Cooperative control method for assisting local power grid frequency and voltage support by wind power system

Abstract

The invention discloses a cooperative control method for assisting a local power grid frequency and voltage support by a wind power system. The method comprises the following steps: 1) considering local load equivalent impedance characteristics and a line power flow influence active power and reactive power demand quantification strategy; 2) a frequency modulation active power demand quantification strategy based on inertia control and droop control and a voltage regulation reactive power demand quantification strategy based on droop control; 3) considering converter station capacity limitation and frequency voltage support strength, and realizing frequency voltage cooperative control; according to the method, on the basis of a novel power system in which an offshore wind plant is connected with an IEEE39 standard node system through a flexible direct-current power transmission system, local load equivalent impedance characteristics and power flow influences are considered to quantify power requirements, and the side with low power grid strength is preferentially supported according to the voltage strength, so that the power grid strength is improved; and frequency and voltage cooperative control under a deep fault condition is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy power systems, and specifically is a coordinated control method for wind power systems to assist local power grid frequency and voltage support. Background Art

[0002] With the rapid development of large-scale wind farms, multi-terminal high-voltage direct current systems based on voltage source converters have become a flexible solution for large-scale integration and absorption of wind farms (Y.Xiong, W.Yao, J.Wen, et al., "Two-Level Combined Control Scheme of Voltage Support C-MTDC Integrated Of Wind Farms for Onshore System Frequency Support," IEEE Transactions on Power Systems, vol. 36, no. 1, pp. 781-792, Jan. 2021). As the scale of wind farms continues to expand and replace some traditional synchronous generators, the grid's rotational inertia and corresponding frequency regulation capabilities are reduced. If the grid does not have enough rotational inertia, its frequency deviation and frequency change rate will increase, leading to frequency over-limit problems (Z. Lu, Y. Ye and Y. Qiao, "An Adaptive Frequency Regulation Method With Grid-Friendly Restoration for Voltage Support C-HVDC Integrated of Frequency Support Wind Farms," ​​IEEE Transactions on Power Systems, vol. 34, no. 5, pp. 3582-3593, Sept. 2019). However, the traditional centralized control strategy enables wind farms to participate in active frequency support methods without considering the differentiated frequency support capacity of wind turbines caused by different wind speeds due to the wake effect. The average active frequency support power provided by wind turbines is likely to lead to insufficient frequency support capacity of low-speed wind turbines, while the frequency support capacity of high-speed wind turbines is sufficient or even excessive. At the same time, the centralized control strategy needs to be highly dependent on communication reliability and is easily affected by communication delays and communication failures (Z. Wang and W. Wu, "Coordinated Control Method for DFIG-Based Wind Farm to Provide Primary Frequency Regulation Service," IEEE Transactions on Power Systems, vol. 33, no. 3, pp. 2644-2659, May 2018). Therefore, a method for quantifying the active frequency support capability of wind farms that takes into account the wake effect is urgently needed.However, the current wind farm consistency control method only considers the influence of rotor kinetic energy (Y.Xiong, W Yao, Y.Yao, et al., "Distributed Cooperative Control of Wind Farms Integrated via MTDC System for Fast Frequency Support," IEEE Transactions on Industrial Electronics, vol. 70, no. 5, pp. 4693-4704, May 2023), ignoring the problem of wind turbine operating point offset caused by active frequency support, and cannot accurately quantify the active frequency support capability of wind turbines. In order to quantify the active frequency support capability of wind farms under the influence of wake effect, and at the same time maximize the provision of active frequency support services for wind farms;.

[0003] Therefore, some people are currently applying for the following application to solve the above problems;

[0004] Application number: CN109494811B, Patent name: "A power control method and system for wind farm units participating in frequency and voltage regulation" Technical comparison:

[0005] The technical solution of "a power control method and system for wind farm units participating in frequency and voltage regulation" involves a power control method and system for wind farm units participating in frequency and voltage regulation. Active power-frequency droop control is used to perform a frequency regulation, and the system is adaptively regulated based on the conditions of the wind farm itself. And reactive power-voltage droop control is used to perform a voltage regulation to achieve the regulation of the grid-connected voltage.

[0006] In order to solve the problem of weakened voltage and frequency regulation capability of offshore wind farms after flexible direct current grid connection, this paper proposes a coordinated control method for wind power system to assist local grid frequency and voltage support, which takes into account the grid strength and power flow demand of the system. The active and reactive power demands of local load, power flow demand and line loss are considered respectively. Based on the grid criteria, the construction of node frequency and voltage disturbance information, the active power demand during frequency regulation and the reactive power demand during voltage regulation are considered to obtain the reactive power demand during frequency regulation and the active power demand during voltage regulation. The capacity limitation of the converter station is considered. When the capacity of the converter station is insufficient, the grid strength (frequency support strength and voltage support strength) of the converter station node is used to coordinate the active and reactive power distribution.

[0007] Application number: CN114614504A, Patent name: Technical comparison of "Quantitative evaluation method of energy storage capacity considering frequency support requirements and low voltage ride-through":

[0008] The technical solution of "Quantitative evaluation method of energy storage capacity considering frequency support demand and low voltage ride-through" discloses a quantitative evaluation method of energy storage capacity considering frequency support demand and low voltage ride-through. The active power variation characteristics and system frequency dynamic characteristics are analyzed when the wind turbine grid-connected voltage drops to different degrees; the control parameters are configured according to the frequency response index; the energy storage capacity required by the wind power transmission system is quantitatively analyzed considering low voltage ride-through and system frequency restrictions.

[0009] This paper focuses on the coordinated control of wind power systems to assist local power grid frequency and voltage support. It analyzes the scenario of simultaneous frequency and voltage drops when large-scale wind farms are connected to the local power grid. It considers the active and reactive power demands of local loads, power flow demands, and line losses, as well as the capacity limitations of converter stations. A frequency and voltage coordinated control method based on power grid strength is proposed.

[0010] Application number: CN115800296B, patent name "Voltage and frequency coordinated support method for offshore wind power grid-connected system via VSC-MTDC" Technical comparison:

[0011] The technical proposal of "Voltage and frequency coordinated support method for offshore wind power grid-connected system via VSC-MTDC" discloses a coordinated control method for wind power system to assist local power grid frequency and voltage support. The coordinated allocation index is constructed by frequency and voltage deviation and change rate, and static load is used to quantify the active power and reactive power demand of the local power grid, and then the coordinated support of frequency and voltage is achieved through converter station capacity limitation.

[0012] Aiming at the coordinated control of the frequency and voltage support of the local power grid assisted by the wind power system, this paper proposes a coordinated control method for the frequency and voltage support of the local power grid assisted by the wind power system, which takes into account the system grid strength and power flow demand. The active and reactive power demands of the local load, power flow demand and line loss are considered respectively, and the capacity limitation of the converter station is considered. When the capacity of the converter station is insufficient, the active and reactive power are coordinatedly distributed through the grid strength (frequency support strength and voltage support strength) of the converter station node.

[0013] In order to solve the problem of weakened frequency and voltage support capacity of the system after large-scale wind farms are connected to the grid through flexible direct current (FDC), this paper proposes a coordinated control method for wind power system to assist local power grid frequency and voltage support. The method takes into account the active and reactive power demand of local load, flow demand and line loss, and is based on the grid strength (frequency support strength and voltage support strength) of the converter station grid connection node. The capacity limitation of the converter station is taken into account, so that the frequency and voltage can be coordinated to support the frequency and voltage after the converter station reaches the capacity limit, thereby improving the stability of the power grid. Summary of the invention

[0014] In order to solve the above technical problems, the present invention proposes a coordinated control method for wind power system to assist local power grid frequency and voltage support. The present invention is based on a new power system in which an offshore wind farm is connected to the IEEE39 standard node system via a flexible direct current transmission system. The method takes into account the local load equivalent impedance characteristics and the influence of power flow on quantified power demand, and by giving priority to supporting the side with lower grid strength according to the voltage strength, frequency and voltage coordinated control is achieved in the case of deep faults.

[0015] To achieve the above object, the technical solution adopted by the present invention is:

[0016] A coordinated control method for wind power system-assisted local power grid frequency and voltage support, the method comprising the following steps:

[0017] 1) A strategy to quantify active and reactive power demand considering the equivalent impedance characteristics of local loads and the influence of line power flow;

[0018] 2) Quantification strategy of active power demand for frequency regulation based on inertia control and droop control and quantification strategy of reactive power demand for voltage regulation based on droop control;

[0019] 3) It is proposed to realize frequency and voltage coordinated control by taking into account the capacity limitation of converter stations and the frequency and voltage support strength.

[0020] As a further improvement of the present invention, step 1) is specifically as follows;

[0021] The offshore wind farm is connected to the onshore power grid through VSC-MTDC. The onshore power grid contains multiple local synchronous generators, complex networks and various loads. When a large disturbance occurs in the onshore power grid, all power sources including synchronous generators, new energy converter stations and loads will participate in frequency support and voltage support to restore frequency and voltage with reasonable power flow. Assuming that the new energy converter station is connected to the i-th node of the onshore power grid, under large disturbances, in order to restore the frequency and voltage of the i-th node by injecting support power, the power flow should be re-formed, while considering the fU recovery power demand of the local load and the power flow and power loss of the adjacent transmission lines. Assuming that there are N adjacent nodes directly connected to the i-th node through transmission lines. The total power demand for fU recovery is expressed as:

[0022]

[0023] In the formula, ΔP require,i and ΔQ require,i represents the total active / reactive power demand of node i, ΔP load,i and ΔQ load,i represents the active / reactive power demand of local load i, ΔP trans,i and ΔQ trans,irepresents the active / reactive power transfer demand from local load i to neighboring node j, ΔP loss,ij and ΔQ loss,ij represents the active / reactive power loss from node i to j;

[0024] 1. Consider the power requirements of local load characteristics;

[0025] The equivalent impedance of the node load can be calculated as follows:

[0026]

[0027]

[0028] In the formula, R ei,i , X ei,i are the real-time equivalent resistance and inductance of the local load at node i respectively; U i is the real-time voltage at node i; P i , Q i are the real-time active power and reactive power injected into the node load, respectively;

[0029] According to the static characteristics of load frequency, the relationship between real-time load power and frequency fluctuation is in the form of a first-order equation:

[0030]

[0031]

[0032] Where Δf i represents the frequency deviation at node i. pf and k qf Represents the correlation coefficient between active / reactive power and frequency, P * load,i and Q * load,i Represents the rated active / reactive power of node i. load,i and Q load,i represents the active / reactive power demand of node i;

[0033] According to (12)-(15), the active / reactive power requirements for frequency support / voltage support considering the static characteristics of load frequency and local load characteristics are as follows:

[0034]

[0035]

[0036] Where U * i represents the rated voltage of node i;

[0037] 2. Consider the power demand of the line flow;

[0038] In a complex network, the frequency support / voltage support performance of the i-th node of the integrated new energy converter station is closely related to the impedance and power flow of the adjacent transmission line. Therefore, the fU recovery power requirement of the i-th node should also consider the power outflow from node i to the adjacent nodes and the line power loss;

[0039] Based on the Newton-Raphson method for power flow analysis of onshore power systems, the received power transmitted from node i to adjacent node j can be expressed as:

[0040]

[0041]

[0042] In the formula, I ij Represents the line transmission current from node i to j. δ i represents the power angle of node i, X ij P represents the line resistance from node i to j. j and Q j represents the active / reactive power outflow from node i to adjacent node j;

[0043] According to the transmission line impedance and power flow, the voltage of the jth adjacent node is calculated as follows:

[0044]

[0045] Where P ij and Q ij represents the active / reactive power outflow from node i to j, R ij represents the line resistance from node i to j, ΔU ij represents the voltage drop between nodes i and j;

[0046] The active / reactive power requirements of FS / VS considering the load frequency static characteristics and power flow are as follows:

[0047]

[0048]

[0049] The transmission line power loss from node i to adjacent node j is calculated as follows:

[0050]

[0051]

[0052] As a further improvement of the present invention, step 2) is as follows:

[0053] When a large disturbance occurs, the quantification of active power and reactive power for frequency support and voltage support is important for fU recovery, expressed as,

[0054] ΔP require,i =ΔP FS,i +ΔP VS,i +ΔP SG,i (16)

[0055] ΔQ require,i =ΔQ FS,i +ΔQ VS,i +ΔQ SG,i (17)

[0056] In the formula, ΔP FS,i and ΔP VS,i Indicates the active power injected for frequency support / voltage support; ΔQ FS,i and ΔQ VS,i Represents the reactive power injected for frequency support / voltage support; ΔP SG,i and ΔQ SG,i represents the increment of active and reactive power injected by the synchronous generator into node i, which can be calculated based on the power flow before and after the fault;

[0057] 1) Provide active power and reactive power for frequency support:

[0058] For OWF connected to VSC-MTDC, the frequency support service of OWF is actually implemented by the new energy converter station. Since there are multiple new energy converter stations, it is necessary to consider the power distribution problem between the new energy converter stations. At present, the power coordination between the new energy converter stations considers the PCC frequency information and cannot fully coordinate the energy flow between different DC links. Therefore, the power distribution between the new energy converter stations needs to further consider the DC bus voltage;

[0059] The relationship between frequency and active power can be obtained through the mechanical angular motion equation:

[0060]

[0061] In the formula, H GSVSC,i represents the inertia time constant of the i-th renewable energy converter station, P m and P e Represent mechanical power and electromagnetic power respectively, ΔP 1 It represents the virtual kinetic energy absorbed or released by the new energy converter station inertia during the frequency support process;

[0062] The relationship between DC voltage and active power is expressed by the capacitor dynamic equation:

[0063]

[0064] Where P in and P out Respectively represent the input and output power of the new energy converter station. dc,i Indicates the DC capacitor capacity, S GSVSC,i represents the rated capacity of the i-th renewable energy converter station, N i represents the number of DC capacitors in the i-th renewable energy converter station, ΔP 2 Indicates the dynamic electrostatic power stored or released by the HVDC capacitor;

[0065] Due to the power change ΔP of the new energy converter station 1 Equal to the capacitor power change ΔP 2 , so the relationship between DC voltage and onshore grid frequency is expressed as:

[0066]

[0067] From the above formula, we can see that the larger the frequency deviation, the greater the U dc Therefore, by considering the DC voltage difference, the ΔP between new energy converter stations is constructed GSVSC,i The power distribution is as follows:

[0068] ΔU dc,ij =U dc,i -U dc,j (twenty one)

[0069] ΔP GSVSC,i =k GSVSC,i (ΔU dc,ij0 -ΔU dc,ij ) (twenty two)

[0070] In the formula, ΔU dc,ij0 and ΔU dc,ij They represent the DC voltage differences before and after the disturbance of DC lines i to j respectively;

[0071] Then, based on the existing frequency-voltage-power droop and inertia control of the new energy converter station, the power distribution between the new energy converter stations is considered, and the active power of the frequency support of the new energy converter station is quantified as follows:

[0072]

[0073] In the formula, k inertia,i and k droop,i are the inertia coefficient and droop coefficient of the frequency support respectively. vp,i is the Udc-P droop coefficient of the i-th renewable energy converter station;

[0074] According to the reactive power relationship in formula (26) and the capacity limit of the new energy converter station, the reactive power supply ΔQ of the frequency support FS,i It can be expressed as:

[0075]

[0076] In the formula, S GSVSCi,n and S GSVSC,i Indicates the rated capacity and real-time injected power of the new energy converter station;

[0077] 2) Active power and reactive power provided for voltage support;

[0078] When a large disturbance occurs, the voltage of the grid-connected node of the new energy converter station exceeds the voltage threshold of 0.9pu~1.1pu, triggering the droop control, which can be expressed as:

[0079]

[0080] In the formula, k VS,i Indicates the droop coefficient of voltage support;

[0081] According to the active power relationship in formula (25) and the capacity limitation of the new energy converter station, the active power supply ΔP of the voltage support VS,i Expressed as:

[0082]

[0083] As a further improvement of the present invention, step 3) is as follows:

[0084] Different degrees of frequency and voltage drops will require the new energy converter station to support different active and reactive powers. Considering the capacity limitations of the new energy converter station, the active / reactive power reference of the new energy converter station should consider the following two scenarios of frequency support and voltage support.

[0085] Scenario 1: The rated capacity of the new energy converter station is sufficient. When the frequency and voltage on the new energy converter station side drop slightly, the support power may not exceed the rated capacity. The reference active and reactive power of the new energy converter station is the sum of the active and reactive power of the frequency support and voltage support, that is:

[0086] P ref,i =P i0 +ΔP FS.i +ΔP VS.i (27)

[0087] Q ref,i =Q i0 +ΔQ FS.i +ΔQ VS.i (28)

[0088] Where P ref,i and Q ref,i Respectively represent the active power and reactive power reference values ​​of the new energy converter station. i0 and Q i0 They respectively represent the initial active power and reactive power of the new energy converter station.

[0089] Scenario 2: The rated capacity of the new energy converter station is insufficient. When the frequency and voltage on the new energy converter station side drop severely, the support power may exceed the rated capacity, and there will be a frequency-voltage coordination problem. The active and reactive power of frequency support and voltage support need to be redistributed. The principle of coordination is that whoever drops more will have more support power. A frequency-voltage coordinated support method is proposed to construct a coordination coefficient considering the strength of the power grid. The formula is:

[0090]

[0091]

[0092] In the formula, α FVSS,i represents the coordination coefficient of node i.

[0093] The frequency-voltage coordination coefficient can be obtained by the node frequency strength SQ f,i and voltage intensity SQ v,i The calculation is as follows:

[0094]

[0095] Compared with the prior art, the present invention has the following beneficial effects:

[0096] The present invention proposes a coordinated control method for wind power system-assisted local power grid frequency and voltage support, which takes into account the local load equivalent impedance characteristics and power flow characteristics of the local power grid during a fault, fully analyzes the active power demand and reactive power demand under system fault conditions, takes into account the capacity limitation of the converter station under deep fault conditions, and performs frequency and voltage coordinated support according to the frequency and voltage support strength of the local power grid nodes. It has high practical value in the frequency and voltage coordinated support of new power systems containing large-scale wind farms. BRIEF DESCRIPTION OF THE DRAWINGS

[0097] Figure 1 is a system control flow chart of the present invention;

[0098] Figure 2 This is the principle diagram of quantification of active and reactive power demand;

[0099] Figure 3 is the topological structure diagram of the example;

[0100] Figure 4It is a diagram of distributed consistency control strategy for offshore wind farms;

[0101] Figure 5 It is the small signal stability analysis diagram of the example model;

[0102] Figure 6 It is a comparison chart of voltage support strategies at different frequencies under short-circuit faults;

[0103] Figure 7 It is the response diagram of offshore wind farm and new energy converter station under short circuit fault;

[0104] Figure 8 This is a comparison chart of voltage support strategies at different frequencies under load surge faults;

[0105] Fig. 9 This is the response diagram of the offshore wind farm and new energy converter station under a sudden load increase fault. DETAILED DESCRIPTION

[0106] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments:

[0107] The present invention discloses a method for quantifying the active frequency support capability of a wind farm considering the wake effect, including evaluation of the operating status of a single wind turbine, distributed consistency control of wind turbines in a wind farm based on a leader-follower algorithm, and a method for quantifying the active frequency support capability of a wind farm, wherein the system control flow chart is as follows: Figure 1 As shown in the figure, this method first calculates the power demand of local load, line flow and line loss, and then calculates the active power supply during frequency regulation and the reactive power supply during voltage regulation according to the grid code. Finally, based on the power demand and grid code, the reactive power demand during frequency regulation and the active power demand during voltage regulation are calculated. Finally, considering the capacity limitation of the converter station, the coordinated control of frequency and voltage is achieved according to the grid strength.

[0108] The principle diagram of active and reactive power demand quantification is as follows Figure 2 As shown, according to the system power flow, when the system has frequency and voltage deviations, the converter station provides coordinated support for frequency and voltage, and needs to consider the local load, line flow and line loss requirements during the frequency and voltage recovery process.

[0109] The topological structure of the example is as follows Figure 3As shown in the figure, the example system is a 4-machine 11-node system. In this test system, there are two wind farms, each of which contains 25 wind turbines arranged in a matrix. The wind farm is connected to the 4-machine (G1~G4) 11-node onshore power grid through the voltage support C-MTDC system. Three land converter stations (converter station 1~converter station 2) are connected to the onshore grid nodes 7 and 9. Two offshore converter stations (converter station 3, converter station 4) integrate two wind farms. The rated frequency and voltage of the onshore power grid are 50Hz and 220kV, respectively.

[0110] according to Figure 4 The wind turbine communication network constructed, the wind farm wind turbines usually operate in the maximum efficiency tracking state. When the land system fails, the wind farm wind turbines will switch from the maximum power operation state to the frequency support state. The wind turbines will release their own rotor kinetic energy to increase the active power output to provide frequency support services.

[0111] In order to analyze the influence of the DC voltage droop control parameters of the converter station 1 on the system stability, the control platform is linearized using the model linearization toolbox in MATLAB / Simulink to analyze the changes in its eigenvalues. According to the small signal analysis, the input and output points are selected to construct the zero-pole diagram with different control parameters. The root locus trend of the eigenvalues ​​in the zero-pole diagram under different control parameters is shown in the figure. Figure 5 As shown. It can be seen that as the DC voltage droop control parameter of converter station 1 changes between 5 and 25, the damping ratio (DR) of most critical eigenvalues ​​in its zero-pole diagram is still greater than 0.06. This proves that when the DC voltage droop control parameter of voltage support C1 changes, the system has strong robustness.

[0112] The technical solution of the present invention is verified by a specific example below. Figure 4 is the example structure,

[0113] The frequency and voltage support performance during short circuit fault is as follows: Figure 6As shown in the figure, the short-circuit fault occurred at bus No. 2 at 10s and was removed after 625ms. The effects of five different control methods (1: frequency and voltage coordinated control method (FVSS); 2, traditional frequency and voltage coordinated control method (traditional FVSS); 3, independent frequency support method; 4, independent voltage support method; 5, no support method) were analyzed and compared. It can be seen that taking the new energy converter station 1 as an example, based on the frequency and voltage support performance under the no support method, the frequency troughs of the five different methods are 49.55Hz, 49.52Hz, 49.51Hz, 49.43Hz, and 49.48Hz respectively; the voltage troughs of the five different methods are 0.626pu, 0.624pu, 0.613pu, 0.618pu, and 0.610pu respectively. The active power and reactive power injection of the no support method are constant before and after the fault. Independent frequency support or independent voltage support only considers active power injection or reactive power injection, resulting in independent frequency support having better frequency response but weaker voltage recovery; independent voltage support has better voltage recovery but weaker frequency response; the traditional FVSS method can obtain better frequency support and voltage support than independent frequency support and IVS; but compared with traditional FVSS, the proposed FVSS method will provide more active power and reactive power due to the consideration of grid strength and load characteristics, thereby achieving faster response speed and better frequency and voltage support performance.

[0114] Figure 7 The performance of the VSC-MTDC integrated wind farm under the proposed FVSS method is given. It can be seen that under a short circuit fault, in order to provide frequency and voltage support services, all leader wind turbines and follower wind turbines will reduce the rotor speed and release the rotor kinetic energy to increase the active power output ( Figure 7 (a)). Based on the distributed consistency strategy and Figure 7 (a), according to the consensus coefficient, the rotor speed reduction rate of different wind turbines is the same / consistent ( Figure 7 (b)). Therefore, a fan with a high rotor speed will release more power ( Figure 7 (c)). All the released power of the wind farm is injected into the MTDC system. Due to the different electrical distances and frequency / voltage fluctuations between different renewable energy converter stations, according to the proposed FVSS method, the renewable energy converter station close to the fault point will output more active power ( Figure 7 (d)).

[0115] Frequency and voltage support performance under load surge scenario Figure 8As shown in the figure, at 10s, node 3 has a large active and reactive load disturbance. The effects of five different control methods (1: frequency and voltage coordinated control method (FVSS); 2, traditional frequency and voltage coordinated control method (traditional FVSS); 3, independent frequency support method; 4, independent voltage support method; 5, no support method) are compared and analyzed. It can be seen that taking the new energy converter station as an example, based on the frequency and voltage support performance under the no support method, the frequency troughs of the five different methods are 49.55Hz, 49.52Hz, 49.51Hz, 49.44Hz, and 49.48Hz respectively; the voltage troughs of the five different methods are 0.808pu, 0.804pu, 0.794pu, 0.799pu, and 0.790pu respectively. Since the load did not recover after the sudden increase, the steady-state values ​​of frequency and voltage were lower than the rated values. The no support method does not provide frequency support and voltage support, so the active power and reactive power output remain unchanged before and after the load is increased. Frequency support alone can improve frequency response performance, but the lack of reactive power support leads to weak voltage recovery. Voltage support alone can improve voltage recovery, but the lack of active power support leads to further frequency deterioration. Since the grid strength as well as load characteristics and line flow are considered in the coordination, the proposed FVSS method obtains better frequency and voltage response performance than the traditional FVSS. Therefore, the proposed FVSS has better frequency support and voltage support performance.

[0116] Fig. 9 The performance of the VSC-MTDC integrated wind farm under the proposed FVSS method is given. It can be seen that when the load increases, the frequency and voltage fluctuations of the onshore power grid will be converted into DC voltage changes in the renewable energy converter station. Affected by the electrical distance, the closer the electrical distance between the renewable energy converter station and the fault node, the greater the DC bus voltage drop ( Fig. 9 (a)), the more active power increment injected by the corresponding new energy converter station ( Fig. 9 (b)). Depending on the DC voltage deviation, the leading and following wind turbines will release different rotor kinetic energy and increase active power injection to provide frequency support and voltage support services ( Fig. 9 (c)). Based on the DCS strategy, the consistency coefficients of different wind turbines are consistent so that wind turbines with different rotor speeds release different powers for support services ( Fig. 9 (d)). Therefore, wind farms can provide the greatest support capacity.

[0117] The above description is only a preferred embodiment of the present invention and does not constitute any other form of limitation to the present invention. Any modification or equivalent change made based on the technical essence of the present invention still falls within the scope of protection required by the present invention.

Claims

1. A coordinated control method for wind power system to assist local power grid frequency and voltage support, characterized in that: The method comprises the following steps: 1) A strategy to quantify active and reactive power demand considering the equivalent impedance characteristics of local loads and the influence of line power flow; 2) Quantification strategy of active power demand for frequency regulation based on inertia control and droop control and quantification strategy of reactive power demand for voltage regulation based on droop control; 3) It is proposed to realize frequency and voltage coordinated control by taking into account the capacity limitation of converter stations and the frequency and voltage support strength.

2. The coordinated control method for frequency and voltage support of a wind power system assisting a local power grid according to claim 1, characterized in that: Step 1) is as follows: The offshore wind farm is connected to the onshore power grid through VSC-MTDC. The onshore power grid contains multiple local synchronous generators, complex networks and various loads. When a large disturbance occurs in the onshore power grid, all power sources including synchronous generators, new energy converter stations and loads will participate in frequency support and voltage support to restore frequency and voltage with reasonable power flow. Assuming that the new energy converter station is connected to the i-th node of the onshore power grid, under large disturbances, in order to restore the frequency and voltage of the i-th node by injecting support power, the power flow should be re-formed, while considering the fU recovery power demand of the local load and the power flow and power loss of the adjacent transmission lines. Assuming that there are N adjacent nodes directly connected to the i-th node through transmission lines. The total power demand for fU recovery is expressed as: In the formula, ΔP require,i and ΔQ require,i represents the total active / reactive power demand of node i, ΔP load,i and ΔQ load,i represents the active / reactive power demand of local load i, ΔP trans,i and ΔQ trans,i represents the active / reactive power transfer demand from local load i to neighboring node j, ΔP loss,ij and ΔQ loss,ij represents the active / reactive power loss from node i to j; 1. Consider the power requirements of local load characteristics; The equivalent impedance of the node load can be calculated as follows: In the formula, R ei,i , X ei,i are the real-time equivalent resistance and inductance of the local load at node i respectively; U i is the real-time voltage at node i; P i , Q i are the real-time active power and reactive power injected into the node load, respectively; According to the static characteristics of load frequency, the relationship between real-time load power and frequency fluctuation is in the form of a first-order equation: Where Δf i represents the frequency deviation at node i. pf and k qf Represents the correlation coefficient between active / reactive power and frequency, P * load,i and Q * load,i Represents the rated active / reactive power of node i. load,i and Q load,i represents the active / reactive power demand of node i; According to (12)-(15), the active / reactive power requirements for frequency support / voltage support considering the static characteristics of load frequency and local load characteristics are as follows: Where U * i represents the rated voltage of node i; 2. Consider the power demand of the line flow; In a complex network, the frequency support / voltage support performance of the i-th node of the integrated new energy converter station is closely related to the impedance and power flow of the adjacent transmission line. Therefore, the fU recovery power requirement of the i-th node should also consider the power outflow from node i to the adjacent nodes and the line power loss; Based on the Newton-Raphson method for power flow analysis of onshore power systems, the received power transmitted from node i to adjacent node j can be expressed as: In the formula, I ij Represents the line transmission current from node i to j. δ i represents the power angle of node i, X ij P represents the line resistance from node i to j. j and Q j represents the active / reactive power outflow from node i to adjacent node j; According to the transmission line impedance and power flow, the voltage of the jth adjacent node is calculated as follows: Where P ij and Q ij represents the active / reactive power outflow from node i to j, R ij represents the line resistance from node i to j, ΔU ij represents the voltage drop between nodes i and j; The active / reactive power requirements of FS / VS considering the load frequency static characteristics and power flow are as follows: The transmission line power loss from node i to adjacent node j is calculated as follows:

3. The coordinated control method for frequency and voltage support of a wind power system assisting a local power grid according to claim 1, characterized in that: Step 2) is as follows: When a large disturbance occurs, the quantification of active power and reactive power for frequency support and voltage support is important for fU recovery, expressed as, ΔP require,i =ΔP FS,i +ΔP VS,i +ΔP SG,i (16) ΔQ require,i =ΔQ FS,i +ΔQ VS,i +ΔQ SG,i (17) In the formula, ΔP FS,i and ΔP VS,i Represents the active power injected for frequency support / voltage support; ΔQ FS,i and ΔQ VS,i Represents the reactive power injected for frequency support / voltage support; ΔP SG,i and ΔQ SG,i represents the increment of active and reactive power injected by the synchronous generator into node i, which can be calculated based on the power flow before and after the fault; 1) Provide active power and reactive power for frequency support: For OWF connected to VSC-MTDC, the frequency support service of OWF is actually implemented by the new energy converter station. Since there are multiple new energy converter stations, it is necessary to consider the power distribution problem between the new energy converter stations. At present, the power coordination between the new energy converter stations considers the PCC frequency information and cannot fully coordinate the energy flow between different DC links. Therefore, the power distribution between the new energy converter stations needs to further consider the DC bus voltage; The relationship between frequency and active power can be obtained through the mechanical angular motion equation: In the formula, H GSVSC,i represents the inertia time constant of the i-th renewable energy converter station, P m and P e represent mechanical power and electromagnetic power respectively, ΔP1 represents the virtual kinetic energy inertially absorbed or released by the new energy converter station during the frequency support process; The relationship between DC voltage and active power is expressed by the capacitor dynamic equation: Where P in and P out Respectively represent the input and output power of the new energy converter station. dc,i Indicates the DC capacitor capacity, S GSVSC,i represents the rated capacity of the i-th renewable energy converter station, N i represents the number of DC capacitors in the i-th renewable energy converter station, ΔP2 represents the dynamic electrostatic power stored or released by the HVDC capacitor; Since the power change ΔP1 of the new energy converter station is equal to the power change ΔP2 of the capacitor, the relationship between the DC voltage and the onshore grid frequency is expressed as: From the above formula, we can see that the larger the frequency deviation, the greater the U dc Therefore, by considering the DC voltage difference, the ΔP between new energy converter stations is constructed GSVSC,i The power distribution is as follows: ΔU dc,ij =U dc,i -U dc,j (21) ΔP GSVSC,i =k GSVSC,i (ΔU dc,ij0 -ΔU dc,ij ) (22) In the formula, ΔU dc,ij0 and ΔU dc,ij They represent the DC voltage differences before and after the disturbance of DC lines i to j respectively; Then, based on the existing frequency-voltage-power droop and inertia control of the new energy converter station, the power distribution between the new energy converter stations is considered, and the active power of the frequency support of the new energy converter station is quantified as follows: In the formula, k inertia,i and k droop,i are the inertia coefficient and droop coefficient of the frequency support respectively. vp,i is the Udc-P droop coefficient of the i-th renewable energy converter station; According to the reactive power relationship in formula (26) and the capacity limitation of the new energy converter station, the reactive power supply ΔQ of the frequency support FS,i It can be expressed as: In the formula, S GSVSCi,n and S GSVSC,i Indicates the rated capacity and real-time injected power of the new energy converter station; 2) Active power and reactive power provided for voltage support; When a large disturbance occurs, the voltage of the grid-connected node of the new energy converter station exceeds the voltage threshold of 0.9pu~1.1pu, triggering the droop control, which can be expressed as: In the formula, k VS,i Indicates the droop coefficient of voltage support; According to the active power relationship in formula (25) and the capacity limitation of the new energy converter station, the active power supply ΔP of the voltage support VS,i Expressed as:

4. The coordinated control method for frequency and voltage support of a local power grid assisted by a wind power system according to claim 1, characterized in that: Step 3) is as follows: Different degrees of frequency and voltage drops will require the new energy converter station to support different active and reactive powers. Considering the capacity limitations of the new energy converter station, the active / reactive power reference of the new energy converter station should consider the following two scenarios of frequency support and voltage support. Scenario 1: The rated capacity of the new energy converter station is sufficient. When the frequency and voltage on the new energy converter station side drop slightly, the support power may not exceed the rated capacity. The reference active and reactive power of the new energy converter station is the sum of the active and reactive power of the frequency support and voltage support, that is: P ref,i =P i0 +ΔP FS.i +ΔP VS.i (27) Q ref,i =Q i0 +ΔQ FS.i +ΔQ VS.i (28) Where P ref,i and Q ref,i Respectively represent the active power and reactive power reference values ​​of the new energy converter station. i0 and Q i0 They respectively represent the initial active power and reactive power of the new energy converter station. Scenario 2: The rated capacity of the new energy converter station is insufficient. When the frequency and voltage on the new energy converter station side drop severely, the support power may exceed the rated capacity, and there will be a frequency-voltage coordination problem. The active and reactive power of frequency support and voltage support need to be redistributed. The principle of coordination is that whoever drops more will have more support power. A frequency-voltage coordinated support method is proposed to construct a coordination coefficient considering the strength of the power grid. The formula is: In the formula, α FVSS,i represents the coordination coefficient of node i. The frequency-voltage coordination coefficient can be obtained by the node frequency strength SQ f,i and voltage intensity SQ v,i The calculation is as follows:

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