Stability control method for grid-connected inverters based on capacitor voltage feedforward integral compensation under weak grid conditions

Abstract

This invention discloses a stable control method for grid-connected inverters based on capacitor voltage feedforward integral compensation under weak grid conditions. It addresses the low-frequency negative damping and low-frequency oscillation problems introduced by phase-locked loops and capacitor voltage feedforward under weak grid conditions. By introducing the grid-side filter capacitor voltage and feeding it forward through a proportional-derivative link to the current regulator output, and then using a capacitor voltage feedforward integral compensation control method, the low-frequency negative damping effect of the grid-connected inverter's dq-axis output admittance is mutually canceled. Furthermore, an improved high-order integral term in the feedforward integral compensation control equation is proposed, enabling the grid-connected inverter to effectively offset the influence of grid frequency offset while operating under extremely weak grid conditions with very high grid impedance, significantly improving the operational stability of the grid-connected inverter.

Description

Technical Field

[0001] This invention relates to a control method for grid-connected inverter systems, specifically to a capacitor voltage feedforward integral compensation control structure and its parameter design method. Background Technology

[0002] Three-phase grid-connected inverters, as power conversion devices, play a crucial role in applications such as photovoltaic power generation, wind power generation, and energy storage systems. With the rapid development of new energy power generation grid connection and energy storage systems, the penetration rate of power electronic power conversion devices in the power grid is constantly increasing. The interaction between the equivalent output impedance of power electronic devices and the grid impedance causes various oscillation phenomena, which seriously affect the power generation quality of new energy grid-connected systems and the operational safety of the power grid.

[0003] Research has found that the oscillation frequency is related to the frequency-domain negative damping region of the inverter output admittance. The negative damping region of the inverter output admittance is affected by various control loop parameters and system control delays. For example, a phase-locked loop introduces low-frequency negative damping, a grid-side voltage feedforward loop introduces mid-frequency negative damping, and system control delays introduce high-frequency negative damping.

[0004] Several academic papers have analyzed and proposed solutions to the negative damping effect and oscillation phenomenon of inverter output admittance introduced by control loop and system delay, for example:

[0005] 1. The article entitled "Input-Admittance Calculation and Shaping for Controlled Voltage-Source Converters", L. Harnefors, M. Bongiorno, S. Lundberg, IEEE Transactions on Industrial Electronics, vol. 54, pp. 3323-3334, 2007. ("Input Admittance Calculation and Shaping for Controlled Voltage-Source Converters", IEEE Transactions on Industrial Electronics, vol. 54, pp. 3323-3334, 2007) analyzes in detail the negative damping effect of various control loops and system delays on inverter output admittance, and the coupling effect between them. However, it does not compensate for the negative damping effect in inverter output admittance, and the broadband oscillation problem between the inverter and the grid remains unresolved.

[0006] 2. The article entitled "Virtual damping control design of three-phase grid-tied PVinverters for passivity enhancement", Z. Yang, C. Shah, T. Chen, J. Teichrib, RW DeDoncker, IEEE Transactions on Power Electronics, vol. 36, pp. 6251-6264, 2021. ("Virtual damping control design of three-phase grid-tied PVinverters for passivity enhancement", IEEE Power Electronics Journal, Vol. 36, pp. 6251-6264, 2021) introduces virtual admittance into the control loop to compensate for the low-frequency negative damping effect caused by the phase-locked loop. However, it does not analyze or compensate for the low-frequency negative damping effect caused by the capacitor voltage feedforward used during inverter startup, and the resulting low-frequency oscillation problem is not completely solved.

[0007] 3. The article titled "Passivity-based multisampled converter-side current control of LCL-filtered VSCs", S. He, D. Zhou, X. Wang, F. Blaabjerg, IEEE Transactions on Power Electronics, vol. 37, pp. 13848-13860, 2022, points out that introducing capacitor voltage feedforward, while compensating for the high-frequency negative damping effect, will aggravate the low-frequency negative damping effect. The article analyzes and compensates for the high-frequency negative damping effect to effectively suppress the high-frequency oscillation problem of the inverter, but does not make corresponding compensation designs for the low-frequency negative damping effect, and the low-frequency oscillation problem still exists.

[0008] 4. The article entitled “Improved Design of PLL Controller for LCL-Type Grid-Connected Converter in Weak Grid”, D. Zhu, S. Zhou, X. Zou, Y. Kang, IEEE Transactions on Power Electronics, vol. 35, pp. 4715-4727. (“Improved Design of PLL Controller for LCL-Type Grid-Connected Converter in Weak Grid”, IEEE Power Electronics Journal, Vol. 35, pp. 4715-4727, 2020) reduces the negative damping region of the inverter output admittance by designing the parameters of the phase-locked loop, but reduces the bandwidth of the phase-locked loop, affecting the dynamic performance of the system.

[0009] In summary, the existing technology has the following problems:

[0010] 1. Under weak grid conditions, existing literature mainly focuses on the compensation design for low-frequency negative damping caused by phase-locked loops, while the analysis and compensation design for low-frequency negative damping caused by capacitor voltage feedforward has not been mentioned, and the low-frequency oscillation problem of inverter system has not been completely solved.

[0011] 2. In weak power grids, the low-frequency negative damping and low-frequency oscillation problems caused by the coupling of phase-locked loop and capacitor voltage feedforward have not yet been analyzed and compensated using a unified architecture.

[0012] 3. In weak grid environments, when the grid frequency deviates, the interaction between the inverter system and the grid intensifies, and oscillation problems become prominent. Existing literature has not yet proposed a complete solution to compensate for and eliminate the stability problems of grid-connected inverters caused by grid frequency deviation. Summary of the Invention

[0013] To overcome the limitations of the aforementioned technical solutions, this paper proposes a stable control method for grid-connected inverters based on capacitor voltage feedforward integral compensation under weak grid conditions, addressing the low-frequency negative damping (i.e., low-frequency oscillation) problem caused by phase-locked loops and capacitor voltage feedforward. This method aims to mutually cancel the low-frequency negative damping effect of the d-axis and q-axis output admittances of the grid-connected inverter. Furthermore, to address the frequency offset problem during grid operation, an improved high-order integral term in the feedforward integral compensation control equation is proposed. This enables the grid-connected inverter to operate under extremely weak grid conditions with high grid impedance while effectively offsetting the impact of grid frequency offset, significantly improving the operational stability of the grid-connected inverter.

[0014] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0015] The present invention provides a method for stabilizing grid-connected inverters under weak grid conditions based on capacitor voltage feedforward integral compensation, characterized by the following steps:

[0016] Step 1: Collect the inductance L1 of the inverter bridge arm and the three-phase current of the inverter system. And the capacitor C of the grid-side filter, and the three-phase voltage.

[0017] Step 2: Calculate the three-phase voltage of the capacitor C of the grid-side filter in the three-phase stationary coordinate system. After performing the Park transformation, the d-axis and q-axis voltage variables in the synchronous rotating coordinate system are obtained. The d-axis and q-axis voltage variables After passing through the phase-locked loop (PLL), the phase angle θ of the grid-side capacitor voltage is obtained;

[0018] Step 3: Based on the phase angle θ of the grid-side capacitor voltage, convert the three-phase current in the three-phase stationary coordinate system... After performing the Park transformation, the d-axis and q-axis current variables in the rotating coordinate system are obtained.

[0019] Step 4: Based on the d-axis and q-axis voltage variables and current variables The inverter control signal is obtained using equation (2) through the AC current control loop ACC, the proportional-derivative feedforward loop based on the grid-side capacitor voltage, and the integral compensation loop based on the grid-side capacitor voltage.

[0020]

[0021] In equation (2), The d-axis and q-axis grid current command signals are set; s is the Laplace operator, ω g The rated angular frequency of the grid voltage. It is a current loop PI controller. This is the proportional adjustment coefficient for the current loop. G is the integral adjustment coefficient of the current loop. CVF (s) represents the control equations for the proportional-differential feedforward link based on the grid-side capacitor voltage; G C (s) represents the control equation for the integral compensation stage of the grid-side capacitor voltage, and we have:

[0022]

[0023]

[0024] In equations (3) and (4), This is the feedforward regulation coefficient for the grid-side capacitor voltage. D0, D1, D2, and D3 are the feedforward differential adjustment coefficients of the grid-side capacitor, and the proportional adjustment coefficients and the first-order, second-order, and third-order integral adjustment coefficients in the feedforward integral controller of the grid-side capacitor voltage, respectively.

[0025] Step 5: Based on the grid-side capacitance C, the three-phase voltage With the inductor L1 of the inverter bridge arm, the three-phase current Design the adjustment coefficients D0, D1, D2, and D3 in the feedforward integral compensation control of the grid-side capacitor voltage;

[0026] Step 6: Based on the phase angle θ of the grid-side capacitor voltage, adjust the control signal for the inverter. After performing the inverse Park transform, the control signal in the three-phase stationary coordinate system is obtained.

[0027] Step 7: Control signal After pulse width modulation (PWM), switching signals for the power devices in the inverter are generated and then passed through the drive circuit to control the power devices to turn on and off.

[0028] The characteristic of the grid-connected inverter stability control method based on capacitor voltage feedforward integral compensation under weak grid conditions described in this invention is that the phase angle θ of the grid-side capacitor voltage is obtained using equation (1) in step 2:

[0029]

[0030] In equation (1), It is a phase-locked loop PI controller. This is the proportional adjustment coefficient of the phase-locked loop. ω is the integral adjustment coefficient of the phase-locked loop. g ω is the rated angular frequency of the grid voltage, and s is the Laplace operator.

[0031] The control equations for the integral compensation stage of the grid-side capacitor voltage in step 4 include: the control equations for the first type of grid-side capacitor voltage feedforward integral compensation stage, as shown in equation (5). and the control equation of the second type of grid-side capacitor voltage feedforward integral compensation link as shown in equation (6).

[0032]

[0033]

[0034] In equation (5), ω c1 This is the cutoff angular frequency of the first type of low-pass filter;

[0035] In equation (6), ω c2This is the cutoff angular frequency of the second type of low-pass filter.

[0036] Step 5 includes:

[0037] Step 5.1: Set the initial value of the proportional adjustment coefficient D0 in the feedforward integral compensation control of the grid-side capacitor voltage;

[0038] Step 5.2: Calculate the inductor current of the inverter bridge arm using equation (7).

[0039]

[0040] In equation (7), S n V is the rated capacity of the inverter. g This represents the amplitude of the grid voltage.

[0041] The grid-side capacitor voltage is obtained using equation (8).

[0042]

[0043] Step 5.3: Calculate the first-order, second-order, and third-order integral adjustment coefficients D1, D2, and D3 using equation (9):

[0044]

[0045] The present invention provides an electronic device, including a memory and a processor, wherein the memory is used to store a program that supports the processor in executing any of the grid-connected inverter stability control methods, and the processor is configured to execute the program stored in the memory.

[0046] The present invention discloses a computer-readable storage medium on which a computer program is stored, wherein the computer program, when executed by a processor, performs any of the steps of the grid-connected inverter stability control method.

[0047] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0048] 1. This invention introduces the grid-side filter capacitor voltage, feeds it forward through a proportional-derivative link to the output of the current regulator, and then uses a capacitor voltage feedforward integral compensation control method to achieve mutual cancellation of the effects of the phase-locked loop and grid-side capacitor voltage feedforward on the low-frequency negative damping effect of the grid-connected inverter output admittance in a unified control architecture. This effectively improves the grid-connected inverter's ability to suppress various low-frequency oscillations, enabling the grid-connected converter to maintain stable operation under extremely weak grid conditions.

[0049] 2. This invention enables the grid-connected inverter to maintain stable operation when the grid frequency deviates significantly by introducing a low-pass filter in the capacitor voltage feedforward integral stage.

[0050] 3. This invention only requires designing the relevant adjustment coefficients based on the system operating voltage, current, and related control loop parameters. It significantly improves the grid-connected inverter's ability to suppress low-frequency oscillations caused by phase-locked loops and grid-side capacitor voltage feedforward, and eliminates stability problems caused by grid frequency deviation. The implementation method is simple and effective. Attached Figure Description

[0051] Figure 1 The topology diagram of the three-phase LCL grid-connected inverter used in the implementation of this invention is shown below;

[0052] Figure 2 A block diagram of the control structure of a three-phase LCL grid-connected inverter with capacitor voltage feedforward integral compensation control added;

[0053] Figure 3 The output current waveforms of the grid-connected inverter before and after applying the control strategy of this invention are shown in the figure, under a grid short-circuit ratio of 1.1 (SCR = 1.1, extremely weak grid).

[0054] Figure 4 The output current waveforms of the grid-connected inverter before and after applying the control strategy of this invention are shown when the grid short-circuit ratio is 1.1 (SCR = 1.1, extremely weak grid) and the grid frequency is 51Hz. Detailed Implementation

[0055] This embodiment presents a grid-connected inverter stability control method based on capacitor voltage feedforward integral compensation under weak grid conditions. This method addresses the low-frequency oscillation problem in grid-connected inverters caused by phase-locked loops (PLLs) and capacitor voltage feedforward under weak grid conditions, as present in existing technologies. The method feeds forward the d- and q-axis grid-side filter capacitor voltages to the current regulator output in a proportional-derivative form. By introducing a capacitor voltage feedforward integral control method, the low-frequency negative damping region of the grid-connected inverter's d- and q-axis output admittance is offset, eliminating the low-frequency oscillation problem. Simultaneously, improved control equations for the first and second types of grid-side capacitor voltage feedforward integral compensation stages are proposed to eliminate inverter stability problems caused by grid frequency deviation. This invention can suppress low-frequency negative damping and low-frequency oscillation problems caused by PLLs and capacitor voltage feedforward under weak grid conditions, while maintaining stable system operation even when the grid frequency deviates. The technical solution of this invention will be clearly and completely described below with reference to the accompanying drawings.

[0056] The topology used in this embodiment is as follows: Figure 1As shown. This topology includes a three-phase full-bridge inverter, inverter arm inductor L1, filter capacitor C, grid-side inductor L2, and grid impedance L. g In this embodiment, L1 = 2mH, C = 6μF, L2 = 1mH, L g =30mH.

[0057] Figure 2 This is a schematic diagram of the control structure of the present invention based on grid-side capacitor voltage feedforward integral compensation control. Figure 2 As can be seen, the method of the present invention consists of the following steps:

[0058] Step 1: Collect the inductance L1 of the inverter bridge arm and the three-phase current. And grid-side filter capacitor C, three-phase voltage

[0059] Step 2: Calculate the three-phase voltage of the capacitor C of the grid-side filter in the three-phase stationary coordinate system. Performing the Park transformation yields the d-axis and q-axis voltage variables in the synchronously rotating coordinate system. The d-axis and q-axis voltage variables After passing through the phase-locked loop (PLL), the phase angle θ of the grid-side capacitor voltage is obtained;

[0060] The formula for calculating the phase angle θ of the grid-side capacitor voltage is shown in equation (1):

[0061]

[0062] In equation (1), It is a phase-locked loop PI controller. This is the proportional adjustment coefficient of the phase-locked loop. ω is the integral adjustment coefficient of the phase-locked loop. g Here, ω is the rated angular frequency of the grid voltage, and s is the Laplace operator. In this embodiment... ω g = 100πrad / s.

[0063] Step 3: Based on the phase angle θ of the grid-side capacitor voltage, convert the three-phase current in the three-phase stationary coordinate system... After performing the Park transformation, the d-axis and q-axis current variables in the rotating coordinate system are obtained.

[0064] Step 4: Based on the d-axis and q-axis voltage variables and current variables The inverter control signal is obtained using equation (2) through the AC current control loop ACC, the proportional-derivative feedforward loop based on the grid-side capacitor voltage, and the integral compensation loop based on the grid-side capacitor voltage.

[0065]

[0066] In equation (2), The d-axis and q-axis grid current command signals are set according to the inverter system capacity; s is the Laplace operator, ω g The rated angular frequency of the grid voltage. It is a current loop PI controller. This is the proportional adjustment coefficient for the current loop. G is the integral adjustment coefficient of the current loop. CVF (s) is the proportional-differential feedforward control equation for the grid-side capacitor voltage based on multi-sampling. G C (s) In this embodiment, the second type of grid-side capacitor voltage feedforward integral compensation control equation is adopted to eliminate the instability problem of the grid-connected inverter caused by grid frequency deviation, and has:

[0067]

[0068]

[0069] In equations (3) and (4), This is the feedforward regulation coefficient for the grid-side capacitor voltage. ω represents the feedforward differential control coefficient of the grid-side capacitor, D0, D1, D2, and D3 are the proportional control coefficient and the first-order, second-order, and third-order integral control coefficients in the feedforward integral control of the grid-side capacitor voltage, respectively. c1 ω c2 These are the cutoff angular frequencies of the first and second type low-pass filters, respectively. In this embodiment, a design is used... ω c1 =2πrad / s, ω c2 = 2πrad / s;

[0070] Step 5: Based on the grid-side capacitor voltage and the inductor current of the inverter arm, design the various adjustment coefficients in the grid-side capacitor voltage feedforward integral compensation control.

[0071] Step 5.1: Design the proportional adjustment coefficient D0 = 0.1 for the grid-side capacitor voltage feedforward integral compensation control;

[0072] Step 5.2: Calculate the inductor current of the inverter bridge arm during stable operation using the formula.

[0073]

[0074] In equation (5), S n V is the rated capacity of the inverter. g This refers to the voltage amplitude of the power grid.

[0075] The grid-side capacitor voltage during stable operation of the inverter is approximated using equation (6).

[0076]

[0077] In this embodiment, based on the inverter capacity S n =3.5kW, grid voltage amplitude V g =110V, obtained through calculation using formulas and .

[0078] Step 5.3: Based on the inductor current of the inverter bridge arm during stable inverter operation. grid-side capacitor voltage during inverter stable operation The proportional adjustment coefficient D0 of the grid-side capacitor voltage feedforward integral compensation control and the control parameters of the current control loop are calculated using equation (7) to obtain the first-order, second-order, and third-order integral adjustment coefficients D1, D2, and D3:

[0079]

[0080] In this embodiment, the following values ​​were obtained through calculation: D0 = 0.1, D1 = 68.8, D2 = 10300, D3 = 375000;

[0081] Step 6: Based on the phase angle θ of the grid-side capacitor voltage, adjust the control signal for the inverter. After performing the inverse Park transform, the control signal in the three-phase stationary coordinate system is obtained.

[0082] Step 7: Control signal After pulse width modulation (PWM), switching signals for the power devices in the inverter are generated, and the power devices are controlled to turn on and off via the drive circuit.

[0083] In this embodiment, an electronic device includes a memory and a processor. The memory stores a program that supports the processor in executing the above-described method, and the processor is configured to execute the program stored in the memory.

[0084] In this embodiment, a computer-readable storage medium stores a computer program, which is executed by a processor to perform the steps of the above method.

[0085] Figure 3 The output current waveforms of the grid-connected inverter before and after applying the control strategy of this invention are shown under a grid short-circuit ratio of 1.1 (SCR = 1.1, extremely weak grid). Figure 3It can be clearly observed that after adopting this invention, the grid-connected inverter under extremely weak power grids can operate stably with no distortion in the output current. However, when this invention is withdrawn, the output current of the grid-connected inverter is significantly distorted, the harmonic content increases significantly, and the low-frequency oscillation problem of the grid-connected inverter is aggravated.

[0086] Figure 4 The output current waveforms of the grid-connected inverter before and after applying the control strategy of this invention are shown under a grid short-circuit ratio of 1.1 (SCR = 1.1, extremely weak grid) and a grid frequency deviation of 51Hz. Figure 4 It can be clearly observed that after adopting this invention, the grid-connected inverter can still maintain stable operation under the dual influence of extremely weak power grid and power grid frequency deviation, and the output current has no obvious distortion. When this invention is withdrawn, the output current of the grid-connected inverter is obviously distorted, the harmonic content is significantly increased, and the oscillation problem of the grid-connected inverter is aggravated.

[0087] The grid-connected inverter stability control method based on capacitor voltage feedforward integral compensation under weak grid conditions proposed in this invention can effectively improve the grid connection quality of inverters under weak grid conditions and enhance the stability of grid-connected inverters.

Claims

1. A weak network-based capacitor voltage feed-forward integral compensation-based grid-connected inverter stable control method, characterized in that, Includes the following steps: Step 1: Collecting the inductance of the inverter bridge arm of the inverter system of the three-phase current , , , and the capacitance of the grid-side filter of the three-phase voltage , , ; Step 2: Adjust the capacitance of the grid-side filter in the three-phase stationary coordinate system. Three-phase voltage , , After performing the Park transformation, the d-axis and q-axis voltage variables in the synchronous rotating coordinate system are obtained. , ; The d-axis and q-axis voltage variables , After passing through a phase-locked loop (PLL), the phase angle of the grid-side capacitor voltage is obtained. ; Step 3, based on the phase angle of the grid-side capacitor voltage. The three-phase current in the three-phase stationary coordinate system , , After performing the Park transformation, the d-axis and q-axis current variables in the rotating coordinate system are obtained. , ; Step 4: Based on the d-axis and q-axis voltage variables , and current variables , The inverter control signal is obtained using equation (2) through the AC current control loop ACC, the proportional-derivative feedforward loop based on the grid-side capacitor voltage, and the integral compensation loop based on the grid-side capacitor voltage. , : (2) In equation (2), , The set d-axis and q-axis grid current command signals; For the Laplace operator, The rated angular frequency of the grid voltage. It is a current loop PI controller. This is the proportional adjustment coefficient for the current loop. This is the integral adjustment coefficient of the current loop. The control equations are based on the proportional-differential feedforward link of the grid-side capacitor voltage; The control equations for the integral compensation stage of the grid-side capacitor voltage are: (3) (4) In equations (3) and (4), This is the feedforward regulation coefficient for the grid-side capacitor voltage. This represents the feedforward differential adjustment coefficient of the grid-side capacitor. , , , These are the proportional control coefficient and the first-order, second-order, and third-order integral control coefficients in the feedforward integral controller for the grid-side capacitor voltage; Step 5: Based on the capacitance on the grid side Three-phase voltage , , Inductance of the inverter bridge arm Three-phase current , , Design the adjustment coefficient in the feedforward integral compensation control of the grid-side capacitor voltage. , , , ; Step 6: Based on the phase angle of the grid-side capacitor voltage Control signals for the inverter , After performing the inverse Park transform, the control signal in the three-phase stationary coordinate system is obtained. , , ; Step 7: Control signal , , After pulse width modulation (PWM), switching signals for the power devices in the inverter are generated and then passed through the drive circuit to control the power devices to turn on and off.

2. The grid-connected inverter stability control method based on capacitor voltage feedforward integral compensation under weak grid conditions according to claim 1, characterized in that, In step 2, the phase angle of the grid-side capacitor voltage is obtained using equation (1). : (1) In equation (1), It is a phase-locked loop PI controller. This is the proportional adjustment coefficient of the phase-locked loop. For the integral adjustment coefficient of the phase-locked loop, The rated angular frequency of the grid voltage. For the Laplace operator.

3. The grid-connected inverter stability control method based on capacitor voltage feedforward integral compensation under weak grid conditions according to claim 2, characterized in that, The control equations for the integral compensation stage of the grid-side capacitor voltage in step 4 include: the control equations for the first type of grid-side capacitor voltage feedforward integral compensation stage, as shown in equation (5). and the control equation of the second type of grid-side capacitor voltage feedforward integral compensation link as shown in equation (6). : (5) (6) In equation (5), This is the cutoff angular frequency of the first type of low-pass filter; In equation (6), This is the cutoff angular frequency of the second type of low-pass filter.

4. The grid-connected inverter stability control method based on capacitor voltage feedforward integral compensation under weak grid conditions according to claim 3, characterized in that, Step 5 includes: Step 5.1: Set the proportional adjustment coefficient in the feedforward integral compensation control of the grid-side capacitor voltage. Initial value; Step 5.2: Calculate the inductor current of the inverter bridge arm using equation (7). ; (7) In equation (7), The rated capacity of the inverter. This represents the amplitude of the grid voltage. The grid-side capacitor voltage is obtained using equation (8). : (8) Step 5.3: Calculate the first-order, second-order, and third-order integral adjustment coefficients using equation (9). , , : (9)。 5. An electronic device, comprising a memory and a processor, characterized in that, The memory is used to store programs that support the processor in executing any of the grid-connected inverter stability control methods according to claims 1-4, and the processor is configured to execute the programs stored in the memory.

6. A computer-readable storage medium storing a computer program thereon, characterized in that, When the computer program is run by the processor, it executes the steps of the grid-connected inverter stability control method according to any one of claims 1-4.

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