Fault-tolerant control method and system for grid-connected converter based on model prediction

Abstract

The invention provides a grid-connected converter fault-tolerant control system and method based on model prediction, and belongs to the technical field of converter control, and the method comprises the steps: determining a fault device in a fault phase of a grid-connected converter based on the measured AC side current and PCC point current of the grid-connected converter; after a power device fault of a certain phase bridge arm of the grid-connected converter is confirmed, a direct-current side capacitor sharing type fault-tolerant control algorithm is put into use, gate signals of all power devices of the fault bridge arm are blocked at the same time, and a direct-current side capacitor replaces the fault bridge arm to support controllable operation of the converter.

Description

Technical Field

[0001] The present invention belongs to the technical field of converter control, and in particular relates to a fault-tolerant control method and system for a grid-connected converter based on model prediction. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] In recent years, the power electronics industry has entered a stage of rapid development. As the penetration rate of new energy sources in my country continues to rise, the types and number of converters used in power grids are increasing. At the same time, the power grid has higher and higher requirements for the efficiency, miniaturization and reliability of power electronics equipment. i C.G. a The third generation of wide bandgap semiconductor materials such as N is still under development. The second generation of power electronic devices, insulated gate bipolar transistors (IGBTs), have been widely used in power conversion and transmission due to their advantages such as fast switching speed, small on-state voltage drop, small switching loss, large safe operating area, resistance to pulse current shocks, and low driving power.

[0004] However, in practical applications, the switching frequency of power electronic converters is as high as hundreds of kilohertz. Under such high-frequency working conditions, it is easy to cause problems such as over-limit junction temperature of switching devices, performance degradation, and electromagnetic interference, which can lead to switch failure and other faults. When a converter fails, abnormal operating conditions such as phase loss and voltage reduction will occur, and even endanger the reliability and safety of the entire grid-connected system. Therefore, the fault-tolerant operation of the grid-connected converter is crucial. In short, converters usually operate under high voltage, strong current, strong electromagnetic interference, high-frequency switching speed, and limited heat dissipation conditions. Therefore, power module failures and commutation failures caused by abnormal power electronic switching devices or disconnection of connecting wires are frequent, and even cause serious economic losses. Therefore, it is very necessary to study the fault-tolerant control method and device of the grid-connected converter.

[0005] The mainstream device used in the bridge arm switch device of the three-level converter is the insulated gate bipolar transistor (IGBT), and its main fault types are short circuit fault and open circuit fault. The main cause of open circuit fault is overcurrent, poor contact or drive damage. The three common fault types of the power module of the three-level converter are: drive signal loss, IGBT open circuit / short circuit fault, and diode open circuit / short circuit fault.

[0006] As an indispensable part of the converter open circuit or short circuit fault response mechanism, the main goal of the fault-tolerant control strategy is to ensure the safe and controllable operation of the converter after the fault until it is shut down for maintenance. At present, the fault-tolerant control strategies for converters are mainly divided into two categories: redundant and non-redundant.

[0007] The existing literature "Karimi S, Gaillard A, Poure P and Saadate S. FPGA-Based Real-Time Power Converter Failure Diagnosis for Wind Energy Conversion Systems[J]. IEEE Transactions on Industrial Electronics, vol.55, no.12, pp.4299-4308" adopts a redundant fault-tolerant control strategy, which is mainly divided into three types of redundant fault-tolerant control strategies: system-level, bridge-arm-level and device-level. One or more mirror devices are configured in the converter system. When the system is operating normally, the mirror device remains silent; when the system fails, the mirror device replaces the faulty device to ensure the safe and controllable operation of the converter system. However, this strategy is costly to implement and has a high space occupancy rate. Its application is limited to fields such as aerospace that are extremely demanding on reliability.

[0008] The existing literature "Jlassi I and Cardoso A. Fault-Tolerant Back-to-Back Converter for Direct-Drive PMSG Wind Turbines Using Direct Torque and Power Control Techniques [J]. IEEE Transactions on Power Electronics, vol. 34, no. 11, pp. 11215-11227" applies a fault-tolerant control strategy under a direct control framework for a two-level back-to-back converter system, connects the three-phase bridge arm output terminals on the grid side and the motor side accordingly, and realizes fault-tolerant operation of the converter system. However, it does not adopt the model predictive control of rolling optimization, and cannot eliminate the influence of uncertain factors such as electromagnetic interference. Summary of the invention

[0009] In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a fault-tolerant control method for a grid-connected converter based on model prediction, which adopts a non-redundant fault-tolerant control strategy - a DC side capacitor sharing fault-tolerant control strategy, which is a method that takes into account both cost and diagnostic effect.

[0010] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions: In a first aspect, a fault-tolerant control method for a grid-connected converter based on model prediction is disclosed, comprising: Confirm that a power device failure occurs in a phase bridge arm of the converter; Afterwards, if the fault type is open circuit, the AC side phase voltage of the faulty phase of the converter is connected to the neutral point N of the two capacitors through the bidirectional thyristor, and the remaining normal switching devices are reconfigured and the energy storage of the DC side capacitor is utilized; Then, the two-phase four-switch voltage vector synthesis is reconstructed to achieve continued stable operation. At the same time, the gate signals of all power devices in the faulty bridge arm are blocked, and the DC side capacitor replaces the faulty bridge arm to support the controllable operation of the converter.

[0011] As a further technical solution, it also includes: when the converter is operating normally, the bidirectional thyristors of the converter are all in the off state, and the voltage, current and phase angle signals on the AC side of the converter are obtained by measurement, and the three-phase AC voltage and current are converted into static two-phase coordinate values ​​through Clark coordinate transformation, and the relationship between current and voltage is calculated through state space expression, and the current value at the next sampling under all possible switching states is predicted, and the cost function of each switching state is evaluated, and the switching state that minimizes the cost function is selected, and the switching state is used to control the converter PWM modulation signal.

[0012] In the above scheme, when predicting the current value at the next sampling under all possible switch states: based on all possible switch combinations (000~111) of the converter, the current value of the next sampling period of the ɑ-β coordinate is calculated using the three-phase voltage balance equation. For details, see (3) in step 7 of the implementation example.

[0013] As a further technical solution, the process of confirming that a power device failure occurs in a phase bridge arm of the converter is as follows: Measure the current on the AC side of the converter and the current at the PCC point; Determine whether the first condition is met based on the measurement data, and if the first condition is met, calculate the power monitoring value on the converter side according to the switch state; Determine whether the second condition is met based on the power monitoring value on the converter side, and if the second condition is met, start a hypothesis verification process to locate the fault; Then determine whether the third condition is met. If the third condition is met, the phase with the minimum absolute value of the converter power update is the fault phase, and the power device that was originally turned on in the fault phase is the fault device.

[0014] As a further technical solution, it is confirmed that a power device failure occurs in a phase bridge arm of the converter. If the fault type is a short circuit, the fuse will operate to convert the short circuit into an open circuit.

[0015] As a further technical solution, the startup hypothesis verification process is used to locate the fault, and the specific steps are as follows: Assuming that the open-circuit fault occurs on the three-phase bridge arms respectively, combined with the current converter switch state and the direction of the three-phase current on the grid side, the quasi-power update expression after the open-circuit fault occurs on the three-phase bridge arms is obtained; If the updated quasi-power value corresponding to the X-phase open-circuit fault is less than the diagnostic threshold, it means that the quasi-power expression matches the actual connection mode of the converter, and the assumption is established; Subsequently, the power device that was originally conducting current in the phase bridge arm is identified as a faulty device. This positioning process is called hypothesis verification.

[0016] In a second aspect, a model prediction-based grid-connected converter fault-tolerant control system is disclosed, comprising: The fault device determination module is configured to: confirm that a power device fault occurs in a phase bridge arm of the converter; The fault-tolerant control module is configured to: if the fault type is an open circuit, connect the AC side phase voltage of the faulty phase of the converter to the neutral points N of the two capacitors through a bidirectional thyristor, reconfigure the remaining normal switch devices and utilize the energy storage of the DC side capacitor; The two-phase four-switch voltage vector synthesis is reconstructed to achieve continued stable operation. At the same time, the gate signals of all power devices in the faulty bridge arm are blocked, and the DC side capacitor replaces the faulty bridge arm to support the controllable operation of the converter.

[0017] As a further technical solution, it also includes: a normal operation module of the converter, which is configured as follows: when the converter is operating normally, the bidirectional thyristors of the converter are all in the off state, the voltage, current and phase angle signals on the AC side of the converter are obtained by measurement, the three-phase AC voltage and current are converted into static two-phase coordinate values ​​through Clark coordinate transformation, the relationship between current and voltage is calculated through state space expression, the current value at the next sampling under all possible switching states is predicted, the cost function of each switching state is evaluated, the switching state that minimizes the cost function is selected, and the switching state is applied to control the converter PWM modulation signal.

[0018] One or more of the above technical solutions have the following beneficial effects: In the technical solution of the present invention, when the converter is operating normally, the bidirectional thyristors TRi are all in the off state, and the system controller executes the standard model predictive control algorithm. After confirming that a power device failure occurs in a certain phase bridge arm, the controller immediately inputs the DC side capacitor sharing fault-tolerant control algorithm, and at the same time blocks the gate signals of all power devices in the faulty bridge arm. The DC side capacitor replaces the faulty bridge arm to support the controllable operation of the converter.

[0019] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0021] Figure 1 It is a flow chart of the steps for implementing the present invention; Figure 2 This is a hardware structure diagram of a DC side capacitor sharing type fault-tolerant control converter according to an embodiment of the present invention; Figure 3 This is a structural diagram of a fault-tolerant converter for a phase C fault according to an embodiment of the present invention; Figure 4 A spatial voltage vector diagram of a fault-tolerant converter according to an embodiment of the present invention; Figure 5 A spatial voltage vector diagram of a fault-tolerant converter with a C-phase bridge arm fault according to an embodiment of the present invention; Figure 6 Grid-connected current curve of the converter system under normal operating conditions; Figure 7 Grid-connected voltage diagram under normal operating conditions; Figure 8 Current THD diagram under normal operating conditions; Fig. 9 Grid-connected current curve of the converter system under fault-tolerant operation; Fig.10 Grid-connected voltage diagram under fault-tolerant operation state; Fig.11 Current THD diagram under fault-tolerant operation state; Fig.12 DC capacitor voltage Vdc1, Vdc2 diagram; Fig.13 This is a flow chart of a fault diagnosis method under a predictive control framework according to an embodiment of the present invention. DETAILED DESCRIPTION

[0022] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0023] It should be noted that the terms used herein are for describing specific embodiments only and are not intended to be limiting of exemplary embodiments according to the present invention.

[0024] In the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0025] Grid-connected converter: It is a power electronic device that connects a DC power source and an AC power grid, converting DC power into AC power that is synchronized with the grid frequency and phase. It is widely used to effectively connect renewable energy power generation such as photovoltaic and wind power to the public power grid. It not only realizes the conversion between DC and AC, but also ensures that the output AC power is synchronized with the mains to ensure the stable operation of the power system.

[0026] Model predictive control: Model predictive control requires the establishment of a reasonable prediction model for the system. At each sampling moment, the controller needs to be able to combine the current measurement information and the prediction model to accurately predict the dynamics of the system in a finite time domain. By solving the optimization problem in the prediction time domain at each sampling moment, the optimal control sequence that makes the predicted value of the system dynamics meet the optimization goal is obtained, and the first element of the optimal control sequence is applied to the controlled object.

[0027] Embodiment 1 See attached Figure 2 As shown, this embodiment discloses a DC side capacitor sharing type fault-tolerant control converter, which, when working, includes: When the converter is operating normally, the bidirectional thyristors TRi are all in the off state, and the system controller executes the standard model predictive control algorithm. Using the measured AC side voltage, current, and phase angle signals, the three-phase AC voltage and current are converted into static two-phase coordinate values ​​through Clark coordinate transformation. The relationship between current and voltage is calculated through the state space expression, and the current value at the next sampling under all possible switching states is predicted. The cost function of each switching state is evaluated, and the switching states Sa, Sb, and Sc that minimize the cost function are selected. The new switching state is used to control the converter PWM modulation signal.

[0028] The specific steps for the above implementation include: 1. Signal acquisition and coordinate transformation: a. Measure and obtain the voltage, current and phase angle signals on the AC side. b. Use Clark transformation to convert the three-phase voltage and current into values ​​in the αβ stationary coordinate system: [Vα, Vβ] = Clark(Va, Vb, Vc); [Iα, Iβ] = Clark(Ia, Ib, Ic).

[0029] 2. Establish state space model: a. According to the converter topology and parameters, establish the state space equation in αβ coordinate system: dx / dt = Ax + Bu; y = Cx; Among them, x is the state variable, such as current, u is the input, such as switch state and DC voltage, and y is the output.

[0030] 3. Current prediction: a. Discretize the state space equation to obtain a discrete prediction model; b. For all possible switch states, usually 8, 000 to 111, calculate the predicted current at the next sampling moment; I(k+1) = f(I(k), V(k), Sa, Sb, Sc, Vdc).

[0031] 4. Cost function evaluation: a. Define the cost function J, which usually includes current tracking error and other performance indicators; J = (Iref - I(k+1))^2 + λ·other terms; b. For each switching state, calculate the corresponding cost function value.

[0032] 5. Optimal switch state selection: a. Compare the cost function values ​​under all switch states; b. Select the switch state combination (Sa, Sb, Sc)opt that minimizes the cost function J.

[0033] 6. Control signal generation and application: a. Generate the corresponding PWM modulation signal based on the selected optimal switching state; b. Apply the PWM signal to the power switch of the converter.

[0034] 7. Loop repeat: Repeat steps 1-6 in the next control cycle.

[0035] After confirming that a power device failure occurs in a certain phase bridge arm, the controller immediately uses the DC side capacitor sharing fault-tolerant control algorithm, connects the AC side phase voltage of the fault phase to the neutral point N of two capacitors through a bidirectional thyristor, reconfigures the remaining normal switch devices and uses the energy storage of the DC side capacitor to reconstruct the two-phase four-switch voltage vector synthesis to achieve the control strategy of the system to continue stable operation, thereby realizing the system's fault tolerance and improving reliability. At the same time, the gate signals of all power devices in the faulty bridge arm are blocked, and the DC side capacitor replaces the faulty bridge arm to support the controllable operation of the converter.

[0036] Reconfiguration steps: a. Activate the bidirectional thyristor: connect the AC side of the fault phase to the neutral point N of the DC side capacitor through the bidirectional thyristor; b. Reconstruct the remaining switches: reconfigure the four switches of the remaining two healthy phases into a two-phase four-switch structure; c. Update control model: Modify the system mathematical model to reflect the new circuit topology.

[0037] Refactoring steps: a. Redesign the voltage vector synthesis strategy: Calculate a new voltage vector diagram based on a two-phase four-switch structure; design a new switch sequence and PWM strategy; b. Modify the control algorithm: Update the system model in the model predictive control (MPC) or other control algorithms; adjust the control objectives and constraints; c. Design the current controller: modify the current reference generation method and adjust the controller parameters to adapt to the new system dynamics.

[0038] Implement new control strategies: a. Update PWM signal generator: implement new switching modes and PWM strategies; b. Adjust protection strategies: update overcurrent, overvoltage and other protection thresholds; c. Implement current balance control: ensure that the remaining two-phase currents are balanced to avoid system imbalance.

[0039] The control signals of the bidirectional thyristor are shown in Table 1.

[0040] Table 1 Bidirectional thyristor control signal

[0041] In order to better illustrate the technical solution of this application, the fault-tolerant conversion structure design of the grid-connected converter is as follows: Figure 2 As shown, the DC side capacitor sharing fault-tolerant control strategy is not only applicable to two-level and three-level converters, but also to back-to-back structures and separate rectifier or inverter structures. Figure 3 Two capacitors with exactly the same capacity and model are connected in parallel on the DC side. F1-F6 represent fast fuses, which convert short circuit into open circuit when a bridge arm fails. TR1-TR3 are three bidirectional thyristors, which serve as connecting switches to connect the output end of the converter with the midpoint N of the two capacitors on the DC side.

[0042] See attached Figure 1 As shown, the fault-tolerant control method of the grid-connected converter based on model prediction includes: Step 1: Measure the bridge arm current and determine whether it is within the normal range. If so, perform standard model predictive control. If not, proceed to step 2. Step 2: Model-driven open circuit diagnosis steps under the predictive control framework: Model-driven open circuit fault diagnosis technology is mainly based on the mechanism model of the converter, and reveals the internal mechanism of the open circuit fault by analyzing the evolution trajectory of each state parameter under normal and fault conditions to form a fault diagnosis method. This type of method is mainly divided into two vectors, voltage and current, to identify open circuit faults. The voltage detection method of the open circuit fault has the characteristics of fast and accurate, but the disadvantage in practical application is that it requires the installation of additional hardware detection equipment. Therefore, the open circuit fault detection method of the present invention-the open circuit diagnosis method based on Tellegen's theorem, mainly utilizes the output current of the converter, does not need to rely on additional hardware equipment or voltage sensors, and only needs to integrate the diagnostic function into the existing control algorithm. This technical solution that does not require hardware modification is currently a two-pronged method that takes into account both cost and diagnostic effect.

[0043] The space voltage vector modulation of the fault-tolerant grid-connected converter is used to control the converter. In this step, the desired reference voltage vector (Vref) is input and the on-off control signal of the converter arm switch is output: Under normal working conditions, the relationship between the three-phase voltage and the switch state function is:

[0044] S a , S b , S c Respectively represent the three-phase switch states of a, b, and c. When S X =1, indicating that the upper bridge arm switch is turned on and the lower bridge arm switch is turned off; S X =0, which means that the lower arm switch is turned on and the upper arm switch is turned off; the three-phase output voltages of the inverter uan, ubn, and ucn respectively represent the voltages of phase a, phase b, and phase c relative to the neutral point n; Vdc DC bus voltage, that is, the input DC voltage of the inverter.

[0045] In SVPWM, the switching state of each phase bridge arm of the inverter can be represented by a switching function. By combining different switching functions, eight basic space voltage vectors can be obtained to synthesize the required reference voltage vector. a , S b , S c Respectively represent the three-phase switch states of a, b, and c. When S X =1, indicating that the upper bridge arm switch is turned on and the lower bridge arm switch is turned off; S X =0, which means the lower bridge arm switch is turned on and the upper bridge arm switch is turned off.

[0046] When a power switch in the converter fails, the series capacitor replaces the faulty bridge arm and reconstructs the three-phase four-switch structure from the three-phase six-switch structure. This requires fast and accurate reconfiguration of the vector. Figure 2As shown in the figure, it is different from the grid-connected converter in normal working state. It has only two bridge arms and four switching devices. In order to analyze the switching state of the fault-tolerant converter, the C-phase bridge arm fault is taken as an example. When the C-phase fails, point C is directly connected to the midpoint N of the two capacitors. When point N is balanced, the voltage is 0.

[0047] Figure 3 is the reconstructed space voltage vector of the normal operation and fault operation state of the grid-connected converter. The voltage vector is becomes indivual.

[0048] Step 3: Fault diagnosis steps under the predictive control framework: Tellegen's theorem is based on Kirchhoff's current law. The total power absorbed by all branches of a collector circuit is 0. When the converter is operating normally, the actual connection mode is consistent with the switch state, and the apparent power Pg maintains a value of 0. However, an open-circuit fault in the power device will break this consistency. The apparent power Pg calculated based on the switch state will no longer be zero. This feature can be used to diagnose open-circuit faults. This open-circuit fault detection method has strong robustness that is not affected by operating conditions. The core of the open-circuit fault diagnosis algorithm based on Tellegen's theorem is to monitor the absolute value of the apparent power of the converter system in real time. |, this value is close to zero under normal conditions, and jumps after the power device fails. The specific process is as follows Fig.13 As shown: (1) Initialize data.

[0049] (2) Measuring the AC current of the converter With PCC point current , calculate the difference between these two currents Δig = -ig.

[0050] (3) Judgment condition: |Δ [k] |>thr1 and| [k] |<0.1thr1, where thr1 is set to 0.15, indicating that the current at the PCC point is very small, preliminarily confirming that there may be an open circuit fault; (4) If the conditions are met, proceed to the next step; if not, return to step 2 to continue measuring.

[0051] (5) The power monitoring value Pᵐ on the converter side is calculated based on the switch state Si. Pᵐ is the theoretical power value calculated based on the switch state Si and the measured current.

[0052] (6) Judgment conditions: | |>thr0 (m∈{0,1,2,...,7}) Here the diagnostic threshold thr0 is set to 1.00.

[0053] The absolute value of the apparent power of the converter system. When it is significantly greater than zero, it means that there is a power imbalance in the system, which is usually an indication of a fault. Pm is the power value calculated based on the measured voltage, current and switch status. |Pm| represents the absolute value of the calculated power value.

[0054] (7) If the conditions are met, proceed to the next step; if not, return to step (2) and continue measuring.

[0055] (8) Start the hypothesis verification process according to Table 4 to locate the fault.

[0056] (9) Judgment condition: Min| | <thr0 (x ∈ {a, b, c}), is the power value, where x represents the phase, which can be a, b, or c, corresponding to the three phases of the three-phase system. The superscript q indicates that this is a specific type of power calculation or measurement method, which corresponds to the updated power value of the open circuit fault of phase X.

[0057] (10) If the conditions are met, proceed to the next step; if not, return to step (2) and continue measuring.

[0058] (11) Converter side power | |The phase with the smallest absolute value is the fault phase.

[0059] (12) The power device that was originally turned on in the faulty phase is the faulty device.

[0060] After detecting the abnormal operation of the converter, the fault location process is immediately started. At this time, the actual connection mode of the converter is affected by the open circuit fault of the power device and is no longer compatible with its switch state Si. Assuming that the open circuit fault occurs on the three-phase bridge arm, combined with the converter switch state Si and the three-phase current on the grid side, The update expressions of the similar power after the three-phase bridge arm has an open circuit fault can be found in Table 4. If the corresponding X-phase open circuit fault similar power update value is less than the diagnostic threshold thr0, indicating that the power expression The assumption is established because it matches the actual connection mode of the converter. Then the power device that was originally conducting current in the phase bridge arm is identified as a faulty device. This positioning process is called hypothesis verification.

[0061] Table 4 Converter system apparent power Updated expression after an open circuit fault

[0062]

[0063] Step 4: Based on the identified faulty components, a DC side capacitor sharing fault-tolerant model of the grid-connected converter under the predictive control framework is constructed:

[0064] Assuming that Sc does not participate in vector synthesis when phase C fails, we have: And because:

[0065] The relationship between the phase voltages of the three endpoints a, b, and c relative to point n and the AC side line voltage of the grid-connected converter can be obtained as follows:

[0066] According to the different changes of the switch state Si, the line voltage U ac and U bc With S a and S b The change relationship is shown in Table 2.

[0067] Table 2U ac and U bc With S a and S b Change relationship

[0068] Summarizing the relationship in Table 2, it can be written as follows: .

[0069] Point N coincides with the fault point C, so: , ,but

[0070] Assumptions Simplified, we get:

[0071] After Clark coordinate transformation, the above formula is converted to α-β coordinate system: .

[0072] Step 5: Fault phase proportional DC bias current compensation for model predictive fault-tolerant control: like Figure 2 As shown in the figure, since the fault phase is directly connected to the capacitor neutral point lead wire, the neutral point voltage of the DC side series capacitor is not balanced, and the voltages of the upper and lower capacitors fluctuate sinusoidally and the phases are completely opposite. When the DC side midpoint voltage is unbalanced, it is necessary to compensate the fault phase with proportional DC bias current to eliminate the capacitor voltage deviation. The relationship between the switch state and the DC capacitor current is as follows:

[0073] Capacitor current , It can be expressed as:

[0074] Voltage on capacitor and Differentiate the difference and get the fault phase current :

[0075] The integral is:

[0076] In the above formula and It is the initial value, generally considered to be 0.

[0077] A low-pass filter is used to extract the capacitor voltage deviation:

[0078]

[0079] In this step, the fault phase proportional DC bias current compensation is mainly used to improve the system performance under fault conditions. This step is usually performed after reconstruction and reconfiguration, and the impact of DC side voltage imbalance needs to be considered as part of the control strategy.

[0080] The inputs include: fault phase information, indicating which phase has failed; three-phase current reference values ​​during normal operation; current system status, including the currents of the remaining healthy phases; DC bus voltage; system parameters, such as filter inductance, resistance, etc.

[0081] Output: Corrected two-phase current reference value; DC offset compensation current value. Updated control target for model predictive control.

[0083] Step 6: Establish the objective function and traverse the optimal switch state Si to obtain the basic spatial voltage vector relationship of fault-tolerant operation based on neutral point voltage balance, as shown in Table 3.

[0084] Table 3 Basic space voltage vector table of fault-tolerant operation based on neutral point voltage balance

[0085] According to Table 3, we can draw Figure 4The spatial voltage vector distribution diagram shown in the figure shows that the modulation area is a diamond, which is equally divided into four right-angle sectors by four basic spatial voltage vectors (vectors V0, V1, V2, and V3 are divided into sectors I, II, III, and IV respectively). These four non-zero vectors include two short vectors with opposite modulus lengths and two long vectors with opposite modulus lengths. The diamond area symmetrically distributed about the origin center formed by the vertices of the vectors is the maximum modulation range of SVPWM, and the inscribed circle of the diamond is the running trajectory of the spatial voltage vector under the maximum linear modulation degree. The reference voltage satisfies:

[0086] From the above formula, it can be seen that in order to balance the neutral point, the maximum amplitude of the three-phase voltage on the AC side in the linear modulation area under the fault-tolerant state is ,Compared with the SVPWM modulation under normal operation, the DC side bus voltage utilization is reduced by half, and is only applicable to short-term fault operation.

[0087] The sub-technical solution of this embodiment uses model predictive control, solves the robust MPC optimization problem in each control cycle, implements the rolling horizon strategy, and only applies the optimal control input at the current moment. The prediction model considering uncertainty can usually be constructed in the following form: x(k+1) = Ax(k) + Bu(k) + w(k), where w(k) represents uncertainty and interference.

[0088] Step 7: The grid-connected converter model predicts fault-tolerant current control and outputs the control instructions for the converter switch IGBT: (1) Establish the state equation in the stationary three-phase coordinate system: ; (2) Perform Clark transformation: ; (3) Discrete form of load current prediction: ; (4) Minimization of cost function: ; (5) Delay compensation: ;

[0089] When the sampling frequency is much higher than the grid frequency, the grid voltage is considered to satisfy: .

[0090] In the above formula, i α (k+2), i β (k+2): Current component in α-β coordinate system at time k+2. Ts: Sampling period. L: System inductance. uα (k), u β (k): u at time k an , u bn , u cn The voltage components of the three-phase voltage in the α-β coordinate system. α (k+1), e β (k+1): voltage components of the three-phase AC voltages a, b, c on the grid side in the α-β coordinate system at time k+1; R: system resistance value. L: inductance; i a , i b , i c : a, b, c three-phase current; u an , u bn , u cn : the voltage between the three phases a, b, c and the neutral point; e a , e b , e c : a, b, c three-phase AC side voltage; R: resistance; i α , i β : current component in the α-β coordinate system; u α , u β :u an , u bn , u cn The voltage components of the three-phase voltage in the α-β coordinate system; e α , e β :a, b, c are the voltage components of the three-phase AC voltage on the grid side in the α-β coordinate system; k: current sampling time; k+1: next sampling time; Ts: sampling period; u α (k), u β (k): α-β voltage at time k; e α (k), e β (k): α-β back electromotive force at time k; i α (k),i β (k): α-β current at time k; g i : cost function; i α (k+1), i β (k+1): α-β current reference value at the next moment; i α (k+1), i β (k+1): The predicted α-β current value at the next moment.

[0091] Simulation verification experiment: The main circuit includes components: DC power supply, voltmeter, IGBT converter, filter circuit, voltage and current measuring components, and three-phase AC power grid.

[0092] Table 5 Main circuit parameters

[0093] The control circuit includes components: a PLL phase-locked loop, a Clark transformation module, a vector selection module, a data conversion module, and a prediction control function module.

[0094] Figure 6-Figure 8 It is the grid-connected current, voltage and current distortion rate of the converter system under normal operating conditions. Figure 9-11 The grid-connected current, voltage and current distortion rate of the converter system in the fault-tolerant operation state when the C phase is open-circuited. In the simulation experiment, the initial value of the current reference signal is set to 3A, and it jumps to 8A at 0.06s. Fig. 9 It can be seen that the three-phase current waveform in the fault-tolerant state is still a sine wave with a phase difference of 120°, and the assignment changes according to the set reference value, which proves that the fault-tolerant current control under the model predictive control framework is accurate.

[0095] Figure 6 and Fig. 9 It shows that the current distortion rates of the normal operation and fault-tolerant operation of the converter are 2.2% and 3.15% respectively. This shows that the current distortion rate is still very low when the C phase fails. Fig.12 It is the curve of the two capacitor voltages Vdc1 and Vdc2 on the DC side. The DC power supply is 400V, so Vdc / 2=200V. It can be seen that the amplitudes are all over 200±7V. After current bias compensation, the fluctuation is small and the voltage is basically balanced.

[0096] Among them, the calculation method of current distortion rate is: according to Figure 6 and Fig. 9 The current waveform is obtained by using the FFT analysis tool in MATLAB's SignalProcessing Toolbox to obtain the current harmonic analysis results. Perform FFT transformation on the current waveform to obtain the fundamental wave and each harmonic amplitude. The fundamental wave frequency is 50Hz. Figure 6 and Fig. 9 The fundamental amplitude is 7.922, the horizontal axis is the harmonic order (to 100), and the vertical axis is the percentage of each harmonic relative to the fundamental. MATLAB automatically calculates the root mean square value of all harmonic components, divides this value by the fundamental amplitude, and multiplies it by 100% to get the final THD value, i.e., the current distortion rate.

[0097] Embodiment 2 The purpose of this embodiment is to provide a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of the above method when executing the program.

[0098] Embodiment 3 The purpose of this embodiment is to provide a computer-readable storage medium.

[0099] A computer-readable storage medium stores a computer program, which executes the steps of the above method when executed by a processor.

[0100] Embodiment 4 The purpose of this embodiment is to provide a grid-connected converter fault-tolerant control system based on model prediction, including: The faulty device determination module is configured to: determine the faulty device in the faulty phase of the grid-connected converter based on the measured AC side current of the grid-connected converter and the PCC point current; The fault-tolerant control module is configured as follows: after confirming that a power device failure occurs in a certain phase bridge arm of the grid-connected converter, a DC side capacitor sharing fault-tolerant control algorithm is activated, and at the same time, the gate signals of all power devices in the faulty bridge arm are blocked, and the DC side capacitor replaces the faulty bridge arm to support the controllable operation of the converter.

[0101] In terms of specific hardware implementation, two capacitors with exactly the same capacity and model are connected in parallel on the DC side, and each switch device is connected in series with a fast fuse. When a bridge arm fails, the fuse is disconnected and the short-circuit fault is converted into an open-circuit fault; a bidirectional thyristor is connected in series between the midpoint of each bridge arm and point N, which serves as a connecting switch to connect the output end of the converter to the midpoint N of the two capacitors on the DC side. When the converter is operating normally, the bidirectional thyristors TRi are all in the off state, and the system controller executes the standard model predictive control algorithm. After confirming that a power device failure has occurred in a phase bridge arm, the controller immediately enters the DC bus sharing fault-tolerant control algorithm, and at the same time blocks the gate signals of all power devices in the faulty bridge arm. The DC bus replaces the faulty bridge arm to support the controllable operation of the converter.

[0102] Model-driven open circuit diagnosis method under the predictive control framework: The detection method of open circuit fault mainly uses the output current of the converter. It does not need to rely on additional hardware equipment or voltage sensors. It only needs to integrate the diagnostic function into the existing control algorithm. The open circuit fault diagnosis algorithm based on Tellegen's theorem, this technical solution without hardware modification is currently a two-pronged method that takes into account both cost and diagnostic effect. This diagnostic method and the fast fuse are backup for each other, that is, only one of the software and hardware needs to be enabled. After the fuse is disconnected, it cannot be automatically restored and requires hardware repair. The software detection method can automatically restore the normal circuit of the three-phase six switches after the fault is restored.

[0103] Fault-tolerant operation of grid-connected converter space voltage vector modulation: After a fault is detected, the fault-tolerant operation mode is started to find the space voltage vector synthesis relationship of the grid-connected converter again.

[0104] When the fault-tolerant operation mode is started, step six is ​​specifically adopted to obtain the spatial voltage vector synthesis relationship of the grid-connected converter by looking up Table 3.

[0105] DC side separated capacitor voltage modeling for model predictive fault-tolerant control: This method is aimed at the fault phase directly connected to the capacitor neutral point lead wire, the neutral point voltage of the DC side series capacitor is unbalanced, and the voltages of the upper and lower capacitors fluctuate sinusoidally and the phases are completely opposite. When the DC side midpoint voltage is unbalanced, proportional DC bias current compensation is required for the fault phase to eliminate the capacitor voltage deviation.

[0106] Grid-connected converter model predictive current control: The state equation in the fault-tolerant mode is established according to the fault phase, and the real-time status of the three-phase current and voltage on the grid side is obtained by using the phase-locked loop (PLL). The abc three-phase current / voltage on the grid side is converted into the current / voltage under the static two-phase coordinates αβ through Clark coordinate transformation. The optimal output current is calculated through the cost function according to the filter impedance, inductance and other values, and the PWM modulation signal of the converter is selected to control the converter power output.

[0107] Embodiment 5 The purpose of this embodiment is to provide a computer program product containing instructions, which, when running on a computer, enables the computer to execute the methods and functions involved in any of the above embodiments. The steps involved in the apparatus of the above embodiment correspond to the method embodiment 1, and the specific implementation method can refer to the relevant description part of embodiment 1. The term "computer-readable storage medium" should be understood as a single medium or multiple media including one or more instruction sets; it should also be understood to include any medium that can store, encode or carry an instruction set for execution by a processor and enable the processor to execute any method in the present invention.

[0108] Those skilled in the art should understand that the modules or steps of the present invention described above can be implemented by a general-purpose computer device, or alternatively, they can be implemented by a program code executable by a computing device, so that they can be stored in a storage device and executed by the computing device, or they can be made into individual integrated circuit modules, or multiple modules or steps therein can be made into a single integrated circuit module for implementation. The present invention is not limited to any specific combination of hardware and software.

[0109] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A fault-tolerant control method for a grid-connected converter based on model prediction, characterized in that it comprises: Confirm that a power device failure occurs in a phase bridge arm of the converter; Afterwards, if the fault type is open circuit, the AC side phase voltage of the faulty phase of the converter is connected to the neutral point N of the two capacitors through the bidirectional thyristor, and the remaining normal switching devices are reconfigured and the energy storage of the DC side capacitor is utilized; Then, the two-phase four-switch voltage vector synthesis is reconstructed to achieve continued stable operation. At the same time, the gate signals of all power devices in the faulty bridge arm are blocked, and the DC side capacitor replaces the faulty bridge arm to support the controllable operation of the converter.

2. A fault-tolerant control method for a grid-connected converter based on model prediction as claimed in claim 1, characterized in that: include: When the converter is operating normally, the bidirectional thyristors of the converter are all in the off state. The voltage, current and phase angle signals of the AC side of the converter are obtained by measurement. The three-phase AC voltage and current are converted into static two-phase coordinate values ​​through Clark coordinate transformation. The relationship between current and voltage is calculated through state space expression, and the current value at the next sampling under all possible switching states is predicted. The cost function of each switching state is evaluated, and the switching state that minimizes the cost function is selected. The switching state is used to control the converter PWM modulation signal.

3. The fault-tolerant control method for a grid-connected converter based on model prediction according to claim 1, characterized in that: The process of confirming that a power device failure occurs in a phase bridge arm of the converter is as follows: Measure the current on the AC side of the converter and the current at the PCC point; Determine whether the first condition is met based on the measurement data, and if the first condition is met, calculate the power monitoring value on the converter side according to the switch state; Determine whether the second condition is met based on the power monitoring value on the converter side, and if the second condition is met, start a hypothesis verification process to locate the fault; Then determine whether the third condition is met. If the third condition is met, the phase with the minimum absolute value of the converter power update is the fault phase, and the power device that was originally turned on in the fault phase is the fault device.

4. The fault-tolerant control method for grid-connected converter based on model prediction according to claim 1, characterized in that: Confirm that a power device fault occurs in a phase bridge arm of the converter. If the fault type is a short circuit, the fuse will operate and convert the short circuit to an open circuit.

5. The fault-tolerant control method for grid-connected converter based on model prediction according to claim 1, characterized in that: The specific steps of starting the hypothesis verification process to locate the fault are as follows: Assuming that the open-circuit fault occurs on the three-phase bridge arms respectively, combined with the current converter switch state and the direction of the three-phase current on the grid side, the quasi-power update expression after the open-circuit fault occurs on the three-phase bridge arms is obtained; If the updated quasi-power value corresponding to the X-phase open-circuit fault is less than the diagnostic threshold, it means that the quasi-power expression matches the actual connection mode of the converter, and the assumption is established; Subsequently, the power device that was originally conducting current in the phase bridge arm is identified as a faulty device. This positioning process is called hypothesis verification.

6. A fault-tolerant control system for a grid-connected converter based on model prediction, characterized in that: include: The fault device determination module is configured to: confirm that a power device fault occurs in a phase bridge arm of the converter; The fault-tolerant control module is configured to: if the fault type is an open circuit, connect the AC side phase voltage of the faulty phase of the converter to the neutral points N of the two capacitors through a bidirectional thyristor, reconfigure the remaining normal switch devices and utilize the energy storage of the DC side capacitor; The two-phase four-switch voltage vector synthesis is reconstructed to achieve continued stable operation. At the same time, the gate signals of all power devices in the faulty bridge arm are blocked, and the DC side capacitor replaces the faulty bridge arm to support the controllable operation of the converter.

7. A grid-connected converter fault-tolerant control system based on model prediction as claimed in claim 6, characterized in that: include: The normal operation module of the converter is configured as follows: when the converter is operating normally, the bidirectional thyristors of the converter are all in the off state, the voltage, current and phase angle signals of the AC side of the converter are obtained by measurement, the three-phase AC voltage and current are converted into static two-phase coordinate values ​​through Clark coordinate transformation, the relationship between current and voltage is calculated through state space expression, the current value at the next sampling under all possible switching states is predicted, the cost function of each switching state is evaluated, the switching state that minimizes the cost function is selected, and the switching state is applied to control the converter PWM modulation signal.

8. A computer program product, comprising a computer program, characterized in that When the computer program is executed by a processor, the method according to any one of claims 1 to 5 is implemented.

9. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the program, the steps of the method described in any one of claims 1 to 5 are implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the method described in any one of claims 1 to 5 are performed.

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