Model Predictive Control Method and Device for Rectifier Harmonic Suppression Based on Current Variance

Abstract

The present invention discloses a method and device for model predictive control of rectifier harmonic suppression based on current variance. This method realizes the indirect control of current harmonics by controlling the current variance. Through offline experiments, the relationship between the load current and the current variance under different current harmonics is measured, and the functional relationship between the two is obtained by curve fitting, avoiding the harmonic calculation link and reducing the computational load. This method can achieve the equivalent control of current harmonics with a relatively small computational load. In addition, in model predictive control, through hierarchical control, the computational complexity is reduced, the design of weight coefficients is avoided, and the balanced control of the neutral point voltage is achieved. By setting a satisfaction interval, the control target is limited within a certain fluctuation range, enabling the control strategy to obtain a higher control freedom and achieving the global optimum of the control effect.

Description

Technical Field

[0001] The present invention relates to the field of rectifier control, and in particular, to a method and device for predicting control of rectifier harmonic suppression based on current variance. Background Art

[0002] The PWM rectifier is directly connected to the power grid, so its harmonic content is a key performance index. Generally, the total harmonic distortion (THD) is required to be less than 5% to ensure the stability of the power grid and the power supply quality. However, for high-power rectifiers, due to their low switching frequencies, the problem of harmonic pollution to the power grid is particularly prominent. This harmonic pollution will not only affect the overall operation of the power grid, but may also interfere with other devices connected to the same power grid. In severe cases, harmonic pollution may even cause these devices to malfunction, such as misoperation, reduced efficiency or damage.

[0003] In addition, the rectifier harmonics vary with the load size. When the load is large, the rectifier harmonics decrease, and when the load is small, the rectifier harmonics increase. For high-power inverters, even with a small load, the impact of their harmonics is still very serious, and in severe cases, it may even cause the auxiliary power supply system to malfunction due to excessive harmonics. Summary of the Invention

[0004] In order to solve the above problems, the purpose of the embodiments of the present invention is to provide a method and device for predicting control of rectifier harmonic suppression based on current variance. The present invention indirectly controls the current harmonics by controlling the current variance. On the premise of taking into account the requirement of low harmonic content of the rectifier, the switching frequency of the system is reduced.

[0005] In the first aspect, an embodiment of the present application provides a method for predicting control of rectifier harmonic suppression based on current variance. According to the constraint conditions of the control target, the control priority of the control target is divided into three levels;

[0006] The three levels include: high priority, the control target constrains the switching state of the system output to ensure that the output voltage jump is less than half of the DC-side voltage; medium priority, the control target is the balance of the DC-side capacitor voltage; low priority, the control target is the grid current tracking, and the current satisfaction interval of the low priority is adjusted through the relationship between the current variance, the total harmonic distortion rate of the current and the load current.

[0007] Multi-level control is performed by setting the priority order to achieve the suppression of the rectifier grid-side harmonics; the specific process of performing multi-level control by setting the priority order includes:

[0008] Pre-select the switching state by high priority to obtain a pre-selected switching state set;

[0009] Perform midpoint potential control on the obtained preselected switch state set through medium priority to obtain a candidate switch state set, and set the value of the flag bit UN indicating that the midpoint voltage is within the satisfactory interval;

[0010] Perform current tracking control on the obtained candidate switch state set with low priority, and directly select the optimal switch state from the candidate switch state set through the cost function according to the value of the flag bit UN, or select the optimal switch state by adjusting the width of the satisfactory interval of the current through current variance.

[0011] In one implementation, the preselection of the switch state is performed through switch state selection logic, and the selection logic of the switch state includes that the line voltage jump and the phase voltage jump shall not be DC voltage values.

[0012] In one implementation, the performing midpoint voltage control through medium priority to obtain a candidate switch state set and setting the value of the flag bit UN indicating that the midpoint voltage is within the satisfactory interval includes:

[0013] Collect the upper and lower DC voltages and calculate the midpoint voltage; the DC voltage is the capacitor voltage on the DC side of the three-level rectifier;

[0014] Set the satisfactory interval of the midpoint voltage according to the magnitude of the DC side voltage;

[0015] According to the relationship between the midpoint voltage and the satisfactory interval of the midpoint voltage, screen the preselected switch state set to obtain a candidate switch state set, and set the value of the flag bit UN indicating that the midpoint voltage is within the satisfactory interval.

[0016] In one implementation, the screening the preselected switch state set to obtain a candidate switch state set according to the relationship between the midpoint voltage and the satisfactory interval of the midpoint voltage, and setting the value of the flag bit UN indicating that the midpoint voltage is within the satisfactory interval includes:

[0017] Divide the fluctuation of the midpoint voltage into three cases, and narrow down the preselected switch state set in the following three cases respectively;

[0018] Case 1: When the midpoint voltage u o exceeds the satisfactory interval, that is, u o >δu o , at this time, select the small vector that can make the midpoint voltage drop in the preselected switch state set as the candidate switch state set 1, and set the flag bit UN indicating that the midpoint voltage is within the satisfactory interval to UN = 1;

[0019] Case 2: When the midpoint voltage is within the ±δu o interval, that is, -δu o ≤u o ≤δu o, at this time, the midpoint voltage is within the satisfactory range, without processing the preselected switch state set, directly taking the preselected switch state set as the candidate switch state set 2, and setting the flag bit UN indicating that the midpoint voltage is within the satisfactory range to 0;

[0020] Case 3: When the midpoint voltage exceeds the lower limit of the satisfactory range, that is, u o <-δu o , at this time, select the small vector that raises the midpoint voltage in the preselected switch state set as the candidate switch state set 3, and set the flag bit UN indicating that the midpoint voltage is within the satisfactory range to 1.

[0021] In one implementation manner, the obtained candidate switch state set is subjected to current tracking control with a low priority, and the optimal switch state is directly selected from the candidate switch state set through a cost function according to the value of the flag bit UN, or the satisfactory range width of the current is adjusted through the current variance to select the optimal switch state, including:

[0022] If the value of UN is 1, directly select the optimal open state from the candidate switch state sets 1 and 3 through the cost function; if the value of UN is 0, collect the incoming line voltage and current to calculate the predicted values of the active current and reactive current of the rectifier;

[0023] According to the predicted values of the active current and reactive current of the rectifier, calculate the variances of the active current and reactive current;

[0024] Define the current variance as the average value of the reactive current and the active current;

[0025] According to the current variance - load - harmonic curve, adjust the current satisfactory range through the average value of the current variance;

[0026] Judge whether the predicted values of the active and reactive currents are within the current satisfactory range according to the current satisfactory range; if they are within the satisfactory range, continue to use the switch state of the previous moment; otherwise, select the optimal switch state from the candidate switch state set 2 through the cost function.

[0027] In one implementation manner, the calculation of the predicted values of the active current and reactive current of the rectifier is as follows:

[0028]

[0029] Calculate the change rate of the current from the above formula, and calculate the predicted value of the current through the following formula:

[0030]

[0031] Calculate the variances of the active current and reactive current as:

[0032]

[0033] wherein, i d (k - n), i q (k - n) are respectively the calculated values of the reactive current and the active current at the moment of (k - n), are respectively the current reference values of the d-axis and the q-axis, N is the total number of current sampling values counted, and N is an integer multiple of the period; N is the total number of current sampling values counted, and it is an integer multiple of the power grid voltage period; k is the sampling moment, and n is the number of statistical times;

[0034] The variance of the grid current is defined as the average value of the reactive current and the active current:

[0035]

[0036] In one embodiment, the adjustment of the current satisfaction interval by the average value of the current variance is:

[0037]

[0038] wherein, is the current variance reference value, ρ is the satisfaction interval adjustment coefficient, δ i (k), δ i (k + 1) are respectively the current satisfaction interval values at the moments of k and k + 1.

[0039] In one embodiment, the cost function is:

[0040]

[0041] wherein, i d * , i q * are the givens of the active current and the reactive current, i d (k + 1), i q (k + 1) are the predicted values of the active current and the reactive current.

[0042] In one embodiment, the method for judging whether the active current is within the satisfaction interval:

[0043] Case 1: The predicted value of the active current at the moment of k + 1 exceeds the upper limit of the satisfaction interval;

[0044] Case 2: The predicted value of the active current at the moment of k + 1 is within the satisfaction interval;

[0045] Case 3: The predicted value of the active current at the moment of k + 1 exceeds the lower limit of the satisfaction interval;

[0046] In the above formula, i d(k + 1) is the predicted value of the active current, is the given value of the active current, δ i (k + 1) is the satisfactory interval of the current;

[0047] Method for judging whether the reactive current is within the satisfactory interval:

[0048] Case 1: The predicted value of the reactive current at time k + 1 exceeds the upper limit of the satisfactory interval;

[0049] Case 2: The predicted value of the reactive current at time k + 1 is within the satisfactory interval;

[0050] Case 3: The predicted value of the reactive current at time k + 1 exceeds the lower limit of the satisfactory interval.

[0051] In the above formula, i q (k + 1) is the predicted value of the reactive current, is the given value of the reactive current.

[0052] In one implementation, the process of obtaining the current variance - load - harmonic curve includes:

[0053] Run the rectifier. Under different loads, by adjusting the current satisfactory interval, make the output harmonic of the rectifier be the expected value;

[0054] When the output current harmonic is the expected value, calculate the average value of the current variance, and record the average value of the current variance at this time, to obtain a set of current variance - load - harmonic data related to the load size;

[0055] Perform curve fitting on the current variance - load - harmonic data to obtain the variance curve under the same current harmonic content at different loads.

[0056] In a second aspect, the embodiments of the present application provide a rectifier harmonic suppression model predictive control based on current variance. The device includes:

[0057] A high - priority module, used for pre - selecting the switch state to obtain a set of pre - selected switch states;

[0058] A medium - priority module, used for performing mid - point potential control on the obtained set of pre - selected switch states through medium priority to obtain a set of candidate switch states, and setting the flag bit UN value of the mid - point voltage in the satisfactory interval;

[0059] The low-priority module is used to perform current tracking control on the obtained candidate switch state set with low priority, and directly select the optimal switch state from the candidate switch state set through the cost function according to the value of the flag bit UN, or select the optimal switch state by adjusting the width of the satisfactory interval of the current through the current variance.

[0060] In a third aspect, an embodiment of the present application provides a computer-readable storage medium, on which computer program instructions are stored, and when the instructions are executed by a processor, the above-mentioned rectifier harmonic suppression model predictive control method based on current variance is implemented.

[0061] Compared with the prior art, the present invention has the following advantages:

[0062] Through offline experiments, the present invention measures the relationship between the load current and the current variance under different current harmonics, and obtains the functional relationship between the two by using curve fitting, avoiding the harmonic calculation link and reducing the computational complexity. The present invention can achieve equivalent control of current harmonics with a small amount of computation. In addition, through hierarchical control in model predictive control, the computational complexity is reduced, the design of weight coefficients is avoided, and balanced control of the midpoint voltage is achieved. By setting a satisfactory interval, the present invention limits the control target within a certain fluctuation range, enabling the control strategy to obtain a higher degree of control freedom and achieving the global optimum of the control effect. Description of the Drawings

[0063] The drawings, as a part of the present invention, are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention, but do not constitute an improper limitation of the present invention. Obviously, the drawings in the following description are only some embodiments, and those of ordinary skill in the art can obtain other drawings based on these drawings without creative efforts.

[0064] Figure 1 It is a specific process flowchart for multi-level control by setting the priority order in the rectifier harmonic suppression model predictive control method based on current variance provided by an embodiment of the present invention;

[0065] Figure 2 It is a switch state preselection logic diagram provided by an embodiment of the present invention;

[0066] Figure 3 It is a topological structure diagram of an NPC three-level rectifier;

[0067] Figure 4 It is a schematic diagram of the satisfactory interval of the midpoint voltage provided by an embodiment of the present invention;

[0068] Figure 5 It is a fitting curve graph of current variance - load - harmonics provided by an embodiment of the present invention;

[0069] Figure 6 Schematic diagram of the current satisfaction interval provided by an embodiment of the present invention.

[0070] It should be noted that these drawings and textual descriptions are not intended to limit the scope of the concept of the present invention in any way, but to illustrate the concept of the present invention to those skilled in the art by referring to specific embodiments. Detailed implementation manners

[0071] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. The following embodiments are used to illustrate the present invention but are not used to limit the scope of the present invention.

[0072] In high-power application scenarios, on the premise of meeting the requirements of system grid connection, by appropriately reducing the control effects of the grid current and the DC-side capacitor voltage, the system switching frequency can be effectively reduced. The present invention proposes a satisfaction model predictive control based on current variance. Variance is a measure in statistics used to quantify the degree of deviation of each data point in a data set from its mean value. In this method, variance is used to quantify the deviation between the actual current waveform and the ideal current waveform, thereby indirectly evaluating the magnitude of the harmonic content.

[0073] The embodiments of the present disclosure provide a rectifier harmonic suppression model predictive control method based on current variance. According to the constraint conditions of the control objective, the control priorities of the control objective are divided into three levels. The three levels include high priority, medium priority, and low priority. Among them, for high priority, the control objective is to constrain the switching state of the system output to ensure that the voltage jump of the output is less than half of the DC-side voltage. For medium priority, the control objective is the balance of the DC-side capacitor voltage. For low priority, the control objective is grid current tracking, and by adjusting the current satisfaction interval of the low priority through the relationship between the current variance, the total harmonic distortion rate of the current, and the load current, the suppression of the grid-side harmonics of the rectifier is achieved. By setting the priority order for multi-level control, the suppression of the grid-side harmonics of the rectifier is achieved. As Figure 1 shown, the specific process of multi-level control by setting the priority order includes the following steps:

[0074] Step S100: Pre-select the switching state by high priority to obtain a pre-selected switching state set.

[0075] For the NPC rectifier, the direct switching between the bridge arm states of "1" and "-1" is prohibited, which may damage the semiconductor devices. The change in the line voltage is limited within u dc / 2 to reduce the impact on the power grid. Therefore, not all voltage vectors can be used as pre-selected voltage vectors.

[0076] Further, the preselection of the switch state is preselected through the switch state selection logic, and the selection logic of the switch state includes that the line voltage jump and the phase voltage jump must not be the DC voltage value.

[0077] Specifically, in the high priority, Figure 2 The switch state selection logic shown is used for preselecting the switch state. For example, for switch state 16, the selectable switch states include V4, V5, V6, V7, V16, V22, and V23. And the switch states V4, V5, V6, V7, V16, V22, and V23 are used as the preselected switch state set and enter the medium priority.

[0078] Step S200: Perform neutral point potential control on the obtained preselected switch state set through the medium priority, and set the flag bit UN value of the neutral point voltage in the satisfactory interval.

[0079] In the embodiment of the present application, performing neutral point potential control on the obtained preselected switch state set through the medium priority, obtaining the candidate switch state set, and setting the flag bit UN value of the neutral point voltage in the satisfactory interval includes the following steps:

[0080] Step S210: Collect the upper and lower DC voltages and calculate the neutral point voltage.

[0081] The DC voltage is the capacitor voltage on the DC side of the three-level rectifier, as Figure 3 in u c1 , u c2 shown.

[0082] Step S220: Set the satisfactory interval of the neutral point voltage according to the magnitude of the DC side voltage.

[0083] The satisfactory interval of the neutral point voltage is generally within 20% of the DC side voltage. In this embodiment, it is preferably set to 10%.

[0084] Step S230: According to the relationship between the neutral point voltage and the satisfactory interval of the neutral point voltage, screen the preselected switch state set to obtain the candidate switch state set, and set the flag bit UN value of the neutral point voltage in the satisfactory interval.

[0085] In a preferred embodiment, as Figure 4 shown, the fluctuation of the neutral point voltage is divided into three cases, and the preselected switch state set is reduced in the following three cases.

[0086] Case 1: When the neutral point voltage u o exceeds the satisfactory interval, u o > δu o , as Figure 4As shown in I. At this time, select the small vectors in the preselected switch state set that can reduce the midpoint voltage as the candidate switch state set 1. The small vectors are as Figure 4 V4 - V15 in. Reduce the midpoint voltage through the small vectors to bring the midpoint voltage back into the satisfactory range. At this time, delete the discarded small vectors from the preselected switch state set, and transfer the remaining switch states to step S300. Set the flag bit UN indicating that the midpoint voltage is in the satisfactory range to 1.

[0087] Case 2: When the midpoint voltage is in the range of ±δu o interval -δu o ≤u o ≤δu o , as Figure 4 shown in II. At this time, the midpoint voltage is in the satisfactory range. Therefore, the preselected switch state set can be directly used as the candidate switch state set 2 without processing. Set the flag bit UN indicating that the midpoint voltage is in the satisfactory range to 0, and transfer the preselected switch state set in the high priority to step S300.

[0088] Case 3: When the midpoint voltage exceeds the lower limit u of the satisfactory range o < -δu o as shown in III of 4. At this time, select the small vectors in the preselected switch state set that can increase the midpoint voltage as the candidate switch state set 3. The small vectors are as Figure 4 V4 - V15 in. Raise the midpoint voltage through the small vectors to bring the midpoint voltage back into the satisfactory range. At this time, delete the discarded small vectors from the preselected switch state set, and transfer the remaining switch states to step S300. Set the flag bit UN indicating that the midpoint voltage is in the satisfactory range to 1.

[0089] It should be noted that the influence of the small vectors on the midpoint voltage, that is, the redundant small vectors can raise and lower the midpoint voltage, belongs to common knowledge and will not be explained in detail here.

[0090] Step S300: Perform current tracking control on the obtained candidate switch state set with low priority, and directly select the optimal switch state from the candidate switch state set through the cost function according to the value of the flag bit UN, or select the optimal switch state by adjusting the width of the satisfactory range of the current through the current variance.

[0091] In the embodiment of the present application, perform current tracking control on the obtained candidate switch state set with low priority, and directly select the optimal switch state from the candidate switch state set through the cost function according to the value of the flag bit UN, or select the optimal switch state by adjusting the width of the satisfactory range of the current through the current variance, which specifically includes the following steps:

[0092] Step S310: Determine the value of the flag bit UN indicating that the midpoint voltage is within the satisfactory range. If the value of UN is 1, directly select the optimal on-state from the candidate switch states through the cost function; if the value of UN is 0, collect the incoming line voltage and current to calculate the predicted values of the active current and reactive current of the rectifier.

[0093] Step S320: Calculate the variances of the active current and reactive current based on the predicted values of the active current and reactive current of the rectifier.

[0094] Furthermore, the predicted values of the active current and reactive current of the rectifier are:

[0095]

[0096] Calculate the change rate of the current from Equation (1) and calculate the predicted value of the current through Equation (2):

[0097]

[0098] Calculate the variances of the active current and reactive current as:

[0099]

[0100] where i d (k - n), i q (k - n) are the calculated values of the reactive current and active current at time (k - n) respectively, are the current reference values on the d-axis and q-axis respectively. N is the total number of current sampling values statistically, which is an integer multiple of the grid voltage period; k is the sampling time, and n is the number of statistical times.

[0101] Step S330: Define the current variance as the average of the reactive current and active current.

[0102] Furthermore, the variance of the grid current is:

[0103]

[0104] Step S340: Adjust the current satisfactory range according to the current variance - load - harmonic curve through the average value of the current variance.

[0105] Furthermore, the current satisfactory range at time k + 1 is:

[0106]

[0107] where is the current variance reference value, ρ is the satisfactory range adjustment coefficient, δ i (k), δ i (k + 1) are the current satisfactory range values at times k and k + 1 respectively.

[0108] In the embodiment of the present application, the process of obtaining the current variance-load-harmonic curve includes:

[0109] Step S341: Operate the rectifier. Under different loads, by adjusting the current satisfaction interval, make the output harmonic of the rectifier be the expected value. The output harmonic is obtained by testing with a power quality tester.

[0110] Step S342: When the output current harmonic is the expected value, calculate the average value of the current variance, and record the average value of the current variance at this time, to obtain a set of current variance-load-harmonic data related to the load size.

[0111] The calculation of the current variance is shown in Equations (3)-(5).

[0112] Step S343: Perform curve fitting on the current variance-load-harmonic data to obtain the variance curve under the same current harmonic content at different loads. As Figure 5 shown.

[0113] For the traditional method of calculating the current harmonic distortion rate, although an accurate current harmonic distortion rate can be obtained, its computational complexity is large, consuming too much computing resources, resulting in an increase in the control period of the model prediction, and thus affecting the control performance of the model prediction. In this embodiment, the relationship between the variance of the given current and the predicted current is obtained through experiments, and the functional relationship between the two is obtained by means of curve fitting. This method can achieve equivalent control of harmonics with a relatively small amount of computation.

[0114] Step S350: Determine whether the predicted values of the active and reactive currents are within the current satisfaction interval according to the current satisfaction interval; if they are within the satisfaction interval, continue to use the switch state at the previous moment; otherwise, select the optimal switch state from the candidate switch state set 2 through the cost function.

[0115] Furthermore, the cost function is:

[0116]

[0117] The satisfaction interval of the current is as Figure 6 shown. The method for determining whether the predicted value of the active current is within the satisfaction interval:

[0118] Case 1: The predicted value of the active current at time k + 1 exceeds the upper limit of the satisfaction interval;

[0119] Case 2: The predicted value of the active current at time k + 1 is within the satisfaction interval;

[0120] Case 3: The predicted value of the active current at time k+1 exceeds the lower limit of the satisfactory interval.

[0121] In the above formula, i d (k+1) is the predicted value of the active current, is the given value of the active current, and δ i (k+1) is the satisfactory interval of the current;

[0122] Method for judging whether the reactive current is within the satisfactory interval:

[0123] Case 1: The predicted value of the reactive current at time k+1 exceeds the upper limit of the satisfactory interval;

[0124] Case 2: The predicted value of the reactive current at time k+1 is within the satisfactory interval;

[0125] Case 3: The predicted value of the reactive current at time k+1 exceeds the lower limit of the satisfactory interval.

[0126] In the above formula, i q (k+1) is the predicted value of the reactive current, is the given value of the reactive current.

[0127] In an embodiment, a rectifier harmonic suppression model predictive control device is shown, and the device includes:

[0128] A control priority setting module for dividing the control priority of the control target into three levels according to the constraint conditions of the control target;

[0129] The three levels include: high priority, where the control target constrains the switching state of the system output to ensure that the voltage jump of the output is less than half of the DC-side voltage; medium priority, where the control target is the balance of the DC-side capacitor voltage; low priority, where the control target is the grid current tracking, and the current satisfactory interval of the low priority is adjusted through the relationship between the current variance, the total harmonic distortion rate of the current, and the load current;

[0130] A multi-level control module for performing multi-level control by setting the priority order to achieve the suppression of the grid-side harmonics of the rectifier; the multi-level control module includes:

[0131] A high-priority module for preselecting the switching state to obtain a preselected switching state set;

[0132] A medium-priority module for performing midpoint potential control on the obtained preselected switching state set through the medium priority to obtain a candidate switching state set and setting the value of the flag bit UN for the midpoint voltage to be within the satisfactory interval;

[0133] The low-priority module is used to perform current tracking control on the obtained candidate switch state set with low priority, and directly select the optimal switch state from the candidate switch state set through the cost function according to the value of the flag bit UN, or select the optimal switch state by adjusting the width of the satisfactory interval of the current through the current variance.

[0134] It should be noted that when the rectifier harmonic suppression model predictive control device based on current variance provided in the above embodiment executes a rectifier harmonic suppression model predictive control method based on current variance, only the above-mentioned division of each functional module is used as an example for illustration. In practical applications, the above functions can be allocated to different functional modules according to needs, that is, the internal structure of the device is divided into different functional modules to complete all or part of the functions described above. In addition, the rectifier harmonic suppression model predictive control device based on current variance provided in the above embodiment and the rectifier harmonic suppression model predictive control method embodiment belong to the same concept. The implementation process is detailed in the rectifier harmonic suppression model predictive control method embodiment based on current variance, which will not be repeated here.

[0135] In one embodiment, a computer-readable storage medium is provided, on which computer program instructions are stored, and when the instructions are executed by a processor, the above-mentioned rectifier harmonic suppression model predictive control method based on current variance is implemented.

[0136] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above 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 recorded in this specification.

[0137] The above-described embodiments only represent several implementation manners of the present invention, and the description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the present invention patent. It should be pointed out that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent should be subject to the appended claims.

Claims

1. A model predictive control method for rectifier harmonic suppression based on current variance, characterized in that According to the constraint conditions of the control objective, the control priority of the control objective is divided into three levels; the three levels include: High priority, the control objective is to constrain the switching state of the system output to ensure that the output voltage jump is less than half of the DC-side voltage; Medium priority, the control objective is the balance of the DC-side capacitor voltage; Low priority, the control objective is grid current tracking, and the current satisfaction interval of the low priority is adjusted through the relationship between the current variance, the total harmonic distortion rate of the current, and the load current; Multilevel control is performed by setting the priority order to achieve the suppression of the grid-side harmonics of the rectifier; The specific process of performing multilevel control by setting the priority order includes: Preselect the switching state by high priority to obtain a preselected switching state set; Perform midpoint potential control on the obtained preselected switching state set by medium priority to obtain a candidate switching state set, and set the value of the flag bit UN indicating that the midpoint voltage is in the satisfaction interval; Perform current tracking control on the obtained candidate switching state set by low priority, and directly select the optimal switching state from the candidate switching state set through the cost function according to the value of the flag bit UN or select the optimal switching state by adjusting the width of the current satisfaction interval through the current variance.

2. The model predictive control method for rectifier harmonic suppression based on current variance according to claim 1, wherein The performing midpoint voltage control by medium priority to obtain a candidate switching state set and setting the value of the flag bit UN indicating that the midpoint voltage is in the satisfaction interval includes: Collect the upper and lower DC voltages and calculate the midpoint voltage; the DC voltage is the capacitor voltage on the DC side of the three-level rectifier; Set the satisfaction interval of the midpoint voltage according to the magnitude of the DC-side voltage; According to the relationship between the midpoint voltage and the midpoint voltage satisfaction interval, screen the preselected switching state set to obtain a candidate switching state set, and set the value of the flag bit UN indicating that the midpoint voltage is in the satisfaction interval.

3. The model predictive control method for rectifier harmonic suppression based on current variance according to claim 2, wherein, The screening the preselected switching state set to obtain a candidate switching state set according to the relationship between the midpoint voltage and the midpoint voltage satisfaction interval and setting the value of the flag bit UN indicating that the midpoint voltage is in the satisfaction interval includes: Divide the fluctuation of the midpoint voltage into three cases, and narrow the preselected switching state set in the following three cases; Case 1: When the midpoint voltage u o exceeds the satisfactory range, that is, u o > δu o , at this time, select the small vector that makes the midpoint voltage drop in the preselected switch state set as the candidate switch state set 1, and set the flag bit UN = 1 when the midpoint voltage is in the satisfactory range; Case 2, when the midpoint voltage is in the range of ±δu o interval, that is, -δu o ≤u o ≤δu o , at this time, the midpoint voltage is in the satisfactory interval, without processing the preselected switch state set, directly taking the preselected switch state set as the candidate switch state set 2, and setting the flag bit UN indicating that the midpoint voltage is in the satisfactory interval to 0; Case 3: When the midpoint voltage exceeds the lower limit of the satisfactory interval, i.e., u o < -δu o , at this time, select the small vector that makes the midpoint voltage rise in the preselected switch state set as the candidate switch state set 3, and set the flag bit UN indicating that the midpoint voltage is in the satisfactory interval to 1.

4. The model predictive control method for rectifier harmonic suppression based on current variance according to claim 3, characterized in that The performing current tracking control on the obtained candidate switching state set by low priority and directly selecting the optimal switching state from the candidate switching state set through the cost function according to the value of the flag bit UN or selecting the optimal switching state by adjusting the width of the current satisfaction interval through the current variance includes: Judge the value of the flag bit UN indicating that the midpoint voltage is in the satisfaction interval; If the value of UN is 1, directly select the optimal switching state from candidate switching state set 1 and candidate switching state set 3 through the cost function; if the value of UN is 0, collect the incoming line voltage and current to calculate the predicted values of the active current and reactive current of the rectifier; Calculate the variances of the active current and reactive current according to the predicted values of the active current and reactive current of the rectifier; Define the current variance as the average value of the reactive current and the active current; Adjust the current satisfaction interval according to the current variance-load-harmonic curve through the average value of the current variance. Judge whether the predicted values of the active and reactive currents are within the current satisfaction interval according to the current satisfaction interval; if they are within the satisfaction interval, continue to use the switch state at the previous moment; otherwise, select the optimal switch state from the candidate switch state set 2 through the cost function.

5. The predictive control method for rectifier harmonic suppression based on current variance according to claim 4, wherein The predicted values of the active current and reactive current of the rectifier are: Calculate the change rate of the current from the above formula, and calculate the predicted value of the current through the following formula: The variances of the active current and reactive current are: where i d (k - n) and i q (k - n) are respectively the calculated values of the reactive current and the active current at the moment of k - n, are respectively the current reference values of the d - axis and the q - axis. N is the total number of sampled current values statistically, which is an integer multiple of the power grid voltage period; k is the sampling moment, and n is the number of statistical times; The variance of the grid current is defined as the average value of the reactive current and active current:

6. The model predictive control method for rectifier harmonic suppression based on current variance according to claim 4, characterized in that, The formula for adjusting the current satisfaction interval through the average value of the current variance is: Among them, is the reference value of current variance, ρ is the adjustment coefficient of the satisfaction interval, and δ i (k), δ i (k + 1) are the current satisfaction interval values at times k and k + 1 respectively.

7. The model predictive control method for rectifier harmonic suppression based on current variance according to claim 4, characterized in that The cost function is: where, i d * , i q * is the given value of the active current and the reactive current, and i d (k + 1), i q (k + 1) are the predicted values of the active current and the reactive current.

8. The rectifier harmonic suppression model predictive control method based on current variance according to claim 4, characterized in that The method for judging whether the predicted value of the active current is within the satisfaction interval: Case 1: The predicted value of the active current at the (k + 1)-th moment exceeds the upper limit of the satisfactory interval; Case 2: The predicted value of the active current at the k+1 moment is within the satisfactory range; Case 3: The predicted value of the active current at the k+1 moment exceeds the lower limit of the satisfactory interval. In the above formula, i d (k + 1) is the predicted value of the active current, is the given value of the active current, δ i (k + 1) is the satisfactory interval of the current; The method for judging whether the reactive current is within the satisfaction interval: Case 1: The predicted value of the reactive current at the (k + 1)-th moment exceeds the upper limit of the satisfactory interval; Case 2: The predicted value of the reactive current at the k+1 moment is within the satisfactory interval; Case 3: The predicted value of the reactive current at time k+1 exceeds the lower limit of the satisfactory interval; In the above formula, i q (k + 1) is the predicted value of the reactive current, and is the given value of the reactive current.

9. The model predictive control method for rectifier harmonic suppression based on current variance according to claim 4, wherein The process of obtaining the current variance-load-harmonic curve includes: Operate the rectifier, under different loads, by adjusting the current satisfaction interval, so that the output harmonic of the rectifier is the expected value; When the output current harmonic is the expected value, calculate the average value of the current variance, and record the average value of the current variance at this time, to obtain a set of current variance-load-harmonic data related to the load size; Perform curve fitting on the current variance-load-harmonic data to obtain the variance curve under the same current harmonic content at different loads.

10. A rectifier harmonic suppression model predictive control device based on current variance, characterized in that, The device includes: A control priority setting module, which is used to divide the control priority of the control target into three levels according to the constraint conditions of the control target; The three levels include: high priority, the control target constrains the switch state of the system output to ensure that the output voltage jump is less than half of the DC side voltage; medium priority, the control target is the balance of the DC side capacitor voltage; low priority, the control target is grid current tracking, and adjusts the current satisfaction interval of the low priority through the relationship between the current variance, the total current harmonic distortion rate and the load current. A multi-level control module, which is used to perform multi-level control by setting the priority order to achieve the suppression of the grid-side harmonics of the rectifier; the multi-level control module includes: A high-priority module, which is used to perform pre-selection of the switch state to obtain a pre-selected switch state set; A medium-priority module, which is used to perform midpoint potential control on the obtained pre-selected switch state set through the medium priority, obtain a candidate switch state set, and set the flag bit UN value of the midpoint voltage in the satisfaction interval; A low-priority module, which is used to perform current tracking control on the obtained candidate switch state set through the low priority, and directly select the optimal switch state from the candidate switch state set through the cost function according to the value of the flag bit UN or select the optimal switch state by adjusting the width of the current satisfaction interval through the current variance.

11. A computer-readable storage medium, on which computer program instructions are stored, and when the instructions are executed by a processor, the method for predicting and controlling the harmonic suppression of a rectifier based on current variance according to any one of claims 1 to 8 is implemented.

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