MMC AC voltage measurement fault detection method

Abstract

The invention relates to an MMC AC voltage measurement fault detection method, and the method comprises the steps: building an MMC output current observer based on a Kalman filtering algorithm according to a three-phase decoupling differential equation of MMC output current; the AC voltage, the output current, the upper and lower bridge arm capacitor voltage of each phase and the corresponding switching function of the MMC are obtained and input into the MMC output current observer; performing state observation on the output current of the MMC by using the MMC output current observer to obtain an output current observation value; and obtaining a moving effective value of an observation residual error according to the observation residual error of the output current observation value and the output current actual value, and judging an AC voltage measurement fault according to the moving effective value of the observation residual error. According to the method, the actual MMC system information can be described more accurately, the interference of power grid imbalance can be resisted, the detection is more convenient, and the noise influence can be reduced so as to reduce misjudgment.

Description

Technical Field

[0001] This application relates to the technical field of flexible DC power transmission, and particularly to a fault detection method for MMC AC voltage measurement. Background Art

[0002] Benefiting from advantages such as modular design, good scalability, low switching loss, and high power quality, the modular multilevel converter (MMC) has become the core equipment of flexible DC power transmission. Currently, flexible DC power transmission systems based on MMC have been widely applied in fields such as new energy grid connection, asynchronous grid interconnection, and island power supply.

[0003] The safe and stable operation of the MMC converter station is the focus of attention in the academic and industrial circles. The control of the MMC system is inseparable from the accurate measurement of the actual electrical data of the converter station. Among them, the AC voltage measurement device is an important device for obtaining the AC grid voltage on the AC side of the converter station. It sends the measured actual AC voltage into the phase-locked loop, power outer loop, current inner loop, etc. in the MMC control system, which is crucial for the safe and stable operation of the MMC system. Once the AC voltage measurement device has problems, it will lead to AC voltage measurement faults. The incorrect measurement data will affect the normal control and operation of the MMC system. In severe cases, it may cause system instability and ultimately lead to the locking protection of the converter station.

[0004] When an AC voltage measurement fault occurs, if the fault can be quickly detected and an alarm signal can be sent, it can provide support for subsequent fault-tolerant control, etc. Therefore, researching a fast detection method for AC voltage measurement faults is of great significance for the MMC converter station. Summary of the Invention

[0005] Based on this, in view of the above technical problems, it is necessary to provide a fault detection method for MMC AC voltage measurement, which can quickly and accurately detect AC voltage measurement faults.

[0006] An embodiment of this application provides a fault detection method for MMC AC voltage measurement, and the method includes: Based on the three-phase decoupled differential equation of the MMC output current, establish an MMC output current observer based on the Kalman filter algorithm; Obtain the AC voltage, output current, upper and lower arm capacitor voltages of each phase, and corresponding switching functions of the MMC and input them into the MMC output current observer; Use the MMC output current observer to perform state observation on the output current of the MMC to obtain the output current observation value; According to the observation residual between the output current observation value and the actual value of the output current, obtain the moving effective value of the observation residual, and judge the AC voltage measurement fault according to the moving effective value of the observation residual.

[0007] In one of the embodiments, the three-phase decoupled differential equation of the MMC output current is as follows: ; In the formula, represents the AC voltage represents the output current, represents the average value of the capacitor voltage of the upper-arm sub-module, represents the average value of the capacitor voltage of the lower-arm sub-module, represents the switching function of the upper-arm sub-module, represents the switching function of the lower-arm sub-module, , , represent three phases, represents the three-phase zero-sequence voltage compensation term of the MMC, represents the decoupling term of the AC voltage, represents the equivalent inductance, represents the equivalent resistance; where: ; Among them, , , respectively represent the switching functions of the lower-arm sub-module , , for three phases, , , respectively represent the average values of the capacitor voltages of the lower-arm sub-module , , for three phases, , , respectively represent the switching functions of the upper-arm sub-module , , for three phases, , , respectively represent the average values of the capacitor voltages of the upper-arm sub-module , , for three phases; ; Among them, , , respectively represent , , three-phase AC voltages; ; Among them, represents the grid-side inductor, represents the grid-side resistor, represents the MMC arm inductor, represents the MMC arm resistor.

[0008] In one embodiment, the MMC output current observer based on the Kalman filter algorithm established according to the three-phase decoupled differential equation of the MMC output current includes: Discretize the three-phase decoupled differential equation to obtain the discrete three-phase output current equation; Convert the three-phase output current equation into the form of a state-space equation to obtain the prediction equation of the output current as: ; The definitions of the variables in the formula are as follows: ; Among them, represents the predicted value of the output current at time, represents the observed value of the output current at time, represents the sampling period; Based on the Kalman filter algorithm, obtain the MMC output current observer: ; Among them, represents the measured value of the output current at time, represents the Kalman filter gain at time.

[0009] In one embodiment, the Kalman filter gain at time is calculated specifically as: ; Among them, represents the Kalman filter gain of the phase at time, represents the Kalman filter gain of the phase at time, represents the Kalman filter gain of the phase at time.

[0010] In one embodiment, the , , Three-phase Kalman filter gain 、 、 The specific calculation method is as follows: ; Wherein, represents the Kalman filter gain of the phase at time represents the state prediction error covariance at time represents the measurement noise covariance at time

[0011] In one embodiment, the specific calculation method of the state prediction error covariance is as follows: ; Wherein, represents the state error covariance at time represents the process noise covariance at time

[0012] In one embodiment, the specific calculation method of the state error covariance is as follows: ; Wherein, represents the state error covariance at time

[0013] In one embodiment, the state observation of the output current of the MMC by using the MMC output current observer to obtain the output current observation value includes: By the output current observation value of the MMC at time, calculate to obtain the output current prediction value at time; Calculate the state error covariance at time, and calculate according to the calculated the state error covariance at time the Kalman filter gain at time; Correct the output current prediction value according to the Kalman filter gain to obtain the output current observation value at time; According to the output current observation value at time, update the state error covariance at time

[0014] In one embodiment, the judgment of the AC voltage measurement fault according to the moving effective value of the observation residual includes: Compare the maximum value of the moving effective value with a set deviation threshold. When the moving effective value is greater than the deviation threshold, it is determined that an abnormal AC voltage measurement occurs in the corresponding phase.

[0015] In one embodiment, the moving effective value of the observation residual The calculation method is as follows: ; Where is the power frequency period of 20 ms, is the observation residual of the phase output current.

[0016] The above MMC AC voltage measurement fault detection method, by establishing an MMC output current observer based on the Kalman filter algorithm, adapts to the three-phase three-wire MMC system, compensates for the MMC output common-mode voltage and grid voltage imbalance, can more accurately describe the actual MMC system information, and can resist the interference of grid imbalance at the same time; By judging the AC voltage measurement fault through the moving effective value of the observation residual, the residual with AC characteristics during the fault can be converted into a DC component, which is more convenient for detection, and at the same time, the influence of noise can be reduced to reduce misjudgment.

[0017] Details of one or more embodiments of the present application are set forth in the following drawings and description, so that other features, objects, and advantages of the present application will become more concise and understandable. Brief Description of the Drawings

[0018] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The schematic embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation to the present application. In the drawings: Figure 1 is a schematic topological structure diagram of an MMC converter station provided in an embodiment of the present application; Figure 2 is a flowchart of an MMC AC voltage measurement fault detection method provided in an embodiment of the present application; Figure 3 is a flowchart of another MMC AC voltage measurement fault detection method provided in an embodiment of the present application; Figure 4 is a flowchart of an MMC output current state observer construction method provided in an embodiment of the present application; Figure 5 is a flowchart of an MMC output current state observation method provided in an embodiment of the present application; Figure 6 is a schematic diagram of the moving effective value of the three-phase output current observation residual under normal working conditions of AC voltage measurement provided in an embodiment of the present application; Figure 7 It is a schematic diagram of the moving effective value of the three-phase output current observation residual after an AC voltage measurement failure provided in the embodiments of the present application. Detailed implementation manners

[0019] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be described and explained below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments provided in the present application without creative efforts belong to the scope of protection of the present application.

[0020] Obviously, the accompanying drawings in the following description are only some examples or embodiments of the present application. For those of ordinary skill in the art, without creative efforts, the present application can also be applied to other similar scenarios based on these drawings. In addition, it can also be understood that although the efforts made in this development process may be complex and lengthy, for those of ordinary skill in the art related to the content disclosed in the present application, some design, manufacturing or production changes made based on the technical content disclosed in the present application are only conventional technical means and should not be understood that the content disclosed in the present application is insufficient.

[0021] Referring to "embodiments" in the present application means that specific features, structures or characteristics described in connection with the embodiments can be included in at least one embodiment of the present application. The phrase appears in various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those of ordinary skill in the art explicitly and implicitly understand that the embodiments described in the present application can be combined with other embodiments without conflict.

[0022] Unless otherwise defined, the technical terms or scientific terms involved in this application shall have the ordinary meanings understood by those with ordinary skills in the technical field to which this application belongs. The words such as "a", "an", "one kind", "the" and the like involved in this application do not indicate a limitation in quantity and may represent a singular or plural number. The terms "include", "comprise", "have" and any variations thereof involved in this application are intended to cover non-exclusive inclusion; for example, a process, method, system, product or device that includes a series of steps or modules (units) is not limited to the listed steps or units, but may further include steps or units not listed, or may further include other steps or units inherent to these processes, methods, products or devices. The similar words such as "connect", "be connected", "couple" and the like involved in this application are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. The "plurality" involved in this application means two or more than two. "And / or" describes the association relationship of associated objects and indicates that three relationships may exist. For example, "A and / or B" may represent: A exists alone, A and B exist simultaneously, and B exists alone. The character " / " generally represents an "or" relationship between the front and rear associated objects. The terms "first", "second", "third" and the like involved in this application are only used to distinguish similar objects and do not represent a specific order for the objects.

[0023] The MMC AC voltage measurement fault detection method provided by an embodiment of this application is applied to the MMC converter station shown in FIG. 1.

[0024] Among them, the MMC converter station includes three phases, and each phase includes a total of six arms, namely upper and lower arms. The structures of all arms are the same and each is composed of an arm inductor , an arm resistor and N half-bridge sub-modules with the same structure connected in cascade. Each half-bridge sub-module includes two power switching tubes , , and a DC capacitor .

[0025] This embodiment provides an MMC AC voltage measurement fault detection method. As shown in FIG. 2, it includes the following steps: Step S101, establish an MMC output current observer based on the Kalman filter algorithm according to the three-phase decoupled differential equation of the MMC output current.

[0026] Specifically, the three-phase decoupled differential equation of the MMC output current is: ; In the formula, represents the AC voltage represents the output current, represents the average capacitor voltage of the upper-arm sub-module, represents the average capacitor voltage of the lower-arm sub-module, represents the switching function of the upper-arm sub-module, represents the switching function of the lower-arm sub-module, , , represents three phases, represents the three-phase zero-sequence voltage compensation term of the MMC, represents the AC voltage decoupling term, represents the equivalent inductance, represents the equivalent resistance; where: ; where, 、 、 respectively represent the lower-arm sub-module , , switching functions of three phases, 、 、 respectively represent the lower-arm sub-module , , average capacitor voltages of three phases, 、 、 respectively represent the upper-arm sub-module , , switching functions of three phases, 、 、 respectively represent the upper-arm sub-module , , average capacitor voltages of three phases; ; where, 、 、 respectively represent , , three-phase AC voltages; ; where, represents the grid-side inductor, represents the grid-side resistor, represents the MMC arm inductor, represents the MMC arm resistor.

[0027] Step S102: Obtain the AC voltage, output current, capacitor voltages of the upper and lower bridge arms of each phase, and the corresponding switching functions of the MMC, and input them into the MMC output current observer.

[0028] Step S103: Use the MMC output current observer to perform state observation on the output current of the MMC to obtain the observed value of the output current.

[0029] Step S104: According to the observation residual between the observed value of the output current and the actual value of the output current, obtain the moving effective value of the observation residual, and judge the AC voltage measurement fault based on the moving effective value of the observation residual.

[0030] Specifically, compare the maximum value of the moving effective value with the set deviation threshold. When the moving effective value is greater than the deviation threshold, it is determined that there is an abnormal AC voltage measurement in the corresponding phase.

[0031] The moving effective value of the observation residual The calculation method is as follows: ; Wherein, is the power frequency period of 20 ms, is the observation residual of the phase output current.

[0032] The above MMC AC voltage measurement fault detection method, by establishing an MMC output current observer based on the Kalman filter algorithm, can adapt to a three-phase three-wire MMC system, compensate for the MMC output common-mode voltage and grid voltage imbalance, and can more accurately describe the actual MMC system information. At the same time, it can resist the interference of grid imbalance; by judging the AC voltage measurement fault through the moving effective value of the observation residual, the residual with an AC characteristic during the fault can be converted into a DC component, which is more convenient for detection, and at the same time, the influence of noise can be reduced to reduce misjudgment.

[0033] Based on the above embodiment, this embodiment provides an optional example of an MMC AC voltage measurement fault detection method. As Figure 3 shown, the specific implementation process is as follows: The average value of the capacitor voltage of the upper bridge arm sub-module , the average value of the capacitor voltage of the lower bridge arm sub-module , the switching function of the upper bridge arm sub-module , the switching function of the lower bridge arm sub-module , the measured value of the AC voltage , the measured value of the output current are input into the MMC output current observer based on the Kalman filter algorithm to obtain the observed value of the output current ; Based on the observed value of the output current Output current measurement value Calculate the moving effective value of the observation residual Perform fault detection based on the moving effective value of the observation residual

[0034] Based on the above embodiments, the embodiments of the present application will explain the above embodiment S101 in detail. Specifically, the embodiments of the present application involve establishing an MMC output current observer based on the Kalman filter algorithm according to the three-phase decoupled differential equation of the MMC output current, as Figure 4 shown, specifically including the following steps Step S201: Discretize the three-phase decoupled differential equation to obtain the discrete three-phase output current equation

[0035] Step S202: Convert the three-phase output current equation into the form of a state space equation to obtain the prediction equation of the output current ; The definitions of the variables in the formula are as follows ; Among them represents the predicted value of the output current at time represents the observed value of the output current at time represents the sampling period Step S203: Based on the Kalman filter algorithm, obtain the MMC output current observer ; Among them represents the measured value of the output current at time represents the Kalman filter gain at time

[0036] Specifically, the calculation method of the Kalman filter gain at time is ; Among them represents the Kalman filter gain of phase at time represents the Kalman filter gain of phase at time represents the Kalman filter gain of phase at time

[0037] The , , three-phase Kalman filter gain , , The specific calculation method is as follows: ; Wherein, represents the time phase Kalman filter gain, represents the state prediction error covariance at time represents the measurement noise covariance at time

[0038] The specific calculation method of the state prediction error covariance is as follows: ; Wherein, represents the state error covariance at time represents the process noise covariance at time

[0039] The specific calculation method of the state error covariance is as follows: ; Wherein, represents the state error covariance at time

[0040] Based on the above embodiments, the embodiments of the present application further explain the above embodiment S103 in detail. Specifically, the embodiments of the present application relate to using the MMC output current observer to perform state observation on the output current of the MMC to obtain the output current observation value, as Figure 5 shown, which specifically includes the following steps: Step S301, calculate the output current prediction value at time through the output current observation value of the MMC at time ; It should be noted that the initial value of the observation value can be considered as 0 and is slowly adjusted by the MMC output current observer until it converges.

[0041] Step S302, calculate the state error covariance at time , and calculate the Kalman filter gain at time according to the calculated state error covariance at time ;

[0042] Step S303, correct the predicted output current value according to the Kalman filter gain to obtain the output current observation value at the moment.

[0043] Step S304, according to the output current observation value at the moment, update the state error covariance at the moment.

[0044] To verify the effectiveness of the AC voltage measurement fault detection method proposed by the present invention, an MMC simulation model was built in MATLAB / Simulink, and the main parameters of the model are set as shown in Table 1.

[0045] Table 1 Main parameters of the MMC simulation model

[0046] Specifically, when the MMC model is in a normal operating state and there is no fault in the AC voltage measurement, the moving effective value of the three-phase output current observation residual, as Figure 6 shown, the moving effective values MRMS of the three-phase output current observation residuals all remain at a low level and do not trigger fault detection.

[0047] When an AC voltage measurement fault occurs in the MMC model, four fault conditions of the phase AC voltage measurement being 5% larger, 50% larger, 5% smaller, and 50% smaller are respectively simulated. As shown in the figure, (a), (b), (c), and (d) are respectively the changes in the moving effective values MRMS of the three-phase output current observation residuals when the four faults of 5% larger, 50% larger, 5% smaller, and 50% smaller occur. It can be seen that when the fault occurs at 2.1 s, the moving effective value MRMS of the

[0048] phase residual increases rapidly and soon exceeds the set threshold, determining that an AC voltage measurement fault has occurred in the MMC.

[0049] It should be understood that although the steps in the flowcharts involved in the above-described embodiments are shown sequentially in the direction of the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless there is a clear indication in this document, there is no strict order restriction for the execution of these steps, and these steps can be executed in other orders. Moreover, at least a part of the steps in the flowcharts involved in the above-described embodiments may include multiple steps or multiple stages. These steps or stages are not necessarily executed at the same time, but can be executed at different times. The execution order of these steps or stages is not necessarily sequential, but can be executed alternately or in turn with at least a part of other steps or steps or stages in other steps.

[0050] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the various technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0051] The above-described embodiments merely represent several implementation manners of the present application. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the appended claims.

Claims

1. A fault detection method for MMC AC voltage measurement, characterized in that, The method includes: Based on the three-phase decoupled differential equation of the MMC output current, an MMC output current observer based on the Kalman filter algorithm is established; The AC voltage, output current, upper and lower arm capacitor voltages of each phase, and corresponding switching functions of the MMC are obtained and input into the MMC output current observer; The MMC output current observer is used to perform state observation on the MMC output current to obtain the observed value of the output current; Based on the observation residual between the observed value of the output current and the actual value of the output current, the moving effective value of the observation residual is obtained, and the AC voltage measurement fault is judged according to the moving effective value of the observation residual.

2. The method according to claim 1, characterized in that It includes: The three-phase decoupled differential equation of the MMC output current is: ； Wherein, represents the AC voltage represents the output current, represents the average value of the capacitor voltage of the upper-arm sub-module, represents the average value of the capacitor voltage of the lower-arm sub-module, represents the switching function of the upper-arm sub-module, represents the switching function of the lower-arm sub-module, , , represents three phases, represents the three-phase zero-sequence voltage compensation term of the MMC, represents the decoupling term of the AC voltage, represents the equivalent inductance, represents the equivalent resistance; wherein: ； Among them, 、 、 respectively represent the switching functions of the three phases of the lower arm sub-module , , ; 、 、 respectively represent the average capacitor voltages of the three phases of the lower arm sub-module , , ; 、 、 respectively represent the switching functions of the three phases of the upper arm sub-module , , ; 、 、 respectively represent the average capacitor voltages of the three phases of the upper arm sub-module , , ; ； Among them, , , respectively represent , , three-phase AC voltages; ； Among them, represents the grid-side inductor, represents the grid-side resistor, represents the MMC arm inductor, represents the MMC arm resistor.

3. The method according to claim 2, characterized in that, Based on the three-phase decoupled differential equation of the MMC output current, establishing an MMC output current observer based on the Kalman filter algorithm includes: Performing discretization processing on the three-phase decoupled differential equation to obtain the discretized three-phase output current equation; Converting the three-phase output current equation into the form of a state space equation, and obtaining the prediction equation of the output current as: ； The definitions of each variable in the formula are as follows: ； Among them, represents the predicted value of the output current at moment, represents the observed value of the output current at moment, represents the sampling period; Based on the Kalman filter algorithm, an MMC output current observer is obtained: ； in, Indicates the output current is The measured value at the moment, express Kalman filter gain at time t.

4. The method according to claim 3, characterized in that, It includes: The Kalman filter gain at a specific moment is calculated as follows: ； Among them, represents the moment Kalman filter gain of the phase, represents the moment Kalman filter gain of the phase, represents the moment Kalman filter gain of the phase.

5. The method according to claim 4, wherein It includes: The said , , Three-phase Kalman filter gain 、 、 The specific calculation method is as follows: ； Among them, denotes the time phase Kalman filter gain, denotes the state prediction error covariance at time denotes the measurement noise covariance at time.

6. The method according to claim 5, wherein It includes: The specific calculation method of the state prediction error covariance is: ； Among them, represents the state error covariance at the moment, represents the process noise covariance at the moment.

7. The method according to claim 6, characterized in that It includes: The specific calculation method of the state error covariance is: ； Among them, represents the state error covariance at a moment.

8. The method according to claim 1, characterized in that Using the MMC output current observer to perform state observation on the MMC output current to obtain the observed value of the output current includes: Through the observed output current value of the MMC at moment MMC, calculate to obtain the predicted output current value at moment; Calculation The state error covariance at a moment, according to the calculated The state error covariance at a moment is calculated The Kalman filter gain at a moment; Correct the predicted output current value according to the Kalman filter gain to obtain the output current observation value at the moment; According to Output the observed value of the current at the moment and update The state error covariance at the moment.

9. The method according to claim 1, characterized in that, Judging the AC voltage measurement fault according to the moving effective value of the observation residual includes: Comparing the maximum value of the moving effective value with the set deviation threshold. When the moving effective value is greater than the deviation threshold, it is determined that the corresponding phase has an abnormal AC voltage measurement.

10. The method according to claim 1 or 9, characterized in that It includes: The moving effective value of the observed residual The calculation method is as follows: ； Among them, is the power frequency period of 20 ms, is the observed residual of the phase output current.

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