A grid-connected converter stability control method based on complex frequency domain impedance remodeling

Abstract

The application discloses a grid-connected converter stability control method based on complex frequency domain impedance remodeling, and the control method comprises a converter, a public power grid, a phase-locked loop, a stability controller inverse link, a current controller, a stability controller, a coordinate transformation module and a pulse width modulator. The control method adopts a design-oriented analysis method in the complex frequency domain, increases the stability controller in the forward channel of the grid-connected converter current control, simultaneously increases the corresponding stability controller inverse link in the current feedback channel, remodels the output impedance of the grid-connected converter, and thus improves the stability of the grid-connected converter, so that the grid-connected converter can still be stably operated under a weak power grid condition. The control method does not affect the current control performance, only needs to increase the stability controller and the stability controller inverse link in the control system, is convenient to realize, and makes the stability controller improve the system stability margin while only changing the interaction characteristics of the phase-locked loop and the power grid without affecting the current control loop.

Description

Technical Field

[0001] This invention relates to the field of power electronic power system control technology, specifically a grid-connected converter stability control method based on complex frequency domain impedance reshaping. Background Technology

[0002] As grid-connected converters become increasingly prevalent in the power grid, the grid's operational stability faces significant challenges. On one hand, grid-connected converters commonly use phase-locked loops (PLLs) to synchronize the converter's output current with the grid voltage in terms of frequency and phase. However, research indicates that PLLs affect the converter's output impedance, introducing negative damping and reducing the system's stability margin. On the other hand, renewable energy sources are typically located far from load centers. As the number of grid-connected converters increases, the power grid gradually exhibits characteristics of a weak grid. In this case, grid impedance cannot be ignored. When small-signal disturbances occur in the system, the coupling effect between the large grid impedance and the PLL may lead to system instability.

[0003] Existing literature research has found that while increasing the bandwidth of the phase-locked loop (PLL) is beneficial to system dynamics, it also increases the negative damping range of the converter's output impedance, which is detrimental to system stability. Therefore, in weak grid conditions, reducing the PLL bandwidth can maintain the stable operation of the grid-connected converter, but this is not practical for existing renewable energy power plants. Furthermore, reducing the PLL bandwidth also significantly impacts the steady-state and dynamic performance of the grid-connected converter. Summary of the Invention

[0004] The purpose of this invention is to provide a grid-connected converter stability control method based on complex frequency domain impedance reshaping. This method is achieved by introducing a stability controller in the current control channel and a corresponding inverse stability controller in the current feedback channel. This allows the stability controller to improve the system stability margin while only changing the interaction characteristics between the phase-locked loop and the power grid without affecting the current control loop. The stability control method is an easy-to-implement and simple-to-design control strategy that reshapes the converter output impedance, improving the system stability margin while ensuring control performance.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] A stability control method for a grid-connected converter based on impedance reshaping in the complex frequency domain is disclosed. The method includes a converter, a public power grid, a phase-locked loop (PLL), an inverse link of a stability controller, a current controller, a stability controller, a coordinate transformation module, and a pulse width modulator. The specific flow of the stability control method includes the following steps:

[0007] S1. Obtain the PCC voltage of the grid-connected converter. abc and converter output current i abc .

[0008] S2, using a phase-locked loop to control voltage vabc Processing is performed to obtain voltage v abc Phase θ is used in the coordinate transformation module.

[0009] S3, convert the three-phase current i a i b i c After processing by the inverse circuit of the stabilizing controller, the output signal i is obtained. a ′, i b ′, i c ′.

[0010] S4, output signal i from S3 a ′, i b ′, i c After coordinate transformation, the dq-axis current i is obtained. d and i q .

[0011] S5. The dq-axis current i obtained in S4 d i q With reference value i of dq axis current dref i qref进行 The comparison is then processed by the current controller to obtain v. rd and v rq .

[0012] S6, take the v obtained from S5 rd and v rq After processing by the stabilization controller, the dq-axis control voltage v is obtained. r ′ d and v r ′ q .

[0013] S7. The dq axis control voltage v obtained in S6 r ′ d and v r ′ q The voltage is transmitted to the coordinate transformation module to obtain the abc axis modulation voltage, and finally the abc axis modulation voltage is transmitted to the pulse width modulator to obtain the switching signal of the converter.

[0014] Furthermore, the converter is connected to the public power grid, and the converter is connected to the filter inductor L1. The public power grid includes an equivalent impedance and a voltage source. The phase-locked loop is any three-phase voltage phase-locked loop, and the current controller is a proportional-integral controller.

[0015] The phase-locked loop is connected to the coordinate transformation module. One coordinate transformation module is connected to the inverse link of the stabilization controller and the current controller. The other coordinate transformation module is connected to the stabilization controller and the pulse width modulator. The current controller is connected to the stabilization controller. The pulse width modulator outputs a switching signal to control the converter.

[0016] Furthermore, the stabilization controller includes a first-order low-pass filter LPF, a second-order low-pass filter SLPF, and a second-order notch filter NF.

[0017] Furthermore, the frequency domain expression of the first-order low-pass filter LPF is:

[0018]

[0019] The frequency domain expression of the second-order low-pass filter SLPF is:

[0020]

[0021] The frequency domain expression of the second-order notch filter NF is:

[0022]

[0023] Where, ω f For a value lower than the instability frequency, ω fs Select a value lower than the instability frequency, ξ s ω is the damping coefficient. n Set the notch frequency to the instability frequency, and ξ represents the notch depth.

[0024] Furthermore, the small-signal model of the grid-connected converter is obtained through the control method. A stable controller matrix G is added to the forward path of the current controller in the small-signal model of the grid-connected converter. f An inverse matrix of the stabilizing controller was added to the current feedback channel. Their expressions are as follows:

[0025]

[0026]

[0027] Furthermore, the control method eliminates the influence of the stabilizing controller on the current control by introducing an inverse link of the stabilizing controller in the current feedback channel, so that it only affects the phase-locked loop characteristics, resulting in the converter output impedance as follows:

[0028]

[0029] in, This indicates the coupling between the phase-locked loop and the current controller. This refers to the impedance of the current control loop in traditional control systems.

[0030] The beneficial effects of this invention are:

[0031] 1. The stable control method of this invention is based on the analysis method of design in the complex frequency domain. It reshapes the output impedance of the grid-connected converter by adding a stable controller in the control forward channel and adding a corresponding stable controller inverse link in the current feedback channel. The implementation method is simple and enables the grid-connected converter system to maintain stable operation under weak grid conditions.

[0032] 2. The stabilizing controller in the stabilizing control method of the present invention adopts various minimum phase digital filters, including but not limited to first-order low-pass filters, second-order low-pass filters and second-order notch filters. The parameter design process only needs to detect the system instability frequency to determine the stabilizing control parameters, which is simple to design.

[0033] 3. The stabilization control method of this invention is not limited to the type of phase-locked loop (PLL). It is applicable to a variety of common types of PLLs, including synchronous coordinate system PLLs and dual second-order generalized integrator PLLs. The stabilization control will correct the output impedance of the PLL, thereby reshaping the converter impedance and improving system stability, without affecting the impedance of the current control link. Attached Figure Description

[0034] The invention will now be further described with reference to the accompanying drawings.

[0035] Figure 1 This is a block diagram illustrating the control method of the present invention;

[0036] Figure 2 This is a small-signal model diagram of the grid-connected converter after adopting the control method of this invention;

[0037] Figure 3 This is an equivalent small-signal model diagram after the control method is adopted in this invention;

[0038] Figure 4 This is a comparison diagram of the amplitude and phase characteristics of the control method of the present invention before and after using a first-order low-pass filter;

[0039] Figure 5 This is a comparison diagram of the amplitude and phase characteristics of the control method of the present invention before and after using a second-order low-pass filter;

[0040] Figure 6 This is a comparison diagram of the amplitude and phase characteristics of the control method of the present invention before and after using a second-order notch filter;

[0041] Figure 7 These are simulation waveforms of the control method of this invention before and after using a first-order low-pass filter;

[0042] Figure 8 These are simulation waveforms of the control method of the present invention before and after using a second-order low-pass filter;

[0043] Figure 9 These are simulation waveforms of the control method of this invention before and after using a second-order notch filter. Detailed Implementation

[0044] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] A grid-connected converter stability control method based on complex frequency domain impedance reshaping, such as... Figure 1 As shown, Figure 1 The block diagram for implementing the stability control method includes a converter 1, a public power grid 2, a phase-locked loop 3, a stability controller inverse loop 4, a current controller 5, a stability controller 6, a coordinate transformation module 7, and a pulse width modulator 8. The converter 1 is connected to the public power grid 2 and is connected to a filter inductor L1. The public power grid 2 consists of an equivalent impedance and a voltage source. The phase-locked loop 3 can be any type of three-phase voltage phase-locked loop. The phase-locked loop 3 is connected to the coordinate transformation module 7. One coordinate transformation module 7 is connected to the stability controller inverse loop 4 and the current controller 5, while the other coordinate transformation module 7 is connected to the stability controller 6 and the pulse width modulator 8. The current controller 5 is connected to the stability controller 6. The pulse width modulator 8 outputs a switching signal to control the converter 1. The current controller 5 is a proportional-integral controller, achieving closed-loop control.

[0046] The stability control method uses a stability controller 6 and its inverse element 4 to improve the system's stability margin. The stability control method includes the following steps:

[0047] S1. Obtain the voltage v at the common point of coupling (PCC) of the grid-connected converter. abc And the output current i of converter 1 abc ;

[0048] S2, using phase-locked loop 3 to pair voltage v abc Processing is performed to obtain voltage v abc Phase θ is used in coordinate transformation module 7;

[0049] S3, three-phase current i a i b i c After processing by the inverse circuit 4 of the stabilizer controller, its output signal is i a ′, i b ′, i c ′;

[0050] S4, i a ′, i b ′, i cThe dq-axis current i is obtained after coordinate transformation module 7. d and i q ;

[0051] S5, dq axis current i d and i q With reference value i of dq axis current dref and i qref After comparison, the output v is obtained after passing through current controller 5 (PI controller). rd and v rq ;

[0052] S6, v rd and v rq The dq axis control voltage v is obtained after processing by the stabilizing controller 6. r ′ d and v r ′ q ;

[0053] S7, Set the dq axis control voltage v r ′ d and v r ′ q The voltage is transmitted to coordinate transformation module 7 to obtain the abc axis modulation voltage, and then transmitted to pulse width modulator 8 to obtain the switching signal of converter 1.

[0054] The stabilizing controller 6 supports various minimum-phase digital filters. Taking a first-order low-pass filter (LPF), a second-order low-pass filter (SLPF), and a second-order notch filter (NF) as examples, their frequency domain expressions are as follows:

[0055]

[0056]

[0057]

[0058] like Figure 2 The diagram shows the small-signal model of the grid-connected converter obtained after applying the stability control method, as follows: Figure 3 The diagram shows the equivalent small-signal model of the grid-connected converter. Compared to traditional control methods, the small-signal model of the grid-connected converter adds a stabilizing controller matrix G to the forward control channel of the current controller 5. f An inverse matrix of the stabilizing controller 6 was added to the current feedback channel. Their expressions are respectively In the fields of mathematics and control, Figure 2 It can be equivalent to Figure 3 This is equivalent to introducing G into the 3-channel phase-locked loop. f It does not affect the current loop.

[0059] When the converter control system does not have a stabilizer controller 6 and its inverse stabilizer controller 4, its output impedance is:

[0060]

[0061] Z CL This indicates the effect of current controller 5 and filter inductor, Z. pll This indicates the effect of phase-locked loop 3 on the impedance, where, The above equation indicates the coupling between phase-locked loop 3 and current controller 5. For the converter output impedance, as shown, the dd-axis impedance is mainly determined by the current loop and filter, and is not affected by phase-locked loop 3. However, the q-axis impedance is affected by phase-locked loop 3, introducing negative damping. Furthermore, the q-axis impedance mainly depends on… Therefore, the stability control method compensates for this term through the stability controller 6 and its inverse stage 4. Based on the small-signal model of the grid-connected converter, the converter output impedance is obtained as follows:

[0062]

[0063] Compared with the output impedance under traditional control, this invention only adds one term G. f It will not affect the impedance of the current control loop in traditional control.

[0064] Figures 4-6 To improve the impedance ratio Z before and after adding a stability controller 6 in weak grid conditions g The amplitude-phase characteristic diagram of the dominant stability eigenvalues ​​of / Z, Z g Z represents the grid impedance, and Z represents the converter output impedance. The dashed line in the figure is the amplitude and phase characteristic diagram under conventional control. In the range where the amplitude is greater than 0, the phase crosses through -180°, indicating that the system is unstable.

[0065] For a first-order low-pass filter, where ω f Set it to a value lower than the instability frequency; here it is set to 300Hz.

[0066] For a second-order low-pass filter, where ω fs Choose a value lower than the instability frequency, set it to 300Hz, ξ s The damping coefficient is set to...

[0067] For a second-order notch filter, where ω n ξ represents the notch frequency, set to the instability frequency, and ξ represents the notch depth, set to 0.1.

[0068] like Figures 4-6As shown, when the stabilization control method of the present invention is used, the stabilization controller 6 can ensure that there is no phase crossover in the amplitude-phase characteristic diagram within the range where the amplitude is greater than 0, regardless of whether a first-order low-pass filter, a second-order low-pass filter, or a second-order notch filter is used, thus ensuring system stability.

[0069] like Figures 7-9 The simulation results of the grid-connected converter under weak grid conditions, with or without the control method of this invention, show that under traditional control, the converter voltage and current exhibit wide-frequency domain oscillations. Frequency domain analysis reveals that both contain significant harmonic components, indicating that the system is unstable. However, under the grid-connected converter stability control method based on complex frequency domain impedance reshaping described in this invention, the converter voltage and current oscillations disappear, and the system exhibits excellent power quality, thus confirming the effectiveness of the control method of this invention.

[0070] The control method of this invention does not affect the current control performance. It only requires adding a stable control and its inverse link to the control system, which is convenient to implement. In practical applications, the stable controller can be implemented by various low-order and high-order digital filters, and the structure and parameter design are simple.

[0071] The control method of this invention is not limited to the type of phase-locked loop (PLL) and is applicable to a variety of common types of PLLs, including synchronous coordinate system PLLs and dual second-order generalized integrator PLLs. The stable control will correct the output impedance of the PLL, thereby reshaping the converter impedance and improving system stability, without affecting the impedance of the current control link.

[0072] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0073] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A grid-connected converter stability control method based on complex frequency domain impedance reshaping, characterized in that, The control method includes a converter (1), a public power grid (2), a phase-locked loop (3), a stabilization controller inverse link (4), a current controller (5), a stabilization controller (6), a coordinate transformation module (7), and a pulse width modulator (8). The specific process of the stabilization control method includes the following steps: S1. Obtain the PCC voltage of the grid-connected converter common coupling point. v abc and converter (1) output current i abc ; S2, using a phase-locked loop (3) to control the voltage v abc Processing is performed to obtain voltage. v abc phase θ , used for coordinate transformation module (7); S3, convert the three-phase current i a , i b , i c Through the inverse transfer function respectively The output signal is obtained by processing the inverse circuit (4) of the stabilizer controller. , , ; S4, output signal from S3 , , After coordinate transformation module (7), the dq axis current is obtained. i d and i q ; S5. The dq-axis current obtained in S4 i d、 i q Reference value of dq axis current i dref、 i qref The results are compared and then output through the current controller (5). v rd and v rq ; S6, the result obtained in S5 v rd and v rq After passing through the transfer function respectively After processing by the stabilizing controller (6), the dq-axis control voltage is obtained. and ; S7. The dq-axis control voltage obtained in S6 and The voltage is transmitted to the coordinate transformation module (7) to obtain the abc axis modulation voltage. Finally, the abc axis modulation voltage is transmitted to the pulse width modulator (8) to obtain the switching signal of the converter (1).

2. The grid-connected converter stability control method based on complex frequency domain impedance reshaping according to claim 1, characterized in that, The converter (1) is connected to the public power grid (2). The converter (1) is connected to the filter inductor L1. The public power grid (2) includes an equivalent impedance and a voltage source. The phase-locked loop (3) is any three-phase voltage phase-locked loop. The current controller (5) is a proportional-integral controller. The phase-locked loop (3) is connected to the coordinate transformation module (7). One of the coordinate transformation modules (7) is connected to the inverse link of the stabilization controller (4) and the current controller (5). The other coordinate transformation module (7) is connected to the stabilization controller (6) and the pulse width modulator (8). The current controller (5) is connected to the stabilization controller (6). The pulse width modulator (8) outputs a switching signal to control the converter (1).

3. The grid-connected converter stability control method based on complex frequency domain impedance reshaping according to claim 1, characterized in that, The stability controller (6) includes a first-order low-pass filter LPF, a second-order low-pass filter SLPF, and a second-order notch filter NF.

4. The grid-connected converter stability control method based on complex frequency domain impedance reshaping according to claim 3, characterized in that, The frequency domain expression of the first-order low-pass filter LPF is: ； The frequency domain expression of the second-order low-pass filter SLPF is: ； The frequency domain expression of the second-order notch filter NF is: In the frequency domain expression of the first-order low-pass filter LPF, ω f This is a value lower than the instability frequency; in the frequency domain expression of the second-order low-pass filter SLPF, ω fs This is a value lower than the instability frequency. ξ s Let be the damping coefficient; in the frequency domain expression of the second-order notch filter NF, ω f Set the notch filter frequency and the instability frequency. ξ Indicates the depth of the notch.

5. The grid-connected converter stability control method based on complex frequency domain impedance reshaping according to claim 1, characterized in that, After obtaining the small-signal model of the grid-connected converter through the control method, the small-signal model of the grid-connected converter adds a stability controller (6) matrix to the forward control channel of the current controller (5). An inverse matrix of the stabilizing controller (6) was added to the current feedback channel. Their expressions are as follows: 。 6. The grid-connected converter stability control method based on complex frequency domain impedance reshaping according to claim 5, characterized in that, The control method eliminates the influence of the stabilizing controller (6) on the current control by introducing the inverse link (4) of the stabilizing controller in the current feedback channel, so that it only affects the characteristics of the phase-locked loop (3), and the output impedance of the converter is obtained as: in, This indicates the coupling between the phase-locked loop (3) and the current controller (5). This refers to the impedance of the current control loop in traditional control systems.

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