Droop control method and system for offshore wind plant

Abstract

The invention discloses a droop control method and system for an offshore wind plant, and the method comprises the steps: constructing a self-adaptive droop coefficient of a sending-end converter station according to a preset basic droop coefficient of the sending-end converter station, the obtained active power and DC voltage of the sending-end converter station, and the difference degree of the multi-end sending-end converter station; constructing a virtual inertia control model by taking the lowest oscillation amplitude of the power grid frequency as a target based on the change relationship between the active power of the sending-end converter station and the power grid frequency; according to a virtual inertia control model, virtual inertia and the adaptive droop coefficient are introduced into a preset sending end converter station active power model, and a sending end converter station power inertia control model is obtained; and adjusting the output power of the fan side of the offshore wind plant according to the power inertia control model of the sending-end converter station. When the offshore wind plant is connected to the grid, the output power of the fan side of the offshore wind plant is optimally distributed to suppress the frequency fluctuation of the power grid, and the operation stability of the system is maintained.

Description

Technical Field

[0001] The present application belongs to the field of new energy power system control design, and specifically relates to a droop control method and system for an offshore wind farm. Background Art

[0002] The wind speed at sea is usually higher than that on land, and the wind speed is more stable and the wind shear is smaller, which means that offshore wind farms can capture more wind energy and improve power generation efficiency. In order to optimize the operating efficiency of offshore wind farms and ensure the quality of power, a DC transmission network is often constructed to connect multiple wind turbines or wind farms through multiple converters to achieve efficient transmission and flexible control of power, which is convenient for long-distance transmission to the land power grid. However, this kind of offshore wind farm with multiple converters in parallel is prone to grid fluctuations due to uneven distribution of active power output on the wind turbine side when connected to the grid, resulting in unstable grid frequency.

[0003] Existing research has proposed an adaptive droop control method, which changes the droop coefficient of the active power output on the wind turbine side in real time according to the power margin of the converter when connected to the grid, so as to avoid overload of the converter station. However, this method is too conservative in the design of the droop coefficient and the judgment of the power margin is not accurate enough, so the grid frequency regulation effect is not significant enough. Summary of the invention

[0004] The present application proposes a droop control method and system for an offshore wind farm, which suppresses grid frequency fluctuations and maintains system operation stability by optimizing the distribution of output power on the wind turbine side of the offshore wind farm when the offshore wind farm is connected to the grid.

[0005] A first aspect of the present application provides a droop control method for an offshore wind farm, the method comprising:

[0006] According to the preset basic droop coefficient of the sending-end converter station, the acquired active power and DC voltage of the sending-end converter station, and the difference between the multi-end sending-end converter stations, the adaptive droop coefficient of the sending-end converter station is constructed;

[0007] Based on the changing relationship between the active power of the sending-end converter station and the grid frequency, a virtual inertia control model is constructed with the goal of minimizing the oscillation amplitude of the grid frequency.

[0008] According to the virtual inertia control model, the virtual inertia and the adaptive droop coefficient are introduced into the preset active power model of the sending-end converter station to obtain the power inertia control model of the sending-end converter station;

[0009] According to the power inertia control model of the sending-end converter station, the output power of the wind turbine side of the offshore wind farm is adjusted.

[0010] In the above scheme, considering the inconsistency of active power margins between multi-end sending-end converter stations, in order to reduce the fluctuation of active power and the influence of DC voltage of the sending-end converter station, the original basic droop coefficient is optimized to obtain the adaptive droop coefficient. Because the adaptive droop coefficient combines the active power margins between different sending-end converter stations, it can enable different sending-end converter stations to maximize the regulation ability of active power and further reduce the fluctuation of DC voltage. Then, considering the influence of the increase of active power on the oscillation trend of grid frequency, a virtual inertia control model is constructed to quickly stabilize the grid frequency at the rated value, so as to reduce the oscillation amplitude of the grid frequency and maintain the stability of the power system. Then, based on the virtual inertia control model and the adaptive droop coefficient, the active power margin of the sending-end converter station is accurately judged to clarify how to adjust the active power on the wind turbine side to stabilize the grid frequency and DC voltage.

[0011] In a possible implementation method of the first aspect, according to a preset basic droop coefficient of the sending-end converter station, the acquired active power and DC voltage of the sending-end converter station, and combined with the difference of the multi-end sending-end converter stations, an adaptive droop coefficient of the sending-end converter station is constructed, specifically:

[0012] Constructing the basic droop coefficient according to the preset safe fluctuation range of active power and safe fluctuation range of DC voltage of the sending-end converter station;

[0013] Constructing a power margin factor of the sending-end converter station according to the active power of the sending-end converter station and a preset active power deviation from a rated value;

[0014] Constructing a voltage deviation factor of the sending-end converter station according to the DC voltage of the sending-end converter station and a preset DC voltage rated value;

[0015] According to the basic droop coefficient, power margin factor and voltage deviation factor, combined with the differences of multi-end sending-end converter stations, an adaptive droop coefficient of the sending-end converter station is constructed.

[0016] The above scheme first determines the basic droop coefficient that can maintain the active power and DC voltage within the safe range. Then, according to the active power and DC voltage of the sending-end converter station, and the corresponding deviation from the rated value, the power margin factor and voltage deviation factor are constructed to ensure that the active power and DC voltage do not deviate from the rated value even if they fluctuate. Finally, the basic droop coefficient is optimized according to the power margin factor and the voltage deviation factor. The obtained adaptive droop coefficient can maximize the power regulation capability of different sending-end converter stations, so that the active power output of the offshore wind farm when connected to the grid can be adaptively distributed and the fluctuation of DC voltage can be reduced.

[0017] In a possible implementation method of the first aspect, the basic droop coefficient, the power margin factor and the voltage deviation factor are combined with the differences of the multi-end sending-end converter stations to construct an adaptive droop coefficient of the sending-end converter station, specifically:

[0018] According to the difference in active power margin between the multi-end sending-end converter stations, the basic droop coefficient and the power margin factor are combined to obtain a first droop coefficient;

[0019] According to the voltage deviation factor, the first droop coefficient is optimized to obtain the adaptive droop coefficient for the purpose of reducing the fluctuation of the DC voltage caused by the active power fluctuation of the sending-end converter station.

[0020] In a possible implementation method of the first aspect, the adaptive droop coefficient is specifically:

[0021]

[0022] In the formula, k * is the adaptive droop coefficient, k'0 is the first droop coefficient, α2 is the voltage deviation factor, α1 is the power margin factor, P max , P min are the maximum and minimum values ​​of the safe fluctuation range of active power of the sending-end converter station, respectively. dc_max , U dc_min are the maximum and minimum values ​​of the safe fluctuation range of the DC voltage at the sending-end converter station, respectively, and α0 is the margin coefficient.

[0023] In a possible implementation method of the first aspect, based on the changing relationship between the active power of the sending-end converter station and the grid frequency, with the goal of minimizing the oscillation amplitude of the grid frequency, a virtual inertia control model is constructed, specifically:

[0024] According to the preset grid frequency time variation model, determine the variation relationship between the active power of the sending-end converter station and the grid frequency;

[0025] Based on the changing relationship between the active power of the sending-end converter station and the grid frequency, a virtual inertia control model is constructed by introducing and adjusting the value of the virtual inertia to minimize the oscillation amplitude of the grid frequency; wherein the virtual inertia is constructed based on the inertia characteristics of the synchronous motor.

[0026] The above scheme first uses the influence of the active power change of the sending-end converter station on the grid frequency to clarify how to adjust the active power to quickly return the grid frequency to stability. In order to significantly adjust the grid frequency, virtual inertia is introduced to construct a virtual inertia control model, and the grid frequency is directly adjusted by adjusting the value of virtual inertia.

[0027] In a possible implementation method of the first aspect, the virtual inertia control model is specifically:

[0028]

[0029] In the formula, J v is the virtual inertia, J0 is the rated value of the virtual inertia, k1 and k2 are the adaptive adjustment coefficients of the virtual inertia under different conditions, f is the actual frequency of the power grid, t is the moment, Δf is the deviation value of the power grid frequency, Δω is the deviation value of the angular frequency of the synchronous motor, and N is a constant.

[0030] In a possible implementation method of the first aspect, according to the virtual inertia control model, the virtual inertia and the adaptive droop coefficient are introduced into the preset active power model of the sending-end converter station to obtain the power inertia control model of the sending-end converter station, which is specifically:

[0031] Based on the assumption that the active power output of the wind turbine side of the offshore wind farm is equal to the active power income of the sending-end converter station, the sending-end converter station active power model is constructed on the basis of a preset active power model of the wind turbine side of the offshore wind farm;

[0032] According to the relationship between the DC voltage and the grid frequency of the sending-end converter station, the virtual inertia and the adaptive droop coefficient are introduced into the power inertia control model of the sending-end converter station.

[0033] The above scheme first constructs the active power model of the sending-end converter station based on the fact that the active power output of the wind turbine side of the offshore wind farm is equal to the active power income of the sending-end converter station, and also provides data support for adjusting the grid frequency fluctuation by adjusting the active power of the wind turbine side. Then, the virtual inertia and the adaptive droop coefficient are introduced into the active power model of the sending-end converter station to adjust the active power of the sending-end converter station by adjusting the virtual inertia and the adaptive droop coefficient.

[0034] In a possible implementation method of the first aspect, the power inertia control model of the sending-end converter station is specifically:

[0035]

[0036] Where P con is the active power of the sending-end converter station, P con_ref is the active power reference value of the sending-end converter station, k * is the adaptive droop coefficient, u dc is the DC voltage of the sending-end converter station, U dcN is the rated value of the DC voltage of the sending-end converter station, s is the differential operator, T d is the active power delay coefficient of the offshore wind farm, J v is the virtual inertia, f Nis the rated frequency of the power grid, k u is the droop coefficient between the DC voltage and grid frequency at the sending-end converter station, and t is the time.

[0037] In a possible implementation method of the first aspect, the output power of the wind turbine side of the offshore wind farm is adjusted according to the power inertia control model of the sending-end converter station, specifically:

[0038] According to the change of the current grid frequency, determine the DC voltage change amplitude of the sending-end converter station;

[0039] Determining the optimal active power of the sending-end converter station according to the DC voltage variation amplitude;

[0040] The output power of the wind turbine side of the offshore wind farm is adjusted according to the optimal active power of the sending-end converter station.

[0041] The above scheme reflects the fluctuation of grid frequency in the change of DC voltage, and then determines how to adjust the output power of the wind turbine side of the offshore wind farm through the change of DC voltage, so as to affect the active power of the sending-end converter station through the output power of the wind turbine side of the offshore wind farm, so as to stabilize the DC voltage and grid frequency at the rated value as soon as possible, thereby improving the operational stability of the grid.

[0042] The second aspect of the present application provides a droop control system for an offshore wind farm, the system comprising: an adaptive droop coefficient building module, a virtual inertia control model building module, an inertia control model building module and a wind turbine side power regulation module:

[0043] The adaptive droop coefficient construction module constructs the adaptive droop coefficient of the sending-end converter station according to the preset basic droop coefficient of the sending-end converter station, the acquired active power and DC voltage of the sending-end converter station, and the difference between the multi-end sending-end converter stations;

[0044] The virtual inertia control model building module is used to build a virtual inertia control model based on the changing relationship between the active power of the sending-end converter station and the grid frequency, with the goal of minimizing the oscillation amplitude of the grid frequency;

[0045] The inertia control model building module is used to introduce virtual inertia and the adaptive droop coefficient into a preset active power model of a sending-end converter station according to a virtual inertia control model, so as to obtain a power inertia control model of a sending-end converter station;

[0046] The wind turbine side power regulation module is used to regulate the wind turbine side output power of the offshore wind farm according to the power inertia control model of the sending-end converter station. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In order to more clearly illustrate the technical solution of the present application, the drawings required for use in the implementation manner will be briefly introduced below. Obviously, the drawings described below are only some implementation manners of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0048] Figure 1 It is a specific flow chart of a droop control method for an offshore wind farm provided in a certain embodiment of the present application;

[0049] Figure 2 It is a time variation diagram of the power grid frequency of a droop control method for an offshore wind farm provided in a certain embodiment of the present application;

[0050] Figure 3 is a control flow chart of a droop control method for an offshore wind farm provided in a certain embodiment of the present application;

[0051] Figure 4 A structural diagram of a droop control system for an offshore wind farm is provided in accordance with a certain embodiment of the present application. DETAILED DESCRIPTION

[0052] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0053] It should be understood that the step numbers used in this article are only for the convenience of description and are not intended to limit the order in which the steps are executed.

[0054] First embodiment

[0055] As a renewable energy source, offshore wind power does not produce greenhouse gas and other pollutant emissions, which helps to mitigate climate change and improve air quality. In addition, with the advancement of high-voltage direct current transmission technology, it is now common to build a direct current transmission network to connect multiple wind turbines or wind farms to achieve efficient transmission and flexible control of electric energy. At the same time, it helps to reduce power loss, enhance the stability and scalability of the system, and facilitate long-distance transmission to the land power grid. However, due to the isolation of the direct current transmission network, wind farms can hardly perceive the frequency changes of the grid at the receiving end and cannot actively provide active support, which will have a negative impact on the frequency stability of the AC power grid. Although existing studies often use voltage droop control strategies to control the droop relationship between active power and DC voltage to achieve coordinated distribution of active power, the selection of the droop coefficient is too conservative, so that the DC voltage regulation effect is not significant enough and the grid frequency is difficult to return to stability quickly.

[0056] The embodiment of the present application takes into account the differences in the adaptive regulation capabilities of multi-terminal converter stations to active power fluctuations when the offshore wind farm is connected to the grid, and sets an adaptive droop coefficient that can maximize the active power regulation capability of the sending-end converter station, as well as introduces virtual inertia based on the rotational inertia of the synchronous motor to regulate the active power output on the wind turbine side of the offshore wind farm, thereby reducing DC voltage fluctuations and ultimately suppressing grid frequency fluctuations.

[0057] like Figure 1 As shown, Figure 1 A specific flow chart of a droop control method for an offshore wind farm is provided for a certain embodiment of the present application. The droop control method for an offshore wind farm of this embodiment includes steps S1 to S4, which are described in detail as follows:

[0058] Step S1, constructing an adaptive droop coefficient of the sending-end converter station according to the preset basic droop coefficient of the sending-end converter station, the acquired active power and DC voltage of the sending-end converter station, and the difference between the multi-end sending-end converter stations.

[0059] In the embodiment of the present application, a study is conducted on how to maximize the regulating capability of different converter stations on the active power received from the wind turbine side based on the background of multi-end sending-end converter stations being connected in parallel. Therefore, the design of the adaptive droop coefficient also takes into account the active power margin of different sending-end converter stations to determine the proportional value of the droop coefficient.

[0060] For the construction of adaptive droop coefficient, three parameters need to be determined, namely the basic droop coefficient, the power margin factor of the sending-end converter station and the voltage deviation factor. Among them, the basic droop coefficient is used in the multi-end sending-end converter station to adjust the output power of the wind turbine side to cope with the changes in grid frequency and DC voltage.

[0061] Optionally, in the embodiment of the present application, the design of the basic droop coefficient is based on fully considering the influence of the overshoot of the DC voltage transient process in the control process of the sending-end converter station on the control effect, and a certain margin needs to be left for the control object. First, according to the actual situation, the safe fluctuation range of active power and the safe fluctuation range of DC voltage of the sending-end converter station are obtained, and then the basic droop coefficient is constructed with the goal of stabilizing the overshoot of DC voltage within the allowable range.

[0062] The specific expression of the basic droop coefficient is:

[0063]

[0064] Where α0 is the margin coefficient, which can be used to amplify the value of the basic droop coefficient to prevent the instantaneous overshoot of the DC voltage transient process from exceeding the limit range during initial regulation; P max , P min are the maximum and minimum values ​​of the safe fluctuation range of active power at the sending-end converter station, U dc_max , U dc_min are the maximum and minimum values ​​of the safe fluctuation range of the DC voltage at the sending-end converter station, respectively. Because the values ​​of different sending-end converter stations are not necessarily the same, when α0 is introduced, it can effectively prevent the DC voltage overshoot from exceeding the safe fluctuation range of the DC voltage.

[0065] The active power of the sending-end converter station and the set active power deviation from the rated value are obtained, and the power margin factor of the sending-end converter station is calculated.

[0066] The power margin factor is specifically expressed as:

[0067]

[0068] Where P is the actual value of active power at the sending-end converter station, ΔP is the deviation of active power from the rated value, and P N is the rated active power of the sending-end converter station, and α1 is the power margin factor.

[0069] It is worth mentioning that P max , P min P N The center point is symmetrical. max and P min The value of α1 is not necessarily the same, so the value of α1 depends on the P max and P min The value of .

[0070] Furthermore, PP N There is a positive and negative value relationship in , so the present embodiment introduces the sign(ΔP) function to indicate the positive and negative signs.N >0, ΔP>0, sign(ΔP) takes the value of 1. N When ΔP<0, the value of sign(ΔP) is -1. Based on this, the expression of the power margin factor is simplified to obtain:

[0071]

[0072] Constructing the power margin factor can determine the proportional value of the basic droop coefficient by adjusting the active power margin of different converter stations, thereby giving full play to the active power absorption and regulation capabilities of different sending-end converter stations.

[0073] Finally, the voltage deviation factor of the sending-end converter station is designed according to the DC voltage of the sending-end converter station and the preset DC voltage rated value.

[0074] The voltage deviation factor is specifically expressed as:

[0075]

[0076] Where α2 is the voltage deviation factor, u dc is the DC voltage of the sending-end converter station, U dcN is the rated DC voltage of the sending-end converter station, Δu dc is the difference between the DC voltage of the sending-end converter station and the rated value of the DC voltage, U min , U max They are the minimum and maximum values ​​of the DC voltage allowed.

[0077] Wherein, the sign(Δu dc ) is similar to the definition of sign(ΔP). In a multi-terminal sending-end converter station, α2 is only related to the DC voltage deviation value.

[0078] Finally, the basic droop coefficient, power margin factor and voltage deviation factor are combined to obtain the adaptive droop coefficient of the sending-end converter station. First, in order to ensure the adaptive adjustment capability of the sending-end converter station to the active power fluctuation, an improved basic droop coefficient is designed, which is recorded as the first droop coefficient k'0, that is, the basic droop coefficient and the power margin factor are combined first, and the active power margin of different converter stations is used to determine the proportional value of the basic droop coefficient.

[0079] The first droop coefficient k'0 is specifically expressed as:

[0080]

[0081] Secondly, in order to reduce the influence of active power fluctuation on DC voltage, the voltage deviation factor is used as the exponent of the first droop coefficient, and finally the adaptive droop coefficient is obtained. The specific expression is:

[0082]

[0083] In the formula, k * is the adaptive droop coefficient.

[0084] It is worth mentioning that k * The power margin factor and voltage deviation factor of the multi-terminal sending-end converter station are integrated, so that the sending-end converter station can adaptively distribute the active power obtained from the wind turbine side and reduce the fluctuation of the DC voltage.

[0085] Step S2, based on the changing relationship between the active power of the sending-end converter station and the grid frequency, with the goal of minimizing the oscillation amplitude of the grid frequency, construct a virtual inertia control model.

[0086] First, based on historical data, a grid frequency time variation model is obtained. In the embodiment of the present application, Figure 2 A time-varying graph of the power grid frequency is provided. As shown in the figure, the horizontal axis represents the time variation, and the vertical axis represents the power grid frequency variation. N is the rated frequency of the power grid. From time t0, the oscillation trend of the actual frequency f of the power grid oscillates up and down and decays with time until it stabilizes.

[0087] At the same time, because the control of the wind turbine side of the offshore wind farm is a droop control link, it is accompanied by an action delay in active power calculation. Compared with the active power regulation of the synchronous motor, its inertia characteristics are missing. Therefore, in the embodiment of the present application, it is considered to introduce the inertia characteristics of the synchronous motor into the active power model of the wind turbine side of the offshore wind farm. The specific method is to extract the inertia increment in the rotor inertia motion equation of the synchronous motor, and use the inertia increment as a virtual inertia to improve the active power model of the wind turbine side of the offshore wind farm.

[0088] The rotor inertia motion equation of the synchronous motor is specifically expressed as follows:

[0089]

[0090] In the formula, ΔP s is the active power provided to the rotor by the synchronous motor, J s is the rotor inertia of the synchronous motor, ω s is the angular frequency of the synchronous motor, ω Ns is the rated angular frequency of the synchronous motor, D s is the damping coefficient.

[0091] As an improvement of the above scheme, the embodiment of the present application needs to determine the changing relationship between the active power of the sending-end converter station and the grid frequency to construct an adaptive control strategy for the virtual inertia, so as to adjust the value of the virtual inertia to minimize the oscillation amplitude of the grid frequency.

[0092] Further, combined with Figure 2 The provided grid frequency time variation diagram and the active power variation of the sending-end converter station can be used to obtain the relationship between the active power of the sending-end converter station and the grid frequency, as follows:

[0093] At t0~t1, the active power of the sending-end converter station increases, f>f N , df / dt>0, Δf>0, then the virtual inertia J v A larger value should be selected to increase the oscillation time, so that f rises slowly to the highest point, and indirectly reduce the sudden change of f; where Δf is the deviation value of the grid frequency and t is the time.

[0094] At time t1~t2, the active power of the sending-end converter station increases, f>f N , df / dt<0, Δf>0, at this time the virtual inertia J v A smaller value should be selected to reduce the oscillation time and make f drop to the rated value quickly.

[0095] The mechanism of action at t2~t3 is the same as that at t0~t1, but at this moment f <f N , df / dt<0,Δf<0.

[0096] The mechanism of action at t3-t4 is the same as that at t1-t2, but at this moment f <f N , df / dt>0,Δf<0.

[0097] After time t4, the actual frequency f of the power grid changes periodically and repeatedly, and the action mechanism is consistent with that of time t0~t4.

[0098] Based on the above, the virtual inertia J can be established v The adaptive control strategy is recorded as the virtual inertia control model, and the specific expression is:

[0099]

[0100] In the formula, J v is the virtual inertia, J0 is the rated value of the virtual inertia, k1 and k2 are the adaptive adjustment coefficients of the virtual inertia under different conditions, f is the actual frequency of the power grid, t is the moment, Δf is the deviation value of the power grid frequency, Δω is the deviation value of the angular frequency of the synchronous motor, and N is a constant.

[0101] Optionally, in the embodiment of the present application, it is set that the virtual inertia starts to be adaptively adjusted only when Δf>N. This behavior is to avoid the virtual inertia affecting the stability of the system due to frequent adjustments.

[0102] Step S3, according to the virtual inertia control model, the virtual inertia and the adaptive droop coefficient are introduced into the preset active power model of the sending-end converter station to obtain the power inertia control model of the sending-end converter station.

[0103] In the embodiment of the present application, a power inertia control model of the sending-end converter station is first constructed.

[0104] If the transmission loss of the submarine cable is ignored, it can be approximately considered that the active power output of the wind turbine side of the offshore wind farm is equal to the active power income of the sending-end converter station. Therefore, the active power model of the wind turbine side of the offshore wind farm can be constructed first, and then the active power model of the sending-end converter station can be constructed based on this model.

[0105] First, the original wind turbine side active power model of the offshore wind farm is specifically expressed as:

[0106]

[0107] Where P WT is the active power output of the offshore wind farm, P MPPT is the reference value of active power output under maximum power tracking control of offshore wind farm, k f is the fixed droop coefficient of active power droop control, s is the differential operator, T d is the active power delay coefficient of the offshore wind farm, which can be understood as the overall action delay of active power control.

[0108] As described in step S2, in order to supplement the inertia characteristics of the wind turbine side, the inertia increment of the synchronous motor is extracted as the virtual inertia to improve the active power model of the wind turbine side of the offshore wind farm, and the following is obtained:

[0109]

[0110] In the formula, J v is the virtual inertia.

[0111] Combined with the above-mentioned relationship between the wind turbine side active power output of the offshore wind farm and the active power income of the sending-end converter station, the improved wind turbine side active power model of the offshore wind farm can be approximated as the active power model of the sending-end converter station.

[0112] Then, because of the relationship between the DC voltage of the sending-end converter station and the grid frequency, the active power model of the sending-end converter station can be improved again to obtain:

[0113]

[0114] Where P con is the active power of the sending-end converter station, k uis the droop coefficient between the DC voltage of the sending-end converter station and the grid frequency, which is a fixed constant.

[0115] The relationship between the DC voltage of the sending-end converter station and the grid frequency is specifically expressed as follows:

[0116] f=f N +k u (u dc -U dcN );

[0117] In order to further simplify the active power model of the sending-end converter station and more effectively adjust the control target, in the embodiment of the present application, k=k f ·k u is the droop coefficient between the sending-end converter station and the grid frequency, and P MPPT Approximately regarded as a constant, there is no delay effect on the fan side, so P MPPT Corresponding to the active power reference value P of the sending-end converter station con_ref , the active power model of the sending-end converter station is simplified as:

[0118]

[0119] Where P con_ref It is the reference value of active power of the sending-end converter station.

[0120] Because the embodiment of the present application is based on a multi-end sending-end converter station for research, it is necessary to introduce virtual inertia and the adaptive droop coefficient to optimize the active power model of the sending-end converter station, and obtain the final sending-end converter station power inertia control model, so as to achieve real-time response to changes in grid frequency by virtual inertia, and meet the optimal coupling control between different sending-end converter stations and the grid by adaptive droop coefficient, thereby improving the operating stability of the offshore wind power system.

[0121] The power inertia control model of the sending-end converter station is specifically:

[0122]

[0123] Where P con is the active power of the sending-end converter station, P con_ref is the active power reference value of the sending-end converter station, k * is the adaptive droop coefficient, u dc is the DC voltage of the sending-end converter station, U dcN is the rated value of the DC voltage of the sending-end converter station, s is the differential operator, T d is the active power delay coefficient of the offshore wind farm, J v is the virtual inertia, f N is the rated frequency of the power grid, k uis the droop coefficient between the DC voltage and grid frequency at the sending-end converter station, and t is the time.

[0124] Step S4: adjusting the output power of the wind turbine side of the offshore wind farm according to the power inertia control model of the sending-end converter station.

[0125] In the embodiment of the present application, the change of the current grid frequency is first obtained to determine the variation range of the DC voltage of the sending-end converter station. Then, according to the variation range of the DC voltage, the optimal active power of the sending-end converter station is determined through the power inertia control model of the sending-end converter station.

[0126] Finally, according to the optimal active power of the sending-end converter station, the output power of the wind turbine side of the offshore wind farm is adjusted to complete the adaptive power distribution of the offshore wind farm, avoiding grid fluctuations caused by uneven distribution of active power output on the wind turbine side, resulting in unstable grid frequency.

[0127] In order to better demonstrate how to determine the optimal active power of the sending-end converter station, Figure 3 As shown in the figure, the upper part is the effect of the adaptive droop coefficient on the active power of the sending-end converter, and the lower part is the effect of the virtual inertia on the active power of the sending-end converter. The two are coordinated to obtain the optimal active power that can stabilize the DC voltage and the grid frequency.

[0128] Implementing the embodiments of the present application has the following beneficial effects:

[0129] The embodiment of the present application takes into account the inconsistency of the active power margin between the multi-end sending-end converter stations. In order to reduce the fluctuation of the active power of the sending-end converter station and the influence of the DC voltage, the original basic droop coefficient is optimized to obtain an adaptive droop coefficient. Because the adaptive droop coefficient combines the active power margins between different sending-end converter stations, it can enable different sending-end converter stations to maximize their ability to regulate active power and further reduce the fluctuation of DC voltage. Then, considering the impact of the increase or decrease of active power on the oscillation trend of the grid frequency, a virtual inertia control model is constructed to quickly stabilize the grid frequency at the rated value to reduce the oscillation amplitude of the grid frequency and maintain the stability of the power system. Then, based on the virtual inertia control model and the adaptive droop coefficient, the active power margin of the sending-end converter station is accurately judged to clarify how to adjust the active power on the wind turbine side to stabilize the grid frequency and DC voltage.

[0130] Second embodiment

[0131] Further, in order to execute the droop control system of the offshore wind farm corresponding to the above method embodiment to achieve corresponding functions and technical effects, Figure 4A structural diagram of a droop control system for an offshore wind farm is provided. For ease of description, only the parts related to this embodiment are shown. The droop control system for an offshore wind farm provided by the embodiment of the present application includes:

[0132] The adaptive droop coefficient construction module 201 constructs the adaptive droop coefficient of the sending-end converter station according to the preset basic droop coefficient of the sending-end converter station, the acquired active power and DC voltage of the sending-end converter station, and the difference between the multi-end sending-end converter stations.

[0133] In an embodiment of the present application, the basic droop coefficient is constructed according to the preset active power safe fluctuation range and DC voltage safe fluctuation range of the sending-end converter station; the power margin factor of the sending-end converter station is constructed according to the active power of the sending-end converter station and the preset active power deviation from the rated value; the voltage deviation factor of the sending-end converter station is constructed according to the DC voltage of the sending-end converter station and the preset DC voltage rated value; the adaptive droop coefficient of the sending-end converter station is constructed according to the basic droop coefficient, the power margin factor and the voltage deviation factor, combined with the differences of multi-end sending-end converter stations.

[0134] The virtual inertia control model building module 202 is used to build a virtual inertia control model based on the changing relationship between the active power of the sending-end converter station and the grid frequency, with the goal of minimizing the oscillation amplitude of the grid frequency.

[0135] In an embodiment of the present application, according to a preset grid frequency time variation model, the variation relationship between the active power of the sending-end converter station and the grid frequency is determined; based on the variation relationship between the active power of the sending-end converter station and the grid frequency, a virtual inertia control model is constructed by introducing and adjusting the value of the virtual inertia to minimize the oscillation amplitude of the grid frequency; wherein the virtual inertia is constructed based on the inertia characteristics of the synchronous motor.

[0136] The inertia control model building module 203 is used to introduce virtual inertia and the adaptive droop coefficient into the preset active power model of the sending-end converter station according to the virtual inertia control model to obtain the power inertia control model of the sending-end converter station.

[0137] In an embodiment of the present application, based on the fact that the active power output on the wind turbine side of the offshore wind farm is equal to the active power income of the sending-end converter station, the sending-end converter station active power model is constructed on the basis of a preset active power model on the wind turbine side of the offshore wind farm; according to the relationship between the DC voltage and the grid frequency of the sending-end converter station, the virtual inertia and the adaptive droop coefficient are introduced into the power inertia control model of the sending-end converter station.

[0138] The wind turbine side power regulation module 204 is used to regulate the wind turbine side output power of the offshore wind farm according to the power inertia control model of the sending-end converter station.

[0139] In the embodiment of the present application, the change of the current grid frequency is first obtained to determine the variation range of the DC voltage of the sending-end converter station. Then, according to the variation range of the DC voltage, the optimal active power of the sending-end converter station is determined through the power inertia control model of the sending-end converter station.

[0140] Finally, according to the optimal active power of the sending-end converter station, the output power of the wind turbine side of the offshore wind farm is adjusted to complete the adaptive power distribution of the offshore wind farm, avoiding grid fluctuations caused by uneven distribution of active power output on the wind turbine side, resulting in unstable grid frequency.

[0141] In some embodiments, the adaptive droop coefficient construction module 201 is specifically:

[0142] In the embodiment of the present application, a study is conducted on how to maximize the regulating capability of different converter stations on the active power received from the wind turbine side based on the background of multi-end sending-end converter stations being connected in parallel. Therefore, the design of the adaptive droop coefficient also takes into account the active power margin of different sending-end converter stations to determine the proportional value of the droop coefficient.

[0143] For the construction of adaptive droop coefficient, three parameters need to be determined, namely the basic droop coefficient, the power margin factor of the sending-end converter station and the voltage deviation factor. Among them, the basic droop coefficient is used in the multi-end sending-end converter station to adjust the output power of the wind turbine side to cope with the changes in grid frequency and DC voltage.

[0144] Optionally, in the embodiment of the present application, the design of the basic droop coefficient is based on fully considering the influence of the overshoot of the DC voltage transient process in the control process of the sending-end converter station on the control effect, and a certain margin needs to be left for the control object. First, according to the actual situation, the safe fluctuation range of active power and the safe fluctuation range of DC voltage of the sending-end converter station are obtained, and then the basic droop coefficient is constructed with the goal of stabilizing the overshoot of DC voltage within the allowable range.

[0145] The specific expression of the basic droop coefficient is:

[0146]

[0147] Where α0 is the margin coefficient, which can be used to amplify the value of the basic droop coefficient to prevent the instantaneous overshoot of the DC voltage transient process from exceeding the limit range during initial regulation; P max , P min are the maximum and minimum values ​​of the safe fluctuation range of active power at the sending-end converter station, U dc_max , U dc_minare the maximum and minimum values ​​of the safe fluctuation range of the DC voltage at the sending-end converter station, respectively. Because the values ​​of different sending-end converter stations are not necessarily the same, when α0 is introduced, it can effectively prevent the DC voltage overshoot from exceeding the safe fluctuation range of the DC voltage.

[0148] The active power of the sending-end converter station and the set active power deviation from the rated value are obtained, and the power margin factor of the sending-end converter station is calculated.

[0149] The power margin factor is specifically expressed as:

[0150]

[0151] Where P is the actual value of active power at the sending-end converter station, ΔP is the deviation of active power from the rated value, and P N is the rated active power of the sending-end converter station, and α1 is the power margin factor.

[0152] It is worth mentioning that P max , P min P N The center point is symmetrical. max and P min The value of α1 is not necessarily the same, so the value of α1 depends on the P max and P min The value of .

[0153] Furthermore, PP N There is a positive and negative value relationship in , so the present embodiment introduces the sign(ΔP) function to indicate the positive and negative signs. N >0, ΔP>0, sign(ΔP) takes the value of 1. N When ΔP<0, the value of sign(ΔP) is -1. Based on this, the expression of the power margin factor is simplified to obtain:

[0154]

[0155] Constructing the power margin factor can determine the proportional value of the basic droop coefficient by adjusting the active power margin of different converter stations, thereby giving full play to the active power absorption and regulation capabilities of different sending-end converter stations.

[0156] Finally, the voltage deviation factor of the sending-end converter station is designed according to the DC voltage of the sending-end converter station and the preset DC voltage rated value.

[0157] The voltage deviation factor is specifically expressed as:

[0158]

[0159] Where α2 is the voltage deviation factor, u dc is the DC voltage of the sending-end converter station, U dcN is the rated DC voltage of the sending-end converter station, Δu dc is the difference between the DC voltage of the sending-end converter station and the rated value of the DC voltage, U min , U max They are the minimum and maximum values ​​of the DC voltage allowed.

[0160] Wherein, the sign(Δu dc ) is similar to the definition of sign(ΔP). In a multi-terminal sending-end converter station, α2 is only related to the DC voltage deviation value.

[0161] Finally, the basic droop coefficient, power margin factor and voltage deviation factor are combined to obtain the adaptive droop coefficient of the sending-end converter station. First, in order to ensure the adaptive adjustment capability of the sending-end converter station to the active power fluctuation, an improved basic droop coefficient is designed, which is recorded as the first droop coefficient k'0, that is, the basic droop coefficient and the power margin factor are combined first, and the active power margin of different converter stations is used to determine the proportional value of the basic droop coefficient.

[0162] The first droop coefficient k'0 is specifically expressed as:

[0163]

[0164] Secondly, in order to reduce the influence of active power fluctuation on DC voltage, the voltage deviation factor is used as the exponent of the first droop coefficient, and finally the adaptive droop coefficient is obtained. The specific expression is:

[0165]

[0166] In the formula, k * is the adaptive droop coefficient.

[0167] It is worth mentioning that k * The power margin factor and voltage deviation factor of the multi-terminal sending-end converter station are integrated, so that the sending-end converter station can adaptively distribute the active power obtained from the wind turbine side and reduce the fluctuation of the DC voltage.

[0168] In some embodiments, the virtual inertia control model building module 202 is specifically:

[0169] First, based on historical data, a grid frequency time variation model is obtained. In the embodiment of the present application, Figure 2 A time-varying graph of the power grid frequency is provided. As shown in the figure, the horizontal axis represents the time variation, and the vertical axis represents the power grid frequency variation. Nis the rated frequency of the power grid. From time t0, the oscillation trend of the actual frequency f of the power grid oscillates up and down and decays with time until it stabilizes.

[0170] At the same time, because the control of the wind turbine side of the offshore wind farm is a droop control link, it is accompanied by an action delay in active power calculation. Compared with the active power regulation of the synchronous motor, its inertia characteristics are missing. Therefore, in the embodiment of the present application, it is considered to introduce the inertia characteristics of the synchronous motor into the active power model of the wind turbine side of the offshore wind farm. The specific method is to extract the inertia increment in the rotor inertia motion equation of the synchronous motor, and use the inertia increment as a virtual inertia to improve the active power model of the wind turbine side of the offshore wind farm.

[0171] The rotor inertia motion equation of the synchronous motor is specifically expressed as follows:

[0172]

[0173] Where ΔP s is the active power provided to the rotor by the synchronous motor, J s is the rotor inertia of the synchronous motor, ω s is the angular frequency of the synchronous motor, ω Ns is the rated angular frequency of the synchronous motor, D s is the damping coefficient.

[0174] As an improvement of the above scheme, the embodiment of the present application needs to determine the changing relationship between the active power of the sending-end converter station and the grid frequency to construct an adaptive control strategy for the virtual inertia, so as to adjust the value of the virtual inertia to minimize the oscillation amplitude of the grid frequency.

[0175] Furthermore, by combining the change of the grid frequency over time and the change of the active power of the sending-end converter station, the change relationship between the active power of the sending-end converter station and the grid frequency can be obtained, as follows:

[0176] At t0~t1, the active power of the sending-end converter station increases, f>f N , df / dt>0, Δf>0, then the virtual inertia J v A larger value should be selected to increase the oscillation time, so that f rises slowly to the highest point, and indirectly reduce the sudden change of f; where Δf is the deviation value of the grid frequency and t is the time.

[0177] At time t1~t2, the active power of the sending-end converter station increases, f>f N , df / dt<0, Δf>0, at this time the virtual inertia J v A smaller value should be selected to reduce the oscillation time and make f drop to the rated value quickly.

[0178] The mechanism of action at t2~t3 is the same as that at t0~t1, but at this moment f <f N , df / dt<0,Δf<0.

[0179] The mechanism of action at t3-t4 is the same as that at t1-t2, but at this moment f <f N , df / dt>0,Δf<0.

[0180] After time t4, the actual frequency f of the power grid changes periodically and repeatedly, and the action mechanism is consistent with that of time t0~t4.

[0181] Among them, t0 is the start time, t4 is the end time, and t0~t4 is a cycle.

[0182] Based on the above, the virtual inertia J can be established v The adaptive control strategy is recorded as the virtual inertia control model, and the specific expression is:

[0183]

[0184] In the formula, J v is the virtual inertia, J0 is the rated value of the virtual inertia, k1 and k2 are the adaptive adjustment coefficients of the virtual inertia under different conditions, f is the actual frequency of the power grid, t is the moment, Δf is the deviation value of the power grid frequency, Δω is the deviation value of the angular frequency of the synchronous motor, and N is a constant.

[0185] Optionally, in the embodiment of the present application, it is set that the virtual inertia starts to be adaptively adjusted only when Δf>N. This behavior is to avoid the virtual inertia affecting the stability of the system due to frequent adjustments.

[0186] In some embodiments, the inertial control model building module 203 is specifically:

[0187] First, construct the power inertia control model of the sending-end converter station.

[0188] If the transmission loss of the submarine cable is ignored, it can be approximately considered that the active power output of the wind turbine side of the offshore wind farm is equal to the active power income of the sending-end converter station. Therefore, the active power model of the wind turbine side of the offshore wind farm can be constructed first, and then the active power model of the sending-end converter station can be constructed based on this model.

[0189] First, the original wind turbine side active power model of the offshore wind farm is specifically expressed as:

[0190]

[0191] Where P WT is the active power output of the offshore wind farm, P MPPTis the reference value of active power output under maximum power tracking control of offshore wind farm, k f is the fixed droop coefficient of active power droop control, s is the differential operator, T d is the active power delay coefficient of the offshore wind farm, which can be understood as the overall action delay of active power control.

[0192] As described in step S2, in order to supplement the inertia characteristics of the wind turbine side, the inertia increment of the synchronous motor is extracted as the virtual inertia to improve the active power model of the wind turbine side of the offshore wind farm, and the following is obtained:

[0193]

[0194] In the formula, J v is the virtual inertia.

[0195] Combined with the above-mentioned relationship between the wind turbine side active power output of the offshore wind farm and the active power income of the sending-end converter station, the improved wind turbine side active power model of the offshore wind farm can be approximated as the active power model of the sending-end converter station.

[0196] Then, because of the relationship between the DC voltage of the sending-end converter station and the grid frequency, the active power model of the sending-end converter station can be improved again to obtain:

[0197]

[0198] Where P con is the active power of the sending-end converter station, k u is the droop coefficient between the DC voltage of the sending-end converter station and the grid frequency, which is a fixed constant.

[0199] The relationship between the DC voltage of the sending-end converter station and the grid frequency is specifically expressed as follows:

[0200] f=f N +k u (u dc -U dcN );

[0201] In order to further simplify the active power model of the sending-end converter station and more effectively adjust the control target, in the embodiment of the present application, k=k f ·k u is the droop coefficient between the sending-end converter station and the grid frequency, and P MPPT Approximately regarded as a constant, there is no delay effect on the fan side, so P MPPT Corresponding to the active power reference value P of the sending-end converter station con_ref , the active power model of the sending-end converter station is simplified as:

[0202]

[0203] Where P con_ref It is the active power reference value of the sending-end converter station.

[0204] Because the embodiment of the present application is based on a multi-end sending-end converter station for research, it is necessary to introduce virtual inertia and the adaptive droop coefficient to optimize the active power model of the sending-end converter station, and obtain the final sending-end converter station power inertia control model, so as to achieve real-time response to changes in grid frequency by virtual inertia, and meet the optimal coupling control between different sending-end converter stations and the grid by adaptive droop coefficient, thereby improving the operating stability of the offshore wind power system.

[0205] The power inertia control model of the sending-end converter station is specifically:

[0206]

[0207] Where P con is the active power of the sending-end converter station, P con_ref is the active power reference value of the sending-end converter station, k * is the adaptive droop coefficient, u dc is the DC voltage of the sending-end converter station, U dcN is the rated value of the DC voltage of the sending-end converter station, s is the differential operator, T d is the active power delay coefficient of the offshore wind farm, J v is the virtual inertia, f N is the rated frequency of the power grid, k u is the droop coefficient between the DC voltage and grid frequency at the sending-end converter station, and t is the time.

[0208] Implementing the embodiments of the present application has the following beneficial effects:

[0209] The embodiment of the present application takes into account the inconsistency of the active power margin between the multi-end sending-end converter stations. In order to reduce the fluctuation of the active power of the sending-end converter station and the influence of the DC voltage, the original basic droop coefficient is optimized to obtain an adaptive droop coefficient. Because the adaptive droop coefficient combines the active power margins between different sending-end converter stations, it can enable different sending-end converter stations to maximize their ability to regulate active power and further reduce the fluctuation of DC voltage. Then, considering the impact of the increase or decrease of active power on the oscillation trend of the grid frequency, a virtual inertia control model is constructed to quickly stabilize the grid frequency at the rated value to reduce the oscillation amplitude of the grid frequency and maintain the stability of the power system. Then, based on the virtual inertia control model and the adaptive droop coefficient, the active power margin of the sending-end converter station is accurately judged to clarify how to adjust the active power on the wind turbine side to stabilize the grid frequency and DC voltage.

[0210] The specific embodiments described above further describe the purpose, technical solutions and beneficial effects of the present application in detail. It should be understood that the above description is only a specific embodiment of the present application and is not intended to limit the scope of protection of the present application. It is particularly pointed out that for those skilled in the art, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the scope of protection of the present application.

Claims

1. A droop control method for an offshore wind farm, characterized in that: include: According to the preset basic droop coefficient of the sending-end converter station, the acquired active power and DC voltage of the sending-end converter station, and the difference between the multi-end sending-end converter stations, the adaptive droop coefficient of the sending-end converter station is constructed; Based on the changing relationship between the active power of the sending-end converter station and the grid frequency, a virtual inertia control model is constructed with the goal of minimizing the oscillation amplitude of the grid frequency. According to the virtual inertia control model, the virtual inertia and the adaptive droop coefficient are introduced into the preset active power model of the sending-end converter station to obtain the power inertia control model of the sending-end converter station; According to the power inertia control model of the sending-end converter station, the output power of the wind turbine side of the offshore wind farm is adjusted.

2. The droop control method for an offshore wind farm according to claim 1, characterized in that: The adaptive droop coefficient of the sending-end converter station is constructed according to the preset basic droop coefficient of the sending-end converter station, the acquired active power and DC voltage of the sending-end converter station, and the difference between the multi-end sending-end converter stations. Specifically, it is: Constructing the basic droop coefficient according to the preset safe fluctuation range of active power and safe fluctuation range of DC voltage of the sending-end converter station; Constructing a power margin factor of the sending-end converter station according to the active power of the sending-end converter station and a preset active power deviation from a rated value; Constructing a voltage deviation factor of the sending-end converter station according to the DC voltage of the sending-end converter station and a preset DC voltage rated value; According to the basic droop coefficient, power margin factor and voltage deviation factor, combined with the differences of multi-end sending-end converter stations, an adaptive droop coefficient of the sending-end converter station is constructed.

3. The droop control method for an offshore wind farm according to claim 2, characterized in that: According to the basic droop coefficient, power margin factor and voltage deviation factor, combined with the differences of multi-end sending-end converter stations, the adaptive droop coefficient of the sending-end converter station is constructed, which is specifically: According to the difference in active power margin between the multi-end sending-end converter stations, the basic droop coefficient and the power margin factor are combined to obtain a first droop coefficient; According to the voltage deviation factor, the first droop coefficient is optimized to obtain the adaptive droop coefficient for the purpose of reducing the fluctuation of the DC voltage caused by the active power fluctuation of the sending-end converter station.

4. The droop control method for an offshore wind farm according to claim 3, characterized in that: The adaptive droop coefficient is specifically: In the formula, k * is the adaptive droop coefficient, k'0 is the first droop coefficient, α2 is the voltage deviation factor, α1 is the power margin factor, P max , P min are the maximum and minimum values ​​of the safe fluctuation range of active power of the sending-end converter station, respectively. dc_max , U dc_min are the maximum and minimum values ​​of the safe fluctuation range of the DC voltage at the sending-end converter station, respectively, and α0 is the margin coefficient.

5. The droop control method for an offshore wind farm according to claim 1, characterized in that: Based on the changing relationship between the active power of the sending-end converter station and the grid frequency, a virtual inertia control model is constructed with the goal of minimizing the oscillation amplitude of the grid frequency, specifically: According to the preset grid frequency time variation model, determine the variation relationship between the active power of the sending-end converter station and the grid frequency; Based on the changing relationship between the active power of the sending-end converter station and the grid frequency, a virtual inertia control model is constructed by introducing and adjusting the value of the virtual inertia to minimize the oscillation amplitude of the grid frequency; wherein the virtual inertia is constructed based on the inertia characteristics of the synchronous motor.

6. The droop control method for an offshore wind farm according to claim 5, characterized in that: The virtual inertia control model is specifically: In the formula, J v is the virtual inertia, J0 is the rated value of the virtual inertia, k1 and k2 are the adaptive adjustment coefficients of the virtual inertia under different conditions, f is the actual frequency of the power grid, t is the moment, Δf is the deviation value of the power grid frequency, Δω is the deviation value of the angular frequency of the synchronous motor, and N is a constant.

7. The droop control method for an offshore wind farm according to claim 1, characterized in that: According to the virtual inertia control model, the virtual inertia and the adaptive droop coefficient are introduced into the preset active power model of the sending-end converter station to obtain the power inertia control model of the sending-end converter station, which is specifically: Based on the assumption that the active power output of the wind turbine side of the offshore wind farm is equal to the active power income of the sending-end converter station, the sending-end converter station active power model is constructed on the basis of a preset active power model of the wind turbine side of the offshore wind farm; According to the relationship between the DC voltage and the grid frequency of the sending-end converter station, the virtual inertia and the adaptive droop coefficient are introduced into the power inertia control model of the sending-end converter station.

8. The droop control method for an offshore wind farm according to claim 7, characterized in that: The power inertia control model of the sending-end converter station is specifically: Where P con is the active power of the sending-end converter station, P con_ref is the active power reference value of the sending-end converter station, k * is the adaptive droop coefficient, u dc is the DC voltage of the sending-end converter station, U dcN is the rated value of the DC voltage of the sending-end converter station, s is the differential operator, T d is the active power delay coefficient of the offshore wind farm, J v is the virtual inertia, f N is the rated frequency of the power grid, k u is the droop coefficient between the DC voltage and grid frequency at the sending-end converter station, and t is the time.

9. The droop control method for an offshore wind farm according to claim 1, characterized in that: The output power of the wind turbine side of the offshore wind farm is adjusted according to the power inertia control model of the sending-end converter station, specifically: According to the change of the current grid frequency, determine the DC voltage change amplitude of the sending-end converter station; Determining the optimal active power of the sending-end converter station according to the DC voltage variation amplitude; The output power of the wind turbine side of the offshore wind farm is adjusted according to the optimal active power of the sending-end converter station.

10. A droop control system for an offshore wind farm, characterized in that: include: Adaptive droop coefficient building module, virtual inertia control model building module, inertia control model building module and fan side power regulation module; The adaptive droop coefficient construction module constructs the adaptive droop coefficient of the sending-end converter station according to the preset basic droop coefficient of the sending-end converter station, the acquired active power and DC voltage of the sending-end converter station, and the difference between the multi-end sending-end converter stations; The virtual inertia control model building module is used to build a virtual inertia control model based on the changing relationship between the active power of the sending-end converter station and the grid frequency, with the goal of minimizing the oscillation amplitude of the grid frequency; The inertia control model building module is used to introduce virtual inertia and the adaptive droop coefficient into a preset active power model of a sending-end converter station according to a virtual inertia control model, so as to obtain a power inertia control model of a sending-end converter station; The wind turbine side power regulation module is used to regulate the wind turbine side output power of the offshore wind farm according to the power inertia control model of the sending-end converter station.

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