Offshore new energy-DC power grid stability analysis control method and system

Abstract

The invention relates to an offshore new energy-direct current power grid stability analysis control method and system, belongs to the technical field of new energy, and solves the problem that oscillation caused by transmitting / receiving end faults of a multi-end flexible direct transmission system of large-scale new energy cannot be rapidly suppressed. The method comprises the following steps: acquiring instantaneous value variation of voltage and current output by a system port to obtain instantaneous value variation of electrical quantity output by each subsystem port; based on the voltage and current instantaneous value variable quantity and the electrical quantity instantaneous value variable quantity, constructing a dynamic energy model of each subsystem to calculate the interactive energy variable quantity among the subsystems; obtaining an interaction energy change rate between the different subsystems based on the interaction energy change quantity between the subsystems so as to determine key interaction energy for inducing system oscillation instability; and establishing a mapping relation between the key interaction energy and active / reactive power of the wind field / converter station so as to suppress the key interaction energy by adjusting the current instruction value. And stable suppression of system divergent oscillation in different fault scenes is realized.

Description

Technical Field

[0001] This invention relates to the field of new energy technology, and in particular to a method and system for stability analysis and control of offshore new energy DC power grids. Background Technology

[0002] With the rapid development of power electronics technology, transmission via VSC-MTDC (voltage source converter based multi-terminal) power transmission systems has gradually become one of the mainstream solutions for large-scale renewable energy transmission. Compared with traditional AC grid-connected systems, VSC-MTDC systems contain a large number of coupled power electronic devices, have complex control strategies, and experience a surge in the risk of system oscillation and instability under fault scenarios at both the sending and receiving ends. However, existing technologies do not address oscillation suppression strategies under these fault scenarios. Therefore, research is urgently needed on oscillation suppression strategies for direct-drive wind farms (DDWF) transmitted via VSC-MTDC systems under fault scenarios.

[0003] Existing DDWF-VSC-HVDC-based system oscillation suppression strategies can be mainly divided into three categories: parameter optimization and adjustment, additional device-based, and control optimization and adjustment. Although the above methods can suppress system oscillations, they have different problems such as limited parameter adjustment range, complex control strategy design, and poor economy. Furthermore, they cannot be applied to system oscillations caused by large disturbances such as faults. Summary of the Invention

[0004] Based on the above analysis, the embodiments of the present invention aim to provide a method and system for stability analysis and control of offshore new energy DC grid, in order to solve the problems of cumbersome design and difficult parameter tuning of existing direct-drive wind farms connected to the grid via multi-terminal flexible DC grid, which cannot achieve rapid suppression of oscillations caused by faults at the sending / receiving ends of large-scale new energy transmission systems via multi-terminal flexible DC grid.

[0005] On one hand, embodiments of the present invention provide a method for stability analysis and control of offshore new energy-DC grids, comprising: acquiring instantaneous changes in voltage and current outputs at the ports of a direct-drive wind power grid-connected system, using the period of the dominant oscillation mode as the sampling interval, to obtain instantaneous changes in electrical quantities output at the ports of each subsystem in the direct-drive wind power grid-connected system; constructing dynamic energy models for each subsystem based on the instantaneous changes in voltage and current and the instantaneous changes in electrical quantities, to generate interactive energy changes between each subsystem; obtaining the rate of change of interactive energy between different subsystems based on the interactive energy changes between each subsystem, to determine the key interactive energy that induces system oscillation instability; and establishing a mapping relationship between the key interactive energy and the active / reactive power of the wind farm / converter station to suppress the key interactive energy by adjusting the current command value.

[0006] The beneficial effects of the above technical solution are as follows: the oscillation suppression strategy can achieve stable suppression of system divergent oscillations at the level of hundreds of milliseconds under different fault scenarios, without affecting the control performance under normal system operation.

[0007] Further improvements to the above method, based on the amount of interaction energy change between the subsystems, to obtain the rate of interaction energy change between the subsystems in order to determine the key interaction energy that induces system oscillation instability, further include: obtaining the rate of interaction energy change between the different subsystems by calculating the derivative of the amount of interaction energy change between the subsystems with respect to time; and judging the stability of the direct-drive wind power grid-connected system based on the sign and magnitude of the rate of interaction energy change to determine the key interaction energy that induces system oscillation instability.

[0008] Further improvements to the above method, determining the key interactive energy that induces system oscillation and instability, further include: determining the key interactive energy that induces system oscillation based on the rate of change of the interactive energy, wherein, when the rate of change of the interactive energy between different subsystems is negative, the direct-drive wind power grid-connected system maintains stable operation; the smaller the rate of change of the interactive energy between different subsystems, the higher the stability level of the direct-drive wind power grid-connected system; when the rate of change of the interactive energy between different subsystems is zero, the rate of change of the interactive energy between different subsystems is irrelevant to the stability level of the direct-drive wind power grid-connected system; when the rate of change of the interactive energy between different subsystems is positive, it is detrimental to the stable operation of the direct-drive wind power grid-connected system; the larger the rate of change of the interactive energy between different subsystems, the more detrimental it is to maintaining the stability of the direct-drive wind power grid-connected system.

[0009] Further improvements to the above method, establishing a mapping relationship between the key interactive energy and the active / reactive power of the wind farm / converter station to suppress the key interactive energy by adjusting the current command value, further includes: when a fault occurs on the direct-drive wind farm side, after the fault at the sending end is cleared, the interactive energy V between the phase-locked loop (PLL) and the grid-side converter GSC current inner loop q-axis subsystem is... t79 The interaction energy V between the outer voltage loop subsystem and the inner current loop subsystem of the grid-side converter GSC t108 And the interaction energy V between the voltage outer loop subsystem and the current inner loop subsystem of the sending-end converter station. tv_oi_d and V tv_oi_q The key interaction energy V was identified as inducing system oscillations. This interaction energy V was suppressed by adjusting the reference value of the q-axis subsystem of the current inner loop. t108 :

[0010]

[0011] In the formula, This represents the d-axis component of the DC bus voltage of the grid-side converter for a direct-drive wind turbine. This represents the d-axis component of the GSC output current. This is the d-axis reference value for the inner loop of the GSC current. This is the reference value for GSC DC voltage control; k p5 k p6 The dynamic energy coupling coefficient; This is the reference value for reactive current compensation of direct-drive wind turbines; ω pll ω0 is the measured grid angular frequency of the phase-locked loop (PLL); ω0 is the rated grid angular frequency; Q is the instantaneous value of the reactive power output of the direct-drive wind turbine; Q0 ref This is the reactive power reference value; U ppccd k represents the d-axis component of the voltage at the point of common coupling (PCC). paq1 k paq2 Frequency / reactive power compensation coefficient;

[0012] When a fault occurs at the sending end of the DC converter station, after the fault is cleared, the interaction energy Vtv1_oi_d and Vtv1_oi_q between the outer voltage loop subsystem and the inner current loop subsystem of the No. 1 voltage-source converter are identified as the key factors inducing system oscillation. Among these factors, adjusting the current reference value... To suppress the interaction energies Vtv1_oi_d and Vtv1_oi_q:

[0013]

[0014] In the formula, U v1pccdref U is the reference value for the d-axis component of the PCC voltage; v1pccd This is the actual measured value; k vad1 P is the voltage loop integral gain; v1 -P v1ref For active power tracking error; k vad2 U is the power-to-current conversion factor. v1pccqref The constant value is 0 (to maintain the q-axis voltage at zero); Q v1ref This is from an automatic voltage regulator (AVR) command;

[0015] When a fault occurs at the receiving end of the flexible DC converter station, after the receiving end fault is cleared, the interaction energy V between the outer voltage loop subsystem and the inner current loop subsystem of the #3 voltage-source converter VSC, and between the phase-locked loop (PLL) and the q-axis current loop subsystem, will be transferred. tv3_oi_d V t3v_oi_q and V tv3_pq The key interaction energy that induces system oscillation and instability was identified, and this was determined by adjusting the current reference value. and To suppress the interaction energy V tv3_oi_d V t3v_oi_qand V tv3_pq :

[0016]

[0017] In the formula, This is the reference value for the d-axis current of converter station 3; This is a reference value for DC voltage. For actual measurement of DC bus voltage; P v3 The active power output of converter station 3; P v3ref This is a reference value for active power. k represents the d-axis component of the voltage at point PCC. v3a1 k is the proportional coefficient for DC voltage control. v3a2 The active power to current conversion factor; This is the reference value for the q-axis current of converter station 3; This is the reference value for the PCC AC voltage. For actual measurement of PCC AC voltage; Q v3 The reactive power output of converter station 3; Q v3ref This is the reference value for reactive power; k v3a3 This is the proportional coefficient for AC voltage control.

[0018] Further improvements to the above method include collecting the instantaneous voltage and current changes at the port output of the direct-drive wind power grid-connected system, which further includes: collecting the instantaneous voltage and current values ​​at the d-axis and q-axis ports of each voltage-type converter (VSC) grid connection point in the direct-drive wind power grid-connected system; collecting the voltage and current values ​​at the d-axis and q-axis ports of each voltage-type converter (VSC) grid connection point during normal operation in the direct-drive wind power grid-connected system; and generating the instantaneous voltage and current changes at the port output of the direct-drive wind power grid-connected system based on the instantaneous voltage and current values ​​at the d-axis and q-axis ports of the grid connection point and the voltage and current values ​​at the d-axis and q-axis ports during normal operation of the grid connection point.

[0019] Based on a further improvement to the above method, the instantaneous changes in electrical quantities output by each subsystem port further include: the voltage and current changes output by the d-axis subsystem port of the GSC voltage outer loop. Voltage and current changes at the output ports of the GSC voltage outer loop q-axis subsystem The voltage and current changes ΔU at the port output of the GSC current inner loop d-axis subsystem vkd ΔI vd The voltage and current changes ΔU at the port output of the q-axis subsystem in the GSC current inner loop. vkq ΔI vq The voltage and current changes ΔU at the output ports of the VSC voltage outer loop d-axis subsystem vpccd ΔI vkdThe voltage and current changes ΔU at the output ports of the VSC outer loop q-axis subsystem. vpccq ΔI vkq The voltage and current changes ΔU at the output ports of the VSC inner current loop d-axis subsystem vkd ΔI vkd The voltage and current changes ΔU at the output ports of the VSC inner current loop q-axis subsystem vkq ΔI vkq The change in phase-locked angle Δθ at the PLL subsystem port output pll and angular frequency change Δω pll The voltage and current changes Δu at the port output of the AC line d-axis subsystem pccd , Δi ld The voltage and current changes Δu at the port output of the q-axis subsystem of the AC line pccq , Δi lq .

[0020] Further improvements to the above method, obtaining the instantaneous changes in electrical quantities output by each subsystem port in the direct-drive wind power grid-connected system further include: based on the angular velocity ω0 and filter inductance L during stable operation of the direct-drive wind power grid-connected system. vil Instantaneous reference values ​​U of voltage and current at the d-axis port of the #1 voltage-source converter VSC grid connection point v1pccdref and I v1dref Calculate the voltage and current changes ΔU at the d-axis subsystem port of the VSC outer voltage loop of voltage-source converter #1. v1pccd and △I v1d ;

[0021] Based on the proportional coefficient k of the direct drive fan current inner loop p2 and integral coefficient k i2 L p The instantaneous change Δu represents the change in the q-axis voltage of the line filter inductance and the direct-drive wind power grid-connected system. pccq Calculation of the d-axis component of the instantaneous output current of the direct-drive wind turbine during normal operation; calculation of the voltage and current change Δu at the port of the q-axis subsystem of the grid-side converter GSC current inner loop. kq , Δi pq The proportional and integral coefficients k based on the phase-locked loop (PLL) ppll Change in q-axis voltage at the port of the direct-drive fan (k) ipll Calculate the change in phase-locked angle Δθ at the port output of the phase-locked loop (PLL) subsystem. pll and angular frequency change Δω pll Based on filter capacitor C t The angular velocity ω0 of the direct-drive wind power grid-connected system during steady-state operation, and the AC line inductance L. l AC line resistance Rl The change in port current Δi of the inner loop q-axis subsystem of the grid-side converter GSC current. pq Calculate the changes in voltage and current Δu at the ports of the d-axis subsystem of the AC line. pccd , Δi ld Based on filter capacitor C t The angular velocity ω0 of the direct-drive wind power grid-connected system during steady-state operation, and the AC line inductance L. l AC line resistance R l The change in port current Δi of the inner loop q-axis subsystem of the grid-side converter GSC current. pq and the change in voltage and current Δu at the port of the d-axis subsystem of the AC line pccd , Δi ld Calculate the changes in port voltage and current Δu of the q-axis subsystem of the AC line. pccq , Δi lq .

[0022] Based on further improvements to the above method, and based on the instantaneous changes in voltage and current and the instantaneous changes in electrical quantities, the dynamic energy model of each subsystem is further constructed as follows: The dynamic energy model of the direct-drive wind power grid-connected system includes the energy model of the grid-side converter GSC voltage outer loop d-axis subsystem, the energy model of the grid-side converter GSC voltage outer loop q-axis subsystem, the energy model of the grid-side converter GSC current inner loop d-axis subsystem, the energy model of the grid-side converter GSC current inner loop q-axis subsystem, and the energy model of the voltage-source converter VSC voltage outer loop d-axis subsystem. The energy models for the following subsystems are defined: VSC voltage outer loop q-axis subsystem energy model, VSC current inner loop d-axis subsystem energy model, VSC current inner loop q-axis subsystem energy model, PLL subsystem energy model, DC line subsystem energy model, AC line d-axis subsystem energy model, and AC line q-axis subsystem energy model. The energy modules of each subsystem are represented by the following formulas, where the dynamic energy model of each subsystem includes the following stored energy, dissipated energy, and interactive energy:

[0023]

[0024] Where, ΔV s ΔV d ΔV t These represent the changes in stored energy, dissipated energy, and interaction energy for each subsystem; K a K b All coefficients are greater than 0; R k L k C k These represent the equivalent resistance, equivalent inductance, and equivalent capacitance in the subsystem, respectively; ΔU and ΔI represent the instantaneous changes in the output voltage and current at the subsystem ports, respectively.

[0025] Based on the above method, a further improvement is made to the calculation of the interaction energy change between subsystems, which further includes:

[0026] Using the following formula ΔV t =K a K b ∫ΔIΔUdt calculates the change in interaction energy ΔV between different subsystems. ti , where i = 1, 2, 3, 4, 5, 6, 7, to represent different subsystems;

[0027] The interaction energy changes between the subsystems include: the interaction energy V between the GSC voltage outer loop subsystem and the current inner loop subsystem. t108 The interaction energy V between the PLL subsystem and the GSC inner current loop q-axis subsystem t79 The interaction energy V between the outer voltage loop subsystem and the inner current loop subsystem of 1#VSC tv_oi_d V tv_oi_q The interaction energy V between the voltage outer loop d-axis subsystem and the #1 DC line subsystem t_od_1d The interaction energy V between the inner current loop d-axis subsystem and the #1 DC line subsystem t_id_1d The interaction energy V between the voltage outer loop d-axis subsystem and the #3 DC line subsystem t_od_3d The interaction energy V between the inner current loop d-axis subsystem and the DC line subsystem t_id_3d The interaction energy V between the voltage outer loop-current inner loop d-axis subsystem and the #3 DC line subsystem t_oid_3d V t_3d_oid The interaction energy V between the voltage outer loop d-axis subsystem and the #4 DC line subsystem t_od_4d The interaction energy V between the inner current loop d-axis subsystem and the #4 DC line subsystem i_id_4d The #1 DC line subsystem transfers energy V to the #2 DC subsystem via -Udc1. t_v1_v2 The #2 DC line subsystem transfers energy V to the #1 DC subsystem via -Idc1_2. t_v2_v1 .

[0028] On the other hand, embodiments of the present invention provide a marine new energy-DC grid stability analysis and control system, comprising: a data acquisition module, used to acquire the instantaneous changes in voltage and current outputs of the port of the direct-drive wind power grid-connected system at the sampling interval of the dominant oscillation mode period to obtain the instantaneous changes in electrical quantities output by the port of each subsystem in the direct-drive wind power grid-connected system; an interactive energy calculation module, used to construct a dynamic energy model of each subsystem based on the instantaneous changes in voltage and current and the instantaneous changes in electrical quantities to generate the interactive energy changes between each subsystem; a key interactive link determination module, used to obtain the rate of change of interactive energy between different subsystems based on the interactive energy changes between each subsystem to determine the key interactive energy that induces system oscillation instability; and an oscillation suppression module, used to establish a mapping relationship between the key interactive energy and the active / reactive power of the wind farm / converter station to suppress the key interactive energy by adjusting the current command value.

[0029] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0030] 1. The oscillation suppression strategy can achieve stable suppression of system divergent oscillations at the level of hundreds of milliseconds under different fault scenarios, without affecting the control performance under normal system operation.

[0031] 2. By determining the sign and magnitude of the rate of change of energy between each subsystem, the stability of the direct-drive wind power grid-connected system can be quickly and effectively evaluated;

[0032] 3. By using the energy construction module through the interaction of various subsystems, the amount of computation is reduced, the accuracy of computation is improved, and the reliable operation of the system is ensured;

[0033] 4. By using the interaction energy change rate calculation module provided by the interaction energy change rate calculation module between each subsystem, the key interaction items of the oscillation of the direct-drive wind power grid-connected system can be accurately located.

[0034] 5. The oscillation suppression strategy based on the adjustment of the current command value of the grid-side converter of the direct-drive wind turbine can effectively reduce the interactive energy between the voltage outer loop subsystem, the current inner loop subsystem, and the phase-locked loop subsystem, which is beneficial to improving the system stability level. The oscillation suppression strategy based on the adjustment of the current inner loop command value of the sending / receiving end flexible DC converter station can realize the energy transfer between the voltage outer loop subsystem and the current inner loop subsystem, thereby improving the system level.

[0035] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0036] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0037] Figure 1 A flowchart of a method for stability analysis and control of a marine new energy DC power grid according to an embodiment of the present invention;

[0038] Figure 2 Send out the system topology diagram for the four-terminal ring network DDWF+VSC-MTDC;

[0039] Figure 3 This diagram shows the energy interaction path between 1#DDWF and 1#VSC, as well as between the AC and DC sides of 1#VSC.

[0040] Figure 4 This is a diagram showing the energy interaction path between the AC and DC sides of VSC #3.

[0041] Figure 5 This is a diagram showing the energy interaction path between the AC and DC sides of VSC #4.

[0042] Figure 6 A diagram showing the energy interaction paths between the various VSC DC line subsystems;

[0043] Figure 7 GSC control topology diagram;

[0044] Figure 8 The control topology diagram for VSC #1;

[0045] Figure 9 The control topology diagram for VSC #3;

[0046] Figure 10 The time-domain waveform of the DC voltage hardware-in-the-loop experiment for #1 VSC is shown.

[0047] Figure 11 The time-domain waveform of the DC current in the hardware-in-the-loop experiment for VSC #2 is shown.

[0048] Figure 12 Time-domain waveform of the d-axis component of the AC grid-connected voltage at point 3#VSC in a hardware-in-the-loop experiment.

[0049] Figure 13 The waveform diagram in the time domain of the hardware-in-the-loop experiment for DC power of VSC #4 is shown.

[0050] Figure 14 The plot shows the trajectory of Vt_dc.

[0051] Figure 15 The plot shows the trajectory of Vd_dc.

[0052] Figure 16 The plot shows the trajectory of Vs_dc.

[0053] Figure 17 The time-domain waveform of the DC voltage oscillation suppression experiment for VSC #1 is shown.

[0054] Figure 18 The time-domain waveform of the DC current oscillation suppression experiment for VSC #2 is shown below.

[0055] Figure 19 Time-domain waveform of the d-axis component oscillation suppression experiment of the AC grid connection point voltage of 3#VSC;

[0056] Figure 20 The time-domain waveform diagram of the DC power oscillation suppression experiment for VSC #4;

[0057] Figure 21 A diagram showing the energy change trajectory of Vt79;

[0058] Figure 22 This is a diagram showing the energy change trajectory of Vt108.

[0059] Figure 23 The interactive energy change trajectory diagram of Vtv_oi_d;

[0060] Figure 24 The interactive energy change trajectory diagram for Vtv_oi_q;

[0061] Figure 25 The time-domain waveform of the DC voltage oscillation suppression experiment for VSC #1 is shown.

[0062] Figure 26 The time-domain waveform of the DC current oscillation suppression experiment for VSC #2 is shown below.

[0063] Figure 27 Time-domain waveform of the d-axis component oscillation suppression experiment of the AC grid connection point voltage of 3#VSC;

[0064] Figure 28 The time-domain waveform diagram of the DC power oscillation suppression experiment for VSC #4;

[0065] Figure 29 The trajectory diagram of the interaction energy change of Vtv3_oi_d;

[0066] Figure 30 The interactive energy change trajectory diagram for Vtv3_oi_q;

[0067] Figure 31 The interactive energy change trajectory diagram for Vtv3_pq;

[0068] Figure 32This is a schematic diagram of a marine new energy-DC grid stability analysis and control system according to an embodiment of the present invention. Detailed Implementation

[0069] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0070] refer to Figure 1 A specific embodiment of the present invention discloses a method for stability analysis and control of a marine new energy DC grid, comprising: in step S101, using the period of the dominant oscillation mode as the sampling interval, collecting the instantaneous changes in voltage and current output at the port of the direct-drive wind power grid-connected system to obtain the instantaneous changes in electrical quantities output at the port of each subsystem in the direct-drive wind power grid-connected system; in step S102, constructing a dynamic energy model for each subsystem based on the instantaneous changes in voltage and current and the instantaneous changes in electrical quantities to generate the interactive energy changes between each subsystem; in step S103, obtaining the rate of change of interactive energy between different subsystems based on the interactive energy changes between each subsystem to determine the key interactive energy that induces system oscillation instability; and in step S104, establishing a mapping relationship between the key interactive energy and the active / reactive power of the wind farm / converter station to suppress the key interactive energy by adjusting the current command value.

[0071] Compared with existing technologies, the offshore new energy DC grid stability analysis and control method provided in this embodiment can achieve stable suppression of system divergent oscillations at the level of hundreds of milliseconds under different fault scenarios, without affecting the control performance under normal system operation.

[0072] In the following text, refer to Figure 1 The stability analysis and control method for offshore new energy DC power grid according to embodiments of the present invention will be described in detail.

[0073] In step S101, the instantaneous changes in voltage and current output at the ports of the direct-drive wind power grid-connected system are collected at the sampling interval of the dominant oscillation mode period to obtain the instantaneous changes in electrical quantities output at the ports of each subsystem in the direct-drive wind power grid-connected system.

[0074] Step S101 includes dividing the direct-drive wind power grid-connected system into multiple subsystems according to the system control links, and collecting the instantaneous changes in the output voltage and current of the direct-drive wind power grid-connected system ports with the period of the dominant oscillation mode as the sampling interval.

[0075] The structure of the four-terminal ring network DDWF+VSC-MTDC transmission system is as follows: Figure 2As shown, the machine-side section includes a permanent magnet synchronous generator (PMSG), a machine-side converter (MSC), and a grid-side converter (GSC). The grid-side section consists of an AC filter inductor L, an equivalent resistance R, and a filter capacitor C. t Equivalent grid inductance L l and resistance R l The system consists of an AC line section and a DC side section composed of a voltage source converter (VSC) and a DC bus. Based on the system control components, the system is divided into multiple subsystems.

[0076] like Figure 2 As shown, an interconnected model of multiple subsystems is constructed based on the control loop of the grid-connected system. These subsystems are: VSC voltage outer loop d-axis subsystem, VSC voltage outer loop q-axis subsystem, VSC current inner loop d-axis subsystem, VSC current inner loop q-axis subsystem, GSC voltage outer loop d-axis subsystem, GSC voltage outer loop q-axis subsystem, GSC current inner loop d-axis subsystem, GSC current inner loop q-axis subsystem, DC line subsystem, PLL subsystem, AC line d-axis subsystem, and AC line q-axis subsystem, as shown... Figure 3 , Figure 4 and Figure 5 A schematic diagram of the interconnection model of each subsystem is shown.

[0077] The sampling time interval is the period of the dominant oscillation mode, that is, with The instantaneous changes in the output voltage and current of the direct-drive wind power grid-connected system are collected at the sampling period to obtain the instantaneous changes in the output voltage and current of each subsystem. Here, ω represents the angular velocity of the dominant oscillation mode of the direct-drive wind power grid-connected system.

[0078] Specifically, the acquisition of instantaneous voltage and current changes at the ports of the direct-drive wind power grid-connected system further includes: acquiring the instantaneous voltage and current values ​​at the d-axis and q-axis ports of each voltage-type converter (VSC) grid connection point in the direct-drive wind power grid-connected system; acquiring the voltage and current values ​​at the d-axis and q-axis ports of each voltage-type converter (VSC) grid connection point during normal operation in the direct-drive wind power grid-connected system; and generating the instantaneous voltage and current changes at the ports of the direct-drive wind power grid-connected system based on the instantaneous voltage and current values ​​at the d-axis and q-axis ports of the grid connection point and the voltage and current values ​​at the d-axis and q-axis ports during normal operation of the grid connection point. Specifically, the instantaneous values ​​of voltage and current at the ports of the direct-drive unit are acquired by voltage and current sensors installed inside the unit, with the sampling interval being the period of the dominant oscillation mode, thereby improving the accuracy and speed of data acquisition.

[0079] The instantaneous changes in voltage and current at the d-axis and q-axis output ports of the direct-drive wind power system after passing through a multi-terminal flexible DC grid-connected system are ΔU. v1pccd ΔU v1pccd ΔU v2pccd ΔU v2pccq , ΔI v1d ΔI v1q ΔI v2d ΔI v2q , As shown in formula (1).

[0080] Among them, U v1pccd U v1pccq U v2pccd U v2pccd , I v1d I v1q I v2d I v2q , U represents the instantaneous values ​​of the d-axis and q-axis port voltages and currents at the grid connection points of VSCs 1#, 2#, 3#, and 4#. v1pccd0 U v1pccq0 U v2pccd0 U v2pccq0 I v1d0 I v1q0 I v2d0 I v2q0 , The values ​​of the d-axis and q-axis ports are the voltage and current values ​​of the grid connection points 1#VSC, 2#VSC, 3#VSC and 4#VSC when they are operating normally.

[0081]

[0082] Step S101 further includes calculating the instantaneous change in the electrical quantity output by each subsystem port based on the instantaneous changes in the voltage and current output by the grid-connected system port. Specifically, obtaining the instantaneous change in the electrical quantity output by each subsystem port in the direct-drive wind power grid-connected system further includes:

[0083] (1) Voltage and current changes ΔU at the port of the outer loop d-axis subsystem of 1#VSC voltage -> VSC voltage outer loop d-axis subsystem v1pccd ΔI v1kd Based on the angular velocity ω0 and filter inductance L during stable operation of a direct-drive wind power grid-connected system vil Instantaneous reference values ​​U of voltage and current at the d-axis port of the #1 voltage-source converter VSC grid connection point v1pccdref and I v1drefCalculate the voltage and current changes ΔU at the d-axis subsystem ports of voltage source converter VSC (there are four voltage source converters in total, which play a role in converting AC and DC in the power system; voltage source converters 1, 2, 3, and 4 are numbered sequentially). v1pccd and △I v1d ;

[0084]

[0085] (2) Changes in port voltage and current Δu of the q-axis subsystem in the inner current loop of the GSC kq , Δi pq The proportional coefficient k based on the inner loop current of the direct-drive fan. p2 and integral coefficient k i2 L p The instantaneous change Δu represents the change in the q-axis voltage of the line filter inductance and the direct-drive wind power grid-connected system. pccq Calculation of the d-axis component of the instantaneous output current of the direct-drive wind turbine during normal operation; calculation of the voltage and current change Δu at the port of the q-axis subsystem of the grid-side converter GSC current inner loop. kq , Δi pq ;

[0086]

[0087] (3) PLL subsystem port output phase-locked angle change Δθ pll The change in angular frequency at the port output Δω pll The proportional and integral coefficients k based on the phase-locked loop (PLL) ppll Change in q-axis voltage at the port of the direct-drive fan (k) ipll Calculate the change in phase-locked angle Δθ at the port output of the phase-locked loop (PLL) subsystem. pll and angular frequency change Δω pll ;

[0088]

[0089] (4) Changes in voltage and current at the port of the AC line d-axis subsystem Δu pccd , Δi ld Based on filter capacitor C t The angular velocity ω0 of the direct-drive wind power grid-connected system during steady-state operation, and the AC line inductance L. l AC line resistance R l The change in port current Δi of the inner loop q-axis subsystem of the grid-side converter GSC current. pq Calculate the changes in voltage and current Δu at the ports of the d-axis subsystem of the AC line. pccd , Δi ld ;

[0090]

[0091] (5) Changes in voltage and current at the port of the q-axis subsystem of the AC line, Δu pccq , Δi lq Based on filter capacitor C t The angular velocity ω0 of the direct-drive wind power grid-connected system during steady-state operation, and the AC line inductance L. l AC line resistance R l The change in port current Δi of the inner loop q-axis subsystem of the grid-side converter GSC current. pq and the change in voltage and current Δu at the port of the d-axis subsystem of the AC line pccd , Δi ld Calculate the changes in port voltage and current Δu of the q-axis subsystem of the AC line. pccq , Δi lq .

[0092]

[0093] Specifically, the instantaneous changes in electrical quantities output by each subsystem port further include: the voltage and current changes output by the d-axis subsystem port of the GSC voltage outer loop. Voltage and current changes at the output ports of the GSC voltage outer loop q-axis subsystem The voltage and current changes ΔU at the port output of the GSC current inner loop d-axis subsystem vkd ΔI vd The voltage and current changes ΔU at the port output of the q-axis subsystem in the GSC current inner loop. vkq ΔI vq The voltage and current changes ΔU at the output ports of the VSC voltage outer loop d-axis subsystem vpccd ΔI vkd The voltage and current changes ΔU at the output ports of the VSC outer loop q-axis subsystem. vpccq ΔI vkq The voltage and current changes ΔU at the output ports of the VSC inner current loop d-axis subsystem vkd ΔI vkd The voltage and current changes ΔU at the output ports of the VSC inner current loop q-axis subsystem vkq ΔI vkq The change in phase-locked angle Δθ at the PLL subsystem port output pll and angular frequency change Δω pll The voltage and current changes Δu at the port output of the AC line d-axis subsystem pccd , Δi ld The voltage and current changes Δu at the port output of the q-axis subsystem of the AC line pccq , Δilq .

[0094] Substitute equation (1) into equations (2) to (6) respectively to calculate the instantaneous changes in electrical quantities at each subsystem port.

[0095] In step S102, a dynamic energy model for each subsystem is constructed based on the instantaneous changes in voltage and current, as well as the instantaneous changes in electrical quantities, to generate the interactive energy changes between the subsystems. Step S102 includes constructing a dynamic energy model for each subsystem based on the instantaneous changes in output voltage and current at the grid-connected system ports and the instantaneous changes in electrical quantities output at the ports of each subsystem.

[0096] Specifically, based on the instantaneous changes in voltage and current, and the instantaneous changes in electrical quantities, the dynamic energy models of each subsystem are further constructed, including: the dynamic energy model of the direct-drive wind power grid-connected system includes the energy models of the grid-side converter GSC voltage outer loop d-axis subsystem, the grid-side converter GSC voltage outer loop q-axis subsystem, the grid-side converter GSC current inner loop d-axis subsystem, the grid-side converter GSC current inner loop q-axis subsystem, the voltage-source converter VSC voltage outer loop d-axis subsystem, the voltage-source converter VSC voltage outer loop q-axis subsystem, the voltage-source converter VSC current inner loop d-axis subsystem, the voltage-source converter VSC current inner loop q-axis subsystem, the phase-locked loop (PLL) subsystem, the DC line subsystem, the AC line d-axis subsystem, and the AC line q-axis subsystem. The energy modules of each of the above subsystems are represented by the following formulas, where the dynamic energy model of each subsystem includes the following stored energy, dissipated energy, and interactive energy:

[0097]

[0098] Where, ΔV s ΔV d ΔV t These represent the changes in stored energy, dissipated energy, and interaction energy for each subsystem; K a K b All coefficients are greater than 0; R k L k C k These represent the equivalent resistance, equivalent inductance, and equivalent capacitance in the subsystem, respectively; ΔU and ΔI represent the instantaneous changes in output voltage and current at the subsystem ports, respectively. Specifically, K... a K b The value is set to a value greater than 0 and not exceeding 10.

[0099] Step S102 further includes calculating the interaction energy change between each subsystem using the interaction energy change in the dynamic energy model.

[0100] Specifically, the calculation of the interaction energy change between the generating subsystems further includes:

[0101] Using the following formula ΔV t =K a K b ∫ΔIΔUdt calculates the change in interaction energy ΔV between different subsystems. ti , where i = 1, 2, 3, 4, 5, 6, 7, to represent different subsystems;

[0102] The energy changes between the subsystems include:

[0103] The interaction energy V between the GSC voltage outer loop subsystem and the current inner loop subsystem t108 ;

[0104] The interaction energy V between the PLL subsystem and the GSC current inner loop q-axis subsystem t79 ;

[0105] The interaction energy V between the outer voltage loop subsystem and the inner current loop subsystem of 1#VSC tv_oi_d V tv_oi_q .

[0106] The interaction energy V between the voltage outer loop d-axis subsystem and the #1 DC line subsystem t_od_1d ;

[0107] The interaction energy V between the inner current loop d-axis subsystem and the #1 DC line subsystem t_id_1d ;

[0108] The interaction energy V between the voltage outer loop d-axis subsystem and the #3 DC line subsystem t_od_3d ;

[0109] The interaction energy V between the inner current loop d-axis subsystem and the DC line subsystem t_id_3d ;

[0110] The interaction energy V between the voltage outer loop-current inner loop d-axis subsystem and the #3 DC line subsystem t_oid_3d V t_3d_oid ;

[0111] The interaction energy V between the voltage outer loop d-axis subsystem and the #4 DC line subsystem t_od_4d ;

[0112] The interaction energy V between the inner current loop d-axis subsystem and the #4 DC line subsystem t_id_4d ;

[0113] The #1 DC line subsystem transfers energy V to the #2 DC subsystem via -Udc1. t_v1_v2 ;

[0114] The #2 DC line subsystem transfers energy V to the #1 DC subsystem via -Idc1_2. t_v2_v1 .

[0115] In step S103, based on the change in interaction energy between each subsystem, the rate of change of interaction energy between different subsystems is obtained to determine the key interaction energy that induces system oscillation and instability. This step S103 includes calculating the rate of change of interaction energy between each subsystem by taking the derivative based on the change in interaction energy between each subsystem.

[0116] Specifically, based on the amount of interaction energy change between each subsystem, the rate of interaction energy change between each subsystem is obtained to determine the key interaction energy that induces system oscillation instability. This further includes: obtaining the rate of interaction energy change between different subsystems by calculating the derivative of the amount of interaction energy change between each subsystem with respect to time; and judging the stability of the direct-drive wind power grid-connected system based on the sign and magnitude of the rate of interaction energy change to determine the key interaction energy that induces system oscillation instability.

[0117] Based on the energy changes resulting from the interactions between the different subsystems, calculate their time derivative ΔV. ti (i = 1, 2, 3, 4, 5, 6, 7) is defined as the rate of change of interaction energy, and the positive or negative sign of the rate of change of interaction energy is used as the criterion for system stability, as shown in formula (9).

[0118]

[0119] Where i = 1, 2, 3, 4, 5, 6, 7. The sign and magnitude of the rate of change of the interaction energy are used as the criteria for determining the oscillation source.

[0120] For V t108 V t79 V tv_oi_d V tv_oi_q V t_od_1d V t_id_1d V t_3d_oid The derivative of V with respect to time is always greater than 0. t_od_3d V t_id_3d V t_id_4d V t_od_4d The derivative of with respect to time is less than 0.

[0121] Step S103 further includes determining the key interactive energy that induces system oscillation based on the rate of change of interactive energy between the subsystems.

[0122] Determining the key interaction energy that induces system oscillation and instability further includes: determining the key interaction energy that induces system oscillation based on the rate of change of interaction energy, wherein when the rate of change of interaction energy between different subsystems is negative, i.e. At this time, the direct-drive wind power grid-connected system maintains stable operation. The smaller the rate of change of interactive energy between different subsystems, the higher the stability level of the direct-drive wind power grid-connected system; when the rate of change of interactive energy between different subsystems is zero, i.e. When the rate of change of energy between different subsystems is positive, it is independent of the stability level of the direct-drive wind power grid-connected system; when the rate of change of energy between different subsystems is positive, that is... At this time, it is not conducive to the stable operation of the direct-drive wind power grid-connected system. Among them, the greater the rate of change of interaction energy between different subsystems, the more detrimental it is to maintaining the stability of the direct-drive wind power grid-connected system. The subsystem corresponding to the largest rate of change of interaction energy greater than 0 is identified as the oscillating interaction energy that induces system oscillation.

[0123] In step S104, a mapping relationship is established between critical interactive energy and active / reactive power of the wind farm / converter station in order to suppress critical interactive energy by adjusting the current command value.

[0124] Specifically, establishing a mapping relationship between critical interactive energy and the active / reactive power of the wind farm / converter station to suppress critical interactive energy by adjusting the current command value further includes:

[0125] (1) Direct-drive wind farm side fault: After the fault at the sending end is cleared, the interaction energy V between the phase-locked loop (PLL) and the grid-side converter GSC current inner loop q-axis subsystem is transferred. t79 The interaction energy V between the outer voltage loop subsystem and the inner current loop subsystem of the grid-side converter GSC t108 And the interaction energy V between the voltage outer loop subsystem and the current inner loop subsystem of the sending-end converter station. tv_oi_d and V tv_oi_q The key interaction energy V was identified as inducing system oscillations. This interaction energy V was suppressed by adjusting the reference value of the q-axis subsystem of the current inner loop. t108 :

[0126]

[0127] In the formula, The d-axis component of the DC bus voltage of the grid-side converter (GSC) of the direct-drive wind turbine (superscript c indicates the control coordinate system); This represents the d-axis component of the GSC output current. This is the d-axis reference value for the inner loop of the GSC current. This is the reference value for GSC DC voltage control; k p5 k p6This is the dynamic energy coupling coefficient (dimensionless proportionality coefficient); This is the reference value for reactive current compensation of direct-drive wind turbines; ω pll ω0 is the measured grid angular frequency of the phase-locked loop (PLL); ω0 is the rated grid angular frequency (50Hz corresponds to 314 rad / s); Q is the instantaneous value of the reactive power output of the direct-drive wind turbine; Q ref This is the reactive power reference value; U ppccd k represents the d-axis component of the voltage at the point of common coupling (PCC). paq1 k paq2 This is the frequency / reactive power compensation coefficient.

[0128] From equation (10), it can be seen that, Based on the amplitude-phase relationship between the variables and This energy constitutes a significant portion of the overall interactive energy, and the current reference value can be adjusted to suppress it. In this case, the GSC control topology is as follows: Figure 6 As shown.

[0129] (2) Sending-end fault of flexible DC converter station: After the sending-end fault is cleared, the interaction energy Vtv1_oi_d and Vtv1_oi_q between the outer voltage loop subsystem and the inner current loop subsystem of the No. 1 voltage-type converter VSC are identified as the key factors inducing system oscillation. Among them, the current reference value is adjusted. To suppress the interaction energies Vtv1_oi_d and Vtv1_oi_q:

[0130]

[0131] In the formula, U v1pccdref The reference value for the d-axis component of the PCC voltage (usually from the higher-level dispatcher); U v1pccd This is the actual measured value; k vad1 P is the voltage loop integral gain; v1 -P v1ref For active power tracking error; k vad2 U is the power-to-current conversion factor. v1pccqref The constant value is 0 (to maintain the q-axis voltage at zero); Q v1ref This could originate from an AVR (Automatic Voltage Regulator) command.

[0132] Combining the amplitude-phase relationship between each state variable, U v1kd I v1dref I v1d I v1dref U v1kq I v1qref I v1q and I v1qref In V tv1_oi_d and V tv1_oi_qThe current reference value accounts for a relatively large proportion, and its adjustment can suppress interactive energy. The adjustment amount of the current reference value is set. The control topology of 1#VSC is as follows: Figure 7 As shown, the interaction energy represented by Equation (11) is greatly reduced, which effectively suppresses the interaction energy between the voltage outer loop subsystem and the current inner loop subsystem and improves the stability level of the system.

[0133] (3) Receiver-end fault of the flexible DC converter station: After the receiver-end fault is cleared, the interaction energy V between the outer voltage loop subsystem and the inner current loop subsystem of the No. 3 voltage-type converter VSC, and between the phase-locked loop PLL and the q-axis current loop subsystem will be reduced. tv3_oi_d V t3v_oi_q and V tv3_pq The key interaction energy that induces system oscillation and instability was identified, and this was determined by adjusting the current reference value. and To suppress the interaction energy V tv3_oi_d V t3v_oi_q and V tv3_pq :

[0134]

[0135] In the formula, This is the reference value for the d-axis current of converter station 3; This is a reference value for DC voltage. For actual measurement of DC bus voltage; P v3 The active power output of converter station 3; P v3ref This is a reference value for active power. k represents the d-axis component of the voltage at point PCC. v3a1 k is the proportional coefficient for DC voltage control. v3a2 The active power to current conversion factor; This is the reference value for the q-axis current of converter station 3;

[0136] This is the reference value for the PCC AC voltage. For actual measurement of PCC AC voltage; Q v3 The reactive power output of converter station 3; Q v3ref This is the reference value for reactive power; k v3a3 This is the proportional coefficient for AC voltage control.

[0137] An energy compensation branch based on the current inner loop reference value is introduced to suppress energy interaction between the aforementioned subsystems. Combining the amplitude-phase relationship of each variable, it can be obtained that... and This accounts for a significant portion of the aforementioned interactive energy. Therefore, interactive energy can be suppressed by adjusting the reference value of the inner current loop. At this point, the control topology of VSC #3 is as follows: Figure 8As shown, the interaction energies in Equation (12) are significantly reduced, effectively suppressing the transmission of oscillation energy between subsystems and improving the stability of the system.

[0138] To verify the feasibility of the methods described in the above embodiments, a direct-drive wind power grid-connected system via a multi-terminal flexible DC transmission was constructed using simulation. Based on this simulation system, the energy distribution of interactions between different subsystems under fault scenarios was obtained. By comparing this with the energy change rates of interactions between each subsystem calculated in the embodiments of this invention, it was found that the energy change rates of interactions between different subsystems calculated using this invention meet the accuracy requirements, and the positive and negative signs and magnitudes of these rates can reliably pinpoint key interaction terms in system oscillation.

[0139] Specifically, the system simulation structure diagram is as follows: Figure 1 As shown, a simulation model was built on the RT-LAB platform. RT-LAB provides a real-time simulation environment that allows engineers to simulate various systems and control algorithms. It is used to test and verify power electronics, motor control, aircraft control, power systems, and other complex engineering systems. The main system parameters are shown in Table 1.

[0140] Table 1 Main System Parameters

[0141]

[0142] Simulation scenarios: In the effectiveness verification of the dynamic energy stability analysis method, a three-phase ground fault occurred at the grid connection point of 1#DDWF at t=5.1s, with a transition resistance Rg=7Ω. The fault was cleared at t=5.115s. In the effectiveness verification of the sending-end fault oscillation suppression method, the oscillation suppression method proposed in this paper was applied 1.5s after the fault was cleared. In the effectiveness verification of the receiving-end fault oscillation suppression method, a three-phase ground fault occurred at the AC grid connection point of 3#VSC at t=5.1s, with a transition resistance Rg=4Ω. The fault was cleared at t=5.115s. The oscillation suppression method proposed in this paper was applied at t=6.615s.

[0143] Simulation experiments verified the time-domain waveforms and energy change trajectories of each subsystem, such as... Figures 10-31 As shown.

[0144] Figure 10 This represents the time-domain waveform of the DC voltage in the hardware-in-the-loop experiment for 1#VSC. Figure 11 This shows the time-domain waveform of the DC current in the hardware-in-the-loop experiment for VSC #2. Figure 12 This represents the time-domain waveform of the d-axis component of the AC grid-connected voltage at point 3#VSC in a hardware-in-the-loop experiment. Figure 13 This shows the time-domain waveform of the hardware-in-the-loop DC power experiment for VSC #4. Figure 14 V represents t_dc The trajectory of change; Figure 15 V representsd_dc The trajectory of change; Figure 16 V represents s_dc The trajectory of change; Figure 17 This represents the time-domain waveform of the DC voltage oscillation suppression experiment for VSC #1. Figure 18 This shows the time-domain waveform of the DC current oscillation suppression experiment for VSC #2. Figure 19 This represents the time-domain waveform of the d-axis component oscillation suppression experiment of the AC grid connection point voltage of 3#VSC. Figure 20 This shows the time-domain waveform of the DC power oscillation suppression experiment for VSC #4. Figure 21 V represents t79 Interactive energy change trajectory diagram; Figure 22 V represents t108 Interactive energy change trajectory diagram; Figure 23 V represents tv_oi_d Interactive energy change trajectory diagram; Figure 24 V represents tv_oi_q Interactive energy change trajectory diagram; Figure 25 This represents the time-domain waveform of the DC voltage oscillation suppression experiment for VSC #1. Figure 26 This shows the time-domain waveform of the DC current oscillation suppression experiment for VSC #2. Figure 27 This represents the time-domain waveform of the d-axis component oscillation suppression experiment of the AC grid connection point voltage of 3#VSC. Figure 28 This shows the time-domain waveform of the DC power oscillation suppression experiment for VSC #4. Figure 29 V represents tv3_oi_d Interactive energy change trajectory diagram; Figure 30 V represents tv3_oi_q Interactive energy change trajectory diagram; Figure 31 V represents tv3_pq Interactive energy change trajectory diagram.

[0145] Depend on Figures 10-16 It can be seen that when the stored energy of the system increases over time during oscillation, the system will become unstable. t_dc The amplitude is greater than V d_dc The amplitude of V leads to s_dc The amplitude increases with time, thus verifying the correctness of the stability analysis method proposed in this paper; Figures 17-24 It can be seen that V t79 V t108 V tv_oi_d and V tv_oi_q All of these are positive values ​​that increase with time, and after implementing the proposed oscillation suppression strategy, the aforementioned interactive energies quickly reach a steady-state value. Furthermore, it can be seen that the proposed oscillation suppression strategy can achieve stable suppression of system oscillations at the millisecond level. Figures 25-31 It can be seen that V tv3_oi_d V tv3_oi_q and V tv3_pqAll values ​​are positive and increase with time, and after implementing the proposed oscillation suppression strategy, the aforementioned interactive energies quickly reach a steady-state value. Furthermore, the proposed oscillation suppression strategy can achieve stable suppression of system oscillations at the hundred-millisecond level.

[0146] In summary, the oscillation suppression strategy established in this invention can achieve stable suppression of system divergent oscillations at the level of hundreds of milliseconds under different fault scenarios, without affecting the control performance under normal system operation.

[0147] refer to Figure 32 A specific embodiment of the present invention discloses a marine new energy-DC grid stability analysis and control system, comprising: a data acquisition module M1, used to acquire the instantaneous changes in voltage and current output at the ports of the direct-drive wind power grid-connected system at the sampling interval of the dominant oscillation mode period to obtain the instantaneous changes in electrical quantities output at the ports of each subsystem in the direct-drive wind power grid-connected system; an interactive energy calculation module M2, used to construct a dynamic energy model of each subsystem based on the instantaneous changes in voltage and current and the instantaneous changes in electrical quantities to generate the interactive energy changes between each subsystem; a key interactive link determination module M3, used to obtain the rate of change of interactive energy between different subsystems based on the interactive energy changes between each subsystem to determine the key interactive energy that induces system oscillation instability; and an oscillation suppression module M4, used to establish a mapping relationship between the key interactive energy and the active / reactive power of the wind farm / converter station to suppress the key interactive energy by adjusting the current command value.

[0148] The offshore new energy DC grid stability analysis and control system according to an embodiment of the present invention also includes several other modules, which will not be described in detail here as they correspond to the method steps.

[0149] Those skilled in the art will understand that all or part of the processes of the methods described in the above embodiments can be implemented by a computer program instructing related hardware, and the program can be stored in a computer-readable storage medium. The computer-readable storage medium may be a disk, optical disk, read-only memory, or random access memory, etc.

[0150] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for stability analysis and control of a marine new energy DC power grid, characterized in that, include: Using the period of the dominant oscillation mode as the sampling interval, the instantaneous changes in voltage and current output at the ports of the direct-drive wind power grid-connected system are collected to obtain the instantaneous changes in electrical quantities output at the ports of each subsystem in the direct-drive wind power grid-connected system. Based on the instantaneous changes in voltage and current and the instantaneous changes in electrical quantities, a dynamic energy model for each subsystem is constructed to generate the interactive energy changes between the subsystems. Based on the amount of interaction energy change between the subsystems, the rate of interaction energy change between the subsystems is obtained in order to determine the key interaction energy that induces system oscillation and instability. as well as A mapping relationship is established between the key interactive energy and the active / reactive power of the wind farm / converter station to suppress the key interactive energy by adjusting the current command value.

2. The offshore new energy DC grid stability analysis and control method according to claim 1, characterized in that, Based on the change in interaction energy between the subsystems, the rate of change of interaction energy between the subsystems is obtained to determine the key interaction energy that induces system oscillation and instability. This further includes: The rate of change of interaction energy between the different subsystems is obtained by calculating the derivative of the change in interaction energy between each subsystem with respect to time; and The stability of the direct-drive wind power grid-connected system is determined based on the sign and magnitude of the rate of change of the interactive energy to identify the key interactive energy that could induce system oscillations and instability.

3. The offshore new energy DC grid stability analysis and control method according to claim 2, characterized in that, The stability of the direct-drive wind power grid-connected system is determined based on the sign and magnitude of the rate of change of the interactive energy. Further determination of the key interactive energy that induces system oscillation and instability includes: When the rate of change of interaction energy between the different subsystems is negative, the direct-drive wind power grid-connected system maintains stable operation. The smaller the rate of change of interaction energy between the different subsystems, the higher the stability level of the direct-drive wind power grid-connected system. When the rate of change of the interaction energy between the different subsystems is zero, the rate of change of the interaction energy between the different subsystems is independent of the stability level of the direct-drive wind power grid-connected system. When the rate of change of interaction energy between the different subsystems is positive, it is not conducive to the stable operation of the direct-drive wind power grid-connected system. In particular, the greater the rate of change of interaction energy between the different subsystems, the more detrimental it is to the stability of the direct-drive wind power grid-connected system.

4. The offshore new energy DC grid stability analysis and control method according to claim 1, characterized in that, Establishing a mapping relationship between the critical interactive energy and the active / reactive power of the wind farm / converter station to suppress the critical interactive energy by adjusting the current command value further includes: When a fault occurs on the direct-drive wind farm side, after the fault is cleared at the sending end, the interaction energy V between the phase-locked loop (PLL) and the grid-side converter GSC current inner loop q-axis subsystem is transferred. t79 The interaction energy V between the outer voltage loop d-axis subsystem and the inner current loop subsystem d-axis of the grid-side converter GSC t108 And the interaction energy V between the outer voltage loop subsystem and the inner current loop d-axis and q-axis subsystems of the sending-end converter station VSC tv_oi_d and V tv_oi_q The key interaction energy V was identified as inducing system oscillations. This interaction energy V was suppressed by adjusting the reference value of the q-axis subsystem of the current inner loop. t108 : In the formula, This represents the d-axis component of the DC bus voltage of the grid-side converter for a direct-drive wind turbine. This represents the d-axis component of the GSC output current. This is the d-axis reference value for the inner loop of the GSC current. This is the reference value for GSC DC voltage control; k p5 k p6 The dynamic energy coupling coefficient; This is the reference value for reactive current compensation of direct-drive wind turbines; ω pll ω0 is the measured grid angular frequency of the phase-locked loop (PLL); ω0 is the rated grid angular frequency; Q is the instantaneous value of the reactive power output of the direct-drive wind turbine; Q0 ref This is the reactive power reference value; U ppccd k represents the d-axis component of the voltage at the point of common coupling (PCC). paq1 k paq2 Frequency / reactive power compensation coefficient; When a fault occurs at the sending end of the DC converter station, after the fault is cleared, the interaction energy Vtv1_oi_d and Vtv1_oi_q between the outer voltage loop subsystem and the inner current loop d-axis and q-axis subsystems of the No. 1 voltage-source converter are identified as the key factors inducing system oscillation. Among these factors, adjusting the current reference value... To suppress the interaction energy V tv1_oi_d and V tv1_oi_q : In the formula, U v1pccdref U is the reference value for the d-axis component of the PCC voltage; v1pccd This is the actual measured value; k vad1 P is the voltage loop integral gain; v1 -P v1ref For active power tracking error; k vad2 U is the power-to-current conversion factor. v1pccqref The constant value is 0 (to maintain the q-axis voltage at zero); Q v1ref This is from an automatic voltage regulator (AVR) command; When a fault occurs at the receiving end of the flexible DC converter station, after the receiving end fault is cleared, the interaction energy V between the outer voltage loop subsystem and the inner current loop subsystem of the #3 voltage-source converter VSC, and between the phase-locked loop (PLL) and the q-axis current loop subsystem, will be transferred. tv3_oi_d V t3v_oi_q and V tv3_pq The key interaction energy that induces system oscillation and instability was identified, and this was determined by adjusting the current reference value. and To suppress the interaction energy V tv3_oi_d V t3v_oi_q and V tv3_pq : In the formula, This is the reference value for the d-axis current of converter station 3; This is a reference value for DC voltage. For actual measurement of DC bus voltage; P v3 The active power output of converter station 3; P v3ref This is a reference value for active power. k represents the d-axis component of the voltage at point PCC. v3a1 k is the proportional coefficient for DC voltage control. v3a2 The active power to current conversion factor; This is the reference value for the q-axis current of converter station 3; This is the reference value for the PCC AC voltage. For actual measurement of PCC AC voltage; Q v3 The reactive power output of converter station 3; Q v3ref This is the reference value for reactive power; k v3a3 This is the proportional coefficient for AC voltage control.

5. The offshore new energy DC grid stability analysis and control method according to claim 1, characterized in that, The acquisition of instantaneous changes in voltage and current output at the ports of the direct-drive wind power grid-connected system further includes: Collect the instantaneous voltage and current values ​​at the d-axis and q-axis ports of each voltage-source converter (VSC) in the direct-drive wind power grid-connected system. The voltage and current values ​​at the d-axis and q-axis ports of each voltage-source converter (VSC) in the direct-drive wind power grid-connected system are collected during normal operation; and The instantaneous voltage and current values ​​at the d-axis and q-axis ports of the grid connection point, as well as the voltage and current values ​​at the d-axis and q-axis ports during normal operation of the grid connection point, are used to generate the instantaneous voltage and current values ​​output by the ports of the direct-drive wind power grid-connected system.

6. The offshore new energy DC grid stability analysis and control method according to claim 1, characterized in that, The instantaneous changes in electrical quantities output from each subsystem port further include: the voltage and current changes output from the d-axis subsystem port of the GSC voltage outer loop. Voltage and current changes at the output ports of the GSC voltage outer loop q-axis subsystem The voltage and current changes ΔU at the port output of the GSC current inner loop d-axis subsystem vkd ΔI vd The voltage and current changes ΔU at the port output of the q-axis subsystem in the GSC current inner loop. vkq ΔI vq The voltage and current changes ΔU at the output ports of the VSC voltage outer loop d-axis subsystem vpccd ΔI vkd The voltage and current changes ΔU at the output ports of the VSC outer loop q-axis subsystem. vpccq ΔI vkq The voltage and current changes ΔU at the output ports of the VSC inner current loop d-axis subsystem vkd ΔI vkd The voltage and current changes ΔU at the output ports of the VSC inner current loop q-axis subsystem vkq ΔI vkq The change in phase-locked angle Δθ at the PLL subsystem port output pll and angular frequency change Δω pll The voltage and current changes Δu at the port output of the AC line d-axis subsystem pccd , Δi ld The voltage and current changes Δu at the port output of the q-axis subsystem of the AC line pccq , Δi lq .

7. The offshore new energy DC grid stability analysis and control method according to claim 6, characterized in that, Obtaining the instantaneous changes in electrical quantities output from the ports of each subsystem in the direct-drive wind power grid-connected system further includes: Based on the angular velocity ω0 and filter inductance L of the direct-drive wind power grid-connected system during stable operation vil Instantaneous reference values ​​U of voltage and current at the d-axis port of the #1 voltage-source converter VSC grid connection point v1pccdref and I v1dref Calculate the voltage and current changes ΔU at the d-axis subsystem port of the VSC outer voltage loop of voltage-source converter #1. v1pccd and △I v1d ; Based on the proportional coefficient k of the direct drive fan current inner loop p2 and integral coefficient k i2 L p The instantaneous change Δu represents the change in the q-axis voltage of the line filter inductance and the direct-drive wind power grid-connected system. pccq Calculation of the d-axis component of the instantaneous output current of the direct-drive wind turbine during normal operation; calculation of the voltage and current change Δu at the port of the q-axis subsystem of the grid-side converter GSC current inner loop. kq , Δi pq ; Based on the proportional coefficient and integral coefficient k of the phase-locked loop (PLL) ppll Change in q-axis voltage at the port of the direct-drive fan (k) ipll Calculate the change in phase-locked angle Δθ at the port output of the phase-locked loop (PLL) subsystem. pll and angular frequency change Δω pll ; Based on filter capacitor C t The angular velocity ω0 of the direct-drive wind power grid-connected system during steady-state operation, and the AC line inductance L. l AC line resistance R l The change in port current Δi of the inner loop q-axis subsystem of the grid-side converter GSC current. pq Calculate the changes in voltage and current Δu at the ports of the d-axis subsystem of the AC line. pccd , Δi ld ; Based on filter capacitor C t The angular velocity ω0 of the direct-drive wind power grid-connected system during steady-state operation, and the AC line inductance L. l AC line resistance R l The change in port current Δi of the inner loop q-axis subsystem of the grid-side converter GSC current. pq and the change in voltage and current Δu at the port of the d-axis subsystem of the AC line pccd , Δi ld Calculate the changes in port voltage and current Δu of the q-axis subsystem of the AC line. pccq , Δi lq .

8. The offshore new energy DC grid stability analysis and control method according to claim 7, characterized in that, Based on the instantaneous changes in voltage and current, and the instantaneous changes in electrical quantities, the dynamic energy model of each subsystem is further constructed by including: The dynamic energy model of the direct-drive wind power grid-connected system includes the energy models of the grid-side converter GSC voltage outer loop d-axis subsystem, the grid-side converter GSC voltage outer loop q-axis subsystem, the grid-side converter GSC current inner loop d-axis subsystem, the grid-side converter GSC current inner loop q-axis subsystem, the voltage-source converter VSC voltage outer loop d-axis subsystem, the voltage-source converter VSC voltage outer loop q-axis subsystem, the voltage-source converter VSC current inner loop d-axis subsystem, the voltage-source converter VSC current inner loop q-axis subsystem, the phase-locked loop (PLL) subsystem, the DC line subsystem, the AC line d-axis subsystem, and the AC line q-axis subsystem. The energy modules of each of the above subsystems are represented by the following formulas, wherein the dynamic energy model of each subsystem includes the following stored energy, dissipated energy, and interactive energy: Where, ΔV s ΔV d ΔV t These represent the changes in stored energy, dissipated energy, and interaction energy for each subsystem; K a K b All coefficients are greater than 0; R k L k C k These represent the equivalent resistance, equivalent inductance, and equivalent capacitance in the subsystem, respectively; ΔU and ΔI represent the instantaneous changes in the output voltage and current at the subsystem ports, respectively.

9. The offshore new energy DC grid stability analysis and control method according to claim 7, characterized in that, The generation of interaction energy changes between subsystems further includes: Using the following formula ΔV t =K a K b ∫ΔIΔUdt calculates the change in interaction energy ΔV between different subsystems. ti , where i = 1, 2, 3, 4, 5, 6, 7, to represent different subsystems; The energy changes between the subsystems include: The interaction energy V between the GSC voltage outer loop subsystem and the current inner loop subsystem t108 ; The interaction energy V between the PLL subsystem and the GSC current inner loop q-axis subsystem t79 ; The interaction energy V between the outer voltage loop subsystem and the inner current loop subsystem of 1#VSC tv_oi_d V tv_oi_q ; The interaction energy V between the voltage outer loop d-axis subsystem and the #1 DC line subsystem t_od_1d ; The interaction energy V between the inner current loop d-axis subsystem and the #1 DC line subsystem t_id_1d ; The interaction energy V between the voltage outer loop d-axis subsystem and the #3 DC line subsystem t_od_3d ; The interaction energy V between the inner current loop d-axis subsystem and the DC line subsystem t_id_3d ; The interaction energy V between the voltage outer loop-current inner loop d-axis subsystem and the #3 DC line subsystem t_oid_3d V t_3d_oid ; The interaction energy V between the voltage outer loop d-axis subsystem and the #4 DC line subsystem t_od_4d ; The interaction energy V between the inner current loop d-axis subsystem and the #4 DC line subsystem t_id_4d ; The #1 DC line subsystem transfers energy V to the #2 DC subsystem via -Udc1. t_v1_v2 ; The #2 DC line subsystem transfers energy V to the #1 DC subsystem via -Idc1_2. t_v2_v1 .

10. A marine new energy-DC grid stability analysis and control system, characterized in that, include: The acquisition module is used to acquire the instantaneous changes in voltage and current output at the ports of the direct-drive wind power grid-connected system, with the period of the dominant oscillation mode as the sampling interval, to obtain the instantaneous changes in electrical quantities output at the ports of each subsystem in the direct-drive wind power grid-connected system. The interactive energy calculation module is used to construct a dynamic energy model of each subsystem based on the instantaneous changes in voltage and current and the instantaneous changes in electrical quantities, so as to generate the interactive energy changes between each subsystem. The key interaction link determination module is used to obtain the rate of change of interaction energy between different subsystems based on the amount of change of interaction energy between each subsystem, so as to determine the key interaction energy that induces system oscillation and instability. as well as The oscillation suppression module is used to establish a mapping relationship between the critical interactive energy and the active / reactive power of the wind farm / converter station in order to suppress the critical interactive energy by adjusting the current command value.

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