A method, system and medium for predicting and suppressing zero sequence current of a soft direct current converter
By establishing a discrete prediction model for zero-sequence current and a multi-objective optimization function, the zero-sequence differential mode voltage control quantity is optimized in real time, which solves the problem of insufficient zero-sequence current suppression effect of flexible DC converter under asymmetrical operating conditions and improves control accuracy and stability.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- STATE GRID JIANGSU ELECTRIC POWER CO LTD RESEARCH INSTITUTE
- Filing Date
- 2026-06-17
- Publication Date
- 2026-07-14
AI Technical Summary
Existing flexible DC converters have difficulty accurately suppressing zero-sequence current under asymmetrical operating conditions. Existing zero-sequence current suppression technology relies on preset control parameters, resulting in lag in control response and insufficient compensation accuracy.
By acquiring zero-sequence voltage and current in real time, a discrete prediction model for zero-sequence current is established, a multi-objective optimization function is constructed, and the target zero-sequence differential mode voltage control quantity is solved through rolling optimization to generate a modulation voltage command to suppress zero-sequence current.
It achieves adaptive suppression of zero-sequence current under asymmetrical operating conditions, improves control response speed and compensation accuracy, and reduces DC bus voltage fluctuation.
Smart Images
Figure CN122394043A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of converter control technology, and in particular to a method, system and medium for predicting and suppressing zero-sequence current in a flexible DC converter. Background Technology
[0002] Embedded flexible DC converters are power electronic devices used in flexible DC systems to realize AC-DC energy conversion and power flow regulation. Their AC side is usually directly connected to the power grid, and they are characterized by compact structure and easy engineering integration. However, since there is no connecting transformer on the AC side, when a single-phase ground fault or a three-phase asymmetrical fault occurs in the AC system, the zero-sequence voltage and zero-sequence current in the system are difficult to isolate through the transformer. The zero-sequence component may form a propagation loop through the AC side line, the equivalent impedance branch of the converter, and the zero-sequence voltage output channel of the converter, thereby affecting the safe and stable operation of the converter.
[0003] Existing zero-sequence current suppression techniques typically generate a compensation signal by adding a zero-sequence controller and apply this signal to the converter control circuit to reduce the zero-sequence current amplitude under asymmetrical operating conditions. However, these methods often rely on preset control parameters or fixed compensation strategies, making it difficult to dynamically adjust the zero-sequence compensation control quantity based on real-time changes in zero-sequence current, AC grid-side zero-sequence voltage, and converter operating status. This results in lag in control response and insufficient compensation accuracy, making it difficult to balance zero-sequence current suppression effectiveness with control output smoothness. Therefore, existing technologies suffer from the problem of inaccurate zero-sequence current suppression in embedded flexible DC converters under asymmetrical operating conditions.
[0004] The information disclosed in this background section is intended only to enhance the understanding of the general background of this disclosure and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention
[0005] This invention provides a method, system, and medium for predicting and suppressing zero-sequence current in a flexible DC converter, which can effectively solve the problems in the background art.
[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for predicting and suppressing zero-sequence current in a flexible DC converter, the method comprising: Based on the three-phase voltage and three-phase current on the AC side of the embedded flexible DC converter, obtain the zero-sequence voltage and zero-sequence current on the AC grid side at the current sampling time. Based on the zero-sequence equivalent circuit, AC-side equivalent impedance, and sampling period of the embedded flexible DC converter, a discrete prediction model for zero-sequence current is established to predict the zero-sequence current at the next sampling moment. Based on the zero-sequence current discrete prediction model, a multi-objective optimization function is constructed with zero-sequence current tracking error and zero-sequence differential mode voltage control increment as optimization objectives. The zero-sequence differential-mode voltage control quantity at the current sampling time is used as the optimization variable of the multi-objective optimization function, and the multi-objective optimization function is solved by rolling optimization in combination with the zero-sequence current discrete prediction model to obtain the target zero-sequence differential-mode voltage control quantity. The target zero-sequence differential mode voltage control quantity is superimposed on the modulation reference signal of the embedded flexible DC converter to generate a modulation voltage command and control the operation of the embedded flexible DC converter to suppress the zero-sequence current under asymmetrical operating conditions.
[0007] Furthermore, the zero-sequence voltage and zero-sequence current on the AC grid side at the current sampling time are obtained, including: The three-phase voltages at the current sampling time are summed and averaged to obtain the zero-sequence voltage on the AC grid side; The zero-sequence current is obtained by summing and averaging the three-phase currents at the current sampling time. The zero-sequence voltage and zero-sequence current on the AC grid side are used as real-time inputs to the zero-sequence current discrete prediction model.
[0008] Furthermore, a discrete prediction model for zero-sequence current is established to predict the zero-sequence current at the next sampling time, including: When the embedded flexible DC converter is not connected to the AC side and is in an asymmetrical operating condition, the zero-sequence current propagation path is determined by the AC side line, the AC side equivalent impedance branch and the zero-sequence differential mode voltage output path. The zero-sequence equivalent circuit is constructed based on the zero-sequence current propagation channel. The continuous-time relationship between the AC grid-side zero-sequence voltage, the zero-sequence current, the AC-side equivalent impedance, and the zero-sequence differential-mode voltage control quantity is established based on the zero-sequence equivalent circuit.
[0009] Furthermore, the establishment of the zero-sequence current discrete prediction model includes: The continuous-time relationship is discretized according to the sampling period to obtain the zero-sequence current discrete prediction model; The zero-sequence current discrete prediction model is made to take the zero-sequence current at the current sampling time, the AC grid-side zero-sequence voltage at the current sampling time, and the zero-sequence differential mode voltage control quantity at the current sampling time as inputs; The zero-sequence current discrete prediction model is then used to output the predicted value of the zero-sequence current at the next sampling time.
[0010] Furthermore, based on the aforementioned zero-sequence current discrete prediction model, a multi-objective optimization function is constructed with the zero-sequence current tracking error and the zero-sequence differential-mode voltage control increment as optimization objectives, including: The zero-sequence current prediction value at the next sampling time is obtained based on the zero-sequence current discrete prediction model. The zero-sequence current tracking error is determined based on the deviation between the predicted value of the zero-sequence current and the reference value of the zero-sequence current at the next sampling time. The zero-sequence differential mode voltage control increment is determined based on the deviation between the zero-sequence differential mode voltage control quantity at the current sampling time and the zero-sequence differential mode voltage control quantity at the previous sampling time. The multi-objective optimization function is constructed based on the zero-sequence current tracking error, the zero-sequence differential mode voltage control increment, and the corresponding weighting coefficients.
[0011] Furthermore, the construction of the multi-objective optimization function includes: The zero-sequence current reference value is set as a target value for suppressing the zero-sequence current. The degree to which the zero-sequence current prediction value tracks the zero-sequence current reference value is adjusted by the zero-sequence current tracking error weighting coefficient. The variation amplitude of the zero-sequence differential mode voltage control quantity between adjacent sampling times is limited by the weighting coefficient of the zero-sequence differential mode voltage change. The multi-objective optimization function is designed to balance zero-sequence current suppression performance with control output smoothness.
[0012] Furthermore, by combining the aforementioned zero-sequence current discrete prediction model with the multi-objective optimization function through rolling optimization, the target zero-sequence differential-mode voltage control quantity is obtained, including: Substitute the zero-sequence current discrete prediction model into the multi-objective optimization function, and associate the multi-objective optimization function with the zero-sequence differential mode voltage control quantity at the current sampling time; Using the zero-sequence differential-mode voltage control quantity at the current sampling time as the decision variable of the multi-objective optimization function, the stationary point solution of the multi-objective optimization function is performed; The target zero-sequence differential mode voltage control quantity is generated based on the stationary point solution result. In each sampling period, the target zero-sequence differential voltage control quantity is updated based on the updated zero-sequence current, the AC grid-side zero-sequence voltage, and the zero-sequence differential voltage control quantity at the previous sampling time.
[0013] Furthermore, generating modulation voltage commands and controlling the operation of the embedded flexible DC-DC converter includes: The target zero-sequence differential mode voltage control quantity is input as a zero-sequence control command into the zero-sequence control channel of the embedded flexible DC converter. The zero-sequence control command is combined with the positive-sequence control command and the negative-sequence control command of the embedded flexible DC converter to form the modulation voltage command; The embedded flexible DC converter is driven to operate according to the modulation voltage command in order to suppress the propagation of the zero-sequence current under asymmetrical operating conditions and reduce DC bus voltage fluctuations.
[0014] A zero-sequence current prediction and suppression system for a flexible DC converter, the system comprising: The zero-sequence quantity acquisition module acquires the AC grid-side zero-sequence voltage and zero-sequence current at the current sampling moment based on the three-phase voltage and three-phase current on the AC side of the embedded flexible DC converter. The prediction model building module establishes a discrete prediction model for zero-sequence current based on the zero-sequence equivalent circuit, AC-side equivalent impedance, and sampling period of the embedded flexible DC converter to predict the zero-sequence current at the next sampling moment. The objective function construction module constructs a multi-objective optimization function based on the zero-sequence current discrete prediction model, with the zero-sequence current tracking error and the zero-sequence differential mode voltage control increment as optimization objectives. The rolling optimization solution module uses the zero-sequence differential mode voltage control quantity at the current sampling time as the optimization variable of the multi-objective optimization function, and combines the zero-sequence current discrete prediction model to perform rolling optimization solution of the multi-objective optimization function to obtain the target zero-sequence differential mode voltage control quantity. The modulation command generation module superimposes the target zero-sequence differential mode voltage control quantity onto the modulation reference signal of the embedded flexible DC converter, generates a modulation voltage command, and controls the operation of the embedded flexible DC converter to suppress zero-sequence current under asymmetrical operating conditions.
[0015] A computer-readable storage medium storing a computer program, the computer program including program instructions that, when executed by a processor, can implement the aforementioned zero-sequence current prediction and suppression method for flexible DC converters.
[0016] The technical solution of this invention can achieve the following technical effects: By acquiring zero-sequence voltage and zero-sequence current in real time, a discrete prediction model for zero-sequence current is established. Based on the zero-sequence current tracking error and the zero-sequence differential mode voltage control increment, a multi-objective optimization function is constructed. The target zero-sequence differential mode voltage control quantity is solved in a rolling manner and then superimposed on the modulation reference signal to generate a modulation voltage command to achieve zero-sequence current prediction and suppression. This effectively solves the problem that embedded flexible DC converters without AC side connection transformers have difficulty in adaptively generating zero-sequence compensation control quantity under asymmetrical operating conditions, resulting in insufficient zero-sequence current suppression effect.
[0017] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description
[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0019] Figure 1 It is an embedded direct-connect converter topology; Figure 2 This is the zero-sequence current-voltage equivalent circuit; Figure 3 This is a zero-sequence current suppression process based on multi-objective optimization model predictive control; Figure 4 This is a block diagram of zero-sequence current suppression control based on multi-objective optimization model predictive control. Detailed Implementation
[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.
[0021] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.
[0022] Example 1; like Figure 1 As shown, this application provides a method for predicting and suppressing zero-sequence current in a flexible DC converter. The method includes: Based on the three-phase voltage and three-phase current on the AC side of the embedded flexible DC converter, obtain the zero-sequence voltage and zero-sequence current on the AC grid side at the current sampling time; Based on the zero-sequence equivalent circuit, AC-side equivalent impedance, and sampling period of the embedded flexible DC converter, a discrete prediction model for zero-sequence current is established to predict the zero-sequence current at the next sampling moment. Based on the discrete prediction model of zero-sequence current, a multi-objective optimization function is constructed with zero-sequence current tracking error and zero-sequence differential mode voltage control increment as optimization objectives. The zero-sequence differential-mode voltage control quantity at the current sampling time is used as the optimization variable of the multi-objective optimization function, and the multi-objective optimization function is solved by rolling optimization in combination with the zero-sequence current discrete prediction model to obtain the target zero-sequence differential-mode voltage control quantity. The target zero-sequence differential mode voltage control quantity is superimposed on the modulation reference signal of the embedded flexible DC converter to generate a modulation voltage command and control the operation of the embedded flexible DC converter to suppress the zero-sequence current under asymmetrical operating conditions.
[0023] Specifically, this embodiment provides a zero-sequence current prediction and suppression method for flexible DC converters, which is applied to embedded flexible DC converters without AC-side transformers. Embedded flexible DC converters are prone to forming zero-sequence current propagation paths under single-phase grounding faults or three-phase asymmetrical faults. This embodiment generates zero-sequence compensation control quantities through predictive control and applies them to the converter modulation stage to suppress zero-sequence current under asymmetrical operating conditions. The controller collects the three-phase voltage and three-phase current on the AC side of the embedded flexible DC converter in real time, and performs summation and averaging of the three-phase voltage to obtain the zero-sequence voltage on the AC grid side, and summation and averaging of the three-phase current to obtain the zero-sequence current. The controller establishes a discrete prediction model for zero-sequence current based on the zero-sequence equivalent circuit, AC-side equivalent impedance, and sampling period of the embedded flexible DC converter. This model enables the zero-sequence current to predict the zero-sequence current at the next sampling moment based on the zero-sequence current at the current sampling moment, the zero-sequence voltage on the AC grid side, and the zero-sequence differential mode voltage control quantity. The controller constructs a multi-objective optimization function based on the zero-sequence current discrete prediction model, and uses the deviation of the zero-sequence current prediction value from the zero-sequence current reference value as the zero-sequence current tracking error, and uses the change of the zero-sequence differential mode voltage control quantity at the current sampling time relative to the zero-sequence differential mode voltage control quantity at the previous sampling time as the zero-sequence differential mode voltage control increment, so that the multi-objective optimization function takes into account both the zero-sequence current suppression effect and the smoothness of the control output. The controller uses the zero-sequence differential-mode voltage control quantity at the current sampling moment as the optimization variable to perform rolling optimization of the multi-objective optimization function to obtain the target zero-sequence differential-mode voltage control quantity; within each sampling period, the controller re-solves for the target zero-sequence differential-mode voltage control quantity based on the updated AC grid-side zero-sequence voltage, zero-sequence current and the zero-sequence differential-mode voltage control quantity at the previous sampling moment. The controller uses the target zero-sequence differential mode voltage control quantity as a zero-sequence compensation component to superimpose on the modulation reference signal of the embedded flexible DC converter, generates a modulation voltage command, and controls the operation of the embedded flexible DC converter according to the modulation voltage command, thereby realizing online prediction and suppression of zero-sequence current under asymmetrical operating conditions.
[0024] The technical solution of this invention acquires zero-sequence voltage and zero-sequence current in real time to establish a discrete prediction model for zero-sequence current. Based on the zero-sequence current tracking error and the zero-sequence differential mode voltage control increment, a multi-objective optimization function is constructed. After the target zero-sequence differential mode voltage control quantity is solved in a rolling manner, it is superimposed on the modulation reference signal to generate a modulation voltage command to achieve zero-sequence current prediction and suppression. This effectively solves the problem that embedded flexible DC converters without AC side connection transformers have difficulty in adaptively generating zero-sequence compensation control quantity under asymmetrical operating conditions, resulting in insufficient zero-sequence current suppression effect.
[0025] Furthermore, obtaining the AC grid-side zero-sequence voltage and zero-sequence current at the current sampling moment includes: The three-phase voltages at the current sampling time are summed and averaged to obtain the zero-sequence voltage on the AC grid side; The zero-sequence current is obtained by summing and averaging the three-phase currents at the current sampling time. The zero-sequence voltage and zero-sequence current on the AC grid side are used as real-time inputs to the zero-sequence current discrete prediction model.
[0026] As a preferred embodiment of the above, since no connecting transformer is installed on the AC side of the embedded direct-connected flexible DC converter, when a single-phase ground fault or a three-phase asymmetrical fault occurs in the AC system, the zero-sequence component cannot be isolated by the connecting transformer. The zero-sequence voltage can form a closed loop along the AC side line, the equivalent inductance-resistance branch of the AC side of the converter, and the zero-sequence differential mode voltage output channel of the converter, thereby generating a zero-sequence current in the system; the zero-sequence current in the zero-sequence current voltage path of the embedded direct-connected flexible DC converter is as follows: Figure 1 As shown; Among them, the zero-sequence voltage on the AC grid side Defined as one-third of the sum of the phase voltages on the three-phase AC side, zero-sequence current. Defined as one-third of the sum of the phase currents on the three-phase AC side, that is: ; in, , , These are the voltages of each phase on the three-phase AC side; , , These are the currents of each phase on the three-phase AC side; Taking the inverter side as an example, the MMC equivalent circuit is as follows: Figure 1 As shown, by Figure 2 The current-voltage relationship of the zero-sequence circuit can be obtained as follows: ; in This is the zero-sequence voltage on the AC grid side. This is the zero-sequence compensation voltage output by the converter. and These are the equivalent resistance and equivalent inductance on the AC side, respectively.
[0027] Furthermore, a discrete prediction model for zero-sequence current is established to predict the zero-sequence current at the next sampling time, including: When no connecting transformer is installed on the AC side of the embedded flexible DC converter and it is in an asymmetrical operating condition, the zero-sequence current propagation path is determined by the AC side line, the AC side equivalent impedance branch and the zero-sequence differential mode voltage output channel. Construct a zero-sequence equivalent circuit based on the zero-sequence current propagation path; Establish the continuous-time relationship between the zero-sequence voltage, zero-sequence current, AC equivalent impedance, and zero-sequence differential-mode voltage control quantity on the AC grid side based on the zero-sequence equivalent circuit.
[0028] Furthermore, the establishment of the discrete prediction model for zero-sequence current includes: The continuous-time relationship is discretized according to the sampling period to obtain the zero-sequence current discrete prediction model; The zero-sequence current discrete prediction model takes the zero-sequence current at the current sampling time, the AC grid-side zero-sequence voltage at the current sampling time, and the zero-sequence differential mode voltage control quantity at the current sampling time as inputs. This enables the discrete prediction model for zero-sequence current to output the predicted value of zero-sequence current at the next sampling time.
[0029] As a preferred embodiment of the above, the zero-sequence circuit current-voltage relationship is discretized using Euler discretization, yielding the prediction expression for the zero-sequence current at the next sampling time: ; in, This is the predicted value of the zero-sequence current at the next sampling time. This is the zero-sequence current measurement value at the current sampling time. and These are the equivalent AC side resistance and inductance, respectively. The sampling period is This represents the zero-sequence voltage on the AC grid side at the current sampling time. This is the zero-sequence differential mode voltage control quantity currently output by the converter.
[0030] Furthermore, based on the zero-sequence current discrete prediction model, and with the zero-sequence current tracking error and the zero-sequence differential-mode voltage control increment as optimization objectives, a multi-objective optimization function is constructed, including: The predicted value of the zero-sequence current at the next sampling time is obtained based on the zero-sequence current discrete prediction model; The zero-sequence current tracking error is determined based on the deviation between the predicted value of the zero-sequence current and the reference value of the zero-sequence current at the next sampling time. The zero-sequence differential mode voltage control increment is determined based on the deviation between the zero-sequence differential mode voltage control quantity at the current sampling time and the zero-sequence differential mode voltage control quantity at the previous sampling time. A multi-objective optimization function is constructed based on the zero-sequence current tracking error, the zero-sequence differential mode voltage control increment, and the corresponding weighting coefficients.
[0031] Furthermore, the construction of a multi-objective optimization function includes: Set the zero-sequence current reference value as the target value for suppressing zero-sequence current; The degree to which the zero-sequence current prediction value tracks the zero-sequence current reference value is adjusted by the zero-sequence current tracking error weighting coefficient. The variation amplitude of the zero-sequence differential mode voltage control quantity between adjacent sampling times is limited by the weighting coefficient of the zero-sequence differential mode voltage change. This allows the multi-objective optimization function to balance zero-sequence current suppression performance and control output smoothness.
[0032] As a preferred embodiment of the above embodiments, based on the above-described zero-sequence current discrete prediction model, the zero-sequence compensation voltage output by the converter can be... As the decision variable for model predictive control; in order to achieve rapid suppression of zero-sequence current while avoiding drastic fluctuations in control quantity, a multi-objective optimization cost function that balances zero-sequence current tracking performance and control smoothness is constructed. Let the zero-sequence current reference value be... Then the tracking error of the zero-sequence current at the next sampling time can be expressed as: ; in, The zero-sequence current prediction value at the next sampling time is obtained from the discrete prediction model; since the control objective of this invention is to suppress the zero-sequence current, the reference value of the zero-sequence current is usually taken as: ; To ensure that the predicted zero-sequence current approximates the reference value as closely as possible, and to reduce the abrupt changes in the zero-sequence compensation voltage command between adjacent sampling times, a control increment term is introduced: ; in, This is the control quantity for the zero-sequence differential mode voltage output of the current converter; k is the zero-sequence differential mode voltage control quantity output by the converter at the previous sampling time; k is the current sampling time. Therefore, the multi-objective optimization cost function is constructed as follows: ; in, This is the zero-sequence current tracking error weighting coefficient. This is the weighting coefficient for the zero-sequence compensation voltage change, and ; In the aforementioned cost function, the first term measures the degree to which the predicted zero-sequence current at the next sampling time tracks the reference value. The larger its weight, the higher the controller's requirements for zero-sequence current suppression performance. The second term constrains the variation amplitude of the zero-sequence compensation voltage between adjacent sampling times. The larger its weight, the higher the controller's requirements for control output smoothness and system stability. By simultaneously optimizing the above two objectives, a balance can be achieved between the zero-sequence current suppression effect and the smoothness of control quantity changes, thereby improving system operating stability and reducing switching shocks caused by drastic changes in control quantity.
[0033] Furthermore, by combining the zero-sequence current discrete prediction model with the rolling optimization solution of the multi-objective optimization function, the target zero-sequence differential mode voltage control quantity is obtained, including: Substitute the zero-sequence current discrete prediction model into the multi-objective optimization function, and associate the multi-objective optimization function with the zero-sequence differential mode voltage control quantity at the current sampling time; Using the zero-sequence differential-mode voltage control quantity at the current sampling time as the decision variable of the multi-objective optimization function, the stationary point solution of the multi-objective optimization function is performed; Generate the target zero-sequence differential mode voltage control quantity based on the stationary point solution results; Within each sampling period, the target zero-sequence differential voltage control quantity is updated based on the updated zero-sequence current, AC grid-side zero-sequence voltage, and the zero-sequence differential voltage control quantity at the previous sampling time.
[0034] Furthermore, generating modulation voltage commands and controlling the operation of the embedded flexible DC-DC converter includes: The target zero-sequence differential mode voltage control quantity is input as a zero-sequence control command into the zero-sequence control channel of the embedded flexible DC converter. The zero-sequence control command is combined with the positive-sequence and negative-sequence control commands of the embedded flexible DC converter to form a modulation voltage command; The embedded flexible DC converter is driven to operate according to the modulation voltage command in order to suppress the propagation of zero-sequence current under asymmetrical operating conditions and reduce DC bus voltage fluctuations.
[0035] As a preferred embodiment of the above, such as Figure 3 and Figure 4 As shown, substituting the discrete prediction model of the zero-sequence current at the next time step into the multi-objective optimization cost function, we can obtain: ; In the formula, the first term is used to characterize the tracking error of the zero-sequence current prediction value to the reference value at the next sampling time, and the second term is used to constrain the change amplitude of the zero-sequence compensation voltage at adjacent sampling times, so as to balance the zero-sequence current suppression performance and the smoothness of the control output. Due to the cost function and zero-sequence compensation voltage Related, therefore can As an optimization variable, the cost function is adjusted at each sampling time step. Perform online solution to obtain the optimal zero-sequence compensation voltage that minimizes the cost function; To reduce the complexity of online solutions, the cost function is... Regarding zero-sequence compensation voltage Find the partial derivative and set it equal to zero, that is: ; Cost function For zero-sequence compensation voltage Find the partial derivative and set it equal to zero. For ease of expression, let: ; The above formula can then be written as: ; For cost function Regarding zero-sequence compensation voltage Find the partial derivatives, and set the partial derivatives equal to zero: ; After sorting, we can obtain: ; Further expand and organize information about From the terms, we can obtain: ; Therefore, the analytical expression for the target zero-sequence compensation voltage can be obtained as follows: ; Substituting A back, we get: ; Since this invention aims to suppress zero-sequence current, the reference value of the zero-sequence current at the next moment is typically set to satisfy: ; The target zero-sequence compensation voltage can then be further written as: ; From the above equation, it can be seen that the target zero-sequence compensation voltage From the zero-sequence current at the current sampling time AC grid side zero-sequence voltage Zero-sequence compensation voltage and weighting coefficient at the previous sampling time The controller jointly determines the target zero-sequence compensation voltage based on real-time measurements during each sampling period and applies it to the converter's zero-sequence control channel, thereby achieving rolling optimization suppression of the zero-sequence current. The model predictive controller adjusts the cost function at each sampling time step. By performing an online solution, the optimal zero-sequence compensation voltage that minimizes the cost function is obtained, i.e.: ; Calculate the target zero-sequence compensation voltage Then, it is input as a zero-sequence control command into the modular multilevel converter (MMC) control system and superimposed on the converter modulation voltage command to achieve zero-sequence current suppression control; The optimal zero-sequence compensation voltage obtained through the above method can ensure that the zero-sequence current quickly approaches the reference value while taking into account the smoothness of the control quantity change, avoiding system oscillation and switching shock caused by sudden changes in the zero-sequence compensation command, thereby improving the operating stability and control performance of MMC under asymmetrical operating conditions.
[0036] Example 2; Based on the same inventive concept as the zero-sequence current prediction and suppression method for a flexible DC converter in the foregoing embodiments, the present invention also provides a zero-sequence current prediction and suppression system for a flexible DC converter, the system comprising: The zero-sequence quantity acquisition module acquires the AC grid-side zero-sequence voltage and zero-sequence current at the current sampling moment based on the three-phase voltage and three-phase current on the AC side of the embedded flexible DC converter. The prediction model building module establishes a discrete prediction model for zero-sequence current based on the zero-sequence equivalent circuit, AC-side equivalent impedance, and sampling period of the embedded flexible DC converter to predict the zero-sequence current at the next sampling moment. The objective function construction module constructs a multi-objective optimization function based on the zero-sequence current discrete prediction model, with the zero-sequence current tracking error and the zero-sequence differential mode voltage control increment as optimization objectives. The rolling optimization solution module uses the zero-sequence differential mode voltage control quantity at the current sampling time as the optimization variable of the multi-objective optimization function, and combines the zero-sequence current discrete prediction model to perform rolling optimization solution of the multi-objective optimization function to obtain the target zero-sequence differential mode voltage control quantity. The modulation command generation module superimposes the target zero-sequence differential mode voltage control quantity onto the modulation reference signal of the embedded flexible DC converter, generates a modulation voltage command, and controls the operation of the embedded flexible DC converter to suppress zero-sequence current under asymmetrical operating conditions.
[0037] The system described above in this invention can effectively implement a method for predicting and suppressing zero-sequence current in a flexible DC converter. The technical effects it can achieve are as described in the above embodiments, and will not be repeated here.
[0038] Example 3; Based on the same inventive concept as the zero-sequence current prediction and suppression method for a flexible DC converter in the foregoing embodiments, the present invention also provides a computer-readable storage medium storing a computer program, the computer program including program instructions, which, when executed by a processor, can implement the zero-sequence current prediction and suppression method for a flexible DC converter.
[0039] Although this application has been described in conjunction with specific features and embodiments, it is obvious that various modifications and combinations can be made thereto without departing from the spirit and scope of this application. Accordingly, this specification and drawings are merely exemplary illustrations of the application as defined herein, and are to be considered as covering any and all modifications, variations, combinations, or equivalents within the scope of this application. Clearly, those skilled in the art can make various alterations and modifications to this application without departing from its scope. Thus, if such modifications and modifications fall within the scope of this application and its equivalents, this application intends to include such modifications and modifications.
Claims
1. A method for predicting and suppressing zero-sequence current in a flexible DC converter, characterized in that, The method includes: Based on the three-phase voltage and three-phase current on the AC side of the embedded flexible DC converter, obtain the zero-sequence voltage and zero-sequence current on the AC grid side at the current sampling time; Based on the zero-sequence equivalent circuit, AC-side equivalent impedance, and sampling period of the embedded flexible DC converter, a discrete prediction model for zero-sequence current is established to predict the zero-sequence current at the next sampling moment. Based on the zero-sequence current discrete prediction model, a multi-objective optimization function is constructed with zero-sequence current tracking error and zero-sequence differential mode voltage control increment as optimization objectives. The zero-sequence differential-mode voltage control quantity at the current sampling time is used as the optimization variable of the multi-objective optimization function, and the multi-objective optimization function is solved by rolling optimization in combination with the zero-sequence current discrete prediction model to obtain the target zero-sequence differential-mode voltage control quantity. The target zero-sequence differential mode voltage control quantity is superimposed on the modulation reference signal of the embedded flexible DC converter to generate a modulation voltage command and control the operation of the embedded flexible DC converter to suppress the zero-sequence current under asymmetrical operating conditions.
2. The method for predicting and suppressing zero-sequence current in a flexible DC converter according to claim 1, characterized in that, Obtain the AC grid-side zero-sequence voltage and zero-sequence current at the current sampling time, including: The three-phase voltages at the current sampling time are summed and averaged to obtain the zero-sequence voltage on the AC grid side; The zero-sequence current is obtained by summing and averaging the three-phase currents at the current sampling time. The zero-sequence voltage and zero-sequence current on the AC grid side are used as real-time inputs to the zero-sequence current discrete prediction model.
3. The method for predicting and suppressing zero-sequence current in a flexible DC converter according to claim 1, characterized in that, Establish a discrete prediction model for zero-sequence current to predict the zero-sequence current at the next sampling time, including: When the embedded flexible DC converter is not connected to the AC side and is in an asymmetrical operating condition, the zero-sequence current propagation path is determined by the AC side line, the AC side equivalent impedance branch and the zero-sequence differential mode voltage output path. The zero-sequence equivalent circuit is constructed based on the zero-sequence current propagation channel. The continuous-time relationship between the AC grid-side zero-sequence voltage, the zero-sequence current, the AC-side equivalent impedance, and the zero-sequence differential-mode voltage control quantity is established based on the zero-sequence equivalent circuit.
4. The method for predicting and suppressing zero-sequence current in a flexible DC converter according to claim 3, characterized in that, The establishment of the zero-sequence current discrete prediction model includes: The continuous-time relationship is discretized according to the sampling period to obtain the zero-sequence current discrete prediction model; The zero-sequence current discrete prediction model is made to take the zero-sequence current at the current sampling time, the AC grid-side zero-sequence voltage at the current sampling time, and the zero-sequence differential mode voltage control quantity at the current sampling time as inputs; The zero-sequence current discrete prediction model is then used to output the predicted value of the zero-sequence current at the next sampling time.
5. The method for predicting and suppressing zero-sequence current in a flexible DC converter according to claim 1, characterized in that, Based on the aforementioned zero-sequence current discrete prediction model, a multi-objective optimization function is constructed with the zero-sequence current tracking error and the zero-sequence differential-mode voltage control increment as optimization objectives, including: The zero-sequence current prediction value at the next sampling time is obtained based on the zero-sequence current discrete prediction model. The zero-sequence current tracking error is determined based on the deviation between the predicted value of the zero-sequence current and the reference value of the zero-sequence current at the next sampling time. The zero-sequence differential mode voltage control increment is determined based on the deviation between the zero-sequence differential mode voltage control quantity at the current sampling time and the zero-sequence differential mode voltage control quantity at the previous sampling time. The multi-objective optimization function is constructed based on the zero-sequence current tracking error, the zero-sequence differential mode voltage control increment, and the corresponding weighting coefficients.
6. The method for predicting and suppressing zero-sequence current in a flexible DC converter according to claim 5, characterized in that, The construction of the multi-objective optimization function includes: The zero-sequence current reference value is set as a target value for suppressing the zero-sequence current. The degree to which the zero-sequence current prediction value tracks the zero-sequence current reference value is adjusted by the zero-sequence current tracking error weighting coefficient. The variation amplitude of the zero-sequence differential mode voltage control quantity between adjacent sampling times is limited by the weighting coefficient of the zero-sequence differential mode voltage change. The multi-objective optimization function is designed to balance zero-sequence current suppression performance with control output smoothness.
7. The method for predicting and suppressing zero-sequence current in a flexible DC converter according to claim 6, characterized in that, By combining the aforementioned zero-sequence current discrete prediction model with the multi-objective optimization function through rolling optimization, the target zero-sequence differential mode voltage control quantity is obtained, including: Substitute the zero-sequence current discrete prediction model into the multi-objective optimization function, and associate the multi-objective optimization function with the zero-sequence differential mode voltage control quantity at the current sampling time; Using the zero-sequence differential-mode voltage control quantity at the current sampling time as the decision variable of the multi-objective optimization function, the stationary point solution of the multi-objective optimization function is performed; The target zero-sequence differential mode voltage control quantity is generated based on the stationary point solution result. In each sampling period, the target zero-sequence differential voltage control quantity is updated based on the updated zero-sequence current, the AC grid-side zero-sequence voltage, and the zero-sequence differential voltage control quantity at the previous sampling time.
8. The method for predicting and suppressing zero-sequence current in a flexible DC converter according to claim 1, characterized in that, Generating modulation voltage commands and controlling the operation of the embedded flexible DC-DC converter includes: The target zero-sequence differential mode voltage control quantity is input as a zero-sequence control command into the zero-sequence control channel of the embedded flexible DC converter. The zero-sequence control command is combined with the positive-sequence control command and the negative-sequence control command of the embedded flexible DC converter to form the modulation voltage command; The embedded flexible DC converter is driven to operate according to the modulation voltage command in order to suppress the propagation of the zero-sequence current under asymmetrical operating conditions and reduce DC bus voltage fluctuations.
9. A zero-sequence current prediction and suppression system for a flexible DC converter, characterized in that, The system includes: The zero-sequence quantity acquisition module acquires the AC grid-side zero-sequence voltage and zero-sequence current at the current sampling moment based on the three-phase voltage and three-phase current on the AC side of the embedded flexible DC converter. The prediction model building module establishes a discrete prediction model for zero-sequence current based on the zero-sequence equivalent circuit, AC-side equivalent impedance, and sampling period of the embedded flexible DC converter to predict the zero-sequence current at the next sampling moment. The objective function construction module constructs a multi-objective optimization function based on the zero-sequence current discrete prediction model, with the zero-sequence current tracking error and the zero-sequence differential mode voltage control increment as optimization objectives. The rolling optimization solution module uses the zero-sequence differential mode voltage control quantity at the current sampling time as the optimization variable of the multi-objective optimization function, and combines the zero-sequence current discrete prediction model to perform rolling optimization solution of the multi-objective optimization function to obtain the target zero-sequence differential mode voltage control quantity. The modulation command generation module superimposes the target zero-sequence differential mode voltage control quantity onto the modulation reference signal of the embedded flexible DC converter, generates a modulation voltage command, and controls the operation of the embedded flexible DC converter to suppress zero-sequence current under asymmetrical operating conditions.
10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program, the computer program including program instructions, which, when executed by a processor, can implement the zero-sequence current prediction and suppression method for flexible DC converters as described in any one of claims 1-8.