Frequency division coordination control method for wind power and multi-terminal flexible DC transmission system

Abstract

This invention proposes a frequency division coordination control method for wind power and multi-terminal flexible DC transmission systems. The steps are as follows: Based on Kirchhoff's voltage and current laws, a mathematical model of the voltage source converter in a three-phase stationary coordinate system is established; the Park transformation is performed to obtain the voltage source converter in... d‑q Mathematical model in synchronous rotating coordinates, calculated in d‑q Active and reactive power in a synchronous rotating coordinate system; active and reactive power output of the doubly-fed wind turbine generator are obtained based on the rotor-side control structure, and calculations are performed. d shaft and q The magnitude of the shaft current controls the output power of the doubly-fed wind turbine; a frequency divider controller is constructed; an upper-level controller is introduced, which communicates with the frequency divider controller to ensure the sensitive operation of the frequency divider controller and avoid frequent operation of the frequency divider controller. This invention can realize frequency control of AC / DC hybrid systems, thereby improving the frequency stability of the system, and has strong reference value.

Description

Technical Field

[0001] This invention relates to the technical field of additional control for multi-terminal flexible DC transmission systems, and particularly to a frequency division coordination control method for wind power and multi-terminal flexible DC transmission systems, which optimizes the frequency division coordination control of wind power and converter stations in multi-terminal flexible DC transmission systems containing wind power under fault conditions. Background Technology

[0002] As a new generation of DC transmission technology, the flexible high voltage direct current (VSC-HVDC) transmission method, which uses fully controlled devices, is relatively simple and reliable. It solves many inherent technical bottlenecks of conventional high voltage direct current transmission technology and is an important means of interconnecting large power grids.

[0003] For AC / DC hybrid systems, utilizing DC overload capacity and rapid adjustment characteristics to implement additional control after system disturbances can significantly improve system stability. With the increasing number of AC / DC hybrid systems, rapid DC power regulation has become the primary choice for power regulation in these systems. DC power control fully leverages its fast adjustment speed by adding a DC power controller to the AC / DC hybrid system. When a disturbance occurs in the AC system, the DC transmission system adjusts its active power transmission through feedback signals from the AC system, absorbing or compensating for excess or insufficient power in the connected AC system. Therefore, a well-designed DC power controller can significantly improve the frequency stability of AC / DC hybrid systems.

[0004] For AC / DC hybrid systems formed by wind power being connected to a multi-terminal flexible DC transmission system, the flexible power adjustment capability of wind turbines and the power sharing capability of the multi-terminal flexible DC transmission system can be fully utilized, enabling them to respond quickly to frequency disturbances in the AC system and significantly improve the frequency stability of the AC / DC hybrid system.

[0005] Patent application number 201811347915.4 discloses an active power control method and system for a flexible DC transmission system. The control method includes the following steps: upon receiving an active power adjustment signal, determining the converter station's operating condition based on the power reference value of the main pole control system / main unit and the received active power adjustment value; and calculating the active power adjustment values ​​of the main pole control system / main unit and the slave pole control system / slave unit under each operating condition based on the received active power adjustment value, the upper and lower power limits of each pole control system, and the power reference value. This invention adjusts the active power adjustment values ​​of the main pole control system / main unit and the slave pole control system / slave unit according to the control method and operating condition of the flexible DC transmission system, enabling the flexible DC transmission system to meet control requirements for active power component adjustment and improving the reliability of the flexible DC transmission system. However, this invention does not consider wind power participation in the system's frequency regulation, failing to fully utilize the frequency regulation capability of wind power and achieve coordinated control between wind power and the multi-terminal flexible DC transmission system. Summary of the Invention

[0006] To address the technical problem of frequency instability when wind power is connected to the grid via a multi-terminal flexible DC transmission system, this invention proposes a frequency-division coordinated control method for wind power and the multi-terminal flexible DC transmission system. The frequency fluctuation signal of the AC system is superimposed onto the active power control loops of the wind turbine generator and the converter station of the multi-terminal flexible DC transmission system via a frequency divider controller. Based on the AC system disturbance, the power output of the wind turbine generator and the power output of the converter station of the multi-terminal flexible DC transmission system are adjusted, achieving frequency-division coordinated control between wind power and the multi-terminal flexible DC transmission system. This reduces grid investment and construction costs and improves the frequency stability of the AC / DC hybrid system.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows: a frequency division coordination control method for wind power and multi-terminal flexible DC transmission system, comprising the following steps:

[0008] S1: Based on Kirchhoff's voltage and current laws, establish a mathematical model of the voltage source converter in a three-phase stationary coordinate system.

[0009] S2: Perform a Park transformation on the mathematical model from step S1 to obtain the mathematical model of the voltage source converter in the dq synchronous rotating coordinate system, and calculate the active power P in the dq synchronous rotating coordinate system. s and reactive power Q s ;

[0010] S3: The active power P output by the doubly-fed wind turbine is obtained based on the rotor-side control structure. r and reactive power Q r According to the active power P r Reactive power Q rCalculate the magnitude of the current along the d-axis and q-axis to control the output power of the doubly-fed wind turbine.

[0011] S4: Combining steps S2 and S3, construct a frequency divider controller; for power disturbances caused by AC system faults, the frequency divider controller coordinates the doubly fed wind turbine and the multi-terminal flexible DC transmission system to jointly participate in the frequency regulation of the AC system and maintain the stability of the AC system frequency;

[0012] S5: Introducing an upper-level controller, which communicates with the frequency divider controller to ensure the sensitive operation of the frequency divider controller and avoid frequent operation of the frequency divider controller.

[0013] Preferably, in step S1, the mathematical model of the voltage source converter in the three-phase stationary coordinate system is established based on the converter structure on the rectifier side of the converter station.

[0014]

[0015] Among them, L s R s These represent the equivalent inductance and resistance of the converter and line, respectively; i a i b i c Represents the three-phase currents a, b, and c; u a u b u c Indicates the three-phase output voltage of the converter; u sa u sb u sc These represent the three-phase voltages at the busbar of the AC system.

[0016] Preferably, the mathematical model of the voltage source converter in step S2 under the dq synchronous rotating coordinate system is as follows:

[0017]

[0018] Among them, i d i q These represent the dq-axis components of the AC system current; ω represents the angular frequency of the AC system; u sd u sq These are the dq-axis components of the AC system bus voltage, respectively; u d u q These are the dq-axis components of the converter output voltage, respectively.

[0019] When the d-axis is positioned on the AC system bus voltage vector, neglecting converter losses, the active power P in the dq synchronous rotating coordinate system is... s and reactive power Q s for:

[0020]

[0021] The Park transformation matrix P and its inverse matrix P -1 They are respectively:

[0022]

[0023]

[0024] Where ω represents the angular frequency of the AC system.

[0025] Preferably, in step S3, the active power P output by the doubly-fed wind turbine is obtained by combining the electromagnetic relationship of the doubly-fed wind turbine. r Reactive power Q r The relationship with the dq current is as follows:

[0026]

[0027] Among them, L m For the mutual inductance between the stator and rotor, L r For the self-inductance of the equivalent two-phase stator winding, ψ r For stator flux linkage, U r For stator voltage, i rd and i rq These are the rotor d-axis and q-axis currents, respectively, u rd The d-axis component of the stator voltage;

[0028] The magnitudes of the d-axis and q-axis currents are:

[0029]

[0030] Where, k rp k ri P is the proportional-integral coefficient of the power outer-loop PI regulator. rref Q rref These are the reference values ​​for the active and reactive power of a doubly-fed wind turbine, i. rd,ref i rq,ref These are the current reference values ​​for the d-axis and q-axis, respectively.

[0031] Preferably, the method for constructing the frequency divider controller is as follows:

[0032] S4.1 Establish the mathematical relationship between AC system frequency and DC voltage:

[0033]

[0034] Where k represents the coupling coefficient, f s f represents the frequency of the AC system. srefU represents the frequency reference value of the AC system. dcref Indicates the DC voltage reference value;

[0035] After a disturbance occurs in the AC system, the frequency divider controller realizes the synchronization characteristics of the doubly-fed wind turbine, and its relationship expression is:

[0036]

[0037] Where, ω g δ represents the angular frequency of the doubly-fed wind turbine, δ represents the power angle, and P represents the angular frequency of the wind turbine. dc U represents DC output power. dc This represents DC voltage; the above expression represents the frequency change f of the AC system. s and synchronous generator angular frequency ω g Since the changes are consistent, the approach for the rotor side of a doubly-fed induction generator (DFIG) to participate in AC system frequency control is as follows: by changing the rotor angular frequency, the rotational kinetic energy of the DFIG rotor is altered. In other words, the angular frequency ω, which is the additional response to the AC system frequency change, is added to the outer loop of the active power control on the DFIG rotor side. href This enables the doubly fed wind turbine to respond to changes in the frequency of the AC system.

[0038] S4.2 The DC voltage U is acquired by the PMU acquisition device. dc and the DC voltage reference value U dcref The DC current I is obtained after comparison and control by the PI controller. dc Then it passes through a first-order low-pass filter. Frequency division is performed to obtain the low-frequency current signal I. lref and high-frequency current signal I href Low-frequency current signal I lref Multiplied by DC voltage U dc Obtain the active power P lref High-frequency current signal I href Multiplied by DC voltage U dc Obtain the active power P href .

[0039] Preferably, the calculation method for the frequency divider controller is as follows:

[0040]

[0041] P href =I href U dc

[0042] P lref =U lref U dc

[0043] Where, k p4With k i4 Let represent the proportional and integral coefficients of the voltage control of the PI controller, respectively; T be the filtering time constant; s represent the complex domain; and I href =I dc -I lref .

[0044] Preferably, the principle of step S5 is as follows: the AC system frequency f is obtained through the PMU acquisition device. s With AC system frequency reference value f sref The difference is calculated to obtain the frequency change value Δf. When |Δf|≥0.1Hz, the frequency divider controller is put into operation; when |Δf|<0.1Hz, the frequency divider controller is taken out of operation.

[0045] Preferably, the upper-level controller and the frequency divider controller constitute three control modes: Mode 1: the power fluctuation of the system is borne by the active power regulation of the wind turbine; Mode 2: the power fluctuation of the system is entirely borne by the DC transmission system; Mode 3: the power fluctuation of the system is jointly borne by the wind farm and the DC transmission system.

[0046] The frequency change Δf of the AC system disturbance is used to determine if a disturbance has occurred when the frequency change Δf exceeds the stable range. The specific formula for this determination is as follows:

[0047]

[0048] The frequency change value Δf of the AC system is obtained by subtracting the system's rated frequency from the measured operating frequency; f1 and f2 are both constants, representing the trigger signal;

[0049] A dead zone is introduced in the upper-level controller, with the dead zone value set to ±0.1Hz, and the upper and lower limits of frequency operation set to 60±0.1Hz.

[0050] Preferably, when a system fault causes the AC system frequency to exceed the limit, the upper-level controller takes action. The DC voltage droop controller of the VSC control structure senses the unbalanced power generated in the AC system through the frequency feedback loop, and changes the active power modulation signal of the doubly fed wind turbine or multi-terminal flexible DC transmission system through the frequency divider controller to quickly increase or decrease the power output, maintain the active power balance of the system, and improve the frequency stability of the AC system.

[0051] Preferably, the frequency divider controller uses the output of the outer voltage loop as the reference current of the inner current loop. The reference current is divided into two parts by a low-pass filter. Part of the power flowing through the low-pass filter into the DC-side power loop is used as the DC-side power loop command value. The remaining power is added to the rotor active power loop as the rotor angular frequency command value. By changing the rotor kinetic energy, the output power of the wind turbine is changed. The low-frequency power is added to the DC side, while the high-frequency power is added to the wind farm, realizing frequency divider control of the wind farm and the DC side.

[0052] The beneficial effects of this invention are as follows: Taking a three-terminal flexible DC transmission system as the research object, a frequency divider controller is designed to coordinate the power output of wind turbines and converter stations in a multi-terminal flexible DC transmission system, addressing AC system disturbances. The acquired frequency deviation signal is divided into different frequency bands, and different frequency regulation strategies are adopted in different frequency bands to ensure the safe operation of the converter station. Finally, a three-terminal flexible DC transmission system with wind power is built on a PSCAD simulation platform, and simulation verification is performed using the system's bus voltage and frequency as control targets. The results demonstrate the rationality of this coordinated control method, which can achieve frequency control of AC / DC hybrid systems, thereby improving system frequency stability and possessing strong reference value. Attached Figure Description

[0053] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0054] Figure 1 This is the overall control block diagram of the multi-terminal flexible DC transmission system and the doubly fed wind turbine of the present invention.

[0055] Figure 2 This is a structural diagram of the doubly fed wind turbine model of the present invention.

[0056] Figure 3 This is a block diagram of the rotor-side control of the doubly fed wind turbine of the present invention.

[0057] Figure 4 This is a structural diagram of the converter of the multi-terminal flexible DC transmission system of the present invention.

[0058] Figure 5 This is a schematic diagram of the parallel radial three-terminal flexible DC transmission system of the present invention.

[0059] Figure 6 This is a graph showing the DC voltage droop control characteristics in the converter station of this invention.

[0060] Figure 7 This is a schematic diagram of the DC voltage droop controller of the present invention.

[0061] Figure 8 This is a schematic diagram of the principle structure of the frequency divider controller of the present invention.

[0062] Figure 9 This is a flowchart illustrating the upper-level control process of the present invention applicable to a frequency divider controller.

[0063] Figure 10 The graph shows the active power / frequency changes of the AC system connected to converter station VSC1 during a single-phase ground fault, where (a) is the frequency change curve of AC system 1, (b) is the output power change curve of converter station VSC1, (c) is the frequency change curve of AC system 2, and (d) is the output power change curve of converter station VSC2. Detailed Implementation

[0064] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0065] like Figure 1 As shown, a frequency-division coordinated control method for wind power and multi-terminal flexible DC transmission systems is proposed to achieve frequency-division coordinated control of wind power and converter stations under fault conditions. This invention divides the control into wind farm level and converter station level. At the converter station level, DC voltage droop control is employed. At the wind farm level, the wind turbine first provides inertial support by releasing rotor kinetic energy during the frequency support phase, raising the minimum frequency point. For different power disturbances, the acquired frequency deviation signals are divided into different frequency bands, and different frequency adjustment strategies are used for coordinated optimization in different frequency bands.

[0066] This invention realizes frequency-division coordinated control of wind power and converter stations of a multi-terminal flexible DC transmission system under fault conditions. The steps are as follows:

[0067] S1: Based on Kirchhoff's voltage and current laws, establish a mathematical model of the voltage source converter in a three-phase stationary coordinate system.

[0068] like Figure 4The diagram shows the converter structure on the rectifier side of the converter station, consisting of equivalent impedance, converter reactor, capacitor, insulated-gate bipolar thyristor, and transmission line. C is the DC capacitor, which maintains the DC voltage; the insulated-gate bipolar thyristor is responsible for turning on and off the current. According to Kirchhoff's voltage and current laws, the mathematical model of the voltage source converter (VSC) in a three-phase stationary coordinate system is as follows:

[0069]

[0070] Among them, L s R s These represent the equivalent inductance and resistance of the converter and line, respectively; i a i b i c Represents the three-phase currents a, b, and c; u a u b u c Indicates the three-phase output voltage of the converter; u sa u sb u sc These represent the three-phase voltages at the busbar of the AC system.

[0071] S2: Perform Park transformation on the mathematical model obtained in step S1 to obtain the mathematical model of the voltage source converter in the dq synchronous rotating coordinate system.

[0072] To facilitate control system design, the mathematical model obtained in step S1 undergoes a Park transformation, converting its three-phase stationary coordinates into two-phase synchronous rotating coordinates, i.e., dq decomposition. The Park transformation matrix P and its inverse matrix P' are then defined. -1 They are respectively:

[0073]

[0074]

[0075] Parker transformation matrix P and its inverse matrix P -1 The Park transform is used for this purpose. The mathematical model of the voltage source converter (VSC) in dq synchronous rotating coordinates, obtained by the Park transform, is:

[0076]

[0077] Among them, i d i q These represent the dq-axis components of the AC system current; ω represents the angular frequency of the AC system; u sd u sq These are the dq-axis components of the AC system bus voltage, respectively; u d u qThese are the dq-axis components of the converter output voltage, respectively.

[0078] When the d-axis is positioned on the AC system bus voltage vector, neglecting converter losses, the active power P in the dq synchronous rotating coordinate system is... s and reactive power Q s It can be represented as:

[0079]

[0080] S3: The active power P output by the doubly-fed wind turbine is obtained based on the rotor-side control structure. r Reactive power Q r The relationship between the d-axis and q-axis currents is used to control the output power of the doubly-fed wind turbine.

[0081] Depend on Figure 1 Rotor-side control structure block diagram, Figure 3 yes Figure 1 The detailed unfolded diagram of rotor-side current control is shown in the figure. The input of the reactive power control outer loop in the figure is the reactive power Q. s and reactive power reference value Q rref The output is the d-axis current reference value i. dref reactive power Q s The power reference value Q is obtained through the power grid PMU acquisition device. rref This is provided by grid operators based on grid operating conditions. The input to the active power control outer loop is ω. dref and ω s The output is the q-axis current reference value i. qref The inner current loop is based on the actual current and the d-axis current reference value i. dref and q-axis current reference value i qref To take control.

[0082] Based on the electromagnetic relationship of the doubly-fed wind turbine, the active power P output by the doubly-fed wind turbine is obtained. r Reactive power Q r The relationship with the dq current is as follows:

[0083]

[0084] Among them, L m For the mutual inductance between the stator and rotor, L r For the self-inductance of the equivalent two-phase stator winding, ψ r For stator flux linkage, U r For stator voltage, i rd and i rq These are the rotor d-axis and q-axis currents, respectively, u rd This represents the d-axis component of the stator voltage.

[0085] The output power of a doubly-fed wind turbine can be controlled by adjusting the magnitudes of the d-axis and q-axis currents.

[0086]

[0087] Where, k rp k ri P is the proportional-integral coefficient of the power outer-loop PI regulator. rref Q rref These are reference values ​​for the active and reactive power of a doubly-fed induction generator (DFIG), respectively. Specific values ​​will be provided by grid operators based on grid operating conditions. rd,ref i rq,ref These are the current reference values ​​for the d-axis and q-axis, respectively. Adjust the current reference value i. rd,ref i rq,ref The output power of the doubly-fed wind turbine will change accordingly, thus enabling control of the output power of the doubly-fed wind turbine.

[0088] S4: Combining steps S2 and S3, construct as follows Figure 8 The frequency divider controller shown is used to coordinate the doubly-fed wind turbine and the multi-terminal flexible DC transmission system to participate in frequency regulation of the AC system and maintain the stability of the AC system frequency in the event of power disturbances caused by AC system faults.

[0089] The specific implementation steps of step S4 are as follows:

[0090] S4.1 First, establish the mathematical relationship between the AC system frequency and the DC voltage:

[0091]

[0092] Where k represents the coupling coefficient; the closer the connection between the AC and DC systems, the larger the value of k, and vice versa; f s Indicates the frequency of the AC system; f sref This indicates that the frequency reference value for the AC system is the rated value. dcref This indicates the reference value for DC voltage.

[0093] After a disturbance occurs in the AC system, the frequency divider controller will realize the synchronization characteristics of the doubly-fed wind turbine, and its relationship expression is as follows:

[0094]

[0095] Where, ω g δ represents the angular frequency of the doubly-fed wind turbine, δ represents the power angle, and P represents the angular frequency of the wind turbine. dc U represents DC output power. dc This represents the DC voltage. The above expression represents the frequency change f of the AC system. s and synchronous generator angular frequency ωg Since the changes are consistent, the approach to participating in AC system frequency control on the rotor side of a doubly-fed induction generator (DFIG) is to change the rotor's angular frequency, thereby altering the rotor's rotational kinetic energy. Figure 3 As shown, the angular frequency ω is the frequency change of the AC system frequency in the additional response loop of the active power control outer loop on the rotor side of the doubly fed wind turbine generator. href This enables the doubly fed wind turbine to respond to changes in the AC system frequency.

[0096] S4.2 The DC voltage U is acquired by the PMU acquisition device. dc and the DC voltage reference value U dcref The DC current I is obtained after comparison and control by the PI controller. dc Then it passes through a first-order low-pass filter. Frequency division is performed to obtain the low-frequency current signal I. lref and high-frequency current signal I href Low-frequency current signal I lref Multiplied by DC voltage U dc Obtain the active power P lref High-frequency current signal I href Multiplied by DC voltage U dc Obtain the active power P href .

[0097] The numerical relationships are as follows:

[0098]

[0099] P href =I href U dc

[0100] P lref =U lref U dc

[0101] Where, k p4 With k i4 These represent the proportional and integral coefficients of the voltage control in the PI controller, respectively. s represents the complex domain, and I... href =I dc -I lref . Figure 7 It is based on Figure 3 The active power P output by the frequency divider controller lref It is superimposed on the original droop controller.

[0102] According to the principle of first-order low-pass filtering, the value of the filtering time constant affects the passband of the filter. As the filtering time constant T increases, the passband of the filter narrows, the component of power fluctuation smoothing by the wind turbine gradually decreases, and the component of power fluctuation undertaken by the DC system gradually increases. Therefore, the active power of multi-terminal flexible DC transmission system and doubly-fed wind turbine participating in AC system frequency regulation can be adjusted by changing the value of the filtering time constant T.

[0103] The purpose of inner-loop current control is to track the reference current, thereby controlling active and reactive power. Figure 2 This is a simplified diagram of the operating control principle of a doubly-fed wind turbine, clearly demonstrating the operating control principle of a doubly-fed wind turbine. Figure 6 This is the control characteristic diagram of a DC voltage droop controller, based on... Figure 7 The control principle of the droop controller is obtained, demonstrating the specific characteristics of DC voltage droop control.

[0104] S5, based on step S4, an upper-level controller is introduced on the basis of the frequency divider controller to ensure the sensitive operation of the frequency divider controller and to avoid frequent operation of the frequency divider controller. Specifically, the AC system frequency f is obtained through the PMU acquisition device. s With AC system frequency reference value (rated value) f sref The difference is calculated to obtain the frequency change value Δf. When |Δf|≥0.1Hz, the frequency divider controller is put into operation; when |Δf|<0.1Hz, the frequency divider controller is taken out of operation.

[0105] The upper-level controller and the frequency divider controller combine to form three control modes: Mode 1: Power fluctuations in the system are handled by the active power regulation of the wind turbines; Mode 2: Power fluctuations in the system are entirely handled by the DC transmission system; Mode 3: Power fluctuations in the system are jointly handled by the wind farm and the DC transmission system. The control flow is as follows: Figure 9 As shown.

[0106] according to Figure 9 As can be seen, firstly, by detecting the frequency change value Δf of the AC system disturbance, when the frequency change value Δf exceeds the stable range, it is determined that the system has experienced a disturbance. The specific judgment formula is as follows:

[0107]

[0108] The upper-level controller is combined with a frequency divider controller. Here, Δf represents the frequency change of the AC system, obtained by subtracting the system's rated frequency (60Hz) from the measured operating frequency. f1 and f2 are constants representing the trigger signals. The frequency comparison stage is crucial to the overall system sensitivity. If the frequency change Δf is too small, the frequency divider controller will be too sensitive, and normal frequency fluctuations will cause unnecessary controller actions. If the frequency change is too large, the delayed action of the frequency divider controller will not achieve the desired control effect. To ensure good control speed and demonstrate the VSC-multi-terminal flexible DC transmission system's ability to quickly restore frequency stability, a dead zone is introduced before the upper-level controller. The dead zone value is set to ±0.1Hz, and the upper and lower limits of frequency action are set to 60±0.1Hz.

[0109] When a system fault causes the AC system frequency to exceed the limit, the upper-level controller takes action. The DC voltage droop controller of the VSC control structure senses the unbalanced power generated in the system through the frequency feedback loop. By changing the active power modulation signal of the doubly-fed wind turbine or multi-terminal flexible DC transmission system, it quickly increases or decreases the power output to maintain the active power balance of the system and improve the frequency stability of the AC system.

[0110] This invention's frequency divider controller uses the output of the outer voltage loop as the reference current for the inner current loop. This reference current is split into two parts by a low-pass filter. Part of the power passing through the low-pass filter flows into the DC-side power loop, serving as the DC-side power loop command value. The remaining power is added to the rotor active power loop as the rotor angular frequency command value. By changing the rotor kinetic energy, the wind turbine's output power is altered. Low-frequency power is added to the DC side, while high-frequency power is added to the wind farm, thus achieving frequency divider control between the wind farm and the DC side.

[0111] To verify the effectiveness and robustness of the present invention, a simulation model of a three-terminal flexible DC transmission system with a doubly fed wind turbine was built in PSCAD software. The system power fluctuations were adjusted to make the three-terminal flexible DC transmission system operate under different conditions, thereby verifying the effectiveness of the additional frequency divider controller.

[0112] A three-terminal flexible DC transmission system incorporating a wind farm was constructed using PSCAD software. The topology is as follows: Figure 5As shown. Specifically, one sending-end AC system includes a thermal power generator set S1, a doubly-fed wind turbine generator set WF, and a voltage source converter VSC1. S1 and WF are both connected to the voltage source converter VSC1, which is connected to the DC bus. Another sending-end AC system includes a thermal power generator set S2 and a voltage source converter VSC2, which is connected to the DC bus system. The receiving-end AC system includes a voltage source converter VSC3 and a thermal power generator set S3, which is connected to both the DC bus and the thermal power generator set S2.

[0113] The installed capacities of generator sets S1, S2, and S3 are 384, 288, and 288 MW, respectively; the installed capacity of the doubly-fed wind turbine is 100 MW. The rated capacities of the converter stations are 60, 60, and 120 MW, respectively; the rated positive and negative DC voltage of the DC transmission system is 200 kV; VSC1 and VSC2 are rectifier stations, VSC3 is an inverter station, and the wind turbine generator sets are connected to VSC1. In the initial state, the active power command values ​​for each converter station are -40, -35, and 75 MW, respectively. In the three-terminal system, VSC1 and VSC2 are two power transmitting ends, and VSC3 on the inverter side is considered a power receiving end. Assuming the receiving end system (i.e., the receiving-end AC system) is subject to disturbance, insufficient power will cause the rotor speed of the receiving-end generator to decrease, resulting in a lower frequency. Interference will cause the rotor speed of the transmitting-end generator to increase, resulting in a higher frequency.

[0114] The invention is verified by setting up fault scenarios. The simulation results under the conventional DC voltage control strategy are represented by black lines, while the simulation results under the control strategy of this invention are represented by red lines; f1, f2 and P1, P2 represent the frequency and DC active power of AC systems S1 and S2, respectively.

[0115] Example: At time 3s, a single-phase ground fault occurs in the AC system connected to converter station VSC1, which is controlled by DC voltage droop, for a duration of 0.05s. The grounding resistance is 0.05Ω. Figure 10 A comparative analysis of simulation results between the control strategy of this invention and conventional control strategies is presented. Through, as shown in... Figure 10 The changes in bus frequency and DC active power show that, for single-phase grounding faults, the control strategy of this invention can coordinate the output of the doubly fed wind turbine and the power output of the converter station of the multi-terminal flexible DC transmission system, thereby improving the frequency stability of the system and proving the effectiveness of the method proposed in this invention.

[0116] This invention establishes a mathematical model of a multi-terminal flexible DC transmission system including wind power based on the connection relationships of the system. It derives the rotor voltage and flux linkage equations from the mathematical model of a doubly-fed induction generator (DFIG). A frequency divider controller is introduced to coordinate the output of the DFIG wind turbine and the power output of the multi-terminal flexible DC converter station based on the magnitude of system frequency fluctuations. Building upon this frequency divider control, an upper-level controller is introduced to compare trigger signals f1 and f2 with the frequency change Δf, ensuring sensitive operation of the controller. Figure 5 The simulation system shown is configured with a fault to verify the effectiveness of the invention. At time 3s, a single-phase ground fault with a duration of 0.05s and a grounding resistance of 0.05Ω is simulated in AC system S1 connected to VSC1. The simulation results are as follows: Figure 10 The results shown are based on Figure 10 The curve comparison results verified the effectiveness of the present invention.

[0117] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A frequency-division coordinated control method for wind power and multi-terminal flexible DC transmission systems, characterized in that, The steps are as follows: S1: Based on Kirchhoff's voltage and current laws, establish a mathematical model of the voltage source converter in a three-phase stationary coordinate system. S2: Perform a Park transformation on the mathematical model from step S1 to obtain the mathematical model of the voltage source converter in the dq synchronous rotating coordinate system, and calculate the active power P in the dq synchronous rotating coordinate system. s and reactive power Q s ; S3: The active power P output by the doubly-fed wind turbine is obtained based on the rotor-side control structure. r and reactive power Q r According to the active power P r Reactive power Q r Calculate the magnitude of the current along the d-axis and q-axis to control the output power of the doubly-fed wind turbine. S4: Combining steps S2 and S3, construct a frequency divider controller; for power disturbances caused by AC system faults, the frequency divider controller coordinates the doubly fed wind turbine and the multi-terminal flexible DC transmission system to jointly participate in the frequency regulation of the AC system and maintain the stability of the AC system frequency; S5: Introducing an upper-level controller, which communicates with the frequency divider controller to ensure the sensitive operation of the frequency divider controller and avoid frequent operation of the frequency divider controller; In step S1, a mathematical model of the voltage source converter in a three-phase stationary coordinate system is established based on the converter structure on the rectifier side of the converter station. ； Among them, L s R s These represent the equivalent inductance and resistance of the converter and line, respectively; i a i b i c Represents the three-phase currents a, b, and c; u a u b u c Indicates the three-phase output voltage of the converter; u sa u sb u sc These represent the three-phase voltages at the busbar of the AC system; The mathematical model of the voltage source converter in step S2 under the dq synchronous rotating coordinate system is as follows: ； Among them, i d i q These are the dq-axis components of the AC system current, respectively. Represents the angular frequency of an AC system; u sd u sq These are the dq-axis components of the AC system bus voltage, respectively; u d u q These are the dq-axis components of the converter output voltage, respectively. When the d-axis is positioned on the AC system bus voltage vector, neglecting converter losses, the active power P in the dq synchronous rotating coordinate system is... s and reactive power Q s for: ； The Park transformation matrix P and its inverse matrix -1 They are respectively: ； ； in, This represents the angular frequency of the AC system.

2. The frequency division coordination control method for wind power and multi-terminal flexible DC transmission system according to claim 1, characterized in that, In step S3, the active power P output by the doubly-fed wind turbine is obtained by combining the electromagnetic relationship of the doubly-fed wind turbine. r Reactive power Q r The relationship with the dq current is as follows: ； Among them, L m For the mutual inductance between the stator and rotor, L r For the self-inductance of the equivalent two-phase stator winding, ψ r For stator flux linkage, U r For stator voltage, i rd and i rq These are the rotor d-axis and q-axis currents, respectively, u rd The d-axis component of the stator voltage; The magnitudes of the d-axis and q-axis currents are: ； Where, k rp k ri P is the proportional-integral coefficient of the power outer-loop PI regulator. rref Q rref These are the reference values ​​for the active and reactive power of a doubly-fed wind turbine, i. rd,ref i rq,ref These are the current reference values ​​for the d-axis and q-axis, respectively.

3. The frequency division coordination control method for wind power and multi-terminal flexible DC transmission system according to claim 2, characterized in that, The method for constructing the frequency divider controller is as follows: S4.1 Establish the mathematical relationship between AC system frequency and DC voltage: ； Where k represents the coupling coefficient, f s f represents the frequency of the AC system. sref This represents the frequency reference value for the AC system. Indicates the DC voltage reference value; After a disturbance occurs in the AC system, the frequency divider controller realizes the synchronization characteristics of the doubly-fed wind turbine, and its relationship expression is: ； Where, ω g This represents the angular frequency of a doubly-fed wind turbine. Indicates the angle of action. Indicates DC output power. This represents DC voltage; the above expression represents the frequency change f of the AC system. s and synchronous generator angular frequency ω g Since the changes are consistent, the approach for the rotor side of a doubly-fed induction generator (DFIG) to participate in the frequency control of the AC system is as follows: by changing the rotor angular frequency, the rotational kinetic energy of the DFIG rotor is altered. In other words, the angular frequency ω, which is the additional response to the frequency change of the AC system, is added to the outer loop of the active power control on the rotor side of the DFIG. href This enables the doubly fed wind turbine to respond to changes in the frequency of the AC system. S4.2 The DC voltage U is acquired by the PMU acquisition device. dc and the DC voltage reference value U dcref The DC current I is obtained after comparison and control by the PI controller. dc Then it passes through a first-order low-pass filter. Frequency division is performed to obtain the low-frequency current signal I. lref and high-frequency current signal I href Low-frequency current signal I lref Multiplied by DC voltage U dc Obtain the active power P lref High-frequency current signal I href Multiplied by DC voltage U dc Obtain the active power P href .

4. The frequency division coordination control method for wind power and multi-terminal flexible DC transmission system according to claim 3, characterized in that, The calculation method for the frequency divider controller is as follows: ； ； ； Where, k p4 With k i4 Let represent the proportional and integral coefficients of the PI controller, respectively; T be the filtering time constant; and s represent the complex domain. =I dc -I lref .

5. The frequency division coordination control method for wind power and multi-terminal flexible DC transmission system according to claim 3 or 4, characterized in that, The principle behind step S5 is as follows: the AC system frequency f is obtained through the PMU acquisition device. s With AC system frequency reference value f sref By taking the difference, we obtain the frequency change value Δf. The frequency divider controller is put into operation; when The frequency divider controller has been shut down.

6. The frequency division coordination control method for wind power and multi-terminal flexible DC transmission system according to claim 5, characterized in that, The upper-level controller and frequency divider controller constitute three control modes: Mode 1: the power fluctuation of the system is borne by the active power regulation of the wind turbine; Mode 2: the power fluctuation of the system is entirely borne by the DC transmission system; Mode 3: the power fluctuation of the system is jointly borne by the wind farm and the DC transmission system. The frequency change Δf of the AC system disturbance is used to determine if a disturbance has occurred when the frequency change Δf exceeds the stable range. The specific formula for this determination is as follows: ； The frequency change value Δf of the AC system is obtained by subtracting the system's rated frequency from the measured operating frequency; f1 and f2 are both constants, representing the trigger signal. A dead zone is introduced in the upper-level controller, with the dead zone value set to ±0.1Hz, and the upper and lower limits of frequency operation set to 60±0.1Hz.

7. The frequency division coordination control method for wind power and multi-terminal flexible DC transmission system according to claim 6, characterized in that, When a system fault causes the AC system frequency to exceed the limit, the upper-level controller takes action. The DC voltage droop controller of the VSC control structure senses the unbalanced power generated in the AC system through the frequency feedback loop. It changes the active power modulation signal of the doubly-fed wind turbine or multi-terminal flexible DC transmission system through the frequency divider controller, which quickly increases or decreases the power output, maintains the active power balance of the system, and improves the frequency stability of the AC system.

8. The frequency division coordination control method for wind power and multi-terminal flexible DC transmission system according to claim 1 or 7, characterized in that, The frequency divider controller uses the output of the outer voltage loop as the reference current of the inner current loop. The reference current is divided into two parts by a low-pass filter. Part of the power flowing through the low-pass filter into the DC-side power loop is used as the DC-side power loop command value. The remaining power is added to the rotor active power loop as the rotor angular frequency command value. By changing the rotor kinetic energy, the output power of the wind turbine is changed. The low-frequency power is added to the DC side, while the high-frequency power is added to the wind farm, realizing frequency divider control of the wind farm and the DC side.

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