CN115622110A - Offshore wind power flexible direct current grid-connected system and control method thereof - Google Patents

Offshore wind power flexible direct current grid-connected system and control method thereof Download PDF

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CN115622110A
CN115622110A CN202211242537.XA CN202211242537A CN115622110A CN 115622110 A CN115622110 A CN 115622110A CN 202211242537 A CN202211242537 A CN 202211242537A CN 115622110 A CN115622110 A CN 115622110A
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virtual
wind power
converter station
direct current
mmc converter
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Inventor
范彩云
俎立峰
周晓风
韩坤
董朝阳
赵洋洋
刘路路
马俊杰
魏卓
樊宏伟
马太虎
慕小乐
周辉
柴卫强
冯敏
刘静一
李文雅
滕林阳
张壮
任如晨
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Xuji Group Co Ltd
XJ Electric Co Ltd
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Xuji Group Co Ltd
XJ Electric Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/28—The renewable source being wind energy
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/70—Wind energy
    • Y02E10/76—Power conversion electric or electronic aspects

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  • Power Engineering (AREA)
  • Control Of Eletrric Generators (AREA)

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

Offshore wind power flexible direct current grid-connected system and control method thereof
Technical Field
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.
Background
With increasing importance to energy safety, ecological environment, climate change and other problems of all countries in the world, accelerating the development of wind power becomes a common consensus and consistent action for promoting energy transformation development and coping with global climate change in the international society. The offshore wind power generation system has the advantages of stable wind energy, high generation utilization hours, basically no influence from landform and landform, suitability for large-scale development and the like, is close to a power load center, is convenient for a power grid to be consumed on the spot, and avoids long-distance transportation of the wind power. Therefore, the development and utilization of offshore wind power are receiving more and more attention and attention, and have become one of the new growth points and main directions for the development of global renewable energy. According to the latest data of global wind energy institute GWEC, the global offshore wind power accumulated installed capacity reaches 3310 ten thousand kilowatts by the end of 2021, wherein the China offshore wind power accumulated installed capacity reaches 555.5 ten thousand kilowatts. Compared with a land wind power plant, the offshore wind power plant has the advantages that land resources are not occupied, the influence of landform and landform is avoided basically, the wind speed is higher, the single-machine capacity of the wind turbine generator is larger, and the annual utilization hours are higher.
The MMC (Modular Multilevel Converter) Converter has the remarkable characteristic that the MMC-HVDC power transmission technology becomes the only choice for connecting foreign large-scale long-distance offshore wind power plants. With the expansion of development scale, the increase of transmission capacity and transmission distance, the factors of large-scale unit, short circuit level of receiving end power grid, safety and stability of power grid and the like, the development trend of offshore wind power transmission in the direct current direction is more obvious.
The classic flexible direct current transmission system based on active/reactive control is fully applied at home and abroad, and a plurality of flexible direct current projects are established at home and abroad. A traditional flexible direct current system adopts an active/reactive control mode, depends on a connected alternating current power grid and obtains an alternating current power grid synchronous phase through a phase-locked loop technology, and then a double closed-loop proportional integral control function of a d/q space is constructed. However, when the flexible direct-current transmission technology is applied to an offshore wind power transmission scene, the short-circuit capacity (short-circuit ratio) of a connected alternating-current power grid is very small, the characteristic has a great influence on the safe and stable operation of a traditional flexible direct-current transmission system, and even the normal operation of the system cannot be maintained.
Disclosure of Invention
The invention aims to provide an offshore wind power flexible direct current grid-connected system and a control method thereof, which are used for solving the problem that the stable and normal operation of the system cannot be maintained by using a control strategy in the prior art.
In order to solve the technical problem, the invention provides a control method of an offshore wind power flexible direct current grid-connected system, which comprises 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 adjusting 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) Judging whether an alternating current system fault occurs on an alternating current side of the marine MMC converter station and whether the medium-high frequency harmonic content is greater than a set threshold value: 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 converter station to directly obtain a modulation wave of the marine MMC converter station according to the generated virtual rotor rotation angle theta and the virtual internal potential amplitude value E; and controlling the marine MMC convertor station according to the modulation wave of the marine MMC convertor station.
The beneficial effects are as follows: the invention adopts a new control strategy aiming at an offshore MMC converter station, firstly, a phase-locked loop phase is obtained by simulating an electromechanical equation and an excitation voltage equation of a synchronous generator without depending on a connected alternating current power grid, a virtual rotor rotation angle is obtained according to a virtual rotor dynamics link, a virtual internal potential amplitude value is generated according to a virtual excitation adjusting link, then, under the condition that the offshore MMC converter station does not generate alternating current system faults and the medium-high frequency harmonic content is higher, a switch control mode is adopted, the current internal loop is forbidden to be put into use, so that a modulation wave of the offshore converter station is directly obtained according to the virtual rotor rotation angle and the virtual internal potential amplitude value, medium-high frequency harmonic oscillation which possibly occurs under the low-power operation working condition is inhibited, the inherent inertia and damping characteristic of the offshore MMC converter station are increased, the frequency and inertia supporting capability of a wind power generation grid system of an offshore wind power field are improved, the safety and stability of an alternating current-direct current hybrid large power grid system are further improved, and the safe and reliable operation of the alternating current-direct current hybrid large power grid system mainly based on new energy power generation is ensured.
Further, 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 marine MMC converter station, a current inner ring reference value is obtained through dq transformation according to the generated virtual rotor rotation angle theta and the virtual inner potential amplitude value E, and then modulation waves of the marine MMC converter station are obtained through the current inner ring and dq inverse transformation.
The beneficial effects are as follows: under the condition of alternating current system fault or low medium-high frequency harmonic content, a closed-loop control mode is adopted, and a current inner loop is put into use to achieve the effect of inhibiting fault current.
Further, the transfer function of the virtual rotor dynamics link is:
Figure BDA0003885245000000021
Figure BDA0003885245000000022
in the formula, T j Is the inertia time constant; omega is a virtual angular frequency; p is e Is a virtual electromagnetic power; d is a damping factor; p is ref Is a virtual active power reference value, and P ref ＝K f (F N -F)，K f Controlling gain, F, for frequency difference N And F is the rated frequency per unit value and the actual value of the power grid frequency.
The beneficial effects are as follows: the virtual rotor rotation angle is generated by adopting the virtual rotor link, the phase of a phase-locked loop is not required to be acquired by depending on a connected alternating current power grid, and the marine MMC converter station has the inertia and damping characteristics of the traditional synchronous generator.
Further, the transfer function of the virtual excitation adjusting link is as follows:
Figure BDA0003885245000000031
in the formula, Q ref Is a reactive power reference value; q e The actual value of the reactive power is; 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 A reactive power deviation gain factor; k is a radical of formula ac Is an AC voltage deviation gain coefficient; k is a radical of formula p Proportional gain of the virtual excitation controller; t is a unit of i Integrating a time constant for the virtual excitation controller; e 0 Is a virtual internal potential reference value.
The beneficial effects are as follows: and a virtual excitation adjusting link is adopted to generate a virtual internal potential amplitude value, so that the marine MMC convertor station has inertia and damping characteristics of the traditional synchronous generator.
Further, when no alternating current system fault occurs and the medium-high frequency harmonic content is greater than a set harmonic content threshold value, the modulation wave of the marine MMC converter station is as follows:
Figure BDA0003885245000000032
in the formula, m a 、m b And m c Three-phase modulation waves A, B and C are respectively.
Further, 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 reference value of the current inner ring is as follows:
Figure BDA0003885245000000033
Figure BDA0003885245000000034
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 set; 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:
Figure BDA0003885245000000041
in the formula, v d_PI 、v q_PI Respectively d-axis current inner ring proportional-integral controller output and q-axis current inner ring proportional-integral controller output, m a 、m b And m c Three-phase modulated waves A, B and C respectively.
Further, the medium-high frequency harmonic content in the alternating-current side voltage of the marine MMC convertor station is as follows: 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 is 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 converter station h Is the content of high-frequency harmonics in the current, I h1 For original value, K, of alternating current side current of marine MMC converter station LP50 Is a low pass filter function.
The beneficial effects are as follows: by adopting the mode, the high-frequency harmonic content in the alternating-current side voltage/current of the marine MMC convertor station can be quickly obtained.
Further, generating a modulation wave of the onshore MMC converter station by adopting a double-closed-loop control strategy, and controlling the onshore MMC converter station 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.
The beneficial effects are as follows: a land MMC converter station adopts a classic double closed-loop control strategy, and reliable and stable operation of a flexible direct current transmission project can be guaranteed.
Further, judging whether an alternating current system fault occurs by adopting a zero sequence criterion and a three-phase amplitude criterion; 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.
The beneficial effects are as follows: the zero sequence criterion and the three-phase amplitude criterion are used for judging whether the alternating current system fault occurs, so that whether the alternating current system fault occurs can be accurately and comprehensively judged.
In order to solve the technical problem, the invention also provides an offshore wind power flexible direct current grid-connected system which 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 convertor station is used for connecting an offshore wind power plant, and the direct current side of the marine MMC convertor 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 comprises a processor, and 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, and can achieve the same beneficial effects as the method.
Drawings
FIG. 1 is a schematic diagram of an offshore wind power flexible direct current grid-connected system of the invention;
FIG. 2 is a schematic view of a topological structure and a measuring point of an MMC converter valve used in the present invention;
FIG. 3 is a block diagram of land MMC converter station control functionality of the present invention;
FIG. 4 is a block diagram of the current inner loop control and the circulating current suppression control of the present invention;
FIG. 5 is a block diagram of virtual rotor dynamics and virtual excitation link control according to the present invention;
fig. 6 is a schematic diagram of a control flow of the marine MMC converter station of the present invention.
Detailed Description
The basic idea of the invention is that different control strategies are adopted for an offshore MMC converter station and a onshore MMC converter station, particularly for the offshore MMC converter station, the offshore MMC converter station is in a passive island control mode, and the adopted control strategies comprise a plurality of links, namely a virtual rotor dynamics link for generating a virtual rotor rotation angle theta, a virtual excitation adjusting link for generating a virtual internal potential amplitude E, a current internal ring control link and an open-loop and closed-loop control switching link. Under the condition that no alternating current system fault occurs on the alternating current side of the marine MMC converter station but the medium-high frequency harmonic content is high, the open-loop control mode is adopted, the current inner-loop function is forbidden, and the modulation wave of the marine MMC converter station is directly obtained according to the rotation angle theta of the pseudo-rotor and the virtual inner potential amplitude E; under the condition that an alternating current system fault occurs on the alternating current side of the marine MMC convertor station or the medium-high frequency harmonic content is low, a closed-loop control mode is adopted, a current inner ring needs to be put into use, a current inner ring set value is obtained according to the rotation angle theta of the pseudo-rotor and the virtual inner potential amplitude E, and then a modulation wave of the marine MMC convertor station is obtained through the current inner ring.
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are given by way of illustration only, not by way of limitation, i.e., the embodiments described are intended as a selection of the best mode contemplated for carrying out the invention, not as a full mode. Thus, the following detailed description of the embodiments of the present invention, presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without making any creative effort, shall fall within the protection scope of the present invention.
The embodiment of the offshore wind power flexible direct current grid-connected system comprises:
the embodiment of the offshore wind power flexible direct current grid-connected system is applied to a (deep) offshore wind power flexible direct current transmission system, and as shown in fig. 1, the offshore wind power flexible direct current grid-connected system comprises an offshore alternating current booster station, an offshore MMC converter station, a direct current submarine cable, an onshore MMC converter station and a control device. The low pressure side of marine interchange booster station is used for connecting offshore wind power field, and the interchange side of marine MMC converter station is connected to the high-pressure side, and the one end of direct current submarine cable is connected to the direct current side of marine MMC, and the direct current side of land MMC converter station is connected to the other end of direct current submarine cable, and the interchange side of land MMC converter station is used for connecting land alternating current power grid. The topological structures and voltage and current measuring points of the MMC converter valves of the offshore MMC converter station and the onshore MMC converter station are shown in figure 2, and the converter valve sub-modules can adopt typical half-bridge sub-modules or full-bridge sub-modules, and can also adopt a mixed module structure of a half bridge with a certain proportion and a full bridge with a certain proportion. The control device comprises an offshore control device and a land control device, inter-station communication is carried out between the offshore control device and the land control device, the two control devices are embedded industrial control platforms or PC devices, the offshore control device is used for outputting modulation waves to the offshore MMC converter station, and the land control device is used for outputting modulation waves to the land MMC converter station. Both of these control devices include a processor and a memory, and execution of computer instructions stored in the memory by the processor may implement a corresponding converter station control strategy.
For a land MMC converter station, a classical double closed-loop control strategy of a flexible direct-current transmission system is adopted, as shown in fig. 3, a d-axis outer ring selects and fixes voltage control of a port of the marine station of the direct-current system, a q-axis outer ring selects and fixes reactive power control of the land station, and a d-axis inner ring and a q-axis inner ring both select and fix current control, and are direct current control based on alternating current at the circulating valve side of an MMC, as shown in fig. 4.
For an MMC convertor station on the sea, the adopted control strategy comprises a virtual rotor dynamics link, a virtual excitation adjusting link, a current inner ring control link and an open-loop and closed-loop control switching link. As shown in fig. 5, the virtual rotor dynamics link is used to generate a virtual rotor rotation angle θ, the virtual excitation adjusting link is used to generate a virtual internal potential amplitude E, and the open-loop and closed-loop control switching link is used to realize switching between an open-loop control mode and a closed-loop control mode. The control strategy is as follows: when no alternating current system fault occurs on the alternating current side of the marine MMC converter station and medium/high frequency harmonic oscillation occurs, an open-loop control mode is adopted, namely, the current inner-loop control function is forbidden, and the modulating wave of the marine MMC converter station is directly generated according to the virtual inner potential amplitude E and the virtual rotor rotation angle theta; when an alternating current system fault occurs or medium/high frequency harmonic oscillation does not occur on the alternating current side of the marine MMC converter station, a closed-loop control mode is adopted, namely a virtual inner potential amplitude E and a virtual rotor rotation angle theta are subjected to dq conversion to generate a current inner ring reference value, and the output of the current inner ring is subjected to dq inverse conversion to generate an MMC converter station modulation wave. This is done because the current inner loop control element plays an important role in suppressing the fault current, but the input of the current inner loop control element introduces a potential risk of medium-high frequency harmonic resonance.
The virtual rotor dynamics link simulates a rotor motion equation (electromechanical equation) of the synchronous generator, so that a virtual rotor rotation angle theta is generated, and the transfer function of the virtual rotor rotation angle theta is as follows:
Figure BDA0003885245000000061
in the formula, T j The inertia time constant is usually 5-10 seconds; ω is the virtual angular frequency (1 per unit nominal value of 100 π); p is ref Is a virtual active power reference value (per unit value); p is e The virtual electromagnetic power (the actual value of the power of the MMC converter valve is taken, and the actual value of the active power of the valve side of the MMC converter is taken, and the actual value can be obtained by calculating an instantaneous power theory; d is a damping factor; θ is the virtual rotor rotation angle. In particular, the virtual active power reference value is obtained from the active-frequency droop characteristic, that is, the virtual active power reference value simulates the primary frequency modulation function of the synchronous generator, and the relationship is as follows: p is ref ＝K f (F N -F)，K f Controlling gain for frequency difference, F N Is the rated frequency per unit value (the default value is 1), and F is the actual value of the power grid frequency.
The virtual excitation adjusting link simulates excitation control of a synchronous generator, when system voltage fluctuates, excitation voltage is changed through deviation proportion, and finally the exchange of a unit and a system is affected to be idle, so that a virtual internal potential amplitude E is generated, and the transfer function of the virtual internal potential amplitude E is as follows:
Figure BDA0003885245000000071
wherein Q is ref Taking the inductive reactive power of the MMC injection system as positive, wherein the inductive reactive power is a reactive power reference value; q e The actual value of the reactive power is; u shape ac_ref The reference value of the alternating voltage is 1 as a default value; u shape ac Is the actual value of the alternating voltage; k is a radical of formula q A reactive power deviation gain factor; k is a radical of ac Is an AC voltage deviation gain coefficient; k is a radical of p Proportional gain of the virtual excitation controller; t is i Integrating a time constant for the virtual excitation controller; e 0 The default value is 1 for the virtual internal potential reference value.
Under a closed-loop control mode, a current inner-loop control link is composed of a d-axis and q-axis proportional integral control link, d-axis and q-axis current reference values are obtained by formulas (3) and (4), and then a modulation wave of the marine MMC converter station is obtained by a formula (5).
Figure BDA0003885245000000072
Figure BDA0003885245000000073
Figure BDA0003885245000000074
In the open-loop control mode, the modulation wave of the marine MMC converter station is directly obtained by the formula (5).
Figure BDA0003885245000000075
The following introduces the whole flow of the control method adopted by the whole offshore wind power flexible direct current grid-connected system with reference to the control flow chart of the offshore MMC converter station of FIG. 6:
1) And the two MMC convertor stations complete the setting of the initialization running condition, and enter the normal running condition if the initialization is successful. A land MMC converter station selects a double-closed-loop control strategy, a d-axis outer ring in the double-closed-loop control strategy selects and determines voltage control of a marine station port of a direct-current system, a q-axis outer ring selects and determines reactive power control of the land station, and a d-axis inner ring and a q-axis inner ring select current rings. Charging of the direct current side of an MMC sub-module in the marine MMC converter station is completed by the onshore MMC converter station.
2) After the charging of the marine MMC loop station through the direct current side is completed, according to a virtual rotor dynamics link, a virtual rotor rotation angle theta required by dq positive and negative conversion is generated by using a formula (1), and according to a virtual excitation link, a virtual internal potential amplitude E is generated by using a formula (2).
3) The method comprises the steps that an AC voltage signal and an AC current signal are collected in real time by an offshore MMC converter station, and a medium-high frequency harmonic amplitude value, U, is obtained through a fast Fourier transform or low-pass filtering method and other general methods h ＝U h1 *(1-K LP50 )，I 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 converter 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; and judging whether an alternating current side fault occurs by adopting a zero sequence criterion and a three-phase amplitude criterion, wherein the zero sequence criterion is mainly used for judging whether a single-phase alternating current grounding occurs, and the three-phase amplitude criterion is used for judging whether a three-phase grounding or two-phase grounding fault occurs:
if no alternating current system fault is generated and the medium-high frequency harmonic content is larger than a set harmonic content threshold value delta setting1 (5%), an open-loop control mode is adopted by the marine MMC convertor station. The principle of MMC open-loop control is that the current inner loop control function in a closed-loop mode is forbidden, and a three-phase positive sequence modulation wave is generated by a virtual rotor phase angle and an inner potential amplitude value directly according to a formula (6);
if the alternating current system fails or the medium-high frequency harmonic content is less than or equal to the set harmonic content threshold value delta setting1, the marine MMC converter station adopts a closed-loop control mode, and at the moment, a current inner-loop control function needs to be put into operation. And generating a three-phase reference voltage vector according to the generated virtual rotor rotation angle and the virtual internal potential amplitude, generating an internal ring current d/q axis reference value after dq forward conversion, namely formulas (3) to (4), outputting d/q axis voltage after proportional integral control of the internal ring current, and finally generating a trigger modulation wave of the marine MMC converter station through dq inverse conversion, namely formula (5).
In conclusion, the control method of the offshore wind power flexible direct current grid-connected system provided by the invention has the advantages that by simulating an electromechanical equation and an excitation loop equation of a traditional synchronous generator, a phase-locked loop phase is not obtained by depending on a connected alternating current power grid, but a virtual rotor rotation angle is obtained according to a virtual rotor dynamics link, and a virtual internal potential amplitude value is generated by utilizing a virtual excitation regulation link, so that the offshore MMC converter station has inertia and damping characteristics of the traditional synchronous generator, and the closed-loop control is switched to avoid medium-high frequency harmonic oscillation by judging the harmonic amplitude value, so that the new energy consumption capability of the offshore wind power flexible direct current grid-connected system is enhanced, and the safe, reliable and stable operation of a mixed alternating current and direct current large power grid which mainly uses new energy power generation is ensured.
The embodiment of the control method of the offshore wind power flexible direct current grid-connected system comprises the following steps:
the invention discloses a control method embodiment of an offshore wind power flexible direct current grid-connected system, and aims at the offshore wind power flexible direct current grid-connected system shown in figure 1. The method enhances the inertia supporting and damping capacity of the MMC converter station, realizes the reliable and friendly access of the flexible direct-current power transmission system of the wind power on the (deep) open sea to new energy, and finally ensures the flexibility and the safety and the stability of the large alternating-current and direct-current hybrid power grid taking the new energy as the main body. The specific implementation process of the method is consistent with the control method of the offshore wind power flexible direct current grid-connected system introduced in the embodiment of the offshore wind power flexible direct current grid-connected system, the flow is shown in fig. 6, and the whole detailed process is not repeated in this embodiment.
The specific embodiments are given above, but the present invention is not limited to the described embodiments. The basic idea of the present invention lies in the above basic scheme, and it is obvious to those skilled in the art that no creative effort is needed to design various modified models, formulas and parameters according to the teaching of the present invention. Variations, modifications, substitutions and alterations may be made to the embodiments without departing from the principles and spirit of the invention, and still fall within the scope of the 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:
Figure FDA0003885244990000011
Figure FDA0003885244990000012
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:
Figure FDA0003885244990000013
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:
Figure FDA0003885244990000021
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:
Figure FDA0003885244990000022
Figure FDA0003885244990000023
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:
Figure FDA0003885244990000024
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.
CN202211242537.XA 2022-10-11 2022-10-11 Offshore wind power flexible direct current grid-connected system and control method thereof Pending CN115622110A (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN117526397A (en) * 2024-01-04 2024-02-06 国网经济技术研究院有限公司 Flexible direct-current inner-loop-free network construction control method and system with current limiting function

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN117526397A (en) * 2024-01-04 2024-02-06 国网经济技术研究院有限公司 Flexible direct-current inner-loop-free network construction control method and system with current limiting function
CN117526397B (en) * 2024-01-04 2024-04-26 国网经济技术研究院有限公司 Flexible direct-current inner-loop-free network construction control method and system with current limiting function

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