Oscillation suppression control method for network-forming flexible direct-current power transmission system

Abstract

The invention relates to the technical field of network construction type flexible direct-current power transmission system control, in particular to an oscillation suppression control method for a network construction type flexible direct-current power transmission system. The method comprises the following steps of: firstly, establishing a PI controller of a rectifier-side converter through reference voltage of the rectifier-side converter; then, based on virtual synchronous motor control, constructing a mathematical model of a network-forming controller, extracting an angular frequency error and an angular frequency change rate, and designing a power deviation and an adaptive adjustment rule to realize dynamic optimization of control parameters; and on the basis, generating a reference voltage of the alternating-current side network-building type inverter through the final network-building type controller, and further establishing a PI controller of the alternating-current side network-building type inverter. And finally, in combination with a rectification side PI controller, a network-building type controller and an inversion side PI controller, the stable control and oscillation suppression of the network-building type flexible direct current power transmission system are comprehensively realized.

Description

Technical Field

[0001] The present invention relates to the technical field of control of a grid-type flexible direct current power transmission system, and in particular to an oscillation suppression control method for a grid-type flexible direct current power transmission system. Background Art

[0002] In modern power systems, grid-connected flexible direct current transmission systems, as an advanced transmission method, have become one of the key technologies to meet the large-scale grid connection needs of renewable energy with their high reliability, flexibility and adaptability. With the rapid development of renewable energy such as wind power and photovoltaics, the dispatching needs of power grids are becoming increasingly complex. Traditional AC transmission technology is limited by power loss and stability issues in long-distance transmission and is difficult to meet the development requirements of modern power systems.

[0003] Flexible DC transmission technology is based on the fast switching characteristics of power electronic devices. By controlling the working state of the converter, it can achieve precise control of power flow and dynamic reactive power compensation. This technology not only significantly improves the efficiency of power transmission, but also enhances the stability and anti-interference ability of the power grid to a certain extent. In addition, the flexible DC transmission system can quickly respond to the power demand of the power grid, especially when dealing with frequent power fluctuations, showing excellent dynamic performance.

[0004] In the grid-type flexible DC transmission system, due to the inherent dynamic characteristics of power electronic equipment and complex mesh topology, the system is prone to power oscillation when it is subject to external interference such as small signal disturbances, load mutations or line faults. If these power oscillations are not suppressed in a timely and effective manner, they may cause system frequency instability, voltage fluctuations, and even cause large-scale chain failures, endangering the overall safety of the power grid.

[0005] In this context, oscillation suppression of grid-type flexible DC transmission systems has become an important technical challenge. To this end, an oscillation suppression method for grid-type flexible DC transmission systems is proposed, which aims to achieve real-time suppression of power oscillations by establishing a nonlinear grid-type controller combined with a control strategy based on a virtual synchronous machine and an adaptive adjustment rule, thereby ensuring the reliable operation and stable power transmission of the grid-type flexible DC transmission system. Summary of the invention

[0006] The present invention provides an oscillation suppression control method for a grid-type flexible direct current transmission system, which is used to solve the above technical problems. First, a grid-type flexible direct current transmission system includes a rectifier-side converter and an AC-side grid-type inverter. Given the reference voltage of the rectifier-side converter, a rectifier-side converter PI controller is established; secondly, based on virtual synchronous motor control, a grid-type controller mathematical model of the grid-type flexible direct current transmission system is established, and an angular frequency error and an angular frequency change rate are established according to the mathematical model; then, a power deviation is designed based on the angular frequency error; according to the angular frequency error and the angular frequency error change rate, a corresponding adaptive adjustment rule is designed; then, a final grid-type controller is obtained according to the controller and the adaptive adjustment rule, and a reference voltage of the AC-side grid-type inverter is calculated; further, based on the reference voltage of the AC-side grid-type inverter, an AC-side grid-type inverter PI controller is established; finally, the rectifier-side converter PI controller, the final grid-type controller and the AC-side grid-type inverter PI controller are integrated to achieve control and oscillation suppression of the grid-type flexible direct current transmission system.

[0007] An oscillation suppression control method for a grid-type flexible direct current transmission system comprises the following steps:

[0008] S1, a grid-type flexible direct current transmission system, including a rectifier-side converter and an AC-side grid-type inverter, a reference voltage of the rectifier-side converter is given, and a PI controller of the rectifier-side converter is established;

[0009] S2. Based on virtual synchronous motor control, a mathematical model of a grid-forming controller of the grid-forming flexible direct current transmission system is established, and an angular frequency error and an angular frequency change rate are obtained according to the mathematical model;

[0010] S3, designing a power deviation based on the angular frequency error; designing a corresponding adaptive adjustment rule according to the angular frequency error and the angular frequency error change rate;

[0011] S4, obtaining a final grid-type controller according to the rectifier-side converter PI controller and the adaptive adjustment rule, and calculating a reference voltage of the AC-side grid-type inverter;

[0012] S5. Establishing an AC side grid-type inverter PI controller based on the reference voltage of the AC side grid-type inverter;

[0013] S6, integrating the rectifier-side converter PI controller, the final grid-forming controller and the AC-side grid-forming inverter PI controller, to control oscillation suppression of the grid-forming flexible direct current transmission system.

[0014] Further, given the reference voltage of the rectifier-side converter, a PI controller of the rectifier-side converter is established, specifically:

[0015]

[0016] Among them, k 11 , k 12 , k 21 , k 22 , k 31 , k 32 , k 41 and k 42 is the controller parameter, and are the reference currents of the d-axis and q-axis of the rectifier-side converter respectively, and are the reference voltages of the d-axis and q-axis of the rectifier-side converter, u od and u oq are the actual voltages of the d-axis and q-axis of the rectifier-side converter, respectively, cd and i cq are the actual currents of the rectifier-side converter on the d-axis and q-axis, respectively, and u cd and u cq are the modulation signals of the d-axis and q-axis of the rectifier-side converter respectively, They are u od , u oq , i cd , i cq The first derivative of .

[0017] Furthermore, based on the virtual synchronous motor control, a mathematical model of the grid-type controller of the grid-type flexible direct current transmission system is established; the mathematical model of the grid-type controller is:

[0018]

[0019] Where ω is the mechanical angular frequency, θ is the voltage source phase angle, V is the output voltage amplitude, J is the moment of inertia, P ref is the active power reference value, P Δf is the power deviation, P e is the output active power, D p is the damping coefficient, ω0 is the rated angular frequency, U ref is the rated voltage, K Q is the integral gain coefficient, Q ref is the reference value of reactive power, Q e is the output reactive power, and t is the time.

[0020] Furthermore, the angular frequency error and the angular frequency change rate are established according to the mathematical model of the meshed controller; the angular frequency error is e ω =ω-ω0, the angular frequency change rate is

[0021] Furthermore, based on the super-helical sliding mode control strategy, the power deviation is designed using the angular frequency error as follows:

[0022]

[0023] Among them, tanh(e ω ) is the hyperbolic tangent function, expressed as:

[0024]

[0025] Among them, λ1 and λ2 are power deviation parameters, ω is the mechanical angular frequency, P ref is the active power reference value, P Δf is the power deviation, P e is the output active power, D p is the damping coefficient, ω0 is the rated angular frequency, e ω is the angular frequency error, and t is time.

[0026] Further, the adaptive adjustment rule is designed according to the angular frequency error and the angular frequency change rate;

[0027] The adaptive adjustment rule is:

[0028]

[0029] Among them, J w and D w is the steady-state interval threshold, k1, k2 and k3 are adaptive adjustment parameters, J0 is the basic value of J, D0 is the basic value of D p The base value of .

[0030] Further, the final grid-type controller is obtained according to the power deviation and the adaptive adjustment rule, the real-time value of the moment of inertia and the real-time value of the damping coefficient are adjusted by using the adaptive adjustment rule, the power deviation is dynamically adjusted according to the real-time value of the moment of inertia and the real-time value of the damping coefficient, and the final grid-type controller is obtained; according to the final grid-type controller, the voltage source phase angle θ and the output voltage amplitude V are calculated, and the reference voltage of the AC side grid-type inverter is calculated and

[0031] Further, according to the reference voltage of the AC side grid-type inverter and The PI controller of the AC side grid-forming inverter is established, specifically:

[0032]

[0033]

[0034] Among them, k 51 , k 52 , k 61 , k 62 , k 71 , k 72 , k 81 and k 82 are the PI controller parameters of the AC side grid-connected inverter, and are the reference currents of the d-axis and q-axis of the AC-side grid-forming inverter, respectively, and are the reference voltages of the d-axis and q-axis of the AC-side grid-connected inverter, u od and u oq are the actual voltages of the d-axis and q-axis of the AC-side grid-connected inverter, i cd and i cq are the actual currents of the d-axis and q-axis of the AC-side grid-connected inverter, u gd1 and u gq1 are the modulation signals of the d-axis and q-axis of the rectifier-side converter respectively, They are u gd , u gq , i gd , i gq The first derivative of ;

[0035] The rectifier-side converter PI controller, the final grid-forming controller and the AC-side grid-forming inverter PI controller are integrated to achieve control and oscillation suppression of the grid-forming flexible direct current transmission system.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1. The super-helical sliding mode control strategy is applied to the virtual synchronous motor control. This control strategy reduces the system oscillation amplitude caused by load changes or external disturbances by quickly responding to the system dynamic characteristics, and improves the control system's ability to suppress oscillation by optimizing the switching logic.

[0038] 2. Under the virtual synchronous motor control framework, key parameters such as moment of inertia and damping coefficient are dynamically optimized through adaptive adjustment rules. The system can automatically adjust according to different working conditions to ensure real-time adjustment of control parameters when the load changes rapidly or the external grid conditions fluctuate, effectively suppressing power and frequency oscillations and improving the stability and responsiveness of the system.

[0039] 3. The three strategies of super-helical sliding mode control, adaptive rule adjustment and PI control are organically combined to form a multi-level oscillation suppression mechanism. Super-helical sliding mode control has advantages in dynamic response and oscillation suppression, adaptive rules further suppress power and frequency oscillations, and PI control stabilizes the steady-state operation of the system. These three control strategies work together to ensure the smooth operation of the grid-type flexible DC transmission system under various working conditions and effectively suppress system oscillations. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a flow chart of an oscillation suppression control method for a grid-type flexible direct current transmission system proposed by the present invention;

[0041] Figure 2 It is an oscillation suppression control block diagram of a grid-type flexible direct current transmission system proposed by the present invention;

[0042] Figure 3 It is a flow chart of the algorithm of the adaptive adjustment rule of the moment of inertia J proposed by the present invention;

[0043] Figure 4 is the damping coefficient D proposed by the present invention p Flowchart of the adaptive adjustment rule algorithm;

[0044] Figure 5 is a control performance comparison diagram of a grid-type flexible direct current transmission system when the active power reference value changes in steps, provided in the first embodiment of the present invention;

[0045] Figure 6 It is a comparison diagram of the control performance of the grid-forming flexible direct current transmission system when the grid strength changes provided in the second embodiment of the present invention. DETAILED DESCRIPTION

[0046] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0047] In the management of modern power systems, the stability and dynamic response performance of the grid-type flexible direct current transmission system play a key role in the overall operating efficiency and reliability of the system. Traditional control technology can meet the needs in some simple applications, but it is difficult to adapt to the frequently changing system requirements in complex power scenarios such as high proportion of renewable energy grid connection and rapid changes in power demand. To this end, the present invention proposes an oscillation suppression control method for a grid-type flexible direct current transmission system. The method achieves optimization in dynamic response and steady-state accuracy by integrating three control strategies: super-helical sliding mode control, adaptive rule adjustment and PI control. Super-helical sliding mode control is used to quickly respond to system changes and effectively suppress oscillations, adaptive rule adjustment optimizes key control parameters in real time to cope with changes under complex working conditions, and PI control maintains system stability in steady-state operation. Through the synergistic effect of the three, the method can ensure the stability of the system in a changing power environment and effectively suppress oscillation problems caused by unstable factors.

[0048] Embodiment 1

[0049] In the embodiment of the present application, an oscillation suppression method for a grid-type flexible DC transmission system is adopted; the specific implementation process is as follows Figure 1 As shown, first, a grid-type flexible direct current transmission system includes a rectifier-side converter and an AC-side grid-type inverter. Given a reference voltage of the rectifier-side converter, a PI controller of the rectifier-side converter is established; secondly, based on virtual synchronous motor control, a mathematical model of the grid-type controller of the grid-type flexible direct current transmission system is established, and an angular frequency error and an angular frequency change rate are established according to the mathematical model; then, a power deviation is designed based on the angular frequency error; according to the angular frequency error and the angular frequency error change rate, a corresponding adaptive adjustment rule is designed; subsequently, a final grid-type controller is obtained according to the controller and the adaptive adjustment rule, and a reference voltage of the AC-side grid-type inverter is calculated; further, based on the reference voltage of the AC-side grid-type inverter, an AC-side grid-type inverter PI controller is established; finally, the rectifier-side converter PI controller, the final grid-type controller and the AC-side grid-type inverter PI controller are integrated to realize control and oscillation suppression of the grid-type flexible direct current transmission system. This improves the stability of the grid-type flexible DC transmission system and effectively suppresses oscillations caused by changes in system parameters or external interference.

[0050] The control block diagram of the grid-type flexible DC transmission system is as follows: Figure 2 As shown in the figure, the control system receives three-phase AC power from the power grid, converts the components under the abc coordinates into the dq components under the rotating coordinate system through abc / dq coordinate transformation, and then calculates the active power P and reactive power Q. Through power feedback control, the active power and reactive power are respectively compared with the reference value P * and Q* Compare and generate power deviation P Δf ; In the power regulation process, the system uses the feedback regulation mechanism of the angular frequency ω, compares the angular frequency ω with ω0 to obtain the angular frequency error and its rate of change, and introduces adaptive adjustment rules to automatically adjust the moment of inertia J and the damping coefficient D p , in order to optimize the dynamic response characteristics of the system and suppress oscillation. Through adjustment, the system generates a corrected voltage amplitude V and a reference phase angle signal θ, which are input into the PI controller. After the PI controller corrects the reference voltage, the corrected signal is restored to the abc coordinate system through the dq / abc inverse transformation, and the modulation signal of the converter is generated through the SVPWM module to control its output power, and finally achieve stable operation of the entire system.

[0051] In the embodiment of the present application, the superiority of the method of the present invention in the grid-type flexible DC transmission system is verified for the typical operation scenario of the transmission power step change. ref =100MW; at t=0.5s, the active power reference value increases stepwise to P ref =300MW; at t=1.5s, the active power reference value is restored to P ref =100MW.

[0052] Furthermore, the specific implementation process of the embodiment of the present application is as follows:

[0053] Given a reference voltage of the rectifier-side converter, a PI controller of the rectifier-side converter is established, specifically:

[0054]

[0055] Among them, k 11 , k 12 , k 21 , k 22 , k 31 , k 32 , k 41 and k 42 is the controller parameter, which is an empirical parameter. and are the reference currents of the d-axis and q-axis of the rectifier-side converter respectively, and are the reference voltages of the d-axis and q-axis of the rectifier-side converter respectively, and Artificially given, u od and u oq are the actual voltages of the d-axis and q-axis of the rectifier-side converter, respectively, cd and i cqare the actual currents of the rectifier-side converter on the d-axis and q-axis, respectively, and u cd and u cq are the modulation signals of the d-axis and q-axis of the rectifier-side converter respectively, They are u od , u oq , i cd , i cq The first derivative of .

[0056] In the embodiment of the present application, the real-time performance of the control system is improved by the PI controller, so that the rectifier-side converter can operate smoothly under complex working conditions. In addition, the method reduces oscillations during the transition process and avoids stability problems caused by parameter mismatch, thereby improving the stability and reliability of the overall system.

[0057] Furthermore, based on the virtual synchronous motor control, a mathematical model of the grid-type controller of the grid-type flexible direct current transmission system is established; the mathematical model of the grid-type controller is:

[0058]

[0059] Where ω is the mechanical angular frequency, θ is the voltage source phase angle, V is the output voltage amplitude, J is the moment of inertia, P ref is the active power reference value, P Δf is the power deviation, P e is the output active power, D p is the damping coefficient, ω0 is the rated angular frequency, U ref is the rated voltage, D Q is the integral gain coefficient, Q ref is the reference value of reactive power, Q e is the output reactive power, and t is the time.

[0060] In the embodiment of the present application, the mathematical model of the grid-forming controller of the grid-forming flexible direct current transmission system is established based on the virtual synchronous motor control principle, which can realize more flexible and stable power scheduling and control. The introduction of virtual synchronous motor control technology enables the flexible direct current transmission system to have inertia characteristics similar to those of traditional synchronous generators, enhances the disturbance suppression capability of the power grid, and improves the reliability and flexibility of the system. Especially in the environment of large-scale renewable energy access, it can effectively balance the power demand and supply of the system and improve the energy utilization efficiency and stability of the system.

[0061] Furthermore, the angular frequency error and the angular frequency change rate are established according to the mathematical model of the meshed controller; the angular frequency error is eω =ω-ω0, the angular frequency change rate is

[0062] In the embodiment of the present application, the angular frequency error and the angular frequency change rate are established based on the mathematical model to more accurately reflect the dynamic performance and control effect of the system. The angular frequency error can monitor the frequency deviation of the system in a timely manner by comparing it with the difference of the rated angular frequency, thereby providing real-time feedback for power scheduling and system stability optimization. The angular frequency change rate reflects the changing trend of the system frequency fluctuation, which can assist in adjusting the control strategy to ensure that the system frequency change is within a controllable range.

[0063] Furthermore, the power deviation is designed based on the angular frequency error as follows:

[0064]

[0065] Among them, tanh(e ω ) is the hyperbolic tangent function, expressed as:

[0066]

[0067] Among them, λ1 and λ2 are power deviation parameters, which are empirical parameters, ω is the mechanical angular frequency, P ref is the active power reference value, P Δf is the power deviation, P e is the output active power, D p is the damping coefficient, ω0 is the rated angular frequency, e ω is the angular frequency error, and t is time.

[0068] In an embodiment of the present application, a super-helical sliding mode control strategy is applied to virtual synchronous motor control. This control strategy reduces the system oscillation amplitude caused by load changes or external disturbances by quickly responding to the dynamic characteristics of the system, and improves the control system's ability to suppress oscillations by optimizing the switching logic.

[0069] Further, according to the angular frequency error and the angular frequency change rate, an adaptive adjustment rule is designed;

[0070] When designing adaptive adjustment rules based on angular frequency error and angular frequency change rate, it is necessary to fully consider the dynamic characteristics of the system to achieve dynamic optimization of the moment of inertia and damping coefficient, thereby improving system performance. In terms of dynamic characteristics, when the moment of inertia and damping coefficient increase, the system active power output will be more stable, but the response speed may slow down; when both decrease, the response speed will increase, but it may cause oscillation. Therefore, it is necessary to find a balance between steady state and stability. In addition, an increase in the moment of inertia will prolong the peak time, while a decrease will shorten the peak time; an increase in the damping coefficient will shorten the stabilization time, while a decrease will prolong the stabilization time. At the same time, properly adjusting the damping coefficient can effectively reduce the overshoot, but excessive damping may cause response hysteresis, while too small damping may cause oscillation.

[0071] The adaptive adjustment rules are divided into four intervals according to the different states of angular frequency error and rate of change. For the case where the angular frequency error is large and the rate of change is large, in order to suppress the rapid change of the angular frequency, it is necessary to increase the moment of inertia and the damping coefficient at the same time, so as to smooth the system response and stabilize the operation. When the angular frequency error is small but the rate of change is large, the moment of inertia should be reduced to increase the response speed, and the error fluctuation should be quickly weakened by increasing the damping coefficient. When the angular frequency error is large but the rate of change is small, in order to prevent error accumulation and system instability, the moment of inertia and the damping coefficient should be increased to enhance the system's anti-interference ability. When the angular frequency error and the rate of change are both small, in order to improve the response speed and quickly reach a steady state, the moment of inertia can be appropriately reduced and the damping coefficient can be increased.

[0072] Based on the above analysis, the adaptive adjustment rule is designed as follows:

[0073]

[0074] Among them, J w and D w is the steady-state interval threshold, k1, k2 and k3 are adaptive adjustment parameters, which are empirical parameters, J0 is the basic value of J, and D0 is the basic value of D p The basic value of the moment of inertia J is shown in the flowchart of the adaptive adjustment rule algorithm. Figure 3 As shown, the damping coefficient D p The adaptive adjustment rule algorithm flow chart is as follows Figure 4 As shown;

[0075] The steady-state interval threshold and the adaptive adjustment parameter can be set according to simulation experiments; the basic value can be selected according to the following formula:

[0076]

[0077] Among them, T m is the mechanical torque, T e is the electromagnetic torque, Pmax is the maximum active power.

[0078] In the embodiment of the present application, key parameters such as the moment of inertia and the damping coefficient are dynamically optimized through adaptive adjustment rules. The system can automatically adjust according to different working conditions to ensure that the control parameters are adjusted in real time when the load changes rapidly or the external grid conditions fluctuate, thereby effectively suppressing power and frequency oscillations and improving the stability and responsiveness of the system.

[0079] Furthermore, the real-time value of the moment of inertia and the real-time value of the damping coefficient are adjusted by using the adaptive adjustment rule, and the power deviation is dynamically adjusted according to the real-time value of the moment of inertia and the real-time value of the damping coefficient to obtain the final grid-type controller; according to the final grid-type controller, the voltage source phase angle θ and the output voltage amplitude V are calculated to calculate the reference voltage of the AC side grid-type inverter and

[0080] In the embodiment of the present application, by combining power deviation and adaptive adjustment rules, an optimized version of the grid-type controller is finally obtained. By adjusting the moment of inertia and damping coefficient in real time, it is possible to optimize power distribution and frequency regulation when the power system load fluctuates and external disturbances occur, thereby ensuring the stability and reliability of system operation. In addition, the dynamic adjustment of power deviation further improves the power tracking accuracy and stability of the system, reduces frequency fluctuations and overshoot, and effectively ensures the stability and response speed of the system.

[0081] Further, according to the reference voltage of the AC side grid-type inverter and The PI controller of the AC side grid-forming inverter is established, specifically:

[0082]

[0083] Among them, k 51 , k 52 , k 61 , k 62 , k 71 , k 72 , k 81 and k 82 is the controller parameter, which is an empirical parameter. and are the reference currents of the d-axis and q-axis of the AC-side grid-forming inverter, respectively, and are the reference voltages of the d-axis and q-axis of the AC-side grid-connected inverter, u od and u oq are the actual voltages of the d-axis and q-axis of the AC-side grid-connected inverter, icd and i cq are the actual currents of the d-axis and q-axis of the AC-side grid-connected inverter, u gd1 and u gq1 are the modulation signals of the d-axis and q-axis of the rectifier-side converter respectively, They are u gd , u gq , i gd , i gq The first derivative of ;

[0084] The rectifier-side converter PI controller, the final grid-forming controller and the AC-side grid-forming inverter PI controller are integrated to achieve control and oscillation suppression of the grid-forming flexible direct current transmission system.

[0085] In the embodiment of the present application, the three strategies of super-helical sliding mode control, adaptive rule adjustment and PI control are organically combined to form a multi-level oscillation suppression mechanism. Super-helical sliding mode control has advantages in dynamic response and oscillation suppression, adaptive rules further suppress power and frequency oscillations, and PI control stabilizes the steady-state operation of the system. These three control strategies work together to ensure the smooth operation of the grid-type flexible direct current transmission system under various working conditions and effectively suppress system oscillations.

[0086] Specifically, through MATLAB / Simulink simulation, the control effect can be obtained as follows Figure 5 shown. Figure 5 The control performance comparison results of the grid-type flexible DC transmission system when the active power reference value changes in steps. The horizontal axis represents time, and the vertical axis is the active power change curve. In the figure, the solid line represents the tracking result of the grid-type controller being a PI controller under the droop control method; the dotted line represents the tracking result of the grid-type controller being a traditional sliding mode controller under the virtual synchronous motor control method; the dotted line represents the tracking result under the control method proposed in the present invention. Figure 5 This shows that the control law designed in this paper can stably track the desired active power and effectively suppress oscillations.

[0087] Embodiment 2

[0088] In the embodiment of the present application, the superiority of the method of the present invention in the grid-type flexible DC transmission system is verified for the operation scenario when the grid strength changes. At t = 0 to 0.5s, the short-circuit ratio of the receiving-end grid is 1.4, and the grid is weak; at t = 0.5s, the short-circuit ratio of the receiving-end grid is 4.1, and the grid becomes weak; at t = 1.5s, the short-circuit ratio of the receiving-end grid is restored to 1.4.

[0089] The specific implementation process of the embodiment of the present application is as follows:

[0090] Given the reference voltage of the rectifier-side converter, a PI controller of the rectifier-side converter is established, specifically:

[0091]

[0092] Among them, k 11 , k 12 , k 21 , k 22 , k 31 , k 32 , k 41 and k 42 is the controller parameter, which is an empirical parameter. and are the reference currents of the d-axis and q-axis of the rectifier-side converter respectively, and are the reference voltages of the d-axis and q-axis of the rectifier-side converter respectively, and Artificially given, u od and u oq are the actual voltages of the d-axis and q-axis of the rectifier-side converter, respectively, cd and i cq are the actual currents of the rectifier-side converter on the d-axis and q-axis, respectively, and u cd and u cq are the modulation signals of the d-axis and q-axis of the rectifier-side converter respectively, They are u od , u oq , i cd , i cq The first derivative of .

[0093] Furthermore, based on the virtual synchronous motor control, a mathematical model of a grid-type controller of the grid-type flexible DC transmission system is established; the mathematical model of the grid-type controller is:

[0094]

[0095] Where ω is the mechanical angular frequency, θ is the voltage source phase angle, V is the output voltage amplitude, J is the moment of inertia, P ref is the active power reference value, P Δf is the power deviation, P e is the output active power, D p is the damping coefficient, ω0 is the rated angular frequency, U refis the rated voltage, K Q is the integral gain coefficient, Q ref is the reference value of reactive power, Q e is the output reactive power, and t is the time.

[0096] Furthermore, an angular frequency error and an angular frequency change rate are established according to the mathematical model of the meshed controller; the angular frequency error is e ω =ω-ω0, the angular frequency change rate is

[0097] Furthermore, the power deviation is designed based on the angular frequency error as follows:

[0098]

[0099] Among them, tanh(e ω ) is the hyperbolic tangent function, expressed as:

[0100]

[0101] Among them, λ1 and λ2 are power deviation parameters, which are empirical parameters, ω is the mechanical angular frequency, P ref is the active power reference value, P Δf is the power deviation, P e is the output active power, D p is the damping coefficient, ω0 is the rated angular frequency, e ω is the angular frequency error, and t is time.

[0102] Furthermore, according to the angular frequency error and the angular frequency change rate, an adaptive adjustment rule is designed as follows:

[0103]

[0104] Among them, J w and D w is the steady-state interval threshold, k1, k2 and k3 are adaptive adjustment parameters, which are empirical parameters, J0 is the basic value of J, and D0 is the basic value of D p The basic value of the moment of inertia J is shown in the flowchart of the adaptive adjustment rule algorithm. Figure 3 As shown, the damping coefficient D p The adaptive adjustment rule algorithm flow chart is as follows Figure 4 As shown;

[0105] The steady-state interval threshold and the adaptive adjustment parameter can be set according to simulation experiments; the basic value can be selected according to the following formula:

[0106]

[0107] Among them, Tm is the mechanical torque, T e is the electromagnetic torque, P max is the maximum active power.

[0108] Furthermore, a final grid-type controller is obtained according to the power deviation and the adaptive adjustment rule, and the real-time value of the moment of inertia and the real-time value of the damping coefficient are adjusted by using the adaptive adjustment rule, and then the power deviation is dynamically adjusted according to the real-time value of the moment of inertia and the real-time value of the damping coefficient to obtain the final grid-type controller; according to the final grid-type controller, the voltage source phase angle θ and the output voltage amplitude V are calculated to calculate the reference voltage of the AC side grid-type inverter and

[0109] Further, according to the reference voltage of the AC side grid-type inverter and The PI controller of the AC side grid-forming inverter is established, specifically:

[0110]

[0111]

[0112] Among them, k 51 , k 52 , k 61 , k 62 , k 71 , k 72 , k 81 and k 82 is the PI controller parameter of the AC side grid-type inverter, which is an empirical parameter. and are the reference currents of the d-axis and q-axis of the AC-side grid-forming inverter, respectively, and are the reference voltages of the d-axis and q-axis of the AC-side grid-connected inverter, u od and u oq are the actual voltages of the d-axis and q-axis of the AC-side grid-connected inverter, i cd and i cq are the actual currents of the d-axis and q-axis of the AC-side grid-connected inverter, u gd1 and u gq1 are the modulation signals of the d-axis and q-axis of the rectifier-side converter respectively, They are u gd , u gq , i gd , i gq The first derivative of ;

[0113] The rectifier-side converter PI controller, the final grid-forming controller and the AC-side grid-forming inverter PI controller are integrated to achieve control and oscillation suppression of the grid-forming flexible direct current transmission system.

[0114] Specifically, through MATLAB / Simulink simulation, the control effect can be obtained as follows Figure 6 shown. Figure 6 The control performance comparison results of the grid-type flexible DC transmission system when the grid strength changes. The horizontal axis represents time, and the vertical axis represents the active power change curve. In the figure, the solid line represents the tracking result of the grid-type controller being a PI controller under the droop control method; the dotted line represents the tracking result of the grid-type controller being a traditional sliding mode controller under the virtual synchronous motor control method; the dotted line represents the tracking result under the control method proposed in the present invention. Figure 6 This shows that the control law designed in the present invention effectively suppresses the oscillation caused by the change of power grid strength.

[0115] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. An oscillation suppression control method for a grid-type flexible direct current transmission system, characterized in that: The following steps are involved: S1, a grid-type flexible direct current transmission system, including a rectifier-side converter and an AC-side grid-type inverter, a reference voltage of the rectifier-side converter is given, and a PI controller of the rectifier-side converter is established; S2. Based on virtual synchronous motor control, a mathematical model of a grid-type controller of the grid-type flexible direct current transmission system is established, and an angular frequency error and an angular frequency change rate are obtained according to the mathematical model of the grid-type controller; S3, based on the super-helical sliding mode control strategy, using the angular frequency error to design the power deviation, and according to the angular frequency error and the angular frequency error change rate, designing a corresponding adaptive adjustment rule; S4, obtaining a final grid-type controller according to the rectifier-side converter PI controller and the adaptive adjustment rule, and calculating a reference voltage of the AC-side grid-type inverter; S5. Establishing an AC side grid-type inverter PI controller based on the reference voltage of the AC side grid-type inverter; S6, the integrated rectifier side converter PI controller, the final grid-forming controller and the AC side grid-forming inverter PI controller, control and suppress oscillation of the grid-forming flexible direct current transmission system.

2. The oscillation suppression control method of a grid-type flexible direct current transmission system according to claim 1, characterized in that: Given a reference voltage of the rectifier-side converter, a PI controller of the rectifier-side converter is established, specifically: Among them, k 11 , k 12 , k 21 , k 22 , k 31 , k 32 , k 41 and k 42 is the controller parameter, and are the reference currents of the d-axis and q-axis of the rectifier-side converter respectively, and are the reference voltages of the d-axis and q-axis of the rectifier-side converter, u od and u oq are the actual voltages of the d-axis and q-axis of the rectifier-side converter, respectively, cd and i cq are the actual currents of the rectifier-side converter on the d-axis and q-axis, respectively, and u cd and u cq are the modulation signals of the d-axis and q-axis of the rectifier-side converter respectively, They are u od , u oq , i cd , i cq The first derivative of .

3. The oscillation suppression control method of a grid-type flexible direct current transmission system according to claim 1, characterized in that: Based on virtual synchronous motor control, a mathematical model of the grid-type controller of the grid-type flexible DC transmission system is established; the mathematical model of the grid-type controller is: Where ω is the mechanical angular frequency, θ is the voltage source phase angle, V is the output voltage amplitude, J is the moment of inertia, P ref is the active power reference value, P Δf is the power deviation, P e is the output active power, D p is the damping coefficient, ω0 is the rated angular frequency, U ref is the rated voltage, K Q is the integral gain coefficient, Q ref is the reference value of reactive power, Q e is the output reactive power, and t is the time.

4. The oscillation suppression control method of a grid-type flexible direct current transmission system according to claim 1, characterized in that: The angular frequency error and the angular frequency change rate are established according to the mathematical model of the meshed controller; the angular frequency error is e ω =ω-ω0, the angular frequency change rate is 5. The oscillation suppression control method of a grid-type flexible direct current transmission system according to claim 1, characterized in that: Based on the super-helical sliding mode control strategy, the power deviation is designed using the angular frequency error as follows: Among them, tanh(e ω ) is the hyperbolic tangent function, expressed as: Among them, λ1 and λ2 are power deviation parameters, ω is the mechanical angular frequency, P ref is the active power reference value, P Δf is the power deviation, P e is the output active power, D p is the damping coefficient, ω0 is the rated angular frequency, e ω is the angular frequency error, and t is time.

6. The oscillation suppression control method of a grid-type flexible direct current transmission system according to claim 1, characterized in that: Designing the adaptive adjustment rule according to the angular frequency error and the angular frequency change rate; The adaptive adjustment rule is: Among them, J w and D w is the steady-state interval threshold, k1, k2 and k3 are adaptive adjustment parameters, J0 is the basic value of J, D0 is the basic value of D p The basic value, e ω is the angular frequency error.

7. The oscillation suppression control method of a grid-type flexible direct current transmission system according to claim 1, characterized in that: The real-time value of the moment of inertia and the real-time value of the damping coefficient are adjusted by using the adaptive adjustment rule, and the power deviation is dynamically adjusted according to the real-time value of the moment of inertia and the real-time value of the damping coefficient to obtain the final grid-type controller; according to the final grid-type controller, the voltage source phase angle θ and the output voltage amplitude V are calculated to calculate the reference voltage of the AC side grid-type inverter and 8. The oscillation suppression control method of a grid-type flexible direct current transmission system according to claim 1, characterized in that: According to the reference voltage of the AC side grid-type inverter and The PI controller of the AC side grid-forming inverter is established, specifically: Among them, k 51 , k 52 , k 61 , k 62 , k 71 , k 72 , k 81 and k 82 are the PI controller parameters of the AC side grid-connected inverter, and are the reference currents of the d-axis and q-axis of the AC-side grid-forming inverter, respectively, and are the reference voltages of the d-axis and q-axis of the AC-side grid-connected inverter, u od and u oq are the actual voltages of the d-axis and q-axis of the AC-side grid-connected inverter, i cd and i cq are the actual currents of the d-axis and q-axis of the AC-side grid-connected inverter, u gd1 and u gq1 are the modulation signals of the d-axis and q-axis of the rectifier-side converter respectively, They are u gd , u gq , i gd , i gq The first derivative of .

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