Method for suppressing frequency coupling of net-following converter based on improved direct current voltage loop

Abstract

This invention discloses a frequency coupling suppression method for grid-connected converters based on an improved DC voltage loop. The core of this method is to construct two parallel correction branches—phase compensation and reactive power compensation—on the DC voltage loop control path. The phase compensation branch corrects the phase characteristics of the control loop through a series phase compensator, specifically weakening the negative resistance effect in the system admittance that causes grid oscillations. The reactive power compensation branch extracts electrical quantities at the grid connection point, constructs a virtual reactive current reference signal, and actively generates reactive power, making the DC voltage loop control structure more symmetrical and fundamentally reducing coupling strength. This invention can suppress frequency coupling effects simultaneously from both phase characteristics and structural symmetry dimensions without optimizing existing control parameters, significantly improving the stable operation capability of grid-connected converters under weak grid conditions.

Description

Technical Field

[0001] This invention relates to the field of grid connection stability of new energy power generation systems, and in particular to a frequency coupling suppression method for grid-connected converters based on an improved DC voltage loop. Background Technology

[0002] Climate change caused by fossil fuel consumption and ever-increasing electricity demand are jointly driving the development of new energy power generation systems, primarily based on renewable energy sources. Among these, grid-connected interface devices based on power electronic converters, especially grid-connected converters, are seeing a continuous increase in penetration into power systems. However, under weak grid conditions, the nonlinear control characteristics of these converters are prone to interacting with grid impedance, leading to subsynchronous and supersynchronous oscillations. This oscillation problem is often accompanied by significant frequency coupling effects, meaning that mirror frequency harmonics symmetrical about the fundamental frequency are generated simultaneously and influence each other. This effect may render traditional linearized stability analysis methods ineffective, leading to misjudgments of the system's true stability boundary, thus seriously threatening the safe and stable operation of the power grid. Currently, most existing methods for frequency coupling suppression can improve grid-connected system stability or power quality, but they fail to truly decouple the system and cannot fundamentally suppress the frequency coupling effect. Therefore, this invention proposes a frequency coupling suppression method for grid-connected converters based on an improved DC voltage loop. By constructing two parallel modified branches—phase compensation and reactive power compensation—on the DC voltage loop control path, the coupling strength is fundamentally reduced, improving the stable operation capability of weak grids. Summary of the Invention

[0003] To achieve the above objectives, the technical solution provided by this invention is as follows:

[0004] Step 1: On the DC side of the grid-connected converter system, collect the voltage across the DC bus capacitor as the DC voltage. On the AC side, the voltage at the grid connection point is collected. and grid connection point current And obtained through coordinate transformation respectively shaft voltage , shaft voltage , shaft current and shaft current ;Will shaft voltage The phase-locked loop (PLL) output phase is obtained through the PLL. ; Step 2: In the improved DC voltage loop, an AC current loop is generated through a phase compensation branch. Shaft current reference value An AC current loop is generated through the reactive power compensation branch. Shaft current reference value The specific steps are as follows: Step 2-1: Generating an AC current loop in the phase compensation branch Shaft current reference value: derived from DC voltage and DC voltage reference value The difference is obtained by subtracting the values ​​of the DC voltages. After phase compensation, the gain adjustment coefficient and phase compensation controller The processed signal is then passed through a DC voltage loop. controller Obtain the AC current loop Shaft current reference value Alternating current loop Shaft current reference value The expression is: (1) Step 2-2: Generating an AC current loop in the reactive power compensation branch Shaft current reference value: shaft current After the first-order differential process Afterwards, with shaft voltage Add them together and take the opposite, then pass them through the reactive power compensation controller. The reactive power compensation voltage signal is obtained. Extract reactive power compensation voltage signal The imaginary part is then passed through a DC voltage loop. controller Obtain the AC current loop Shaft current reference value Alternating current loop Shaft current reference value The expression is: (2) In equation (2), For the Laplace operator; This is the value of the filter inductance; Represents the imaginary part of a complex number; Step 3: In the AC current loop, the AC current loop Shaft current reference value and shaft current Difference, through the AC current loop controller Afterwards, with shaft current Inductive reactance of the filtered inductor The difference between the obtained voltage signals is used to obtain the AC current loop. Axis output signal Alternating current loop Shaft current reference value and shaft current Difference, through the AC current loop controller Afterwards, with shaft current Inductive reactance of the filtered inductor The obtained voltage signals are added together to obtain the AC current loop. Axis output signal Alternating current loop Axis output signal and AC current loop Axis output signal The expressions are as follows: (3) In equation (3), The fundamental angular frequency; Step 4: Connect the AC current loop Axis output signal After inverse coordinate transformation, the coordinates in the stationary coordinate system are obtained. Three-phase modulation signal After The modulated input is fed into the grid converter to control the converter's on and off states.

[0005] Furthermore, in step 2-1, the phase compensation controller The expression is: (4) In equation (4), For correction factors; The time constant of the phase compensation stage; the expressions are as follows: (5) In equation (5), To correct the angle; For phase compensation center frequency; The adjustment function of the reactive power compensation controller in step 2-2 The expression is: (6) In equation (6), This is the DC side current. For DC bus capacitors; The fundamental frequency component of the grid-connected current has the following specific expression: (7) In equation (7), This represents the amplitude of the fundamental frequency component of the grid-connected current. The initial phase angle of the grid-connected current; It is an imaginary number; When the system frequency coupling effect is only related to the DC voltage loop, establish the positive sequence component of the output voltage disturbance at the grid-connected converter port. and negative order components Regarding the positive and negative sequence disturbance components of grid-connected voltage , With the positive and negative sequence disturbance components of the grid-connected current , Relationship: (8) In equation (8), the coefficients and These represent the relationships between the grid-connected voltage disturbance and the grid-connected current disturbance in the DC voltage loop with respect to the converter port output voltage disturbance; where the subscript " "This indicates the relationship between grid-connected voltage and current disturbances and the positive-sequence disturbance of the converter port output voltage, indicated by the subscript " "" indicates the relationship between grid-connected voltage and current disturbances and the negative-sequence disturbance of the converter port output voltage; while the superscript " "" indicates the relationship between the positive sequence disturbance of grid-connected voltage and current and the disturbance of converter port output voltage, indicated by the superscript " This indicates the relationship between the negative sequence disturbance of the grid-connected voltage and current and the disturbance of the converter port output voltage; the specific expressions for each coefficient are as follows: (9) (10) In equations (9) and (10), The fundamental frequency component of the grid-connected current The conjugate value; V 1 represents the fundamental frequency component of the grid-connected voltage; This is the fundamental frequency component of the modulated signal. for The conjugate value; This represents the DC voltage loop modulation coefficient.

[0006] Compared with existing technologies, the principles and advantages of this solution are as follows: This invention discloses a frequency coupling suppression method for grid-connected converters based on an improved DC voltage loop. The core of this method is to construct two parallel correction branches—phase compensation and reactive power compensation—on the DC voltage loop control path. The phase compensation branch corrects the phase characteristics of the control loop through a series phase compensator, specifically weakening the negative resistance effect in the system admittance that causes grid oscillations. The reactive power compensation branch extracts electrical quantities at the grid connection point, constructs a virtual reactive current reference signal, and actively generates reactive power, making the DC voltage loop control structure more symmetrical and fundamentally reducing coupling strength. This invention can suppress frequency coupling effects simultaneously from both phase characteristics and structural symmetry dimensions without optimizing existing control parameters, significantly improving the stable operation capability of grid-connected converters under weak grid conditions. Attached Figure Description

[0007] Figure 1 This is a topology diagram of the main circuit and control circuit of the grid-connected converter based on the improved DC voltage loop in an embodiment of the present invention; Figure 2 This is the overall control block diagram of the grid-type converter in an embodiment of the present invention; Figure 3 The experimental waveforms of grid-connected current under the conventional method in the embodiments of the present invention and their... Analysis results; Figure 4 The experimental waveforms of the grid-connected current under the method of the present invention in the embodiments of the present invention are as follows: Analysis results; Figure 5 This is a graph showing the changes in grid-connected current overshoot and adjustment time-current loop bandwidth after using the method of the present invention in an embodiment of the invention. Detailed Implementation

[0008] The present invention will be further described below with reference to specific embodiments: Figure 1 The diagram shows the main circuit and control circuit topology of a grid-connected converter based on an improved DC voltage loop, with DC side current... After the DC side capacitor , grid-type converter, filter inductor It is then connected to the grid connection point, and then passes through the grid's equivalent inductance. When connected to the power grid, the control circuit of the grid-connected converter mainly includes an improved DC voltage loop module, an AC current loop module, and a phase-locked loop module. Coordinate transformation module and Modulation module.

[0009] Figure 2 The diagram shown is the overall control block diagram of a grid-connected converter, which includes the following steps: Step 1: On the DC side of the grid-connected converter system, collect the voltage across the DC bus capacitor as the DC voltage. On the AC side, the voltage at the grid connection point is collected. and grid connection point current And obtained through coordinate transformation respectively shaft voltage , shaft voltage , shaft current and shaft current ;Will shaft voltage The phase-locked loop (PLL) output phase is obtained through the PLL. ; Step 2: In the improved DC voltage loop, an AC current loop is generated through a phase compensation branch. Shaft current reference value An AC current loop is generated through the reactive power compensation branch. Shaft current reference value The specific steps are as follows: Step 2-1: Generating an AC current loop in the phase compensation branch Shaft current reference value: derived from DC voltage and DC voltage reference value The difference is obtained by subtracting the values ​​of the DC voltages. After phase compensation, the gain adjustment coefficient and phase compensation controller The processed signal is then passed through a DC voltage loop. controller Obtain the AC current loop Shaft current reference value Alternating current loop Shaft current reference value The expression is: (11) Step 2-2: Generating an AC current loop in the reactive power compensation branch Shaft current reference value: shaft current After the first-order differential process Afterwards, with shaft voltage Add them together and take the opposite, then pass them through the reactive power compensation controller. The reactive power compensation voltage signal is obtained. Extract reactive power compensation voltage signal The imaginary part is then passed through a DC voltage loop. controller Obtain the AC current loop Shaft current reference value Alternating current loop Shaft current reference value The expression is: (12) In equation (12), For the Laplace operator; This is the value of the filter inductance; Represents the imaginary part of a complex number; Step 3: In the AC current loop, the AC current loop Shaft current reference value and shaft current Difference, through the AC current loop controller Afterwards, with shaft current Inductive reactance of the filtered inductor The difference between the obtained voltage signals is used to obtain the AC current loop. Axis output signal Alternating current loop Shaft current reference value and shaft current Difference, through the AC current loop controller Afterwards, with shaft current Inductive reactance of the filtered inductor The obtained voltage signals are added together to obtain the AC current loop. Axis output signal Alternating current loop Axis output signal and AC current loop Axis output signal The expressions are as follows: (13) In equation (13), The fundamental angular frequency; Step 4: Connect the AC current loop Axis output signal After inverse coordinate transformation, the coordinates in the stationary coordinate system are obtained. Three-phase modulation signal After The modulated input is fed into the grid converter to control the converter's on and off states.

[0010] Furthermore, in step 2-1, the phase compensation controller The expression is: (14) In equation (14), For correction factors; The time constant of the phase compensation stage; the expressions are as follows: (15) In equation (15), To correct the angle; For phase compensation center frequency; The adjustment function of the reactive power compensation controller in step 2-2 The expression is: (16) In equation (16), This is the DC side current. For DC bus capacitors; The fundamental frequency component of the grid-connected current has the following specific expression: (17) In equation (17), This represents the amplitude of the fundamental frequency component of the grid-connected current. The initial phase angle of the grid-connected current; It is an imaginary number; When the system frequency coupling effect is only related to the DC voltage loop, establish the positive sequence component of the output voltage disturbance at the grid-connected converter port. and negative order components Regarding the positive and negative sequence disturbance components of grid-connected voltage , With the positive and negative sequence disturbance components of the grid-connected current , Relationship: (18) In equation (18), the coefficients and These represent the relationships between the grid-connected voltage disturbance and the grid-connected current disturbance in the DC voltage loop with respect to the converter port output voltage disturbance; where the subscript " "This indicates the relationship between grid-connected voltage and current disturbances and the positive-sequence disturbance of the converter port output voltage, indicated by the subscript " "" indicates the relationship between grid-connected voltage and current disturbances and the negative-sequence disturbance of the converter port output voltage; while the superscript " "" indicates the relationship between the positive sequence disturbance of grid-connected voltage and current and the disturbance of converter port output voltage, indicated by the superscript " This indicates the relationship between the negative sequence disturbance of the grid-connected voltage and current and the disturbance of the converter port output voltage; the specific expressions for each coefficient are as follows: (19) (20) In equations (19) and (20), The fundamental frequency component of the grid-connected current The conjugate value; V 1 represents the fundamental frequency component of the grid-connected voltage; This is the fundamental frequency component of the modulated signal. for The conjugate value; This represents the DC voltage loop modulation coefficient.

[0011] Figure 3 and Figure 4 The experimental waveforms of grid-connected current under the traditional method and the method of this invention are respectively shown. Analysis results. The injection frequency at the grid connection point is 80. and 90 Voltage disturbance signals, grid strength switching is the switching from a strong grid to a strong grid. A weak power grid of 1.47. Figure 3 and Figure 4 The conditions and parameter settings for both sets of experiments were kept completely identical. Specifically, the parameters in the improved DC voltage loop were: phase compensation gain adjustment coefficient... 0.5, correction angle Phase compensation center frequency 10.5 Filter inductor DC side current A, DC side capacitor DC voltage reference value 700V, DC voltage loop controller .

[0012] Depend on Figure 3 It can be seen that when using the traditional method, the system remains stable under strong power grid conditions. and 10 The harmonic content of the coupling current was 2.02% and 1.61%, respectively, indicating that 80 and 90 The voltage disturbance signal in 20 and 10 This generates coupled current harmonics. When the grid strength switches from a strong grid to a low grid strength... After a weak power grid of 1.47, the total harmonic distortion rate is 20.46%, and the system loses stability under traditional control methods.

[0013] Depend on Figure 4 It can be seen that when using the method of the present invention, the system remains stable under strong power grid conditions. and 10 The harmonic content of the coupled current was 0.01% and 0.02%, respectively, far lower than the 2.02% and 1.61% under the traditional method, indicating that the frequency coupling effect was effectively suppressed. When the grid intensity switches from a strong grid to a low grid... After a weak grid of 1.47, 20 and 10 The harmonic content of the coupling current is 0.11% and 0.14% respectively, which are still relatively small values, and the total harmonic distortion rate is 4.24%, indicating that the method of the present invention can effectively suppress frequency coupling and maintain system stability under weak power grid conditions.

[0014] Figure 5 The figure shows the changes in grid-connected current overshoot and settling time versus current loop bandwidth after applying the method of this invention. When the current loop bandwidth changes, the transient response indicators of the grid-connected current, namely the overshoot and settling time, show very small changes. The overshoot variation range is less than 1%, and the settling time variation range is no more than 0.02 s. This indicates that the method of this invention has strong robustness to the current loop bandwidth parameters, therefore no additional optimization of existing control parameters is required.

[0015] The above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Therefore, any changes made in accordance with the shape and principle of the present invention should be covered within the protection scope of the present invention.

Claims

1. A frequency coupling suppression method for grid-connected converters based on an improved DC voltage loop, characterized in that, Includes the following steps: Step 1: On the DC side of the grid-connected converter system, collect the voltage across the DC bus capacitor as the DC voltage. On the AC side, the voltage at the grid connection point is collected. and grid connection point current And obtained through coordinate transformation respectively shaft voltage , shaft voltage , shaft current and shaft current ;Will shaft voltage The phase-locked loop (PLL) output phase is obtained through the PLL. ; Step 2: In the improved DC voltage loop, an AC current loop is generated through a phase compensation branch. Shaft current reference value An AC current loop is generated through the reactive power compensation branch. Shaft current reference value The specific steps are as follows: Step 2-1: Generating an AC current loop in the phase compensation branch Shaft current reference value: derived from DC voltage and DC voltage reference value The difference is obtained by subtracting the values ​​of the DC voltages. After phase compensation, the gain adjustment coefficient and phase compensation controller The processed signal is then passed through a DC voltage loop. controller Obtain the AC current loop Shaft current reference value Alternating current loop Shaft current reference value The expression is: (1) Step 2-2: Generating an AC current loop in the reactive power compensation branch Shaft current reference value: shaft current After the first-order differential process Afterwards, with shaft voltage Add them together and take the opposite, then pass them through the reactive power compensation controller. The reactive power compensation voltage signal is obtained. Extract reactive power compensation voltage signal The imaginary part is then passed through a DC voltage loop. controller Obtain the AC current loop Shaft current reference value Alternating current loop Shaft current reference value The expression is: (2) In equation (2), For the Laplace operator; This is the value of the filter inductance; Represents the imaginary part of a complex number; Step 3: In the AC current loop, the AC current loop Shaft current reference value and shaft current Difference, through the AC current loop controller Afterwards, with shaft current Inductive reactance of the filtered inductor The difference between the obtained voltage signals is used to obtain the AC current loop. Axis output signal Alternating current loop Shaft current reference value and shaft current Difference, through the AC current loop controller Afterwards, with shaft current Inductive reactance of the filtered inductor The obtained voltage signals are added together to obtain the AC current loop. Axis output signal Alternating current loop Axis output signal and AC current loop Axis output signal The expressions are as follows: (3) In equation (3), The fundamental angular frequency; Step 4: Connect the AC current loop Axis output signal After inverse coordinate transformation, the coordinates in the stationary coordinate system are obtained. Three-phase modulation signal After The modulated input is fed into the grid converter to control the converter's on and off states.

2. The frequency coupling suppression method for grid-connected converters based on an improved DC voltage loop according to claim 1, characterized in that, Phase compensation controller in step 2-1 The expression is: (4) In equation (4), For correction factors; The time constant of the phase compensation stage; the expressions are as follows: (5) In equation (5), To correct the angle; This is the center frequency for phase compensation.

3. The frequency coupling suppression method for grid-connected converters based on an improved DC voltage loop according to claim 1, characterized in that, The adjustment function of the reactive power compensation controller in step 2-2 The expression is: (6) In equation (6), This is the DC side current. For DC bus capacitors; The fundamental frequency component of the grid-connected current has the following specific expression: (7) In equation (7), This represents the amplitude of the fundamental frequency component of the grid-connected current. The initial phase angle of the grid-connected current; It is an imaginary number.

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