All-dc offshore wind power system based on networked dc fan

Abstract

The full direct current offshore wind power system based on network type direct current fan relates to the field of model construction of full direct current offshore wind power system. The existing full direct current OWP system has the problems of too large volume and weight. The full direct current offshore wind power system based on network type direct current fan and uncontrolled direct current transformer is realized, the uncontrolled direct current transformer (U-DCT) is operated in the open loop control mode of the maximum modulation ratio, the internal alternating current voltage can be maximized, the sub-module capacitor value and conduction loss are reduced, and the economy of the full direct current offshore wind power system is effectively improved; the use weight and installation volume of the capacitor device are greatly reduced; the application of the full direct current offshore wind power system is mainly used in the field of offshore wind power.

Description

Technical Field

[0001] This invention relates to the field of model building for all-DC offshore wind power systems. Background Technology

[0002] In recent years, offshore wind power (OWP) has experienced rapid development due to its abundant resources, small footprint, and proximity to coastal load centers. Currently, high-voltage direct current (HVDC) transmission systems based on voltage source converters (VSCs), coupled with AC collection networks, have become the mainstream solution for transmitting offshore wind power. However, with the continuous expansion of wind turbine (WT) capacity and wind farm scale, traditional AC collection methods are gradually showing limitations in power collection capacity, and increased losses due to the large capacitive charging current. Furthermore, offshore converter stations and platforms also face challenges such as large size, heavy weight, and high cost.

[0003] In contrast, an all-DC OWP system employing medium-voltage DC (MVDC) collection and HVDC transmission eliminates reactive power issues, thereby extending collection distances and reducing cable costs. Furthermore, offshore converter stations can utilize DC transformers (DCTs) equipped with internal intermediate frequency transformers to reduce the size and weight of the offshore platform.

[0004] However, current all-DC OWP systems still face many challenges, including the high cost of marine DCTs due to their large size and weight. Therefore, these issues urgently need to be addressed. Summary of the Invention

[0005] The purpose of this invention is to address the problem of excessive size and weight in existing all-DC OWP systems. This invention provides an all-DC offshore wind power system based on a grid-type DC wind turbine.

[0006] A fully DC offshore wind power system based on grid-type DC wind turbines includes at least one grid-type DC wind turbine for converting offshore wind energy into electrical energy.

[0007] Each grid-type DC wind turbine is equipped with a corresponding grid-type controller, which is based on the active power reference value. Generate control signals To adjust the output power and medium-voltage DC voltage of the corresponding grid-type DC fan;

[0008] All grid-type DC wind turbines are connected in parallel to the medium-voltage DC bus via DC submarine cables;

[0009] An uncontrolled DC transformer, whose low-voltage side is connected to the medium-voltage DC bus and whose high-voltage side is connected to the offshore high-voltage DC transmission line, is used to step up the medium-voltage DC voltage to the high-voltage DC voltage.

[0010] The uncontrolled DC transformer operates in open-loop control mode with maximum modulation ratio;

[0011] An onshore converter station is connected between the offshore high-voltage direct current transmission line and the onshore AC power grid, and is used to regulate the voltage of the offshore high-voltage direct current transmission line.

[0012] Preferably, the uncontrolled DC transformer includes a modular multilevel converter (MMC2), an AC filter, an AC transformer, a diode rectifier (DR), and an open-loop controller.

[0013] The modular multilevel converter MMC2 has its DC side connected to the medium-voltage DC bus, and this DC side serves as the low-voltage side of an uncontrolled DC transformer.

[0014] An AC filter is installed on the AC side of the modular multilevel converter MMC2 and the low-voltage side of the AC transformer.

[0015] An AC transformer, the high-voltage side of which is connected to the AC side of a diode rectifier DR;

[0016] The DC side of the diode rectifier DR is connected to the offshore high-voltage DC transmission line as the high-voltage side of the uncontrolled DC transformer.

[0017] Open-loop controller, used to generate open-loop control signals Open-loop control is performed on the modular multilevel converter MMC2.

[0018] Preferably, the open-loop controller generates the open-loop control signal. The implementation method is as follows:

[0019] The circulating current suppression controller is based on the internal circulating current of the modular multilevel converter MMC2. Generate common-mode modulation ratio ;

[0020] The q-axis modulation ratio of the modular multilevel converter MMC2 Set to 0;

[0021] When the high-voltage side voltage of the uncontrolled DC transformer exceeds a preset threshold, the modular multilevel converter (MMC2) is determined to be in ZVRT operation, and the d-axis modulation ratio is adjusted. Set to 0.85; otherwise, ensure the modular multilevel converter MMC2 is operating normally and adjust the d-axis modulation ratio. Set to 1;

[0022] Based on the AC frequency of the modular multilevel converter MMC2 Calculate the phase angle ;

[0023] right , and Perform an inverse coordinate transformation to obtain the modulation ratio vector composed of the modulation ratios of the three axes a, b, and c. ;

[0024] The recent level modulation and submodule capacitor voltage equalization module is used to adjust the voltage according to the current level. and The upper and lower arm modulation reference voltages of the modular multilevel converter (MMC2) are generated, and the upper and lower arm modulation reference voltages are processed using nearest-level modulation and submodule capacitor voltage equalization techniques to generate control signals. .

[0025] Preferably, the phase angle is calculated. The implementation method is as follows:

[0026] right After integration, the data is sent to the modulus extraction module.

[0027] The modulus module processes the received integration results and Perform a modulo operation, and use the result as the phase angle. .

[0028] Preferably, the open-loop control mode for the maximum modulation ratio is a unit modulation ratio operation mode, and the q-axis modulation ratio of the modular multilevel converter MMC2 is... Set to 0, d-axis modulation ratio Set to 1 to maximize the AC voltage inside the uncontrolled DC transformer.

[0029] Preferably, the AC power output from the permanent magnet synchronous generator in the grid-type DC wind turbine is converted into low-voltage DC power by the rectifier VSC2, and then the low-voltage DC power is converted into medium-voltage DC power by the controllable DC transformer and sent to the medium-voltage DC bus.

[0030] The controllable DC transformer includes an inverter VSC1, a step-up transformer, and a modular multilevel converter MMC1. The inverter VSC1 converts the low-voltage DC power output from the rectifier VSC2 into AC power, which is then stepped up by the step-up transformer and sent to the modular multilevel converter MMC1.

[0031] The network-type controller generates corresponding control signals based on the active power reference value. The power and medium-voltage DC output of the modular multilevel converter MMC1 are adjusted and used as the power and medium-voltage DC output of the grid-type DC wind turbine, respectively.

[0032] Preferably, each grid-type controller includes an active power control loop module, a reactive power control loop module, a current loop module, a DC voltage loop module, a circulating current suppression controller, and a nearest-level modulation and sub-module capacitor voltage equalization module.

[0033] The active power control loop module is used to control the active power based on the active power reference value. The power output of the modular multilevel converter MMC1 and the initial reference voltage of the modular multilevel converter MMC1. Generate medium-voltage DC reference voltage ;

[0034] DC voltage loop module, used to base on medium-voltage DC reference voltage and the medium-voltage DC output of the modular multilevel converter MMC1 Generate d-axis reference current ;

[0035] The reactive power control loop module is used to control reactive power based on the reference value. Reactive power of the modular multilevel converter MMC1 Generate q-axis reference current ;

[0036] The current loop module is used to control the current through inner loop technology and in combination with... , , and Generate a modulation ratio vector consisting of the modulation ratios of axes a, b, and c. The and These are the actual d-axis and q-axis currents of the modular multilevel converter MMC1, respectively.

[0037] A circulating current suppression controller is used to suppress the internal circulating current of the modular multilevel converter MMC1. Generate common-mode modulation ratio ;

[0038] The recent level modulation and submodule capacitor voltage equalization module is used to adjust the voltage according to the current level. and The modulated wave reference voltages of the upper and lower arms of the modular multilevel converter (MMC1) are generated. These reference voltages are then processed using nearest-level modulation and submodule capacitor voltage equalization techniques to generate control signals. .

[0039] Preferably, a medium-voltage DC reference voltage is generated. The implementation method is as follows:

[0040] The MPPT algorithm generates an active power reference value based on the current wind speed. ;

[0041] Calculate the reference value of active power With the power output of the modular multilevel converter MMC1 The deviation, after being adjusted by a PI controller, yields the increment of the medium-voltage DC voltage. With the initial DC reference voltage After superposition, the superposition result is used as the medium-voltage DC reference voltage. .

[0042] Preferably, a d-axis reference current is generated. The implementation method is as follows:

[0043] Calculate the medium-voltage DC reference voltage The deviation between the medium-voltage DC output of the modular multilevel converter MMC1 and the d-axis reference current is obtained after adjusting the deviation through a PI regulator. .

[0044] Preferably, a q-axis reference current is generated. The implementation method is as follows:

[0045] Calculate reactive power reference value Reactive power of the modular multilevel converter MMC1 The q-axis reference current is obtained by adjusting the PI controller to determine the q-axis deviation. .

[0046] The beneficial effects of this invention are:

[0047] This invention proposes an integrated grid-type DC wind turbine and an uncontrolled DC transformer (U-DCT) for a fully DC offshore wind power system. The proposed U-DCT operates in an open-loop control mode with maximum modulation ratio, maximizing its internal AC voltage and thus reducing submodule capacitance and conduction losses. This effectively improves the economics of the fully DC offshore wind power system and significantly reduces the weight and installation volume of capacitor components. Specifically, it reduces the peak-to-peak energy fluctuation of the bridge arm by 36.4% (from 0.96MJ to 0.61MJ), and the submodule capacitance is reduced from 23.9mF in the traditional scheme to 15.2mF, a reduction of 36.4%. Furthermore, the energy density of the submodule capacitors in the modular multilevel converter (MMC2) on the medium-voltage side of the U-DCT is reduced to 4.6kJ / MVA, far below the typical value of 10kJ / MVA for a 200Hz MMC system, significantly reducing the weight and installation volume of capacitor components.

[0048] A grid-type controller was designed to control the grid-type DC fan, which can simultaneously adjust the medium-voltage DC output and output power of a single grid-type DC fan, thereby improving the system's steady-state operation stability.

[0049] This invention also significantly enhances the system's zero-voltage ride-through capability, avoids the risk of fault overvoltage, can quickly interrupt power transmission, and effectively suppresses HVDC overvoltage: the proposed ZVRT scheme detects HVDC voltage, and when applied... When the voltage exceeds 1.05 pu, the MMC2 modulation ratio is reduced from 1 to 0.85, causing the diode rectifier (DR) to be reverse biased 50 ms in advance, quickly interrupting the power transmission from MVDC to HVDC. The HVDC overvoltage is limited to 1.1 pu, which is significantly reduced compared to 1.25 pu without this solution, completely avoiding the risk of system shutdown and equipment damage caused by voltage exceeding 1.2 pu. Attached Figure Description

[0050] Figure 1 This is a schematic diagram of the principle of the all-DC offshore wind power system based on a grid-type DC wind turbine as described in this invention; wherein, This refers to the DC voltage on the medium-voltage side of the uncontrolled DC transformer, which is also the DC voltage on the medium-voltage side of the modular multilevel converter MMC2. This refers to the high-voltage side DC voltage of an uncontrolled DC transformer.

[0051] Figure 2 This is a structural schematic diagram of a grid-type DC fan; among which, This refers to the DC side voltage of rectifier VSC2. The DC voltage of the modular multilevel converter MMC1;

[0052] Figure 3 This is a control strategy diagram for an all-DC offshore wind power system based on grid-connected DC wind turbines; among which...

[0053] Figure 3 (a) is a schematic diagram of the control principle of an uncontrolled DC transformer;

[0054] Figure 3 (b) is a schematic diagram of the control principle of a grid-type DC fan;

[0055] Figure 4 This is a waveform diagram of the wind power output variation in the all-DC offshore wind power system based on a grid-type DC wind turbine, according to the present invention; wherein,

[0056] Figure 4 (a) shows a schematic diagram of the changes in active power output from three grid-type DC wind turbines WT1, WT2 and WT3;

[0057] Figure 4 (b) shows the changes in medium-voltage DC voltage of the three grid-type DC wind turbines WT1, WT2 and WT3 and the modular multilevel converter MMC2 during the period of change in the active power of the three grid-type DC wind turbines.

[0058] Figure 4 (c) shows the changes in medium-voltage DC current of WT1, WT2 and WT3 during the period of change in active power of the three grid-type DC wind turbines;

[0059] Figure 4 (d) represents a schematic diagram of the high-voltage DC voltage change of the onshore converter station during the period of change in the active power of the three grid-type DC wind turbines.

[0060] Figure 5 This is a waveform diagram of the modular multilevel converter MMC2 under rated operating conditions; among which,

[0061] Figure 5 (a) shows a schematic diagram of the arm energy variation of the modular multilevel converter MMC2;

[0062] Figure 5 (b) shows a schematic diagram of the voltage variation of the submodule capacitors in the modular multilevel converter MMC2;

[0063] Figure 6 This is a comparative diagram showing the system employing and not employing the ZVRT strategy during an onshore power grid failure; among which,

[0064] Figure 6 (a) is a schematic diagram showing the variation of AC voltage amplitude in the onshore power grid;

[0065] Figure 6 (b) is a schematic diagram of the change in AC current amplitude at the onshore converter station;

[0066] Figure 6 (c) is a schematic diagram of the high-voltage direct current voltage change at the onshore converter station;

[0067] Figure 6 (d) is a schematic diagram of the power variation of the modular multilevel converter MMC2;

[0068] Figure 6 (e) is a schematic diagram of the AC voltage amplitude variation of the modular multilevel converter MMC2;

[0069] Figure 6 (f) is a schematic diagram of the medium-voltage DC voltage variation of the modular multilevel converter MMC1;

[0070] Figure 6 (g) is a schematic diagram of the change in high-voltage direct current at the onshore converter station;

[0071] Figure 6 The diagram in (h) shows the energy changes absorbed by the uncontrolled DC transformer during a fault. Detailed Implementation

[0072] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0073] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0074] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0075] Specific Implementation Method 1: Combination Figure 1 This embodiment describes a fully DC offshore wind power system based on a grid-type DC wind turbine, which includes at least one grid-type DC wind turbine for converting offshore wind energy into electrical energy.

[0076] Each grid-type DC wind turbine is equipped with a corresponding grid-type controller, which is based on the active power reference value. Generate control signals To adjust the output power and medium-voltage DC voltage of the corresponding grid-type DC fan;

[0077] All grid-type DC wind turbines are connected in parallel to the medium-voltage DC bus via DC submarine cables;

[0078] An uncontrolled DC transformer (U-DCT) has its low-voltage side connected to the medium-voltage DC bus and its high-voltage side connected to the offshore high-voltage DC transmission line, used to step up the medium-voltage DC voltage to the high-voltage DC voltage.

[0079] The uncontrolled DC transformer (U-DCT) operates in open-loop control mode with maximum modulation ratio;

[0080] An onshore converter station is connected between the offshore high-voltage direct current transmission line and the onshore AC power grid, and is used to regulate the voltage of the offshore high-voltage direct current transmission line.

[0081] The uncontrolled DC transformer (U-DCT) of this invention operates in the open-loop control mode with the maximum modulation ratio, which can maximize its internal AC voltage. In other words, the modular multilevel converter (MMC2) inside the uncontrolled DC transformer (U-DCT) operates in the maximum AC voltage output state. While simplifying the complexity of the control system, it achieves lower power loss and smaller sub-module capacity, significantly reducing the size and weight of the offshore platform, and effectively improving the economy and reliability of the all-DC offshore wind power system based on grid-type DC wind turbines.

[0082] See Figure 3 In section (a), it is further shown that the uncontrolled DC transformer (U-DCT) includes a modular multilevel converter MMC2, an AC filter, an AC transformer, a diode rectifier DR, and an open-loop controller;

[0083] The modular multilevel converter MMC2 has its DC side connected to the medium-voltage DC bus, and this DC side serves as the low-voltage side of the uncontrolled DC transformer (U-DCT).

[0084] An AC filter is installed on the AC side of the modular multilevel converter MMC2 and the low-voltage side of the AC transformer.

[0085] An AC transformer, the high-voltage side of which is connected to the AC side of a diode rectifier DR;

[0086] The DC side of the diode rectifier DR is connected to the offshore high-voltage DC transmission line as the high-voltage side of the uncontrolled DC transformer (U-DCT).

[0087] Open-loop controller, used to generate open-loop control signals Open-loop control is performed on the modular multilevel converter MMC2.

[0088] The uncontrolled DC transformer (U-DCT) organically integrates a modular multilevel converter (MMC2), an AC filter, an AC transformer, a diode rectifier (DR), and an open-loop controller. This achieves efficient power conversion from a medium-voltage DC bus to a high-voltage DC transmission line at sea. The diode rectifier (DR) serves as the high-voltage side connected to the offshore high-voltage DC transmission line. The AC transformer provides electrical isolation and voltage matching between high and low voltage levels. The AC filter improves the power quality on the AC side of the MMC2. The open-loop controller provides stable open-loop control of the MMC2. The DC side of the MMC2 serves as the low-voltage side connected to the medium-voltage DC bus. The overall structure is simple, control is straightforward, and reliability is high. It can stably perform uncontrolled voltage boosting and power transmission from medium-voltage DC to high-voltage DC, meeting the application requirements of offshore DC transmission systems.

[0089] See Figure 3 In (a), the open-loop control mode for the maximum modulation ratio is the unit modulation ratio operation mode, and the q-axis modulation ratio of the modular multilevel converter MMC2 is... Set to 0, d-axis modulation ratio Set to 1 to maximize the AC voltage inside the uncontrolled DC transformer (U-DCT).

[0090] For details, see Figure 3 In section (a), the open-loop controller generates open-loop control signals. The implementation method is as follows:

[0091] The circulating current suppression controller is based on the internal circulating current of the modular multilevel converter MMC2. Generate common-mode modulation ratio ;

[0092] The q-axis modulation ratio of the modular multilevel converter MMC2 Set to 0;

[0093] When the high-voltage side voltage of the uncontrolled DC transformer (U-DCT) exceeds a preset threshold, the modular multilevel converter (MMC2) is determined to be in ZVRT operation, and the d-axis modulation ratio is adjusted. Set to 0.85; otherwise, ensure the modular multilevel converter MMC2 is operating normally and adjust the d-axis modulation ratio. Set to 1;

[0094] Based on the AC frequency of the modular multilevel converter MMC2 Calculate the phase angle ;

[0095] right , and Perform an inverse coordinate transformation to obtain the modulation ratio vector composed of the modulation ratios of the three axes a, b, and c. ;

[0096] The recent level modulation and submodule capacitor voltage equalization module is used to adjust the voltage according to the current level. and The upper and lower arm modulation reference voltages of the modular multilevel converter (MMC2) are generated, and the upper and lower arm modulation reference voltages are processed using nearest-level modulation and submodule capacitor voltage equalization techniques to generate control signals. .

[0097] The upper and lower bridge arm modulation reference voltages of the modular multilevel converter MMC2 are respectively , Specifically:

[0098] ;

[0099] Furthermore, calculate the phase angle. The implementation method is as follows:

[0100] right After integration, the data is sent to the modulus extraction module.

[0101] The modulus module processes the received integration results and Perform a modulo operation, and use the result as the phase angle. .

[0102] A major challenge facing all-DC offshore wind power systems is zero-voltage ride-through (ZVRT) operation. To achieve fault ride-through during onshore AC grid faults, this paper proposes an improved ZVRT method based on U-DCT open-loop control. When the U-DCT detects that the local high-voltage side voltage exceeds 1.05 pu through the hysteresis module, see [see...]. Figure 3 In (a) U-DCT, the MMC2 actively modulates the modulation ratio during normal operation. From unit value decreased to Here The design aims to ensure reliable reverse bias of the DR. By actively reducing the modulation ratio, the DR switches from the on state to reverse bias. Compared with maintaining a unit modulation ratio, it can interrupt power transmission to the HVDC line earlier, thereby mitigating overvoltage.

[0103] Further, see Figure 2 In the grid-type DC wind turbine, the AC power output by the permanent magnet synchronous generator is converted into low-voltage DC power by the rectifier VSC2, and then the low-voltage DC power is converted into medium-voltage DC power by the controllable DC transformer (DCT) and sent to the medium-voltage DC bus.

[0104] The controllable DC transformer (DCT) includes an inverter VSC1, a step-up transformer, and a modular multilevel converter MMC1. The inverter VSC1 converts the low-voltage DC power output from the rectifier VSC2 into AC power, which is then stepped up by the step-up transformer before being sent to the modular multilevel converter MMC1.

[0105] The network-type controller generates corresponding control signals based on the active power reference value. The power and medium-voltage DC output of the modular multilevel converter MMC1 are adjusted and used as the power and medium-voltage DC output of the grid-type DC wind turbine, respectively.

[0106] In this preferred embodiment, the grid-type DC wind turbine converts the AC power generated by the permanent magnet synchronous generator into low-voltage DC power via rectifier VSC2, and then the voltage is transformed by a controllable DC transformer (DCT) to finally output medium-voltage DC power and connect to the medium-voltage DC bus. The controllable DC transformer (DCT) consists of inverter VSC1, step-up transformer and modular multilevel converter MMC1. Inverter VSC1 first inverts the low-voltage DC power output from VSC2 into AC power, which is then stepped up by the step-up transformer and sent to MMC1. At the same time, the grid-type controller generates control signals based on the active power reference value to adjust the output power and medium-voltage DC voltage of MMC1, so that they serve as the output power and medium-voltage DC voltage of the grid-type DC wind turbine, respectively, to achieve stable power generation and DC voltage support of the DC wind turbine.

[0107] Further, see Figure 3 In (b), each network controller includes an active power control loop module, a reactive power control loop module, a current loop module, a DC voltage loop module, a circulating current suppression controller, and a recent level modulation and sub-module capacitor voltage equalization module;

[0108] The active power control loop module is used to control the active power based on the active power reference value. The power output of the modular multilevel converter MMC1 and the initial reference voltage of the modular multilevel converter MMC1. Generate medium-voltage DC reference voltage ;

[0109] DC voltage loop module, used to base on medium-voltage DC reference voltage and the medium-voltage DC output of the modular multilevel converter MMC1 Generate d-axis reference current The reactive power control loop module is used to control reactive power based on the reference value. Reactive power of the modular multilevel converter MMC1 Generate q-axis reference current ;

[0110] The current loop module is used to control the current through inner loop technology and in combination with... , , and Generate a modulation ratio vector consisting of the modulation ratios of axes a, b, and c. The and These are the actual d-axis and q-axis currents of the modular multilevel converter MMC1, respectively.

[0111] A circulating current suppression controller is used to suppress the internal circulating current of the modular multilevel converter MMC1. Generate common-mode modulation ratio ;

[0112] The recent level modulation and submodule capacitor voltage equalization module is used to adjust the voltage according to the current level. and The modulated wave reference voltages of the upper and lower arms of the modular multilevel converter (MMC1) are generated. These reference voltages are then processed using nearest-level modulation and submodule capacitor voltage equalization techniques to generate control signals. .

[0113] The upper and lower bridge arm modulation reference voltages of the modular multilevel converter MMC1 are respectively , Specifically:

[0114] ;

[0115] Further, see Figure 3 In section (b), a medium-voltage DC reference voltage is generated. The implementation method is as follows:

[0116] The MPPT algorithm generates an active power reference value based on the current wind speed. ;

[0117] Calculate the reference value of active power With the power output of the modular multilevel converter MMC1 The deviation, after being adjusted by a PI controller, yields the increment of the medium-voltage DC voltage. With the initial DC reference voltage After superposition, the superposition result is used as the medium-voltage DC reference voltage. .

[0118] Specifically, generate the d-axis reference current. The implementation method is as follows:

[0119] Calculate the medium-voltage DC reference voltage The deviation between the medium-voltage DC output of the modular multilevel converter MMC1 and the d-axis reference current is obtained after adjusting the deviation through a PI regulator. .

[0120] Specifically, generate the q-axis reference current. The implementation method is as follows:

[0121] Calculate the reference value of active power Reactive power of the modular multilevel converter MMC1 The q-axis reference current is obtained by adjusting the PI controller to determine the q-axis deviation. .

[0122] Verification experiment:

[0123] 1. Normal operation

[0124] The system under consideration consists of three lumped-grid DC wind turbines, designated WT1, WT2, and WT3, with a single unit rated power of 666.7 MW, and each located 20 km from the offshore converter station. Other key parameters are shown in Table 1.

[0125] Table 1. Main parameters of the all-DC offshore wind power system based on grid-type DC wind turbines of this invention.

[0126]

[0127] At the initial moment of the simulation, the all-DC offshore wind power system based on grid-type DC wind turbines of this invention is operating under rated conditions. Figure 4 The dynamic process and corresponding system operating status of three grid-type DC fans WT1, WT2 and WT3 are given, starting from the first second, when their output power decreases from 1 p.u. to 0.2 pu, 0.1 pu and 0 respectively within 200 milliseconds, and then recovers to the rated value in the second second.

[0128] Figure 4 Figure (a) shows the change in active power output of the three grid-type DC wind turbines WT1, WT2, and WT3. Starting from the rated state in the first second, their output power decreased from 1 p.u. to 0.2 p.u., 0.1 p.u., and 0 p.u., respectively, within 200 milliseconds. In the second second, the active power output of the three grid-type DC wind turbines WT1, WT2, and WT3 recovered to 1 p.u. The experimental results verify that the proposed grid-type controller can effectively control the active power of the grid-type DC wind turbines under different operating conditions.

[0129] Figure 4Figure (b) shows the changes in the medium-voltage DC voltage of the three grid-type DC wind turbines WT1, WT2, and WT3 and the modular multilevel converter MMC2 during the period of active power variation. When the three grid-type DC wind turbines are operating at rated power, the medium-voltage DC voltage of the modular multilevel converter MMC2 is 1 p.u., and the medium-voltage DC voltage of the three grid-type DC wind turbines WT1, WT2, and WT3 is 1.02 p.u. As the output active power decreases, the medium-voltage DC voltage of the modular multilevel converter MMC2 drops to 0.907 p.u., and the medium-voltage DC voltage of the three grid-type DC wind turbines WT1, WT2, and WT3 drops to 0.911, 0.909, and 0.907 p.u., respectively.

[0130] Figure 4 (c) represents the change in medium-voltage DC current of WT1, WT2, and WT3 during the period of change in active power of the three grid-type DC wind turbines. According to Figure 4 (b) The reduction in the medium-voltage DC of the grid-type DC fan resulted in a decrease in its medium-voltage DC current to 0.22 pu, 0.11 pu, and 0, respectively. Simulation results verify that the proposed grid-type controller can effectively regulate the medium-voltage DC of the grid-type DC fan under different operating conditions.

[0131] Figure 4 Figure (d) shows the change in the high-voltage direct current (HVDC) voltage of the onshore converter station during the period of change in the active power of the three grid-type DC wind turbines. This figure demonstrates that even when the active power output of the grid-type DC wind turbines changes, the onshore converter station can still ensure that its HVDC voltage remains stable at 1 p.u.

[0132] To verify that the present invention can solve the problems of large size and high construction cost of offshore converter stations, it is compared with the traditional method with a modulation ratio of 0.8.

[0133] Figure 5 This paper presents a comparison of the energy variation of the MMC2 bridge arm with the present invention and a conventional method with a modulation ratio of 0.8, assuming the submodule capacitor voltage changes are the same.

[0134] Figure 5 Figure (b) shows the voltage variation of the submodule capacitors in the modular multilevel converter MMC2. Figure 5Figure (a) illustrates the energy variation of the MMC2 arm of the modular multilevel converter. Compared to the traditional method with a modulation ratio of 0.8, the peak-to-peak energy of the MMC2 arm decreases from 0.94 MJ to 0.61 MJ, representing a 36.4% reduction in the peak-to-peak energy fluctuation (from 0.96 MJ to 0.61 MJ). Due to the positive correlation between capacitor energy and capacitance value, while maintaining the same range of submodule capacitor voltage variation, the reduction in peak-to-peak energy results in a corresponding decrease in the required submodule capacitor value. Therefore, while keeping the submodule capacitor voltage variation range constant (1.8 kV to 2.2 kV, such as...), the energy variation is significantly reduced. Figure 5 Under the premise shown in (b), the required submodule capacitance is reduced from 23.9mF to 15.2mF, a decrease of 36.4%, specifically as follows:

[0135] Submodule capacitor design based on instantaneous capacitor energy Its expression is as follows:

[0136]

[0137] in, For the equivalent bridge arm capacitance, Equivalent bridge arm capacitance Actual voltage, equivalent bridge arm capacitance actual voltage The maximum and minimum values ​​are respectively:

[0138]

[0139] in, The DC voltage for the modular multilevel converter MMC2, Capacitor voltage ripple

[0140] The instantaneous capacitance energy is respectively in the equivalent bridge arm capacitance actual voltage maximum value and minimum value Reaching its maximum value and minimum value Therefore, the peak-to-peak value of the energy change in each bridge arm is:

[0141]

[0142] Among them, capacitor voltage ripple Typically, the DC voltage on the medium-voltage side of the modular multilevel converter MMC2 is selected as 10% of the rated capacitor voltage, i.e., 0.2kV. Due to the peak-to-peak energy of the MMC2 bridge arm of the modular multilevel converter... The submodule capacitance decreased from 0.94 MJ to 0.61 MJ. It is given by the following formula:

[0143] ;

[0144] Therefore, while maintaining the same range of submodule capacitor voltage variation (1.8kV to 2.2kV), the required submodule capacitor is reduced from 23.9mF to 15.2mF, a reduction of 36.4%. This verifies that the all-DC offshore wind power system based on grid-type DC wind turbines of this invention effectively reduces the submodule capacitor, thereby reducing construction costs and helping to achieve a more compact offshore converter station design.

[0145] 2. ZVRT operation

[0146] To evaluate the proposed ZVRT capability, a three-phase metallic ground fault lasting up to 2.5 seconds was applied to the onshore AC grid at 2 seconds (e.g., Figure 1 As shown in the figure, the performance of the system with and without the proposed ZVRT scheme is compared and analyzed. Detailed results are as follows. Figure 6 As shown.

[0147] Figure 6 (a) and (b) show the AC voltage amplitude of the onshore grid and the AC current amplitude of the onshore converter station during the fault. The voltage of the onshore grid dropped sharply, causing the onshore converter station to be unable to transmit power to the onshore grid, while its AC current amplitude was limited to 1.2 pu.

[0148] Figure 6 (c) and (d) illustrate the HVDC voltage and power of the modular multilevel converter (MMC2) at the onshore converter station during the fault. 1) Without the proposed ZVRT scheme, the power surplus generated by the continuous power generation of the wind turbines leads to an increase in the HVDC voltage at the onshore converter station; 2) With the proposed ZVRT scheme, when the HVDC voltage of the uncontrolled rectifier DR rises to 1.05 pu, the modulation ratio of the modular multilevel converter (MMC2) actively decreases from a single value to 0.85, limiting the HVDC overvoltage at the onshore converter station to 1.1 pu, compared to 1.25 pu without the ZVRT scheme. This verifies that the proposed ZVRT scheme can effectively reduce the HVDC overvoltage during the fault, protecting the equipment from damage.

[0149] Figure 6(e) illustrates the AC voltage amplitude of the modular multilevel converter (MMC2) during the fault. This figure demonstrates that: 1) When the proposed ZVRT scheme is not used, due to the unit modulation ratio in the open-loop control, the DC voltage increases, simultaneously raising the AC voltage of the MMC2. As the DC voltage increases, the active power control loop module of the grid-type controller reaches saturation and sets the reference value of the medium-voltage DC voltage of the MMC1 to 1.05 pu. Therefore, the output power of the grid-type DC wind turbine gradually drops to zero at 2.07 seconds; 2) When the proposed ZVRT scheme is used, the modulation ratio of the MMC2 actively decreases from a unit value to 0.85, and the AC voltage of the MMC2 drops to 0.91 pu, thereby accelerating the reverse bias of the uncontrolled rectifier DR. DR enters the reverse bias state at 2.02 seconds, 50 ms earlier than when the proposed control scheme is not used.

[0150] Figure 6 (f) illustrates the medium-voltage DC voltage of the Modular Multilevel Converter (MMC1) during a fault. To maintain power generation, the grid-type controller used in the grid-connected DC wind turbine increases the medium-voltage DC voltage of the MMC1 until the active power control loop module of the grid-type controller reaches saturation, and sets the reference value of the medium-voltage DC voltage of the MMC1 to 1.05 pu. Under the action of the grid-type controller, the medium-voltage DC voltage of the MMC1 stabilizes at 1.05 pu during the fault. The simulation results verify that the proposed grid-type controller can ensure the stability of the medium-voltage DC voltage at sea and avoid overvoltage of the medium-voltage DC voltage under ZVRT operation.

[0151] Figure 6 (g) and (h) illustrate the onshore high-voltage direct current and the energy absorbed by the uncontrolled DC transformer during the fault. With the proposed ZVRT scheme, the faster reverse bias of the diode rectifier DR leads to a rapid reduction in the current of the high-voltage direct current line during the fault, decreasing the energy accumulated in the high-voltage direct current line from 60.5 MJ to 25.3 MJ, a reduction of 58.2%. This verifies that the proposed ZVRT scheme reduces the energy accumulated in the high-voltage direct current line during the fault, alleviates high-voltage direct current overvoltage, and accelerates the system recovery speed after fault clearance.

[0152] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A fully DC offshore wind power system based on grid-type DC wind turbines, characterized in that, Includes at least one grid-connected DC wind turbine for converting offshore wind energy into electrical energy; Each grid-type DC wind turbine is equipped with a corresponding grid-type controller, which is based on the active power reference value. Generate control signals To adjust the output power and medium-voltage DC voltage of the corresponding grid-type DC fan; All grid-type DC wind turbines are connected in parallel to the medium-voltage DC bus via DC submarine cables; An uncontrolled DC transformer, whose low-voltage side is connected to the medium-voltage DC bus and whose high-voltage side is connected to the offshore high-voltage DC transmission line, is used to step up the medium-voltage DC voltage to the high-voltage DC voltage. The uncontrolled DC transformer operates in open-loop control mode with maximum modulation ratio; An onshore converter station is connected between the offshore high-voltage direct current transmission line and the onshore AC power grid, and is used to regulate the voltage of the offshore high-voltage direct current transmission line.

2. The all-DC offshore wind power system based on grid-type DC wind turbines according to claim 1, characterized in that, The uncontrolled DC transformer includes a modular multilevel converter (MMC2), an AC filter, an AC transformer, a diode rectifier (DR), and an open-loop controller. The modular multilevel converter MMC2 has its DC side connected to the medium-voltage DC bus, and this DC side serves as the low-voltage side of an uncontrolled DC transformer. An AC filter is installed on the AC side of the modular multilevel converter MMC2 and the low-voltage side of the AC transformer. An AC transformer, the high-voltage side of which is connected to the AC side of a diode rectifier DR; The DC side of the diode rectifier DR is connected to the offshore high-voltage DC transmission line as the high-voltage side of the uncontrolled DC transformer. Open-loop controller, used to generate open-loop control signals Open-loop control is performed on the modular multilevel converter MMC2.

3. The all-DC offshore wind power system based on a grid-type DC wind turbine according to claim 2, characterized in that, The open-loop controller generates open-loop control signals. The implementation method is as follows: The circulating current suppression controller is based on the internal circulating current of the modular multilevel converter MMC2. Generate common-mode modulation ratio ; The q-axis modulation ratio of the modular multilevel converter MMC2 Set to 0; When the high-voltage side voltage of the uncontrolled DC transformer exceeds a preset threshold, the modular multilevel converter (MMC2) is determined to be in ZVRT operation, and the d-axis modulation ratio is adjusted. Set to 0.85; otherwise, ensure the modular multilevel converter MMC2 is operating normally and adjust the d-axis modulation ratio. Set to 1; Based on the AC frequency of the modular multilevel converter MMC2 Calculate the phase angle ; right , and Perform an inverse coordinate transformation to obtain the modulation ratio vector composed of the modulation ratios of the three axes a, b, and c. ; The recent level modulation and submodule capacitor voltage equalization module is used to adjust the voltage according to the current level. and The upper and lower arm modulation reference voltages of the modular multilevel converter (MMC2) are generated, and the upper and lower arm modulation reference voltages are processed using nearest-level modulation and submodule capacitor voltage equalization techniques to generate control signals. .

4. The all-DC offshore wind power system based on a grid-type DC wind turbine according to claim 3, characterized in that, Calculate the phase angle The implementation method is as follows: right After integration, the data is sent to the modulus extraction module. The modulus module processes the received integration results and Perform a modulo operation, and use the result as the phase angle. .

5. The all-DC offshore wind power system based on a grid-type DC wind turbine according to claim 2, characterized in that, The open-loop control mode for maximum modulation ratio is a unit modulation ratio operation mode, and the q-axis modulation ratio of the modular multilevel converter MMC2 is... Set to 0, d-axis modulation ratio Set to 1 to maximize the AC voltage inside the uncontrolled DC transformer.

6. The all-DC offshore wind power system based on a grid-type DC wind turbine according to claim 1, characterized in that, In a grid-type DC wind turbine, the AC power output from the permanent magnet synchronous generator is converted into low-voltage DC power by rectifier VSC2, and then the low-voltage DC power is converted into medium-voltage DC power by a controllable DC transformer and sent to the medium-voltage DC bus. The controllable DC transformer includes an inverter VSC1, a step-up transformer, and a modular multilevel converter MMC1. The inverter VSC1 converts the low-voltage DC power output from the rectifier VSC2 into AC power, which is then stepped up by the step-up transformer and sent to the modular multilevel converter MMC1. The network-type controller generates corresponding control signals based on the active power reference value. The power and medium-voltage DC output of the modular multilevel converter MMC1 are adjusted and used as the power and medium-voltage DC output of the grid-type DC wind turbine, respectively.

7. The all-DC offshore wind power system based on a grid-type DC wind turbine according to claim 6, characterized in that, Each network-type controller includes an active power control loop module, a reactive power control loop module, a current loop module, a DC voltage loop module, a circulating current suppression controller, and a nearest-level modulation and sub-module capacitor voltage equalization module; The active power control loop module is used to control the active power based on the active power reference value. The power output of the modular multilevel converter MMC1 and the initial reference voltage of the modular multilevel converter MMC1. Generate medium-voltage DC reference voltage ; DC voltage loop module, used to base on medium-voltage DC reference voltage and the medium-voltage DC output of the modular multilevel converter MMC1 Generate d-axis reference current ; The reactive power control loop module is used to control reactive power based on the reference value. Reactive power of the modular multilevel converter MMC1 Generate q-axis reference current ; The current loop module is used to control the current through inner loop technology and in combination with... , , and Generate a modulation ratio vector consisting of the modulation ratios of axes a, b, and c. The and These are the actual d-axis and q-axis currents of the modular multilevel converter MMC1, respectively. A circulating current suppression controller is used to suppress the internal circulating current of the modular multilevel converter MMC1. Generate common-mode modulation ratio ; The recent level modulation and submodule capacitor voltage equalization module is used to adjust the voltage according to the current level. and The modulated wave reference voltages of the upper and lower arms of the modular multilevel converter (MMC1) are generated. These reference voltages are then processed using nearest-level modulation and submodule capacitor voltage equalization techniques to generate control signals. .

8. The all-DC offshore wind power system based on a grid-type DC wind turbine according to claim 7, characterized in that, Generate medium-voltage DC reference voltage The implementation method is as follows: The MPPT algorithm generates an active power reference value based on the current wind speed. ; Calculate the reference value of active power With the power output of the modular multilevel converter MMC1 The deviation, after being adjusted by a PI controller, yields the increment of the medium-voltage DC voltage. With the initial DC reference voltage After superposition, the superposition result is used as the medium-voltage DC reference voltage. .

9. The all-DC offshore wind power system based on a grid-type DC wind turbine according to claim 7, characterized in that, Generate d-axis reference current The implementation method is as follows: Calculate the medium-voltage DC reference voltage The deviation between the medium-voltage DC output of the modular multilevel converter MMC1 and the d-axis reference current is obtained after adjusting the deviation through a PI regulator. .

10. The all-DC offshore wind power system based on a grid-type DC wind turbine according to claim 7, characterized in that, Generate q-axis reference current The implementation method is as follows: Calculate the reference value of reactive power Reactive power of the modular multilevel converter MMC1 The q-axis reference current is obtained by adjusting the PI controller to determine the q-axis deviation. .

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