A low-computational multi-step model predictive circulating current suppression method and system for MMC

Abstract

The present application relates to a kind of low calculation amount's MMC multi-step model prediction circulating current suppression method and system.Firstly, by the method of reverse prediction, the basic level number of upper and lower bridge arm input that satisfies phase current tracking is obtained, then using multi-step model prediction circulating current suppression control, the upper and lower bridge arm compensation level number that realizes circulating current suppression is obtained, by the method of simultaneously compensating the same level number in upper and lower bridge arm, to reach the goal of not affecting phase current tracking accuracy, and suppressing bridge arm internal circulating current, not only can improve the problem that single-step model prediction is not good for circulating current suppression effect, but also consider the goal of phase current tracking, and avoid the problem of large amount of calculation of conventional multi-step model prediction control, only by 8 times calculation, the optimal compensation level number is obtained, greatly reduce the amount of calculation, applicable to engineering application.

Description

Technical Field

[0001] This invention belongs to the field of power engineering technology. Specifically, it relates to a low-computational-load MMC multi-step model prediction circulating current suppression method and system. Background Technology

[0002] The modular design of the Modular Multilevel Converter (MMC) makes it very easy to expand its voltage and power levels, and it is widely used in grid connection of renewable energy generation such as wind power and photovoltaics. However, the imbalance of the total voltage input of the three-phase bridge arm of the MMC will cause negative sequence second harmonic circulating current between the three phases. The existence of circulating current will not only increase the loss of the MMC and reduce its efficiency, but also cause distortion of the output phase current. Therefore, it is very necessary to reduce the circulating current content. Summary of the Invention

[0003] To address the aforementioned issues, this invention presents a low-computation MMC multi-step model prediction circulating current suppression method. By simultaneously compensating the same number of levels in both the upper and lower bridge arms, the method aims to suppress circulating current within the bridge arms without affecting phase current tracking. This improves upon the local optima problem inherent in single-step model prediction control methods used for circulating current suppression, reduces the circulating current content, and significantly lowers the computational load, facilitating engineering applications.

[0004] The technical solution provided by this invention is:

[0005] The design of a low-computation MMC multi-step model predictive circulation suppression controller is characterized by,

[0006] The specific steps are as follows:

[0007] A low-computation-consumption MMC multi-step model prediction circulation suppression method, characterized by comprising the following steps:

[0008] Obtain the base number of upper and lower bridge arm sub-modules that meet the phase current tracking requirements at the current moment. , ;

[0009] Compensate the upper and lower bridge arms with the same number of voltage levels to ensure the final number of submodules deployed. , It satisfies the balance conditions of AC side current tracking and phase-to-phase circulating current suppression.

[0010] In the suppression method described above, the final number of sub-modules deployed is... , satisfy: , ; It is the optimal number of compensation levels for simultaneous compensation of the upper and lower bridge arms.

[0011] In the above-mentioned suppression methods, The steps to determine this are:

[0012] Single-step prediction: Construct a single-step model to predict the circulating current control value function, and use the five-level compensation method to obtain the number of optimal, second-optimal, and third-optimal compensation levels;

[0013] Multi-step prediction: Construct a multi-step model to predict the value function of the circulating current control, and obtain the optimal number of compensation levels based on the number of compensation levels obtained from the single-step prediction.

[0014] In the above-mentioned suppression methods, t k The base number of upper and lower bridge arm input submodules that always meet phase current tracking requirements , Obtained from the external characteristic equation of MMC:

[0015] (1)

[0016] in e j The output phase voltage of the converter ，u uj ，u lj These are the output voltages of the upper and lower bridge arms, respectively. u sj This is the grid-side voltage. i j To determine the output phase current, perform a first-order forward difference on equation (1) to obtain the predicted value (2) of the MMC converter phase current at time k+1.

[0017] (2)

[0018] in , ;

[0019] T s To control the sampling period for model prediction, L arm The equivalent inductance of the bridge arm is used to limit the starting current and short-circuit current. L The equivalent inductance on the grid side, R This is the equivalent resistance on the grid side.

[0020] In the above suppression method, the tracking condition for the AC side current is:

[0021]

[0022] in J 0 is the objective function for AC current tracking.；i j ( k+ 1) For k+ The predicted value of the alternating current at time 1. for k+ The reference value of the alternating current at moment 1, k Calculate the predicted values ​​of all currents at any given time, and select the values ​​that make the predicted values ​​of all currents available. J The minimum voltage of the upper and lower bridge arms is used as the input voltage, and the optimal input voltage of the upper bridge arm is... The optimal voltage for the lower bridge arm is This enables optimal control of the AC current; the upper and lower bridge arm input submodules are respectively denoted as... N uj , N lj .

[0023] In the above suppression method, the output AC tracking and the circulation suppression balance condition are satisfied at the same time. That is, the first-order forward difference of the internal characteristic equation of MMC is performed to obtain the single-step model prediction function of circulation (3).

[0024] (2)

[0025] (3)

[0026] in , They are respectively t k+1 The upper and lower bridge arm voltages must always meet the requirements for phase current tracking. U dc It is a DC voltage; i diffj For equivalent converter j Phase internal current,

[0027] Constructing a single-step model to predict the value function of circulation control J 1. Obtain the number of its optimal, second-optimal, and third-optimal compensation levels;

[0028] Internal current of each phase i diffj It is a phase circulation Composed of one-third of the DC current, to make the circulating current zero, the reference value of the internal current of each phase is... ;

[0029] (4)

[0030] Five-level compensation is used, and the number of compensation levels is denoted as follows: The compensation voltage is denoted as ;

[0031] Define single-step model predictive circulating current control, and upper and lower arm level compensation sets. , To ensure consistent AC current tracking accuracy, the number of compensation levels in the upper and lower bridge arms must be equal, and the compensation voltage is...

[0032] (5)

[0033] Select to make The number of compensation levels corresponding to the smallest, second smallest, and second-smallest values ​​are denoted as follows: d 1. d 2. d 3; among which d 1 represents the number of compensation levels corresponding to the optimal single-step circulating current prediction. d 2 represents the number of compensation levels corresponding to the suboptimal single-step circulating current prediction. d 3 represents the number of compensation levels corresponding to the second-best single-step circulating current prediction.

[0034] In the above suppression methods, the definition is... D 2 represents the multi-step model prediction circulating current control level compensation set. D 2=[ d 1 d 2 d 3]; The number of compensation levels predicted by the multi-step model is denoted as Compensation voltage Its value is:

[0035] (6)

[0036] The internal characteristic equation of MMC is subjected to first-order forward difference, with a sampling period of . T s， get t k+2 Predicted internal current value at time ;

[0037] (7)

[0038] , They are respectively t k+2 The upper and lower bridge arm input voltages must always meet phase current tracking requirements;

[0039] Constructing a multi-step model to predict the value function of circulation control J 2

[0040] (8)

[0041] Select to make The number of compensation levels corresponding to the minimum value is applied to the converter. At that moment, the corresponding compensation level is the optimal compensation level. Therefore, the number of upper and lower levels that satisfy both output AC tracking and circulating current suppression are (9) and (10) respectively.

[0042] (9)

[0043] (10).

[0044] A low-computational-consumption MMC multi-step model prediction circulation suppression system, characterized by comprising the following steps:

[0045] Module 1: Obtain the base number of upper and lower bridge arms that meet the phase current tracking requirements at the current moment. , ;

[0046] The second module: compensates the upper and lower bridge arms with the same number of voltage levels, so that the final number of sub-modules deployed is [number missing]. , It satisfies the conditions for AC side current tracking, phase-to-phase circulating current suppression, and submodule capacitor voltage balance.

[0047] In the aforementioned suppression system, the final number of sub-modules deployed... , satisfy: , ; It is the optimal number of compensation levels for simultaneous compensation of the upper and lower bridge arms.

[0048] In the aforementioned inhibition system, The steps to determine this are:

[0049] Single-step prediction: Construct a single-step model to predict the circulating current control value function, and use the five-level compensation method to obtain the number of optimal, second-optimal, and third-optimal compensation levels;

[0050] Multi-step prediction: Construct a multi-step model to predict the value function of the circulating current control, and obtain the optimal number of compensation levels based on the number of compensation levels obtained from the single-step prediction.

[0051] The present invention has the following advantages: the improved multi-step model predictive control is used for circulating current suppression. By simultaneously compensating the same number of levels in the upper and lower bridge arms, it achieves the goal of suppressing circulating current within the bridge arms without affecting phase current tracking. This not only improves the problem of poor performance of single-step model predictive control for circulating current suppression, but also takes into account the goal of phase current tracking. Furthermore, it avoids the problem of large computational load in conventional multi-step model predictive control. The optimal number of compensation levels is obtained in only 8 calculations, which greatly reduces the computational load and makes it suitable for engineering applications. Attached Figure Description

[0052] Figure 1 Three-phase MMC topology.

[0053] Figure 2 Schematic diagram of improved multi-step model predictive control. Specific implementation methods

[0054] The method for quantitative analysis of battery faults provided by this invention will be described in further detail below.

[0055] This embodiment designs a low-computation MMC multi-step model prediction circulation suppression controller, which mainly includes the following two steps.

[0056] The first step is to obtain the single-step compensation level set by predicting the circulating current suppression control using a single-step model. First, a single-step model prediction function for the circulating current is established. The MMC schematic is shown below. Figure 1 As shown, its differential equation is as follows:

[0057] (1)

[0058] (2)

[0059] in e j The output phase voltage of the converter ，u uj ，u lj These are the output voltages of the upper and lower bridge arms, respectively. u sj This is the grid-side voltage. i j To determine the output phase current, analysis of the external characteristic equation (1) of the MMC shows that the converter output phase current is related to the voltage difference between the upper and lower bridge arms. From equation (1), we can obtain... t k The base number of upper and lower bridge arm input submodules that always meet phase current tracking requirements , Analysis of the internal characteristic equation (2) of the MMC shows that the internal current and DC voltage of the bridge arm are related to the difference between the sum of the voltages of the upper and lower bridge arms. This invention first obtains the base number of upper and lower bridge arm submodules that satisfy phase current tracking using formula (1). , Then, by simultaneously compensating the same level number in both the upper and lower bridge arms, the goal of both not affecting phase current tracking and suppressing bridge arm circulating current is achieved.

[0060] By performing a first-order forward difference on formula (2), we obtain the single-step model prediction function (3) for the circulation.

[0061] (3)

[0062] in , They are respectively t k+1 The upper and lower bridge arm voltages must always meet the phase current tracking requirements.

[0063] Then, a single-step model is constructed to predict the value function of the circulating current control, and its optimal, second-optimal, and third-optimal compensation levels are obtained.

[0064] The internal current of each phase is a phase-to-phase circulating current. It consists of one-third DC current. To suppress circulating current as much as possible, the reference value of the internal current of each phase is... .

[0065] (4)

[0066] This invention employs five-level compensation, with the number of compensation levels denoted as follows: The compensation voltage is denoted as Define single-step model predictive circulating current control, with upper and lower arm level compensation sets. , To reduce computational load and maintain phase current tracking accuracy, the number of compensation levels in the upper and lower bridge arms must be equal, and the compensation voltage is...

[0067] (5)

[0068] Then select to make The number of compensation levels corresponding to the smallest, second smallest, and second-smallest values ​​are denoted as follows: d 1. d 2. d 3. Among them d 1 represents the number of compensation levels corresponding to the optimal single-step circulating current prediction. d 2 represents the number of compensation levels corresponding to the suboptimal single-step circulating current prediction. d 3 represents the number of compensation levels corresponding to the second-best single-step circulating current prediction.

[0069] The second step involves predicting the circulating current suppression control using a multi-step model to obtain the final optimal number of compensation levels. First, define... D 2 represents the multi-step model prediction circulating current control level compensation set, which is known from the first step. D 2=[ d 1 d 2 d 3). The number of compensation levels predicted by the multi-step model is denoted as... Compensation voltage Its value is:

[0070] (6)

[0071] Then, the multi-step model predictive control value function is constructed to obtain the final optimal compensation level number. The improved multi-step model predictive control principle diagram is shown below. Figure 2 .

[0072] Perform a first-order forward difference on equation (2). Based on the first step, the sampling period is... T s， It can be obtained t k+2 Predicted internal current value at any given time.

[0073] (7)

[0074] , They are respectively t k+2 The upper and lower bridge arms are always connected to the phase current tracking voltage.

[0075] Constructing a multi-step model to predict the value function of circulation control

[0076] (8)

[0077] Select to make The number of compensation levels corresponding to the minimum value is applied to the converter. At that moment, the corresponding compensation level is the optimal compensation level. Therefore, the number of upper and lower levels that ultimately satisfy both output AC tracking and circulating current suppression are (9) and (10), respectively;

[0078] (9)

[0079] (10)

[0080] In summary, this invention found the optimal number of compensation levels in only 8 calculations, achieving multi-step model prediction and circulating current suppression. This not only ensures the accuracy of phase current tracking but also effectively suppresses circulating current, making it suitable for engineering applications.

[0081] This embodiment also designs a low-computational-consumption MMC multi-step model prediction circulation suppression system, including the following steps:

[0082] Module 1: Obtain the base number of upper and lower bridge arms that meet the phase current tracking requirements at the current moment. , ;

[0083] The second module: compensates the upper and lower bridge arms with the same number of voltage levels, so that the final number of sub-modules deployed is [number missing]. , It satisfies the conditions for AC side current tracking, phase-to-phase circulating current suppression, and submodule capacitor voltage balance.

[0084] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A low-computational-consumption MMC multi-step model prediction circulation suppression method, characterized in that, Includes the following steps: Obtain the base number of upper and lower bridge arm sub-modules that meet the phase current tracking requirements at the current moment. , ; Compensate the upper and lower bridge arms with the same number of voltage levels to ensure the final number of submodules deployed. , It satisfies the conditions for AC side current tracking and interphase circulating current suppression; The final number of sub-modules deployed , satisfy: , ; It is the optimal number of compensation levels for simultaneous compensation of the upper and lower bridge arms; The steps to determine this are: Single-step prediction: Construct a single-step model to predict the circulating current control value function, and use the five-level compensation method to obtain the number of optimal, second-optimal, and third-optimal compensation levels; Multi-step prediction: Construct a multi-step model to predict the value function of the circulating current control, and obtain the optimal number of compensation levels based on the number of compensation levels obtained from the single-step prediction.

2. The suppression method according to claim 1, characterized in that, t k The base number of upper and lower bridge arm input submodules that always meet phase current tracking requirements , Obtained from the external characteristic equation of MMC: (1) in e j The output phase voltage of the converter ，u uj ，u lj These are the output voltages of the upper and lower bridge arms, respectively. u sj This is the grid-side voltage. i j For output phase current; By performing a first-order forward difference on equation (1), we obtain the predicted value (2) of the phase current of the MMC converter at time k+1. (2) in , ; T s To control the sampling period for model prediction, L arm The equivalent inductance of the bridge arm is used to limit the starting current and short-circuit current. L The equivalent inductance on the grid side, R This is the equivalent resistance on the grid side.

3. The suppression method according to claim 1, characterized in that, The tracking conditions for the AC side current are: in J 0 is the objective function for AC current tracking. ；i j ( k+ 1) For k+ The predicted value of the alternating current at time 1. for k+ The reference value of the alternating current at moment 1, k Calculate the predicted values ​​of all currents at any given time, and select the values ​​that make the predicted values ​​of all currents available. J The minimum voltage of the upper and lower bridge arms is used as the input voltage, and the optimal input voltage of the upper bridge arm is... The optimal voltage for the lower bridge arm is This enables optimal control of the AC current; the upper and lower bridge arm input submodules are respectively denoted as... N uj , N lj .

4. The suppression method according to claim 1, characterized in that, Simultaneously satisfying the output AC tracking and circulation suppression balance conditions, that is: performing a first-order forward difference on the internal characteristic equation of MMC to obtain the single-step model prediction function of circulation (3). (2) (3) in , They are respectively t k+1 The upper and lower bridge arm voltages must always meet the requirements for phase current tracking. U dc It is a DC voltage; i diffj For equivalent converter j Phase internal current, Constructing a single-step model to predict the value function of circulation control J 1. Obtain the number of its optimal, second-optimal, and third-optimal compensation levels; Internal current of each phase i diffj It is a phase circulation Composed of one-third of the DC current, to make the circulating current zero, the reference value of the internal current of each phase is... ; (4) Five-level compensation is used, and the number of compensation levels is denoted as follows: The compensation voltage is denoted as ; Define single-step model predictive circulating current control, and upper and lower arm level compensation sets. , To ensure consistent AC current tracking accuracy, the number of compensation levels in the upper and lower bridge arms must be equal, and the compensation voltage is... (5) Select to make The number of compensation levels corresponding to the smallest, second smallest, and second-smallest values ​​are denoted as follows: d 1. d 2. d 3; among which d 1 represents the number of compensation levels corresponding to the optimal single-step circulating current prediction. d 2 represents the number of compensation levels corresponding to the suboptimal single-step circulating current prediction. d 3 represents the number of compensation levels corresponding to the second-best single-step circulating current prediction.

5. The suppression method according to claim 1, characterized in that, definition D 2 represents the multi-step model prediction circulating current control level compensation set. D 2=[ d 1 d 2 d 3]; The number of compensation levels predicted by the multi-step model is denoted as Compensation voltage Its value is: (6) The internal characteristic equation of MMC is subjected to first-order forward difference, with a sampling period of . T s， get t k+2 Predicted internal current value at time ; (7) , They are respectively t k+2 The upper and lower bridge arm input voltages must always meet phase current tracking requirements; Constructing a multi-step model to predict the value function of circulation control J 2 (8) Select to make The number of compensation levels corresponding to the minimum value is applied to the converter. At that moment, the corresponding compensation level is the optimal compensation level. Therefore, the number of upper and lower levels that satisfy both output AC tracking and circulating current suppression are (9) and (10) respectively. (9) (10) 。 6. A low-computational-consumption MMC multi-step model prediction circulation suppression system, characterized in that, Includes the following steps: Module 1: Obtain the base number of upper and lower bridge arms that meet the phase current tracking requirements at the current moment. , ; The second module: compensates the upper and lower bridge arms with the same number of voltage levels, so that the final number of sub-modules deployed is [number missing]. , The balance condition of tracking AC side current and suppressing interphase circulating current must be met; The final number of sub-modules deployed , satisfy: , ; It is the optimal number of compensation levels for simultaneous compensation of the upper and lower bridge arms; The steps to determine this are: Single-step prediction: Construct a single-step model to predict the circulating current control value function, and use the five-level compensation method to obtain the number of optimal, second-optimal, and third-optimal compensation levels; Multi-step prediction: Construct a multi-step model to predict the value function of the circulating current control, and obtain the optimal number of compensation levels based on the number of compensation levels obtained from the single-step prediction.

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