A method for suppressing low-frequency oscillation of a network-structured converter in a direct-current power distribution network

Abstract

This invention relates to the field of converter control, specifically to a method for suppressing low-frequency oscillations in a grid-type converter in a DC distribution network. The method includes acquiring the DC-side voltage of the grid-type converter in a point-to-point symmetrical DC distribution network; adjusting the acquired DC-side voltage and feeding it forward to the reactive power control loop of the grid-type converter. This invention feeds the DC voltage forward to the reactive power control loop of the grid-type converter, thereby dynamically introducing the DC voltage of the DC distribution network into the control strategy of the grid-type converter, enhancing the damping effect of the grid-type converter on low-frequency oscillations, and improving the stability of the grid-type converter.

Description

Technical Field

[0001] This invention relates to the field of converter control, and more specifically to a method for suppressing low-frequency oscillations in grid-type converters in DC distribution networks. Background Technology

[0002] DC distribution networks are a new type of power distribution architecture that distributes electrical energy from upstream power sources or backbone networks to end users in the form of direct current (DC). Compared to traditional AC distribution networks, DC distribution networks can significantly reduce conversion stages, lower line losses and harmonic levels, and more efficiently accommodate DC-type distributed energy sources such as photovoltaics and energy storage, as well as DC loads. They represent an important development direction for building modern power distribution systems with high power supply reliability, high power quality, and high energy utilization efficiency. Typical application scenarios include industrial parks, commercial building clusters, data centers, and urban power supply networks. However, DC distribution networks are complex systems with low inertia and strong coupling, consisting of a large number of power electronic converters. The dynamic stability of the system, especially the broadband oscillation risk caused by multi-machine interaction of converters, has become a key technical bottleneck restricting its safe and stable operation and large-scale application.

[0003] In DC distribution networks, the key interface for energy exchange with the upstream AC main grid is the grid-connected converter. Based on their core control objectives within the system, they can be mainly divided into two categories: one type focuses on maintaining a constant DC bus voltage, acting as the voltage support foundation of the system; the other type emphasizes precise and proactive control of power (flow) exchange between the main grid and the DC distribution network. The grid-connected converter is a typical representative of the latter type. Unlike traditional grid-following converters, grid-connected converters autonomously construct the amplitude and frequency of their output voltage through internal algorithms (such as virtual synchronous machines and droop control), thereby actively setting and adjusting the active and reactive power exchanged with the main grid. This "proactive grid-connection" characteristic makes them insensitive to changes in the strength of the main grid. When facing complex operating conditions such as weak grids or main grid impedance fluctuations, they exhibit significantly better stability and robustness than grid-following converters, greatly enhancing the DC distribution network's adaptability and proactive support capabilities under various grid conditions. However, while simulating the external characteristics of a synchronous generator, the grid-type converter also inherits the low-frequency oscillation problem similar to that of a synchronous generator. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for suppressing low-frequency oscillations in grid-type converters in DC distribution networks. This method can improve the damping effect of grid-type converters on low-frequency oscillations and enhance the stability of grid-type converters in DC distribution networks.

[0005] The technical solution adopted in this invention is as follows: A method for suppressing low-frequency oscillations in grid-type converters in DC distribution networks, comprising the following steps: In a point-to-point symmetrical DC distribution network, the DC-side voltage of the grid-type converter is collected; The collected DC-side voltage is regulated and then fed forward to the reactive power control loop of the grid converter.

[0006] Furthermore, the point-to-point type symmetrical DC distribution network includes: a DC voltage-controlled converter, a DC transmission line, and a grid-type converter; wherein, the DC voltage-controlled converter is used to rectify the DC power in the transmission network into DC power; the DC transmission line is used to transmit the rectified DC power to the grid-type converter; and the grid-type converter is used to invert the DC power into AC power to supply the load at a remote end.

[0007] Furthermore, the collected DC-side voltage u dc The reactive power control loop fed forward to the grid-type converter after adjustment specifically includes: The reactive power deviation is obtained based on the reactive power reference value of the grid converter and the measured reactive power value. At the same time, the DC voltage compensation amount is obtained by adjusting the compensation gain based on the collected DC side voltage of the grid converter. The reactive power deviation is superimposed with the DC voltage compensation and then input into the PI regulator to obtain the d-axis voltage reference value, while the q-axis voltage reference value and the zero-axis voltage reference value are both set to zero.

[0008] Furthermore, the DC voltage compensation amount is achieved through the following formula:

[0009] in This indicates the DC voltage compensation amount, i.e., the adjusted DC side voltage; It is the adjustment coefficient.

[0010] Furthermore, the grid-type converter also includes an active power control loop, which, together with the reactive power control loop, controls the output of the grid-type converter.

[0011] Furthermore, the active power control loop is implemented through the following steps: Obtain the active power reference value and active power measurement value of the grid-type converter, and calculate the active power deviation; The active power deviation is processed through a virtual inertia and damping circuit to obtain the output angular velocity, and the output phase angle is obtained by integrating the output angular velocity.

[0012] Furthermore, in the step of processing the active power deviation through a virtual inertia and damping element, the transfer function of the virtual inertia and damping element is as follows: H(s) = 1 / (Js + D) Where H(s) represents the transfer function of the virtual inertia and damping element, and J is the virtual inertia coefficient. Here, is the Laplace operator, and D is the damping coefficient.

[0013] Furthermore, the active power control loop and the reactive power control loop jointly control the output of the grid-type converter, specifically including the following steps: The output phase angle is obtained from the active power control loop of the grid converter; The d-axis voltage reference value, q-axis voltage reference value, and zero-axis voltage reference value are obtained from the reactive power control loop of the grid-type converter. Based on the output phase angle, d-axis voltage reference value, q-axis voltage reference value and zero-axis voltage reference value, the three-phase voltage reference value is obtained through dq0 to abc coordinate transformation; The three-phase voltage reference values ​​are modulated by PWM to generate switching signals to control the output of the grid converter.

[0014] Furthermore, the three-phase voltage reference values ​​are obtained through coordinate transformation from dq0 to abc, specifically including the following transformation formulas: E a = E d cosθ GFM E b = E d cos (θ GFM 2π / 3) E c = E d cos (θ GFM +2π / 3) Among them, E a E is the reference value for phase A voltage. b E is the reference value for phase B voltage. c E is the reference value for phase C voltage. d θ is the reference value for the d-axis voltage. GFM This is the output phase angle.

[0015] As can be seen from the above scheme, the present invention discloses a method for suppressing low-frequency oscillations in grid-type converters in DC distribution networks, which has the following beneficial effects: This invention relates to a method for suppressing low-frequency oscillations in grid-type converters in DC distribution networks. This method improves the damping effect of grid-type converters on low-frequency oscillations and enhances the stability of grid-type converters by feeding forward the DC voltage of the DC distribution network to the reactive power control loop of the grid-type converter without changing the main control structure and control parameters of the grid-type control. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the structure of a grid-type converter in a DC distribution network provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the control strategy structure of a grid-type converter in a DC distribution network provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the control strategy structure of a DC voltage control converter in a DC distribution network provided in an embodiment of the present invention; Figure 4 This is a coordinate system relationship diagram provided in the embodiments of the present invention; Figure 5 These are the parameters provided in the embodiments of the present invention. When changing Changes in position; Figure 6 This is the active power curve of the grid-type converter when the converter control parameters for controlling DC voltage in the DC distribution network are perturbed, as provided in the embodiments of the present invention. Figure 7 This is the FFT analysis result of the active power curve of the grid-type converter provided in the embodiment of the present invention; Figure 8 This is a schematic diagram of the low-frequency oscillation suppression method for grid-type converters in DC distribution networks provided in this embodiment of the invention; Figure 9 These are the parameters provided in the embodiments of the present invention. When changing A diagram illustrating the changes in position; Figure 10 This is the active power curve of the improved grid-type converter provided in the embodiment of the present invention. Detailed Implementation

[0017] An embodiment of the present invention will be further described below with reference to the accompanying drawings.

[0018] like Figure 1 The diagram shows the structure of a grid-type converter in a DC distribution network. The grid-type converter adopts a grid-type control strategy. In this invention, the grid-type control strategy refers to virtual synchronous generator control.

[0019] Figure 1 middle and The values ​​represent the voltages of the two power grids on either side. It can be seen that the circuit topology of the DC distribution network composed of the two converters is symmetrical. Therefore, the subscripts "1" and "2" in the variables are only used to distinguish their positions; their physical meanings are the same. For simplicity, only the variables are described here. For grid current, This is the grid connection interface voltage. This refers to the converter bridge arm voltage. For the converter output current, , These are the equivalent inductance and equivalent resistance of the power grid, respectively. , , These are the converter's filter capacitor, filter inductor, and parasitic resistance of the filter inductor, respectively. It is a voltage regulator capacitor. It is angular frequency.

[0020] like Figure 2 The diagram shows the control strategy structure of a grid-type converter in a DC distribution network. It is the output angle of the active ring. It is the output angular frequency of the active power loop. and These are the coefficient of rotational inertia and the damping coefficient, respectively. It is the voltage reference value output by the reactive power loop. , These are the proportional and integral coefficients of the reactive power loop PI controller. This is the active power reference value. This is a reactive power reference value. It is the active power measurement value. This is the measured value of reactive power.

[0021] like Figure 3 The diagram shows the control strategy structure of a DC voltage control converter in a DC distribution network. This is the DC voltage reference value. This is the DC voltage value. , These are the proportional and integral coefficients of the outer loop PI controller. , for Components in the dq coordinate system , These are the proportional and integral coefficients of the inner-loop PI controller. It is the output angle of the phase-locked loop. This is the rated value of the power grid angular velocity.

[0022] according to Figure 1Based on Kirchhoff's voltage and current laws, a small-signal mathematical model of a DC distribution network circuit can be established, as shown in equations (1) to (7). Here, the subscript 0 represents the steady-state value of the variable. Small signals representing variables.

[0023] (1) (2) (3) (4) (5) (6) (7) in, (8) (9) (10) according to Figure 2 and Figure 3 A small-signal model of the control section of the DC distribution network is established, as shown in equations (11) to (17), where, For the Laplace operator, It is an intermediate variable.

[0024] (11) (12) (13) (14) (15) (16) (17) in, (18) (19) (20) (twenty one) and Its main function is to transform the three-phase system coordinate system variables into dq coordinate system variables and vice versa. Ideally, the system coordinate system and the control coordinate system coincide, but in reality, due to the lag effect of the control loop, there is usually an angular difference between the two coordinate systems, such as... Figure 4 As shown, where Let represent a certain variable. Then the transformation relationship of the variable is shown in equations (22) to (25).

[0025] (twenty two) (twenty three) (twenty four) (25) By combining equations (1) to (25) and rearranging these expressions in the form of state equations, we can obtain the small-signal mathematical model of the state space of the flexible interconnected system, as shown in equation (26).

[0026] (26) Among them, state variables The form is shown in equation (27).

[0027] (27) Input The form is shown in equation (28).

[0028] (28) The matrix is ​​a 21×21 order state parameter matrix. The matrix is ​​a 21×8 order input parameter matrix.

[0029] The specific form of the matrix is ​​shown in equation (29), where for The element in the i-th row and j-th column of the matrix; all other unlisted elements are 0.

[0030] (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46) (47) (48) (49) The parameters of the DC distribution network are shown in Table 1. The eigenvalues ​​of the matrix are obtained using the formula shown in equation (50). Accordingly, The oscillation frequency and damping ratio of the oscillation mode of the matrix are obtained using the formula shown in equation (51), where the system eigenvalues ​​are complex numbers. The oscillation frequency is Damping ratio is .

[0031] (50) (51) The characteristic values ​​and oscillation modes of the DC distribution network are shown in Table 2.

[0032] Table 1 Basic parameters of DC distribution network

[0033] Table 2 System eigenvalues ​​and oscillation modes

[0034] set up If the value is reduced from 0.2 to 0.0002 in steps of 0.0001, then... Positional changes on the complex plane are as follows Figure 5 As shown.

[0035] Depend on Figure 5 It can be seen that, with The decrease, It gradually moves from the left half to the right half of the complex plane. Correspondingly, the DC distribution network gradually changes from stable to unstable.

[0036] Install DC voltage control converter in DC distribution network If the value decreases from 0.2 to 0.0002 at 4s, the active power curve of the grid-type converter is as follows: Figure 6 As shown in the figure, it can be seen that when the control parameters of the converter controlling the DC voltage are perturbed, the grid-type converter exhibits a low-frequency oscillation of 1.8Hz, such as... Figure 7 As shown, the structure The grid converter is unstable.

[0037] Figure 8 This is a schematic diagram of a method for suppressing low-frequency oscillations in a grid-type converter in a DC distribution network. It is the adjustment coefficient. The first input terminal of the adjustment module is connected to the DC side voltage signal, and the first output terminal of the adjustment module is attached to the output port of the PI controller of the reactive power control loop of the grid-type converter.

[0038] set up , If the value is increased from 0.1 to 1.1 in increments of 0.1, then... Positional changes on the complex plane are as follows Figure 9 As shown.

[0039] Figure 9 Display with The increase First move to the left, when hour, It then changed direction and moved to the right, therefore when At this time, the stability of the DC distribution network is at its best.

[0040] Install converters to control DC voltage in DC distribution networks. The value decreases from 0.2 to 0.0002 at 4s. If the adjustment module is connected at 12s, the active power curve of the grid-type converter is as follows: Figure 10 As shown in the diagram, before the adjustment module was connected, the grid-type converter experienced low-frequency power oscillations after parameter perturbation. At 12 seconds, due to the connection of the adjustment module, the grid-type converter quickly recovered to a stable operating state. This indicates that the adjustment module improved the stability of the grid-type converter.

[0041] In this embodiment of the invention, the method of the present invention, without changing the main control structure and control parameters of the grid-type control, can avoid low-frequency oscillations in the grid-type converter caused by the perturbation of the converter control parameters controlling the DC voltage in the DC distribution network by feeding the DC voltage forward into the reactive power control loop of the grid-type converter, thereby improving the stability of the grid-type converter.

[0042] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for suppressing low-frequency oscillations in grid-type converters in DC distribution networks, characterized in that, Includes the following steps: In a point-to-point symmetrical DC distribution network, the DC-side voltage of the grid-type converter is collected; The collected DC-side voltage is regulated and then fed forward to the reactive power control loop of the grid converter.

2. The method for suppressing low-frequency oscillations in a grid-type converter in a DC distribution network as described in claim 1, characterized in that, The point-to-point symmetrical DC distribution network includes: a DC voltage-controlled converter, a DC transmission line, and a grid-type converter; wherein, the DC voltage-controlled converter is used to rectify DC power in the transmission network into DC power; the DC transmission line is used to transmit the rectified DC power to the grid-type converter; and the grid-type converter is used to invert DC power into AC power to supply distant loads.

3. The method for suppressing low-frequency oscillations in a grid-type converter in a DC distribution network as described in claim 1, characterized in that, The collected DC side voltage u dc The reactive power control loop fed forward to the grid-type converter after adjustment specifically includes: The reactive power deviation is obtained based on the reactive power reference value of the grid converter and the measured reactive power value. At the same time, the DC voltage compensation amount is obtained by adjusting the compensation gain based on the collected DC side voltage of the grid converter. The reactive power deviation is superimposed with the DC voltage compensation and then input into the PI regulator to obtain the d-axis voltage reference value, while the q-axis voltage reference value and the zero-axis voltage reference value are both set to zero.

4. The method for suppressing low-frequency oscillations in a grid-type converter in a DC distribution network as described in claim 3, characterized in that, The DC voltage compensation amount is achieved through the following formula: in This indicates the DC voltage compensation amount, i.e., the adjusted DC side voltage; It is the adjustment coefficient.

5. The method for suppressing low-frequency oscillations in a grid-type converter in a DC distribution network as described in claim 1, characterized in that, The grid-type converter also includes an active power control loop, which, together with the reactive power control loop, controls the output of the grid-type converter.

6. The method for suppressing low-frequency oscillations in a grid-type converter in a DC distribution network as described in claim 5, characterized in that, The active power control loop is implemented through the following steps: Obtain the active power reference value and active power measurement value of the grid-type converter, and calculate the active power deviation; The active power deviation is processed through a virtual inertia and damping circuit to obtain the output angular velocity, and the output phase angle is obtained by integrating the output angular velocity.

7. The method for suppressing low-frequency oscillations in a grid-type converter in a DC distribution network as described in claim 6, characterized in that, In the process of handling the active power deviation through a virtual inertia and damping element, the transfer function of the virtual inertia and damping element is as follows: H(s) = 1 / (Js + D) Where H(s) represents the transfer function of the virtual inertia and damping element, and J is the virtual inertia coefficient. Here, is the Laplace operator, and D is the damping coefficient.

8. The method for suppressing low-frequency oscillations in a grid-type converter in a DC distribution network as described in claim 5, characterized in that, The active power control loop and reactive power control loop jointly control the output of the grid-type converter, specifically including the following steps: The output phase angle is obtained from the active power control loop of the grid converter; The d-axis voltage reference value, q-axis voltage reference value, and zero-axis voltage reference value are obtained from the reactive power control loop of the grid-type converter. Based on the output phase angle, d-axis voltage reference value, q-axis voltage reference value and zero-axis voltage reference value, the three-phase voltage reference value is obtained through dq0 to abc coordinate transformation; The three-phase voltage reference values ​​are modulated by PWM to generate switching signals to control the output of the grid converter.

9. A method for suppressing low-frequency oscillations in a grid-type converter in a DC distribution network as described in claim 8, characterized in that, The three-phase voltage reference values ​​are obtained by transforming the coordinates from dq0 to abc, specifically using the following transformation formulas: AND a = And d cosθ GFM E b = E d cos (θ GFM 2π / 3) E c = E d cos (θ GFM +2π / 3) Among them, E a E is the reference value for phase A voltage. b E is the reference value for phase B voltage. c E is the reference value for phase C voltage. d θ is the reference value for the d-axis voltage. GFM This is the output phase angle.

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