Harmonic compensation method for power converter

Abstract

The invention discloses a harmonic compensation method for a power converter, and the method comprises the steps: sampling the voltage of a power grid and the current of a harmonic load, and carrying out the calculation in a dq coordinate system through Park conversion, and generating a reference current of an active power filter; respectively calculating an actual current one-step predicted value and a reference current one-step predicted value of the active power filter in the alpha-beta coordinate system through Clark conversion; constructing a target function of one-step prediction of the active power filter; and traversing eight switching voltage vectors of the active power filter, selecting the switching voltage vector which enables the one-step prediction objective function to be minimum, and acting the switching voltage vector as the optimal switching voltage vector on the active power filter. According to the method, the total harmonic distortion of the power grid side can be effectively reduced, and the safety and economical efficiency of the power grid are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of power harmonic compensation, and more specifically, to a power converter harmonic compensation method. Background Art

[0002] With the increase of linear and nonlinear loads in more and more large enterprises, the voltage distortion, current waveform distortion and phase angle deviation in the distribution network are becoming more and more serious. At present, due to the limitation of the rated power of power electronic equipment, there is a contradiction between the voltages that need to be controlled in the distribution network. Therefore, the diversification of active power filters has become the focus of research. As an important part of the power system, the reactive power compensation device is an important measure to improve the power factor of the system, ensure the safe operation of the power system, and improve the quality of electric energy. However, with the application of a large number of nonlinear electrical equipment, the problem of harmonic pollution in the power system is becoming more and more serious. These harmonic currents threaten the safe operation of the reactive power compensation device to a certain extent, making the failure rate of the compensation capacitor higher and higher, the capacitor current increases, and the service life of the capacitor is shortened. At the same time, the reactive power compensation capacitor is easy to resonate with the system, which will amplify the harmonics and seriously threaten the safety of the equipment and the stability of the system. Therefore, power management personnel must clearly understand the hazards caused by harmonic pollution, correctly set the capacitor, and ensure the safety of the reactive power compensation device and the power system.

[0003] Nonlinear loads generate harmonic currents with relatively low power factors, which poses a serious problem for power supply systems. Traditionally, passive filters have been used to suppress harmonics. However, these devices can produce resonances. To solve the problems of passive filters, active filters have been developed. They are more effective in harmonic compensation and have better performance. However, for large power systems, active filters are more expensive and require higher power converters. Therefore, how to provide low-cost harmonic compensation and improve the performance of the filter is of great significance. Summary of the invention

[0004] The object of the present invention is to provide a harmonic compensation method for a power converter to solve the problems raised in the above background technology.

[0005] To achieve the above objectives, the present invention provides the following technical solutions:

[0006] A method for harmonic compensation of a power converter, comprising:

[0007] Sampling the grid voltage and harmonic load current, and calculating and generating the reference current of the active power filter in the dq coordinate system through Park transformation;

[0008] The actual current one-step prediction value and the reference current one-step prediction value of the active power filter are calculated in the αβ coordinate system by Clark transformation;

[0009] Construct the objective function of one-step prediction of active power filter;

[0010] The eight switching voltage vectors of the active power filter are traversed, and the switching voltage vector that minimizes the one-step prediction objective function is selected and used as the optimal switching voltage vector to act on the active power filter.

[0011] Preferably, it also includes:

[0012] Constructing a two-step prediction value of actual current and a two-step prediction value of reference current of an active power filter;

[0013] Construct the objective function of two-step prediction of active power filter;

[0014] The eight switching voltage vectors of the active power filter are traversed, and the switching voltage vector that minimizes the two-step prediction objective function is selected and used as the optimal switching voltage vector for the active power filter.

[0015] Preferably, the calculation to generate a reference current of the active power filter comprises:

[0016] Sampling the three-phase voltage v of the power grid abc , through the phase-locked loop control module, the phase angle θ of the power grid is given;

[0017] Based on the phase angle information θ of the power grid, the three-phase current i of the harmonic load is transformed by Park transformation. abc Transform to the dq coordinate system and obtain the current information i of the dq axis dq ;

[0018] The current value i of the d-axis d The d-axis current value i after the low-pass filter d_LPF Make a difference and get the d-axis current i corresponding to the harmonic current d_H ;

[0019] Based on the current information i d_H and i q , the reference current i* of the active power filter is obtained by inverse Park transformation abc .

[0020] Preferably, the actual current one-step prediction value and the reference current one-step prediction value of the active power filter are calculated in the αβ coordinate system by Clark transformation, including:

[0021] Assuming that there is a balanced three-phase voltage in the power grid and according to Kirchhoff's voltage law, the voltage equation of the active power filter is obtained:

[0022] The voltage equation of the active power filter in the abc coordinate system can be converted to the dq coordinate system through Clark transformation;

[0023] Discretize the voltage equation of the active power filter in the dq coordinate system to obtain a one-step prediction value of the actual current of the active power filter;

[0024] The Lagrange interpolation prediction method is used to calculate the one-step prediction value of the reference current of the active power filter.

[0025] Preferably, the voltage equation of the active power filter is:

[0026] ;

[0027] Among them, u an 、u bn and u cn are the three-phase voltages of the active power filter, i a 、i b and i c are the three-phase currents, R and L are the resistance and reactance of the active power filter, respectively, e a 、e b and e c They are the three-phase voltages of the power grid.

[0028] Preferably, constructing an objective function for one-step prediction of an active power filter comprises:

[0029] The objective function of the one-step prediction of the active power filter is the modulus of the difference between the one-step prediction value of the reference current corresponding to the active power filter and the one-step prediction value of the actual current, and the formula is as follows:

[0030] , where k is a time constant.

[0031] Preferably, the traversing eight switch voltage vectors of the active power filter and selecting the switch voltage vector that minimizes the one-step prediction objective function comprises:

[0032] The active power filter adopts a three-bridge structure, each bridge arm contains two switch tubes, which correspond to eight switching vectors, namely: V 0 =0, , , , , , and V 7 =0;

[0033] By substituting the eight vectors into the objective function respectively, a switching vector which minimizes the objective function is selected, which is the optimal switching vector for one-step prediction.

[0034] Preferably, the constructing of the two-step prediction value of the actual current of the active power filter comprises:

[0035] The actual current two-step prediction value of the k+2 cycle is predicted based on the prediction value of the k+1 cycle to reduce the load current ripple and reduce the total harmonic distortion in the grid current, wherein the actual current two-step prediction value is expressed as:

[0036] ;

[0037] Among them, T s To control the cycle.

[0038] Preferably, the objective function of constructing the two-step prediction of the active power filter includes:

[0039] The objective function of the two-step prediction of the active power filter is the modulus of the difference between the two-step prediction value of the reference current and the two-step prediction value of the actual current. The formula is as follows:

[0040] .

[0041] Preferably, traversing eight switch voltage vectors of the active power filter and selecting a switch voltage vector that minimizes the two-step prediction objective function comprises:

[0042] By substituting the eight vectors corresponding to the active power filter into the objective function of the two-step prediction respectively, the switching vector that minimizes the objective function is selected, which is the optimal switching vector of the two-step prediction.

[0043] The beneficial effects of the present invention are as follows: the present invention discloses a harmonic compensation method for a power converter, adopts an active power filter to perform harmonic control, and determines the optimal switching voltage vector to act on the active power filter according to the predicted value of the actual current of the active power filter and the predicted value of the reference current, so as to effectively compensate and suppress the harmonics of the power grid, thereby effectively reducing the total harmonic distortion on the power grid side, and improving the safety and economy of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the specific embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below.

[0045] Figure 1 The present invention provides a flow chart of a method for harmonic compensation of a power converter.

[0046] Figure 2 A schematic diagram of reference current generation of an active power filter provided by an embodiment of the present invention.

[0047] Figure 3An execution diagram of a power converter harmonic compensation method based on two-step finite set predictive control provided in an embodiment of the present invention.

[0048] Figure 4 The THD comparison of current harmonics between the proposed algorithm and the traditional algorithm provided in the embodiment of the present invention is shown. DETAILED DESCRIPTION

[0049] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention. The present invention is described in more detail by way of example with reference to the accompanying drawings in the following paragraphs. The advantages and features of the present invention will become clearer according to the following description and claims. It should be noted that the drawings are all in a very simplified form and are not in precise proportions, and are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention.

[0050] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0051] In view of the high cost of using active filters to suppress harmonics in current power supply systems, the present invention provides a harmonic compensation method for a power converter, which uses an active power filter to control harmonics, and determines the optimal switching voltage vector to act on the active power filter based on the predicted value of the actual current of the active power filter and the predicted value of the reference current, so as to effectively compensate and suppress the harmonics of the power grid, thereby effectively reducing the total harmonic distortion on the power grid side and improving the economic efficiency of the safety of the power grid.

[0052] like Figure 1 As shown, a method for harmonic compensation of a power converter comprises:

[0053] S1: Sample the grid voltage and harmonic load current, and calculate the reference current of the active power filter in the dq coordinate system through Park transformation.

[0054] S2: Calculate the actual current one-step prediction value and the reference current one-step prediction value of the active power filter in the αβ coordinate system through Clark transformation.

[0055] S3: Construct the objective function of one-step prediction of active power filter.

[0056] S4: traverse the eight switch voltage vectors of the active power filter, select the switch voltage vector that minimizes the one-step prediction objective function, and use it as the optimal switch voltage vector to act on the active power filter.

[0057] Furthermore, the method further comprises:

[0058] S5: constructing a two-step prediction value of the actual current and a two-step prediction value of the reference current of the active power filter;

[0059] S6: Construct the objective function of two-step prediction of active power filter;

[0060] S7: traverse the eight switch voltage vectors of the active power filter, select the switch voltage vector that minimizes the two-step prediction objective function, and use it as the optimal switch voltage vector to act on the active power filter.

[0061] Further, the following steps are included to realize generating a reference current of an active power filter:

[0062] Please see attached Figure 2 , sampling the three-phase voltage v of the power grid abc , through the phase-locked loop control module, the phase angle θ of the power grid is given;

[0063] Based on the phase angle information θ of the power grid, the three-phase current i of the harmonic load is transformed by Park transformation. abc Transform to the dq coordinate system and obtain the current information i of the dq axis dq ; Further, the current value i of the d-axis d The d-axis current value i after the low-pass filter d_LPF Make a difference and get the d-axis current i corresponding to the harmonic current d_H ;

[0064] Based on the current information i d_H and i q , the reference current i of the active power filter is obtained by inverse Park transformation * abc ;

[0065] Further, the following steps are included to calculate the actual current one-step prediction value and the reference current one-step prediction value of the active power filter:

[0066] Please see attached Figure 3 , assuming that there are balanced three-phase voltages in the grid and according to Kirchhoff's voltage law, the following equation can be determined for the active power filter:

[0067] ; (1)

[0068] Among them, u an 、u bn and u cn are the three-phase voltages of the active power filter, i a 、i b and i care the three-phase currents, R and L are the resistance and reactance of the active power filter, respectively, e a 、e b and e c They are the three-phase voltages of the power grid.

[0069] The voltage equation in the above abc coordinate system can be converted to the dq coordinate system through Clark transformation:

[0070] ; (2)

[0071] Discretizing the above formula yields:

[0072] ; (3)

[0073] Furthermore, the actual current one-step prediction value of the active power filter can be expressed as:

[0074] ; (4)

[0075] The Lagrange interpolation prediction method is used to calculate the one-step prediction value of the reference current of the active power filter:

[0076] ; (5)

[0077] The advantage of using the Lagrange interpolation prediction method is that it reduces the delay in the calculation process. Using the second-order interpolation prediction, equation (5) can be expressed as:

[0078] ; (6)

[0079] Further, the following steps are included to construct an objective function for one-step prediction of the active power filter:

[0080] The objective function of the one-step prediction of the active power filter is the modulus of the difference between the one-step prediction value of the reference current and the one-step prediction value of the actual current:

[0081] ; (7)

[0082] Further, the following steps are included to select the switch voltage vector that minimizes the one-step prediction objective function:

[0083] The active power filter adopts a three-bridge structure, each bridge arm contains two switch tubes, which correspond to eight switching vectors, namely: V 0 =0, , , , , , as well as ;

[0084] By substituting the eight vectors into the objective function (7) respectively, the switching vector that minimizes the objective function is selected, which is the optimal switching vector for one-step prediction;

[0085] Further, the following steps are included to calculate the two-step predicted value of the actual current and the objective function of the two-step prediction:

[0086] The two-step prediction is based on the prediction value of the k+1 cycle to predict the prediction value of the k+2 cycle to further reduce the load current ripple and reduce the total harmonic distortion in the grid current. The actual current two-step prediction value can be expressed as:

[0087] ; (8)

[0088] The objective function of the two-step prediction of the active power filter is the modulus of the difference between the two-step prediction value of the reference current and the two-step prediction value of the actual current.

[0089] ; (9)

[0090] Further, the following steps are included to select a switch voltage vector that minimizes the two-step prediction objective function:

[0091] The active power filter adopts a three-bridge structure, each bridge arm contains two switch tubes, which correspond to eight switching vectors, namely: V 0 =0, , , , , , as well as ;

[0092] By substituting the eight vectors into the objective function formula (9) respectively, the switching vector that minimizes the objective function is selected, which is the optimal switching vector for two-step prediction;

[0093] The simulation waveform of the active power filter is built in Matlab / Simulink, and the effects of the proposed power converter harmonic compensation method based on two-step finite set predictive control are compared with the traditional method. Figure 4 A comparison of the harmonic characteristics of the active power filter under two methods is given, one of which is the traditional method and the other is the power converter harmonic compensation method based on two-step finite set predictive control. As can be seen from the figure, the proposed power converter harmonic compensation method based on two-step finite set predictive control can reduce the current harmonics of the active power filter from 24.41% to 2.5%, greatly reducing the system's harmonic THD and achieving effective harmonic suppression and compensation.

[0094] It can be seen that the present invention discloses a harmonic compensation method for a power converter, which adopts an active power filter to perform harmonic control, and determines the optimal switching voltage vector to act on the active power filter according to the predicted value of the actual current of the active power filter and the predicted value of the reference current, so as to effectively compensate and suppress the harmonics of the power grid, thereby effectively reducing the total harmonic distortion on the power grid side and improving the safety and economy of the power grid.

[0095] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any ordinary technician in the industry can smoothly implement the present invention as shown in the drawings and described above. However, any equivalent changes, modifications and evolutions made by technicians familiar with the profession without departing from the scope of the technical solution of the present invention using the technical content disclosed above are all equivalent embodiments of the present invention. At the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the technical solution of the present invention.

Claims

1. A method for harmonic compensation of a power converter, characterized in that: include: Sampling the grid voltage and harmonic load current, and calculating and generating the reference current of the active power filter in the dq coordinate system through Park transformation; The actual current one-step prediction value and the reference current one-step prediction value of the active power filter are calculated in the αβ coordinate system by Clark transformation; Construct the objective function of one-step prediction of active power filter; The eight switching voltage vectors of the active power filter are traversed, and the switching voltage vector that minimizes the one-step prediction objective function is selected and used as the optimal switching voltage vector to act on the active power filter.

2. The power converter harmonic compensation method according to claim 1, characterized in that: Also includes: Constructing a two-step prediction value of actual current and a two-step prediction value of reference current of an active power filter; Construct the objective function of two-step prediction of active power filter; The eight switching voltage vectors of the active power filter are traversed, and the switching voltage vector that minimizes the two-step prediction objective function is selected and used as the optimal switching voltage vector for the active power filter.

3. The method for harmonic compensation of a power converter according to claim 2, characterized in that: The calculation generates a reference current of the active power filter, comprising: Sampling the three-phase voltage v of the power grid abc , through the phase-locked loop control module, the phase angle θ of the power grid is given; Based on the phase angle information θ of the power grid, the three-phase current i of the harmonic load is transformed by Park transformation. abc Transform to the dq coordinate system and obtain the current information i of the dq axis dq ; The current value i of the d-axis d The d-axis current value i after the low-pass filter d_LPF Make a difference and get the d-axis current i corresponding to the harmonic current d_H ; Based on the obtained current information i d_H and i q , the reference current i* of the active power filter is obtained by inverse Park transformation abc .

4. The method for harmonic compensation of a power converter according to claim 3, characterized in that: The actual current one-step prediction value and the reference current one-step prediction value of the active power filter are calculated in the αβ coordinate system by Clark transformation, including: Assuming that there is a balanced three-phase voltage in the power grid and according to Kirchhoff's voltage law, the voltage equation of the active power filter is obtained: The voltage equation of the active power filter in the abc coordinate system can be converted to the dq coordinate system through Clark transformation; Discretize the voltage equation of the active power filter in the dq coordinate system to obtain a one-step prediction value of the actual current of the active power filter; The Lagrange interpolation prediction method is used to calculate the one-step prediction value of the reference current of the active power filter.

5. The power converter harmonic compensation method according to claim 4, characterized in that: The voltage equation of the active power filter is: ； Among them, u an 、u bn and u cn are the three-phase voltages of the active power filter, i a 、i b and i c are the three-phase currents, R and L are the resistance and reactance of the active power filter, respectively. a 、e b and e c They are the three-phase voltages of the power grid.

6. The method for harmonic compensation of a power converter according to claim 5, characterized in that: The objective function of the one-step prediction of the active power filter is constructed, including: The objective function of the one-step prediction of the active power filter is the modulus of the difference between the one-step prediction value of the reference current corresponding to the active power filter and the one-step prediction value of the actual current, and the formula is as follows: , where k represents the time constant.

7. The power converter harmonic compensation method according to claim 6, characterized in that: The traversing eight switch voltage vectors of the active power filter and selecting the switch voltage vector that minimizes the one-step prediction objective function includes: The active power filter adopts a three-bridge structure, each bridge arm contains two switch tubes, which correspond to eight switching vectors: V0=0, , , , , , and V7=0; By substituting the eight vectors into the objective function respectively, a switching vector which minimizes the objective function is selected, which is the optimal switching vector for one-step prediction.

8. The power converter harmonic compensation method according to claim 7, characterized in that: The actual current two-step prediction value of the active power filter is constructed, comprising: The actual current two-step prediction value of the k+2 cycle is predicted based on the prediction value of the k+1 cycle to reduce the load current ripple and reduce the total harmonic distortion in the grid current, wherein the actual current two-step prediction value is expressed as: ； Among them, T s To control the cycle.

9. The power converter harmonic compensation method according to claim 8, characterized in that: The objective function of constructing the two-step prediction of the active power filter includes: The objective function of the two-step prediction of the active power filter is the modulus of the difference between the two-step prediction value of the reference current and the two-step prediction value of the actual current. The formula is as follows: 。 10. The power converter harmonic compensation method according to claim 9, characterized in that: The traversing eight switch voltage vectors of the active power filter and selecting the switch voltage vector that minimizes the two-step prediction objective function includes: By substituting the eight vectors corresponding to the active power filter into the objective function of the two-step prediction respectively, the switching vector that minimizes the objective function is selected, which is the optimal switching vector of the two-step prediction.

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