A far-sea wind power flexible direct transmission grid-connected system based on hybrid control converter of follow-the-grid and build-the-grid

Abstract

The application discloses a kind of based on follow network-grid hybrid control converter's far sea wind power flexible sending and grid-connected system, belong to offshore wind power flexible sending and grid-connected system technical field.The system, receiving end converter includes data acquisition and transformation module, synchronous angle fusion module, current reference value generation module, current inner loop module and modulation module;Synchronous angle fusion module will power synchronous ring generation network frequency increment and the weighted fusion that phase-locked loop generates follow network frequency increment obtains hybrid synchronous angle;Current reference value generation module will follow network outer loop and the current reference value that network reactive power-voltage and virtual impedance link generation is weighted fusion and obtains final current instruction.The application realizes the deep fusion of follow network and network control in double link, can be adaptively switched control characteristics under wide range grid intensity change, give consideration to strong network fast tracking and weak network active support ability, eliminate the risk of oscillation instability of single control mode, guarantee system stable operation.

Description

Technical Field

[0001] This invention belongs to the technical field of large-scale offshore wind power flexible DC transmission and grid connection system, specifically relating to an offshore wind power flexible DC transmission and grid connection system based on a grid-connected-grid hybrid control converter. Background Technology

[0002] Flexible DC transmission technology is widely recognized as the best technical means for large-scale transmission and grid connection of offshore wind power. Flexible DC grid-connected converters directly connect offshore wind farm clusters to the power grid, serving as a crucial link in ensuring the safe grid connection of offshore wind power and the safe and stable operation of the power grid.

[0003] Currently, the offshore wind power flexible DC transmission projects already in operation adopt a grid-connected control scheme, which has advantages such as mature technology, fast power regulation speed, and simple parameter setting. However, it cannot meet the needs of weak grid scenarios with frequent operation mode adjustments and numerous fault disturbances. To improve the grid support capability and operational stability of flexible DC grid-connected converters under weak grid conditions, a grid-connected control scheme has been piloted in some offshore wind power flexible DC transmission projects. However, under operating modes with high short-circuit capacity or good interconnection conditions, the impedance matching and control interaction between the flexible DC grid-connected converter and the grid are quite complex. Improper parameter configuration will lead to problems such as low-frequency power angle oscillation or subsynchronous oscillation.

[0004] The randomness and volatility of renewable energy output cause the operating conditions of generating units to exhibit time-varying characteristics, which may further lead to fluctuations in the grid strength of the system. However, it is difficult to simultaneously meet the comprehensive requirements of flexible DC grid-connected converters in terms of stability, grid support capacity, and economy under the wide range of "strong grid to weak grid" operating conditions: grid-connected control has insufficient stability and support capacity in the case of local weak grid at the receiving end, while grid-connected control may introduce new oscillation risks under strong grid or medium short-circuit ratio conditions, and there are engineering constraints in terms of grid operation coordination.

[0005] Therefore, there is an urgent need to propose a flexible DC-DC grid-connected system that can adapt to different grid intensities, and to improve the operating performance and grid adaptability of the flexible DC-DC grid-connected converter by integrating grid-following and grid-building control strategies. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a flexible direct-connection system for offshore wind power based on a grid-connected and grid-structured hybrid control converter. This invention can adapt to different grid intensities, maintain rapid power point tracking performance under strong grid conditions, and possess active grid support capabilities under weak grid conditions.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a flexible DC transmission and grid connection system for offshore wind power based on a grid-connected hybrid control converter, comprising an offshore wind farm, a sending-end MMC, a DC transmission submarine cable, a receiving-end grid-connected hybrid control converter, and an onshore AC power grid. The sending-end MMC uses VF control to establish the voltage and frequency on the offshore wind farm side. The receiving-end grid-connected hybrid control converter is the core receiving-end equipment of the system, and it includes a data acquisition and transformation module, a synchronization angle fusion module, a current reference value generation module, a current inner loop module, and a modulation module.

[0008] The data acquisition and transformation module is used to acquire the AC voltage and current at the receiving-end MMC grid connection point, which is the connection point between the receiving-end MMC and the receiving-end AC power grid. The module performs a dq transformation on the AC voltage and current at the receiving-end MMC grid connection point to obtain the d-axis and q-axis components of the voltage and current at the receiving-end MMC grid connection point in the dq rotating coordinate system. Based on the grid connection point voltage and current, the actual active power value P is calculated. The module also acquires the DC voltage output by the receiving-end MMC, the voltage at the receiving-end MMC grid connection point, the actual value Q of the reactive power, and the reference value Q. ref .

[0009] The synchronization angle fusion module is used to determine the active power reference value P based on the actual active power value P and the active power reference value P generated by the additional DC voltage control. ref Frequency increments for generating network control The frequency increment for grid-connected control is generated based on the AC voltage at the receiving-end MMC grid connection point. The frequency increment of the network control Frequency increments with network control Through the fusion coefficient k h Weighted fusion is performed to obtain the mixed angular velocity. For mixed angular velocity The hybrid synchronization angle θ of the receiving-end MMC is obtained by integration. h .

[0010] The current reference value generation module is used to generate current based on the actual value Q of the receiving-end MMC grid connection point voltage and reactive power, and the reference value Q. ref Generate d-axis current reference values ​​for grid control respectively. and q-axis current reference value and the d-axis current reference value for network control. and q-axis current reference value The d-axis current reference value of the grid control branch will be used. q-axis current reference value Reference value of d-axis current of the network control branch q-axis current reference value Through the fusion coefficient kh Weighted fusion is performed to obtain the d-axis current reference value i at the receiving-end MMC grid connection point. dref and q-axis current reference value i qref .

[0011] The current inner loop module is used to determine the d-axis current reference value i. dref and q-axis current reference value i qref Combined with the d-axis component i of the received-end MMC output current d and q-axis component i q and mixed angular velocity The d-axis voltage reference value v is generated by the inner current loop to obtain the differential mode voltage of the MMC at the receiving end. cdref and q-axis voltage reference value v cqref .

[0012] The modulation module is used to convert the d-axis voltage reference value v of the differential mode voltage of the receiving-end MMC into a voltage value v. cdref and q-axis voltage reference value v cqref By transforming the dq / abc coordinates, the reference value v of the modulation voltage of the receiving-end MMC in the abc coordinate system is obtained. cref And based on the reference value v of the modulation voltage cref By utilizing pulse width modulation, corresponding pulse signals are generated to achieve hybrid control of the flexible DC receiving end and the network structure.

[0013] Furthermore, in the synchronization angle fusion module, the frequency increment of the network control... It is generated by the power synchronization loop based on the active power deviation, and the calculation formula is:

[0014]

[0015] Where s is the Laplace operator, J is the virtual inertia coefficient, D is the damping coefficient, ω0 is the rated value of the angular frequency of the internal potential of the receiving-end MMC, and P is the actual power value at the receiving-end grid connection point. ref This is a reference value for active power.

[0016] Furthermore, in the synchronization angle fusion module, the frequency increment of the network control... It is generated by the phase-locked loop based on the q-axis component of the AC voltage at the receiving end of the grid. The calculation formula is:

[0017]

[0018] Where s is the Laplace operator, k ppll and k ipll These are the proportional and integral parameters of the phase-locked loop proportional-integral controller, v fq This represents the q-axis component of the AC voltage at the receiving end of the grid.

[0019] Furthermore, in the synchronized angle fusion module, the mixed angular velocity... The calculation formula is:

[0020]

[0021] Where ω0 is the rated value of the angular frequency of the internal potential of the receiving-end MMC, k h The fusion coefficient is... and These are the frequency increments generated by network construction and network tracking control, respectively;

[0022] The hybrid synchronization angle θ h The calculation formula is:

[0023] .

[0024] Furthermore, in the current reference value generation module, the d-axis current reference value of the network control... and q-axis current reference value The generation method is as follows:

[0025] An internal potential E is generated through the QU reactive-voltage link, and then based on the internal potential E and the d-axis component of the AC voltage at the receiving end grid connection point... and q-axis components It is generated through a virtual impedance control circuit, and the calculation formula is:

[0026]

[0027] Among them, R v and L v These are virtual resistance and virtual reactance, respectively. This represents the actual value of the AC power grid frequency at the receiving end.

[0028] Furthermore, the formula for calculating the internal potential E is:

[0029]

[0030] in, This represents the amplitude of the no-load internal potential. Here, s is the reactive power loop integral parameter, s is the Laplace operator, and Q is the actual reactive power value at the receiving end grid connection point. ref This is a reference value for reactive power control. This is the reactive power-voltage droop factor. The rated voltage amplitude of grid-connected AC voltage. This refers to the amplitude of the grid-connected AC voltage.

[0031] Furthermore, in the current reference value generation module, the d-axis current reference value i at the receiving end MMC grid connection point...dref and q-axis current reference value i qref The calculation formula is:

[0032] .

[0033] Furthermore, in the current inner loop module, the d-axis voltage reference value v of the differential mode voltage of the receiving-end MMC is... cdref and q-axis voltage reference value v cqref Generated by the inner current loop, the calculation formula is:

[0034]

[0035] Among them, v cdref and v cqref These are the d-axis and q-axis voltages of the differential-mode voltage of the receiving-end MMC, respectively; i d and i q These are the d-axis current and q-axis current of the receiving-end MMC output current, respectively; L represents the actual value of the AC power grid frequency at the receiving end. f For the receiving end MMC connection reactance parameter; k pi and k ii These are the proportional and integral parameters of the current inner-loop proportional-integral controller, respectively; i dref and i qref These are the d-axis current reference value and q-axis current reference value for the MMC output current at the receiving end, respectively.

[0036] Furthermore, the fusion coefficient k h Adaptive adjustment is achieved using the Sigmoid function curve, and a fusion coefficient k is established. h The quantitative relationship between the short-circuit ratio (SCR) and the grid short-circuit ratio is expressed as follows:

[0037]

[0038]

[0039] SCR weak and SCR stiff These are the lower and upper limits of SCR under extremely weak and extremely strong power grids, respectively, and α is the curve slope coefficient.

[0040] Furthermore, the fusion coefficient k h The following adaptive adjustment rule applies to changes in the grid short-circuit ratio (SCR):

[0041] When SCR ≤ SCR weak At that time, k h = 0, the network characteristics have reached saturation;

[0042] When SCRweak ≤ SCR ≤ SCR stiff At that time, k h Adjustments are made based on the Sigmoid function curve; when SCR is low, the fusion control is characterized by meshing characteristics, and as SCR increases, k... h As the curve increases, the mesh characteristics gradually strengthen and eventually tend to saturate;

[0043] When SCR ≥ SCR stiff At that time, k h = 1, the network characteristics have reached saturation;

[0044] When SCR decreases, k h As the curve decreases, the following properties gradually weaken, while the network-forming properties gradually strengthen and eventually tend to saturate.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] (1) This invention proposes a flexible direct-to-grid transmission system for offshore wind power based on a grid-following-grid hybrid control converter. The synchronization angle link and the voltage outer loop are integrated into the design, and a common current inner loop is adopted. The integration of the synchronization angle can simultaneously reduce the risk of instability of the phase-locked loop under weak grid conditions and the risk of instability of the power synchronization control link under strong grid conditions; the integration of the voltage outer loop enables the hybrid control to maintain the fast tracking characteristics of grid-following control under strong grid conditions and the voltage support characteristics of grid-connecting control under weak grid conditions.

[0047] (2) This invention proposes the basic principle that the degree of integration changes monotonically with the short-circuit ratio, and establishes a quantitative relationship between the degree of integration and the grid short-circuit ratio. The integration coefficient is adaptively adjusted according to the short-circuit ratio to ensure the stability of the system over a wide range of short-circuit ratios. Under strong grid conditions, the weight of grid-following characteristics is increased to reduce the risk of subsynchronous oscillations caused by parallel voltage sources. Under weak grid conditions, the weight of grid-building characteristics is increased to weaken the negative damping effect dominated by the PLL and enhance the voltage frequency support capability.

[0048] (3) This invention fully utilizes the advantages of grid-connected control and grid-connected control, solving the problem that existing control strategies are unable to simultaneously meet the comprehensive requirements of flexible DC grid-connected converters in terms of stability, grid support capacity, and economy under the wide range of "strong grid-weak grid" operating conditions at the receiving end. Theoretical analysis and simulation verification show that this offshore wind power flexible DC grid-connected system can adapt to the wide range of short-circuit ratio operating conditions of the receiving end "strong grid-weak grid" and maintain stable system operation. It provides an effective control solution for the safe and stable operation of offshore wind power flexible DC grid-connected systems and has high practical value and economic benefits. Attached Figure Description

[0049] Figure 1This is a schematic diagram of the offshore wind power flexible direct transmission grid connection system in an embodiment of the present invention;

[0050] Figure 2 This is a block diagram illustrating the control principle of the receiving-end grid-network hybrid control converter in an embodiment of the present invention.

[0051] Figure 3 This is a block diagram illustrating the control principle of the synchronization angle fusion module in an embodiment of the present invention.

[0052] Figure 4 This is a block diagram illustrating the control principle of the current reference value generation module in an embodiment of the present invention.

[0053] Figure 5 The fusion coefficient k in this embodiment of the invention h Line graph;

[0054] Figure 6 This is a waveform comparison of simulation results between single net-following control and the net-following-net hybrid control proposed in this invention.

[0055] Figure 7 This is a waveform comparison of simulation results between single mesh control and the mesh-mesh hybrid control proposed in this invention, as shown in the embodiments of the present invention. Detailed Implementation

[0056] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0057] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0058] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0059] This embodiment provides a flexible DC transmission and grid connection system for offshore wind power based on a grid-connected and grid-structured hybrid control converter. Its specific structure is as follows: Figure 1 As shown.

[0060] This offshore wind power flexible DC transmission and grid connection system comprises an offshore wind farm, a sending-end MMC (Modular Multilevel Converter), a DC transmission submarine cable, a receiving-end grid-connected hybrid control converter, and an onshore AC power grid. The connection between the offshore wind farm and the MMC is the sending end, corresponding to the sending-end MMC. The connection between the onshore AC power grid and the MMC is the receiving end, corresponding to the receiving-end grid-connected hybrid control converter. The sending-end MMC uses VF (Voltage-Frequency Control) to establish the voltage and frequency on the offshore wind farm side. The receiving-end grid-connected hybrid control converter is the core receiving-end equipment of this system.

[0061] The receiver-end grid-network hybrid control converter includes a data acquisition and transformation module, a synchronization angle fusion module, a current reference value generation module, a current inner loop module, and a modulation module. Its control principle block diagram is shown below. Figure 2 As shown. The specific implementation methods of each module are as follows.

[0062] 1. Data Acquisition and Transformation Module

[0063] The data acquisition and transformation module acquires the AC voltage and current at the receiving-end MMC grid connection point. The receiving-end MMC grid connection point is the connection point between the receiving-end MMC and the receiving-end AC power grid. The AC voltage and current at the receiving-end MMC grid connection point are subjected to dq transformation to obtain the d-axis and q-axis components of the voltage and current at the receiving-end MMC grid connection point in the dq rotating coordinate system. The actual active power value P is calculated based on the grid connection point voltage and current. The module also acquires the DC voltage output by the receiving-end MMC, the voltage at the receiving-end MMC grid connection point, the actual reactive power value Q, and the reference value Q. ref .

[0064] 2. Synchronous Angle Fusion Module

[0065] Hybrid synchronization angle θ h Generated by the synchronization angle fusion module, such as Figure 3 As shown. The synchronization angle fusion module consists of an additional DC voltage control, a PSL power synchronization loop, and a PLL phase-locked loop.

[0066] Additional DC voltage control generates active power reference value P ref This serves as the input to the PSL power synchronization loop. The PSL power synchronization loop generates the frequency increment for grid control based on the active power deviation. The calculation formula is as follows:

[0067]

[0068] Where s is the Laplace operator, J is the virtual inertia coefficient, D is the damping coefficient, ω0 is the rated value of the angular frequency of the internal potential of the receiving-end MMC, and P is the actual power value at the receiving-end grid connection point. ref This is a reference value for active power.

[0069] The PLL (phase-locked loop) takes the AC voltage at the grid connection point as input and generates the frequency increment ΔωGFL through grid-following control. The calculation formula is as follows:

[0070]

[0071] Where s is the Laplace operator, k ppll and k ipll These are the proportional and integral parameters of the phase-locked loop proportional-integral controller, v fq This represents the q-axis component of the AC voltage at the receiving end of the grid.

[0072] The frequency increments of network control and network tracking control are fused through the fusion coefficient k. h Weighted angular velocity is obtained The calculation formula is as follows:

[0073]

[0074] Where ω0 is the rated value of the angular frequency of the internal potential of the receiving-end MMC, k h The fusion coefficient is... and These are the frequency increments generated by the network construction and network tracking controls, respectively.

[0075] Hybrid synchronization angle θ h By measuring the mixed angular velocity The integral yields the following formula:

[0076] .

[0077] 3. Current reference value generation module

[0078] Reference value of d-axis current i at the receiving end MMC grid connection point dref and q-axis current reference value i qref Generated by the current reference value generation module, such as Figure 4 As shown.

[0079] The active and reactive power control loops of the grid voltage outer loop generate d-axis current reference values ​​for grid-connected control, respectively. and q-axis current reference value The reactive-voltage link in QU generates the internal electromotive force E for grid control, and the calculation formula is as follows:

[0080]

[0081] in, This represents the amplitude of the no-load internal potential. Here, s is the reactive power loop integral parameter, s is the Laplace operator, and Q is the actual reactive power value at the receiving end grid connection point. ref This is a reference value for reactive power control. This is the reactive power-voltage droop factor. The rated voltage amplitude of grid-connected AC voltage. This refers to the amplitude of the grid-connected AC voltage.

[0082] Based on the internal potential E and the d-axis component of the AC voltage at the receiving end grid connection point... and q-axis components The d-axis current reference value for network control is generated through a virtual impedance control loop. and q-axis current reference value :

[0083]

[0084] Among them, R v and L v These are virtual resistance and virtual reactance, respectively. This represents the actual value of the AC power grid frequency at the receiving end.

[0085] The network control branch and the network construction control branch are connected by a weight coefficient k. h The weighted fusion yields the d-axis current reference value i at the receiving-end MMC grid connection point. dref and q-axis current reference value i qref The calculation formula is as follows:

[0086] .

[0087] 4. Inner current loop module

[0088] Reference value of d-axis voltage v of the differential mode voltage of the receiving end MMC cdref and q-axis voltage reference value v cqref Generated by the inner current loop module, the calculation formula is as follows:

[0089]

[0090] Among them, v cdref and v cqref These are the d-axis and q-axis voltages of the differential-mode voltage of the receiving-end MMC, respectively; i d and i q These are the d-axis current and q-axis current of the receiving-end MMC output current, respectively; L represents the actual value of the AC power grid frequency at the receiving end. f For the receiving end MMC connection reactance parameter; k pi and kii These are the proportional and integral parameters of the current inner-loop proportional-integral controller, respectively; i dref and i qref These are the d-axis current reference value and q-axis current reference value for the MMC output current at the receiving end, respectively.

[0091] 5. Modulation module

[0092] Reference value v of the modulation voltage of the receiving end MMC in the abc coordinate system cref The d-axis voltage reference value v of the differential mode voltage of the receiving end MMC cdref and q-axis voltage reference value v cqref Obtained through dq / abc coordinate transformation. Based on the modulation voltage v of the receiving-end MMC. cref By utilizing pulse width modulation, corresponding trigger signals are generated for each switching device in the MMC, thereby realizing hybrid control of the flexible DC receiving end and the network structure.

[0093] Adaptive adjustment of fusion coefficient

[0094] This invention uses the Sigmoid function curve to realize the fusion coefficient k h Adaptive adjustment, establishing fusion coefficient k h Quantitative relationship with grid short-circuit ratio (SCR), such as Figure 5 As shown, its expression is:

[0095]

[0096]

[0097] SCR weak and SCR stiff These are the lower and upper limits of SCR under extremely weak and extremely strong power grids, respectively, and α is the curve slope coefficient.

[0098] When the strength of the external power grid changes, the integrated control follows Figure 5 The rules shown are adaptively adjusted, specifically:

[0099] 1) Extremely weak power grid (SCR ≤ SCR) weak In ), k h = 0, the network characteristics have reached saturation.

[0100] 2) SCR weak ≤ SCR ≤ SCR stiff At that time, k h Adjustments are made based on the Sigmoid function curve. When the SCR is low, the fusion control almost entirely characterizes the meshing properties; as the SCR increases, k... h The curve will increase, the network characteristics will gradually strengthen and eventually tend to saturate.

[0101] 3) When the grid strength is further enhanced to become an extremely strong grid (SCR ≥ SCRstiff), k h = 1, the network characteristics have reached saturation.

[0102] 4) When SCR decreases, k h As the curve decreases, the network characteristics gradually weaken, and the network construction characteristics gradually approach saturation.

[0103] Simulation verification

[0104] To verify the technical effects of this invention, this embodiment uses a ±525kV / 2100MW offshore wind power flexible direct transmission grid-connected system as the application object for simulation verification. The system topology is as follows: Figure 1 As shown in the diagram. In this system, the sending-end MMC uses VF control to establish the voltage and frequency on the offshore wind farm side, while the receiving end is based on a grid-connected / grid-connected hybrid control converter. Its control principle block diagram is shown below. Figure 2 As shown in Table 1, the main parameters used in this embodiment and their values ​​are as follows.

[0105] Table 1 Main Parameters

[0106] Parameter name Value <![CDATA[Power grid voltage U s / kV]]> 525 <![CDATA[DC voltage U dc / kV]]> 1050 <![CDATA[Submodule capacitor voltage U sm / kV]]> 2.2 Number of submodules in a single bridge arm N 480 <![CDATA[Submodule capacitance value C / mF]]> 9 <![CDATA[Arm inductor L MMC / mH]]> 100 <![CDATA[Virtual resistance R v / Ω]]> 2 <![CDATA[Virtual reactance L v / mH]]> 6 <![CDATA[Strong grid critical value SCR stiff > 16 <![CDATA[Extremely weak power grid critical value SCR weak > 1.6

[0107] First, the initial SCR value is set to 16, and then the SCR is decreased to 10, 6, and 1.6 at 0.5s, 1s, and 1.5s respectively. The active power curves of the receiving-end MMC using single grid-following control and the grid-following-network hybrid control proposed in this invention are shown below. Figure 6 The blue dashed line and red solid line in the diagram illustrate this. It can be seen that when the SCR is high, the converter can operate stably; however, as the SCR decreases, disturbances cause increasingly noticeable power fluctuations in the converter. When the SCR drops to 1.6, meaning the AC grid becomes an extremely weak grid, traditional grid-connected control cannot maintain the converter's stability. In contrast, the grid-connected-grid hybrid control proposed in this invention can maintain a stable output power from the converter throughout the entire SCR reduction process.

[0108] On the other hand, the initial SCR value was set to 1.6, and then increased to 6, 10, and 16 at 0.5s, 1s, and 1.5s respectively. The converter active power curves under grid-based control and fusion control are shown below. Figure 7The blue dashed line and red solid line in the diagram illustrate this. It can be seen that when the SCR is low, the converter can operate stably; however, as the SCR increases, disturbances cause increasingly noticeable power fluctuations in the converter, and grid control struggles to eliminate low-frequency power oscillations in a short time. When the SCR reaches 16, meaning the AC side becomes an extremely strong grid, grid control cannot maintain the converter's stability. However, the grid-connection hybrid control proposed in this invention can maintain a stable output power from the converter throughout the entire SCR reduction process.

[0109] The simulation results show that the offshore wind power flexible direct transmission grid connection system based on grid-connection and grid-building hybrid control converter proposed in this invention adaptively exhibits grid-connection or grid-building characteristics according to changes in grid strength, and has good adaptability to grids of different strengths, thus verifying the correctness and effectiveness of the system.

[0110] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A flexible DC transmission and grid connection system for offshore wind power based on a grid-connected hybrid control converter, comprising an offshore wind farm, a sending-end MMC, a DC transmission submarine cable, a receiving-end grid-connected hybrid control converter, and an onshore AC power grid, characterized in that, The receiving-end grid-network hybrid control converter includes: The data acquisition and transformation module is used to acquire the AC voltage and current at the receiving-end MMC grid connection point, which is the connection point between the receiving-end MMC and the receiving-end AC power grid. It performs dq transformation on the AC voltage and current at the receiving-end MMC grid connection point to obtain the d-axis and q-axis components of the voltage and current at the receiving-end MMC grid connection point in the dq rotating coordinate system. Based on the grid connection point voltage and current, it calculates the actual active power value P. It also acquires the DC voltage output by the receiving-end MMC, the voltage at the receiving-end MMC grid connection point, the actual value Q of the reactive power, and the reference value Q. ref ; The synchronization angle fusion module is used to determine the active power based on the actual active power value P and the active power reference value P generated by the additional DC voltage control. ref Frequency increments for generating network control The frequency increment for grid-connected control is generated based on the AC voltage at the receiving-end MMC grid connection point. The frequency increment of the network control Frequency increments with network control Through the fusion coefficient k h Weighted fusion is performed to obtain the mixed angular velocity. For mixed angular velocity The hybrid synchronization angle θ of the receiving-end MMC is obtained by integration. h ; The current reference value generation module is used to generate current reference values ​​based on the actual value Q of the receiving-end MMC grid connection point voltage and reactive power, and the reference value Q. ref Generate d-axis current reference values ​​for grid control respectively. and q-axis current reference value and the d-axis current reference value for network control. and q-axis current reference value The d-axis current reference value of the grid control branch will be used. q-axis current reference value Reference value of d-axis current of the network control branch q-axis current reference value Through the fusion coefficient k h Weighted fusion is performed to obtain the d-axis current reference value i at the receiving-end MMC grid connection point. dref and q-axis current reference value i qref ; The inner current loop module is used to determine the d-axis current reference value i. dref and q-axis current reference value i qref Combined with the d-axis component i of the received-end MMC output current d and q-axis component i q and mixed angular velocity The d-axis voltage reference value v is generated by the inner current loop to obtain the differential mode voltage of the MMC at the receiving end. cdref and q-axis voltage reference value v cqref ; The modulation module is used to convert the d-axis voltage reference value v of the differential mode voltage of the receiving-end MMC. cdref and q-axis voltage reference value v cqref By transforming the dq / abc coordinates, the reference value v of the modulation voltage of the receiving-end MMC in the abc coordinate system is obtained. cref And based on the reference value v of the modulation voltage cref The corresponding pulse signal is generated by using pulse width modulation.

2. The offshore wind power flexible direct transmission and grid connection system based on a grid-connected hybrid control converter according to claim 1, characterized in that, In the synchronization angle fusion module, the frequency increment of the network control It is generated by the power synchronization loop based on the active power deviation, and the calculation formula is: Where s is the Laplace operator, J is the virtual inertia coefficient, D is the damping coefficient, ω0 is the rated value of the angular frequency of the internal potential of the receiving-end MMC, and P is the actual power value at the receiving-end grid connection point. ref This is a reference value for active power.

3. The offshore wind power flexible direct transmission and grid connection system based on a grid-connected hybrid control converter according to claim 1, characterized in that, In the synchronization angle fusion module, the frequency increment of the network control is as follows. It is generated by the phase-locked loop based on the q-axis component of the AC voltage at the receiving end of the grid. The calculation formula is: Where s is the Laplace operator, k ppll and k ipll These are the proportional and integral parameters of the phase-locked loop proportional-integral controller, v fq This represents the q-axis component of the AC voltage at the receiving end of the grid.

4. The offshore wind power flexible direct transmission and grid connection system based on a grid-connected hybrid control converter according to claim 1, characterized in that, In the synchronized angle fusion module, the mixed angular velocity The calculation formula is: Where ω0 is the rated value of the angular frequency of the internal potential of the receiving-end MMC, k h The fusion coefficient is... and These are the frequency increments generated by network construction and network tracking control, respectively; The hybrid synchronization angle θ h The calculation formula is: 。 5. The offshore wind power flexible direct transmission and grid connection system based on a grid-connected hybrid control converter according to claim 1, characterized in that, In the current reference value generation module, the d-axis current reference value of the network control is... and q-axis current reference value The generation method is as follows: An internal potential E is generated through the QU reactive-voltage link, and then based on the internal potential E and the d-axis component of the AC voltage at the receiving end grid connection point... and q-axis components It is generated through a virtual impedance control circuit, and the calculation formula is: Among them, R v and L v These are virtual resistance and virtual reactance, respectively. This represents the actual value of the AC power grid frequency at the receiving end.

6. The offshore wind power flexible direct transmission and grid connection system based on a grid-connected hybrid control converter according to claim 5, characterized in that, The formula for calculating the internal potential E is: in, This represents the amplitude of the no-load internal potential. Here, s is the reactive power loop integral parameter, s is the Laplace operator, and Q is the actual reactive power value at the receiving-end grid connection point. ref This is a reference value for reactive power control. This is the reactive power-voltage droop factor. This refers to the rated voltage amplitude of grid-connected AC voltage. This refers to the amplitude of the grid-connected AC voltage.

7. The offshore wind power flexible direct transmission and grid connection system based on a grid-connected hybrid control converter according to claim 1, characterized in that, In the current reference value generation module, the d-axis current reference value i at the receiving end MMC grid connection point dref and q-axis current reference value i qref The calculation formula is: 。 8. The offshore wind power flexible direct transmission and grid connection system based on a grid-connected hybrid control converter according to claim 1, characterized in that, In the current inner loop module, the d-axis voltage reference value v of the differential mode voltage of the receiving-end MMC is... cdref and q-axis voltage reference value v cqref Generated by the inner current loop, the calculation formula is: Among them, v cdref and v cqref These are the d-axis and q-axis voltages of the differential-mode voltage of the receiving-end MMC, respectively; i d and i q These are the d-axis current and q-axis current of the receiving-end MMC output current, respectively; L represents the actual value of the AC power grid frequency at the receiving end. f For the receiving end MMC connection reactance parameter; k pi and k ii These are the proportional and integral parameters of the current inner-loop proportional-integral controller, respectively; i dref and i qref These are the d-axis current reference value and q-axis current reference value for the MMC output current at the receiving end, respectively.

9. A flexible direct-to-grid transmission system for offshore wind power based on a grid-connected hybrid control converter as described in claim 1, characterized in that, The fusion coefficient k h Adaptive adjustment is achieved using the Sigmoid function curve, and a fusion coefficient k is established. h The quantitative relationship between the short-circuit ratio (SCR) and the grid short-circuit ratio is expressed as follows: SCR weak and SCR stiff These are the lower and upper limits of SCR under extremely weak and extremely strong power grids, respectively, and α is the curve slope coefficient.

10. A flexible direct-to-grid transmission system for offshore wind power based on a grid-connected hybrid control converter according to claim 9, characterized in that, The fusion coefficient k h The following adaptive adjustment rule applies to changes in the grid short-circuit ratio (SCR): When SCR ≤ SCR weak At that time, k h = 0, the network characteristics have reached saturation; When SCR weak ≤ SCR ≤ SCR stiff At that time, k h Adjustments are made based on the Sigmoid function curve; when SCR is low, the fusion control is characterized by meshing characteristics, and as SCR increases, k... h As the curve increases, the mesh characteristics gradually strengthen and eventually tend to saturate; When SCR ≥ SCR stiff At that time, k h = 1, the network characteristics have reached saturation; When SCR decreases, k h As the curve decreases, the following properties gradually weaken, while the network-forming properties gradually strengthen and eventually tend to saturate.

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