Input ripple suppression method of three-phase voltage type inverter under nonlinear load

Abstract

This invention relates to a method for suppressing input ripple in a three-phase voltage-source inverter under nonlinear loads. When the load is a nonlinear load such as a rectifier or AC voltage regulator, it is powered by the output line voltage of the three-phase inverter, and the inverter output current contains... n ±1st harmonic current, load pulsating power contains n The second harmonic component causes the inverter input current to contain... n Subharmonic components. To suppress low-frequency ripple in the input current, the common point of the three output filter capacitors of a traditional three-phase voltage-source inverter is grounded and used as a power decoupling capacitor to detect nonlinear loads. n The secondary pulsating power is used to obtain the compensation required on the decoupling capacitor. n The subharmonic voltage is generated by superimposing a DC voltage, a symmetrical fundamental voltage, and an identical subharmonic voltage onto the three decoupling capacitors through power decoupling dual-loop control and output voltage dual-loop control. n Subharmonic power and load n The sum of the pulsating power of each pulse is zero, thereby suppressing low-frequency ripple in the input current.

Description

Technical Field

[0001] This invention belongs to the field of three-phase inverter technology and relates to a method for suppressing input ripple in a three-phase voltage-source inverter under nonlinear load. Background Technology

[0002] With the gradual depletion of traditional fossil fuels and the increasing prominence of environmental pollution, coupled with the ever-growing demand for energy, the research and application of clean energy will be an inevitable path for the progress of social civilization. Against this backdrop, developing a low-carbon economy has become imperative, and increasing the application of new energy technologies is a crucial support for my country's sustainable development strategy for a low-carbon economy. Driven by the sustainable development strategy, new energy technologies have developed rapidly. Compared with traditional power generation methods, new energy power generation technologies have advantages such as renewability, zero pollution, high power generation efficiency, low input costs, and high reliability. When new energy sources such as photovoltaic cells or fuel cells supply power to AC loads, DC power is typically converted to AC power via an inverter. When the AC load is a nonlinear power electronic device such as a rectifier or AC voltage regulator, its load characteristics often exhibit dynamic and nonlinear behavior. This not only distorts the inverter's output voltage but also generates a large amount of low-frequency harmonic current on the input side or intermediate DC side, causing voltage fluctuations on the input side or intermediate DC bus, affecting system stability and power transmission efficiency, and generating various power quality problems.

[0003] Inverters can be divided into single-phase inverters and three-phase inverters, and most of them use pulse width modulation (PWM) technology to control the switching transistors to turn on and off. Single-phase inverters can be further divided into single-stage, quasi-single-stage, and two-stage inverters. When a single-phase inverter is connected to a linear load, the input current ripple is mainly a second-order ripple. When connected to a nonlinear load, the input current ripple is mainly a low-frequency even-order harmonic. In a three-phase inverter, when the load is a linear symmetrical load, the sum of the three-phase load pulsating power is zero, and the input current does not contain low-frequency harmonics. When the load is a linear asymmetrical load, the sum of the three-phase load pulsating power is not zero, and the input current contains corresponding low-frequency harmonic components. When the load is a nonlinear load such as a rectifier or AC voltage regulator, because there is no path for zero-sequence harmonics, the inverter output current only contains n±1 (n=6, 12, 18, 24…) order harmonic currents, and the load pulsating power is n (n=6, 12, 18, 24…) order components, resulting in the DC input current also containing corresponding n (n=6, 12, 18, 24…) order current ripple. Low-frequency ripple on the DC side reduces the power generation efficiency of fuel cells and the service life of batteries; in photovoltaic power generation systems, it also affects the maximum power point tracking (MPPT) performance, reducing photovoltaic power output. Therefore, suppressing low-frequency ripple in the inverter's input current is of great significance for improving the efficiency and reliability of new energy power generation systems.

[0004] Traditional passive filtering techniques typically use inductors and capacitors to filter out harmonic components of certain frequencies in the input current. Theoretically, increasing the values ​​of inductors and capacitors can completely suppress input current ripple. However, in practical applications, limitations in size and weight mean that the ripple suppression effect becomes less significant when the inductor and capacitor values ​​increase to a certain extent. Furthermore, LC parameters are quite sensitive and can easily lead to series-parallel resonance in the circuit, generating excessive ripple current. Therefore, the effectiveness of passive filtering techniques in suppressing input current ripple is limited. In low-voltage input inverter applications, a DC / DC boost converter is usually introduced between the inverter and the DC power supply. Active control technology is used to make the intermediate DC bus capacitor bear almost all the low-frequency ripple current in the input current of the subsequent inverter, thereby reducing the low-frequency ripple component in the DC input current. However, this requires additional control circuitry, making the control parameter design process complex. Additionally, the intermediate DC bus capacitor must be a large-capacity electrolytic capacitor, which reduces the lifespan of the inverter.

[0005] Active DC filtering technology adds switching transistors and energy storage elements to the input, DC bus, or output sides of the inverter to form an active filter circuit topology. By controlling the switches, the energy of the energy storage elements is controlled, thus suppressing low-frequency ripple current in the input current. Although the filter circuit topologies used in active DC filtering technology differ, the underlying principle of suppressing input-side current ripple is consistent: by controlling the power switches, the energy storage elements balance the pulsating power in the load. When the instantaneous load power is less than the average load power, the energy storage elements absorb excess energy; when the instantaneous load power is greater than the average load power, the energy storage elements release the stored energy. If the energy storage elements and the load are considered as a whole, their instantaneous power equals the average load power, with no pulsating component; therefore, the input side does not contain low-frequency pulsating current.

[0006] Besides the operation of a single-phase inverter with a linear load, three-phase inverters in islanded mode operating with an unbalanced three-phase load are prone to three-phase output voltage imbalance. Secondary pulsating power is also generated on the AC side and coupled to the DC side, leading to reduced system performance and affecting system lifespan. To decouple the secondary pulsating power on the AC and DC sides, inverters typically employ passive power decoupling schemes such as adding DC-side capacitors or secondary resonant branches, resulting in increased cost and size. For three-phase inverters with unbalanced loads, some researchers have proposed a decoupling control strategy based on virtual three-phase instantaneous power. By controlling the virtual three-phase instantaneous power pulsation component to zero, the capacitor in the LCL filter is used as a decoupling capacitor to provide the required secondary pulsating power to the load without adding switching devices. This control strategy does not require source current detection; its detection quantity is the same as that of a traditional converter, and it can achieve decoupling of secondary pulsating power on both the AC and DC sides.

[0007] Current research on inverter input current ripple suppression mainly focuses on single-phase inverters with linear loads and three-phase inverters with unbalanced loads. In actual microgrid systems, there are numerous nonlinear loads such as thyristor rectifiers and thyristor AC voltage regulators. These hybrid loads significantly impact the stable operation of the microgrid: nonlinear loads cause distortion of the distributed generation output voltage waveform, and the low thermal tolerance of semiconductor switching devices leads to low system overload capacity. While there is considerable research on the control of the output voltage waveform of inverters under nonlinear loads, research on the suppression of low-frequency input current ripple is relatively limited. However, under nonlinear loads, not only does the load current contain a large number of odd harmonics that easily cause output voltage distortion, but the input side also contains a large number of low-frequency even harmonics that easily lead to reduced power generation efficiency and harmonic pollution. Therefore, research on inverter input current ripple suppression under nonlinear loads is of great significance. Summary of the Invention

[0008] The purpose of this invention is to effectively solve the problem of a large number of low-frequency harmonics in the input current of a three-phase voltage source inverter when it is connected to a nonlinear load such as a rectifier or AC voltage regulator. This invention provides a method for suppressing input ripple in a three-phase voltage source inverter under nonlinear load. This method can reduce the low-frequency harmonic components of the input current, effectively improve the system operating efficiency, and improve the overall power generation efficiency and safety of the new energy power generation system.

[0009] To achieve the above objectives, the technical solution of the present invention is: a method for suppressing input ripple in a three-phase voltage-source inverter under nonlinear load, providing a circuit for suppressing input ripple in a three-phase voltage-source inverter under nonlinear load, including an input DC power supply U. i Input-side filter capacitor C i Fully controlled inverter bridge arm switches S1, S2, S3, S4, S5, S6; three-phase filter inductor L a L b L c and parasitic resistance r a r b r c Three-phase output filter capacitor C a C b C c and nonlinear loads; input DC power supply U i Input-side filter capacitor C i Connected to the input terminal of the three-phase inverter bridge, the A, B, and C phase outputs of the three-phase inverter bridge are respectively filtered by the three-phase filter inductor L. a L b L c and parasitic resistance r a r b r c The circuit is connected to a nonlinear load and then through a three-phase filter capacitor C. a C b C c With input DC power supply U i Negative terminal connection; this method connects the three-phase output filter capacitor C a C b C c Simultaneously, as a power decoupling capacitor, it achieves the effect of suppressing input-side current ripple by superimposing a DC voltage and n (n = 6, 12, 18, 24...) harmonic voltages on the three-phase output filter capacitor.

[0010] In one embodiment of the present invention, the method is implemented as follows: the nonlinear load power p oAfter extracting the pulsating components, the even-order harmonic voltages that need to be superimposed on the three-phase output filter capacitors are obtained. The extracted 6k harmonic voltages are added to the DC bias and used as the reference voltage for the three-phase output filter capacitors, i.e., the power decoupling capacitors. Both power decoupling control and output voltage control employ voltage outer loop and current inner loop control to ensure that the power decoupling capacitor voltage tracks the reference voltage, limiting the pulsating power loop to the AC output side and suppressing low-frequency current harmonics on the inverter input side. Power decoupling control uses the average value of the three capacitor voltages as the outer loop tracking signal and the average value of the three inductor currents as the inner loop tracking signal to ensure that the capacitor voltage tracks the 6k harmonic voltages that need to be superimposed. Output voltage control uses the average value of the three capacitor voltages minus their respective values ​​as the outer loop tracking signal and the average value of the three inductor currents minus their respective values ​​as the inner loop tracking signal to ensure that the output voltage tracks the reference sinusoidal voltage.

[0011] In one embodiment of the present invention, the method for extracting the even-order harmonic voltages that need to be compensated on the three-phase output filter capacitor, i.e., the power decoupling capacitor, is as follows:

[0012] The instantaneous value of the three-phase power decoupling capacitor voltage u Ca u Cb u Cc Instantaneous value of three-phase output load current i oa i ob i oc Obtain the nonlinear load power p o The instantaneous value of the capacitor voltage u on the power decoupling capacitor. Ca u Cb u Cc and instantaneous value of capacitor current i Ca i Cb i Cc The instantaneous total power p generated on the power decoupling capacitor is obtained. c_all The voltage of the power decoupling capacitor consists of DC and AC components. An n (n = 6, 12, 18, 24…) harmonic voltage is superimposed on the power decoupling capacitor to generate n (n = 6, 12, 18, 24…) order power to compensate for the nonlinear load power p. o It contains n (n = 6, 12, 18, 24...) power pulsation components;

[0013] To obtain the nth (n = 6, 12, 18, 24…) harmonic voltage that needs to be superimposed on the power decoupling capacitor, it is necessary to adjust the nonlinear load power p. o The various pulsation components are separated. Assuming T is the fundamental period of the output voltage, the pulsation power p is first separated. α (p α =p o Delay T / 4n to obtain p β Then, through coordinate transformation matrix A...n For p α p β Transforming to a rotating coordinate system that rotates counterclockwise with an angular velocity of nω, we obtain the power p. dn p qn :

[0014]

[0015] Where ω is the fundamental angular frequency of the AC side output voltage;

[0016] The obtained power p dn p qn The DC power quantity is obtained after passing through a mean filter stage, which includes both DC and AC quantities. Then use matrix A n T right Perform an inverse transform to obtain the separated nth pulsating power component p. dnω p qnω :

[0017]

[0018] The separated nth output pulsating power component p dnω Delaying by T / 4n, the nth harmonic voltage that needs to be superimposed on the power decoupling capacitor is:

[0019]

[0020] Where: C is the capacitance value of the power decoupling capacitor, and the three power decoupling capacitors have equal capacitance values; U dc U is the DC voltage component across the power decoupling capacitor. dc =U i / 2;

[0021] The nth harmonic voltage u that needs to be superimposed on the power decoupling capacitor Cn After adding them together, the DC voltage component U on the power decoupling capacitor is also added. dc The reference signal u of the outer loop voltage of the power compensation loop is then obtained. * 6k Because the DC component and harmonic voltage of the three power decoupling capacitor voltages are the same, a (u) is constructed. Ca +u Cb +u Cc The voltage of the outer loop of the power compensation loop is used as the feedback signal, and multi-resonant quasi-PR control is adopted to obtain the reference signal i of the inner loop of the power decoupling control loop current. * 6k Based on the fact that the average value of the three-phase output fundamental current over one cycle is equal to 0, construct (i La +i Lb+i Lc The ) / 3 is used as the feedback signal for the inner loop current of the power decoupling control loop. It also employs multi-resonant quasi-PR control, ensuring that the n (n = 6, 12, 18, 24…) harmonic voltage superimposed on the power decoupling capacitor tracks the n (n = 6, 12, 18, 24…) harmonic voltage components extracted from the pulsating power. The output voltage control uses u… Ca -(u Ca +u Cb +u Cc ) / 3、u Cb -(u Ca +u Cb +u Cc ) / 3、u Cc -(u Ca +u Cb +u Cc Using ) / 3 as the voltage outer loop feedback signal, multi-quasi-resonant PR control is employed to obtain the reference signal for the current inner loop, thus constructing the inner loop current feedback signal i. La -(i La +i Lb +i Lc ) / 3、i Lb -(i La +i Lb +i Lc ) / 3、i Lc -(i La +i Lb +i Lc ) / 3, also using a quasi-resonant PR controller, makes the three-phase output voltage track the reference voltage.

[0022] Compared with the prior art, the present invention has the following beneficial effects: the present invention can reduce the low-frequency harmonic components of the input current, effectively improve the system operating efficiency, and improve the overall power generation efficiency and safety of the new energy power generation system. Attached Figure Description

[0023] Figure 1 This is the circuit topology diagram of a three-phase voltage source inverter under nonlinear load.

[0024] Figure 2 This is a schematic diagram of the active ripple suppression strategy control principle for a three-phase voltage source inverter under nonlinear load.

[0025] Figure 3 The input current i of a traditional three-phase voltage-source inverter without active filtering circuitry i Waveform.

[0026] Figure 4 The input current i of a three-phase voltage-source inverter under nonlinear load after adopting the control method proposed in this invention is... i Waveform.

[0027] Figure 5 When using a traditional three-phase voltage-source inverter without an active filter circuit and the control method proposed in this invention, the input current i i Spectrum diagram. Detailed Implementation

[0028] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0029] The key to this invention lies in the inverter load power p o After the n (n = 6, 12, 18, 24...) order pulsation components are calculated by mean filtering, the n (n = 6, 12, 18, 24...) order harmonic voltages that need to be superimposed on the filter capacitor are extracted. The extracted n (n = 6, 12, 18, 24...) order harmonic voltages are added to the DC bias and used as the reference voltage of the decoupling capacitor. By designing a power decoupling dual closed-loop control, the decoupling capacitor voltage is made to track the reference voltage, and the pulsating power loop is limited to the AC output side, thereby suppressing the low-frequency current harmonics on the inverter input side.

[0030] Figure 1 This is the circuit topology diagram of a three-phase voltage-source inverter under nonlinear load, including the input DC power supply U. i DC input current i i Input-side filter capacitor C i Fully controlled inverter bridge arm switches S1~S6, inverter-side output filter inductor L a L b L c and parasitic resistance r a r b r c Output inductor current i La i Lb i Lc Inverter-side output filter capacitor C a C b C c Output capacitor voltage u Ca u Cb u Cc Output three-phase load current i oa i ob i oc and nonlinear loads. The output filter capacitor also serves as a power decoupling capacitor. By superimposing a DC voltage and n (n = 6, 12, 18, 24...) harmonic voltages on the filter capacitor, the input-side current ripple is suppressed.

[0031] The output voltage of a three-phase voltage-source inverter under nonlinear loads is supplied by the line voltage. The load current contains only n±1 (n=6, 12, 18, 24…) harmonics, and the higher the harmonic order, the smaller the harmonic amplitude. Under the influence of the fundamental voltage, the load pulsating power contains only n (n=6, 12, 18, 24…) pulsating power components, requiring a DC bias voltage U to be superimposed on the three output decoupling capacitors. dc It compensates for the nth (n = 6, 12, 18, 24...) harmonic voltages of the load, and the line voltage is not affected, that is, it does not affect the normal operation of the rectifier.

[0032] The method for extracting the nth (n=6k) harmonic voltage that needs to be compensated on the decoupling capacitor is as follows:

[0033] Assume the voltages superimposed on the decoupling capacitor are as follows:

[0034]

[0035] Among them U dc To decouple the DC bias voltage component on the capacitor. U o U is the effective value of the AC side output phase voltage. n The amplitude of the nth harmonic voltage. Let ω be the initial phase angle of the nth harmonic voltage, and ω be the fundamental angular frequency of the AC output voltage (ω = 100π rad / s).

[0036] Assuming the capacitance of the decoupling capacitor is C, the current through the capacitor is as follows:

[0037]

[0038] From the above, we can obtain the instantaneous total power p generated on the decoupling capacitor. c_all (t)

[0039]

[0040] When U dc When the value is relatively large, the harmonic power generated by the interaction between the nth harmonic voltage and harmonic current superimposed on the capacitor is very small and can be ignored. The above formula can be simplified to:

[0041]

[0042] The nth pulsation power across the three output capacitors is:

[0043]

[0044] Since the load current mainly contains harmonic currents of order n ± 1 (n = 6, 12, 18, 24…), let the load current ioa i ob i oc for:

[0045]

[0046] Among them, I o To output the amplitude of the fundamental component of the load current, I n±1 φ n±1 These represent the amplitude and initial phase angle of the n±1 harmonic current component in the load current, respectively.

[0047] Then the load power p o for:

[0048]

[0049] Load power p o The nth pulsating power component in (t) is:

[0050]

[0051] Assuming T is the fundamental period of the output voltage, to separate the nth (n = 6, 12, 18, 24…) pulsation power component, the pulsation power p needs to be... o Delay for T / 4n, then transform using coordinate matrix A n The pulsating power is transformed into a rotating coordinate system rotating counterclockwise with an angular velocity of nω. The DC power component in the pulsating power of the rotating coordinate system is extracted by a mean filter. Finally, the DC power is transformed by the inverse coordinate transformation matrix A. n T Reconstructing the original data yields the nth pulsating power component that needs to be extracted. The transformation matrix A... n and A n T They are respectively:

[0052]

[0053] For load pulsating power p α (p α =p o Delaying by 1 / 4n fundamental frequency periods yields the power of the nth delayed pulsation:

[0054]

[0055] Using matrix A n For p α p βn Perform the transformation to obtain the transformed power p. dn p qn :

[0056]

[0057] From the above formula, we can see that the power p dn (t), p qn The power (t) contains both AC and DC components. The AC component is removed using a mean filter to obtain the DC power.

[0058]

[0059] Using matrix A n T DC power Perform an inverse transform to obtain the nth pulsating power component p that needs to be extracted. dnω (t), p qnω (t) is:

[0060]

[0061] Comparing equation (16) with equation (11), we can obtain p dnω (t)=p o_nω (t).

[0062] To compensate for the nth harmonic component in the nonlinear load's pulsating power by superimposing the nth harmonic voltage across the decoupling capacitor, the sum of the pulsating power generated on the decoupling capacitor and the load's pulsating power should be zero.

[0063] p o_nω (t)+p Cn (t)=p dnω (t)+p Cn (t)=0 (17)

[0064] Substituting equations (8) and (16) into equation (17), we get

[0065]

[0066] The extracted nth pulsation power p dnω The component (t) is delayed by T / 4n, and we can obtain

[0067]

[0068] The nth harmonic voltage that needs to be compensated across the decoupling capacitor is:

[0069]

[0070] Furthermore, from equation (16), it can be seen that

[0071]

[0072] Therefore, equation (20) can be rewritten as:

[0073]

[0074] From equation (22), it can be seen that the nth harmonic voltage u that needs to be superimposed on the decoupling capacitor Cn (t) can be derived from p qnω (t) is obtained through simple calculations; there is no need to further calculate p. dnω (t) A delay of T / 4n can reduce the amount of data stored and speed up the detection process.

[0075] The nth harmonic voltage u that needs to be superimposed on the decoupling capacitor Cn After adding (t), the DC bias voltage U is added. dc As the reference signal for the power compensation loop, a DC voltage and n (n = 6, 12, 18, 24...) harmonic voltages are superimposed on the three output decoupling capacitors through dual closed-loop control to compensate for the n (n = 6, 12, 18, 24...) order pulsating power of the load, so that the pulsating power is limited to the inverter output side and the input power contains only DC power, thereby suppressing the inverter's input current ripple.

[0076] The control principle diagram of the active ripple suppression strategy for a three-phase voltage-source inverter under nonlinear loads is shown below. Figure 2 As shown. The inverter main circuit adopts a dual closed-loop control with an outer loop for capacitor voltage and an inner loop for inductor current. The active power compensation section calculates the total output power p of the inverter using the load phase voltage and phase current. o The nth (n = 6, 12, 18, 24…) order pulsating power is extracted using a harmonic detection algorithm, thereby obtaining the nth (n = 6, 12, 18, 24…) order harmonic voltage that needs to be superimposed on the decoupling capacitor. The power decoupling control loop uses the extracted harmonic voltage as a reference to construct a control loop based on the average voltage u of the decoupling capacitor. C_av Average current i of filter inductor L_av This involves dual closed-loop control with feedback signals from the outer and inner loops, respectively. The output voltage control loop is based on a three-phase symmetrical voltage and is constructed with u as the reference. Ca -u C_av u Cb -u C_av u Cc -u C_av As the voltage outer loop feedback signal, i La -i L_av i Lb -i L_av i Lc -i L_avThe dual closed-loop control serves as the inner loop feedback signal. Under three-phase decoupling control, the decoupling capacitor voltage will have a DC bias. The output voltage control loop signal, the power decoupling control loop signal, and the DC bias are added together to obtain the SPWM modulation signal, which controls the switching of the three-phase inverter bridge arm switches. This not only makes the output voltage track the reference, but also makes the sum of the n (n = 6, 12, 18, 24...) pulsating power components on the inverter output side zero. The pulsating power is limited to the AC output side, thereby effectively suppressing the DC input current ripple.

[0077] Because proportional-integral (PI) controllers cannot achieve zero steady-state error control of sinusoidal signals, while proportional-resonant (PR) controllers, based on the internal model principle, have infinite gain at the resonant frequency, thus enabling zero steady-state error tracking of sinusoidal signals. However, PR controllers are highly sensitive to resonant frequency deviations, resulting in a small stability margin. Therefore, in practical engineering applications, quasi-proportional-resonant (QPR) controllers are often used instead of PR controllers. QPR controllers not only possess the advantages of PR controllers but also broaden the bandwidth of PR controllers at the gain. This invention requires compensation for n (n = 6, 12, 18, 24…) harmonics; therefore, a multi-resonant quasi-PR controller is used, with the transfer function as follows:

[0078]

[0079] Among them, K p The proportionality coefficient affects the system's stability; ω0 is the fundamental angular frequency (ω0 = 100π rad / s), and nω0 (n = 6, 12, 18, 24…) are the nth (n = 6, 12, 18, 24…) harmonic angular frequencies that need to be compensated; K i K is the resonance coefficient. i The value of ω affects the system's tracking performance on AC signals. c ω is the cutoff frequency, which acts like adding damping to an ideal PR controller, effectively solving the problem of small bandwidth in ideal PR controllers, while also providing significant gain at the fundamental frequency. c Increasing ω reduces the gain variation of the controller near the fundamental frequency, increases the bandwidth, and improves system stability; c Decreasing the gain of the controller near the fundamental frequency increases the gain variation, leading to increased system error and slower response speed.

[0080] Figure 3 The input current i of a traditional three-phase voltage-source inverter without active filtering circuitry i Waveform. Figure 4 The input current i of a three-phase voltage-source inverter under nonlinear load after adopting the control method proposed in this invention is... i Waveform. Figure 5When using a traditional three-phase voltage-source inverter without an active filter circuit and the control method proposed in this invention, the input current i i Spectrum diagram.

[0081] The above are preferred embodiments of the present invention. Any changes made to the technical solution of the present invention that do not exceed the scope of the technical solution of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for suppressing input ripple in a three-phase voltage-source inverter under nonlinear load, characterized in that, Provides an input ripple suppression circuit for a three-phase voltage-source inverter under nonlinear load, including an input DC power supply U i Input-side filter capacitor C i Fully controlled inverter bridge arm switches S1, S2, S3, S4, S5, S6, three-phase filter inductor L a L b L c and parasitic resistance r a r b r c Three-phase output filter capacitor C a C b C c and nonlinear loads; input DC power supply U i Input-side filter capacitor C i Connected to the input terminal of the three-phase inverter bridge, the A, B, and C phase outputs of the three-phase inverter bridge are respectively filtered by the three-phase filter inductor L. a L b L c and parasitic resistance r a r b r c The circuit is connected to a nonlinear load and then through a three-phase filter capacitor C. a C b C c With input DC power supply U i Negative terminal connection; this circuit connects the three-phase output filter capacitor C a C b C c Simultaneously, as a power decoupling capacitor, it suppresses input-side current ripple by superimposing a DC voltage and an nth harmonic voltage on the three-phase output filter capacitor, where n = 6, 12, 18, 24...; The method is implemented by: converting the nonlinear load power p o After extracting the nth pulsation component, the nth harmonic voltage that needs to be superimposed on the three-phase output filter capacitor is obtained. The extracted nth harmonic voltage is added to the fundamental voltage and the DC bias to serve as the reference voltage for the three-phase output filter capacitor, i.e., the power decoupling capacitor. Both power decoupling control and output voltage control adopt voltage outer loop and current inner loop control to make the power decoupling capacitor voltage track the reference voltage, limit the pulsating power loop to the AC output side, and suppress low-frequency current harmonics on the inverter input side. The power decoupling control uses the average value of the three capacitor voltages (u Ca +u Cb +u Cc ) / 3 is used as the outer loop tracking signal, and the average value of the three inductor currents (i La +i Lb +i Lc ) / 3 is used as the inner loop tracking signal to enable the capacitor voltage to track the superimposed 6k harmonic voltage; the output voltage control uses the instantaneous values ​​u of the three capacitor voltages. Ca u Cb u Cc Subtract their average value (u) respectively Ca +u Cb +u Cc The signal obtained by ) / 3, i.e., u Ca -(u Ca +u Cb +u Cc ) / 3、u Cb -(u Ca +u Cb +u Cc ) / 3、u Cc -(u Ca +u Cb +u Cc ) / 3 serves as the outer loop tracking signal, and the three inductor currents i La i Lb i Lc Subtract its average (i) La +i Lb +i Lc The signal obtained by ) / 3, i.e., i La -(i La +i Lb +i Lc ) / 3、i Lb -(i La +i Lb +i Lc ) / 3、i Lc -(i La +i Lb +i Lc ) / 3 serves as the inner current loop tracking signal, enabling the output voltage to track the reference sinusoidal voltage; The method for extracting the even-order harmonic voltages that need to be compensated on the three-phase output filter capacitor, i.e., the power decoupling capacitor, is as follows: The instantaneous value of the three-phase power decoupling capacitor voltage u Ca u Cb u Cc Instantaneous value of three-phase output load current i oa i ob i oc Obtain the nonlinear load power p o The instantaneous value of the capacitor voltage u on the power decoupling capacitor. Ca u Cb u Cc and instantaneous value of capacitor current i Ca i Cb i Cc The instantaneous total power p generated on the power decoupling capacitor is obtained. c_all The voltage of the power decoupling capacitor consists of DC and AC components. An nth harmonic voltage is superimposed on the power decoupling capacitor to generate nth-order power to compensate for the nonlinear load power p. o The nth power pulsation component contained therein; To obtain the nth harmonic voltage that needs to be superimposed on the power decoupling capacitor, it is necessary to adjust the nonlinear load power p. o The various pulsation components are separated. Assuming T is the fundamental period of the output voltage, the pulsation power p is first separated. α Delay T / 4n to get p β , where p α =p o Then, through coordinate transformation matrix A... n For p α p β Transforming to a rotating coordinate system that rotates counterclockwise with an angular velocity of nω, we obtain the power p. dn p qn : （1） Where ω is the fundamental angular frequency of the AC side output voltage; The obtained power p dn p qn The DC power quantity is obtained after passing through a mean filter stage, which includes both DC and AC quantities. , Then use matrix A n T right , Perform an inverse transform to obtain the separated nth pulsating power component p. dnω p qnω : （2） The separated nth output pulsating power component p dnω Delaying by T / 4n, the nth harmonic voltage that needs to be superimposed on the power decoupling capacitor is: （3） Where: C is the capacitance value of the power decoupling capacitor, and the three power decoupling capacitors have equal capacitance values; U dc U is the DC voltage component across the power decoupling capacitor. dc =U i / 2;

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