CN115622110A - A flexible DC grid-connected system for offshore wind power and its control method - Google Patents

Abstract

The invention belongs to the technical field of offshore wind power, and particularly relates to an offshore wind power flexible direct current grid-connected system and a control method thereof. In the operation process of the offshore wind power flexible direct current grid-connected system, firstly, a virtual rotor rotation angle is generated according to a virtual rotor dynamics link, and a virtual internal potential amplitude is generated according to a virtual excitation adjusting link; and then under the condition that the AC side of the marine MMC converter station is judged not to have AC system fault and the medium-high frequency harmonic content is larger than a set harmonic content threshold value, adopting an open-loop control mode for the marine MMC converter station, or else adopting a closed-loop control mode to directly obtain a modulation wave of the marine MMC converter station according to the generated virtual rotor rotation angle and the virtual inner potential amplitude. The invention inhibits the medium-high frequency harmonic oscillation which possibly occurs under the working condition of low-power operation, improves the wind power consumption capability of the offshore wind power plant, and further improves the safety and stability of the AC-DC hybrid large power grid system.

Description

A flexible DC grid-connected system for offshore wind power and its control method

technical field

The invention belongs to the technical field of offshore wind power, and in particular relates to an offshore wind power flexible DC grid-connected system and a control method thereof.

Background technique

As countries around the world pay more and more attention to issues such as energy security, ecological environment, and climate change, accelerating the development of wind power has become the general consensus and concerted action of the international community to promote energy transformation and development and respond to global climate change. Offshore wind power has the advantages of stable wind energy, high utilization hours of power generation, basically unaffected by terrain and landform, and suitable for large-scale development. It is also close to the power load center, which is convenient for local consumption by the grid and avoids long-distance transportation of wind power. Therefore, the development and utilization of offshore wind power has received more and more attention and attention, and has become one of the new growth points and main directions of global renewable energy development. According to the latest data from the Global Wind Energy Association GWEC, by the end of 2021, the cumulative installed capacity of offshore wind power in the world will reach 33.1 million kilowatts, of which the cumulative installed capacity of offshore wind power in China is 5.555 million kilowatts. Compared with onshore wind farms, the advantages of offshore wind farms are mainly that they do not occupy land resources, are basically not affected by topography, have higher wind speeds, larger single-machine capacity of wind turbines, and higher annual utilization hours.

The remarkable features of the MMC (Modular Multilevel Converter) converter make the MMC-HVDC power transmission technology the only choice for foreign large-scale long-distance offshore wind farms to be connected to the grid. With the expansion of development scale, the increase of transmission capacity and transmission distance, the large-scale unit, the short-circuit level of the receiving end power grid, the safety and stability of the power grid and other factors, the development trend of the direct current direction of offshore wind power transmission is becoming more and more obvious.

The classic flexible DC transmission system based on active/reactive power control has been fully applied at home and abroad, and many flexible DC projects have been established at home and abroad. The traditional flexible DC system adopts the active/reactive power control mode, relies on the connected AC grid and obtains the synchronous phase of the AC grid through the phase-locked loop technology, and then constructs the double closed-loop proportional-integral control function of d/q space. However, when the flexible DC transmission technology is applied to offshore wind power transmission scenarios, the short-circuit capacity of the connected AC grid is very small (the short-circuit ratio is very small), which has had a great impact on the safe and stable operation of the traditional flexible DC transmission system, and even cannot Maintain the normal operation of the system.

Contents of the invention

The purpose of the present invention is to provide an offshore wind power flexible direct current grid-connected system and its control method to solve the problem that the control strategy in the prior art cannot maintain the stable and normal operation of the system.

In order to solve the above technical problems, the present invention provides a control method for an offshore wind power flexible DC grid-connected system, including the following steps:

1) During the operation of the offshore wind power flexible DC grid-connected system, the virtual rotor rotation angle θ is generated according to the virtual rotor dynamics link, and the virtual internal potential amplitude E is generated according to the virtual excitation adjustment link; wherein, the virtual rotor dynamics link It is used to simulate the rotor motion equation of the synchronous generator, and the virtual excitation adjustment link is used to simulate the excitation control of the synchronous generator;

2) Judging whether there is an AC system failure on the AC side of the offshore MMC converter station and whether the medium and high frequency harmonic content is greater than the set threshold: if there is no AC system failure and the medium and high frequency harmonic content is greater than the set harmonic content threshold, then the offshore The MMC converter station adopts the open-loop control mode to directly obtain the modulation wave of the offshore MMC converter station according to the generated virtual rotor rotation angle θ and the virtual internal potential amplitude E; The converter station is controlled.

Its beneficial effects are: the present invention adopts a new control strategy for the offshore MMC converter station, firstly by simulating the electromechanical equation and the excitation voltage equation of the synchronous generator, no longer relying on the connected AC grid to obtain the phase-locked loop phase, but The virtual rotor rotation angle is obtained according to the virtual rotor dynamics link, and the virtual internal potential amplitude is generated according to the virtual excitation adjustment link, and then according to the fact that there is no AC system failure in the offshore MMC converter station and the high-frequency harmonic content is high, adopt The switch control mode prohibits the current inner loop from being put into use, so as to directly obtain the modulation wave of the offshore MMC converter station according to the virtual rotor rotation angle and the virtual inner potential amplitude, which suppresses the mid-high frequency harmonic oscillation that may occur under low-power operating conditions , increase the inherent inertia and damping characteristics of the offshore MMC converter station, enhance the frequency and inertia support capacity of the wind power grid-connected system, improve the wind power consumption capacity of the offshore wind farm, and further improve the safety and stability of the AC-DC hybrid large power grid system It ensures the safe, reliable and stable operation of the AC-DC hybrid power grid with new energy power generation as the main body.

Further, if an AC system fault occurs or the medium and high frequency harmonic content is less than or equal to the set harmonic content threshold, the closed-loop control mode is adopted for the offshore MMC converter station to generate virtual rotor rotation angle θ and virtual internal potential amplitude E obtains the reference value of the current inner loop through dq transformation, and then obtains the modulation wave of the offshore MMC converter station through the current inner loop and dq inverse transformation.

The beneficial effect is: in the case of an AC system fault or a low-frequency harmonic content, the closed-loop control mode is adopted, and the current inner loop is put into use, so as to suppress the fault current.

Furthermore, the transfer function of the virtual rotor dynamics link is:

In the formula, T _j is the inertial time constant; ω is the virtual angular frequency; P _e is the virtual electromagnetic power; D is the damping factor; P _ref is the reference value of virtual active power, and P _ref =K _f (F _N -F), K _f is the frequency difference control gain, F _N is the per unit value of the rated frequency, and F is the actual value of the grid frequency.

The beneficial effects are: the virtual rotor link is used to generate the virtual rotor rotation angle, the phase-locked loop phase is no longer dependent on the connected AC grid, and the offshore MMC converter station has the inertia and damping characteristics of the traditional synchronous generator.

Furthermore, the transfer function of the virtual excitation regulation link is:

In the formula, Q _ref is the reference value of reactive power; Q _e is the actual value of reactive power; U _{ac_ref} is the reference value of AC voltage; U _ac is the actual value of AC voltage; k _q is the deviation gain coefficient of reactive power; k _ac is AC voltage deviation gain coefficient; k _p is the proportional gain of the virtual excitation controller; T _i is the integral time constant of the virtual excitation controller; E ₀ is the reference value of the virtual internal potential.

The beneficial effect is: the virtual excitation adjustment link is used to generate the virtual internal potential amplitude, and the offshore MMC converter station has the inertia and damping characteristics of the traditional synchronous generator.

Further, when there is no AC system failure and the medium and high frequency harmonic content is greater than the set harmonic content threshold, the modulation wave of the offshore MMC converter station is:

In the formula, ma, m _b and m _c are A, B and _C three-phase modulation waves respectively.

Further, when an AC system fault occurs or the medium and high frequency harmonic content is less than or equal to the set harmonic content threshold, the reference value of the current inner loop is:

In the formula, v _d and v _q are the d-axis and q-axis reference values of the current inner loop respectively; v _a , v _b and v _c are the reference modulation waves generated by the virtual rotor dynamics link and the virtual excitation adjustment link respectively ;

And the modulation wave of the offshore MMC converter station is:

In the formula, v _{d_PI} and v _{q_PI} are the outputs of d-axis and q-axis current inner-loop proportional-integral controllers respectively, and ma, m _b and m _c are three-phase modulation waves of A, B and _C , respectively.

Furthermore, the high-frequency harmonic content in the AC side voltage of the offshore MMC converter station is: U _h =U _h1 *(1-K _LP50 ), and the high-frequency harmonic content in the AC side current of the offshore MMC converter station is: I _h =I _h1 *(1-K _LP50 ), U _h is the high-frequency harmonic content of the voltage, U _h1 is the original voltage value of the AC side of the offshore MMC converter station, I _h is the high-frequency harmonic content of the current, and I _h1 is the offshore MMC converter The original value of the AC side current of the station, and K _LP50 is a low-pass filter function.

The beneficial effect is that the high-frequency harmonic content in the AC side voltage/current of the offshore MMC converter station can be quickly obtained by adopting the above-mentioned method.

Further, the modulation wave of the onshore MMC converter station is generated by using the double closed-loop control strategy, and the onshore MMC converter station is controlled according to the modulation wave of the onshore MMC converter station; wherein, in the double closed-loop control strategy, the d-axis The ring selects the port voltage control of the offshore station of the DC system, the outer ring of the q-axis selects the reactive power control of the land station, and the inner ring of the d-axis and the q-axis selects the AC current control of the fixed valve side.

The beneficial effect is that the onshore MMC converter station adopts a classic double closed-loop control strategy, which can ensure the reliable and stable operation of the flexible direct current transmission project.

Further, the zero-sequence criterion and the three-phase amplitude criterion are used to judge whether an AC system fault occurs; among them, the zero-sequence criterion is used to judge whether a single-phase AC grounding occurs, and the three-phase amplitude criterion is used to judge whether a fault occurs Three-phase-to-ground or two-phase-to-ground fault.

The beneficial effect is that: using two criteria of the zero sequence criterion and the three-phase amplitude criterion to judge whether an AC system fault occurs, it is possible to accurately and comprehensively judge whether an AC system fault occurs.

In order to solve the above technical problems, the present invention also provides an offshore wind power flexible DC grid-connected system, including an offshore MMC converter station, a DC submarine cable, an onshore MMC converter station and a control device; the offshore MMC converter station The AC side is used to connect to the offshore wind farm, and the DC side is connected to one end of the DC submarine cable; the DC side of the onshore MMC converter station is connected to the other end of the DC submarine cable, and the AC side is used to connect to the power grid; the control device includes processing The processor is used to execute computer program instructions to realize the control method of the offshore wind power flexible direct current grid-connected system described above, and can achieve the same beneficial effect as the method.

Description of drawings

Fig. 1 is a schematic diagram of the offshore wind power flexible DC grid-connected system of the present invention;

Fig. 2 is a schematic diagram of the topological structure and measuring points of the MMC converter valve used in the present invention;

Fig. 3 is a control functional block diagram of the land MMC converter station of the present invention;

Fig. 4 is a block diagram of current inner loop control and circulation suppression control of the present invention;

Fig. 5 is a control block diagram of virtual rotor dynamics and virtual excitation links of the present invention;

Fig. 6 is a schematic diagram of the control flow of the offshore MMC converter station of the present invention.

detailed description

The basic idea of the present invention is to adopt different control strategies for the offshore MMC converter station and the land MMC converter station, especially for the offshore MMC converter station, which is in the passive island control mode, and the adopted control strategy includes multiple The links are the virtual rotor dynamics link for generating the virtual rotor rotation angle θ, the virtual excitation adjustment link for generating the virtual internal potential amplitude E, the current inner loop control link, and the open-loop and closed-loop control switching links. In the case that there is no AC system fault on the AC side of the offshore MMC converter station but the high-frequency harmonic content is high, the open-loop control mode is adopted, and the current inner-loop function is prohibited. According to the pseudo-rotor rotation angle θ and the virtual internal potential amplitude E Directly obtain the modulated wave of the offshore MMC converter station; when the AC system failure occurs on the AC side of the offshore MMC converter station or the content of medium and high frequency harmonics is low, the closed-loop control mode is adopted, and the current inner loop needs to be put into use. The given value of the current inner loop is obtained by the rotation angle θ and the virtual inner potential amplitude E, and then the modulated wave of the offshore MMC converter station is obtained through the current inner loop.

In order to make the object, technical solution and advantages of the present invention more clear, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, and are not intended to limit the present invention, that is, the described embodiments are only some of the embodiments of the present invention, but not all of the embodiments. Accordingly, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative efforts belong to the protection scope of the present invention.

Embodiment of offshore wind power flexible DC grid-connected system:

An embodiment of an offshore wind power flexible DC grid-connected system of the present invention is applied to a (deep) offshore wind power flexible DC transmission system. As shown in Figure 1, the offshore wind power flexible DC grid-connected system includes an offshore AC booster station, an offshore MMC converter stations, DC submarine cables, onshore MMC converter stations and control devices. The low-voltage side of the offshore AC booster station is used to connect to the offshore wind farm, the high-voltage side is connected to the AC side of the offshore MMC converter station, the DC side of the offshore MMC is connected to one end of the DC submarine cable, and the other end of the DC submarine cable is connected to the onshore MMC converter. The DC side of the converter station and the AC side of the onshore MMC converter station are used to connect to the onshore AC grid. The MMC converter valve topology and voltage and current measurement points of the offshore MMC converter station and onshore MMC converter station are shown in Figure 2. The converter valve sub-module can adopt a typical half-bridge sub-module or full-bridge sub-module structure , can also adopt a certain proportion of half bridge and certain proportion of full bridge hybrid module structure. The control device includes an offshore control device and an onshore control device, and the inter-station communication between the offshore control device and the onshore control device, both of which are embedded industrial control platforms or PC devices, and the offshore control device is used to control the offshore MMC The converter station outputs modulated waves, and the onshore control device is used to output modulated waves to the onshore MMC converter station. These two control devices both include a processor and a memory, and the processor executes computer instructions stored in the memory to implement a corresponding converter station control strategy.

For the onshore MMC converter station, the classic double closed-loop control strategy of the flexible HVDC transmission system is adopted, as shown in Figure 3, the d-axis outer loop is selected to control the port voltage of the DC system offshore station, and the q-axis outer loop is selected to set the onshore station without For power control, the d-axis and q-axis inner rings both select constant current control, which is a direct current control based on the alternating current at the side of the MMC circulation valve, as shown in Figure 4.

For offshore MMC converter stations, the control strategies adopted include virtual rotor dynamics link, virtual excitation regulation link, current inner loop control link, open-loop and closed-loop control switching link. As shown in Figure 5, the virtual rotor dynamics link is used to generate the virtual rotor rotation angle θ, the virtual excitation adjustment link is used to generate the virtual internal potential amplitude E, and the open-loop and closed-loop control switching link is used to realize the open-loop control mode and closed-loop control mode. Control mode switching. The specific control strategy is as follows: when there is no AC system failure and medium/high frequency harmonic oscillation occurs on the AC side of the offshore MMC converter station, the open-loop control mode is adopted, that is, the current inner-loop control function is prohibited, and according to the virtual internal potential amplitude The value E and the virtual rotor rotation angle θ directly generate the modulation wave of the offshore MMC converter station; when the AC system fault occurs on the AC side of the offshore MMC converter station or no medium/high frequency harmonic oscillation occurs, the closed-loop control mode is adopted, that is, The reference value of the current inner loop is generated by the dq transformation of the virtual inner potential amplitude E and the virtual rotor rotation angle θ, and the output of the current inner loop is generated by the dq inverse transformation to generate the modulation wave of the MMC converter station. The reason for this is that the current inner loop control link plays an important role in suppressing the fault current, but the input of the current inner loop control link will introduce potential medium and high frequency harmonic resonance risks.

The virtual rotor dynamics link simulates the rotor motion equation (electromechanical equation) of the synchronous generator, thereby generating a virtual rotor rotation angle θ, and its transfer function is:

In the formula, T _j is the inertial time constant, which usually takes 5-10 seconds; ω is the virtual angular frequency (the rated value of 1 per unit is 100π); P _ref is the virtual active power reference value (per unit value); P _e is the virtual Electromagnetic power (take the actual value of the active power of the MMC converter valve, per unit value), take the actual value of the active power at the valve side of the MMC converter, which can be calculated from the instantaneous power theory; D is the damping factor; θ is the virtual rotor rotation angle. In particular, the virtual active power reference value is obtained from the active power-frequency droop characteristic, that is, the virtual active power reference value simulates the primary frequency modulation function of the synchronous generator, and its relationship is: P _ref =K _f (F _N -F), K _f is Frequency difference control gain, F _N is the per-unit value of the rated frequency (the default value is 1), and F is the actual value of the grid frequency.

The virtual excitation adjustment link simulates the excitation control of the synchronous generator. When the system voltage fluctuates, the excitation voltage is changed through the deviation ratio, which finally affects the reactive power exchange between the unit and the system, thereby generating the virtual internal potential amplitude E. The transfer function is:

Among them, Q _ref is the reference value of reactive power, and the inductive reactive power injected into the system by MMC is positive; Q _e is the actual value of reactive power; U _{ac_ref} is the reference value of AC voltage, the default value is 1; U _ac is the actual value of AC voltage value; k _q is the reactive power deviation gain coefficient; k _ac is the AC voltage deviation gain coefficient; k _p is the proportional gain of the virtual excitation controller; T _i is the integral time constant of the virtual excitation controller; E ₀ is the reference value of the virtual internal potential , the default value is 1.

In the closed-loop control mode, the current inner loop control link is composed of the d-axis and q-axis proportional-integral control links. The d-axis and q-axis current reference values are obtained by formulas (3) and (4), and then obtained by formula (5). Modulated wave of MMC converter station.

In the open-loop control mode, the modulated wave of the offshore MMC converter station can be obtained directly from formula (5).

The following is an introduction to the entire process of the control method adopted by the entire offshore wind power flexible DC grid-connected system in combination with the control flow chart of the offshore MMC converter station in Figure 6:

1) The two MMC converter stations have completed the initialization operation condition setting, and if the initialization is successful, they will enter the normal operation condition. The onshore MMC converter station chooses a double closed-loop control strategy. In the double closed-loop control strategy, the outer ring of the d-axis chooses to control the port voltage of the offshore station of the DC system, and the outer ring of the q-axis chooses the reactive power control of the land station. The d-axis and q-axis The inner loop selects the current loop. The charging of the DC side of the MMC sub-module in the offshore MMC converter station is completed by the onshore MMC converter station.

2) After the charging of the offshore MMC loop station is completed through the DC side, according to the virtual rotor dynamics link, use the formula (1) to generate the virtual rotor rotation angle θ required for dq positive and negative transformation, and according to the virtual excitation link, use the formula (2) A virtual internal potential magnitude E is generated.

3) The offshore MMC converter station collects AC voltage and current signals in real time, and obtains the amplitude of medium and high frequency harmonics through general methods such as fast Fourier transform or low-pass filtering, U _h = U _h1 *(1-K _LP50 ), I _h ＝I _h1 *(1-K _LP50 ), U _h is the high-frequency harmonic content of the voltage, U _h1 is the original voltage value of the AC side of the offshore MMC converter station, I _h is the high-frequency harmonic content of the current, and I _h1 is the offshore The original value of the AC side current of the MMC converter station, K _LP50 is a low-pass filter function; and the zero-sequence criterion and the three-phase amplitude criterion are used to judge whether the AC side fault occurs, and the zero-sequence criterion is mainly used to judge whether a single-phase AC grounding, the three-phase amplitude criterion is used to judge whether a three-phase grounding or two-phase grounding fault occurs:

If no AC system fault occurs at this time and the medium and high frequency harmonic content is greater than the set harmonic content threshold Δsetting1 (5%), the offshore MMC converter station adopts the open-loop control mode. The principle of the MMC open-loop control is to prohibit the current inner-loop control function in the closed-loop mode, and directly according to the formula (6), the three-phase positive sequence modulation wave is generated by the virtual rotor phase angle and the internal potential amplitude;

If the AC system fails at this time or the medium and high frequency harmonic content is less than or equal to the set harmonic content threshold Δsetting1, the offshore MMC converter station adopts the closed-loop control mode, and the current inner-loop control function needs to be put into use at this time. According to the above-mentioned generated virtual rotor rotation angle and virtual internal potential amplitude, a three-phase reference voltage vector is generated, and the d/q axis reference value of the inner ring current is generated after dq forward transformation, that is, formulas (3) to (4). After the proportional integral control of the ring current, the d/q axis voltage is output, and then the dq inverse transformation is performed to finally generate the trigger modulation wave of the offshore MMC converter station, that is, the formula (5).

To sum up, the control method of the offshore wind power flexible DC grid-connected system of the present invention, by simulating the electromechanical equation and the excitation circuit equation of the traditional synchronous generator, no longer depends on the connected AC grid to obtain the phase-locked loop phase, but according to the virtual rotor dynamics link to obtain the virtual rotor rotation angle, and use the virtual excitation adjustment link to generate the virtual internal potential amplitude, which realizes the inertia and damping characteristics of the traditional synchronous generator in the offshore MMC converter station, and switches the open-closed loop control by judging the harmonic amplitude to avoid The medium and high frequency harmonic oscillation enhances the new energy absorption capacity of the offshore wind power flexible DC grid-connected system, and ensures the safe, reliable and stable operation of the AC-DC hybrid power grid with new energy power generation as the main body.

Embodiment of control method for offshore wind power flexible DC grid-connected system:

An embodiment of a control method for an offshore wind power flexible DC grid-connected system of the present invention is aimed at the offshore wind power flexible DC grid-connected system shown in Figure 1. The overall idea of the method is to obtain the virtual rotor rotation according to the virtual rotor dynamics link angle, and use the virtual excitation adjustment link to generate the virtual internal potential amplitude, according to the obtained virtual rotor rotation angle and virtual internal potential amplitude, and then according to whether the offshore MMC converter station has an AC system fault and the content of medium and high frequency harmonics, different The modulation wave of the offshore MMC converter station is obtained by the method. This method enhances the inertial support and damping capacity of the MMC converter station, realizes the reliable and friendly access of new energy to the (deep) offshore wind power flexible direct current transmission system, and finally guarantees the new energy-based AC-DC hybrid large-scale The flexibility and safety and stability of the power grid. The specific implementation process of this method is consistent with the control method of the offshore wind power flexible DC grid-connected system introduced in the embodiment of the offshore wind power flexible DC grid-connected system.

Specific embodiments have been given above, but the present invention is not limited to the described embodiments. The basic idea of the present invention lies in the above-mentioned basic scheme. For those of ordinary skill in the art, according to the teaching of the present invention, it does not need to spend creative labor to design various deformation models, formulas, and parameters. Changes, modifications, substitutions and variations to the implementations without departing from the principle and spirit of the present invention still fall within the protection scope of the present invention.

Claims (10)

1. A control method for an offshore wind power flexible direct current grid-connected system is characterized by comprising the following steps:

1) In the operation process of the offshore wind power flexible direct current grid-connected system, generating a virtual rotor rotation angle theta according to a virtual rotor dynamics link, and generating a virtual internal potential amplitude E according to a virtual excitation regulation link; the virtual rotor dynamics link is used for simulating a rotor motion equation of the synchronous generator, and the virtual excitation adjusting link is used for simulating excitation control of the synchronous generator;

2) Whether alternating current system faults occur on the alternating current side of the marine MMC convertor station or not and whether medium-high frequency harmonic content is larger than a set threshold value or not are judged: if the alternating current system fault does not occur and the medium-high frequency harmonic content is larger than the set harmonic content threshold value, adopting an open-loop control mode for the marine MMC convertor station to directly obtain a modulation wave of the marine MMC convertor station according to the generated virtual rotor rotation angle theta and the virtual internal potential amplitude E; and controlling the marine MMC converter station according to the modulation wave of the marine MMC converter station.

2. The offshore wind power flexible direct current grid-connected system control method according to claim 1, characterized in that if an alternating current system fault occurs or the medium-high frequency harmonic content is less than or equal to a set harmonic content threshold value, a closed-loop control mode is adopted for the offshore MMC converter station, a current inner-loop reference value is obtained through dq transformation according to the generated virtual rotor rotation angle theta and the virtual inner potential amplitude E, and then a modulation wave of the offshore MMC converter station is obtained through current inner-loop and dq inverse transformation.

3. The offshore wind power flexible direct current grid-connected system control method according to claim 1, characterized in that the transfer function of the virtual rotor dynamics link is:

in the formula, T _j Is the inertia time constant; omega is a virtual angular frequency; p _e Is a virtual electromagnetic power; d is a damping factor; p _ref Is a virtual active power reference value, and P _ref ＝K _f (F _N -F)，K _f Controlling gain for frequency difference, F _N And F is the per unit value of the rated frequency and the actual value of the grid frequency.

4. The offshore wind power flexible direct current grid-connected system control method according to claim 1, characterized in that the transfer function of the virtual excitation adjusting link is:

in the formula, Q _ref Is a reactive power reference value; q _e The actual value of the reactive power is obtained; u shape _{ac_ref} Is an AC voltage reference value; u shape _ac Is the actual value of the alternating voltage; k is a radical of _q Is a reactive power deviation gain coefficient; k is a radical of _ac Is an AC voltage deviation gain coefficient; k is a radical of formula _p Proportional gain of the virtual excitation controller; t is _i Integrating a time constant for the virtual excitation controller; e ₀ Is a virtual internal potential reference value.

5. The offshore wind power flexible direct current grid-connected system control method according to claim 1, characterized in that when an alternating current system fault does not occur and the medium-high frequency harmonic content is greater than a set harmonic content threshold value, the modulation wave of the offshore MMC converter station is:

in the formula, m _a 、m _b And m _c Three-phase modulation waves A, B and C are respectively.

6. The offshore wind power flexible direct current grid-connected system control method according to claim 2, wherein when an alternating current system fault occurs or the medium-high frequency harmonic content is less than or equal to a set harmonic content threshold value, the current inner loop reference value is as follows:

in the formula, v _d And v _q Current inner ring reference values of a d axis and a q axis of the current inner ring are respectively; v. of _a 、v _b And v _c Respectively generating reference modulation waves by a virtual rotor dynamics link and a virtual excitation regulation link;

and the modulation wave of the marine MMC convertor station is as follows:

in the formula, v _{d_PI} 、v _{q_PI} D-axis and q-axis currents respectivelyInner loop proportional integral controller output, m _a 、m _b And m _c Three-phase modulation waves A, B and C are respectively.

7. The offshore wind power flexible direct current grid-connected system control method according to any one of claims 1 to 6, characterized in that the high frequency harmonic content in the alternating current side voltage of the offshore MMC converter station is: u shape _h ＝U _h1 *(1-K _LP50 ) And the medium-high frequency harmonic content of the alternating current side current of the marine MMC converter station is as follows: I.C. A _h ＝I _h1 *(1-K _LP50 )，U _h Is the harmonic content of high and medium frequency in voltage, U _h1 For original value, I, of alternating-current side voltage of marine MMC convertor station _h Is the content of high-frequency harmonic in current, I _h1 For original value, K, of alternating current side current of marine MMC converter station _LP50 Is a low pass filter function.

8. The offshore wind power flexible direct current grid-connected system control method according to any one of claims 1 to 6, characterized in that a double closed-loop control strategy is adopted to generate a modulation wave of a onshore MMC converter station, and the onshore MMC converter station is controlled according to the modulation wave of the onshore MMC converter station; in the double closed-loop control strategy, the d-axis outer ring selects fixed direct-current system offshore station port voltage control, the q-axis outer ring selects fixed onshore station reactive power control, and the d-axis inner ring and the q-axis inner ring both select fixed valve side alternating current control.

9. The offshore wind power flexible direct current grid-connected system control method according to any one of claims 1 to 6, characterized by adopting a zero sequence criterion and a three-phase amplitude criterion to judge whether an alternating current system fault occurs; the zero-sequence criterion is used for judging whether single-phase alternating current grounding occurs, and the three-phase amplitude criterion is used for judging whether three-phase grounding or two-phase grounding faults occur.

10. A flexible direct current grid-connected system for offshore wind power comprises an offshore MMC converter station, a direct current submarine cable, a land MMC converter station and a control device; the alternating current side of the marine MMC converter station is used for being connected with an offshore wind power plant, and the direct current side of the marine MMC converter station is connected with one end of a direct current submarine cable; the direct current side of the onshore MMC converter station is connected with the other end of the direct current submarine cable, and the alternating current side of the onshore MMC converter station is used for connecting a power grid; the control device is characterized by comprising a processor, wherein the processor is used for executing computer program instructions to realize the control method of the offshore wind power flexible direct current grid-connected system according to any one of claims 1 to 9.

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