Method and device for improving stability of flexible interconnection device and medium

Abstract

The invention discloses a stability improving method and device for a flexible interconnection device and a medium, and the method comprises the steps: introducing a capacitor voltage feed-forward term of series low-pass filtering into a first control loop of an inverter, and introducing a capacitor voltage feedback term into a second control loop of the inverter, a capacitance current feed-forward item is introduced into the first control loop and the second control loop to obtain a first inverter; and controlling the flexible interconnection device comprising the first inverter to start running. The invention provides a stability improving method and device for a flexible interconnection device and a medium, and the method comprises the steps: introducing a capacitor voltage feed-forward item and a capacitor voltage feedback item of series low-pass filtering into a first control loop and a second control loop respectively, and introducing a capacitor current feed-forward item into the two control loops, the output impedance characteristic can be improved, the system robustness is enhanced, the control precision is improved, and the problem of instability caused by power grid impedance change when the inverter of the flexible interconnection device works in a current source mode can be solved.

Description

Technical Field

[0001] The present invention relates to the field of power electronics, and particularly to a method, device and medium for improving the stability of a flexible interconnection device. Background Art

[0002] A flexible interconnection device (FID) based on power electronic equipment can replace the tie switch, flexibly interconnect distribution networks with multiple voltages, frequencies, amplitudes and phases, and can achieve power mutual assistance, voltage support and rapid fault handling; the inverter is the energy interaction interface between the FID and the heavy-load substation area power grid, and the main control objective is to transmit active power to the heavy-load substation area load; however, the grid impedance changes greatly, the inverter is prone to instability, and the power of the inverter and the rectifier will cross-couple, and these factors are likely to cause the entire FID system to collapse. At present, the main method is to reshape the output impedance of the converter to change the passive characteristics of the system, so as to maintain the system stability. The active damping method among them establishes a new virtual impedance of the system by changing the control loop, and then optimizes the passive characteristics of the converter system, mainly including the feedforward or feedback of the grid voltage / capacitor voltage, capacitor current and inverter-side current.

[0003] However, in addition to the grid-side current sensor, the common active damping method also requires additional sensors such as capacitor current, capacitor voltage or inverter-side current to provide damping effect, and less consideration is given to the elimination of the wide-band negative impedance region, and the improvement of the system stability margin is not comprehensive enough, resulting in instability when the inverter of the flexible interconnection device operates in the current source mode. Summary of the Invention

[0004] The present invention provides a method, device and medium for improving the stability of a flexible interconnection device, so as to solve the instability problem caused by the change of the grid impedance when the inverter of the flexible interconnection device operates in the current source mode.

[0005] Obtain a flexible interconnection device; wherein, the flexible interconnection device includes an inverter.

[0006] Introduce the capacitor voltage feedforward term of the series low-pass filter into the first control loop of the inverter, introduce the capacitor voltage feedback term into the second control loop of the inverter, and introduce the capacitor current feedforward term into the first control loop and the second control loop together to obtain a first inverter.

[0007] Control the flexible interconnection device including the first inverter to start running.

[0008] In the present invention, for the first control loop, the capacitor voltage feedforward term of the series low-pass filter can improve the phase characteristic of the inverter output impedance in the low-frequency band; by introducing the feedforward control, the system can respond more quickly to the change of current, thereby reducing the phase lag and improving the stability of the system. Moreover, when the grid impedance is large, low-frequency harmonics are likely to appear in the inverter output current. By introducing the capacitor voltage feedforward term, the output of the first control loop can be adjusted in real time, effectively suppressing the generation of these low-frequency harmonics, thereby avoiding system instability. For the second control loop, the introduction of the capacitor voltage feedback term helps to enhance the stability of the second control loop; through the feedback control, the change of current can be sensed in real time and corresponding adjustments can be made, thereby improving the robustness of the system. Moreover, the capacitor voltage feedback term can also optimize the output impedance characteristic of the inverter. By adjusting the parameters of the feedback control, the characteristics of the output impedance in different frequency bands can be further improved, especially in the frequency band related to the target axis in the second control loop, thereby improving the overall performance of the system. The introduction of the capacitor current feedforward term in the first and second control loops also helps to suppress the high-frequency harmonics of the grid-side current caused by the change of the grid impedance.

[0009] Compared with the prior art, by respectively introducing the capacitor voltage feedforward term of the series low-pass filter and the capacitor voltage feedback term in the first and second control loops, and jointly introducing the capacitor current feedforward term in the two control loops, the present invention can improve the output impedance characteristic, suppress the current harmonics, enhance the system robustness and improve the control accuracy, so as to solve the instability problem caused by the change of the grid impedance when the inverter of the flexible interconnection device operates in the current source mode.

[0010] As a preferred solution, introducing the capacitor voltage feedforward term of the series low-pass filter into the first control loop of the inverter specifically includes:

[0011] In the first control loop of the inverter, the feedforward component of the capacitor on the d-axis is introduced into the output side of the first control loop through a proportional controller;

[0012] A low-pass filter is connected in series at the output end of the proportional controller;

[0013] Wherein, the value of the proportional controller is a negative real number, and the feedforward component is used to reshape the impedance in the low-frequency band of the first control loop.

[0014] In this preferred solution, the design of introducing the feedforward component enables the first control loop to have specific impedance characteristics in the low-frequency band. By adjusting the value of the proportional controller and the parameters of the low-pass filter, the magnitude and phase of the impedance in the low-frequency band can be precisely controlled, thus meeting the requirements of specific application scenarios. By introducing the capacitor voltage feedforward term, the system can respond more quickly to changes in current, which helps to maintain the stability of the system. The design of the proportional controller as a negative real number helps to achieve negative feedback control of the system, further suppressing the oscillation and instability phenomena of the system; at the same time, the addition of the low-pass filter can filter out high-frequency noise and interference, improving the anti-interference ability and robustness of the flexible interconnection device.

[0015] As a preferred solution, the capacitor current feedforward term consists of a proportional coefficient, a capacitor, and a high-pass filter;

[0016] Among them, the proportional coefficient is used to adjust the influence degree of the capacitor current feedforward term in the control system, the capacitor is used to adjust the output impedance of the inverter, and the high-pass filter is used to extract the high-frequency components in the capacitor current.

[0017] In this preferred solution, the proportional coefficient, as an adjustment factor, can flexibly adjust the influence degree of the capacitor current feedforward term in the control system, which enables the control system to precisely control the role of the capacitor current feedforward according to different working conditions and requirements, thereby achieving a more refined adjustment effect. By selecting an appropriate capacitor value, the output impedance characteristics of the inverter in a specific frequency range can be optimized, thereby improving the stability and performance of the system. The high-pass filter is used to extract the high-frequency components in the capacitor current, which helps the control system to more accurately sense and respond to high-frequency disturbances.

[0018] As a preferred solution, the capacitor voltage feedback term is a proportional controller;

[0019] Among them, the capacitor voltage feedback term is used to reshape the impedance of the low-frequency band of the second control loop.

[0020] In this preferred solution, in the flexible interconnection device, changes in impedance characteristics often affect the stability of the system; by adjusting the impedance, the risk of system oscillation can be reduced, and the robustness of the system can be improved.

[0021] As a preferred solution, the first inverter includes a d-axis closed-loop output impedance and a q-axis closed-loop output impedance;

[0022] Among them, the expression of the d-axis closed-loop output impedance is:

[0023]

[0024] Among them, Y ui (s) is the actual filter circuit model, Q(s) is the low-pass filter, r1 is the damping coefficient of passive current control, G feed (s) is the ideal circuit model corresponding to Y ui (s). Gdel(s) represents the system dynamic delay or disturbance caused by various factors. G C (s) is the transfer function of the capacitor voltage feedforward term, G lp (s) is a low-pass filter, K ad is a proportional controller.

[0025] The present application also provides a stability improvement device for a flexible interconnection device, including an acquisition module, an adjustment module, and a start module;

[0026] Among them, the acquisition module is used to acquire a flexible interconnection device; among them, the flexible interconnection device includes an inverter.

[0027] The adjustment module is used to introduce the capacitor voltage feedforward term of series low-pass filtering into the first control loop of the inverter, introduce the capacitor voltage feedback term into the second control loop of the inverter, and jointly introduce the capacitor current feedforward term into the first control loop and the second control loop to obtain a first inverter;

[0028] The start module is used to control the flexible interconnection device including the first inverter to start running.

[0029] As a preferred solution, the adjustment module includes a component unit and a filtering unit;

[0030] Among them, the component unit is used to introduce the feedforward component of the capacitor on the d-axis into the output side of the first control loop of the inverter through a proportional controller in the first control loop of the inverter;

[0031] The filtering unit is used to connect a low-pass filter in series at the output end of the proportional controller;

[0032] Among them, the value of the proportional controller is a negative real number, and the feedforward component is used to reshape the low-frequency impedance of the first control loop.

[0033] As a preferred solution, the capacitor current feedforward term is composed of a proportional coefficient, a capacitor, and a high-pass filter;

[0034] Among them, the proportional coefficient is used to adjust the influence degree of the capacitor current feedforward term in the control system, the capacitor is used to adjust the output impedance of the inverter, and the high-pass filter is used to extract the high-frequency components in the capacitor current.

[0035] As a preferred solution, the capacitor voltage feedback term is a proportional controller;

[0036] Among them, the capacitor voltage feedback term is used to reshape the impedance of the low-frequency segment of the second control loop.

[0037] As a preferred solution, the first inverter includes a d-axis closed-loop output impedance and a q-axis closed-loop output impedance;

[0038] Among them, the expression of the d-axis closed-loop output impedance is:

[0039]

[0040] Among them, Y ui (s) is the actual filter circuit model, Q(s) is the low-pass filter, r 1 is the damping coefficient of the passive current control, G feed (s) is the ideal circuit model corresponding to Y ui (s), Gdel(s) represents the system dynamic delay or disturbance caused by various factors, G C (s) is the transfer function of the capacitor voltage feedforward term, G lp (s) is the low-pass filter, K ad is the proportional controller.

[0041] This application also provides a storage medium, on which a computer program is stored. The computer program is called and executed by a computer to implement the method for improving the stability of a flexible interconnection device as described above. Description of the Drawings

[0042] Figure 1 is a schematic flow chart of a method for improving the stability of a flexible interconnection device provided by an embodiment of this application;

[0043] Figure 2 is a three-phase T-type three-level inverter topology diagram provided by an embodiment of this application;

[0044] Figure 3 is a closed-loop small-signal control block diagram based on DOPBC provided by an embodiment of this application;

[0045] Figure 4 is a small-signal control block diagram of an inverter based on an improved impedance reshaping method provided by an embodiment of this application;

[0046] Figure 5 is a Bode diagram of the output impedance in different cases provided by an embodiment of this application;

[0047] Figure 6 is a schematic structural diagram of a device for improving the stability of a flexible interconnection device provided by an embodiment of this application. Detailed Embodiments

[0048] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts shall fall within the protection scope of the present application.

[0049] In the description of the present application, it should be understood that the terms "first", "second", "third", and "fourth" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first", "second", "third", and "fourth" may explicitly or implicitly include one or more of such features. In the description of the present application, unless otherwise specified, the meaning of "several" is two or more.

[0050] A method for improving the stability of a flexible interconnection device provided by an embodiment of the present application is mainly applied to reshaping the output impedance of an inverter in the flexible interconnection device, improving the system stability margin, and overcoming the instability caused by the change of the low-voltage distribution network impedance when the inverter operates in the current source mode.

[0051] Embodiment 1:

[0052] Please refer to Figure 1 , an embodiment of the present application provides a method for improving the stability of a flexible interconnection device, including S1 to S3, and the specific implementation steps are as follows:

[0053] S1. Obtain a flexible interconnection device; wherein, the flexible interconnection device includes an inverter.

[0054] Step S1 in the embodiment of the present application is specifically:

[0055] Obtain a flexible interconnection device based on a back-to-back power electronic converter from a power system or an energy conversion system; wherein, the flexible interconnection device includes an inverter and a rectifier.

[0056] It should be noted that the flexible interconnection device based on the back-to-back power electronic converter includes a rectifier and an inverter. The rectifier mainly operates in the DC voltage source mode and is responsible for stabilizing the DC bus voltage. The operating modes of the inverter include the current source mode and the regulated voltage source mode. In the current source mode, the inverter operates under grid-following control and adjusts the power flow between the two end substations according to the command. In the regulated voltage source mode, the inverter operates under grid-forming control and operates off-grid to support the load power consumption of the substation. The present application mainly solves the instability problem caused by the change of the low-voltage distribution network impedance when the inverter operates in the current source mode. Therefore, the hardware circuit and control method in this embodiment are all about the inverter.

[0057] For the application of the embodiments of the present application, please refer to Figures 2-3 , Figure 2 which is the topology diagram of a three-phase T-type three-level inverter provided by the embodiments of the present application, representing the topology of an inverter based on a three-phase three-wire T-type three-level; Figure 3 which is the closed-loop small-signal control block diagram based on DOPBC provided by the embodiments of the present application, representing the control block diagram of the traditional Disturbance Observer Passivity Based Control (DOPBC) method;

[0058] In Figure 2 , the AC side is connected to the power grid through an LC filter, and the DC side is connected to the DC bus generated by the rectifier. i 1a , i 1b and i 1c are the inverter-side currents, u Ca , u Cb and u Cc are the filter capacitor voltages, u a , u b and u c are the inverter-side voltages, L 1 and R 1 are the filter inductor and resistor respectively.

[0059] In Figure 3 , the first part represents the d-axis, the second part represents the q-axis, and the dashed box is the disturbance observer part; where Y ui (s) is the actual filter circuit model, G feed (s) is the ideal circuit model corresponding to Y ui (s), and Q(s) is a low-pass filter. r 1 and r 2 are the damping coefficients of the passive current control, I 1d and I 1q are the magnitudes of the dq-axis current components i 1d and i 1q respectively, u Cd and u Cq are the feedforward components of the capacitor voltage on the d-axis and q-axis respectively, G PLL (s) is the transfer function of the phase-locked loop, and are the reference values of the d-axis and q-axis current closed-loop controls respectively. In addition, in order to make the disturbance observer have better robustness and improve the sensitivity of the system to measurement noise, Q(s) uses the following third-order low-pass filter:

[0060]

[0061] Among them, τ = 0.0001, representing the time constant of the filter; s is the complex variable of the Laplace transform.

[0062] Through Figure 3 , the closed-loop output impedance Z ddo (s) of the inverter system on the d-axis and the closed-loop output impedance Z qdo (s) on the q-axis can be derived as follows:

[0063]

[0064]

[0065] It can be seen from the above derivation results that, compared with traditional passive control, the passive current control method based on a disturbance observer has better robustness and steady-state performance. However, the addition of the disturbance observer will cause the output impedance of the inverter to have a negative impedance. Furthermore, the inverter impedance and the grid impedance will generate harmonic resonance in the negative impedance frequency band, resulting in unstable grid connection of the system. In order to expand the passive region of the inverter output impedance, some scholars have adopted the method of impedance reshaping, reducing the non-passive regions of the inverter output impedance in the low-frequency band and the high-frequency band based on the capacitor current feedforward method and the capacitor voltage feedback method. Although this impedance reshaping method partially reshapes the impedance in the low-frequency band and the high-frequency band, there is still a negative impedance region in the high-frequency band of its d-axis output impedance, and the negative impedance region in the low-frequency band is still large, which is extremely likely to cause low-frequency harmonics, high-frequency harmonic components, and even resonance instability in the grid-connected current when the grid impedance changes in the inverter system;

[0066] Therefore, this application proposes a method for improving the stability of a flexible interconnection device, aiming to eliminate the negative impedance region of the inverter d-axis output impedance near the switching frequency band, increase the positive impedance region in the low-frequency band, and effectively improve the grid connection stability of the inverter system. Therefore, it can also be called an impedance reshaping method.

[0067] S2. Introduce the capacitor voltage feedforward term of the series low-pass filter into the d-axis control loop of the inverter, introduce the capacitor voltage feedback term into the q-axis control loop of the inverter, and jointly introduce the capacitor current feedforward term into the d-axis control loop and the q-axis control loop to obtain the first inverter.

[0068] Step S2 of the embodiment of this application includes S2.1 to S2.3, specifically:

[0069] S2.1. In the d-axis control loop of the inverter, through the proportional controller K ad introduce the feedforward component of the capacitor on the d-axis (capacitor voltage feedforward term) to the output side of the d-axis control loop, which will help introduce a positive impedance to the d-axis output impedance; among them, the d-axis control loop refers to the first control loop;

[0070] In series with the output of the proportional controller K ad is a low-pass filter G lp (s), obtaining the first transitional inverter; and, G lp (s) = (5T l s + 1) / (2(T l s) 2 + 2T l s + 1), T l = 0.0001, s represents the complex variable in the Laplace transform, and the low-pass filter G lp (s) is used to eliminate the influence of the introduced proportional controller K ad on the negative impedance region in the high-frequency band;

[0071] Among them, the value of the proportional controller K ad is a negative real number, and the capacitor voltage feedforward term is used to reshape the d-axis low-frequency impedance of the d-axis control loop.

[0072] This embodiment S2.1 is used to reshape the output impedance of the d-axis low-frequency band.

[0073] S2.2, Introduce the capacitor voltage feedback term into the q-axis control loop of the first transitional inverter to obtain the second transitional inverter; among them, the q-axis control loop refers to the second control loop;

[0074] Among them, the transfer function of the capacitor voltage feedback term is G qad (s), which is essentially a proportional controller and is used to reshape the q-axis low-frequency impedance of the q-axis control loop; and, G qad (s) = k qad , k qad is the proportional gain. Through G qad (s), the output impedance of the q-axis low-frequency band can be reshaped, and the negative impedance region of the q-axis low-frequency band can be reduced.

[0075] This embodiment S2.2 is used to reshape the low-frequency impedance of the q-axis. In the flexible interconnected device, the change of the impedance characteristic often affects the stability of the system; by adjusting the impedance, the risk of system oscillation can be reduced, and the robustness of the system can be improved.

[0076] S2.3, Based on the second transitional inverter, introduce the capacitor current feedforward term into both the d-axis control loop and the q-axis control loop to obtain the first inverter; among them, the first inverter includes a d-axis closed-loop output impedance and a q-axis closed-loop output impedance;

[0077] Among them, the transfer function G C (s) of the capacitor current feedforward term is composed of the proportional coefficient k c , the filter capacitor value Cs, and the high-pass filter G high (s); and, GC (s) = k c CsG high (s), reshapes the output impedance of the inverter in the high - frequency band through G C (s), and can reduce the negative impedance region in the high - frequency band;

[0078] The proportional coefficient k c is used to adjust the influence degree of the capacitor current feed - forward term in the control system. The capacitor Cs is used to adjust the output impedance of the inverter; The high - pass filter G high (s) is used to extract the high - frequency components in the capacitor current; And, G high (s) = s / (s + ω h ), s is the complex variable in the Laplace transform, ω h is the cut - off frequency of the high - pass filter.

[0079] Among them, the expression of the d - axis closed - loop output impedance is:

[0080]

[0081] The expression of the q - axis closed - loop output impedance is:

[0082]

[0083] Among them, Y ui (s) is the actual filter circuit model, Q(s) is the low - pass filter, r 1 is the damping coefficient of the passive current control, G feed (s) is the ideal circuit model corresponding to Y ui (s), G del (s) represents the system dynamic delay or disturbance caused by various factors, G C (s) is the transfer function of the capacitor voltage feed - forward term, G lp (s) is the low - pass filter, G PLL (s) is the transfer function of the phase - locked loop, G qf (s) is equal to G qad (s)r 1 , U Cd is the steady - state value of the feed - forward component of the capacitor voltage in the d - axis, K ad is the proportional controller.

[0084] For applying the embodiments of the present application, please refer to Figures 4-5 , Figure 4 is the small - signal control block diagram of the inverter based on the improved impedance reshaping method provided by the embodiments of the present application, representing the control relationship after adjusting the inverter based on a pair of flexible interconnection devices in this embodiment; Figure 5It is the Bode plot of the output impedance under different conditions provided by the embodiments of the present application, showing the Bode plot of the output impedance of the inverter after impedance reshaping by the DOPBC method and the present application.

[0085] In Figure 4 , the third part represents the d-axis and the fourth part represents the q-axis; through Figure 4 , the closed-loop output impedance Z dm (s) of the inverter system on the d-axis and the closed-loop output impedance Z qm (s) on the q-axis can be derived, as shown in the above expressions.

[0086] In Figure 5 , the green dashed line part represents the Bode plot of the d-axis and the red dashed line part represents the Bode plot of the q-axis; it can be seen from Figure 5 that after the impedance reshaping of the present application, the passive region of the output impedance of the inverter on the d-axis extends from f d2 -f d3 to the region of f d1 -f d4 , and the passive region of the output impedance on the q-axis extends from f q2 -f q3 to the region of f q1 -f q4 . Through the capacitor current feedforward, there is no non-passive region in the frequency range of 1 kHz - 8 kHz for the output impedance, which can effectively reduce the high-frequency harmonics and even high-frequency oscillation instability generated by the interaction between the inverter and the power grid. After connecting a low-pass filter in series in the proportional controller K ad , the output impedance of the inverter on the d-axis not only reduces the non-passive region in the high-frequency band but also reduces the non-passive region in the frequency band of 170 Hz - 500 Hz, with remarkable effects.

[0087] It should be noted that in the present application, the adjustment steps for the d-axis control loop and the q-axis control loop are not in a specific order.

[0088] This embodiment S2.3 is used to reshape the impedance in the high-frequency band. By jointly introducing the capacitor current feedforward term G C (s) in the d-axis and the q-axis, the high-frequency output impedance of the inverter can be reshaped, reducing the negative impedance region in the high-frequency band;

[0089] Among them, the proportional coefficient is used as an adjustment factor, which can flexibly adjust the influence degree of the capacitor current feedforward term in the control system, enabling the control system to accurately control the role of the capacitor current feedforward according to different working conditions and requirements, thereby achieving a more refined adjustment effect. By selecting an appropriate capacitance value, the output impedance characteristics of the inverter within a specific frequency range can be optimized, thereby improving the stability and performance of the system. The high-pass filter is used to extract the high-frequency components in the capacitor current, which helps the control system to more accurately sense and respond to high-frequency disturbances.

[0090] S3. Control the flexible interconnection device including the first inverter to start running.

[0091] Step S3 in the embodiment of the present application is specifically as follows:

[0092] After receiving the dispatching instruction, control the flexible interconnection device including the first inverter to start running.

[0093] Overall, this embodiment has the following beneficial effects:

[0094] In the present application, for the d-axis control loop, the capacitor voltage feedforward term in series with the low-pass filter can improve the phase characteristic of the inverter output impedance in the low-frequency band; by introducing feedforward control, the system can respond more quickly to the change of the d-axis current, thereby reducing the phase lag and improving the stability of the system. Moreover, when the grid impedance is large, low-frequency harmonics are likely to appear in the inverter output current. By introducing the capacitor voltage feedforward term, the output of the d-axis control loop can be adjusted in real time, effectively suppressing the generation of these low-frequency harmonics, thereby avoiding system instability. For the q-axis control loop, the introduction of the capacitor voltage feedback term helps to enhance the stability of the q-axis control loop; through feedback control, the change of the q-axis current can be sensed in real time and corresponding adjustments can be made, thereby improving the robustness of the system. Moreover, the capacitor voltage feedback term can also optimize the output impedance characteristic of the inverter. By adjusting the parameters of the feedback control, the characteristics of the output impedance in different frequency bands can be further improved, especially in the frequency bands related to the q-axis, thereby improving the overall performance of the system. The introduction of the capacitor current feedforward term in the d-axis and q-axis control loops also helps to suppress the high-frequency harmonics of the grid-side current caused by the change of the grid impedance.

[0095] In summary, through the improved impedance reshaping method, the present application reshapes the impedance of the inverter system in the flexible interconnection device in the low-frequency band and high-frequency band under the premise of ensuring the robustness and steady-state performance of the traditional passive current control, expands the adaptation range of the inverter system to the grid impedance, and enables the inverter to still operate stably in the scenario of grid impedance change. The inverter and the rectifier are a strongly coupled series system. When the stable range of the inverter is improved, the stability of the entire FID system is also correspondingly improved. In addition, this solution is applicable to most grid-connected converter devices and has universality and generality.

[0096] Embodiment 2:

[0097] Please refer to Figure 6 , the embodiment of the present application provides a device for improving the stability of a flexible interconnection device, including an acquisition module 10, an adjustment module 20, and a start module 30;

[0098] Among them, the acquisition module 10 is used to acquire a flexible interconnection device; among them, the flexible interconnection device includes an inverter.

[0099] The adjustment module 20 is used to introduce the capacitor voltage feedforward term of the series low-pass filter into the first control loop of the inverter, introduce the capacitor voltage feedback term into the second control loop of the inverter, and commonly introduce the capacitor current feedforward term into the first control loop and the second control loop to obtain a first inverter.

[0100] The startup module 30 is used to control the flexible interconnection device including the first inverter to start running.

[0101] In one embodiment, the acquisition module 10 is specifically:

[0102] Acquire a flexible interconnection device based on a back-to-back power electronic converter from a power system or an energy conversion system; among them, the flexible interconnection device includes an inverter and a rectifier.

[0103] It should be noted that the flexible interconnection device based on the back-to-back power electronic converter includes a rectifier and an inverter. The rectifier mainly operates in the DC voltage source mode and is responsible for stabilizing the DC bus voltage. The operating modes of the inverter include the current source mode and the voltage stabilizing source mode. In the current source mode, the inverter operates under grid-following control and adjusts the power flow between the two end substations according to the command; in the voltage stabilizing source mode, the inverter operates under grid-forming control and operates off-grid to support the power consumption of the substation load. This application mainly solves the instability problem caused by the change of the low-voltage distribution network impedance when the inverter operates in the current source mode. Therefore, the hardware circuit and control method in this embodiment are all about the inverter.

[0104] To apply the embodiments of the present application, please refer to Figures 2-3 , Figure 2 is the three-phase T-type three-level inverter topology diagram provided by the embodiments of the present application, representing the inverter topology based on the three-phase three-wire T-type three-level; Figure 3 is the closed-loop small-signal control block diagram provided by the embodiments of the present application based on DOPBC, representing the control block diagram of the traditional disturbance observer passivity based control (DOPBC) method;

[0105] In Figure 2 , the AC side is connected to the power grid through an LC filter, the DC side is connected to the DC bus generated by the rectifier, i 1a , i 1b and i 1c are the inverter-side currents, u Ca , u Cb and u Cc are the filter capacitor voltages, ua , u b and u c are the voltages on the inverter side, L 1 and R 1 are the filter inductor and resistor respectively.

[0106] In Figure 3 , the first part represents the d-axis, the second part represents the q-axis, and the dashed box is the disturbance observer part; where, Y ui (s) is the actual filter circuit model, G feed (s) is the ideal circuit model corresponding to Y ui (s), and Q(s) is a low-pass filter. r 1 and r 2 are the damping coefficients of the passive current control, I 1d and I 1q are the magnitudes of the dq-axis current components i 1d and i 1q respectively, u Cd and u Cq are the feedforward components of the capacitor voltage on the d-axis and q-axis respectively, G PLL (s) is the transfer function of the phase-locked loop, and are the reference values of the d-axis and q-axis current closed-loop controls respectively. In addition, in order to make the disturbance observer have better robustness and improve the sensitivity of the system to measurement noise, Q(s) uses the following third-order low-pass filter:

[0107]

[0108] where, τ = 0.0001, representing the time constant of the filter; s is the complex variable of the Laplace transform.

[0109] Through Figure 3 , the closed-loop output impedance Z ddo (s) of the inverter system on the d-axis and the closed-loop output impedance Z qdo (s) on the q-axis can be derived, which are respectively:

[0110]

[0111]

[0112] As can be seen from the above derivation results, compared with traditional passive control, the passive current control method based on a disturbance observer has better robustness and steady-state performance. However, the addition of the disturbance observer will cause the output impedance of the inverter to have a negative impedance, and then the inverter impedance and the grid impedance will generate harmonic resonance in the negative impedance frequency band, resulting in unstable grid connection of the system. In order to expand the passive region of the inverter output impedance, some scholars have adopted the method of impedance reshaping, which reduces the non-passive regions of the inverter output impedance in the low-frequency band and the high-frequency band based on the capacitor current feedforward method and the capacitor voltage feedback method. Although this impedance reshaping method partially reshapes the impedance in the low-frequency band and the high-frequency band, there is still a negative impedance region in the high-frequency band of the d-axis output impedance, and the negative impedance region in the low-frequency band is still large, which is extremely likely to cause low-frequency harmonics and high-frequency harmonic components or even resonance instability in the grid-connected current when the grid impedance changes in the inverter system;

[0113] Therefore, this application proposes a stability improvement device for a flexible interconnection device, aiming to eliminate the negative impedance region of the inverter d-axis output impedance near the switching frequency band, increase the positive impedance region in the low-frequency band, and effectively improve the grid connection stability of the inverter system. Therefore, it can also be called an impedance reshaping device.

[0114] In one embodiment, the adjustment module 20 includes a component unit, a filtering unit, an adjustment unit, and an introduction unit;

[0115] Among them, the component unit is used to introduce the feedforward component of the capacitor in the d-axis (capacitor voltage feedforward term) to the output side of the d-axis control loop in the d-axis control loop of the inverter, which will help introduce a positive impedance to the d-axis output impedance; among them, the d-axis control loop refers to the first control loop; ad In the d-axis control loop of the inverter, the feedforward component of the capacitor in the d-axis (capacitor voltage feedforward term) is introduced to the output side of the d-axis control loop through the proportional controller K, which will help introduce a positive impedance to the d-axis output impedance; among them, the d-axis control loop refers to the first control loop;

[0116] The filtering unit is used to connect a low-pass filter G ad in series at the output end of the proportional controller K lp (s) to obtain a first transitional inverter; and, G lp (s) = (5T l s + 1) / (2(T l s) 2 + 2T l s + 1), T l = 0.0001, s represents the complex variable in the Laplace transform, and the low-pass filter G lp (s) is used to eliminate the influence of the introduced proportional controller K ad on the negative impedance region in the high-frequency band;

[0117] Among them, the value of the proportional controller K ad is a negative real number, and the capacitor voltage feedforward term is used to reshape the d-axis low-frequency band impedance of the d-axis control loop.

[0118] In this embodiment, the component unit and the filtering unit are used to reshape the output impedance of the d-axis low-frequency band.

[0119] An adjustment unit is configured to introduce a capacitor voltage feedback term into the q-axis control loop of the first transition inverter to obtain a second transition inverter; wherein, the q-axis control loop refers to the second control loop.

[0120] Wherein, the transfer function of the capacitor voltage feedback term is G qad (s), which is essentially a proportional controller and is used to reshape the q-axis low-frequency band impedance of the q-axis control loop; and, G qad (s)=k qad , k qad is the proportional gain. Through G qad (s), the output impedance of the q-axis low-frequency band can be reshaped, and the negative impedance region of the q-axis low-frequency band can be reduced.

[0121] The adjustment unit of this embodiment is used to reshape the low-frequency band impedance of the q-axis. In a flexible interconnected device, changes in impedance characteristics often affect the stability of the system; by adjusting the impedance, the risk of system oscillation can be reduced, and the robustness of the system can be improved.

[0122] An introduction unit is configured to commonly introduce a capacitor current feedforward term into the d-axis control loop and the q-axis control loop based on the second transition inverter to obtain a first inverter; wherein, the first inverter includes a d-axis closed-loop output impedance and a q-axis closed-loop output impedance.

[0123] Wherein, the transfer function G C (s) of the capacitor current feedforward term is composed of a proportional coefficient k c , a filter capacitor value Cs, and a high-pass filter G high (s); and, G C (s)=k c CsG high (s). Through G C (s), the output impedance of the high-frequency band of the inverter can be reshaped, and the negative impedance region of the high-frequency band can be reduced.

[0124] The proportional coefficient k c is used to adjust the influence degree of the capacitor current feedforward term in the control system, the capacitor Cs is used to adjust the output impedance of the inverter; the high-pass filter G high (s) is used to extract the high-frequency components in the capacitor current; and, G high (s)=s / (s + ω h ), s is the complex variable in the Laplace transform, and ω h is the cut-off angular frequency of the high-pass filter.

[0125] Among them, the expression of the d-axis closed-loop output impedance is:

[0126]

[0127] The expression of the q-axis closed-loop output impedance is:

[0128]

[0129] Among them, Y ui (s) is the actual filter circuit model, Q(s) is the low-pass filter, r 1 is the damping coefficient of the passive current control, G feed (s) is the ideal circuit model corresponding to Y ui (s), G del (s) represents the system dynamic delay or disturbance caused by various factors, G C (s) is the transfer function of the capacitor voltage feedforward term, G lp (s) is the low-pass filter, G PLL (s) is the transfer function of the phase-locked loop, G qf (s) is equal to G qad (s)r 1 , U Cd is the steady-state value of the feedforward component of the capacitor voltage on the d-axis, K ad is the proportional controller.

[0130] For the application of the embodiments of the present application, please refer to Figures 4-5 , Figure 4 is the small-signal control block diagram of the inverter based on the improved impedance reshaping method provided by the embodiments of the present application, representing the control relationship after the adjustment of the inverter based on a pair of flexible interconnection devices in this embodiment; Figure 5 is the Bode diagram of the output impedance in different cases provided by the embodiments of the present application, representing the Bode diagram of the output impedance of the inverter after impedance reshaping by the DOPBC method and the present application.

[0131] In Figure 4 , the third part represents the d-axis, and the fourth part represents the q-axis; through Figure 4 , the closed-loop output impedance Z dm (s) of the inverter system on the d-axis and the closed-loop output impedance Z qm (s) on the q-axis can be derived, as shown in the above expressions.

[0132] In Figure 5 , the green dotted line part represents the d-axis Bode diagram, and the red dotted line part represents the q-axis Bode diagram; from Figure 5 it can be seen that after the impedance reshaping of the present application, the passive region of the output impedance of the inverter on the d-axis extends from f d2 -f d3 to fd1 -f d4 region, from f in the passive region of the output impedance on the q-axis q2 -f q3 extends to f q1 -f q4 region. Through capacitive current feedforward, there is no non-passive region in the output impedance in the frequency range of 1 kHz - 8 kHz, which can effectively reduce high-frequency harmonics and even high-frequency oscillation instability generated by the interaction between the inverter and the power grid. After connecting a low-pass filter in series in the proportional controller K ad , the output impedance of the inverter on the d-axis not only reduces the non-passive region in the high-frequency band, but also reduces the non-passive region in the frequency band of 170 Hz - 500 Hz, with significant effects.

[0133] It should be noted that in this application, the adjustment steps of the d-axis control loop and the q-axis control loop are not in a sequential order.

[0134] The present embodiment introduces a unit for reshaping the impedance in the high-frequency band. By jointly introducing a capacitive current feedforward term G C (s) in the d-axis and the q-axis, the output impedance of the inverter in the high-frequency band can be reshaped, and the negative impedance region in the high-frequency band can be reduced;

[0135] Among them, the proportional coefficient, as an adjustment factor, can flexibly adjust the influence degree of the capacitive current feedforward term in the control system, which enables the control system to accurately control the role of the capacitive current feedforward according to different working conditions and requirements, so as to achieve a more refined adjustment effect. By selecting an appropriate capacitance value, the output impedance characteristics of the inverter in a specific frequency range can be optimized, thereby improving the stability and performance of the system. The high-pass filter is used to extract the high-frequency components in the capacitive current, which helps the control system to more accurately sense and respond to high-frequency disturbances.

[0136] In one embodiment, the startup module 30 is specifically:

[0137] After receiving the dispatching instruction, control the flexible interconnection device including the first inverter to start running.

[0138] Overall, the present embodiment has the following beneficial effects:

[0139] In this application, for the d-axis control loop, the capacitor voltage feedforward term in series with the low-pass filter can improve the phase characteristics of the inverter output impedance in the low-frequency band; by introducing feedforward control, the system can respond more quickly to changes in the d-axis current, thereby reducing the phase lag and improving the stability of the system. Moreover, when the grid impedance is large, low-frequency harmonics are likely to appear in the inverter output current. By introducing the capacitor voltage feedforward term, the output of the d-axis control loop can be adjusted in real time, effectively suppressing the generation of these low-frequency harmonics and thus avoiding system instability. For the q-axis control loop, the introduction of the capacitor voltage feedback term helps to enhance the stability of the q-axis control loop; through feedback control, the changes in the q-axis current can be sensed in real time and corresponding adjustments can be made, thereby improving the robustness of the system. Moreover, the capacitor voltage feedback term can also optimize the output impedance characteristics of the inverter. By adjusting the parameters of the feedback control, the characteristics of the output impedance in different frequency bands can be further improved, especially in the frequency bands related to the q-axis, thereby improving the overall performance of the system. The introduction of the capacitor current feedforward term in the d-axis and q-axis control loops also helps to suppress the high-frequency harmonics of the grid-side current caused by changes in the grid impedance.

[0140] In summary, through the improved impedance reshaping method, this application reshapes the impedance of the inverter system of the flexible interconnection device in the low-frequency and high-frequency bands while ensuring the robustness and steady-state performance of the traditional passive current control, expanding the adaptability range of the inverter system to the grid impedance, so that the inverter can still operate stably under the scenario of grid impedance change. The inverter and the rectifier are a strongly coupled series system. When the stable range of the inverter is increased, the stability of the entire FID system is correspondingly improved. In addition, this solution is applicable to most grid-connected converter devices and has generality and universality.

[0141] Embodiment 3:

[0142] The embodiment of this application provides a computer-readable storage medium, which includes a stored computer program. When the computer program runs, it controls the device where the computer-readable storage medium is located to execute the method for improving the stability of a flexible interconnection device.

[0143] Among them, for the method for improving the stability of the flexible interconnection device, when it is implemented in the form of a software functional unit and used as an independent product, it can be stored in a computer-readable storage medium. Based on such an understanding, to implement all or part of the processes in the above-mentioned embodiment methods of the present invention, it can also be completed by instructing relevant hardware through a computer program. The computer program can be stored in a computer-readable storage medium. When the computer program is executed by a processor, the steps of the above-mentioned various method embodiments can be implemented. Among them, the computer program includes computer program code, and the computer program code can be in the form of source code, object code, executable file or some intermediate form, etc. The computer-readable medium can include: any entity or device capable of carrying the computer program code, recording medium, USB flash drive, mobile hard disk, magnetic disk, optical disk, computer memory, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), electrical carrier signal, telecommunication signal, and software distribution medium, etc.

[0144] The above are the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art in the technical field, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements are also regarded as the protection scope of the present invention.

Claims

1. A method for improving the stability of a flexible interconnection device, characterized in that: include: Acquire a flexible interconnection device; wherein the flexible interconnection device includes an inverter; Introducing a capacitor voltage feedforward term of a series low-pass filter into a first control loop of the inverter, introducing a capacitor voltage feedback term into a second control loop of the inverter, and introducing a capacitor current feedforward term into both the first control loop and the second control loop to obtain a first inverter; The flexible interconnection device including the first inverter is controlled to start running.

2. A method for improving the stability of a flexible interconnection device according to claim 1, characterized in that: The capacitor voltage feedforward term of the series low-pass filter is introduced into the first control loop of the inverter, specifically: In the first control loop of the inverter, a feedforward component of the capacitor on the d-axis is introduced into the output side of the first control loop through a proportional controller; A low-pass filter is connected in series to the output end of the proportional controller; The value of the proportional controller is a negative real number, and the feedforward component is used to reshape the low-frequency impedance of the first control loop.

3. The method for improving the stability of a flexible interconnection device according to claim 1, characterized in that: The capacitor current feedforward term is composed of a proportionality coefficient, a capacitor and a high-pass filter; The proportionality coefficient is used to adjust the influence of the capacitor current feedforward term in the control system, the capacitor is used to adjust the output impedance of the inverter, and the high-pass filter is used to extract high-frequency components in the capacitor current.

4. The method for improving the stability of a flexible interconnection device according to claim 1, characterized in that: The capacitor voltage feedback term is a proportional controller; The capacitor voltage feedback term is used to reshape the low-frequency impedance of the second control loop.

5. A method for improving the stability of a flexible interconnection device according to any one of claims 1 to 4, characterized in that: The first inverter includes a d-axis closed-loop output impedance and a q-axis closed-loop output impedance; Wherein, the expression of the d-axis closed-loop output impedance is: Among them, Y ui (s) is the actual filter circuit model, Q(s) is the low-pass filter, r1 is the damping coefficient of the passive current control, G feed (s) corresponds to Y ui (s) is the ideal circuit model, G del (s) represents the system dynamic delay or disturbance caused by various factors, G C (s) is the transfer function of the capacitor voltage feedforward term, G lp (s) is a low-pass filter, K ad It is a proportional controller.

6. A device for improving the stability of a flexible interconnection device, characterized in that: Includes acquisition module, adjustment module and startup module; Wherein, the acquisition module is used to acquire a flexible interconnection device; wherein, the flexible interconnection device includes an inverter; The adjustment module is used to introduce the capacitor voltage feedforward term of the series low-pass filter into the first control loop of the inverter, introduce the capacitor voltage feedback term into the second control loop of the inverter, and introduce the capacitor current feedforward term into the first control loop and the second control loop to obtain a first inverter; The starting module is used to control the flexible interconnection device including the first inverter to start running.

7. The device for improving the stability of a flexible interconnection device according to claim 6, characterized in that: The adjustment module includes a component unit and a filter unit; The component unit is used to introduce the feedforward component of the capacitor on the d-axis into the output side of the first control loop through a proportional controller in the first control loop of the inverter; The filtering unit is used to connect a low-pass filter in series at the output end of the proportional controller; The value of the proportional controller is a negative real number, and the feedforward component is used to reshape the low-frequency impedance of the first control loop.

8. The device for improving the stability of a flexible interconnection device according to claim 6, characterized in that: The capacitor current feedforward term is composed of a proportionality coefficient, a capacitor and a high-pass filter; The proportionality coefficient is used to adjust the influence of the capacitor current feedforward term in the control system, the capacitor is used to adjust the output impedance of the inverter, and the high-pass filter is used to extract high-frequency components in the capacitor current.

9. A device for improving the stability of a flexible interconnection device according to any one of claims 6 to 8, characterized in that: The first inverter includes a d-axis closed-loop output impedance and a q-axis closed-loop output impedance; Wherein, the expression of the d-axis closed-loop output impedance is: Among them, Y ui (s) is the actual filter circuit model, Q(s) is the low-pass filter, r1 is the damping coefficient of the passive current control, G feed (s) corresponds to Y ui (s), Gdel(s) represents the system dynamic delay or disturbance caused by various factors, G C (s) is the transfer function of the capacitor voltage feedforward term, G lp (s) is a low-pass filter, K ad It is a proportional controller.

10. A storage medium, characterized in that: The storage medium stores a computer program, which is called and executed by a computer to implement a method for improving the stability of a flexible interconnection device as described in any one of claims 1 to 5 above.

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