Photovoltaic grid-connected converter impedance modeling method considering frequency coupling

Abstract

The invention discloses a photovoltaic grid-connected converter impedance modeling method considering frequency coupling, and the method comprises the steps: S1, carrying out the equivalence of a DC side of a grid-connected converter according to the structure of the grid-connected converter, and building a main circuit equation according to a main circuit topological structure; s2, establishing a transfer function expression under frequency disturbance for the phase-locked loop according to a frequency coupling phenomenon caused by asymmetric control of the phase-locked loop; s3, according to the voltage disturbance relation, response relational expressions after voltage disturbance is applied to the alternating-current side and the direct-current side are worked out respectively; and S4, based on the mathematical expressions of the output power, the equivalent admittance and the disturbance admittance obtained in the steps S1, S2 and S3, establishing the equivalent admittance / impedance of the photovoltaic power generation system. According to the method, multi-time scale control and full-working-condition frequency domain port characteristics of the photovoltaic power generation system can be effectively represented, and the equivalent impedance model can accurately analyze the stability of the system.

Description

Technical Field

[0001] The present application relates to the field of renewable energy direct current transmission, and in particular to a photovoltaic grid-connected converter impedance modeling method considering frequency coupling. Background Art

[0002] After the "dual carbon" goals were proposed, the power system dominated by generators gradually transformed into a power system dominated by new energy converters. In the past decade, the cumulative installed capacity of photovoltaic power generation has shown a rapid growth trend.

[0003] Since photovoltaic power plants contain a large number of power electronic equipment, with the widespread application of these power electronic equipment, the problem of broadband oscillation in new energy bases when connected to the power system has become increasingly prominent, seriously threatening the safe and stable operation of the power system and causing significant economic losses. Therefore, in order to reduce the oscillation risk of future high photovoltaic penetration power systems, it is of great significance to conduct theoretical research on the oscillation laws of photovoltaic power generation systems connected to the grid.

[0004] Among the existing analysis methods for broadband oscillation problems in power electronic systems, the impedance method is an effective method.

[0005] In summary, in order to utilize energy more effectively and fully, improve the level of photovoltaic consumption, ensure the continuous and high-quality transmission of solar energy to high-load areas, and transmit electricity at lower cost and line loss, there is an urgent need for an impedance modeling method for photovoltaic grid-connected converter systems based on impedance analysis, so as to analyze the factors affecting the safety and stability of photovoltaic grid-connected systems, determine the safe and stable operation range of the system, and put forward effective operation suggestions to ensure the safety and stability of the system. Summary of the invention

[0006] The purpose of the present application is to overcome the defects in the prior art and to provide a method for modeling the impedance of a photovoltaic grid-connected converter taking into account frequency coupling.

[0007] The purpose of this application is achieved through the following technical solutions:

[0008] A photovoltaic grid-connected converter impedance modeling method considering frequency coupling, the photovoltaic grid-connected converter impedance modeling method comprising:

[0009] S1: Equivalence the DC side of the grid-connected converter according to its structure, and establish the main circuit equation according to the main circuit topology;

[0010] S2: According to the frequency coupling phenomenon caused by the asymmetric control of the phase-locked loop, the transfer function expression of the phase-locked loop under frequency disturbance is established;

[0011] S3: According to the voltage disturbance relationship, the response relationship after applying voltage disturbance on the AC side and the DC side is obtained respectively;

[0012] S4: Based on the mathematical expressions of output power, equivalent admittance, and disturbance admittance obtained in steps S1, S2, and S3, an equivalent admittance / impedance of the photovoltaic power generation system is established.

[0013] According to a preferred embodiment, in step S1, when the grid-connected converter operates in an ideal condition, the main circuit equation is as follows:

[0014]

[0015] Where m a 、m b 、m c is the modulation coefficient corresponding to the three, L is the filter inductance of the grid-connected converter, i a 、i b 、i c is the three-phase current output by the converter, v a 、v b 、v c is the three-phase grid voltage, v dc is the equivalent DC voltage source, and t represents the system operating time.

[0016] According to a preferred embodiment, in step S2, the closed-loop transfer function T of the phase-locked loop pll for:

[0017]

[0018] Among them, V 1 is the AC voltage amplitude, k ppll , k ipll is the phase-locked loop parameter, ω p is the disturbance frequency, j is the imaginary unit.

[0019] According to a preferred embodiment, in step S3, when the grid voltage is ω p When the voltage disturbance of frequency is caused, the grid connection point will have a frequency of ω p and 2ω 0 -ω p The disturbance current response, ω 0 Indicates the fundamental frequency, and there will also be ω on the DC side p -ω 0 The disturbance current, the disturbance transfer relationship is as follows:

[0020]

[0021] Among them, V pis the input disturbance voltage, I p (ω p ), I p (ω p -2ω 0 ) is the AC side disturbance current corresponding to different frequencies, I pdc (ω p -ω 0 ) is the disturbance current on the DC side, Y ac1_PV , Y ac2_PV , Y ac3_PV Introduce ω at the grid connection point p When the voltage is disturbed by frequency, ω p ,ω p -2ω 0 The response current on the AC side at the frequency, and the admittance under the response current on the DC side.

[0022] According to a preferred embodiment, according to the control structure of the grid-connected converter, the disturbance expression of the modulation coefficient is as follows:

[0023]

[0024] Among them, M dq (ω p -ω 0 )、M dq (ω 0 -ω p ) is the complex variable form of the modulation coefficient corresponding to the disturbance frequency, V dc0 is the DC side voltage stability value, K gi is the current inner loop control parameter, H gi is the current controller, I dq is the current response under different disturbance frequencies, j is an imaginary unit;

[0025] Thus, the admittance under different current responses is obtained:

[0026]

[0027] Among them, T pll is the closed-loop transfer function of the phase-locked loop, H v is the DC voltage controller, M dq0 ,I dq0 is the steady-state modulation coefficient and steady-state output current value under the steady-state dq axis.

[0028] According to a preferred embodiment, in step S3, when there is ω on the DC side p When the voltage disturbance of frequency is caused, it will bring two-frequency current disturbance to the AC side. The disturbance transfer relationship is expressed as follows:

[0029]

[0030] in, is the disturbance voltage on the DC side, is the DC side current response corresponding to the DC side voltage disturbance, They are the current responses of the grid-connected converter output at different frequencies, Y dc1_PV , Y dc2_PV , Y dc3_PV They represent the introduction of ω on the DC side. p When the voltage is disturbed by a frequency, the DC side responds to the current and ω p +ω 0 ,ω p -ω 0 The system admittance corresponding to the response current on the AC side at the frequency.

[0031] According to a preferred embodiment, the expression of the modulation coefficient obtained according to the circuit structure is:

[0032]

[0033] Among them, M pdq It represents the modulation coefficient disturbance under the dq axis coordinate, V dc0 is the DC side voltage stability value, K gi is the current inner loop control parameter, H v is the DC voltage controller, H gi is the current controller, I pdq is the current disturbance in the dq axis coordinate system, V pdc is the DC side disturbance voltage, ω p is the disturbance frequency, j is the imaginary unit;

[0034] Thus, the admittance under different current responses is obtained as:

[0035]

[0036] Among them, I p is the AC side disturbance current, M dq0 ,I dq0 is the steady-state modulation coefficient and steady-state output current value under the steady-state dq axis, V dc0 is the DC side voltage stability value, ω 0 is the fundamental frequency, Indicates H v (ω p ), Indicates H gi (ω p ) is the conjugate value of .

[0037] According to a preferred embodiment, the self-admittance Y of the grid-connected converter is obtained according to the transfer relationship between the equivalent impedance and the self-admittance and the accompanying admittance. PV_sef (ω p ) and the adjoint admittance Y PV_aef (ω p )as follows:

[0038]

[0039] Among them, Y ms is the admittance corresponding to the bus capacitance on the DC side of the grid-connected converter, Y C is the admittance of the DC bus capacitor, Y ac1_PV (ω p ), Y ac3_PV (ω p ), Y dc2_PV (ω p -ω 0 ), Y dc3_PV (ω p -ω 0 )'s conjugate value;

[0040] Based on the influence of grid impedance, the equivalent admittance Y of the grid-connected converter with frequency coupling effect is obtained. PV_inv , Impedance Z PV_inv as follows:

[0041]

[0042] Z s is the grid impedance, Z s The conjugate value of Y PV_aef The conjugate value of Y PV_sef The conjugate value of .

[0043] The aforementioned main scheme of the present application and its further options can be freely combined to form multiple schemes, all of which are schemes that can be adopted and claimed for protection in the present application. After understanding the scheme of the present application, those skilled in the art can understand that there are multiple combinations based on the prior art and common knowledge, all of which are technical schemes to be protected by the present application, and they are not exhaustively listed here.

[0044] Beneficial effects of this application:

[0045] The present invention can effectively characterize the multi-time scale control and full-operating frequency domain port characteristics of photovoltaic power generation systems, and the equivalent impedance model can accurately analyze system stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 is a flow chart of the impedance modeling method of the present invention;

[0047] Figure 2 It is a schematic diagram of the grid-connected structure of the grid-connected converter of the present invention;

[0048] Figure 3 is a simplified grid-connected structure diagram of a grid-connected converter of the present invention;

[0049] Figure 4 This is a control structure diagram of the grid-connected converter of the present invention. DETAILED DESCRIPTION

[0051] The following describes the embodiments of the present application through specific examples, and those skilled in the art can easily understand other advantages and effects of the present application from the contents disclosed in this specification. The present application can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present application. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict.

[0052] Example 1

[0053] refer to Figure 1 As shown, the present invention discloses a photovoltaic grid-connected converter impedance modeling method considering frequency coupling phenomenon, comprising the following steps:

[0054] S1. Equivalent the DC side of the grid-connected converter according to its structure, and establish the main circuit equation according to the main circuit topology.

[0055] S2. According to the frequency coupling phenomenon caused by the asymmetric control of the phase-locked loop, the transfer function expression of the phase-locked loop under frequency disturbance is established.

[0056] S3. Based on the voltage disturbance relationship, respectively determine the response relationship after applying voltage disturbance on the AC side and the DC side.

[0057] S4. Use the mathematical expressions of output power, equivalent admittance, and disturbance admittance obtained by S1, S2, and S3 to finally establish the equivalent admittance / impedance of the photovoltaic power generation system.

[0058] Preferably, in step S1, according to the topological structure of the converter, Figure 3 , when the grid-connected converter works in ideal conditions, the main circuit equation is as follows:

[0059]

[0060] Where, L is the filter inductance of the grid-connected converter, i a 、i b 、i c is the three-phase current output by the grid-connected converter, v a 、v b 、v c is the three-phase grid-connected point voltage, m a 、m b 、m c is the modulation coefficient corresponding to the three, v dc is the equivalent DC voltage source, and t is the system operating time.

[0061] Preferably, in step S2, in a three-phase stationary coordinate system, ω is applied p The grid connection point voltage after the frequency disturbance voltage can be expressed as:

[0062]

[0063] Where V a 、V b 、V c is the three-phase grid voltage, V 1 is the output voltage amplitude, V p is the disturbance voltage amplitude, ω p is the disturbance frequency, ω 0 is the base frequency, and t is the system running time.

[0064] After transforming the above formula to the dq coordinate system, it is expressed in the form of complex variables as follows:

[0065]

[0066] Where V dq is the complex variable of grid-connected voltage under dq axis, j is the imaginary unit, V d =V 1 e j0 is the voltage phasor component under the d-axis, is the voltage phasor component under the q axis, V 1 is the output voltage amplitude, V p is the disturbance voltage amplitude, ω p is the disturbance frequency, ω 0 is the base frequency, and t is the system running time.

[0067] Furthermore, when the disturbance component causes disturbance of the phase-locked loop and thus leads to frequency coupling, the current conversion between the grid-connected converters has the following relationship:

[0068]

[0069] Here, X refers to variables such as voltage, current, and modulation coefficient. dq0is the steady-state value of the variable, X p Represents the disturbance of variables such as voltage, current and modulation coefficient, V p is the disturbance voltage, ω p is the disturbance frequency, ω 0 is the fundamental frequency, the values ​​in brackets are different perturbation frequency variables, the variable with * in the upper right corner represents the conjugate value, T pli is the closed-loop transfer function of the phase-locked loop, and its expression is as follows:

[0070]

[0071] Where V 1 is the AC voltage amplitude, k ppll , k ipll is the phase-locked loop parameter, ω p is the disturbance frequency.

[0072] Preferably, in step S3, when the grid voltage is ω p When the voltage disturbance of frequency is caused, the grid connection point will have a frequency of ω p and 2ω 0 -ω p The disturbance current response is ω. p -ω 0 Therefore, the disturbance transfer relationship is as follows:

[0073]

[0074] Among them, V p is the input disturbance voltage, I p (ω p ), I p (ω p -2ω 0 ) is the AC side disturbance current corresponding to different frequencies, I pdc (ω p -ω 0 ) is the disturbance current on the DC side, Y ac1_PV , Y ac2_PV , Y ac3_PV Introduce ω at the grid connection point p When the voltage is disturbed by frequency, ω p ,ω p -2ω 0 The response current on the AC side under the frequency and the admittance under the response current on the DC side.

[0075] According to the control structure of the grid-connected converter, the disturbance expression of the modulation coefficient is as follows:

[0076]

[0077] Among them, M dq (ω p -ω 0 )、M dq (ω 0 -ω p ) is the complex variable form of the modulation coefficient corresponding to the disturbance frequency, V dc0 is the DC side voltage stability value, K gi is the current inner loop control parameter, H gi is the current controller, I dq is the current response under different disturbance frequencies, ω p is the disturbance frequency, ω 0 is the fundamental frequency and j is the imaginary unit.

[0078] Combining the above formula, we can get the admittance under different current responses:

[0079]

[0080] Among them, Y ac1_PV , Y ac2_PV , Y ac3_PV Introduce ω at the grid connection point p When the voltage is disturbed by frequency, ω p ,ω p -2ω 0 The response current on the AC side at the frequency and the corresponding system admittance under the response current on the DC side, V p is the input disturbance voltage, I p (ω p ), I p (ω p -2ω 0 ) is the AC side disturbance current corresponding to different frequencies, I pdc (ω p -ω 0 ) is the DC side disturbance current, T pll is the closed-loop transfer function of the phase-locked loop, K gi is the current inner loop control parameter, H v is the DC voltage controller, H gi is the current controller, M dq0 ,I dq0 is the steady-state modulation coefficient and steady-state output current value under the steady-state dq axis, V dc0 is the DC side voltage stability value, ω p is the disturbance frequency, ω 0 is the fundamental frequency and j is the imaginary unit.

[0081] Preferably, in step S3, when there is ω on the DC side p When the voltage disturbance of frequency is caused, it will also bring two-frequency current disturbance to the AC side. The disturbance transmission relationship can be expressed as follows:

[0082]

[0083] in, is the disturbance voltage on the DC side, They are the DC side current response corresponding to the DC side voltage disturbance and the current response of the grid-connected converter output side at different frequencies. dc1_PV , Y dc2_PV , Y dc3_PV They represent the introduction of ω on the DC side. p When the voltage is disturbed by a frequency, the DC side responds to the current and ω p +ω 0 ,ω p -ω 0 The system admittance corresponding to the response current on the AC side at the frequency, ω p is the disturbance frequency, ω 0 is the base frequency.

[0084] According to the circuit structure, the expression of the modulation coefficient can be obtained as follows:

[0085]

[0086] Among them, M pdq It represents the modulation coefficient disturbance under the dq axis coordinate, V dc0 is the DC side voltage stability value, K gi is the current inner loop control parameter, H v is the DC voltage controller, H gi is the current controller, I pdq is the current disturbance in the dq axis coordinate system, V pdc is the DC side disturbance voltage, ω p is the disturbance frequency, j is the imaginary unit.

[0087] Combining the above formula, the admittance under different current responses can be obtained as:

[0088]

[0089] Among them, Y dc1_PV , Y dc2_PV , Y dc3_PV They represent the introduction of ω on the DC side. p When the voltage is disturbed by a frequency, the DC side responds to the current and ω p +ω 0 ,ω p -ω 0 The system admittance corresponding to the response current on the AC side at the frequency, I p is the AC side disturbance current, V pdc is the DC side disturbance voltage, M dq0,I dq0 is the steady-state modulation coefficient and steady-state output current value under the steady-state dq axis, V dc0 is the DC side voltage stability value, H v is the DC voltage controller, H gi is the current controller, K gi is the current inner loop control parameter. The variable with * in the upper right corner represents the conjugate value. p is the disturbance frequency, ω 0 is the fundamental frequency and j is the imaginary unit.

[0090] Preferably, in step S4, according to the transfer relationship between the equivalent impedance and the self-admittance and the accompanying admittance, Figure 4 , the self-admittance and adjoint admittance of the grid-connected converter can be obtained as follows:

[0091]

[0092] Among them, Y PV_sef (ω p ), Y PV_aef (ω p ) are the self-admittance and adjoint admittance of the grid-connected converter, Y ac1 , Y ac2 , Y ac3 Introduce ω at the grid connection point p When the voltage is disturbed by frequency, ω p ,ω p -2ω 0 The response current of the AC side under the frequency and the admittance of the response current of the DC side, Y dc1 , Y dc2 , Y dc3 Respectively represent the introduction of ω on the DC side p When the voltage is disturbed by a frequency, the DC side responds to the current and ω p +ω 0 ,ω p -ω 0 The system admittance corresponding to the response current on the AC side at the frequency, Y ms is the admittance corresponding to the bus capacitance on the DC side of the grid-connected converter, Y C is the admittance of the DC bus capacitor. The variable with * in the upper right corner represents the conjugate value, ω p is the disturbance frequency, ω 0 is the base frequency.

[0093] At this time, considering the influence of grid impedance, the equivalent admittance (impedance) of the grid-connected converter taking into account the frequency coupling effect is obtained as follows:

[0094]

[0095] Among them, Y PV_inv , ZPV_inv is the equivalent admittance and impedance of the grid-connected converter, Y PV_sef , Y PV_aef is the self-admittance and adjoint admittance of the grid-connected converter, Z s is the grid impedance, the variable with * in the upper right corner represents the conjugate, ω p is the disturbance frequency, ω 0 is the base frequency.

[0096] The present invention comprehensively considers that small signal modeling of the converter in the photovoltaic power generation grid-connected system must overcome the time-varying characteristics of the converter and the asymmetry of the converter's dq axis, and proposes a photovoltaic grid-connected converter impedance modeling method considering frequency coupling. The impedance model provides a model basis for the stability analysis of the interconnection between the photovoltaic system and the new energy system.

[0097] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A photovoltaic grid-connected converter impedance modeling method considering frequency coupling, characterized in that: The photovoltaic grid-connected converter impedance modeling method comprises: S1: Equivalence the DC side of the grid-connected converter according to its structure, and establish the main circuit equation according to the main circuit topology; S2: According to the frequency coupling phenomenon caused by the asymmetric control of the phase-locked loop, the transfer function expression of the phase-locked loop under frequency disturbance is established; S3: According to the voltage disturbance relationship, the response relationship after applying voltage disturbance on the AC side and the DC side is obtained respectively; S4: Based on the mathematical expressions of output power, equivalent admittance, and disturbance admittance obtained in steps S1, S2, and S3, an equivalent admittance / impedance of the photovoltaic power generation system is established.

2. The photovoltaic grid-connected converter impedance modeling method according to claim 1, characterized in that: In step S1, when the grid-connected converter operates in an ideal condition, the main circuit equation is as follows: Where m a 、m b 、m c is the modulation coefficient corresponding to the three, L is the filter inductance of the grid-connected converter, i a 、i b 、i c is the three-phase current output by the converter, v a 、v b 、v c is the three-phase grid voltage, v dc is the equivalent DC voltage source, and t represents the system operating time.

3. The photovoltaic grid-connected converter impedance modeling method according to claim 2, characterized in that: In step S2, the closed-loop transfer function T of the phase-locked loop pll for: Where V1 is the AC voltage amplitude, k ppll , k ipll is the phase-locked loop parameter, ω p is the disturbance frequency, j is the imaginary unit.

4. The photovoltaic grid-connected converter impedance modeling method according to claim 3, characterized in that: In step S3, when the grid voltage is p When the voltage disturbance of frequency is caused, the grid connection point will have a frequency of ω p and 2ω0-ω p The disturbance current response of ω0 represents the fundamental frequency, and there will also be ω on the DC side. p -ω0 disturbance current, the disturbance transfer relationship is as follows: Among them, B p is the input disturbance voltage, I p (ω p ), I p (ω p -2ω0) is the AC side disturbance current corresponding to different frequencies, I pdc (ω p -ω0) is the disturbance current on the DC side, Y ac1_PV , Y ac2_PV , Y ac3_PV Introduce ω at the grid connection point p When the voltage is disturbed by frequency, ω p ,ω p The response current on the AC side at a frequency of -2ω0, and the admittance under the response current on the DC side.

5. The photovoltaic grid-connected converter impedance modeling method according to claim 4, characterized in that: According to the control structure of the grid-connected converter, the disturbance expression of the modulation coefficient is as follows: Among them, M dq (ω p -ω0)、M dq (ω0-ω p ) is the complex variable form of the modulation coefficient corresponding to the disturbance frequency, V dc0 is the DC side voltage stability value, K gi is the current inner loop control parameter, H gi is the current controller, I dq is the current response under different disturbance frequencies, j is an imaginary unit; Thus, the admittance under different current responses is obtained: Among them, T pll is the closed-loop transfer function of the phase-locked loop, H v is the DC voltage controller, M dq0 ,I dq0 is the steady-state modulation coefficient and steady-state output current value under the steady-state dq axis.

6. The photovoltaic grid-connected converter impedance modeling method according to claim 5, characterized in that: In step S3, when there is ω on the DC side p When the voltage disturbance of frequency is caused, it will bring current disturbance of two frequencies to the AC side. The disturbance transfer relationship is expressed as follows: in, is the disturbance voltage on the DC side, is the DC side current response corresponding to the DC side voltage disturbance, They are the current responses of the grid-connected converter output at different frequencies, Y dc1_PV , Y dc2_PV , Y dc3_PV Respectively represent the introduction of v on the DC side p When the voltage is disturbed by a frequency, the DC side responds to the current and v p +ω0、ω p The system admittance corresponding to the response current on the AC side at -v0 frequency.

7. The photovoltaic grid-connected converter impedance modeling method according to claim 6, characterized in that: The expression of modulation coefficient obtained according to the circuit structure is: Among them, M pdq It represents the modulation coefficient disturbance under the dq axis coordinate, V dc0 is the DC side voltage stability value, K gi is the current inner loop control parameter, H v is the DC voltage controller, H gi is the current controller, I pdq is the current disturbance in the dq axis coordinate system, V pdc is the DC side disturbance voltage, v p is the disturbance frequency, j is the imaginary unit; Thus, the admittance under different current responses is obtained as: Among them, I p is the AC side disturbance current, M dq0 ,I dq0 is the steady-state modulation coefficient and steady-state output current value under the steady-state dq axis, V dc0 is the DC side voltage stability value, ω0 is the base frequency, Indicates H v (ω p ), Indicates H gi (ω p ) is the conjugate value of .

8. The photovoltaic grid-connected converter impedance modeling method according to claim 7, characterized in that: According to the transfer relationship between equivalent impedance, self-admittance and adjoint admittance, the self-admittance Y of the grid-connected converter is obtained: PV_sef (ω p ) and the adjoint admittance Y PV_aef (ω p )as follows: Among them, Y ms is the admittance corresponding to the bus capacitance on the DC side of the grid-connected converter, Y C is the admittance of the DC bus capacitor, Y ac1_PV (ω p ), Y ac3_PV (ω p ), Y dc2_PV (ω p -ω0), Y dc3_PV (v p -ω0) conjugate value; Based on the influence of grid impedance, the equivalent admittance Y of the grid-connected converter with frequency coupling effect is obtained. PV_inv , Impedance Z PV_inv as follows: Z s is the grid impedance, Z s The conjugate value of Y PV_aef The conjugate value of Y PV_sef The conjugate value of .

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