge submodules for voltage equalization, this method redetermines the specific half-bridge submodules to be switched on or off based on predicted junction temperature trends during each switching period and relevant thermal balance factors. This allows for temperature equalization simultaneously with voltage equalization. Furthermore, it does not add additional control steps, is simple to control, and easy to implement. While ensuring voltage equalization, it effectively improves the uneven distribution of thermal stress among MMC half-bridge submodules. In addition, this invention can predict the junction temperature change of the corresponding switching device in the next control cycle of the half-bridge submodule based on the direction of the bridge arm current. It selects the optimal half-bridge submodule to be switched on by ranking the junction temperatures, and determines the half-bridge submodule whose switching state most urgently needs to be changed in the current cycle by combining a weighting factor with the capacitor voltage of the half-bridge submodule. This invention has a certain degree of self-adaptability and considers both voltage and temperature, which greatly helps improve the reliability of the system. Furthermore, this invention also considers the temperatures of the four switching devices (T1, T2, D1, D2) in the half-bridge submodule. Therefore, when the MMC system operating mode changes (rectification, inversion, STATCOM), thermal balance control among the half-bridge submodules can still be achieved based on the different current and device temperature values. This makes it widely applicable and effective in multiple operating conditions and scenarios.

[0065] This invention can simply and effectively improve the thermal balance problem between MMC half-bridge sub-modules without adding extra control steps.

[0066] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method for thermal equilibrium control between MMC half-bridge submodules, characterized in that, The MMC includes multiple bridge arms, each of which includes multiple half-bridge sub-modules with the same structure; each half-bridge sub-module includes two switching devices connected in series, and a capacitor connected in parallel with the two switching devices. The voltage of the half-bridge sub-module is the voltage across the capacitor in the half-bridge sub-module. This method includes the following steps: Step 1: Sample the temperature of each switching device in different half-bridge submodules on the same bridge arm, and collect the capacitor voltage of each half-bridge submodule on the bridge arm. Step 2: Based on the sign of the bridge arm current and the charging and discharging data of each half-bridge submodule, analyze the temperature change trend of each switching device on the half-bridge submodule. Step 3, based on the voltage balance principle and the junction temperature balance principle, calculate the thermal balance factor considering the capacitor voltage of the half-bridge sub-module and the temperature of each switch tube; and determine the number N of half-bridge sub-modules according to the capacitor voltage fluctuation requirement of the half-bridge sub-module b ; Step 4: Divide the half-bridge sub-modules of the bridge arm into an engaged group and an unengaged group according to their engagement and disengagement status; sort the half-bridge sub-modules in the engaged group in ascending order of the thermal balance factor of their respective switching transistor temperatures; sort the half-bridge sub-modules in the disengaged group in ascending order of the thermal balance factor of their respective switching transistor temperatures. Step 5, record the number of half-bridge sub-modules n put in current control period onk The number of half-bridge sub-modules n put in last control period onk-1 ; and calculate the increment Δn of half-bridge sub-modules put in current adjacent control period ref =n onk - n onk-1 ; Step 6, collect the real-time bridge arm current i of MMC arm , according to the bridge arm current i arm and the positive and negative of Δn ref , according to the preset switching strategy based on the voltage balance principle and the junction temperature balance principle, combined with the sorting of the input group and the removed group in step 4, carry out thermal balance control; Wherein, the voltage balance principle is: when bridge arm current i arm >0, the half-bridge submodule with high capacitor voltage is preferentially cut off, and the half-bridge submodule with low capacitor voltage is preferentially put into; bridge arm current i arm <0, the half-bridge submodule with high capacitor voltage is preferentially put into, and the half-bridge submodule with low capacitor voltage is preferentially cut off; The junction temperature balance principle is as follows: prioritize the operation of switching devices whose temperature is about to rise, in order to balance the junction temperature differences of switching devices between different half-bridge sub-modules.

2. The thermal balance control method between MMC half-bridge submodules as described in claim 1, characterized in that: Each half-bridge submodule includes two IGBTs and two diode switches D, namely, a first IGBT switch T1 and a second IGBT switch T2, and a first diode switch D1 and a second diode switch D2. The first diode switch D1 and the second diode switch D2 are connected in anti-parallel between the collector and emitter of the first IGBT switch T1 and the second IGBT switch T2, respectively. Each IGBT switch and its parallel diode constitute a switching device. The emitter of the first IGBT switch T1 is connected to the collector of the second IGBT switch T2. The collector of the first IGBT switch T1 is connected to the positive terminal of a capacitor in the half-bridge submodule, and the emitter of the second IGBT switch T2 is connected to the negative terminal of a capacitor in the half-bridge submodule. The emitters of the first IGBT switch T1 and the second IGBT switch T2 serve as two connection terminals of the half-bridge submodule. In step 3, the thermal balance factors of each switch tube temperature of the calculated half-bridge sub-module include the thermal balance factor Z corresponding to the first IGBT switch tube T1 T1 , the thermal balance factor Z corresponding to the second IGBT switch tube T2 T2 , the thermal balance factor Z corresponding to the first diode switch tube D1 D1 , and the thermal balance factor Z corresponding to the second diode switch tube D2 D2 . In step 4, the half-bridge submodules in the input group are respectively processed according to Z... T1 Z T2 Z D1 and Z D2 Sort in ascending order; divide the half-bridge submodules in the resection group according to Z... T1 Z T2 Z D1 and Z D2 Sort them in ascending order.

3. The thermal balance control method between MMC half-bridge submodules as described in claim 2, characterized in that: In step 2, the temperature change trends of each switching device on the half-bridge submodule include: bridge arm current i arm When the value is greater than 0 and the half-bridge submodule changes from the on state to the off state, the temperature of the second IGBT switch T2 increases; the bridge arm current i arm When the voltage is >0 and the half-bridge submodule changes from the off state to the on state, the temperature of the first diode switch D1 increases; the bridge arm current i arm When the voltage is less than 0 and the half-bridge submodule changes from the active state to the deactivated state, the temperature of the second diode switch D2 increases; the bridge arm current i arm When the temperature of the first IGBT switch T1 increases when the temperature of the half-bridge module changes from the off state to the on state and the temperature of the half-bridge module changes from the off state to the on state, the temperature of the half-bridge module T1 increases.

4. The thermal balance control method between MMC half-bridge submodules as described in claim 3, characterized in that: In step 3, the formula for calculating the thermal balance factor considering the capacitor voltage of the half-bridge submodule and the temperature of each switching transistor is as follows: ； In the formula, This represents the capacitor voltage of the half-bridge submodule and the thermal balance factor of the first IGBT switch T1. This represents the capacitor voltage of the half-bridge submodule and the thermal balance factor of the second IGBT switch T2. This represents the capacitor voltage of the half-bridge submodule and the thermal balance factor of the first diode switch D1. The capacitor voltage of the half-bridge submodule is represented by the thermal balance factor of the second diode switch D2; c represents the weighting factor of temperature and voltage, and c=0.5; This represents the capacitor voltage of the i-th half-bridge submodule; This represents the average capacitor voltage of all half-bridge submodules in the bridge arm; Let T1 be the temperature of the i-th half-bridge submodule; This represents the average temperature of all half-bridge submodules T1 within the bridge arm; Let T2 be the temperature of the i-th half-bridge submodule; This represents the average temperature of all half-bridge submodules T2 within the bridge arm; Let be the temperature of the i-th half-bridge submodule D1; This represents the average temperature of all half-bridge submodules D1 within the bridge arm; Let be the temperature of the i-th half-bridge submodule D2; This represents the average temperature of all half-bridge submodules D2 within the bridge arm.

5. The thermal balance control method between MMC half-bridge submodules as described in claim 4, characterized in that: In step 6, the preset switching strategy based on voltage balance and junction temperature balance principles includes: If the bridge arm current i arm >0 and Δn ref If ≥0, then Δn is selected from the resection group. ref +N b Z D1 The smallest half-bridge submodule is set as the input; N is selected from the input group. b Z T2 The smallest half-bridge submodule is set to be cut off; When the bridge arm current i arm >0 and Δn ref If the value is less than 0, then |Δn| is selected from the input group. ref |+N b Z T2 The smallest half-bridge submodule is set as the cut; N is selected from the cut group. b Z D1 The smallest half-bridge submodule is set to input; When the bridge arm current i arm <0 and Δn ref If ≥0, then Δn is selected from the resection group. ref +N b Z T1 The smallest half-bridge submodule is set as the input; N is selected from the input group. b Z D2 The smallest half-bridge submodule is set to be cut off; When the bridge arm current i arm <0 and Δn ref If the value is less than 0, then |Δn| is selected from the input group. ref |+N b Z D2 The smallest half-bridge submodule is set as the cut; N is selected from the cut group. b Z T1 The smallest half-bridge submodule is set to input.

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