A fault diagnosis method and system for a cascaded h-bridge converter

Abstract

The application relates to the technical field of electrical engineering fault diagnosis, and provides a fault diagnosis method and system of a cascade H-bridge converter. g dc The system is determined whether an open-circuit fault of a power device occurs and a suspicious fault tube pair is determined by comparing with a preset threshold value; the DC bus voltage equalization control is cut off from the system, the DC bus voltage equalization control of the remaining modules except the fault module is restored after the fault module is located; the fault module is short-circuited by changing the driving signal of the power device of the fault module, and the specific fault switch tube is located; the system is restored to stable work by using the redundancy of the modulation coefficients of other modules. After the open-circuit fault occurs, the fault device is quickly, efficiently and reliably located and the system is restored to normal work by temporarily cutting off the voltage equalization control.

Description

Technical Field

[0001] This invention relates to the field of electrical engineering fault diagnosis technology, specifically to a fault diagnosis method and system for a cascaded H-bridge converter. Background Technology

[0002] Compared to traditional two-level converters, multilevel converters (MLBs) have the main advantage of smaller output voltage steps, resulting in lower harmonic content and better electromagnetic compatibility. Cascaded H-bridge MLBs are widely used in photovoltaic systems, static synchronous compensators (SSCs), and other fields due to their high modularity, high voltage capacity, low harmonic content, and strong fault tolerance. However, CHBs contain a large number of power devices, which significantly increases the likelihood of power device failure. Power device failures are categorized into open-circuit and short-circuit faults. Although the time interval between a short-circuit fault and its significant impact is relatively short, requiring rapid detection and isolation, its fault characteristics are quite obvious and can be quickly and accurately detected and located using hardware integrated into the power device. However, open-circuit faults are much more subtle and difficult to identify quickly and accurately. Among these, the probability of a single power device failing is the highest, making it a hot topic in previous research.

[0003] The 2019 publication, "A State Estimator-Based Approach for Open-Circuit Fault Diagnosis in Single-Phase Cascaded H-Bridge Rectifiers," published in IEEE Transactions on Industry Applications, 2019, 55(2):1608-1618, proposes a fault diagnosis strategy based on grid current residuals to locate open-circuit faulted power devices. This diagnostic scheme derives an estimated expression for the grid current by mathematically modeling the cascaded H-bridge inverter system. The grid current residual is then obtained by subtracting the estimated and measured values, and subsequently used to diagnose and locate the fault. However, this scheme requires high-precision voltage / current sensors due to the high accuracy requirements for capacitor voltage and grid current, resulting in high diagnostic costs.

[0004] The 2014 publication, "Research on Fault Diagnosis and Fault-Tolerant Control of Cascaded STATCOM" (Yang Xiaodong, China University of Mining and Technology), proposes an artificial intelligence-based fault diagnosis scheme. This scheme utilizes wavelet analysis to perform multi-resolution analysis of the DC bus voltage, and then uses artificial intelligence to classify the feature vectors obtained from wavelet decomposition under different fault types, thereby diagnosing and locating the fault. However, this scheme requires training on a large dataset and is significantly affected by parameters.

[0005] Some scholars have also proposed fault diagnosis schemes based on capacitor voltage. These schemes all locate the fault based on the rise of the DC bus voltage of the faulty module after an open-circuit fault in the power device. The paper "A FaultySubmodule Mathematical Model-Based Localization Strategy for Switch Open-Circuit Fault of Module Multilevel Converter" published in 2023 (IEEE Transactions on Power Electronics, 2023, 38(3): 3899-3916) derives an estimated expression for the capacitor voltage through mathematical modeling and calculates the capacitor voltage residual to locate the faulty module. The rising trend of the capacitor voltage over a period of time is monitored to identify the faulty module. The 2022 publication, "Online Diagnosis and Ride-Through Operation for Cascaded H-Bridge Converter Based STATCOM With a Single Open-Circuit IGBT" (IEEE Transactions on Industrial Electronics, 2022, 69(8):7549-7559), monitors the rising trend of capacitor voltage over a period of time to identify faulty modules. The 2023 publication, "A Novel Detection and Localization Approach of Open-Circuit Switch Fault for the Grid-Connected Modular Multilevel Converter" (IEEE Transactions on Industrial Electronics, 2023, 70(1):112-124), proposes an improved Pauta criterion to monitor abnormal capacitor voltage data and thus locate faulty modules. Both methods can effectively locate faulty modules.

[0006] However, the DC voltage equalization control in the cascaded H-bridge system suppresses the fault characteristic of capacitor voltage rise, reducing the robustness and reliability of the fault diagnosis strategy. Furthermore, in existing fault diagnosis schemes, the location of the faulty power device and fault tolerance are separated, affecting the speed at which the system recovers. Summary of the Invention

[0007] The technical problem to be solved by this invention is to improve the efficiency and reliability of open-circuit fault devices in a cascaded H-bridge converter.

[0008] The present invention solves the above-mentioned technical problems through the following technical means:

[0009] This invention provides a fault diagnosis method for a cascaded H-bridge converter, comprising:

[0010] S10. Use a current sensor to collect the transient AC grid current and extract its DC component. The DC component The absolute value is compared with the preset DC component threshold. If it exceeds the DC component threshold, it is determined that the system has an open circuit fault in the power device, the suspected faulty transistor pair is identified, and step S20 is continued; otherwise, step S10 is returned to be executed.

[0011] S20. Disconnect the DC bus voltage equalization control from the system and locate the faulty module;

[0012] S30. Restore the DC bus voltage equalization control of all modules except the faulty module; by changing the drive signal of the power device of the faulty module and monitoring the change trend of the capacitor voltage of the faulty module under the drive signal, the faulty module is short-circuited; by utilizing the redundancy of the modulation coefficients of other modules, the system is restored to stable operation.

[0013] S40. Based on the execution process of step S30 and the suspected faulty transistor pair determined in step S10, locate the specific faulty switch transistor.

[0014] Furthermore, the DC component threshold is specifically as follows:

[0015]

[0016] In the formula, β is the threshold coefficient, which takes a value between 10% and 20%.

[0017] Furthermore, the specific method for determining the faulty transistor pair is as follows:

[0018] For the aforementioned DC component If the polarity is positive, the suspected faulty transistor pair is S2 and S3; if the polarity of the DC component is negative, the suspected faulty transistor pair is S1 and S4.

[0019] Furthermore, the fault location module in step S20 includes the following steps:

[0020] (1) Record the value of capacitor voltage using a DC-side capacitor voltage sensor;

[0021] (2) Compare the capacitor voltage of each module with the preset capacitor voltage threshold. If the voltage exceeds the capacitor voltage threshold, the corresponding module is judged to have an open circuit fault.

[0022] Furthermore, the method of recording the capacitor voltage value using a DC-side capacitor voltage sensor specifically involves:

[0023] In the converter topology, N sensors are used to detect and record the capacitor voltage signal; the sensors have the same sampling frequency, and the number of sampling points in one cycle is:

[0024]

[0025] In the formula, T represents the period of the sensor output signal. s For controller cycles;

[0026] Define a data storage matrix, denoted as matrix A, to record the sampled values ​​of each capacitor voltage signal over the past cycle. i Where k is any sampling time t;

[0027] A i =[V Ci (kL-1) V Ci (kL) … V Ci (k-1) V Ci (k)]

[0028] The above data storage matrix is ​​periodically slid to obtain a periodic sliding window for acquiring the sampled values ​​of the three-phase current; the sampling window length L is the number of current sampling points within one period, L = 1 / (T s ·f0), where T s Let f be the controller period and f0 be the frequency of the output current. By storing the sampling points of the three-phase grid current within one cycle into three matrices, each new sampling deletes the oldest sampling point in the matrix and adds the new sampling point to the end of the matrix, thereby updating the matrix.

[0029] Furthermore, the setting of the capacitor voltage threshold is specifically as follows:

[0030]

[0031] In the formula, Here, α is the reference value for capacitor voltage, and α is the capacitor voltage threshold coefficient, which takes a value of 1.3-1.5.

[0032] The capacitor voltage fluctuates due to power pulsation:

[0033]

[0034] Further, step S30, which involves changing the drive signal of the power device of the faulty module and monitoring the change trend of the capacitor voltage of the faulty module under the drive signal, to short-circuit the faulty module, specifically involves:

[0035] Adjust the drive signal of the faulty module so that the switching transistors S1 and S3 of the faulty module remain on, while the switching transistors S2 and S4 remain off.

[0036] Monitor the voltage change trend of the faulty module. If the voltage remains unchanged or decreases slowly, it is determined that the switches S1 and S3 are normally turned on and the faulty module is short-circuited. Otherwise, adjust the drive signal of the faulty module so that the switches S2 and S4 of the faulty module remain on and the switches S1 and S3 remain off.

[0037] Furthermore, step S30, which utilizes the redundancy of modulation coefficients from other modules to restore the system to stable operation, specifically involves:

[0038] Assuming the current remains constant during normal system operation, a single-phase system must satisfy the following conditions to operate:

[0039] v a =v g +jωLi g

[0040] In the formula v a The output voltage of the cascaded H-bridge, v g For grid voltage, i g V is the mains current; where v a It is derived from the sum of the output voltages of each H-bridge:

[0041]

[0042] v is expressed as a modulus. i :

[0043] v i =(m di +m qi V ref

[0044] In the formula m di and m qi These are the modulation coefficients for the active and reactive components of the i-th H-bridge, respectively; and the modulation coefficients must satisfy the following constraints:

[0045]

[0046] v a It can be represented as:

[0047]

[0048] Furthermore:

[0049]

[0050] Therefore, the active and reactive power outputs of the cascaded H-bridge multilevel converter can be expressed as:

[0051]

[0052] Assuming the system is running normally, i g Remain unchanged, that is:

[0053]

[0054] With redundancy in the system's modulation coefficients, if a single module fails, the modulation coefficients of the remaining modules are increased, so that the cascaded H-bridge converter outputs active and reactive power that ensures stable system operation.

[0055] Further, step S40 specifically includes:

[0056] (1) When switch S1 and switch S3 remain on, the fault module is short-circuited; and the suspected faulty transistor pair determined in step S10 is S2 and S3, then the faulty switch is S2.

[0057] (2) When switch S1 and switch S3 remain on, the fault module is short-circuited; and the suspected faulty transistor pair determined in step S10 is S1 and S4, then the faulty switch is S4.

[0058] (3) When switch S2 and switch S4 remain on, the fault module is short-circuited; and the suspected faulty transistor pair determined in step S10 is S2 and S3, then the faulty switch is S3.

[0059] (4) When switch S2 and switch S4 remain on, the fault module is short-circuited; and the suspected faulty transistor pair determined in step S10 is S1 and S4, then the faulty switch is S1.

[0060] This invention also provides a fault diagnosis system for a cascaded H-bridge converter. The system executes the above-described method during operation and includes the following modules:

[0061] The system fault diagnosis module is used to collect the transient AC grid current using a current sensor and extract its DC component. The DC component The absolute value is compared with the preset DC component threshold. If it exceeds the DC component threshold, it is determined that the system has an open circuit fault in the power device, the suspected faulty transistor pair is identified, and the fault module location module continues to be executed; otherwise, it returns to the system fault determination module.

[0062] The fault module location module is used to disconnect the DC bus voltage equalization control from the system and locate the fault module.

[0063] The fault recovery module is used to restore the DC bus voltage equalization control of the modules other than the faulty module. It changes the drive signal of the power device of the faulty module and monitors the change trend of the capacitor voltage of the faulty module under the drive signal, causing the faulty module to short-circuit; and utilizes the redundancy of the modulation coefficients of other modules to restore the system to stable operation.

[0064] The fault device location module is used to locate the specific faulty switch tube based on the execution process of the fault recovery module and the suspected faulty tube pairs determined by the system fault determination module.

[0065] The advantages of this invention are:

[0066] (1) After a fault is diagnosed, the capacitor voltage equalization control is immediately removed from the system, which amplifies the fault characteristics of capacitor voltage rise and improves the speed and robustness of fault module diagnosis.

[0067] (2) Fault tolerance is achieved by utilizing the system’s modulation redundancy, without the need to invest in idle H-bridge modules, thus saving system operating costs.

[0068] (3) Combining the location of specific power devices with fault tolerance improves the speed of fault diagnosis.

[0069] (4) The data used for fault diagnosis and fault tolerance are the same amounts of data required in the control system. Therefore, there is no need to add additional voltage / current sensors, saving costs. Attached Figure Description

[0070] Figure 1 This is a flowchart illustrating a fault diagnosis method for a cascaded H-bridge converter according to an embodiment of the present invention;

[0071] Figure 2 This is a topology diagram of a single-phase cascaded H-bridge multilevel converter in an embodiment of the present invention;

[0072] Figure 3 This is a detailed flowchart of step S10 in an embodiment of the present invention;

[0073] Figure 4This is a schematic diagram of the output voltage of normal and faulty HB in an embodiment of the present invention, as well as its fundamental and DC output voltage components;

[0074] Figure 5 This is the DC equivalent circuit for power device failure in the embodiments of the present invention;

[0075] Figure 6 This is a performance analysis of the H-bridge when S1 fails in this embodiment of the invention;

[0076] Figure 7 This is a control block diagram of the cascaded H-bridge multilevel converter according to an embodiment of the present invention;

[0077] Figure 8 This is a schematic block diagram of the fault-clearing voltage equalization control in an embodiment of the present invention;

[0078] Figure 9 This is a schematic block diagram of the voltage balancing control of the health recovery module in an embodiment of the present invention;

[0079] Figure 10 These are the power grid current waveforms before and after the S1 open-circuit fault in this embodiment of the invention;

[0080] Figure 11 This is the DC component waveform of the power grid current before and after the S1 open circuit fault in this embodiment of the invention;

[0081] Figure 12 This embodiment of the invention compares the capacitor voltage before and after the voltage equalization control is removed under the conditions of healthy operation and power device single tube failure (S1 open circuit) when the module parameters are inconsistent.

[0082] Figure 13 This is the trend of capacitor voltage change of the fault module before and after the fault in the embodiment of the present invention;

[0083] Figure 14 This is a curve showing the change in capacitor voltage of the fault module during the fault tolerance process in this embodiment of the invention;

[0084] Figure 15 These are the capacitor voltage curves of each module after the faulty module is short-circuited in this embodiment of the invention. Detailed Implementation

[0085] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0086] Example 1

[0087] This embodiment provides a fault diagnosis method for a cascaded H-bridge converter, the process of which is as follows: Figure 1 As shown, it includes:

[0088] S10. Use a current sensor to collect the transient AC grid current and extract its DC component. The DC component The absolute value is compared with the preset DC component threshold. If it exceeds the DC component threshold, it is determined that the system has an open circuit fault in the power device, the suspected fault pair is identified, and step S20 is continued; otherwise, step S10 is returned to be executed.

[0089] For the aforementioned DC component If the polarity is positive, the suspected faulty transistor pair is S2 and S3; if the polarity of the DC component is negative, the suspected faulty transistor pair is S1 and S4.

[0090] It should be noted that the topology of a single-phase cascaded H-bridge multilevel converter is as follows: Figure 2 As shown, by changing the drive signals of each power device—specifically, changing the drive signal that rapidly switches between high and low levels during normal operation to a continuous low level—an open-circuit fault in the power device is simulated. Therefore, the output voltage of the cascaded H-bridge converter undergoes a transient change, which in turn causes the grid current to generate a DC component related to the open-circuit fault in the power device.

[0091] The specific process of S10 is as follows: Figure 3 As shown, this embodiment introduces an open-circuit fault in a power device during normal operation of the cascaded H-bridge system. The open-circuit fault is simulated by changing the drive signal of the power device. When the drive signal of the faulty power device is set to a low level, its open-circuit fault is simulated, causing a DC bias in the AC side current. Within a safety margin, this generates a DC component that can be identified and extracted by a low-precision current sensor. Because the damage caused by the open-circuit fault is hidden, the system will not immediately collapse. However, if the open-circuit fault is left unchecked, it may increase the damage to the system, increasing the current or voltage stress on other devices, leading to secondary system failures or even system collapse. Therefore, fault diagnosis needs to be fast, which places requirements on the selection of threshold values.

[0092] Detailed analysis of the output voltage components after different power devices in the HB (H-Bridge) fail, as follows: Figure 4 As shown. From Figure 4As can be seen, the output voltage of a normal HB (Hydraulic Module) obtains its fundamental component after passing through a low-pass filter (LPF), while the output voltage of a faulty HB is a superposition of the fundamental component and the DC component. In other words, the output voltage of a faulty HB can be considered as a reduced-order AC voltage source with DC voltage disturbance. It is worth noting that the polarity of the DC voltage disturbance in the faulty module depends on the location of the faulty power device. Figure 4 As shown, when an open circuit fault occurs in S1 or S4, the DC voltage disturbance is negative; when an open circuit fault occurs in S2 or S3, the DC voltage disturbance is positive.

[0093] Circuit analysis is performed when dealing with the DC voltage component generated by the faulty H-bridge. The simplified DC equivalent circuit of the system is as follows: Figure 5 As shown, the grid voltage U g Filter inductor L f Treating the healthy H-bridge as a DC short circuit, the fault HB is equivalent to a DC voltage source. It can be seen that the abnormal DC component in the grid current originates from the DC voltage of the faulty module, and the polarity of the output DC current of the phase containing the fault HB is always opposite to that of the other phases. Furthermore, as... Figure 5 As shown in the table at the bottom, the polarity of the DC current is determined solely by the position of the fault switch.

[0094] Generally, the ground threshold for the DC component of the grid current should be selected as follows:

[0095]

[0096] In the formula, β is the threshold coefficient, which takes a value between 10% and 20%.

[0097] S20. Disconnect the DC bus voltage equalization control from the system and locate the fault module; locating the fault module includes the following steps:

[0098] (1) Record the value of the capacitor voltage using a DC-side capacitor voltage sensor; specifically:

[0099] In the converter topology, N sensors are used to detect and record the capacitor voltage signal; the sensors have the same sampling frequency, and the number of sampling points in one cycle is:

[0100]

[0101] In the formula, T represents the period of the sensor output signal. s For controller cycles;

[0102] Define a data storage matrix, denoted as matrix A, to record the sampled values ​​of each capacitor voltage signal over the past cycle. i Where k is any sampling time t;

[0103] A i =[V Ci (kL-1)V Ci (kL)…V Ci (k-1)V Ci (k)]

[0104] The above data storage matrix is ​​periodically slid to obtain a periodic sliding window for acquiring the sampled values ​​of the three-phase current; the sampling window length L is the number of current sampling points within one period, L = 1 / (T s ·f0), where T s Let f be the controller period and f0 be the frequency of the output current. By storing the sampling points of the three-phase grid current within one cycle into three matrices, each new sampling deletes the oldest sampling point in the matrix and adds the new sampling point to the end of the matrix, thereby updating the matrix.

[0105] (2) Compare the capacitor voltage of each module with a preset capacitor voltage threshold. If the voltage exceeds the threshold, the corresponding module is determined to have an open-circuit fault. The capacitor voltage threshold is set as follows:

[0106]

[0107] In the formula, Here, α is the reference value for capacitor voltage, and α is the capacitor voltage threshold coefficient, which takes a value of 1.3-1.5.

[0108] The capacitor voltage fluctuates due to power pulsation:

[0109]

[0110] It should be noted that, as Figure 6 The figure shows the performance analysis of the H-bridge when S1 is faulty. λ represents the current direction, and when λ = 1, i g When i > 0 and λ = 0 g <0 (current flowing out of the H-bridge is in the positive direction)). [g1,g2,g3,g4] represents the switching sequence of the power devices. Figure 6 The upper and lower sub-diagrams show the current flow paths of S1 under normal and fault conditions, respectively. Figure 6 In (a), the current is in the positive direction. When an open circuit fault occurs in S1, the current does not flow through the capacitor, that is, the capacitor no longer discharges. Figure 6 In (b), λ = 1. When S1 fails, the capacitor switches from freewheeling to charging. However, when the current is in the negative direction, the faulty transistor S1 has no effect on HB. Figure 6As shown in (c), it can be seen that the output performance of the H-bridge is only affected when λ = 1. In short, when S1 fails, the capacitor voltage will rise, and this effect only lasts for half a current cycle. Similarly, it can be seen that any failure of any power device will lead to an increase in the capacitor voltage.

[0111] The control block diagram of the single-phase cascaded H-bridge in this example is as follows: Figure 7 As shown, it consists of three controllers: a cluster voltage controller, a current controller, and a DC bus voltage equalization controller. The d-axis reference value of the grid current is generated by the cluster voltage controller, which regulates the active power output of the inverter. The grid current sample is delayed by 1 / 4 cycle to generate the orthogonal component required for coordinate system transformation, thus obtaining the q-axis component of the sampled current. The q-axis component of the reference current controls the reactive power. Then, the modulation voltage reference value is obtained by the current controller through the adjustment result of the PI controller in the dq coordinate system using the reference grid current. To ensure equal capacitor voltages in each H-bridge, the modulation voltage of each submodule is obtained by feeding it into the DC bus voltage equalization controller.

[0112] The individual capacitor voltage controller in this system balances the capacitor voltage by controlling the active power flowing into each H-bridge capacitor. Therefore, this controller introduces an additional term into the AC voltage reference value for each H-bridge to control the active power distribution between the H-bridges. Furthermore, to ensure that the operation of this controller does not interfere with the grid current regulated by different controllers, the conditions necessary for decoupling the individual capacitor voltage controller from the rest of the control system must also be met.

[0113]

[0114] Violating the above formula will change the reference voltage generated by the current controller, thus directly affecting the current, and will also cause disturbances to the output of the cluster voltage controller, thus indirectly affecting the current.

[0115] To amplify the fault characteristic of a rise in capacitor voltage after a fault, it is necessary to switch the control mode of the cascaded H-bridge, as shown in the diagram. Figure 8 As shown. This ensures that the modulation voltage of each module equals the reference voltage output of the current controller, without injecting the capacitor voltage equalization control output adjustment amount. As mentioned in step S10 above, the grid current i g A noticeable DC bias will appear after the fault. However, after temporarily disabling the capacitor voltage equalization control at this point, i g There will be almost no noticeable changes; that is, temporarily disabling capacitor voltage equalization control will not exacerbate the system's fault or affect its health.

[0116] An open-circuit fault in the power device within the module causes a transient rise in the module's capacitor voltage. However, the suppression effect of the DC bus voltage balancing control makes this fault characteristic less noticeable. Therefore, by switching control, the DC bus voltage balancing control is removed from the system, amplifying the fault characteristic of the rising module capacitor voltage. Furthermore, by recording the transient capacitor voltage changes and comparing them with a pre-set capacitor voltage threshold, the faulty module can be identified. This embodiment amplifies the fault characteristic, thus avoiding the use of high-precision voltage sensors and eliminating the need for additional hardware, thereby saving monitoring costs.

[0117] S30. Restore DC bus voltage balancing control for all modules except the faulty module, such as... Figure 9 As shown, by changing the drive signal of the power device in the faulty module and monitoring the voltage change trend of the capacitor in the faulty module under this drive signal, the faulty module is short-circuited; by utilizing the redundancy of the modulation coefficients of other modules, the system is restored to stable operation; the specific steps are as follows:

[0118] Adjust the drive signal of the faulty module so that the switching transistors S1 and S3 of the faulty module remain on, while the switching transistors S2 and S4 remain off.

[0119] Monitor the voltage change trend of the faulty module. If the voltage remains unchanged or decreases slowly, it is determined that the switches S1 and S3 are normally turned on and the faulty module is short-circuited. Otherwise, adjust the drive signal of the faulty module so that the switches S2 and S4 of the faulty module remain on and the switches S1 and S3 remain off.

[0120] When a faulty module is short-circuited, the redundancy of the modulation coefficients of other modules is needed to restore the system to stable operation. The specific steps are as follows:

[0121] Assuming the current remains constant during normal system operation, a single-phase system must satisfy the following conditions to operate:

[0122] v a =v g +jωLi g

[0123] In the formula v a The output voltage of the cascaded H-bridge, v g For grid voltage, i g V is the mains current; where v a It is derived from the sum of the output voltages of each H-bridge:

[0124]

[0125] v is expressed as a modulus. i :

[0126] v i =(m di +m qi V ref

[0127] In the formula m di and m qi These are the modulation coefficients for the active and reactive components of the i-th H-bridge, respectively; and the modulation coefficients must satisfy the following constraints:

[0128]

[0129] v a It can be represented as:

[0130]

[0131] Furthermore:

[0132]

[0133] Therefore, the active and reactive power outputs of the cascaded H-bridge multilevel converter can be expressed as:

[0134]

[0135] Assuming the system is running normally, i g Remain unchanged, that is:

[0136]

[0137] With redundancy in the system's modulation coefficients, if a single module fails, the modulation coefficients of the remaining modules are increased, so that the cascaded H-bridge converter outputs active and reactive power that ensures stable system operation.

[0138] S40. Based on the execution process of step S30 and the suspected faulty transistor pair identified in step S10, locate the specific faulty switching transistor. The specific steps are as follows:

[0139] (1) When switch S1 and switch S3 remain on, the fault module is short-circuited; and the suspected faulty transistor pair determined in step S10 is S2 and S3, then the faulty switch is S2.

[0140] (2) When switch S1 and switch S3 remain on, the fault module is short-circuited; and the suspected faulty transistor pair determined in step S10 is S1 and S4, then the faulty switch is S4.

[0141] (3) When switch S2 and switch S4 remain on, the fault module is short-circuited; and the suspected faulty transistor pair determined in step S10 is S2 and S3, then the faulty switch is S3.

[0142] (4) When switch S2 and switch S4 remain on, the faulty module is short-circuited; and the suspected faulty transistor pair determined in step S10 is S1 and S4, then the faulty switch is S1.

[0143] This embodiment further elaborates on the proposed method by using a single-phase cascaded H-bridge multilevel converter operating in STATCOM (Static Synchronous Compensator) mode, and provides Matlab simulation results to verify the effectiveness of the proposed method.

[0144] Simulation parameters:

[0145] This is a single-phase cascaded H-bridge multilevel converter system. The number of H-bridge modules (N) is 4. The DC bus voltage is 20V, the effective value of the mains voltage is 50V, the mains voltage frequency is 50Hz, the filter inductor is 4mH, the rated DC capacitor of the H-bridge module is 1.7mF, the mains current amplitude is 4A, and the power factor is 0. Modulation uses single-frequency carrier phase-shift modulation technology with a carrier frequency of 10kHz. The equivalent switching frequency of the cascaded H-bridges is 20kHz.

[0146] Step (1) Issue an open-circuit fault command for the power device, collect the grid current, and determine the fault based on the DC component of the grid current:

[0147] A power device open-circuit fault command is issued, which changes the drive signal of power device S1 in the first module. Specifically, the drive signal, which normally switches rapidly between high and low levels, is changed to a continuous low level to simulate an open-circuit fault in the power device. The AC-side current sensor records the grid current and extracts the DC component. The DC component is compared with a pre-set threshold; if it exceeds the threshold, an open-circuit fault in the power device is determined. Based on the polarity of the DC component of the grid current, suspected faulty transistor pairs are identified. Specifically, if the polarity of the DC component is positive, the suspected faulty transistor pairs are S2 and S3; if the polarity of the DC component is negative, the suspected faulty transistor pairs are S1 and S4.

[0148] like Figure 10 and Figure 11 The figures show the waveforms of the grid current and its DC component before and after an open-circuit fault in the first module power device S1. It can be seen that an open-circuit fault occurred in S1 at t = 0.5s. After the fault, the grid current exhibited a negative DC bias, and its DC component exceeded the threshold, indicating an open-circuit fault at this point. Furthermore, based on the negative polarity of this DC component, S1 and S4 are identified as a suspected faulty transistor pair.

[0149] Step (2) Modify the parameter values ​​of the capacitors to make the parameters of each module inconsistent, and verify whether it affects the fault characteristics.

[0150] Specifically, the capacitor value of module 2 is changed to 1.6mF, the capacitor value of module 3 is changed to 1.8mF, and the capacitor value of module 4 is changed to 1.5mF. Therefore, the capacitor voltages of each module are different, meaning the parameters between the modules are inconsistent. For example... Figure 12 As shown, when module parameters are inconsistent, Figure 12 (a) and Figure 12 (b) Comparison of capacitor voltage before and after disabling voltage equalization control under healthy operation and single-transistor fault (S1 open circuit) conditions, respectively. From Figure 12 As shown in (a), due to parameter inconsistencies, the capacitor voltages of healthy CHB modules only deviate after voltage balancing control is released at t=0.5s, without a significant increase. This is because the active power borne by each SB is different when parameters are inconsistent. However, when an open-circuit fault occurs simultaneously with the release of voltage balancing control at t=0.5s, the capacitor voltage of the faulty module increases significantly. Therefore, parameter inconsistencies alone will not trigger a false diagnosis after the release of voltage balancing control.

[0151] Step (3) Switch the control mode of the H-bridge to remove the DC capacitor voltage equalization control from the system, amplify the fault characteristics, and locate the faulty module:

[0152] First, switch the control mode of the cascaded H-bridge to remove the DC bus voltage equalization control from the system. Specifically, remove the component output by the capacitor voltage equalization control from the modulation voltage of the four modules.

[0153] Then, the capacitor voltage value is recorded using a DC-side capacitor voltage sensor. The sampling frequency is set to 10kHz, and a sliding window is used for real-time sampling to obtain the sampling matrix.

[0154] Finally, the capacitor voltage is compared with a preset capacitor voltage threshold. If the capacitor voltage of a module exceeds the threshold, the module is considered faulty. The average capacitor voltage of the four modules is calculated using the sampling matrix described above. Based on the capacitor voltage threshold formula, α = 1.3 is selected, resulting in...

[0155] like Figure 13 As shown, the trend of capacitor voltage change of the faulty module before and after the fault is presented. After diagnosing the fault, the control mode of the H-bridge is switched, and the capacitor voltage of the faulty module rises rapidly. When it exceeds the threshold, the faulty module is located.

[0156] Step (4) Utilize system modulation redundancy to restore the system to healthy operation and locate the faulty power device during this process:

[0157] After locating the faulty module, DC bus voltage equalization control of the remaining modules was restored. By changing the drive signal of the power device in the faulty module, the module was short-circuited. Utilizing the redundancy of the modulation coefficients of other modules, the system was restored to normal operation. Simultaneously, the capacitor voltage change trend of the faulty module was monitored, and the specific faulty power device was located by combining this information with suspected faulty transistors.

[0158] like Figure 14 As shown, the change in capacitor voltage of the faulty module during the fault tolerance process is illustrated. After locating the faulty module, the drive signal of the faulty module is immediately changed to [1, 0, 1, 0]. That is, after turning on its S1 and S3, the capacitor voltage of the faulty module still shows an upward trend, indicating that the fault tolerance has failed. Combining the suspected faulty transistor pair S1 and S4 determined in step (1), the faulty power device S1 can be located.

[0159] Next, the drive signal of the faulty module was changed to [0, 1, 0, 1], which turned on S2 and S4, causing the faulty module to short-circuit and thus restoring the system to normal operation. Figure 15 The figure shows the capacitor voltage curves of each module after the faulty module is short-circuited. It can be seen that after the faulty module is short-circuited, the capacitor voltage of the faulty module decreases slowly; the capacitor voltages of the remaining modules fluctuate around the reference value, and the system returns to healthy operation.

[0160] Example 2

[0161] It should be further explained that, based on the same inventive concept, the present invention also provides a fault diagnosis system for a cascaded H-bridge converter. When the system is running, it executes the method described in Embodiment 1, and includes the following modules:

[0162] The system fault diagnosis module is used to collect the transient AC grid current using a current sensor and extract its DC component. The DC component The absolute value is compared with the preset DC component threshold. If it exceeds the DC component threshold, it is determined that the system has an open circuit fault in the power device, the suspected faulty transistor pair is identified, and the fault module location module continues to be executed; otherwise, it returns to the system fault determination module.

[0163] The fault module location module is used to disconnect the DC bus voltage equalization control from the system and locate the fault module.

[0164] The fault recovery module is used to restore the DC bus voltage equalization control of the modules other than the faulty module. It changes the drive signal of the power device of the faulty module and monitors the change trend of the capacitor voltage of the faulty module under the drive signal, causing the faulty module to short-circuit; and utilizes the redundancy of the modulation coefficients of other modules to restore the system to stable operation.

[0165] The fault device location module is used to locate the specific faulty switch tube based on the execution process of the fault recovery module and the suspected faulty tube pairs determined by the system fault determination module.

[0166] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A fault diagnosis method of a cascaded H-bridge converter, characterized in that, include: S10. Use a current sensor to collect the transient AC grid current and extract its DC component I. g dc DC component I g dc The absolute value is compared with the preset DC component threshold. If it exceeds the DC component threshold, it is determined that the system has an open circuit fault in the power device, the suspected faulty transistor pair is identified, and S20 is continued; otherwise, return to S10. S20. Disconnect the DC bus voltage equalization control from the system and locate the faulty module, including the following steps: The value of the capacitor voltage is recorded using a DC-side capacitor voltage sensor; The capacitor voltage of each module is compared with the preset capacitor voltage threshold. If it exceeds the capacitor voltage threshold, the corresponding module is judged to have an open circuit fault. S30. Restore the DC bus voltage equalization control of all modules except the faulty module; by changing the drive signal of the power device of the faulty module and monitoring the change trend of the capacitor voltage of the faulty module under the drive signal, the faulty module is short-circuited; by utilizing the redundancy of the modulation coefficients of other modules, the system is restored to stable operation. The method described above involves changing the drive signal of the power device in the faulty module and monitoring the voltage change trend of the capacitor in the faulty module under the drive signal, thereby short-circuiting the faulty module. Adjust the drive signal of the faulty module so that the switching transistors S1 and S3 of the faulty module remain on, while the switching transistors S2 and S4 remain off. Monitor the voltage change trend of the fault module's capacitor. If the capacitor voltage remains unchanged or decreases slowly, it is determined that switches S1 and S3 are normally turned on and the fault module is short-circuited. Otherwise, adjust the drive signal of the fault module so that switches S2 and S4 of the fault module remain on, while switches S1 and S3 remain off. The aforementioned use of modulation coefficient redundancy from other modules to restore system stability is specifically as follows: Assuming the current remains constant during normal system operation, a single-phase system must satisfy the following conditions to operate: wherein v a is the output voltage of the cascaded H-bridge, v g is the grid voltage, i g is the grid current; wherein v a is the output voltage of the cascaded H-bridge, with modulation degree v i : In the formula m di and m qi They are the first i The modulation coefficients of the active and reactive components of each H-bridge; and the modulation coefficients must satisfy the following constraints: v a may be expressed as: Furthermore: Therefore, the active and reactive power outputs of the cascaded H-bridge multilevel converter can be expressed as: Assuming normal operation of the system i g remains unchanged, i.e.: With redundancy in the system's modulation coefficients, after a single module fails, the modulation coefficients of the remaining modules are increased, so that the cascaded H-bridge converter outputs the active and reactive power necessary for stable system operation. S40. Based on the execution process of S30 and the suspected faulty transistor pair identified in S10, locate the specific faulty switching transistor, including: When switching transistors S1 and S3 remain on, the faulty module is short-circuited; and the suspected faulty transistor pair determined in step S10 is S2 and S3, then the faulty switching transistor is S2. When switching transistors S1 and S3 remain on, the faulty module is short-circuited; and the suspected faulty transistor pair determined in step S10 is S1 and S4, then the faulty switching transistor is S4. When switch S2 and switch S4 remain on, the faulty module is short-circuited; and the suspected faulty transistor pair determined in step S10 is S2 and S3, then the faulty switch is S3. When switching transistors S2 and S4 remain on, the faulty module is short-circuited; and the suspected faulty transistor pair determined in step S10 is S1 and S4, then the faulty switching transistor is S1.

2. The fault diagnosis method of the cascaded H-bridge converter according to claim 1, characterized in that, The DC component threshold is specifically: In the formula, is a threshold coefficient, taking a value between 10% and 20%.

3. The fault diagnosis method of the cascaded H-bridge converter according to claim 1, characterized in that, The specific steps for identifying suspected faulty transistor pairs are: For the direct current component I g dc If the polarity is positive, the suspicious faulty tube pair is S2 and S3. If the polarity of the DC component is negative, then the suspected faulty transistor pair is S1 and S4.

4. The fault diagnosis method of the cascaded H-bridge converter according to claim 1, characterized in that, The method of using a DC-side capacitor voltage sensor to record the capacitor voltage value specifically involves: In the converter topology, N Each sensor detects and records the capacitor voltage signal; the sensors have the same sampling frequency, and the number of sampling points in one cycle is: In the formula T The period of the sensor output signal. T s For controller cycles; The definition data storage matrix, respectively, records the sampling value of each capacitor voltage signal in the past one cycle, recorded as matrix A i ; wherein k is any sampling time t ; By periodically sliding the aforementioned data storage matrix, a periodic sliding window is obtained to acquire the sampled values ​​of the three-phase current; the sampling window length is... L 1 represents the number of current sampling points within one cycle. L 1 = 1 / ( T s · f 0), of which T s For the controller's cycle, f 0 represents the frequency of the output current; by storing the sampling points of the three-phase grid current within one cycle into three matrices, each new sampling deletes the oldest sampling point in the matrix and adds the new sampling point to the end of the matrix, thereby updating the matrix.

5. The fault diagnosis method of the cascaded H-bridge converter according to claim 1, characterized in that, The capacitor voltage threshold is set as follows: In the formula, is a capacitor voltage reference value, is a capacitor voltage threshold coefficient, and is 1.3-1.5; is an inherent fluctuation of the capacitor voltage due to power pulsation, and can be expressed as: 。 6. A fault diagnosis system of a cascaded H-bridge converter, characterized in that, When the system is running, it executes the method according to any one of claims 1-5, comprising the following modules: The system fault diagnosis module is used to collect the transient AC grid current using a current sensor and extract its DC component I. g dc The DC component I g dc The absolute value is compared with the preset DC component threshold. If it exceeds the DC component threshold, it is determined that the system has an open circuit fault in the power device, the suspected faulty transistor pair is identified, and the fault module location module continues to be executed; otherwise, it returns to the system fault determination module. The fault module location module is used to disconnect the DC bus voltage equalization control from the system and locate the fault module. The fault recovery module is used to restore the DC bus voltage equalization control of the modules other than the faulty module. It changes the drive signal of the power device of the faulty module and monitors the change trend of the capacitor voltage of the faulty module under the drive signal, causing the faulty module to short-circuit; and utilizes the redundancy of the modulation coefficients of other modules to restore the system to stable operation. The fault device location module is used to locate the specific faulty switch tube based on the execution process of the fault recovery module and the suspected faulty tube pairs determined by the system fault determination module.

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