Frequency coordination control method based on doubly-fed wind field and flexible direct-current power transmission system

Abstract

The invention discloses a frequency coordination control method based on a doubly-fed wind field and a flexible direct-current power transmission system, and solves the problems that the frequency modulation capability of a doubly-fed fan is limited and parameters of the flexible direct-current system are solidified through adaptive virtual inertia control of the flexible direct-current system and a layered optimization strategy of the doubly-fed wind field. According to the method, the kinetic energy reserve of the rotor is maximized before the fault, the control parameters are dynamically adjusted after the fault, the system frequency stability under the high wind power permeability is remarkably improved, and the method has the engineering practical value.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy grid connection and power system control, and particularly relates to a frequency coordinated control method based on a doubly-fed wind farm and a flexible DC transmission system, which is used for optimizing the coordinated participation of the doubly-fed wind farm and the flexible DC system in power grid frequency regulation. Background Art

[0002] With the large-scale development of offshore wind power, flexible DC transmission has become a typical solution for offshore wind farm grid connection due to its advantages of low loss and long-distance transmission. However, the power electronic devices of the flexible DC system isolate the electrical coupling between the offshore wind farm and the power grid, resulting in the inability of the wind farm to provide inertial support for the power grid and affecting the frequency stability of the system. In the prior art, the frequency modulation strategy of the permanent magnet direct drive wind farm has been relatively mature, but its control method cannot be directly applied to the doubly-fed wind farm.

[0003] The doubly-fed wind turbine controls the power output through the rotor-side converter, and there are significant differences in the rotor kinetic energy management between the doubly-fed wind turbine and the permanent magnet direct drive wind turbine. The frequency modulation strategies of traditional doubly-fed wind turbines are mostly based on rotor kinetic energy release or power reserve control, but there are the following problems:

[0004] 1. Limited rotor kinetic energy: The rotor speed range of the doubly-fed wind turbine is relatively narrow, the overspeed load shedding ability is limited, and the frequency modulation duration is short;

[0005] 2. Insufficient coordination: The frequency modulation capabilities of the units in the wind farm are uneven due to wind speed differences, and there is a lack of a dynamic distribution mechanism;

[0006] 3. Fixed parameters of the flexible DC system: The virtual inertia coefficient of the existing flexible DC system is fixed and cannot be dynamically adjusted according to the frequency deviation, resulting in insufficient energy storage release or DC voltage over-limit. Summary of the Invention

[0007] The present invention proposes a frequency coordinated control method for a doubly-fed wind farm and a flexible DC transmission system, which solves the problems of insufficient frequency modulation ability of the doubly-fed wind farm and fixed parameters of the flexible DC system through adaptive virtual inertia control and wind farm hierarchical optimization strategy, realizes the coordinated frequency modulation of the wind farm and the flexible DC system, and improves the dynamic frequency response ability of the system. The specific technical solutions are as follows:

[0008] Adaptive virtual inertia control of the flexible DC system based on frequency deviation, and hierarchical optimization power distribution and dynamic droop coefficient adjustment of the doubly-fed wind farm.

[0009] Optionally, the adaptive virtual inertia control dynamically adjusts the virtual inertia coefficient H DC, realize the coupling response of DC voltage and grid frequency; the virtual inertia is dynamically adjusted by making the energy of the system equal to the energy stored in the flexible DC transmission system at the rated frequency. When the frequency deviation changes, the corresponding virtual inertia coefficient also changes: when the frequency deviation is large and the deviation from the lowest frequency is small, the virtual inertia coefficient is large; when the frequency deviation is small and the deviation from the lowest frequency is large, the virtual inertia coefficient is correspondingly small.

[0010] Optionally, before a fault occurs, the power reserve method of the doubly-fed wind farm preferably adopts overspeed load shedding control. The control objective at the wind farm level is to maximize the total reserve power of the wind farm, and optimize the load shedding power distribution according to the wind speed difference and safe operating range of each wind turbine, so as to finally reach the target reserve power.

[0011] Optionally, after a fault occurs, at the control level of the wind turbine, the dynamic droop coefficient adjustment is calculated in real time based on the rotor kinetic energy of each unit to achieve differential matching of the frequency modulation ability; the implementation process of the dynamic droop control coefficient is: based on the calculation formula of the rotor kinetic energy of the doubly-fed wind turbine, multiply the reciprocal of the droop coefficient corresponding to the rated speed by the ratio of the real-time rotor kinetic energy to the maximum rotor kinetic energy. When the wind turbine speed is high and the reserve power is large, the droop coefficient is amplified, and more reserve power is sent out to achieve rapid response; when the wind turbine speed is low and the reserve power is small, the droop coefficient is reduced to maintain the normal operation of the unit and less reserve power is sent out.

[0012] 1. Adaptive virtual inertia control of the flexible DC system

[0013] DC voltage - frequency mapping: Convert the grid frequency deviation (Δf) into the change of the DC voltage of the flexible DC system through droop control, and establish a dynamic virtual inertia model:

[0014]

[0015] where f n , H n are the rated frequency of the system and the virtual inertia coefficient corresponding to the rated frequency respectively; f min is the lower limit of the safe operating frequency of the system; H DC is the virtual inertia coefficient of the flexible DC system, which is dynamically adjusted with the frequency deviation to ensure that the DC voltage does not exceed the limit while fully releasing the capacitor energy storage. The specific process is: when f is close to f n , that is, the degree of frequency drop is low, H DC is small; when the drop amplitude of f is large, H DC is amplified, which can achieve rapid support for the frequency.

[0016] At the same time, set the maximum value of the virtual inertia coefficient to prevent the change of the DC voltage of the flexible DC system from exceeding the safety threshold. When the variable virtual inertia coefficient exceeds the maximum value, take the maximum value.

[0017] 2. Hierarchical Optimization Control of Doubly Fed Wind Farm

[0018] Before fault: Improved power reserve method

[0019] Aiming at maximizing the rotor kinetic energy of the wind farm, overspeed load shedding control is preferentially adopted, and the load shedding power is dynamically distributed based on the wind speed difference:

[0020] Optimization objective:

[0021] H ω,i and ω op,i are the inertia time constant and rotational speed corresponding to the i-th doubly fed wind turbine respectively.

[0022] The constraint conditions include the safe range of rotor speed and the total load shedding power demand, ensuring that high-wind-speed units undertake more load shedding tasks and reducing wind curtailment losses.

[0023] After fault: Adaptive droop coefficient adjustment

[0024] According to the real-time rotor kinetic energy of each unit Dynamically tune the droop coefficient:

[0025]

[0026] where, ω max,i and ω min,i are the maximum rotational speed and the minimum rotational speed corresponding to the i-th wind turbine respectively. K n is the droop control coefficient corresponding to the rated rotational speed. Taking its reciprocal can not only enable high-kinetic-energy units to respond to frequency fluctuations preferentially, but also greatly improve the frequency response speed within the safe operating range and enhance the overall frequency regulation ability of the wind farm.

[0027] 3. System-level communication-free coordination

[0028] Indirectly transmit the grid frequency information to the wind farm through the change of the flexible DC voltage, trigger the power adjustment of the doubly fed wind turbines, avoid relying on high-speed communication, and reduce the system complexity. Description of the drawings

[0029] The drawings are used to provide a further understanding of the present invention, and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation to the present invention.

[0030] Figure 1 is the grid connection topology and coordinated control framework of the doubly fed wind farm and the flexible DC system mentioned in the present invention;

[0031] Figure 2 is the improved power reserve control flow chart mentioned in the present invention. Specific implementation manners

[0032] To better understand the technical solution of this application, the embodiments of this application will be described in detail below with reference to the accompanying drawings.

[0033] The present invention proposes a frequency coordinated control method based on a doubly-fed wind farm and a flexible DC system, and the specific implementation manner is as follows:

[0034] Adaptive virtual inertia control of the flexible DC system, as well as hierarchical optimization of power distribution and dynamic droop coefficient adjustment of the doubly-fed wind farm.

[0035] In the embodiment of the present invention, the adaptive virtual inertia control realizes the coupled response of the DC voltage and the grid frequency by dynamically adjusting the virtual inertia coefficient H DC , and the way of dynamically adjusting the virtual inertia is to make the energy of the system equal to the energy stored in the flexible DC transmission system under the rated frequency. When the frequency deviation changes, the corresponding virtual inertia coefficient also changes: when the frequency deviation is large and the deviation from the lowest frequency is small, the virtual inertia coefficient is large; when the frequency deviation is small and the deviation from the lowest frequency is large, the virtual inertia coefficient is also correspondingly small.

[0036] In the embodiment of the present invention, before a fault occurs, the power reserve method of the doubly-fed wind farm preferentially adopts overspeed load shedding control. The control target at the wind farm level is to maximize the total reserve power of the wind farm, and optimize the load shedding power distribution according to the wind speed difference and the safe operation range of each wind turbine, so that the final target reserve power is achieved.

[0037] In the embodiment of the present invention, after a fault occurs, at the control level of the wind turbine, the dynamic droop coefficient adjustment is calculated in real time based on the rotor kinetic energy of each unit to achieve differential matching of the frequency modulation ability; the implementation process of the dynamic droop control coefficient is: based on the calculation formula of the rotor kinetic energy of the doubly-fed wind turbine, multiply the reciprocal of the droop coefficient corresponding to the rated speed by the ratio of the real-time rotor kinetic energy to the maximum rotor kinetic energy. When the wind turbine speed is high and the reserve power is large, the droop coefficient is amplified and more reserve power is sent out to achieve fast response; when the wind turbine speed is low and the reserve power is small, the droop coefficient is reduced to maintain the normal operation of the unit and less reserve power is sent out.

[0038] 1. Adaptive virtual inertia control of the flexible DC system

[0039] DC voltage - frequency mapping: Convert the grid frequency deviation (Δf) into the change of the DC voltage of the flexible DC system through droop control, and establish a dynamic virtual inertia model:

[0040]

[0041] Among them, f n , H n are the system rated frequency and the virtual inertia coefficient corresponding to the rated frequency respectively; f minThe frequency lower limit for safe operation of the system; H DC It is the virtual inertia coefficient of the flexible DC system, which is dynamically adjusted with the frequency deviation to ensure that the DC voltage does not exceed the limit while fully releasing the capacitor energy.

[0042] At the same time, the maximum value of the virtual inertia coefficient is set to prevent the DC voltage change of the flexible DC system from exceeding the safety threshold. When the variable virtual inertia coefficient exceeds the maximum value, the maximum value is taken.

[0043] 2. Doubly fed wind farm stratified optimization control

[0044] Before the Failure: Improving Power Backup Methods

[0045] With the goal of maximizing the kinetic energy of the wind farm rotor, overspeed load reduction control is preferred, and load reduction power is dynamically allocated based on wind speed differences:

[0046] Optimization goal:

[0047] H ω,i and ω op,i are the inertia time constant and speed corresponding to the i-th doubly-fed wind turbine set respectively.

[0048] The constraints include the safe range of rotor speed and the total load reduction power requirement, ensuring that high wind speed units take on more load reduction tasks and reduce wind curtailment losses.

[0049] After a fault: Adaptive droop coefficient adjustment

[0050] According to the real-time rotor kinetic energy of each unit Dynamically adjust the droop coefficient:

[0051]

[0052] Among them, ω max,i and ω min,i are the maximum speed and minimum speed corresponding to the i-th wind turbine. n is the droop control coefficient corresponding to the rated speed. Taking its reciprocal can not only enable high kinetic energy units to respond to frequency fluctuations first, but also greatly improve the frequency response speed within the safe operating range and enhance the overall frequency regulation capability of the wind farm.

[0053] 3. No communication coordination at system level

[0054] The grid frequency information is indirectly transmitted to the wind farm through the change of flexible DC voltage, triggering the power adjustment of the doubly fed wind turbine, avoiding dependence on high-speed communication and reducing system complexity.

[0055] A frequency coordination control strategy based on a doubly-fed wind farm and a flexible DC transmission system. This control strategy demonstrates a reasonable allocation of reserve power in a doubly-fed wind farm and a frequency coordination strategy with a flexible DC transmission system. First, before a system fault, the doubly-fed wind farm adopts an improved power reserve method, preferentially using overspeed load shedding control, and allocating reserve power in combination with the operating conditions of each unit; when detecting system frequency fluctuations, through the proposed frequency coordination control with an adaptive control coefficient, each doubly-fed unit provides power compensation according to its own situation to achieve frequency support.

[0056] Figure 1 For the grid connection topology and coordinated control framework of the doubly-fed wind farm and the flexible DC system mentioned in the present invention, as Figure 1 shown, the system topology includes a doubly-fed wind farm, a wind farm side converter (WFVSC), and a grid side converter (GSVSC). Figure 1 Multiple wind turbines are simplified and shown as one wind turbine in the figure;

[0057] Figure 1 where f DF and f DF0 are the real-time value and the initial value of the frequency of the doubly-fed wind turbine, P DFref and P DF0 are the reference value and the initial value of the power generated by the doubly-fed wind turbine, K DF is the adaptive droop coefficient, and the droop coefficient can be adjusted according to the rotor speed of the wind turbine; V dc1 and V dc10 are the real-time value and the initial value of the DC voltage of the capacitor of the wind farm side converter, f wf and f wf0 are the reference value and the initial value of the frequency of the AC quantity on the WFVSC side, K wf is the magnitude of the adaptive droop control coefficient on the WFVSC side and is the reciprocal of K DC ; f and f0 are the measured value and the initial value of the grid frequency, V dc2ref and V dc20 are the reference value and the initial value of the capacitor voltage of the grid side converter, K DC is the adaptive droop control coefficient on the GSVSC side;

[0058] The traditional control method is that the WFVSC adopts constant voltage and constant frequency control, and the GSVSC adopts constant DC bus voltage control. The specific implementation method of the communication-free frequency coordination control strategy adopted by the present invention is:

[0059] When the frequency of the onshore power grid deviates, it is converted into a change in the DC voltage on the GSVSC side through droop control on the GSVSC side. Assuming that the two capacitor voltages are equal, the DC voltage is then converted into a change in the AC side frequency of the WFVSC through droop control on the WFVSC side. Finally, power control is achieved through droop control of the machine-side converter of the doubly-fed generator set, and the entire process completes the perception of the onshore frequency change by the far-sea wind farm.

[0060] Figure 2 This is the improved power reserve control flow chart mentioned in the present invention. As Figure 2 shown, the system continuously detects the grid frequency status. If there is no fault in the system, the power reserve mode is started. The wind speed and rotor speed of each unit are collected, the maximum rotor kinetic energy optimization target is calculated, and the load shedding power is dynamically allocated according to the operating conditions of each unit: the low-wind-speed units are given priority for overspeed load shedding, the power command of the unit is adjusted, and the rotor kinetic energy is stored. Continuously monitor the grid frequency status. If the system frequency is normal, it remains in the reserve mode all the time. If the system frequency is abnormal, it enters the frequency modulation mode.

Claims

1. A frequency coordinated control method based on a doubly-fed wind farm and a flexible DC transmission system, characterized in that Including: Adaptive virtual inertia control of the VSC-HVDC system based on frequency deviation, and hierarchical optimization of power distribution and dynamic droop coefficient adjustment of the DFIG wind farm.

2. The method according to claim 1, wherein The adaptive virtual inertia control realizes the coupling response of the DC voltage and the grid frequency by dynamically adjusting the virtual inertia coefficient H DC , and the way of dynamically adjusting the virtual inertia is to make the energy of the system equal to the energy stored in the flexible DC transmission system at the rated frequency. When the frequency deviation changes, the corresponding virtual inertia coefficient also changes: if the frequency deviation is large and the deviation from the lowest frequency is small, the virtual inertia coefficient is large; if the frequency deviation is small and the deviation from the lowest frequency is large, the virtual inertia coefficient is also correspondingly small.

3. The method according to claim 1, characterized in that, Before the fault, the power reserve method of the DFIG wind farm preferentially adopts overspeed load shedding control. The control objective at the wind farm level is to maximize the total reserve power of the wind farm, and optimize the load shedding power distribution according to the wind speed difference and safe operating range of each wind turbine, so as to finally reach the target reserve power.

4. The method according to claim 1, wherein After the fault occurs, at the control level of the wind turbine, the dynamic droop coefficient adjustment is calculated in real time based on the rotor kinetic energy of each unit to achieve differential matching of the frequency modulation ability. The implementation process of the dynamic droop control coefficient is as follows: based on the calculation formula of the rotor kinetic energy of the DFIG wind turbine, multiply the reciprocal of the droop coefficient corresponding to the rated speed by the ratio of the real-time rotor kinetic energy to the maximum rotor kinetic energy. When the wind turbine speed is high and the reserve power is large, the droop coefficient is amplified, and more reserve power is sent out to achieve rapid response; when the wind turbine speed is low and the reserve power is small, the droop coefficient is reduced to maintain the normal operation of the unit, and less reserve power is sent out.

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