High-gain proportional resonance PWM rectifier DC voltage ripple suppression method

Abstract

The invention relates to a high-gain proportional resonance PWM rectifier direct current voltage ripple suppression method, and belongs to the technical field of rectifier control in power electronics. In the prior art, when three-phase voltage and three-phase current of a three-phase PWM rectifier are not accurately sampled, secondary ripples and grid-side current harmonics contained in direct-current bus voltage cannot be effectively suppressed. According to the method provided by the invention, a transfer function between negative-sequence current and DC voltage secondary ripples is established according to a power conservation relation of an AC side and a DC side, and the negative-sequence current is adjusted through negative feedback of the DC voltage secondary ripples so as to achieve the purpose of voltage ripple suppression. According to the method, direct-current voltage secondary ripple and grid-side current harmonic can be rapidly and accurately suppressed, the influence of sampling errors and inaccurate system parameters according to a mathematical model and complex calculation is avoided, and the robustness of the system parameters and the sampling accuracy is better.

Description

Technical Field

[0001] The present invention belongs to the field of power electronics control technology, and specifically relates to a method for suppressing DC voltage ripple of a high-gain proportional resonance PWM rectifier. Background Art

[0002] PWM rectifiers are widely used in microgrids, motor drives, high-voltage direct current transmission, and electric vehicle charging due to their advantages such as controllable output voltage, low input current harmonic distortion, adjustable power factor, and bidirectional operation. However, most existing control strategies are designed under the assumption that the three-phase voltage is completely balanced, which is often not true in practical applications. When the grid voltage is unbalanced, the input filter is asymmetric, and the current sampling multiple has deviations, the use of traditional control methods may cause problems such as grid-side input current distortion, DC-side output voltage containing secondary ripples, power fluctuations, and system instability. Generally, a large DC capacitor method is used to alleviate the impact of an unbalanced grid on the rectifier output voltage, but a larger capacitor not only increases the size of the converter and reduces the power density, but also increases unnecessary costs. Existing control methods generally have defects, cannot suppress the DC voltage secondary ripple caused by the grid-side voltage and current sampling errors, and rely on the accuracy of positive and negative sequence decomposition and sampling. The control structure is relatively complex and is not suitable for most application scenarios.

[0003] As for the improvement of the control method under asymmetry, the six-power robust model predictive control method (see patent CN114649965B), the three-phase PWM converter under voltage unbalanced conditions and its optimization control method (see patent CN106411161B) and the document "Generalized Clarke Transformation and Enhanced Dual-Loop Control Scheme for Three-Phase PWM Converters Under the Unbalanced Utility Grid" can achieve lower current total harmonic distortion when the grid voltage is asymmetric, but it requires a large number of formula calculations, increases the control complexity, adds additional power devices, reduces the power density of the system, and has poor ability to suppress secondary voltage ripple.

[0004] Among the current methods, one type requires the use of additional power devices or devices to absorb the secondary pulsation of power and eliminate the secondary ripple on the DC voltage, which increases the cost and volume; the other type increases the restriction conditions by improving the control method to improve the ripple suppression performance, but in essence, they all rely on the calculation of the rectifier power model and mathematical model, which is large in calculation and complex, and the control performance is restricted by parameters such as sampling accuracy and inductance value. Therefore, it is necessary to conduct in-depth research on the DC voltage ripple suppression method of PWM rectifier, quickly realize the complete suppression of the DC voltage secondary ripple, and suppress the grid-side current harmonics at the same time, and it needs to have strong robustness and stability. Summary of the invention

[0005] To achieve the above purpose, the technical solution adopted by the present invention is: a high-gain proportional resonant PWM rectifier DC voltage ripple suppression method, which adds control of DC voltage secondary ripple on the basis of traditional dual closed-loop vector control. The ripple suppression method generally includes four steps: DC voltage secondary ripple calculation part, high-gain proportional resonant controller part, negative sequence active current delay part and traditional dual closed-loop vector control part. The specific steps are as follows:

[0006] (1) DC voltage secondary ripple calculation part: Extracting DC voltage tracking error u dcref -u dc As input to a high-gain proportional resonant controller;

[0007] (2) High-gain proportional resonant controller: The proportional resonant controller is modified to have a larger gain and an adjustable bandwidth. After the input secondary ripple passes through the high-gain proportional resonant controller, the grid-side negative-sequence active current reference value is output;

[0008] (3) Negative sequence active current delay part: the output negative sequence active current is delayed by an inherent time to obtain the negative sequence reactive current, which is combined with the positive sequence current;

[0009] (4) Traditional dual closed-loop vector control part: It includes a DC voltage outer loop and an AC current inner loop. The output of the current inner loop then enters the vector modulation part.

[0010] Further, in step (1), the DC voltage u dc Divided into DC part and secondary ripple When the system is stable, the DC part Equal to the DC voltage reference value u dcref , suppressing voltage secondary ripple Equivalent to the voltage secondary ripple reference value is 0, through the DC voltage u dc and DC voltage reference value u dcrefCalculate the DC voltage ripple component as follows.

[0011]

[0012] In step (2), a high gain proportional resonant controller G is used. HGPR (s), its transfer function is as follows.

[0013]

[0014] Compared with the quasi-proportional resonant controller, the high-gain proportional resonant controller adds a parameter k. Reasonable setting of the parameters can increase the controller gain without changing the bandwidth, which significantly improves the controller performance. The other parameters are the proportional coefficient K p , resonance coefficient K r , resonant frequency ω 0 , cut-off frequency ω c , where the resonant frequency ω 0 is the secondary ripple frequency, and the cutoff frequency is set according to the bandwidth requirement. The proportional coefficient, resonance coefficient and parameter k are selected according to the required controller gain.

[0015] When designing the voltage secondary ripple control structure, it is necessary to fully consider the various parameters of the actual controlled system. These parameters include but are not limited to load characteristics (such as the size of the load, the dynamic changes of the load, etc., because the change of the load will directly affect the generation and change of the voltage ripple in the system), DC capacitor parameters (such as the capacitance of the capacitor, the equivalent series resistance, etc., these parameters determine the role of the capacitor in filtering and energy storage, and thus affect the amplitude and frequency characteristics of the voltage ripple) and sampling time (the length of the sampling time determines the detection accuracy and response speed of the control system to the signal. The appropriate sampling time can enable the control system to obtain the information of the voltage ripple more accurately, so as to carry out effective control in a timely manner).

[0016] Based on the parameters of these actual controlled systems, the transfer function of the designed voltage secondary ripple control structure is deduced through mathematical modeling and analysis. The transfer function reflects the relationship between the input control signal and the output voltage ripple in the voltage secondary ripple control structure. Through this transfer function, we can clearly understand how the control signal affects the voltage ripple and how the voltage ripple changes with the change of the control signal.

[0017] Then, the controller is designed according to the deduced transfer function. The design of the controller is the key link of the entire control system. Its purpose is to effectively control the voltage secondary ripple through reasonable control strategies and algorithms to meet the performance requirements of the system. According to the transfer function, the stability and dynamic performance of the system can be analyzed, and the parameters of the controller can be determined to ensure that the controller can accurately control the voltage secondary ripple.

[0018] The overall transfer function of the voltage secondary ripple control structure is as follows. This transfer function is the core mathematical expression of the entire control structure. It integrates the dynamic characteristics of each part of the system (including the controlled object, controller, etc.) and fully describes the relationship between the input and output of the entire control structure.

[0019]

[0020] Among them, G T (s) is the sampling delay, T samp is the sampling period, G dc (s) is the transfer function between the DC side capacitance and the load, R L is the load, C is the DC capacitor, G PI (s) is the transfer function of the DC voltage loop PI controller, K pu is the proportionality coefficient, K iu is the integration coefficient.

[0021] The gain of the high-gain proportional resonant controller is calculated according to the following formula, and the proportional coefficient, resonant coefficient and parameter k of the controller are designed in combination with the control structure transfer function and the secondary ripple suppression requirements.

[0022]

[0023] In step (3), the current reference value is calculated according to the following formula.

[0024]

[0025] in, is the negative sequence active current, is the negative sequence reactive current, e -τs is a delay link, τ is a delay time, which is 7.5 ms in the present invention, i.e., 3 / 4 of the period corresponding to the double power frequency.

[0026] Calculate the current reference value according to the following formula.

[0027]

[0028] Among them, i dref is the active current reference value, is the positive sequence active current reference value, is the negative sequence active current reference value, i qref is the reactive current reference value, is the positive sequence reactive current reference value, It is the negative sequence reactive current reference value.

[0029] In step (4), the current reference value is input into the current inner loop control part, and it is controlled according to the traditional proportional resonant current loop control method to achieve DC voltage secondary ripple suppression.

[0030] The effects of the present invention are as follows: the control structure of the system is simple, and positive and negative sequence decomposition and a large amount of calculation are not required. Better dynamic and static control performance is obtained, ripple can be quickly suppressed, and steady-state ripple is completely suppressed, and current THD is also improved. The suppression effect will not be affected by the sampling error of the grid-side voltage and grid-side current and the unknown inductance value. No additional power devices are required, and the power density and cost of the system will not be affected. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The gain characteristics of the high-gain proportional resonance controller of the present invention are compared with those of the conventional proportional resonance and quasi-proportional resonance;

[0032] Figure 2 It is the voltage secondary ripple control structure diagram;

[0033] Figure 3 It is a control structure diagram of the whole system of the present invention;

[0034] Figure 4 The present invention discloses a method for suppressing the dynamic process waveform of DC voltage ripple. DETAILED DESCRIPTION

[0035] A specific embodiment of the present invention is described below in conjunction with the accompanying drawings, and the technical solution principle and effect of the present invention are further explained.

[0036] The present invention provides a method for suppressing DC voltage ripple of a high-gain proportional resonance PWM rectifier. The gain characteristics thereof are compared with proportional resonance and quasi-proportional resonance. Figure 1 As shown in Figure 1, it combines the advantages of the latter two, and can maintain a high gain while having an individually adjustable bandwidth under the same parameters. The closed-loop control method of high-gain proportional resonant control is shown in Figure 1. Figure 2 As shown, the DC voltage secondary ripple is taken as the controlled object and is achieved by adjusting the negative sequence current. Figure 3 For the purpose of suppressing the secondary ripple of the DC voltage, the DC voltage ripple suppression proposed by the present invention includes the following steps:

[0037] (1) Calculate the DC voltage ripple. Due to the existence of the DC voltage loop, the DC voltage secondary ripple does not need to be calculated additionally. It can be directly calculated by taking the opposite of the DC voltage tracking error. When the secondary ripple reference value is set to 0, the negative sign can be offset, that is, the DC voltage tracking error, that is, the input of the DC voltage loop PI controller, is directly used as the input of the high gain proportional resonant controller.

[0038] (2) High-gain proportional resonant controller, which tracks the DC voltage error u e As input, after passing through the high-gain proportional resonant controller, the grid-side negative sequence active current reference value is output. Since the designed controller gain is large and has a certain bandwidth, the ripple suppression controller here has a small steady-state error and a fast dynamic speed. The controller parameters are adjusted according to the actual voltage ripple suppression effect. The parameters of the selected high-gain proportional resonant controller are as follows: Proportional coefficient K p is 0.05, the resonance coefficient K r is 1, the resonant frequency ω 0 is 628rad / s, cutoff frequency ω c is 3.14 rad / s, and the parameter k is 0.8. Here the transfer function of the high gain proportional resonant controller is as follows:

[0039]

[0040] When digitally implementing this controller, it is necessary to discretize the transfer function of the above equation and transform it to the z domain. Let the coefficient of the 0th order term of s in the numerator of the transfer function be A 0 , the coefficient of the first-order term is A 1 , the coefficient of the second-order term is A 2 , the coefficient of the 0th order term of s in the denominator is B 0 , the coefficient of the first-order term is B 1 , the coefficient of the second-order term is B 2 , after transformation to the z domain, the coefficient of the 0th order term of z in the numerator is C 0 , the coefficient of the -1 term is C 1 , the coefficient of the -2 term is C 2 , the coefficient of the 0th order term of z in the denominator is D 0 , the coefficient of the -1 term is D 1 , the coefficient of the -2 term is D 2 , the form of its transfer function in the discrete domain becomes

[0041]

[0042] In a discrete control system, it is necessary to record the input and output values ​​of the previous two control cycles corresponding to the current control cycle. The input and output of the current control cycle are defined as u e (n) and The input and output of the last control cycle are u e (n-1) and The input and output of the previous control cycle are u e (n-2) and Before the system starts running, their initial values can be set to 0 to prevent the impact during startup. The formulas for the input and output of the high-gain proportional-resonant controller in the final discrete control system can be expressed as:

[0043]

[0044] The parameters of the control system are as follows: Sampling period T samp is 0.1 ms, load R L is 130 Ω, DC capacitor C is 200 μF, and the proportional coefficient K pu of the DC voltage loop PI controller is 0.02, and the integral coefficient K iu is 6.

[0045] (3) According to the relationship between the negative-sequence active and reactive currents, the negative-sequence reactive current reference value is obtained, that is, the negative-sequence active current reference value is delayed by 7.5 ms to obtain the negative-sequence reactive current reference value. In the discrete control system, the negative-sequence active current reference values of each control period can be stored by defining an array, and the negative-sequence reactive current reference value is the array data before the corresponding control period number. The number of periods of the previous data required in the example of the present invention is 75, that is, the delay time 7.5 ms is the multiple of the control period 0.1 ms.

[0046] (4) Next, the negative-sequence current is combined with the positive-sequence current. Since the negative-sequence reactive current here is also calculated in the positive-sequence rotating coordinate system, the reference current is the direct addition result of the negative-sequence current and the positive-sequence current. That is, the reference current is:[[]]

[0047]

[0048] In the reference current here, is the positive-sequence active current reference value

[0049]

[0050] output by the voltage loop PI controller, which is obtained by multiplying the DC voltage tracking error by the proportional coefficient and adding the sum of the accumulations in each control period multiplied by the integral coefficient. It can be expressed by the following formula: The positive-sequence reactive current reference value is 0, the negative-sequence active current reference value is output by the high-gain proportional-resonant controller, and the negative-sequence reactive current reference value is obtained by delay. After obtaining the reference current, the positive-sequence reference current is a DC quantity, and the negative-sequence reference current is an AC quantity with twice the power grid frequency, which is not suitable for dual-PI current control. Here, the active reference current i dref and the reactive reference current i qrefTransformed to the two-phase static coordinate system, that is, the αβ coordinate system, the reference current at this time becomes an alternating current with a phase difference of 90° but unequal amplitude, and is equal to the grid frequency. Then, the dual PR current control link is used, combined with the grid voltage feedforward, to quickly track the grid current to the reference current and achieve steady-state tracking without static error. The output V of the dual PR current loop is PRα ,V PRβ The grid-side voltage e α ,e β After feedforward, it is the input of space vector modulation, that is, the reference value v of the rectifier AC terminal voltage αref ,v βref , and its corresponding formula is as follows:

[0051]

[0052] (5) When the reference value of the rectifier AC terminal voltage is input into the space vector modulation, two mutually orthogonal AC quantities form a vector. This vector can be linearly represented by the vectors corresponding to other switch states. Its range is within a regular hexagon and is divided into 6 sectors. It is necessary to determine the position of the sector where this vector is located to determine which two vectors are used to synthesize the target rectifier AC terminal voltage vector, and it is necessary to determine the vector amplitude V according to the vector amplitude V. m The phase θ determines the action time t of the two basic vectors and the zero vector 1 ,t 2 and t 0 , the corresponding calculation method is as follows:

[0053]

[0054] t 0 =T samp -t 1 -t 2

[0055] The resultant vector v and the basic vector v 1 , v 2 The relationship is as follows:

[0056] v=v 1 t 1 +v 2 t 2

[0057] The duty cycle of each phase power switch is determined by the switching state of the basic vector and the corresponding action time. Let the duty cycle of the upper bridge arm power switch of phase a be D a , the duty cycle of the upper bridge arm power switch of phase b is D b , the duty cycle of the upper bridge arm power switch of phase c is D c To facilitate calculation, the commonly used duty cycle D is defined as 1～4 as follows:

[0058] D 1 =t 0 / 2T samp

[0059] D 2 =(t 0 +2t 1 ) / 2T samp

[0060] D 3 =(t 0 +2t 2 ) / 2T samp

[0061] D 4 =(t 0 +2t 1 +2t 2 ) / 2T samp

[0062] The duty ratios corresponding to the positions of each sector are as follows:

[0063]

[0064] After obtaining the duty cycle of each phase, the PWM modulation link is used to generate a drive signal to control the power switch of each phase. Finally, under the regulation of the controller, the DC voltage ripple is basically completely suppressed, and the grid-side current harmonics caused by the secondary ripple of the DC side voltage are also significantly reduced.

[0065] In order to demonstrate the remarkable effects of the present invention, this embodiment provides some results obtained by using the embodiments. Figure 4 The waveform of the dynamic startup process of the method of the present invention for suppressing the secondary ripple of the DC voltage is given. Within 0-0.4s, the control method proposed by the present invention is not added, the peak-to-peak value of the secondary ripple of the DC voltage is 5.90V, and there is an obvious ripple fluctuation of twice the power frequency; the effective values ​​of the three-phase voltage are 66V, 110V, and 110V respectively, and the DC component of the DC bus voltage is 300V. The effective values ​​of the three-phase current on the grid side are 3.47A, 3.30A, and 3.54A respectively, and the total harmonic distortion of the current is 4.0%, 3.6%, and 3.5% respectively, and the current distortion is serious. After 0.4s, the DC voltage ripple suppression of this method begins. Thereafter, the peak-to-peak value of the secondary ripple of the DC voltage is 0.01V, and the ripple fluctuation of twice the power frequency is effectively suppressed; the three-phase current THD is 2.5%, 2.4%, and 2.4% respectively, and the current harmonics are significantly reduced. Therefore, the method described in the present invention can not only effectively suppress the secondary ripple of the DC voltage, but also effectively eliminate the harmonics of the grid-side current.

[0066] The experimental results show that the method described in the present invention can not only effectively suppress the DC bus voltage ripple, but also effectively eliminate the grid-side current harmonics.

[0067] This embodiment provides the implementation effect of suppressing the secondary ripple of the DC voltage of a three-phase PWM rectifier, but the present invention is not limited to the three-phase voltage-type full-bridge rectifier topology. The present invention is also applicable to suppressing the secondary ripple of the DC voltage of any three-phase rectifier topology.

[0068] The present invention may be implemented in other specific forms without departing from its spirit or essential characteristics. The described embodiments are considered to be illustrative and not restrictive in all aspects (e.g., DC voltage secondary ripple closed-loop control, DC voltage ripple value calculation, and high-gain proportional resonant control, etc.). Therefore, the scope of the present invention is indicated by the appended claims rather than the above description. All changes within the meaning and scope of the equivalent technical solutions of the claims are included in their scope.

Claims

1. A high-gain proportional resonant PWM rectifier DC voltage ripple suppression method adds control of DC voltage secondary ripple on the basis of traditional dual closed-loop vector control. The ripple suppression method generally includes four steps: DC voltage secondary ripple calculation part, high-gain proportional resonant controller part, negative sequence active current delay part and traditional dual closed-loop vector control part. The specific steps are as follows: (1) DC voltage secondary ripple calculation part: Extracting DC voltage tracking error u dcref -u dc As input to a high-gain proportional resonant controller; (2) High-gain proportional resonant controller: The proportional resonant controller is modified to have a larger gain and an adjustable bandwidth. After the input secondary ripple passes through the high-gain proportional resonant controller, the grid-side negative-sequence active current reference value is output; (3) Negative sequence active current delay part: the output negative sequence active current is delayed by an inherent time to obtain the negative sequence reactive current, which is combined with the positive sequence current; (4) Traditional dual closed-loop vector control part: It includes a DC voltage outer loop and an AC current inner loop. The output of the current inner loop then enters the vector modulation part.

2. The method for suppressing DC voltage ripple of a high-gain proportional resonant PWM rectifier according to claim 1, characterized in that: In part (1), the DC voltage is divided into the DC part and secondary ripple When the system is stable, the DC part Equal to the DC voltage reference value u dcref , suppressing voltage secondary ripple Equivalent to the voltage secondary ripple reference value is 0, through the DC voltage u dc and DC voltage reference value u dcref Calculate the DC voltage ripple component as follows:

3. The method for suppressing DC voltage ripple of a high-gain proportional resonant PWM rectifier according to claim 1, characterized in that: In part (2), a high gain proportional resonant controller G is used. HGPR (s), its transfer function is as follows: Compared with the quasi-proportional resonant controller, the high-gain proportional resonant controller adds the parameter k (k>0), which can increase the controller gain without changing the bandwidth, significantly improving the controller performance. The other parameters are the proportional coefficient K p , resonance coefficient K r , resonant frequency ω0, cutoff frequency ω c , where the resonant frequency ω0 is the secondary ripple frequency, the cutoff frequency is set according to the bandwidth requirement, and the proportional coefficient, resonance coefficient and parameter k are selected according to the required controller gain.

4. The method for suppressing DC voltage ripple of a high-gain proportional resonant PWM rectifier according to claim 3, characterized in that: When designing the voltage secondary ripple control structure, it is necessary to fully consider the various parameters of the actual controlled system, including load characteristics, parameters of DC capacitors, including: the capacitance and equivalent series resistance of the capacitor. The DC capacitor parameters determine the role of the capacitor in filtering and energy storage, which in turn affects the amplitude and frequency characteristics of the voltage ripple and the sampling time. The length of the sampling time determines the detection accuracy and response speed of the control system to the signal. Based on the parameters of these actual controlled systems, the transfer function of the designed voltage secondary ripple control structure is deduced through mathematical modeling and analysis methods. The transfer function reflects the relationship between the input control signal and the output voltage ripple in the voltage secondary ripple control structure. Through this transfer function, we can clearly understand how the control signal affects the voltage ripple, and how the voltage ripple changes with the change of the control signal; Then, the controller is designed according to the deduced transfer function. The design of the controller is the key link of the entire control system. Its purpose is to effectively control the voltage secondary ripple through reasonable control strategies and algorithms to meet the performance requirements of the system; according to the transfer function, the stability and dynamic performance of the system are analyzed, and the parameters of the controller are determined to ensure that the controller can accurately control the voltage secondary ripple. The overall transfer function of the voltage secondary ripple control structure is as follows. This transfer function is the core mathematical expression of the entire control structure. It integrates the various parts of the system, including the controlled object and the dynamic characteristics of the controller, and fully describes the relationship between the input and output of the entire control structure: Among them, G T (s) is the sampling delay, T samp is the sampling period, G dc (s) is the transfer function between the DC side capacitance and the load, R L is the load, C is the DC capacitor, G PI (s) is the transfer function of the DC voltage loop PI controller, K pu is the proportionality coefficient, K iu is the integration coefficient.

5. The method for suppressing DC voltage ripple of a high-gain proportional resonant PWM rectifier according to claim 3, characterized in that: The gain of the high-gain proportional resonant controller is calculated according to the following formula, and the proportional coefficient, resonant coefficient and parameter k of the controller are designed in combination with the control structure transfer function and the secondary ripple suppression requirements.

6. The method for suppressing DC voltage ripple of a high-gain proportional resonant PWM rectifier according to claim 1, characterized in that: In part (3), the negative sequence reactive current is calculated according to the following formula, in, is the negative sequence active current, is the negative sequence reactive current, e -τs is the delay link, τ is the delay time, which is 7.5ms, i.e. 3 / 4 of the period corresponding to double the industrial frequency.

7. The method for suppressing DC voltage ripple of a high-gain proportional resonant PWM rectifier according to claim 1, characterized in that: In part (3), the current reference is calculated according to the following formula, Among them, i dref is the active current reference value, is the positive sequence active current reference value, is the negative sequence active current reference value, i qref is the reactive current reference value, is the positive sequence reactive current reference value, It is the negative sequence reactive current reference value.

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