Seamless fault-tolerant control method for solid-state transformer based on cascaded H-bridge pre-charging

Abstract

The invention discloses a solid-state transformer seamless fault-tolerant control method based on cascaded H-bridge pre-charging, and the method comprises the steps: carrying out the pre-charging of a bus capacitor of a redundant unit module based on a cascaded H-bridge, dynamically adjusting the modulation wave of a preceding-stage H-bridge of each unit module, and achieving the impact-free charging of the bus capacitor of the redundant unit module. And after the pre-charging of the redundant unit module is finished, controlling a modulation wave signal and a driving pulse of a pre-stage H bridge of the redundant unit module to intermittently charge a bus capacitor of the redundant unit module. According to the invention, the problems of long charging time of the redundant unit module, overload of the residual unit module and overlarge current impact of the alternating current inductor in the process of removing the fault unit module and inserting the redundant unit module in the cascade converter are solved. According to the method, no additional circuit needs to be added, the method is suitable for a power bidirectional flow and unidirectional flow system, quick and smooth switching of the unit modules can be achieved when a fault occurs, and reliable and stable operation of the whole system is guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of medium-voltage distribution power electronic converters and relates to a solid-state transformer control method, in particular to a seamless fault-tolerant control method for a solid-state transformer based on cascaded H-bridge precharging. Background Art

[0002] Solid-state transformers, as core equipment connecting medium-voltage and low-voltage power grids, have their front-end directly connected to the medium-voltage grid, enabling power conversion from medium-voltage AC to low-voltage DC. Solid-state transformer systems utilize a large number of power switching devices, which have a high failure rate under high-frequency switching stress, resulting in a higher probability of failure in solid-state transformers than traditional power-frequency transformers. If a failure in a unit module in the system is not promptly addressed, the fault may spread to other units, preventing the solid-state transformer from operating normally. Severe failures can even affect the grid and loads, threatening the safety of the entire medium-voltage distribution system. Therefore, to ensure reliable and stable power supply for the entire distribution system, effective fault-tolerant control is crucial when a unit module in the solid-state transformer fails.

[0003] For cascade-structured systems like solid-state transformers, a commonly used fault-tolerance solution is hardware redundancy, which involves inserting redundant unit modules into the system to replace the faulty unit modules and continue operation after a fault occurs.

[0004] Depending on whether the redundant unit module is put into the system to run with other unit modules under normal operation, the redundant unit solution can be divided into hot standby solution and cold standby solution.

[0005] For the hot standby solution, during normal operation, the redundant unit modules operate together with the other unit modules. When a fault occurs, the faulty unit module is bypassed, the total number of unit modules is reduced, and the remaining unit modules need to increase the bus voltage to avoid overmodulation, which will increase the voltage stress of the power switching devices. In addition, the gain of the subsequent isolated DC-DC converter will change, which will cause it to deviate from the rated operating point. For solid-state transformers with isolated DC-DC converters (such as LLC converters) in the subsequent stage, this hot redundancy solution will reduce the efficiency of the system. "Efficiency enhancement of DC solid-state transformers by dynamically adjusting active cells," IEEE Transactions on Power Electronics, vol. 38, no. 12, pp. 14942-14955, Dec. 2023. For DC solid-state transformer topologies, controlling the duty cycle to change slowly to smoothly bypass and insert unit modules can effectively reduce current overshoot. However, this method is suitable for active switching situations and is not suitable for fault-tolerant control scenarios. "Fault-tolerant design and control strategy for cascaded H-bridge multilevel converter-based STATCOM," IEEE Transactions on Industrial Electronics, vol. 57, no. 8, pp. 2700–2708, Aug. 2010. This approach avoids current surges by boosting the modulation wave at the switching moment. However, after bypassing the faulty module, the bus voltage of the remaining healthy modules needs to be increased. When the bus voltage of the remaining unit modules rises to the specified value, the modulation wave directly returns to the normal value.In "Seamless fault-tolerant control for cascaded H-bridge converters based battery energy storage system," IEEE Transactions on Industrial Electronics, vol. 70, no. 4, pp. 3803–3813, April 2023, and "Redistributed pulse width modulation of MMC battery energy storage system under submodule fault condition," IEEE Transactions on Power Electronics, vol. 35, no. 3, pp. 2284–2294, March 2020, when a submodule fails and is bypassed, the missing voltage level of the faulty submodule is compensated by other healthy submodules in the faulty phase. This method does not require modifying the modulation wave or phase shift angle.

[0006] Unlike the hot standby solution, the number of unit modules in the cold standby solution remains unchanged before and after fault-tolerant control, and the power switching devices have the same voltage stress without reducing the efficiency of the solid-state transformer. Therefore, the cold standby solution can theoretically fully restore normal operating conditions after fault-tolerant control. However, the disadvantage of the cold standby solution is that the bus capacitors of the redundant unit modules must be charged to the rated bus voltage to work properly. Therefore, there will be a current surge at the moment of unit module switching. In addition, during the period from the faulty unit module being removed to the time when its bus capacitors charge to the rated voltage, the remaining unit modules need to additionally share the original power of the faulty unit module. The charging time of the redundant unit module bus capacitor is related to the capacitance value of the capacitor. Usually, in order to suppress the ripple of the H-bridge rectifier output current, the capacitance value of the H-bridge DC bus capacitor is very large, resulting in a long charging time for the redundant unit modules. "Seamless transition control for modular multilevel converters when inserting a cold-reserve redundant submodule," IEEE Transactions on Power Electronics, vol. 30, no. 8, pp. 4052–4057, August 2015. A fault-tolerant control method for modular multilevel converters is proposed. This method seamlessly bypasses the faulty module and inserts a redundant submodule, but this method requires a long charging time for the redundant submodule. To achieve rapid fault-tolerant control, the busbar capacitors of the redundant submodule are precharged to maintain their rated voltage. Once the faulty submodule is removed, the redundant submodule can be quickly inserted into the system for operation.In "Improved fault-tolerant method and control strategy based on reverse charging for the power electronic traction transformer," IEEE Transactions on Industrial Electronics, vol. 65, no. 3, pp. 2672–2682, Mar. 2018. and "A short-time transition and cost-saving redundancy scheme for medium-voltage three-phase cascaded H-bridge electronic power transformer," IEEE Transactions on Power Electronics, vol. 33, no. 11, pp. 9242–9252, Nov. 2018., reverse precharging of the busbar capacitors is performed using a dual active bridge converter downstream of the redundant unit module to ensure fast and smooth fault-tolerant control. However, this method is not suitable for systems with unidirectional power transmission. Therefore, current cold standby schemes struggle to simultaneously shorten the charging time of the redundant unit modules, reduce the fluctuations caused by unit module switching, and be applicable to systems with both bidirectional and unidirectional power flow. Summary of the Invention

[0007] The purpose of the present invention is to provide a seamless fault-tolerant control method for solid-state transformers based on cascaded H-bridge precharging. By precharging the bus capacitors of redundant unit modules, fast and smooth unit module switching is achieved when a system failure occurs, avoiding the problems of input current surge, overload of remaining unit modules, and excessive charging time of redundant unit modules caused by traditional cold standby fault-tolerant control methods. The method proposed by the present invention is applicable to systems with unidirectional and bidirectional power flow. Compared to the reverse charging of the bus capacitors of redundant unit modules from the rear-stage isolated DC-DC converter, which requires two full bridges and an intermediate isolation transformer, the method based on the present invention charges the bus capacitors of redundant unit modules from the front-stage H-bridge only through one full bridge, resulting in less loss.

[0008] The present invention provides a seamless fault-tolerant control method for a solid-state transformer based on cascaded H-bridge precharging. The solid-state transformer is composed of a plurality of cascaded unit modules with input ends connected in series and output ends connected in parallel. The input end of each cascaded unit module is connected in series with a filter inductor and then connected to an AC power grid. Each unit module includes a front-stage H-bridge converter and a rear-stage isolated LLC resonant converter. The method includes four stages:

[0009] In phase 1, the normal unit module is in a stable operating state;

[0010] In phases 2 and 3, the busbar capacitors of the redundant unit modules are precharged. During the precharging period, the CHB controller controls the output voltage. To maintain voltage stability, excess power is required to charge the redundant unit modules. Considering the constant grid voltage, the AC input current will increase, resulting in a surge. To avoid the surge in input current, the CHB controller switches from the voltage-current dual loop to the input current single loop during the precharging phase. After the precharging is complete, the controller switches back to the voltage-current dual loop mode. Here,

[0011] In phase 2: As the bus voltage of the redundant unit modules increases, the modulation waves of all unit modules are reduced in real time to ensure that the total input voltage of all unit modules remains unchanged;

[0012] In stage three: to ensure the smoothness of the redundant unit being bypassed after the pre-charging is completed, the modulation wave of the redundant unit after the pre-charging is completed needs to be zero. Therefore, after the redundant unit module is charged to a certain value, on the basis of the reduction of the modulation wave of the redundant unit module in stage two, the modulation wave of the redundant unit module is multiplied by a coefficient k that changes linearly from one to zero. t , so that the modulation wave of the redundant unit module becomes zero after the pre-charging is completed. At the same time, in order to ensure that the total input voltage of all unit modules remains unchanged, the modulation wave of the normal unit module needs to be dynamically adjusted;

[0013] After the redundant unit module is pre-charged and bypassed, it enters stage four: After the redundant unit is pre-charged, when its bus capacitor voltage drops slightly due to natural discharge, the redundant unit module is reinserted into the system for charging and bypassed again after charging is complete to maintain its bus voltage constant. To maintain smooth switching in and out of the redundant unit module, its modulation wave is close to zero in this stage. When a unit module in the system fails, the redundant unit module can quickly switch into the system and work after the faulty module is removed.

[0014] The specific implementation of the control circuit of the solid-state transformer seamless fault-tolerant control method based on cascade H-bridge pre-charging can be as follows: it includes setting a front-stage CHB controller (referred to as the front-stage controller) and a rear-stage LLC controller (referred to as the rear-stage controller). The front-stage controller is used to control the front-stage H-bridge converter in the cascade module, and the rear-stage controller controls the rear-stage isolated LLC converter of the cascade unit module. The front-stage controller is used to control the output voltage and realize grid-side power factor correction, and the rear-stage controller is used to control the bus voltage in the middle of the cascade unit module. More importantly, in the present invention, the solid-state transformer seamless fault-tolerant control method based on cascade H-bridge pre-charging is divided into four stages. In each stage, the front-stage controller and the rear-stage controller need to be appropriately improved and adjusted.

[0015] Optionally, since the CHB controller is responsible for controlling the output voltage, in order to maintain voltage stability during the pre-charging of the redundant unit modules in phases 2 and 3, it is necessary to use excess power to charge the redundant unit modules. Since the grid voltage is constant, this process will cause the AC input current to rise, thereby generating a surge. To avoid input current surges, during the pre-charging phase of the redundant unit modules, the CHB controller needs to switch from a voltage-current dual loop to a single input current loop. Figure 5 As shown, a pre-stage control switching module is introduced into the CHB controller. In the pre-charging stage of the redundant unit module, the control mode of the CHB controller is switched with the help of the pre-stage control switching module (505). Specifically, "Precharge" is used as a pre-charging flag. When the system is in the pre-charging stage (i.e., stage two and stage three), "Precharge" is set to 1, and the CHB controller switches from the voltage-current dual-loop mode to the input current single-loop mode. When the system is not in the pre-charging stage, "Precharge" is set to 0, and the CHB controller switches back to the original voltage-current dual-loop mode.

[0016] Optionally, in stage 2, the modulation wave v of all unit modules mp As the redundant unit module bus voltage increases, it decreases, such as Figure 5 As shown, a modulation wave reduction module is introduced into the CHB controller, and the modulation wave signal v calculated by the CHB controller is reduced by the modulation wave reduction module (504). m Perform dynamic adjustment. Modulation wave signal v mp With v m The relationship is:

[0017]

[0018] Among them, N is the number of normal unit modules, V buspr Redundant unit module M r The bus voltage changes as the redundant unit module is charged during the fault-tolerant control process, Vbuspi Unit module M under fault-tolerant control i The bus voltage, V bus_rated It is the rated value of the bus voltage, which is also the bus voltage value of each module during normal operation.

[0019] Optionally, in stage 3, to ensure that the modulation wave of the redundant unit module is just reduced to zero when the bus voltage of the redundant unit module is charged to the rated value, after the bus voltage of the redundant unit module rises to a certain level, the modulation wave of the redundant unit module is multiplied by a coefficient k that changes linearly from one to zero based on the change of the modulation wave in stage 2. t , so that the modulation wave v of the redundant unit module mr′ After the pre-charge is completed, it becomes zero. At the same time, in order to ensure that the total input voltage of all unit modules remains unchanged, it is necessary to dynamically correct the modulation wave of the normal unit module. After correction, it is v mp′ .like Figure 6 As shown, a modulation wave correction module is introduced into the CHB controller, and the modulation wave correction module (604) is used to correct the modulation wave signal v of the previous CHB. m Make dynamic corrections.

[0020] Define the modulation wave coefficient k m :

[0021]

[0022] Modulation wave v of the redundant unit module in stage three mr′ The expression is:

[0023] v mr' =k t ·k m ·v m

[0024] where k t is a function that varies linearly with time from one to zero.

[0025] Modulation wave v of redundant unit module mr′ After the change, in order to ensure that the total input voltage of all unit modules remains unchanged, the modulation wave of the normal unit module needs to be adjusted accordingly. In the third stage, the modulation wave v of the normal unit module mp′ The expression is:

[0026] v mp' =((1-k m )·(1-k t )+k m )·v m

[0027] Optionally, in stage 4, after the redundant unit is pre-charged, when the bus capacitor voltage decreases slightly due to natural discharge, the bus capacitor of the redundant unit module is intermittently charged, the redundant unit module is reinserted into the system for charging, and bypassed again after charging is completed to maintain the bus voltage of the redundant unit module unchanged. To ensure the smoothness of switching in and out of the redundant unit module, the value of its modulation wave v mr′' At this stage, it is close to zero. When a unit module (such as unit module 3) fails in the system and is bypassed, the redundant unit module is quickly inserted into the system to replace the failed unit module. Figure 7 As shown, a drive control module is introduced into the CHB controller, and the drive control module (704) is used to control the drive pulses of the redundant unit module and the faulty unit module. "Undervoltage" in the drive control module (704) is the undervoltage flag of the redundant unit module bus voltage. When the bus voltage drops to the undervoltage threshold of the bus capacitor voltage due to natural discharge, "Undervoltage" is set to 1, and the switch S1 (7041) switches to "a". Otherwise, the value of "Undervoltage" is 0, and the switch S1 (7041) switches to "b". "Fault_Flag" is the system fault flag. When a system fault occurs (only the fault of unit module 3 is used as an example in the figure), "Fault_Flag" is set to 1, switch S2 (7042) is opened, and switch S3 (7043) switches to "e". Otherwise, "Fault_Flag" is set to 0, switch S2 (7042) is closed, and switch S3 (7043) switches to "d".

[0028] Optionally, when a unit module (for example, unit module 3) in the system fails, Figure 8 As shown, the back-stage controller of the faulty unit module stops running, and the back-stage controller of the redundant unit module starts running.

[0029] The present invention addresses the problem of unit module failure in a solid-state transformer system by precharging the bus capacitors of the redundant unit modules, and dividing the entire control process into four stages. The present invention can ensure the stability of the system input voltage and current in each stage and between stages. The insertion and bypass process of the redundant unit modules has almost no impact on the system, and seamless fault-tolerant control can be performed. The present invention can effectively solve the problems of long charging time of redundant unit modules, overload of remaining unit modules, and excessive impact of AC inductor current generated in the process of removing the faulty unit module and inserting the redundant unit module in the cascade converter. The method does not require additional circuits and is applicable to systems with bidirectional and unidirectional power flow. It can achieve rapid and smooth switching of unit modules when a fault occurs, ensuring reliable and stable operation of the entire system. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a circuit diagram of a solid-state transformer composed of N normal unit modules and one redundant unit module in an embodiment of the present invention.

[0031] Figure 2 This is a circuit diagram of a solid-state transformer composed of three normal unit modules and one redundant unit module in one embodiment of the present invention.

[0032] Figure 3 yes Figure 2 The front-end control block diagram of the instance under normal operating conditions.

[0033] Figure 4 yes Figure 2 The back-end control block diagram of the instance during normal operation.

[0034] Figure 5 Yes Figure 2 The example shows a front-stage control block diagram of the first half stage (stage 2) of pre-charging using the control method of the present invention.

[0035] Figure 6 Yes Figure 2 The example shows a front-stage control block diagram of the second half of the pre-charging stage (stage three) using the control method of the present invention.

[0036] Figure 7 Yes Figure 2 The example adopts the control method of the present invention in the front-stage control block diagram under fault state.

[0037] Figure 8 yes Figure 2 The following is the block diagram of the post-stage control of a normal unit module under an example fault state.

[0038] Figure 9 This is the phasor diagram of the voltage phasors on the input side under the condition of stable operation of normal unit modules in stage one.

[0039] Figure 10 1 is a phasor diagram of the voltage phasors on the input side during precharging of the redundant unit module.

[0040] Figure 11 Yes Figure 2 Example: A phasor diagram of the voltage phasors on the input side during precharging of a redundant unit module using the control method of the present invention.

[0041] Figure 12 Yes Figure 2 Example A schematic diagram of the variation curves of the modulation waves of all unit modules and the bus voltage of the redundant unit modules during the entire pre-charging process using the control method of the present invention. DETAILED DESCRIPTION

[0042] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The embodiments are only used to illustrate the specific implementation and corresponding effects of the present invention and are not intended to limit the present invention.

[0043] The method of the present invention is useful for Figure 1 The solid-state transformer shown (composed of multiple cascaded unit modules with input ends connected in series and output ends connected in parallel, the input end of each cascaded unit module is connected in series with a filter inductor and then connected to the AC power grid, and each unit module includes a front-stage H-bridge converter and a rear-stage isolated LLC resonant converter) can all be applied.

[0044] According to a specific embodiment of the present invention, Figure 2 The circuit diagram of a single-phase solid-state transformer is composed of three normal unit modules and one redundant unit module, wherein the unit modules M1, M2 and M3 are the three unit modules in normal operation. r It is a redundant unit module. The input of each cascade unit module is connected in series with the filter inductor and then connected to the AC grid. The output adopts a parallel structure to maintain a stable DC voltage. The cascade unit module consists of a front-stage H-bridge converter (CHB) and a rear-stage isolated LLC resonant converter. The front-stage H-bridge converter of each cascade unit module processes the energy of the medium-voltage grid, reduces the equivalent input voltage of each cascade unit by series connection, and converts it to a lower DC bus voltage. The energy is then fed into the rear-stage LLC resonant converter. After high-frequency isolation, the output of all cascade units is connected in parallel to provide energy for the DC load.

[0045] The method includes four stages: stage one is the stage of stable operation of normal unit modules; stages two and three are the stages of precharging redundant unit modules. In the precharging stage, to avoid overshoot of input current, the CHB controller is switched from the voltage-current dual loop to the input current single loop during the precharging stage of the redundant unit modules, and is switched back to the voltage-current dual loop mode after the precharging is completed. Stage four is the stage after the precharging of the redundant unit modules is completed. In stage four, the redundant unit modules are intermittently charged, and fault-tolerant control is performed when a unit module fails. Figure 3 This is the control block diagram of the front-stage CHB of the solid-state transformer during the first stage. Figure 4 This is a control block diagram of the subsequent LLC converter of each unit module of the solid-state transformer under normal system operation. Figure 5 This is the front-stage control block diagram of the second stage. As the bus voltage of the redundant unit module increases, the modulation wave of all unit modules is dynamically reduced to ensure the impact-free charging process. Figure 6This is the front-stage control block diagram of stage 3. To ensure that the modulation wave of the redundant unit module is reduced to zero when the bus voltage of the redundant unit module is charged to the rated value, after the redundant unit module is charged to a certain value, the modulation wave of the redundant unit module is multiplied by a coefficient k that changes linearly from one to zero based on the reduction of the modulation wave of the redundant unit module in stage 2. t , so that the modulation wave of the redundant unit module becomes zero after the pre-charging is completed. At the same time, in order to ensure that the total input voltage of all unit modules remains unchanged, the modulation wave of the normal unit module needs to be dynamically adjusted. Figure 7 This is the front-end control block diagram of stage 4. After the bus capacitor of the redundant unit module is pre-charged to the rated voltage, its bus capacitor voltage will drop slightly due to natural discharge. The redundant unit module is inserted into the system for intermittent charging. After the redundant unit module is fully charged, it is bypassed again. To ensure that the insertion and bypass of the redundant unit module are impact-free, its modulation wave v mr′' At this stage, the voltage is close to zero. When a unit module in the solid-state transformer fails, the redundant unit module is put into use. The redundant unit module and the failed unit module generate the same driving pulse to ensure that the redundant unit module can quickly replace the failed unit module. Figure 8 This is a block diagram of the back-end control when a fault occurs. When a fault occurs, the back-end controller of the faulty unit module is stopped and the back-end controller of the redundant unit module is started.

[0046] Alternatively, as Figure 3 As shown, the front-stage CHB controller in the first stage comprises three parts: a voltage control module (301), a current control module (302) and a carrier phase shift SPWM module (303); the voltage control module (301) is used to control the output voltage V of the entire solid-state transformer system. o , which is specifically: the output voltage reference value V o_ref Subtract the output voltage sampling value V o The error value obtained is used as the input of the voltage controller, and the output signal of the voltage controller is used as the input signal of the current control module (302), wherein the voltage controller is a proportional integral controller (PI controller). The current control module (302) is used to control the grid current to achieve power factor correction, specifically as follows: First, the grid current i g Perform abc / dq coordinate transformation to obtain d-axis component and q-axis component, and use the output signal of the voltage control module (301) as the reference value i of the d-axis component of the grid current. d_ref , and the reference value of the q-axis component is 0, the d-axis and q-axis reference values are respectively subtracted from the d-axis component and q-axis component of the grid current, and the resulting errors are respectively passed through the current controller to generate the d-axis component v of the modulation signal md and the q-axis component v mq , and then v md and vmq Perform dq / abc coordinate transformation according to the grid voltage phase to obtain the duty cycle fundamental modulation signal v of the front-stage H-bridge converter. m , wherein the current controller is a proportional integral controller (PI controller). The carrier phase shift SPWM module (303) is used to generate switching pulses for all the switch tubes of all the H-bridge converter modules in the preceding stage, compare the fundamental wave modulation wave signal with the high-frequency carrier signal, and obtain a driving pulse signal that changes with the modulation wave signal as the switching signal for the switch tubes of the H-bridge converter modules in the preceding stage.

[0047] Optionally, since the CHB controller is responsible for controlling the output voltage, in order to maintain voltage stability during the pre-charging of the redundant unit modules in phases 2 and 3, it is necessary to use excess power to charge the redundant unit modules. Since the grid voltage is constant, this process will cause the AC input current to rise, thereby generating a surge. To avoid input current surges, during the pre-charging phase of the redundant unit modules, the CHB controller needs to switch from a voltage-current dual loop to a single input current loop. Figure 5 As shown, in the pre-charge phase of the redundant unit module, the control mode of the CHB controller is switched with the help of the front-stage control switching module (505). Specifically, "Precharge" is used as the pre-charge flag. When the system is in the pre-charge phase (i.e., phase two and phase three), "Precharge" is set to 1, and the CHB controller switches from the voltage-current dual-loop mode to the input current single-loop mode. When the system is not in the pre-charge phase, "Precharge" is set to 0, and the CHB controller switches back to the original voltage-current dual-loop mode.

[0048] Optionally, in stage 2, the modulation wave v of all unit modules mp As the redundant unit module bus voltage increases, it decreases, such as Figure 5 As shown, the modulation wave reduction module (504) is used to calculate the modulation wave signal v of the CHB controller. m Make dynamic adjustments.

[0049] Optionally, in stage 3, to ensure that the modulation wave of the redundant unit module is just reduced to zero when the bus voltage of the redundant unit module is charged to the rated value, after the bus voltage of the redundant unit module rises to a certain level, the modulation wave of the redundant unit module is multiplied by a coefficient k that changes linearly from one to zero based on the change of the modulation wave in stage 2. t , so that the modulation wave v of the redundant unit module mr′ After the pre-charge is completed, it becomes zero. At the same time, in order to ensure that the total input voltage of all unit modules remains unchanged, the modulation wave of the normal unit module needs to be dynamically corrected to v mp′ .like Figure 6As shown, the modulation wave correction module (604) is used to dynamically correct the modulation waves of the normal unit module and the redundant unit module.

[0050] Optionally, in phase 4, after the redundant unit is pre-charged, when the bus capacitor voltage is slightly reduced due to discharge, the bus capacitor of the redundant unit module is intermittently charged, and when a unit module (for example, unit module 3) in the system fails, fast fault-tolerant control is performed. At this time, the front-stage CHB control block diagram is as follows Figure 7 As shown, the driving control module (704) is used to control the driving of the redundant unit module and the faulty unit module.

[0051] Optionally, when a unit module (for example, unit module 3) in the system fails, Figure 8 As shown, the subsequent controller of the faulty unit module stops running, and the subsequent controller of the redundant unit module starts running.

[0052] Optionally, the modulation wave signal of each unit module adjusted during the second and third stages is based on the bus voltage V buspi (i=1,2,3,r), original modulated wave signal v m and coefficient k t The specific derivation process is as follows:

[0053] Under normal unit module operation, the phasor diagram of each voltage phasor on the input side is as follows: Figure 9 As shown, where v Lg is the voltage on the grid inductor, v si (i=1,2,3) is the input voltage of each module. Lg The phasor representation of is as follows:

[0054]

[0055] The phasor diagram of each voltage phasor on the input side during the pre-charging period of the redundant unit module is as follows: Figure 10 As shown, where v Lgp is the voltage on the grid inductor during the pre-charging period (indicated by the subscript p). Lgp The phasor representation of is as follows:

[0056]

[0057] In order to avoid input current fluctuations during pre-charging, the voltage on both sides of the inductor must remain constant. Therefore, assuming that the inductor voltage remains unchanged, the voltage phasor diagram during the pre-charging process of the redundant unit module is as follows: Figure 11 As shown, the voltage phasor is represented as follows:

[0058]

[0059] Formula (1) can be written as:

[0060]

[0061] Substituting equation (4) into equation (3), we can obtain the scalar form since the voltage direction of each unit module is the same during normal operation and pre-charging:

[0062] v s1p +v s2p +v s3p +v srp =v s1 +v s2 +v s3 (5)

[0063] Taking the average of the switching cycles at both ends of equation (5) yields:

[0064]

[0065] Under normal operating conditions, take unit module 1 as an example, and derive its input voltage v s1 and bus voltage V bus1 The relationship is as follows. In different switching states, v s1 They are:

[0066]

[0067] Assume S 11 The duty cycle is D. When bipolar sinusoidal pulse width modulation (SPWM) is used, equation (7) can be written as:

[0068]

[0069] Where T s is the switching period, and the average of the switching period in formula (8) is:

[0070] <v s1 > Ts =V bus1 ·D+(-V bus1 )·(1-D)=V bus1 (2D-1) (9)

[0071] In one switching cycle, the amplitude of the modulation wave can be considered constant. According to the basic principle of SPWM, the duty cycle D can be expressed as follows:

[0072]

[0073] where v mis the modulation wave of the front-stage H bridge of the unit module, V tri is the amplitude of the triangular carrier. Substituting equation (10) into equation (9) yields:

[0074]

[0075] Set the carrier amplitude V tri If is 1, then formula (11) can be simplified to:

[0076] <v s1 > Ts =V bus1 ·v m (12)

[0077] The input voltage v of the normal unit module under normal operating conditions can be written si and their respective bus voltages V busi and modulation wave v m Relationship (i=1,2,3):

[0078]

[0079] Similarly, the input voltage v of all unit modules during the pre-charging process can be written as sip and their respective bus voltages V buspi and modulation wave v mpi Relationship (i=1,2,3,r):

[0080]

[0081] Substituting equations (13) and (14) into equation (6), we can obtain:

[0082]

[0083] Under normal working conditions, under the control of the post-stage controller of each unit module, the bus voltage of each unit module is basically stable at the rated bus voltage value, so it can be regarded as a constant. Then formula (15) can be further written as:

[0084] V busp1 ·v mp1 +V busp2 ·v mp2 +V busp3 ·v mp3 +V buspr ·v mpr =3V bus_rated ·v m (16)

[0085] Where V bus_rated is the rated value of the unit module bus voltage.

[0086] Because the front-stage CHB controller adopts carrier phase shift modulation, the modulation waves of all unit modules on the left side of equation (16) are equal:

[0087] v mp1 =v mp2 =v mp3 =v mpr =v mp (17)

[0088] Substituting formula (17) into formula (16) yields:

[0089] (V busp1 +V busp2 +V busp2 +V buspr )·v mp =3V bus_rated ·v m (18)

[0090] From the above formula, we can deduce the modulation wave v in the second stage mp Relationship with other quantities:

[0091]

[0092] Where V buspr V is the bus voltage of the redundant module during the pre-charging process, which changes with the charging of the redundant unit module. busp1 、V busp2 and V busp3 They are the bus voltages of normal unit modules M1, M2 and M3 during the pre-charging process respectively.

[0093] During the pre-charging process of the redundant unit module, the value of the modulation wave calculated by the control loop v m According to formula (19), dynamic adjustment can be made according to the bus voltage of each unit module to maintain the stability of the system voltage and current.

[0094] To ensure that the redundant unit module can be bypassed without impact after the pre-charging is completed, the modulation wave of the redundant unit module must be zero. Therefore, when the bus capacitance of the redundant unit module is charged to a certain value, it enters the third stage, and the modulation wave of the redundant unit module is multiplied by a linear transformation coefficient k. t At the same time, the modulation wave of the normal unit module needs to be corrected in real time to maintain the total input voltage of all unit modules unchanged.

[0095] like Figure 12 Assume that the time of stage one starts at t0, ends at t1 and enters stage two, ends at t2 and enters stage three, and ends at t3 and enters stage four.

[0096] The duration of stage 2 Δt1 and the duration of stage 3 Δt2 are defined as follows:

[0097]

[0098] Define the coefficient k t as follows:

[0099]

[0100] Define the modulation wave coefficient k m :

[0101]

[0102] In order to suppress the ripple of the cascade H-bridge rectifier output, the bus capacitor of each unit module is usually very large. Combined with the function of controlling the bus voltage by the post-stage LLC controller, the bus voltage of the normal unit module can be approximately the same as the rated bus voltage V bus_rated Equal. Formula (22) can be rewritten as follows:

[0103]

[0104] At the same time, the modulation wave v of each unit module in stage 2 in formula (19) mp Rewritten as:

[0105] v mp =k m ·v m (twenty four)

[0106] Modulation wave v of the redundant unit module in stage three mr′ The expression is:

[0107] v mr' =k t ·k m ·v m (25)

[0108] The total input voltage of all unit modules in phase 2 and phase 3 is V s and V s 'Respectively:

[0109]

[0110] where v mp′ It is the modulation wave of the normal unit module in stage three.

[0111] Normal unit module bus voltage V buspi (i=1,2,3) can be approximated as the rated bus voltage V bus_rated , formula (26) is rewritten as:

[0112]

[0113] Let the total input voltage of all unit modules in stage 2 and stage 3 be equal:

[0114] V s =V s' (28)

[0115] Combining equations (24), (25), (27) and (28), we can get the modulation wave v of the normal unit module in the third stage: mp′ :

[0116] v mp' =((1-k m )·(1-k t )+k m )·v m (29)

[0117] The schematic diagram of the modulation wave of all unit modules and the bus voltage of redundant unit modules during the entire pre-charging process is shown in the following figure: Figure 12 As shown, the analysis is as follows:

[0118] 1) The period from time t0 to t1 is stage 1, and the normal unit module maintains stable operation. At this time, the modulation wave of the normal unit module is the value v calculated by the CHB controller m ;

[0119] 2) The period from time t1 to t2 is the second stage. At t1, the bus capacitor of the redundant unit module begins to charge. At this time, the modulation wave of the redundant unit module is consistent with that of the normal unit module. As the bus voltage of the redundant unit module rises, the modulation wave v of all unit modules increases. mp Dynamically reduce to ensure that the total input voltage of all unit modules remains unchanged and avoid input current shock;

[0120] 3) The period from t2 to t3 is stage 3. When the redundant unit module is charged to t2, its bus voltage reaches V buspr1 , enter the third stage, based on the reduction of the redundant unit module modulation wave in the second stage, give the redundant unit module modulation wave v mr′ Multiply by a coefficient k that changes linearly from one to zero t , at this time the modulation wave v of the redundant unit module mr′ The faster it drops, the faster the total input voltage of all unit modules will decrease. To ensure that the total input voltage of all unit modules remains unchanged, the modulation wave v of the normal unit modules needs to be adjusted. mp' Perform dynamic adjustment, the modulation wave v of the normal unit module mp' Will gradually rise, when time reaches t3, the modulation wave v of the redundant unit module mr′ Has become zero, the modulation wave v of the normal unit module mp'Return to normal value at stage 1 m , the bus voltage of the redundant unit module is charged to the rated value V bus_rated , at this time, the redundant unit module is bypassed;

[0121] 4) After time t3 comes stage 4, when the modulation wave v of the redundant unit module mr′' Close to zero, the modulation wave of the normal unit is the value v calculated by the CHB controller m .

[0122] Then, the duration of phase 2 Δt1 and phase 3 Δt2 are derived. The bus voltage of the redundant unit rises from zero to V at the end of phase 2. buspr1 At the end of stage three, the bus voltage rises to the rated value V bus_rated .

[0123] In SPWM, it is assumed that the carrier wave is a triangle wave and the switching frequency is f s , in a carrier period T s Internal, on-time t on Proportional to the modulation ratio m, that is, t on (t) = m·T s On a longer time scale, the H-bridge DC side current i dc (t) is considered to be composed of a series of pulses. The alternating current is i g (t), whose amplitude is I m , in each carrier cycle, considering full-wave rectification, the average value of the DC side current pulse is:

[0124]

[0125] The modulation ratio of the redundant unit module in the second stage is m·k m , the capacitor current after full-wave rectification in stage 2 is:

[0126] I dcr =m·k m I m |sin(ωt)| (31)

[0127] The capacitor charging current is the average current after full-wave rectification:

[0128]

[0129] At the beginning of Phase 2, V buspr (t1)=0, when V buspr (t2) = V buspr1 , separate the variables and integrate equation (32):

[0130]

[0131] The integral result on the left side of formula (33) is:

[0132]

[0133] The average value of the integral on the right side of formula (33) |sin(ωt)| within one power frequency period T = 2π / ω is:

[0134]

[0135] Then the integral on the right side of formula (33) is approximately:

[0136]

[0137] Combining equations (33), (34) and (36), we can get the duration of stage 2, Δt1:

[0138]

[0139] Next, the duration Δt2 of phase three is derived. The modulation ratio of the redundant unit module in phase three is m·k m ·k t , the average current after H-bridge full-wave rectification of the redundant unit module in stage three is:

[0140]

[0141] The charging current of the redundant unit module bus capacitor is equal to the average current after full-wave rectification:

[0142]

[0143] Let τ = t - t2, then τ∈[0,Δt2], and Equation (39) can be rewritten as:

[0144]

[0145] In formula (40), at the beginning of stage 3, V buspr (0) = V buspr1 , at the end of V buspr (Δt2)=V bus_rated , then we can separate the variables and integrate equation (40) to obtain:

[0146]

[0147] The integral result on the left side of formula (41) is:

[0148]

[0149] Assuming that the period of |sin(ω(τ+t2))| on the right side of equation (41) is T=2π / ω, which is shorter than the capacitor charging time, the coupling integral on the right side of equation (41) can be approximately decomposed into:

[0150]

[0151] Where <|sin(ωτ)|> is the average value of the integral of |sin(ω(τ+t2))|, which is 2 / π as obtained from formula (35).

[0152] Then the integral result on the right side of formula (41) is approximately:

[0153]

[0154] Combining equations (41), (42) and (44) yields Δt2:

[0155]

[0156] This paper studies fault-tolerant methods for unit modules in solid-state transformers. To address the long charging times and high current surges experienced by redundant unit modules during traditional cold standby switching, the present invention proposes a fault-tolerant control method for precharging redundant unit modules. This method precharges the busbar capacitors of redundant unit modules using a cascaded H-bridge. The modulation waveforms of each unit module are adjusted in real time based on the redundant unit module's busbar voltage and the busbar voltage of a normal unit module, ensuring smooth precharging of the redundant module. After the redundant unit module is precharged, the modulation waveform and drive pulses of the H-bridge preceding the redundant unit module are controlled to intermittently charge its busbar capacitors to maintain a constant voltage. When a normal unit module fails, the redundant unit module can be quickly and smoothly inserted into the system to replace the faulty unit module. This method effectively avoids the long charging times of redundant unit modules, overload of the remaining unit modules, and excessive AC inductive current surges that occur during cold standby fault-tolerant control. The method is applicable to systems with both bidirectional and unidirectional power flow.

[0157] The embodiments described above are only partial solutions of the present invention and are not intended to limit the present invention. Persons skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent substitution or equivalent transformation falls within the scope of protection of the present invention.

Claims

1. A seamless fault-tolerant control method for a solid-state transformer based on cascaded H-bridge precharging, wherein the solid-state transformer comprises a plurality of cascaded unit modules, wherein the input ends of the modules are connected in series and the output ends are connected in parallel to form a cascade structure, wherein the input ends of the cascaded unit modules are connected in series with a filter inductor and then connected to an AC power grid, and each cascaded unit module comprises a front-stage H-bridge converter and a rear-stage isolated LLC resonant converter; characterized in that: The method comprises four stages: In phase 1, the normal unit modules work in a stable operating state; In phases two and three, the busbar capacitors of the redundant cell modules are precharged. During this process, the CHB controller used to control the cascaded H-bridge is regulated to avoid input current surges. In phase two, as the busbar voltage of the redundant cell modules rises, the modulation waves of all cell modules are reduced in real time. In phase three, to ensure smooth bypass of the redundant cells after precharging, the modulation waves of the redundant cell modules are controlled to zero after precharging. At the same time, the modulation waves of the normal cell modules are dynamically adjusted to ensure no input current surges. After the redundant unit module is pre-charged and bypassed, it enters phase 4, where the redundant unit module is intermittently charged to maintain its bus voltage unchanged. When a unit module in the system fails and is bypassed, the redundant unit module can quickly switch into the system and work.

2. A solid-state transformer seamless fault-tolerant control method based on cascaded H-bridge precharging according to claim 1, characterized in that: To avoid input current shock, during the pre-charging phase of the redundant unit module in phases 2 and 3, the CHB controller needs to switch from the voltage and current dual loop to the input current single loop. When the system is not in the pre-charging phase, the CHB controller switches back to the voltage and current dual loop mode.

3. The method for seamless fault-tolerant control of a solid-state transformer based on cascaded H-bridge precharging according to claim 2, characterized in that: A pre-stage control switching module (505) is introduced into the CHB controller, and the pre-stage control switching module (505) switches the control mode of the CHB controller. In the pre-stage control switching module, "Precharge" is used as a pre-charging flag. When the system is in the pre-charging stage, i.e., stage two and stage three, "Precharge" is set to 1, and the CHB controller is switched from the voltage-current dual-loop mode to the input current single-loop mode. When the system is not in the pre-charging stage, "Precharge" is set to 0, and the CHB controller switches back to the original voltage-current dual-loop mode.

4. The method for seamless fault-tolerant control of a solid-state transformer based on cascaded H-bridge precharging according to claim 1, characterized in that: In the second stage, the modulation wave v of all unit modules mp As the bus voltage of the redundant unit module increases, it decreases. Based on the CHB controller in stage 1, the modulation wave signal v is calculated. m Perform dynamic adjustment to obtain a new modulation wave signal v mp , v mp With v m The relationship is: Among them, N is the number of normal unit modules, V buspr Redundant unit module M r The bus voltage changes as the redundant unit module is charged during the fault-tolerant control process, V buspi Unit module M under fault-tolerant control i The bus voltage, V bus_rated It is the rated value of the bus voltage, which is also the bus voltage value of each module during normal operation.

5. A solid-state transformer seamless fault-tolerant control method based on cascaded H-bridge precharging according to claim 4, characterized in that: In order to ensure that the redundant unit module can be bypassed without impact after the pre-charging is completed, the modulation wave of the redundant unit module needs to be reduced to zero at this time. Therefore, after the bus voltage of the redundant unit module rises to a certain level, it enters stage three. On the basis of the modulation wave change in stage two, the modulation wave of the redundant unit module is multiplied by a coefficient k that changes linearly from one to zero. t , so that the modulation wave v of the redundant unit module mr' After the pre-charge is completed, it becomes zero. At the same time, in order to ensure that the total input voltage of all unit modules remains unchanged, the modulation wave of the normal unit module is dynamically corrected to v mp′ ;have: Define the modulation wave coefficient k m : Modulation wave v of the redundant unit module in stage three mr′ The expression is: v mr' =k t ·k m ·v m where k t is a function that varies linearly with time from one to zero; The modulation wave v of the normal unit module during the third stage mp′ The expression is: v mp' =((1-k m )·(1-k t )+k m )·v m。 6. The method for seamless fault-tolerant control of solid-state transformers based on cascaded H-bridge precharging according to claim 1, characterized in that: In the fourth stage, after the redundant unit is pre-charged, when the bus capacitor voltage is slightly reduced due to natural discharge, the bus capacitor of the redundant unit module is intermittently charged, and the redundant unit module is reinserted into the system for charging. After the charging is completed, it is bypassed again to maintain the bus voltage of the redundant unit module unchanged. In order to ensure the smoothness of the redundant unit module switching in and out, the value of its modulation wave v mr″ At this stage, the voltage is close to zero. When a unit module in the system fails and is bypassed, the redundant unit module is quickly inserted into the system to replace the failed unit module.

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