A method for diagnosing and locating open-circuit fault of MMC switching tube based on double sliding mode observer

Abstract

The application discloses a kind of based on double sliding mode observer's MMC switch tube open-circuit fault diagnosis and positioning method, comprising: (1) the first i sub-module of three-phase upper and lower bridge arms in MMC is conceptually combined to form subunit;(2) when MMC normal operation, the capacitor voltage of sub-module in each subunit is added, and the capacitor voltage and U si Of subunit is obtained;The sliding mode observer of capacitor voltage and is designed, whether the switch tube of this subunit occurs open-circuit fault is judged according to the residual error of its estimated value and actual value;(3) the sliding mode observer of bridge arm current is designed, and the residual error of the estimated value and actual value of this bridge arm current is detected to judge whether the switch tube of this bridge arm occurs upper or lower open-circuit fault;(4) when two sliding mode observers both detect that switch tube appears open-circuit fault, the sub-module that occurs fault can be positioned.The application can effectively reduce the time of fault diagnosis and positioning process, so that MMC system has faster detection speed without increasing additional hardware circuit.

Description

Technical Field

[0001] This invention relates to the field of fault diagnosis and location of power electronic equipment, and in particular to a method for diagnosing and locating open-circuit faults in MMC switching transistors based on a dual sliding mode observer. Background Technology

[0002] MMC is considered one of the most promising power electronic topologies for flexible DC transmission and currently occupies a very important position in the field of medium- and high-voltage flexible DC transmission. MMC's high modularity eliminates the need for expensive and bulky transformers, demonstrating excellent development prospects.

[0003] MMC (Multi-Module Control) consists of numerous submodules, and the switching performance of each submodule directly affects the reliable operation of the system. Therefore, taking measures to ensure the normal operation of the MMC is essential. When a submodule experiences an open-circuit fault, failure to locate the faulty submodule in time can lead to widespread system damage and more serious safety issues. Fault diagnosis is crucial to protecting the MMC from the effects of faults. Open-circuit faults in submodules directly affect the capacitor voltage of the MMC submodules. Commonly used methods for MMC fault diagnosis include Kalman filtering, neural networks, and sliding mode observers.

[0004] Kalman filtering utilizes the state equations of a linear system to make an optimal estimate of the system state using system input and output observation data, but it must preserve the previous state values. For example, Chinese patent document CN108872790A discloses an MMC fault diagnosis method based on Kalman filtering and support vector machines, which detects and locates MMC faults by collecting circulating current and phase voltage data.

[0005] Neural networks, primarily used to construct MMC (Multi-Level Cell) neural networks, evolved from traditional observers. Neural networks possess excellent learning capabilities and have unique applications in fault diagnosis. After training, a neural network can be used to observe the residuals that appear when an MMC fault occurs. The advantages of this method are its ability to accurately determine faults in real time, its fast operating speed, and its excellent performance even in noisy environments. However, implementing the neural network structure is difficult, and the algorithm design is relatively complex.

[0006] Sliding mode observers are a class of dynamic systems that derive estimates of state variables from the actual values ​​of the system's input and output variables. Because sliding mode observers inherently handle system uncertainties and exhibit good robustness to disturbances, they not only provide a practical possibility for the technical implementation of state feedback but also find practical applications in many aspects of control engineering. By measuring the input and output of the actual system, sliding mode observers can estimate the current state value of the system. They use nonlinear high-gain feedback to force the estimated state to approximate a hyperplane, making the estimated output equivalent to the true output. However, traditional submodule sliding mode observers have slow detection speeds and cannot quickly detect faults, increasing the risk to the system.

[0007] In summary, how to efficiently diagnose and locate faults when a submodule experiences an open-circuit fault in its switching transistor, and how to locate open-circuit faults in multiple submodules, has become an urgent problem to be solved. Summary of the Invention

[0008] To overcome the shortcomings of slow detection speed, this invention provides a method for diagnosing and locating open-circuit faults in MMC switching transistors based on dual sliding mode observers. This method can flexibly achieve MMC fault diagnosis and location without adding additional sensors and peripheral circuits, by using a sliding mode observer of the sub-unit capacitor voltage and a sliding mode observer of the bridge arm current in a sub-module combination.

[0009] A method for diagnosing and locating open-circuit faults in MMC switching transistors based on a dual sliding mode observer includes the following steps:

[0010] (1) Conceptually combine the i-th sub-module of the upper and lower arms of the three-phase bridge in MMC to form a new three-phase observation unit (here called a sub-unit);

[0011] (2) When the MMC is running normally, the sum of the capacitor voltages of all submodules in each subunit is U. si ;

[0012] Design a sliding mode observer for the capacitor voltage of a sub-unit, and determine whether the switching transistor of the sub-unit has an open circuit fault based on whether the residual between its estimated value and the actual value exceeds the normal fluctuation range.

[0013] (3) Design a sliding mode observer for the bridge arm current to detect whether the residual between the estimated value and the actual value of the bridge arm current exceeds the normal fluctuation range, thereby determining whether the upper / lower switch of the bridge arm has an open circuit fault.

[0014] (4) When both the sliding mode observer of the sub-unit capacitor voltage and the sliding mode observer of the bridge arm current detect an open circuit fault in the switching transistor, the faulty sub-module can be located.

[0015] In step (1), the modulation wave phase difference of the i-th sub-module of each phase upper and lower bridge arm is 180°, the phase of the voltage fluctuation of the upper and lower bridge arm capacitors is opposite, and the fluctuation of the voltage of the upper and lower bridge arm capacitors is reduced.

[0016] The phase difference between the capacitor voltage fluctuations of the i-th submodules of the three-phase upper / lower bridge arms is 60°, and the total capacitor voltage U si The fluctuations were greatly suppressed, and the value was basically maintained at a constant level.

[0017] The specific process of step (2) is as follows:

[0018] (2-1) When the MMC is working normally, according to the MMC topology, the differential equations of the capacitor voltage of the i-th submodule in the upper and lower arms of phase j (j=u,v,w) are as follows:

[0019]

[0020] In the formula, S jpi and S jni These are the switching functions for the i-th submodule of the upper and lower bridge arms, respectively; i jpi and i jni These are the bridge arm currents of the upper and lower bridge arms of phase j, respectively; i j The output current of phase j on the AC side; u jpi and u jni C represents the upper / lower bridge arm voltage of the i-th sub-unit of phase j, respectively. u For capacitors;

[0021] (2-2) For the i-th sub-unit, design the observation voltage U si A sliding mode observer; wherein, one sub-unit consists of six sub-modules, and the sub-unit observes the voltage U. si This is the sum of the capacitor voltages of all submodules within a subunit, i.e.:

[0022] U si =u wpi +u wni +u vpi +u vni +u upi +u uni (2)

[0023] Based on equations (1) and (2), we obtain:

[0024]

[0025] The above equation can be expressed as a state equation:

[0026]

[0027] In the formula, This represents the estimated sum of the voltages across the sub-unit capacitors, and:

[0028] i = i dc [1...1] T i a =i u [1...1] T i b =i v [1...1] T i c =i w [1...1] T

[0029] Choose x as the observed variable, and define... This leads to the sliding mode observer equation for the sum of the sub-unit capacitor voltages:

[0030]

[0031] The saturation function is:

[0032]

[0033] Where M is the gain value of the sliding mode observer, the larger the value of M, the stronger the convergence ability, and h is the threshold of the sub-unit sliding mode observer;

[0034] (2-3) Set the threshold h of the sub-unit sliding mode observer to half of the peak-to-peak value of the normal voltage fluctuation of the sub-unit. Based on the residual between the estimated value and the actual value of the observed voltage of the sub-unit and the set threshold, perform fault monitoring on the sub-unit.

[0035] If the residual between the two exceeds the set threshold and persists for one millisecond, it can be determined that the sub-unit has experienced an open-circuit fault in the switching transistor.

[0036] The specific process of step (3) is as follows:

[0037] (3-1) For the sampling current of the upper and lower arms of phase j, according to Kirchhoff's voltage law, we have:

[0038]

[0039]

[0040]

[0041] In the formula, u jp and u jn These are the upper and lower bridge arm voltages of phase j, respectively, and i jp and i jnThese are the upper / lower bridge arm currents of phase j, and u jo It is the output voltage of phase j, U dc It is the DC side voltage, u jpi and u jni These are the capacitor voltages of the i-th submodule in the upper / lower arm of phase j, respectively, and S jpi and S jni These are the switching functions for the i-th submodule of the j-phase upper / lower arm, respectively.

[0042] (3-2) Design a sliding mode observer for the current of each phase arm, and rewrite equation (8) as follows:

[0043]

[0044] In the formula, This represents the estimated value of the bridge arm current, and:

[0045]

[0046] definition This establishes a sliding mode observer for the bridge arm current:

[0047]

[0048] Wherein, the saturation function is:

[0049]

[0050] Where M is the gain value of the sliding mode observer, the larger the value of M, the stronger the convergence ability, and h is the threshold of the sliding mode observer;

[0051] (3-3) Set the threshold h of the bridge arm current sliding mode observer to half of the peak-to-peak value of the normal fluctuation of the bridge arm current. Based on the residual between the estimated value and the actual value of the bridge arm current and the set threshold, perform fault detection on the bridge arm current.

[0052] When an open-circuit fault occurs in the upper switch and the current is less than zero, the current forms a loop through the anti-parallel diode of the lower switch, and the estimated value is too small. If the residual current of the bridge arm is detected to be greater than the set threshold and lasts for one millisecond, and the estimated value of the bridge arm current is less than the actual value, then it is determined that an open-circuit fault has occurred in the upper switch of that bridge arm.

[0053] Conversely, when a fault occurs in the lower switch and the current is greater than zero, the current forms a loop through the anti-parallel diode of the upper switch, resulting in an extra voltage level being applied, leading to an overestimation of the voltage level. If the residual is detected to be greater than the set threshold for one millisecond, and the estimated value of the bridge arm current is greater than the actual value, it can be determined that an open-circuit fault has occurred in the lower switch of that bridge arm.

[0054] The specific process of step (4) is as follows:

[0055] The sliding mode observers for capacitor voltage and current are used to determine if the i-th sub-unit has a fault, and the sliding mode observers for arm current are used to determine if the arm has an open circuit fault in the upper / lower switch. When both sliding mode observers detect the fault at the same time, the i-th sub-module of the arm can be located to have an open circuit fault in the upper / lower switch.

[0056] Compared with the prior art, the present invention has the following beneficial effects:

[0057] 1. This invention improves the sensitivity of the sliding mode observer, introduces a highly efficient dual sliding mode observer into MMC, and is simple in principle and easy to implement.

[0058] 2. This invention can quickly achieve MMC fault diagnosis and location without adding additional hardware circuits.

[0059] 3. This invention can be applied to fault diagnosis and location under open circuit faults of different numbers of submodules in MMC. The method is highly adaptable, easy to expand, and has low computational load.

[0060] 4. Under the threshold conditions set by this invention, the maximum time required for fault detection and location by this invention shall not exceed 15ms. Attached Figure Description

[0061] Figure 1 Examples of three-phase MMC circuit topologies and their combinations under normal conditions;

[0062] Figure 2 Flowchart of sub-unit voltage fault diagnosis method;

[0063] Figure 3 Flowchart of bridge arm current fault diagnosis method;

[0064] Figure 4 Flowchart for locating faulty submodules;

[0065] Figure 5 Simulation diagram of estimated and actual values ​​for voltage fault diagnosis in the first sub-unit under open-circuit fault of the upper switch transistor;

[0066] Figure 6 Simulation diagram of estimated and actual values ​​of the upper arm current of phase U under open circuit fault of the upper switch transistor;

[0067] Figure 7 Simulation diagram of estimated and actual values ​​for voltage fault diagnosis in the first sub-unit under open-circuit fault of the lower switching transistor;

[0068] Figure 8 Simulation diagram showing the estimated and actual values ​​of the upper arm current fault diagnosis of phase u under the condition of open circuit fault of the lower switch transistor. Detailed Implementation

[0069] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the embodiments described below are intended to facilitate the understanding of the present invention and do not constitute any limitation thereof.

[0070] like Figure 1 The diagram shows the MMC topology for each submodule and the most common combination method. Each phase has two arms, upper and lower, for a total of 6 arms in the topology. Each arm has N (N is a natural number greater than or equal to 1) submodules. Taking the i-th submodule combination in each phase as an example, six submodules form a complete subunit, with submodules of the same color forming one subunit. The sum of the capacitor voltages of all submodules in each subunit is U. si .

[0071] A method for diagnosing and locating open-circuit faults in MMC switching transistors based on a dual sliding mode observer includes the following steps:

[0072] Step 1: Conceptually combine the i-th sub-module of the upper and lower arms of the three-phase bridge in the MMC to form a new three-phase observation unit (referred to as a sub-unit).

[0073] Step 2: During normal operation of the MMC, the capacitor voltages of the sub-modules in each sub-unit are summed to obtain the sub-unit capacitor voltage and U. si ;

[0074] Design a sliding mode observer for the sub-unit capacitor voltage. Based on whether the residual between the estimated and actual values ​​exceeds the normal fluctuation range, if the residual is detected to be greater than half the peak-to-peak value of the normal voltage fluctuation of the sub-unit and remains so for one millisecond, a switching open-circuit fault in the sub-unit can be detected. The specific process is as follows: Figure 2 As shown

[0075] Step 3: Design a sliding mode observer for the bridge arm current to detect whether the residual between the estimated and actual values ​​of the bridge arm current exceeds the normal fluctuation range. When the residual is detected to be greater than half of the peak-to-peak value of the normal fluctuation of the bridge arm current, and remains so for one millisecond, an open-circuit fault in the upper / lower switching transistor of the bridge arm can be detected. The specific process is as follows: Figure 3 As shown.

[0076] Step 4: When both the sliding mode observers of the sub-unit capacitor voltage and the bridge arm current detect an open-circuit fault in the switching transistor, the i-th sub-module of that bridge arm can be located to have an open-circuit fault in the upper / lower switching transistor. The specific process is as follows: Figure 4 As shown.

[0077] like Figure 5The figure shows a simulation of the estimated and actual values ​​of the voltage fault diagnosis for the first sub-unit when the upper switching transistor experiences an open-circuit fault at 0.805s. After the fault occurs, the residual between the estimated and actual values ​​of the sub-unit sliding mode observer gradually increases. When the residual exceeds the set threshold, it indicates that the residual has exceeded half of the peak-to-peak value of the normal fluctuation. After maintaining this residual for one millisecond, it can be determined that an open-circuit fault has occurred in this sub-unit.

[0078] like Figure 6 The figure shows a simulation of the estimated and actual values ​​of the upper arm current fault diagnosis for phase u when the upper switch experiences an open-circuit fault at 0.805s. When the residual exceeds the set threshold, it indicates that the residual is greater than half of the normal peak-to-peak fluctuation. After maintaining this for one millisecond, it can be determined that the upper arm switch has experienced an open-circuit fault. When the upper switch fails and the current is less than zero, the fault submodule cannot be activated, according to the arm current state equation:

[0079]

[0080] u jp It is the voltage of the upper arm of phase j, i jp It is the current in the upper arm of phase j, u jo It is the output voltage of phase j, U dc It is the DC side voltage, u jpi S is the capacitor voltage of the i-th submodule of the j-phase upper bridge arm. jpi This is the switching function of the i-th submodule of the j-phase upper arm. It can be concluded that when the current is less than zero and the estimated value is less than the actual value, it can be determined that the submodule of that arm has experienced an open-circuit fault in its upper switching transistor.

[0081] If both the 2ms sub-unit and the bridge arm current sliding mode observer detect the fault, it can be located that the first sub-module of the bridge arm has an open circuit fault in the upper switch transistor.

[0082] like Figure 7 The figure shows a simulation of the estimated and actual values ​​of the voltage fault diagnosis for the first sub-unit when the lower switching transistor experiences an open-circuit fault at 0.815s. After the fault occurs, the residual between the estimated and actual values ​​of the sub-unit analog observer gradually increases. When the residual exceeds the set threshold, it indicates that the residual has exceeded half of the peak-to-peak value of the normal fluctuation. After maintaining this residual for one millisecond, it can be determined that an open-circuit fault has occurred in this sub-unit.

[0083] like Figure 8The figure shows a simulation of the estimated and actual values ​​of the upper arm current fault diagnosis for phase u when the lower switch experiences an open-circuit fault at 0.815s. When the residual exceeds the set threshold, it indicates that the residual has exceeded half of the normal peak-to-peak fluctuation. After maintaining this for one millisecond, an open-circuit fault is determined to have occurred in this arm. When the lower switch fails and the current is greater than zero, the submodule cannot switch out, according to the arm current state equation:

[0084]

[0085] u jn It is the voltage of the lower arm of phase j, i jn It is the lower arm current of phase j, u jo It is the output voltage of phase j, U dc It is the DC side voltage, u jni S is the capacitor voltage of the i-th submodule of the j-phase upper bridge arm. jni It is the switching function of the i-th submodule of the j-phase lower arm.

[0086] It can be concluded that when the current is greater than zero and the estimated value is greater than the actual value, it can be determined that the submodule of the bridge arm has experienced an open circuit fault of the lower switch transistor.

[0087] If the fault is detected by both the sub-unit voltage and the bridge arm current sliding mode observer within 2ms, it can be located that the first sub-module of the bridge arm has an open circuit fault in the lower switch transistor.

[0088] Table 1 shows the specific parameters of the three-phase MMC circuit topology in the embodiments of the present invention.

[0089] Table 1

[0090]

[0091] The embodiments described above provide a detailed explanation of the technical solutions and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for diagnosing and locating open-circuit fault of MMC switching tube based on double sliding mode observer, characterized in that, The method comprises the following steps: (1) combining the i-th sub-module of the upper and lower bridge arms of three phases in MMC conceptually to form a new three-phase observation unit, referred to as a sub-unit; (2) When the MMC is in normal operation, the capacitor voltage of each submodule of each subunit is U si ; a sliding mode observer of the capacitor voltage and of the sub-unit is designed, and whether the residual error between the estimated value and the actual value exceeds the normal fluctuation range is judged to determine whether the switch tube of the sub-unit has an open circuit fault; the specific process is as follows: (2-1) when the MMC is normally working, according to the MMC topology, the differential equation of the capacitor voltage of the i-th sub-module of the upper and lower bridge arms of phase j (j = u, v, w) is obtained as follows: In the formula, S jpi and S jni are the switching functions of the i-th sub-module of the upper and lower arms, respectively; i jpi and i jni are the arm currents of the j-phase upper and lower arms, respectively; i j is the j-phase AC side output current; u jpi and u jni are the i-th sub-unit upper / lower arm voltages of the j-phase, respectively; C u is the capacitor; (2-2) For the i-th subunit, the design observation voltage U si of the sliding mode observer; wherein one subunit is composed of six submodules, and the subunit observation voltage U si is the sum of the capacitor voltages of each submodule inside a subunit, that is: U si = u wpi + u wni + u vpi + u vni + u upi + u uni (2) According to formula (1) and formula (2), the following formula is obtained: The above formula is expressed as a state equation: wherein represents an estimate of the sub-cell capacitance voltage and is given by: i = i dc [1...1] T , i a = i u [1...1] T , i b = i v [1...1] T , i c = i w [1...1] T The x is selected as the observation variable, and the definition is The sliding mode observer equation of the subunit capacitor voltage and is established as follows: wherein the saturation function is: wherein M is the gain value of the sliding mode observer, the greater the value of M, the stronger the convergence ability, and h is the threshold value of the sliding mode observer of the sub-unit; (2-3) the threshold value h of the sliding mode observer of the sub-unit is set as half of the peak-to-peak value of the voltage fluctuation of the sub-unit, and the residual error between the estimated value and the actual value of the observation voltage of the sub-unit and the set threshold value are used to monitor the fault of the sub-unit; (3) a sliding mode observer of the bridge arm current is designed to detect whether the residual error between the estimated value and the actual value of the bridge arm current exceeds the normal fluctuation range, so as to determine whether the upper / lower switch tube of the bridge arm has an open circuit fault; (4) when the sliding mode observer of the capacitor voltage and of the sub-unit and the sliding mode observer of the bridge arm current both detect that the switch tube has an open circuit fault, the sub-module having the fault can be located; the specific process is as follows: The sliding mode observer of the capacitor voltage and is used to determine that the i-th sub-unit has a fault, and the sliding mode observer of the bridge arm current is used to determine that the upper / lower switch tube of the bridge arm has an open circuit fault; when the two sliding mode observers both detect that a fault occurs, the i-th sub-module of the bridge arm is determined to have an upper / lower switch tube open circuit fault.

2. The MMC switching tube open-circuit fault diagnosis and positioning method based on a double sliding mode observer according to claim 1, characterized in that, In step (1), the phase difference of the modulation wave of the i-th sub-module of the upper and lower bridge arms of each phase is 180°.

3. The MMC switching tube open-circuit fault diagnosis and positioning method based on a double sliding mode observer according to claim 1, characterized in that, In step (1), the phase difference of the capacitor voltage fluctuation of the i-th sub-module of the upper and lower bridge arms of three phases is 60°.

4. The MMC switching tube open-circuit fault diagnosis and positioning method based on a double sliding mode observer according to claim 1, characterized in that, In step (2-3), when it is detected that the residual error exceeds the set threshold value and lasts for one millisecond, it is determined that the switch tube of the sub-unit has an open circuit fault.

5. The MMC switching tube open-circuit fault diagnosis and positioning method based on a double sliding mode observer according to claim 1, characterized in that, The specific process of step (3) is as follows: (3-1) for the sampled current of the bridge arm of phase j, according to the Kirchhoff voltage law, the following formula is obtained: wherein u jp and u jn are the upper / lower bridge arm voltages of phase j, i jp and i jn are the upper / lower bridge arm currents of phase j, u jo is the output voltage of phase j, U dc is the DC side voltage, u jpi and u jni are the capacitor voltages of the i-th sub-module of the upper / lower bridge arm of phase j, S jpi and S jni are the switching functions of the i-th sub-module of the upper / lower bridge arm of phase j; (3-2) a sliding mode observer of the bridge arm current of each phase is designed, and formula (8) is rewritten as the following equation: In the formulae, denotes the bridge arm current estimate, and: Definitions From this the bridge leg current sliding mode observer equation is established: wherein the saturation function is: wherein M is the gain value of the sliding mode observer, the greater the value of M, the stronger the convergence ability, and h is the threshold value of the sliding mode observer; (3-3) the threshold value h of the sliding mode observer of the bridge arm current is set as half of the peak-to-peak value of the normal fluctuation of the bridge arm current, and the residual error between the estimated value and the actual value of the bridge arm current and the set threshold value are used to detect the fault of the bridge arm current.

6. The MMC switching tube open-circuit fault diagnosis and positioning method based on a double sliding mode observer according to claim 5, characterized in that, In step (3-3), if it is detected that the residual error of the bridge arm current is greater than the set threshold value and lasts for one millisecond, and the estimated value of the bridge arm current is less than the actual value, it is determined that the upper switch tube of the bridge arm has an open circuit fault; If the bridge arm current residual is detected to be greater than a set threshold and lasts for a millisecond, and the estimated value of the bridge arm current is greater than the actual value, it can be judged that the lower switch tube of the bridge arm has an open circuit fault.

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