Anti-interference frequency mirror method without communication for grid-forming flexible direct current system with wind power access

Abstract

The application provides a network-constructed flexible system anti-interference frequency mirror method for wind power access, a receiving end converter station adopts network-constructed control, self-synchronization of grid phase and perception of grid frequency are realized by constructing equivalent mapping of sub-module energy-synchronous generator rotor; through AC control, DC control and / or equalization modulation control, positive sequence circulating current is injected in three-phase bridge arm of the receiving end converter station, and / or DC component in upper and lower bridge arm sub-module insertion index is adjusted, positive and negative DC voltage is independently adjusted and controlled; in the receiving end converter station, voltage difference of positive and negative DC voltage of the adjusted and controlled DC transmission system is mapped and grid frequency is transmitted; the sending end converter station perceives DC voltage change of the positive and negative poles, and the grid frequency is restored therefrom, and the AC frequency of the offshore wind farm is controlled to be the same as the grid frequency. The application can offset the influence of voltage drop on transmission accuracy caused by line resistance and reactance, and realize rapid anti-interference mapping of the grid frequency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power grid, in particular, to a communication-free anti-interference frequency mirror method, system, medium and terminal for a grid-forming flexible direct system for wind power access. BACKGROUND

[0002] In recent years, with the continuous development of offshore wind power resources, offshore wind power development will gradually enter deep and remote sea areas, and the use of flexible direct current transmission technology to realize large-scale offshore wind power sending out will become the main offshore wind power development scenario. When sending out through flexible direct current transmission, the frequency of the offshore wind farm is decoupled from the onshore alternating current main grid. When the frequency of the alternating current main grid changes, the offshore wind farm cannot perceive the frequency change and cannot provide inertia response to support the grid. The replacement of synchronous generators by wind power-flexible direct current grid-connected systems reduces the overall inertia of the grid, and under the background of GW-level single-capacity offshore wind power-flexible direct current system, it will seriously affect the frequency stability of the grid operation. If a new energy system dominated by wind and solar power sources is to be built, the wind power-flexible direct current system needs to be transformed from a following-type power source to a leading-type power source, to some extent, like a synchronous generator, to take on the obligation of maintaining real-time energy balance of the system. From the cost point of view, it is the most economical and reliable strategy for wind turbine generators to provide frequency response for the grid. Wind turbine generators can increase or release kinetic energy in the blades and rotor by variable speed operation to realize the inertia response function of the grid, and can also realize primary frequency modulation function through active reserve when necessary. However, when sending out through flexible direct current transmission and grid connection, due to the isolation effect of the flexible direct current system, the wind turbine generator cannot perceive the grid frequency, and the grid frequency needs to be transmitted to the wind turbine generator.

[0003] According to the search, the Chinese patent with application number CN202310747803.2 provides a flexible direct current transmission system and a primary frequency modulation control method and device thereof. The frequency transmission needs to erect a dedicated communication line to realize real-time communication between the onshore converter station and the offshore wind farm energy management system, which greatly increases the construction cost of the offshore wind-flexible direct current grid-connected system. At the same time, the delay from the wind farm energy management system issuing an instruction to control the wind turbine generator to adjust the output power to the completion of the wind turbine generator power adjustment slows down the grid frequency response speed of the offshore wind farm, and the support effect on the grid becomes poor.

[0004] The application number CN202311594378.4 provides a kind of passive power grid frequency control system and method of flexible direct current transmission of wind farm, it does not need to help communication, but when using traditional scheme to control direct current voltage, its control speed is slower, possibly make the frequency information of transmission appear lag. Meanwhile, when direct current voltage transmits power grid frequency, it will be affected by the voltage drop on line impedance, when active power changes, the voltage drop on line impedance will change, resulting in the voltage change of offshore converter station direct current side, possibly be misjudged as power grid frequency change, make wind turbine generator set appear frequency response misoperation. SUMMARY

[0005] In view of the defects in the prior art, the purpose of the present application is to provide a communication-free anti-interference frequency mirroring method for network-type flexible direct current system for wind power access.

[0006] According to one aspect of the present application, a communication-free anti-interference frequency mirroring method for network-type flexible direct current system for wind power access is provided, comprising:

[0007] The receiving end converter station adopts network-type control, and self-synchronization of power grid phase and perception of power grid frequency are realized by constructing equivalent mapping of sub-module energy-synchronous generator rotor;

[0008] Based on the perception results obtained by the network self-synchronization and frequency detection, positive sequence circulating current is injected in the three-phase bridge arm of the receiving end converter station through AC control, DC control and / or balanced modulation control, and / or the DC component in the upper and lower bridge arm sub-module insertion index is adjusted, and the positive and negative DC voltages are independently adjusted and controlled;

[0009] By respectively controlling the DC voltage set value of the upper and lower bridge arms of the receiving end converter station, the voltage difference between the positive and negative poles of the adjusted and controlled DC transmission system is used to map and transmit the power grid frequency; the sending end converter station perceives the positive and negative DC voltage changes, and restores the power grid frequency therefrom, controls the offshore wind farm AC frequency to be the same as the power grid frequency, and realizes power grid frequency mirroring.

[0010] Preferably, the receiving end converter station adopts network-type control, and self-synchronization of power grid phase and perception of power grid frequency are realized by constructing equivalent mapping of sub-module energy-synchronous generator rotor, comprising:

[0011] detecting the positive DC bus voltage U dcr+ of the receiving end converter station, the negative DC bus voltage U dcr- , the grid-connected point AC voltage u pcc , the grid-connected point AC current i pcc , the capacitor voltage U sm of all sub-modules of the converter station, and the two-frequency circulating current i cirj of the three-phase bridge arm of the receiving end converter station, j=a, b, c;

[0012] According to u pcc And i pcc The output active power P g And the reactive power Q g Of the AC side of the receiving end converter station are calculated according to all the upper bridge arm sub-module capacitor voltages U sm The total energy of the upper bridge arm sub-modules, the total energy of the lower bridge arm sub-modules and the total energy of the sub-modules of the receiving end converter station are respectively calculated, specifically as follows:

[0013]

[0014] W MMC =W MMCu +W MMCl

[0015] Wherein, W MMCu Is the total energy of the upper bridge arm sub-modules of the receiving end converter station, W MMCu Is the total energy of the lower bridge arm sub-modules of the receiving end converter station, W MMC Is the total energy of the sub-modules of the receiving end converter station, C SM Is the sub-module capacitor voltage, N is the number of sub-modules of each bridge arm of the converter, U SMaui Is the capacitor voltage of the i-th sub-module of the upper bridge arm of phase a of the receiving end converter station, U SMbui Is the capacitor voltage of the i-th sub-module of the upper bridge arm of phase b of the receiving end converter station, U SMcui Is the capacitor voltage of the i-th sub-module of the upper bridge arm of phase c of the receiving end converter station, U SMali Is the capacitor voltage of the i-th sub-module of the lower bridge arm of phase a of the receiving end converter station, U SMbli Is the capacitor voltage of the i-th sub-module of the lower bridge arm of phase b of the receiving end converter station, U SMcli Is the capacitor voltage of the i-th sub-module of the lower bridge arm of phase c of the receiving end converter station;

[0016] The total energy W MMC Of the sub-modules of the flexible DC converter is calculated, and the difference between the total energy W MMCref And the rated total energy is divided by 2H v , H v Is a virtual inertia coefficient, and the rated frequency ω0 is added to obtain a reference value ω c Of the output AC frequency of the converter; and the reference value θ of the output AC phase of the converter is obtained by integrating ω c .

[0017] Preferably, the AC control comprises:

[0018] The difference between W MMC And W MMCref Is multiplied by a virtual damping coefficient D v , and then divided by 2H vand P ac0 , where P ac0 is the current active power output of the receiving end converter station, and finally the AC voltage damping compensation term △u ac is obtained. g The measured value of the reactive power Q ref is subtracted from the given value Q Q , multiplied by k Q , which is the reactive power control droop coefficient, and added to the rated value, plus the rated value of the AC voltage u a0 and the AC voltage damping compensation term △u ac , to obtain the reference value of the output AC voltage u acre f ;

[0019] The reference value of the AC voltage u acref is subtracted from u d , and then added to the first reference value of the q-axis current i q1 multiplied by the rated frequency ω0, and divided by a virtual admittance R v +sL v , where Rv is a virtual resistance and Lv is a virtual inductance, and s is the Laplace operator, to obtain the first reference value of the d-axis current id1.

[0020] The value 0 is subtracted from u q , and then subtracted from i d1 multiplied by the rated frequency ω0, and divided by a virtual admittance R v +sL v , to obtain the first reference value of the q-axis current i q1 .

[0021] Then, i d1 and i q1 are passed through a current limiting link to obtain the second reference values of the d-axis and q-axis output currents i dref and i qref . The principle of the current limiting link is that if the square root of the sum of the squares of i d1 and i q1 is less than the current limiting amplitude i lim , then i dref =i d1 and i qref =i q1 ; if the square root of the sum of the squares of i d1 and i q1 is greater than the current limiting amplitude i lim , then i d1 and i q1 are reduced by a factor of n until the square root of the sum of the squares of i d1 / n and i q1 / n is equal to i lim . At this time, i dref= i d1 / n, i qref = i q1 / n;

[0022] The second reference value i dref of the d-axis current and the q-axis current is subtracted from the measured value i qref , i d , and after passing through a proportional-integral controller, respectively, the dq-axis decoupling term ω0L q i c and ω0L d i c and the grid-connected point AC voltage feedforward term u q and u d are added, respectively, to obtain the AC side dq modulation voltage E q of the receiving end converter station, where ω0 is the rated angular frequency, and L c is the filter inductance. d and E q , and after inverse PARK transformation at phase θ, the AC side three-phase modulation voltage E a , E b , and E c of the receiving end converter station is obtained.

[0023] Preferably, the DC control includes:

[0024] The grid frequency ω g at this time is obtained;

[0025] After obtaining the grid frequency ω g , the difference between ω g and the rated frequency ω0 is multiplied by the mapping coefficient K dc to obtain the DC voltage deviation value ΔU dc . The rated value U dc0+ of the positive DC voltage of the HVDC system is added to ΔU dc to obtain the reference value U dcrref+ of the positive DC voltage. The rated value U dc0- of the negative DC voltage of the HVDC system is subtracted from ΔU dc to obtain the reference value U dcrref- of the negative DC voltage.

[0026] The difference between the reference value U dcrref+ of the positive DC voltage of the HVDC system and the measured value U dcr+ of the positive DC voltage of the HVDC system is passed through a proportional-integral controller and added to the feedforward U dcr+ to obtain the positive DC voltage modulation wave E dcrref+ of the receiving end converter station.

[0027] The difference between the reference value U dcrref-The negative DC voltage measurement value U of the HVDC system dcr- The difference between the two, after a proportional-integral controller, plus the feedforward U dcr- , to obtain the negative DC voltage modulation wave E of the receiving end converter station dcrref- .

[0028] Preferably, the grid frequency ω g is obtained in the following one or more ways:

[0029] Obtained by communication with the grid;

[0030] Under grid-following control, the AC frequency ω pcc of the PCC point is obtained by a phase-locked loop, and is used as the grid frequency ω g ;

[0031] Under grid-forming control, the frequency ω c of the receiving end converter station itself is used as the grid frequency ω g .

[0032] Preferably, the equalization modulation includes:

[0033] The difference between the total energy W MMCu of the upper bridge arm sub-modules of the receiving end converter station and the total energy W MMCl of the lower bridge arm, after a proportional-integral controller, obtains the upper and lower bridge arm balance voltage amplitude U b , and then the anti-PARK change is performed at the angle of θ+π / 2 to obtain the upper and lower bridge arm balance voltage modulation wave E bj， j=a, b, c;

[0034] The difference between the three-phase double-frequency circulating current component i cirj and 0, after a proportional-resonant controller, obtains the circulating current suppression voltage modulation wave u cirj , j=a, b, c;

[0035] The average voltage U SMavu of the upper bridge arm sub-modules of the receiving end converter station is calculated, specifically:

[0036]

[0037] The average voltage U SMavl of the lower bridge arm sub-modules of the receiving end converter station is calculated, specifically:

[0038]

[0039] The positive and negative DC voltage modulation waves E dcrref+ and E dcrref- , the three-phase modulation voltage E a , E b and Ec , circulating current suppression modulation wave u cirj , upper and lower arm balance voltage modulation wave E bj The reference voltages of the six bridge arms, i.e., the a-phase upper bridge arm, the a-phase lower bridge arm, the b-phase upper bridge arm, the b-phase lower bridge arm, the c-phase upper bridge arm, and the c-phase lower bridge arm, are generated in combination; then the reference voltages E Nuj， j=a, b, c of the a-phase upper bridge arm, the b-phase upper bridge arm, and the c-phase upper bridge arm are added to obtain the average voltage U SMavu of the three-phase upper bridge arms uj， j=a, b, c; the reference voltages E Nlj， j=a, b, c of the a-phase lower bridge arm, the b-phase lower bridge arm, and the c-phase lower bridge arm are added to obtain the average voltage U SMavl of the three-phase lower bridge arms lj， j=a, b, c;

[0040] The sub-module capacitor voltages of the six bridge arms are sorted, and the bridge arm currents are measured; when the bridge arm current is positive, the n sub-modules with lower capacitor voltages are put into use; when the bridge arm current is negative, the n sub-modules with higher capacitor voltages are put into use, and n is the same as the insertion index of the bridge arm.

[0041] Preferably, the voltage difference between the positive and negative poles of the adjusted and controlled DC power transmission system is used to map and transfer the grid frequency by respectively controlling the DC voltage given values of the upper and lower bridge arms of the receiving end converter station, which comprises the following steps:

[0042] After the grid frequency ω g is obtained, the difference between ω g and the rated frequency ω0 is multiplied by a mapping coefficient K dc to obtain the deviation value △U dc of the DC voltage. dc0+ The rated value U dc of the positive pole DC voltage of the flexible DC system is added by △U dcrref+ to obtain the reference value U dc0- of the positive pole DC voltage. dc The rated value U dcrref- of the negative pole DC voltage of the flexible DC system is subtracted by △U dcrref+ to obtain the reference value U dcrref- of the negative pole DC voltage.

[0043] The sending end converter station senses the changes of the positive and negative pole DC voltages, from which the grid frequency is restored, the AC frequency of the offshore wind farm is controlled to be the same as the grid frequency, and the grid frequency mirroring is realized.

[0044] Based on the received positive and negative DC voltage, detect its positive voltage to ground U dcs+ With the negative voltage to ground U dcs- ;

[0045] The positive voltage to ground U dcs+ With the negative voltage to ground U dcs- Difference, and divided by the mapping coefficient K dc After the DC voltage, the grid frequency variation information is restored, and the rated frequency ω0 is added, which is used as the output AC frequency reference value ω of the sending end converter station ref That is, the grid frequency to the offshore wind farm frequency is mirrored.

[0046] According to the second aspect of the present application, a network type flexible DC system communication-free anti-interference frequency mirroring system for wind power access is provided, comprising:

[0047] Network self-synchronization and frequency detection module: the receiving end converter station adopts network type control, and the grid phase self-synchronization and grid frequency sensing are realized by constructing the equivalent mapping of the energy-synchronous generator rotor sub-module;

[0048] Bipolar DC voltage independent decoupling control module based on sub-module decoupling: based on the sensing result obtained by the network self-synchronization and frequency detection, through AC control, DC control and / or balanced modulation control, positive sequence circulating current is injected in the three-phase bridge arm of the receiving end converter station, and / or the DC component in the upper and lower bridge arm sub-module insertion index is adjusted, the positive and negative DC voltage is independently adjusted and controlled;

[0049] The grid frequency anti-interference mirroring control module based on differential mode mapping is: by respectively controlling the DC voltage given value of the upper and lower bridge arms of the receiving end converter station, the voltage difference of the positive and negative DC voltage of the adjusted and controlled DC transmission system is used to map and transfer the grid frequency; the sending end converter station senses the positive and negative DC voltage variation, and restores the grid frequency therefrom, controls the offshore wind farm AC frequency to be the same as the grid frequency, and realizes the grid frequency mirroring.

[0050] According to the third aspect of the present application, a terminal is provided, comprising a memory, a processor and a computer program stored in the memory and executable on the processor, wherein the processor executes the program to execute the network type flexible DC system communication-free anti-interference frequency mirroring method for wind power access, or, runs the network type flexible DC system communication-free anti-interference frequency mirroring for wind power access.

[0051] According to a fourth aspect of the present application, there is provided a computer readable storage medium having stored thereon a computer program which, when executed by a processor, is adapted to perform the method for communication-free disturbance frequency mirroring of grid-forming VSCs for wind power integration or to operate the system for communication-free disturbance frequency mirroring of grid-forming VSCs for wind power integration.

[0052] Compared with the prior art, the embodiments of the present application have at least one of the following beneficial effects:

[0053] The method for communication-free disturbance frequency mirroring of grid-forming VSCs for wind power integration in the embodiments of the present application is based on sensing the grid frequency in the grid-forming mode, and independently decoupling the control of the bipolar DC voltage based on the sub-module decoupling control, and then mapping the grid frequency to the offshore wind farm based on the bipolar differential mode, the frequency mirroring process offsets the influence of voltage drop on transmission accuracy due to line resistance and reactance, and realizes fast disturbance mapping of the grid frequency. BRIEF DESCRIPTION OF DRAWINGS

[0054] Other features, objects, and advantages of the present application will become more apparent from the following detailed description of non-limiting embodiments thereof, read in conjunction with the accompanying drawings:

[0055] Figure 1 A schematic diagram of the offshore wind power-flexible HVDC grid-connected system in an embodiment of the present application;

[0056] Figure 2 A schematic diagram of the detection and calculation part of the receiving converter station in a preferred embodiment of the present application;

[0057] Figure 3 A schematic diagram of the grid-forming AC control part of the receiving converter station in a preferred embodiment of the present application;

[0058] Figure 4 A schematic diagram of the grid-forming DC control part of the receiving converter station in a preferred embodiment of the present application;

[0059] Figure 5 A schematic diagram of the three-phase upper and lower arm balanced voltage generation in a preferred embodiment of the present application;

[0060] Figure 6 A schematic diagram of the circulating current suppression voltage generation in a preferred embodiment of the present application;

[0061] Figure 7 A schematic diagram of the three-phase upper and lower arm insertion index generation in a preferred embodiment of the present application;

[0062] Figure 8 A schematic diagram of the electrical quantity detection of the sending converter station in a preferred embodiment of the present application;

[0063] Figure 9This is a frequency mirror simulation verification result in a specific embodiment of the present invention. Detailed Implementation

[0064] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0065] like Figure 1 The diagram shows an offshore wind power-flexible DC transmission grid connection system, comprising an AC grid, receiving-end converters, sending-end converters, and an offshore wind farm. Based on this system, one embodiment of the present invention provides a frequency mirroring method for a grid-connected flexible DC system without communication interference, which mainly includes three steps:

[0066] S100, the receiving-end converter station adopts grid-type control, which realizes the self-synchronization of grid phase and the sensing of grid frequency by constructing the equivalent mapping of sub-module energy-synchronous generator rotor;

[0067] S200, based on the sensing results obtained from the network self-synchronization and frequency detection, injects positive sequence circulating current into the three-phase bridge arm of the receiving-end converter station through AC control, DC control and / or equalization modulation control, and / or adjusts the DC component in the insertion index of the upper and lower bridge arm sub-modules to independently adjust and control the positive and negative DC voltages.

[0068] The S300, by controlling the DC voltage setpoints of the upper and lower arms of the receiving-end converter station respectively, uses the voltage difference between the positive and negative poles of the DC transmission system after adjustment and control to map and transmit the grid frequency; the sending-end converter station senses the changes in the DC voltage of the positive and negative poles, restores the grid frequency from it, and controls the AC frequency of the offshore wind farm to be the same as the grid frequency, thus realizing the grid frequency mirroring.

[0069] The above embodiments are based on sensing the grid frequency under the grid construction mode and independently decoupling the bipolar DC voltage based on the sub-module decoupling control. Then, the grid frequency is mapped to the offshore wind farm based on the bipolar differential mode. This frequency mirroring process cancels the influence of line resistance and reactance voltage drop on the transmission accuracy, and realizes the fast anti-interference mapping of the grid frequency.

[0070] In a preferred embodiment of the present invention, in implementation S100, the receiving-end converter station adopts grid-based control, and achieves self-synchronization of grid phase and sensing of grid frequency by constructing an equivalent mapping between sub-module energy and synchronous generator rotors. Figure 2 As shown, the specific process is as follows:

[0071] S101, Detect the voltage U between the positive DC bus and ground at the receiving end converter station.dcr+ , negative DC bus voltage to ground U dcr- , grid point AC voltage u pcc , grid point AC current i pcc , all sub-module capacitor voltage U sm and double frequency circulating current i of three-phase bridge arm of receiving end converter station cirj (j = a, b, c).

[0072] S102, according to u pcc and i pcc , the output active power P g and reactive power Q g of the AC side of the receiving end converter station are calculated. sm According to all sub-module capacitor voltage U MMC , the total energy of the upper bridge arm sub-modules, the total energy of the lower bridge arm sub-modules and the total energy of the sub-modules of the receiving end converter station are calculated respectively, and the calculation method is as follows:

[0073]

[0074] W MMCu = W MMCl

[0075] Wherein, W MMCu is the total energy of the upper bridge arm sub-modules of the receiving end converter station, W MMCu is the total energy of the lower bridge arm sub-modules of the receiving end converter station, W MMC is the total energy of the sub-modules of the receiving end converter station, C SM is the sub-module capacitor voltage, N is the number of sub-modules of each bridge arm of the converter, U SMaui is the capacitor voltage of the i-th sub-module of the upper bridge arm of phase a of the receiving end converter station, U SMbui is the capacitor voltage of the i-th sub-module of the upper bridge arm of phase b of the receiving end converter station, USMcui is the capacitor voltage of the i-th sub-module of the upper bridge arm of phase c of the receiving end converter station, U SMali is the capacitor voltage of the i-th sub-module of the lower bridge arm of phase a of the receiving end converter station, U SMbli is the capacitor voltage of the i-th sub-module of the lower bridge arm of phase b of the receiving end converter station, U SMcli is the capacitor voltage of the i-th sub-module of the lower bridge arm of phase c of the receiving end converter station.

[0076] S103, the difference between the total energy W MMC of the sub-modules of the flexible HVDC converter and the rated total energy W MMCref is calculated, and the obtained difference is divided by 2H v , H v is a virtual inertia coefficient, generally about 100ms, plus the rated frequency ω0 (usually 50Hz), that is, the reference value ω c of the AC frequency of the converter output is obtained. For ωc Integrating, we obtain the reference value θ of the AC phase of the converter output.

[0077] In a preferred embodiment of the present invention, the AC control section in S200 is implemented, such as... Figure 3 As shown, specifically:

[0078] First, W MMC With W MMCref The difference multiplied by the virtual damping coefficient D v Then divide by 2H v and P ac0 , where P ac0 The current active power output of the receiving-end converter station is used to obtain the AC voltage damping compensation term Δu. ac Then, the measured value of reactive power Q... g With a given value Q ref Divide by k Q (k Q (This is the reactive power control droop factor). Add the result to the rated value, and then add the rated value of the AC voltage, u. a0 And AC voltage damping compensation term Δu ac Obtain the reference value u of the output AC voltage. acref .

[0079] Next, the AC voltage reference value u acref with u d Subtract the values ​​and add the first reference value i of the q-axis current. q1 Multiply by the rated frequency ω0, then divide by a virtual admittance R v +sL v Where Rv is the virtual resistance, Lv is the virtual inductance, and s is the Laplace operator, obtaining the first reference value i of the d-axis current. d1 .

[0080] At the same time, 0 and u q Subtract i from the difference. d1 Multiply by the rated frequency ω0, then divide by a virtual admittance R v +sL v Obtain the first reference value i of the q-axis current. q1 .

[0081] Then, i d1 and i q1 After passing through a current-limiting circuit, the second reference values ​​i of the d-axis and q-axis output currents are obtained. dref and i qref The principle of the current limiting process is that if i d1 and i q1 The square root of the sum of squares is less than the current limiting amplitude i. lim(Generally 1.2 to 1.5 times the rated current), then i dref =i d1 i qref =i q1 . If i d1 and i q1 The square root of the sum of squares is greater than the current limiting value i. lim Then i d1 and i q1 Scaling down proportionally by a factor of n until (i d1 / n) and (i q1 The square root of the sum of the squares of (n) equals i. lim At this time, i dref =(i d1 / n), i qref =(i q1 / n).

[0082] Finally, the second reference value i of the d-axis current and the q-axis current is compared. dref i qref With the measured value i d i q The difference is calculated, and after passing through the proportional-integral controller, the dq-axis decoupling term ω0L is added to each. c i d With ω0L c i q and the AC voltage feedforward term u at the grid connection point d with u q ω0 is the rated angular frequency (usually taken as 314.15 rad / s), L c The filter inductor is used to obtain the dq modulation voltage E on the AC side of the receiving-end converter station. d With E q Then, an inverse PARK transformation is performed at phase θ to obtain the AC-side three-phase modulation voltage E of the receiving-end converter station. a E b With E c .

[0083] In a preferred embodiment of the present invention, a DC control section is implemented, such as... Figure 4 As shown, specifically:

[0084] First, obtain the current power grid frequency ω. g ω g There are two ways to obtain the frequency ω. The first is through communication with the power grid's SCADA system, and the second is under grid-type control, which can use the frequency ω of the receiving-end converter station itself. c As the power grid frequency ω g .

[0085] Then, after obtaining the power grid frequency ω g Then, ωg Subtract the rated frequency ω0 and multiply by the mapping coefficient K. dc Then, the deviation value ΔU of the DC voltage is obtained. dc The rated value U of the positive DC voltage of the flexible DC system dc0+ Add △U dc To obtain the reference value U of the positive DC voltage. dcrref+ The rated value U of the negative DC voltage of the flexible DC system dc0- Subtract △U dc To obtain the reference value U of the negative DC voltage. dcrref- .

[0086] Next, the reference value U of the positive DC voltage of the flexible DC system is... dcrref+ Measurement value of positive DC voltage U of flexible DC system dcr+ The difference is calculated, and after passing through a proportional-integral controller, a feedforward U is added. dcr+ E, the modulated DC voltage wave at the positive terminal of the receiving-end converter station, is obtained. dcrref+ .

[0087] Finally, the reference value U of the negative DC voltage of the flexible DC system is... dcrref- Measurement value of negative DC voltage U of flexible DC system dcr- The difference is calculated, and after passing through a proportional-integral controller, a feedforward U is added. dcr- E, the DC voltage modulation wave of the negative terminal of the receiving-end converter station is obtained. dcrref- .

[0088] In a preferred embodiment of the present invention, the modulation and equalization portion in S200 is implemented, such as... Figure 5 , 6 As shown in 7, specifically:

[0089] First, calculate the total energy W of the upper arm submodule of the receiving-end converter station. MMCu Total energy W of the lower bridge arm MMCl The difference, after passing through a proportional-integral controller, yields the balanced voltage amplitude U of the upper and lower bridge arms. b Then, by performing the inverse PARK transformation at an angle of θ+π / 2, the balanced voltage E of the three-phase upper and lower bridge arms is obtained. bj (j = a, b, c), such as Figure 5 As shown.

[0090] Then, the second harmonic circulating current component i of the three phases cirj After subtracting from 0, the signal passes through a proportional-resonant controller (PR controller) to obtain the modulated wave u of the circulating current suppression voltage. cirj (j = a, b, c), such as Figure 6 As shown.

[0091] Next, the average voltage U of the upper arm submodule of the receiving-end converter station is calculated. SMavuThe calculation method is as follows:

[0092]

[0093] Simultaneously, the average voltage U of the lower bridge arm submodule of the receiving-end converter station is calculated. SMavl The calculation method is as follows:

[0094]

[0095] Finally, the reference voltages for six bridge arms (phase a upper bridge arm, phase a lower bridge arm, phase b upper bridge arm, phase b lower bridge arm, phase c upper bridge arm, and phase c lower bridge arm) are generated by combining the positive and negative DC voltage modulation waves, the three-phase AC voltage modulation waves, the circulating current suppression modulation waves, and the upper and lower bridge arm balance voltage modulation waves. Then, the reference voltages E for phase a upper bridge arm, phase b upper bridge arm, and phase c upper bridge arm are... Nuj (j=a,b,c) Excluding the average voltage U of the above bridge arm submodules SMavu Obtain the insertion index N of the three-phase upper arm. uj (j=a,b,c). The reference voltage E of the lower bridge arm of phase a, the lower bridge arm of phase b, and the lower bridge arm of phase c. Nlj (j=a,b,c) Excluding the following bridge arm submodule average voltage U SMavl Obtain the insertion index N of the three-phase lower arm. lj (j = a, b, c), such as Figure 7 As shown.

[0096] Finally, the submodule capacitor voltages of the six bridge arms are sorted, and the bridge arm currents are measured. When the bridge arm current is positive, n submodules with lower capacitor voltages are connected, and when the bridge arm current is positive, n submodules with higher capacitor voltages are connected, where n is the same as the insertion index of the bridge arm.

[0097] In a preferred embodiment of the present invention, S300 is implemented, specifically:

[0098] S301, by controlling the DC voltage setpoints of the upper and lower bridge arms of the receiving-end converter station respectively, utilizes the voltage difference between the positive and negative poles of the regulated and controlled DC transmission system to map and transmit the grid frequency, that is, after obtaining the grid frequency ω g Then, ω g Subtract the rated frequency ω0 and multiply by the mapping coefficient K. dc Then, the deviation value ΔU of the DC voltage is obtained. dc The rated value U of the positive DC voltage of the flexible DC system dc0+ Add △U dc To obtain the reference value U of the positive DC voltage. dcrref+ The rated value U of the negative DC voltage of the flexible DC system dc0- Subtract △U dc To obtain the reference value U of the negative DC voltage.dcrref- Reference value U for transmitting positive DC voltage to the receiving-end converter station dcrref+ Reference value U of negative DC voltage dcrref- .

[0099] S302, for the sending-end converter station, the first step is to check its positive terminal to ground voltage U. dcs+ Voltage U of negative terminal to ground dcs- ,like Figure 8 As shown. Then, the positive terminal to ground voltage U dcs+ Voltage U of negative terminal to ground dcs- Subtract from each other and then divide by the mapping coefficient K. dc Then, the grid frequency variation information can be reconstructed from the DC voltage, and after adding the rated frequency ω0, it can be used as the output AC frequency reference value ω of the sending-end converter station. ref This allows for a mirror mapping of the grid frequency to the offshore wind farm frequency. Because this mapping uses the differential modulus of positive and negative DC voltages, it can offset the impact of line resistance and reactance voltage drop on transmission accuracy, achieving rapid anti-interference mapping of the grid frequency.

[0100] Based on the same inventive concept, in other preferred embodiments of the present invention, a frequency mirroring system without communication interference immunity for a grid-connected flexible DC system for wind power access is provided, comprising:

[0101] Grid self-synchronization and frequency detection module: The receiving-end converter station adopts grid-type control, which realizes grid phase self-synchronization and grid frequency sensing by constructing an equivalent mapping of sub-module energy-synchronous generator rotor;

[0102] The bipolar DC voltage independent decoupling control module based on submodule decoupling: Based on the sensing results obtained from the network self-synchronization and frequency detection, positive sequence circulating current is injected into the three-phase bridge arm of the receiving-end converter station through AC control, DC control and / or equalization modulation control, and / or the DC component in the insertion index of the upper and lower bridge arm submodules is adjusted to independently regulate and control the positive and negative DC voltages.

[0103] The grid frequency anti-interference mirroring control module based on bipolar differential mode mapping works as follows: by controlling the DC voltage setpoints of the upper and lower bridge arms of the receiving-end converter station respectively, the grid frequency is mapped and transmitted using the voltage difference between the positive and negative poles of the DC transmission system after adjustment and control; the sending-end converter station senses the changes in the positive and negative DC voltages, restores the grid frequency from them, and controls the AC frequency of the offshore wind farm to be the same as the grid frequency, thereby realizing grid frequency mirroring.

[0104] The specific implementation techniques of each module / unit in the above examples of the present invention can be referred to the steps of the frequency mirroring method for the grid-type flexible DC system without communication interference in the above embodiments, which will not be repeated here.

[0105] Based on the same inventive concept, in another preferred embodiment of the present application, a terminal is provided, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor, when executing the program, is configured to execute the anti-interference frequency mirroring method for grid-connected flexible DC system with wind power access, or to run the anti-interference frequency mirroring system for grid-connected flexible DC system with wind power access.

[0106] Based on the same inventive concept, in another preferred embodiment of the present application, a computer readable storage medium is provided, which stores a computer program executable by a processor to execute the anti-interference frequency mirroring method for grid-connected flexible DC system with wind power access, or to run the anti-interference frequency mirroring system for grid-connected flexible DC system with wind power access.

[0107] To verify the feasibility and effect of the anti-interference frequency mirroring method and system for grid-connected flexible DC system with wind power access in the above embodiments, in one specific embodiment of the present application, a simulation model of the system as shown in Figure 1 is established in the software tool PSCAD / EMTDC (Power Systems Computer Aided Design / Electromagnetic Transient Program) for power system simulation, and the system parameters are as shown in the following table:

[0108] Parameter name Value Rated active power / MW 1100 Rated grid voltage / kV 500 Rated voltage on wind farm side / kV 220 Rated DC voltage / kV ±400 Number of submodules per bridge arm 400 + 50 (redundancy) Submodule capacitance / mF 11 Bridge arm reactance / mH 150 Linking transformer capacity / MVA 1400 Linking transformer leakage reactance / p.u. 0.14 Converter 1 transformer ratio 416 / 500 Converter 2 transformer ratio 416 / 220

[0109] The output power of the wind farm is 500 MW, and at t = 1 s, the grid frequency drops from 50 Hz to 49.5 Hz, at which time the grid frequency, the positive and negative DC voltage difference, and the wind farm frequency are as shown in Figure 9 It can be found that the grid frequency can be well mirrored into the offshore wind farm through the positive and negative DC voltage difference.

[0110] The specific embodiments of the present application are described above. It should be understood that the present application is not limited to the above specific embodiments, and those skilled in the art can make various modifications or changes within the scope of the claims, which does not affect the essential content of the present application. The above preferred features can be used in combination as long as they are not in conflict with each other.

Claims

1. A communication-free anti-interference frequency mirror method for a meshed VSC system with wind power access, characterized in that, Comprise: The receiving end converter station adopts network type control, and self-synchronization of grid phase and perception of grid frequency are realized by constructing equivalent mapping of submodule energy-synchronous generator rotor; Based on the perception results obtained by the network self-synchronization and frequency detection, through AC control, DC control and / or balanced modulation control, positive sequence circulating current is injected in the three-phase bridge arm of the receiving end converter station, and / or the DC component in the insertion index of the upper and lower bridge arm submodules is adjusted, and the positive and negative DC voltages are independently adjusted and controlled; By respectively controlling the DC voltage given value of the upper and lower bridge arms of the receiving end converter station, the voltage difference of the positive and negative poles of the adjusted and controlled DC power transmission system is used to map and transfer the grid frequency; the sending end converter station perceives the change of the positive and negative DC voltages, and restores the grid frequency therefrom, controls the AC frequency of the offshore wind farm to be the same as the grid frequency, and realizes the grid frequency mirroring.

2. The non-communication anti-interference frequency mirror method for network-constructed flexible DC system with wind power integration according to claim 1, characterized in that, The receiving end converter station adopts network type control, and self-synchronization of grid phase and perception of grid frequency are realized by constructing equivalent mapping of submodule energy-synchronous generator rotor, comprising: detecting the positive dc bus voltage u of the receiving end converter station dcr+ , the negative dc bus voltage u of the receiving end converter station dcr- , the grid-connected point ac voltage u pcc , the grid-connected point ac current i pcc , the capacitor voltage u of all sub-modules of the converter station sm , and the double-frequency circulating current i of the three-phase bridge arm of the receiving end converter station cirj , j = a, b, c; According to u pcc And i pcc The output active power P g And the reactive power Q g Of the AC side of the receiving end converter station are calculated; according to all the upper bridge arm sub-module capacitor voltages U sm The total energy of the upper bridge arm sub-modules, the total energy of the lower bridge arm sub-modules and the total energy of the sub-modules of the receiving end converter station are respectively calculated, which are specifically: W MMC = W MMCu + W MMCl wherein, W MMCu is the total energy of the upper bridge arm sub-modules of the receiving end converter station, W MMCu is the total energy of the lower bridge arm sub-modules of the receiving end converter station, W MMC is the total energy of the sub-modules of the receiving end converter station, C SM is the sub-module capacitor voltage, N is the number of sub-modules per bridge arm of the converter, U SMaui is the capacitor voltage of the i-th sub-module of the upper bridge arm of phase a of the receiving end converter station, U SMbui is the capacitor voltage of the i-th sub-module of the upper bridge arm of phase b of the receiving end converter station, U SMcui is the capacitor voltage of the i-th sub-module of the upper bridge arm of phase c of the receiving end converter station, U SMali is the capacitor voltage of the i-th sub-module of the lower bridge arm of phase a of the receiving end converter station, U SMbli is the capacitor voltage of the i-th sub-module of the lower bridge arm of phase b of the receiving end converter station, U SMcli is the capacitor voltage of the i-th sub-module of the lower bridge arm of phase c of the receiving end converter station; Calculate the total energy W of the submodule of the flexible DC converter. MMC With rated total energy W MMCref The difference, divide the resulting difference by 2H v H v Adding the virtual inertia coefficient to the rated frequency ω0 yields the reference value ω for the converter output AC frequency. c ; for ω c Integrating, we obtain the reference value θ of the AC phase of the converter output.

3. The non-communication anti-interference frequency mirror method for network-constructed HVDC system with wind power integration according to claim 2, characterized in that, The AC control comprises: W MMC is the difference between the measured value of the active power W MMCref and the given value W v , D v is the virtual damping coefficient, 2H ac0 is the equivalent reactance of the AC system, P ac0 is the current output active power of the receiving end converter station, and the AC voltage damping compensation term △u ac is obtained by multiplying the difference between W g and W ref by D v , dividing the result by 2H v and P ac0 ; the difference between the measured value of the reactive power Q g and the given value Q ref is multiplied by k Q , k Q is the reactive power control droop coefficient, and the result is added to the rated value, the rated value of the AC voltage u a0 and the AC voltage damping compensation term △u ac to obtain the reference value of the output AC voltage u acref ; AC voltage reference value u acref is subtracted from u d is subtracted from q-axis current first reference value i q1 is multiplied by rated frequency ω0, and then divided by a virtual admittance R v +sL v where Rv is a virtual resistance, Lv is a virtual inductance, s is a Laplace operator, to obtain a first reference value id1 of d-axis current. Subtract i q Subtract i d1 Divide by a virtual admittance R v +sL v Obtain the first reference value i q1 of the q-axis current i d1 and i q1 After a current limiting link, the second reference values of d-axis and q-axis output currents i dref and i qref are obtained, the principle of the current limiting link is that if the square root of the sum of i d1 and i q1 is less than the current limiting amplitude i lim , then i dref =i d1 , i qref =i q1 ; if the square root of the sum of i d1 and i q1 is greater than the current limiting amplitude i lim , then i d1 and i q1 are reduced by n times in proportion, until the square root of the sum of i d1 / n and i q1 / n is equal to i lim ; at this time, i dref =i d1 / n, i qref =i q1 / n; The second reference value i of the d-axis current and the q-axis current. dref i qref With the measured value i d i q The difference is calculated, and after passing through the proportional-integral controller, the dq-axis decoupling term ω0L is added to each. c i d With ω0L c i q and the AC voltage feedforward term u at the grid connection point d with u q ω0 is the rated angular frequency, L c The filter inductor is used to obtain the dq modulation voltage E on the AC side of the receiving-end converter station. d With E q Then, an inverse PARK transformation is performed at phase θ to obtain the AC-side three-phase modulation voltage E of the receiving-end converter station. a E b With E c .

4. The non-communication anti-interference frequency mirror method for network-constructed HVDC system with wind power integration according to claim 2, characterized in that, The DC control comprises: obtaining the grid frequency ω at this time g ; After obtaining the power grid frequency ω g Then, ω g Subtract the rated frequency ω0 and multiply by the mapping coefficient K. dc Then, the deviation value ΔU of the DC voltage is obtained. dc The rated value U of the positive DC voltage of the flexible DC system dc0+ Add △U dc To obtain the reference value U of the positive DC voltage. dcrref+ The rated value U of the negative DC voltage of the flexible DC system dc0- Subtract △U dc To obtain the reference value U of the negative DC voltage. dcrref- ; The reference value U of the positive DC voltage of the flexible DC system dcrref+ Measurement value of positive DC voltage U of flexible DC system dcr+ The difference is calculated, and after passing through a proportional-integral controller, a feedforward U is added. dcr+ E, the modulated DC voltage wave at the positive terminal of the receiving-end converter station, is obtained. dcrref+ ; The reference value of the negative DC voltage of the HVDC system U dcrref- The measured value of the negative DC voltage of the HVDC system U dcr- The difference between the reference value and the measured value is input into a proportional-integral controller and a feedforward value U dcr- The modulated wave of the negative DC voltage of the rectifier station E dcrref- is obtained.

5. The non-communication anti-interference frequency mirror method for network-constructed HVDC system with wind power integration according to claim 4, characterized in that, The grid frequency ω g The acquisition mode includes one or more of the following: Obtained by communication with the grid; Under the control of the grid type, through the phase-locked loop to obtain the PCC point of the alternating frequency ω pcc , and this as the grid frequency ω g ; Under the construction network type control, using the frequency ω c As the grid frequency ω g .

6. The non-communication anti-interference frequency mirror method for network-constructed HVDC system with wind power integration according to claim 2, characterized in that, The balanced modulation comprises: The total energy W of the bridge arm sub-modules on the receiving end converter station is calculated MMCu The difference between the total energy W of the lower bridge arm and the total energy W of the upper bridge arm MMCl passes through a proportional-integral controller to obtain the upper and lower bridge arm balanced voltage amplitude U b After the inverse PARK transformation at the angle of θ+π / 2, the upper and lower bridge arm balanced voltage modulation wave E is obtained bj , j=a, b, c; The three-phase double-frequency circulating current component i cirj After being subtracted by 0, the modulated wave u of circulating current suppression voltage is obtained through a proportional-resonant controller cirj , j = a, b, c; The average voltage U of the bridge arm sub-module on the receiving end converter station is calculated SMavu Specifically, The average voltage U of the lower bridge arm sub-module of the receiving end converter station is calculated SMavl Specifically, modulation wave E of the positive and negative direct current voltage dcrref+ and E dcrref- , modulation voltage E of the three-phase of the alternating side a , E b and E c , circulation suppression modulation wave u cirj , upper and lower bridge arm balance voltage modulation wave E bj The reference voltage of the six bridge arms, including the a-phase upper bridge arm, the a-phase lower bridge arm, the b-phase upper bridge arm, the b-phase lower bridge arm, the c-phase upper bridge arm and the c-phase lower bridge arm, is generated by combination; then the reference voltage E of the a-phase upper bridge arm, the b-phase upper bridge arm and the c-phase upper bridge arm Nuj， j=a, b, c, the average voltage U of the above bridge arm sub-module is excluded SMavu , the insertion index N of the three-phase upper bridge arm is obtained uj j=a, b, c; the reference voltage E of the a-phase lower bridge arm, the b-phase lower bridge arm and the c-phase lower bridge arm Nlj j=a, b, c, the average voltage U of the below bridge arm sub-module is excluded SMavl , the insertion index N of the three-phase lower bridge arm is obtained lj， j=a, b, c; The submodule capacitor voltages of the six bridge arms are sorted, and the bridge arm current is measured, when the bridge arm current is positive, the n submodules with lower capacitor voltage are put into, when the bridge arm current is positive, the n submodules with higher capacitor voltage are put into, and n is the same as the insertion index of the bridge arm.

7. The non-communication anti-interference frequency mirror method for network-constructed flexible DC system with wind power integration according to claim 1, characterized in that, The receiving end converter station adopts network type control, and self-synchronization of grid phase and perception of grid frequency are realized by constructing equivalent mapping of submodule energy-synchronous generator rotor, comprising: After obtaining the power grid frequency ω g Then, ω g Subtract the rated frequency ω0 and multiply by the mapping coefficient K. dc Then, the deviation value ΔU of the DC voltage is obtained. dc The rated value U of the positive DC voltage of the flexible DC system dc0+ Add △U dc To obtain the reference value U of the positive DC voltage. dcrref+ The rated value U of the negative DC voltage of the flexible DC system dc0－ Subtract △U dc To obtain the reference value U of the negative DC voltage. dcrref- Reference value U for transmitting positive DC voltage to the receiving-end converter station dcrref+ Reference value U of negative DC voltage dcrref- ; The sending end converter station perceives the change of the positive and negative DC voltages, and restores the grid frequency therefrom, controls the AC frequency of the offshore wind farm to be the same as the grid frequency, and realizes the grid frequency mirroring, comprising: Based on the received positive and negative DC voltage, the positive voltage to ground U dcs+ and the negative voltage to ground U dcs- ; The positive electrode voltage U dcs+ is subtracted from the negative electrode voltage U dcs- , and the result is divided by the mapping coefficient K dc . The grid frequency variation information is then restored from the DC voltage, and the rated frequency ω0 is added to obtain the output AC frequency reference value ω ref of the sending converter station, i.e. the mirror mapping of the grid frequency to the offshore wind farm frequency is achieved.

8. A communication-free anti-interference frequency mirror system for a meshed network type flexible DC system with wind power access, characterized in that, Comprise: The network self-synchronization and frequency detection module: the receiving end converter station adopts network type control, and self-synchronization of grid phase and perception of grid frequency are realized by constructing equivalent mapping of submodule energy-synchronous generator rotor; The bipolar DC voltage independent decoupling control module based on submodule decoupling: based on the perception results obtained by the network self-synchronization and frequency detection, through AC control, DC control and / or balanced modulation control, positive sequence circulating current is injected in the three-phase bridge arm of the receiving end converter station, and / or the DC component in the insertion index of the upper and lower bridge arm submodules is adjusted, and the positive and negative DC voltages are independently adjusted and controlled; The grid frequency anti-disturbance mirroring control module based on bipolar differential mode mapping: by respectively controlling the DC voltage given value of the upper and lower bridge arms of the receiving end converter station, the voltage difference of the positive and negative poles of the adjusted and controlled DC power transmission system is used to map and transfer the grid frequency; the sending end converter station perceives the change of the positive and negative DC voltages, and restores the grid frequency therefrom, controls the AC frequency of the offshore wind farm to be the same as the grid frequency, and realizes the grid frequency mirroring.

9. A terminal comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, characterized in that, The processor executes the program and can be used to execute the method in any one of claims 1-7, or run the system in claim 8.

10. A computer-readable storage medium having stored thereon a computer program, characterized in that, The program, when executed by the processor, is operable to perform the method of any one of claims 1 to 7, or to operate the system of claim 8.

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