Offshore wind power direct current collection circuit and low-voltage fault ride-through method thereof

Abstract

The invention relates to an offshore wind power direct current collection circuit and a low-voltage fault ride-through method thereof. The direct-driven fan is connected to the direct-current collection system through a fan AC / DC converter and a fan DC / DC converter, the fan AC / DC converter realizes maximum power point tracking of the fan, and the fan DC / DC converter controls the output voltage of the fan AC / DC converter to be stable; the energy storage DC / DC converter controls the energy storage device to absorb unbalanced power between the offshore direct current collection system and the shore alternating current power grid, and the voltage of the direct current collection bus is controlled to be stable in the low voltage ride-through period. And the power balancing unit is connected between the adjacent fan DC / DC converters or energy storage DC / DC converters, so that output voltage balancing of series ports and power output of the direct current collection system are realized. And under a low-voltage fault, the starting fan adopts an overspeed load shedding control strategy, and meanwhile, the working mode of the energy storage device is switched into a charging state to absorb unbalanced active power of the direct-current bus. Compared with the prior art, the method has the advantages of high autonomous balance, fast response, high reliability and the like.

Description

Technical Field

[0001] The present invention relates to the field of offshore wind power DC collection, transmission and control, and in particular to an offshore wind power DC collection circuit and a low voltage fault ride-through method thereof. Background Art

[0002] As offshore wind power develops towards deeper waters and larger scale, high-voltage direct current (HVDC) transmission systems have become the mainstream technology due to their advantages such as low transmission loss and large capacity. However, existing series-connected HVDC transmission systems face significant challenges in practical operation. First, when multiple DC wind turbines are connected in series to the DC bus via DC / DC converters, wind speed variations and fluctuations cause dynamic variations in turbine output power, leading to voltage imbalances at the series ports. Traditional control strategies, relying on fixed parameters or open-loop regulation, struggle to achieve timely autonomous voltage balancing, which can easily lead to localized overvoltage or component damage. Second, when a short circuit or other fault in the onshore grid causes a voltage drop, the onshore converter station's power acceptance capacity plummets, while the offshore wind turbines maintain their original power output due to inertia. This creates a severe system power imbalance, causing the DC bus voltage to rise sharply. Existing solutions often suppress voltage fluctuations through passive energy dissipation or single-link optimization. These solutions suffer from response lag, energy waste, and a lack of coordinated control, making it difficult to prevent turbine disconnection or even cascading failures. Furthermore, during low voltage ride-through (LVRT), traditional control strategies fail to coordinate wind turbine power with the dynamic demands of the grid and continue to operate in maximum power point tracking (MPPT) mode, exacerbating overpower. Relying on protective devices like crowbar circuits to forcibly cut power not only reduces wind energy utilization but also accelerates equipment wear. While current technologies attempt to improve performance through topology or algorithmic refinements, they are generally limited by high system complexity and the difficulty in balancing dynamic response and economic efficiency. This leads to a disconnect between voltage control and power control, making it impossible to meet the reliability requirements for multi-unit coordination and fault ride-through under complex operating conditions.

[0003] Chinese patent application CN116565941A discloses a low voltage ride-through strategy for large-capacity direct-drive wind turbines based on supercapacitor energy storage, and proposes a low voltage ride-through strategy based on supercapacitor energy storage. The buck-boost supercapacitor energy storage circuit improves the voltage level and capacity of the energy storage system through cascading. However, this application focuses on the coordinated control of a single large-capacity direct-drive wind turbine and supercapacitor energy storage, and does not address the voltage balancing problem when multiple wind turbines are connected in series to the DC bus. When the outputs of multiple units do not match due to wind speed differences or power fluctuations, the solution lacks a dynamic adjustment mechanism, which may cause local overvoltage or undervoltage. Therefore, how to balance voltage balancing, dynamic power regulation and low voltage ride-through capability, and ensure voltage balance in a system with multiple units in series, is a technical problem that needs to be solved. Summary of the Invention

[0004] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide an offshore wind power DC collection circuit and a low voltage fault ride-through method thereof. The power balancing unit balances the voltage of the output ports of the series-connected wind turbines and the series-connected energy storage device, avoids the output port voltage mismatch caused by the change of wind speed of the wind turbine, and during the low voltage ride-through period, the wind turbine adopts overspeed reduction control to reduce the output power, and reduces the power difference between the offshore DC collection system and the onshore AC power grid by increasing the wind turbine speed.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] According to one aspect of the present invention, there is provided an offshore wind power DC collection circuit, the circuit comprising a direct-drive wind turbine, a wind turbine AC / DC converter, a wind turbine DC / DC converter, an energy storage device, an energy storage DC / DC converter, and a power balancing unit;

[0007] The direct-drive wind turbine is connected to the DC collection system through the wind turbine AC / DC converter and the wind turbine DC / DC converter. The wind turbine AC / DC converter realizes the maximum power point tracking of the wind turbine, and the wind turbine DC / DC converter controls the output voltage stability of the wind turbine AC / DC converter; the energy storage device is connected to the DC collection system in series through the energy storage DC / DC converter and the power balancing unit. The energy storage DC / DC converter controls the energy storage device to absorb the unbalanced power between the offshore DC collection system and the onshore AC power grid, and controls the DC collection bus voltage stability during low voltage ride-through; the power balancing unit is connected between adjacent wind turbine DC / DC converters or energy storage DC / DC converters. The power balancing unit includes an inductor, a capacitor and a switching element. By adjusting the inductor current direction and duty cycle, the mismatched power between adjacent converters is transmitted, maintaining the output voltage balance of the series port and the power delivery of the DC collection system; the wind turbine DC / DC converter and the energy storage DC / DC converter are dual active bridge converter structures.

[0008] Furthermore, the energy transmission path of the circuit includes power transmission of the wind turbine DC / DC converter and power transmission of the power balancing unit.

[0009] Furthermore, in the power transmission of the wind turbine DC / DC converter, the power transmission of the wind turbine DC / DC converter is adjusted by a first phase shift duty cycle. The first phase shift is obtained by comparing the voltage at the input port of the wind turbine DC / DC converter with a reference value through a PI controller. The transmission power of the wind turbine DC / DC converter is controlled to maintain the voltage at its input port stable. When the first phase shift duty cycle is greater than 0, the output power P of the wind turbine DC / DC converter is SM,i The expression is:

[0010]

[0011] When the duty cycle of the first phase shift is less than 0, the output power P of the wind turbine DC / DC converter SMi The expression is:

[0012]

[0013] Among them, V port,i is the input voltage of the DC / DC converter of the i-th wind turbine, V out,i is the output voltage of the DC / DC converter of the i-th wind turbine, T s is the switching period of the wind turbine DC / DC converter, n T is the transformer ratio, L r,i is the resonant inductance value, d SM,i is the first phase shift duty cycle.

[0014] Furthermore, the power balancing unit power transmission is such that when the output powers of the DC / DC converters of the wind turbines are unequal, the power balancing unit adjusts the inductor current to achieve output balancing of the DC / DC converters of the wind turbines, and adjusts the second phase shift duty cycle, wherein the second phase shift duty cycle is the phase shift duty cycle between the switch drive signals of the DC / DC converters of adjacent wind turbines;

[0015] When the second phase shift duty cycle is greater than 0, the unbalanced power between the DC / DC converters is the output power ΔP of the power balancing unit. i The expression is:

[0016]

[0017] When the second phase shift duty cycle is less than 0, the unbalanced power between the DC / DC converters is the output power ΔP of the power balancing unit. i The expression is:

[0018]

[0019] Among them, V HV is the DC bus voltage, T s is the switching period of the wind turbine DC / DC converter, L B,i is the energy storage inductance value of the balancing unit, d B,i is the second phase shift duty cycle, and N is the number of wind turbine DC / DC converters connected in series.

[0020] Furthermore, the energy storage device includes a full-bridge switch structure. When the power grid is normal, only the secondary side rear half bridge arm switch tube works to achieve voltage equalization control and DC collection system energy delivery; when the grid voltage drops, the primary side switch tube is unlocked and switched to a charging state to absorb the unbalanced power between the DC collection bus and the shore power grid.

[0021] According to another aspect of the present invention, a low-voltage fault ride-through method for an offshore wind power DC collection circuit is provided, comprising: continuously detecting a voltage drop in the onshore power grid; when the DC collection bus voltage rises and the output power of the wind power generation system is greater than the power received by the power grid, a voltage detection module triggers a low-voltage fault ride-through control mode; starting the wind turbine adopts an overspeed load reduction control strategy, and at the same time, the working mode of the energy storage device is switched to a charging state to absorb the excess active power generated by the wind turbine; during the fault period, continuously monitoring the DC bus voltage state until the bus voltage returns to the rated value and remains stable; when the grid voltage recovers, the wind turbine control system slowly increases the active power output to restore the MPPT working state, and at the same time, the energy storage device stops charging and returns to normal mode, thereby realizing the transition of the wind farm from low-voltage fault ride-through to steady-state operation.

[0022] Furthermore, if the fan speed reaches the maximum speed during the fault duration, the rotor acceleration process is stopped and the fan maintains the maximum speed.

[0023] Furthermore, the overspeed load reduction control strategy is to adjust the fan operating state, increase the fan speed, make the fan run at the overspeed point under the wind speed, reduce the fan output power, and assist the energy storage system to achieve dynamic power balance.

[0024] Furthermore, the fan's overspeed load reduction operating power P d for:

[0025]

[0026] Among them, P m is the fan capacity in stable working state, ρ is the air density, A is the fan impeller area, v is the fan wind speed, λ is the blade tip speed ratio, β is the pitch angle, C d is the fan coefficient, d% is the fan load reduction rate, and the expression is:

[0027]

[0028] Where ΔP m is the load reduction capacity of the fan, P2 is the capacity of the fan under overspeed load reduction state, P n It is the fan capacity before overspeed load reduction state.

[0029] Furthermore, the energy storage device is in a charging state, and the error between the bus voltage reference value and the actual value is input into the voltage proportional-integral controller, whose output is used as the input current reference value of the energy storage device converter, and is input into the current proportional-integrator together with the actual value of the converter input current to generate the phase shift angle of the primary side of the converter, thereby realizing tracking control of the DC bus voltage.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) Improved autonomous voltage balancing capability of multiple wind turbines in series: The power balancing unit dynamically adjusts the mismatch power between adjacent DC / DC converters, and utilizes the synergistic effect of inductors, capacitors, and switching devices to adjust the inductor current direction and duty cycle in real time. This solves the voltage imbalance problem of the series ports of multiple wind turbines caused by wind speed differences or power fluctuations, effectively avoids the risk of local overvoltage or undervoltage, and improves the voltage stability and operational reliability of the system.

[0032] (2) Optimization of power coordination control during low voltage ride-through: A coordinated strategy of wind turbine overspeed load reduction control and energy storage device is adopted. When the grid voltage drops, the wind turbine output power is preferentially reduced, and the remaining unbalanced power is absorbed only by the energy storage. This not only suppresses the rapid rise of the DC bus voltage, but also reduces the capacity configuration requirements of the energy storage system, taking into account the dynamic response speed and economy, while avoiding the energy waste problem of the traditional passive energy dissipation scheme.

[0033] (3) Enhanced robustness of system topology and control strategy: Based on the power transmission architecture of dual active bridge converters, combined with closed-loop regulation of phase-shift duty cycle, efficient energy conversion and electrical isolation between wind turbines and energy storage devices are achieved; through the combined action of power balancing unit and bus voltage tracking control, the dynamic power balance of the DC collection system is maintained during fault ride-through, thereby improving the system's anti-disturbance capability and overall reliability under complex working conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a simplified equivalent circuit diagram of the DC collection system;

[0035] Figure 2 This is the topology diagram of the DC collection system of the wind farm with energy storage;

[0036] Figure 3 This is the working process diagram of the wind turbine DC / DC converter;

[0037] Figure 4 This is the working waveform of the wind turbine DC / DC converter;

[0038] Figure 5 This is the working process diagram of the power balancing unit;

[0039] Figure 6 This is the working waveform of the power balancing unit;

[0040] Figure 7 Schematic diagram of converter control drive signal;

[0041] Figure 8 This is the converter control strategy block diagram;

[0042] Figure 9This is the topology diagram of the DC / DC converter for the energy storage device;

[0043] Figure 10 This is the relationship diagram between the power and speed of the direct drive fan;

[0044] Figure 11 This is the control strategy block diagram of the AC / DC converter for direct-drive wind turbines;

[0045] Figure 12 This is the control strategy diagram of the energy storage device;

[0046] Figure 13 The waveform diagram of the speed and power of fan 1 and fan 2 under wind speed changes;

[0047] Figure 14 The voltage waveforms at the input and output ports of the DC / DC converter;

[0048] Figure 15 Switch S for submodule SM1 1,3 and Q 1,3 Drive signal and inductor L r,1 Voltage and current waveforms;

[0049] Figure 16 Switch Q for the power balancing branch LCPB1 1,3 , Q 2,3 Drive signal and inductor L B,1 Voltage and current waveforms;

[0050] Figure 17 The overspeed reduction speed of the DC fan before and after the fault ω WT Power P WT Waveform graph;

[0051] Figure 18 V is the voltage of the DC fan and the DC / DC converter input and output ports before and after the fault port 、V out , DC bus voltage V HV Waveform graph. DETAILED DESCRIPTION

[0052] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0053] Example 1

[0054] The fundamental reason why the grid voltage drop destroys the system stability is the power imbalance between the offshore DC collection system and the onshore AC grid, which in turn causes the DC collection bus voltage to rise. The key to ensuring grid-connected operation during wind turbine faults is to control the stability of the DC collection bus voltage. Figure 1 The simplified equivalent circuit diagram of the DC collection system is shown in Figure 1. The power of each part satisfies the following relationship:

[0055] P mt =P g +P Co ,

[0056] Among them, P mt is the total power transmitted from offshore wind farm turbines to the shore; P g is the power received by the onshore grid; P Co is stored in the support capacitor C o The energy in is expressed as:

[0057]

[0058] Among them, C o is the equivalent support capacitance on the output side of the DC converter, I Co is the current flowing through the capacitor, U Co is the voltage across the capacitor.

[0059] When the system is in a stable working state, the voltage of the capacitor on the DC output side of the wind farm remains unchanged. In steady state, the wind farm output power P mt Equal to the power P received by the onshore grid g ,Right now

[0060] P mt =P g ,

[0061] When the grid voltage drops, the onshore converter station reduces its output power due to its current limiting protection mechanism, resulting in a decrease in grid-side power. mt Still remains unchanged, resulting in the system power balance being broken. At this time, the system power relationship can be expressed as:

[0062]

[0063] Among them, P gd is the power received by the shore grid after the grid voltage drops. g The wind farm output power P mt Still remains unchanged, the unbalanced power of the system causes the DC output side capacitor C of the wind farm o Voltage U CoThis causes the voltage at the output ports of the series-connected wind turbines in the wind farm to rise.

[0064] Therefore, this embodiment provides an offshore wind power DC collection circuit, such as Figure 2 The circuit includes a direct-drive fan, a fan AC / DC converter, a fan DC / DC converter, an energy storage device, an energy storage DC / DC converter, and a power balancing unit.

[0065] The direct-drive wind turbine is connected to the DC collection system through the wind turbine AC / DC converter and the wind turbine DC / DC converter. The wind turbine AC / DC converter realizes the wind turbine MPPT, and the wind turbine DC / DC converter controls the output voltage stability of the wind turbine AC / DC converter; the energy storage device is connected to the DC collection system through the energy storage DC / DC converter. The energy storage DC / DC converter controls the energy storage device to absorb the unbalanced power between the offshore DC collection system and the onshore AC power grid, and controls the DC collection bus voltage stability during low voltage ride-through; the power balancing unit is connected between adjacent wind turbine DC / DC converters or energy storage DC / DC converters. The power balancing unit includes inductors, capacitors and switching elements. It transmits the mismatched power between adjacent converters by adjusting the inductor current direction and duty cycle, maintaining the output voltage balance of the series port and the power delivery of the DC collection system; the wind turbine DC / DC converter and the energy storage DC / DC converter are dual active bridge converter structures.

[0066] The energy transmission path of the circuit includes the power transmission of the wind turbine DC / DC converter and the power transmission of the power balancing unit.

[0067] In the power transmission of the wind turbine DC / DC converter, the power transmission of the wind turbine DC / DC converter is adjusted by a first phase shift duty cycle. The first phase shift is obtained by comparing the voltage at the input port of the wind turbine DC / DC converter with a reference value and then passing through a PI controller. The voltage at the input port of the wind turbine DC / DC converter is maintained stable by controlling the transmission power of the wind turbine DC / DC converter.

[0068] When the first phase shift duty cycle is greater than 0, its working state and switch working waveform are as follows: Figure 3 and Figure 4 As shown. port,i and I port,i are the DC input voltage and current of the DC / DC converter of the i-th wind turbine respectively. V out,i Indicates the output voltage of the DC / DC converter, v r,i and i r,i They are energy storage inductor L r,i Voltage and current. The converter switching period and dead time are represented by T s and T d Indicated by Figure 3 It can be seen that the wind turbine DC / DC converter input current I port,i(t) is expressed as:

[0069]

[0070] Inductance L in the first half cycle r,i Current i r,i for

[0071]

[0072] Therefore, the average value of the DC / DC converter input current Iport,i within one cycle is:

[0073]

[0074] Output power P of wind turbine DC / DC converter SMi The expression is:

[0075]

[0076] When the duty cycle of the first phase shift is less than 0, the output power P of the wind turbine DC / DC converter SMi The expression is:

[0077]

[0078] Among them, V port,i is the input voltage of the DC / DC converter of the i-th wind turbine, V out,i is the output voltage of the DC / DC converter of the i-th wind turbine, T s is the switching period of the wind turbine DC / DC converter, n T is the transformer ratio, L r,i is the resonant inductance value, d SM,i is the first phase shift duty cycle.

[0079] The power transmission of the power balancing unit is as follows: when the output power between the DC / DC converters of each wind turbine is unequal, the inductor current is adjusted by the power balancing unit to achieve output balance of the DC / DC converters of the wind turbines, and the second phase shift duty cycle is adjusted. The second phase shift duty cycle is the phase shift duty cycle between the switch drive signals of the DC / DC converters of adjacent wind turbines.

[0080] Through the first DC / DC converter SM i , the second DC / DC converter SM i+1 and power balancing unit LCPB i The working principle of the mismatch power between DC / DC converters is analyzed by using the example of LCPB. i In the equivalent circuit, the first DC / DC converter SM i and the second DC / DC converter SM i+1are all expressed as controlled current sources, such as Figure 5 In states 2 and 4, the power balancing unit inductor L B,i The current i LB,i The direction changes, and there is no active power transmission at this time. The mismatch power between adjacent DC / DC converters in states 1 and 3 passes through capacitor C B,i For transmission, capacitor C B,i It plays the role of an intermediate point in the power balance process. The average working voltage is obtained by the voltage second balance principle:

[0081] v CB,i =V HV / n,

[0082] Among them, V HV is the DC bus voltage, n is the number of series ports. i and SM i+1 The output voltage should be equal to V HV / n. SM i and SM i+1 The output currents are

[0083]

[0084] Among them, P i and P i+1 SM i and SM i+1 Output power of LCPB i Working status Figure 6 Available

[0085]

[0086] When the DC / DC converter SM i and SM i+1 When the output power is not equal, that is, ΔP i ≠0, the power balancing unit adjusts the inductor current i LB,i Implementing SM i and SM i+1 Unbalanced power transmission ultimately achieves a balanced output voltage of the DC / DC converter. B,i The working waveform when ≥0 is as follows Figure 6 As shown. By the state space averaging method, the inductance L can be obtained B,i Average current I LB,i,ave The expression is

[0087]

[0088] When the second phase shift duty cycle is greater than 0, the unbalanced power between the DC / DC converters is the output power ΔP of the power balancing unit. i The expression is:

[0089]

[0090] When the second phase shift duty cycle is less than 0, the unbalanced power between the DC / DC converters is the output power ΔP of the power balancing unit. i The expression is:

[0091]

[0092] Among them, V HV is the DC bus voltage, T s is the switching period of the wind turbine DC / DC converter, L B,i is the energy storage inductance value of the balancing unit, d B,i is the second phase shift duty cycle, and N is the number of wind turbine DC / DC converters connected in series.

[0093] The energy storage device includes a full-bridge switch structure. When the grid is normal, only the secondary side switches S7 and S8 work to achieve voltage balancing control and DC collection system energy delivery. When the grid voltage drops, the primary side switches unlock and switch to the charging state to absorb the unbalanced power between the DC collection bus and the shore grid. The energy storage device is connected to the DC collection system in series through a DC / DC converter and a power balancing unit. Its DC / DC converter topology is as follows: Figure 9 As shown. The DC / DC converter of the energy storage device is divided into two working states. When the grid voltage remains normal and the system is in a stable working state, the output power of the wind power generation system is balanced with the power received by the grid, that is, P mt =P g At this time, the energy storage device DC / DC converter in the DC collection system only has the secondary side full-bridge switch tubes S7 and S8 working, and cooperates with the power balancing unit to realize the DC / DC converter output voltage equalization and wind turbine energy delivery. In steady-state operation, the energy storage device DC / DC converter control strategy only outputs equalization control, and the power balancing unit works in the same way as before; when the grid voltage drops, there is P mt >P g At this time, the DC bus voltage regulation control of the energy storage device DC / DC converter in the DC collection system unlocks the DC / DC converter switch tube, and the energy storage device is in a charging state, absorbing the power imbalance between the offshore DC collection system and the onshore AC power grid, preventing the DC bus voltage from rising beyond the threshold. After the fault is resolved, the offshore wind power DC collection system returns to a stable working state.

[0094] Example 2

[0095] This embodiment provides a low-voltage fault ride-through method for an offshore wind power DC collection circuit, including: continuously detecting a voltage drop in the onshore grid; when the DC collection bus voltage rises and the wind power generation system output power is greater than the power received by the grid, the voltage detection module triggers a low-voltage fault ride-through control mode; starting the wind turbine using an overspeed load reduction control strategy, while the energy storage device switches its working mode to a charging state to absorb the excess active power generated by the wind turbine; while the fault persists, continuously monitoring the DC bus voltage state until the bus voltage returns to the rated value and remains stable; when the grid voltage recovers, the wind turbine control system slowly increases the active power output to restore the MPPT working state, while the energy storage device stops charging and returns to normal mode, achieving the transition of the wind farm from low-voltage fault ride-through to steady-state operation. If the wind turbine speed reaches the maximum speed during the fault, the rotor acceleration process is stopped and the wind turbine maintains maximum speed operation.

[0096] The overspeed load reduction control strategy is to adjust the fan operating state, increase the fan speed, make the fan run at the overspeed point under the wind speed, reduce the fan output power, and assist the energy storage system to achieve dynamic power balance. The power and speed relationship diagram of the direct drive wind turbine is as follows: Figure 10 As shown. In the stable working state, the fan operates in MPPT mode and operates at point A at the rated wind speed v1. The fan speed ω1 corresponding to point A is the optimal speed at this wind speed, and the power P1 is the maximum power output at this wind speed. Overspeed control will increase the speed of the direct-drive fan to the speed ω2 corresponding to point B. The overspeed load reduction operation power P of the fan d for:

[0097]

[0098] Among them, P m is the fan capacity in stable working state, ρ is air density, A is fan impeller area, v is fan wind speed, λ is blade tip speed ratio, β is blade pitch angle, C d is the fan coefficient, C d =(1-d%)C p , C p (λ,β) is the wind energy utilization coefficient at the MPPT point, and its value depends on the tip speed ratio λ and the pitch angle β. d% is the load reduction rate of the wind turbine, and its expression is:

[0099]

[0100] Where ΔP m is the load reduction capacity of the fan, P2 is the capacity of the fan under overspeed load reduction state, P n It is the fan capacity before overspeed load reduction state.

[0101] When the grid voltage drops, the wind turbine control strategy switches from MPPT control strategy to overspeed load reduction control. By reducing the wind energy utilization coefficient and increasing the speed of the wind turbine, the wind turbine output power decreases to reduce the unbalanced power between the offshore DC collection system and the onshore AC grid. When the grid voltage recovers, the wind turbine smoothly transitions to normal power output. The control strategy of the AC / DC converter connected to the wind turbine is as follows: Figure 11 The wind turbine control strategy switches to overspeed load reduction control, which reduces the output power of the direct-drive wind turbine by increasing the wind turbine speed and cooperates with the energy storage device to complete the low voltage ride-through process of the wind power generation system. At this time, the wind energy utilization coefficient of the wind turbine will drop by d%.

[0102] During low voltage fault ride-through, the energy storage device is in the charging state, and the error between the bus voltage reference value and the actual value is input to the voltage proportional integral controller. Its output is used as the input current reference value of the energy storage device converter, and the actual value of the converter input current is input to the current proportional integrator to generate the phase shift angle of the primary side of the converter, thereby achieving tracking control of the DC bus voltage. When the DC bus voltage exceeds its reference threshold, the energy storage device switches to the charging working state. Its control strategy is as follows: Figure 12 As shown. HVref V is the reference threshold of the DC bus voltage connected to the DC collection system (usually set to 1.1 pu), HV is the actual voltage value of the DC bus. The error between the bus voltage reference value and the actual value is input to the voltage proportional integral (PI) controller, and its output is used as the input current reference value of the energy storage device converter. It is input together with the actual value of the converter input current to the current proportional integrator to generate the phase shift angle of the primary side of the converter, thereby achieving tracking control of the DC bus voltage. The power balancing unit connected to the energy storage DC / DC converter realizes the voltage balancing control of the output voltage between the energy storage device and the series wind turbine during the fault ride-through process. The control strategy is the same as Figure 8 The pressure equalization control strategy shown is the same.

[0103] Using the circuit and method of the present invention, a simulation test was conducted. During the period of 0.5s-0.7s, the wind speed of fan 1 increased to 13.5m / s, while the wind speed of fan 2 decreased to 7.5m / s. The voltage of the DC / DC converter input and output ports of the DC fan was 5kV. The speeds of fans 1 and 2 under the wind speed change were ω. WT and power P WT The waveform diagram is as follows Figure 13 The corresponding DC / DC converter input and output port voltage waveforms are shown as follows: Figure 14 As shown in Figure 1, during the wind speed change of wind turbine 1, the corresponding submodule SM1 switch S 1,3 , Q 1,3 Drive signal and inductor L r,1 Voltage and current waveforms are as follows Figure 15 As shown. Power balancing branch LCPB1 switch Q1,3 , Q 2,3 Drive signal and inductor L B,1 Voltage and current waveforms are as follows Figure 16 As shown. A simulation of low voltage ride-through was conducted by setting up six DC fans and one energy storage device in series. The DC fans and energy storage DC / DC converter had input and output voltages of 5kV, and the DC bus voltage was 35kV. Fault ride-through was achieved within 1.5s-2s, with the DC bus voltage rising to 38.5kV (1.1pu). The DC fan overspeed reduction speed ω before and after the fault WT Power P WT Waveform diagram Figure 17 As shown, the voltage V at the input and output ports of the DC fan and energy storage DC / DC converter before and after the fault port 、V out , DC bus voltage V HV Waveform diagram Figure 18 shown.

[0104] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and such modifications or substitutions are intended to be within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. An offshore wind power DC collection circuit, characterized in that: The circuit includes a direct-drive fan, a fan AC / DC converter, a fan DC / DC converter, an energy storage device, an energy storage DC / DC converter and a power balancing unit; the direct-drive fan is connected to a DC collection system through the fan AC / DC converter and the fan DC / DC converter, the fan AC / DC converter realizes maximum power point tracking of the fan, and the fan DC / DC converter controls the output voltage of the fan AC / DC converter to be stable; the energy storage device is connected to a DC collection system in series through the energy storage DC / DC converter and the power balancing unit, and the energy storage DC / DC converter controls the output voltage of the fan AC / DC converter to be stable. The energy storage device absorbs the unbalanced power between the offshore DC collection system and the onshore AC power grid, and controls the DC collection bus voltage to be stable during low voltage ride-through. The power balancing unit is connected between adjacent wind turbine DC / DC converters or energy storage DC / DC converters. The power balancing unit includes inductors, capacitors, and switching elements. It transmits the mismatched power between adjacent converters by adjusting the inductor current direction and duty cycle, maintaining the output voltage balance of the series port and the power delivery of the DC collection system. The wind turbine DC / DC converter and energy storage DC / DC converter are dual active bridge converter structures.

2. The offshore wind power DC collection circuit according to claim 1, characterized in that: The energy transmission path of the circuit includes power transmission of the wind turbine DC / DC converter and power transmission of the power balancing unit.

3. The offshore wind power DC collection circuit according to claim 2, characterized in that: In the power transmission of the wind turbine DC / DC converter, the power transmission of the wind turbine DC / DC converter is adjusted by a first phase shift duty cycle. The first phase shift is obtained by comparing the voltage at the input port of the wind turbine DC / DC converter with a reference value through a PI controller. The transmission power of the wind turbine DC / DC converter is controlled to maintain the voltage at its input port stable. When the first phase shift duty cycle is greater than 0, the output power P of the wind turbine DC / DC converter is SM,i The expression is: When the duty cycle of the first phase shift is less than 0, the output power P of the wind turbine DC / DC converter SM,i The expression is: Among them, V port,i is the input voltage of the DC / DC converter of the i-th wind turbine, V out,i is the output voltage of the DC / DC converter of the i-th wind turbine, T s is the switching period of the wind turbine DC / DC converter, n T is the transformer ratio, L r,i is the resonant inductance value, d SM,i is the first phase shift duty cycle.

4. The offshore wind power DC collection circuit according to claim 2, characterized in that: The power balancing unit power transmission is as follows: when the output powers of the DC / DC converters of the wind turbines are unequal, the inductor current is adjusted by the power balancing unit to achieve output balancing of the DC / DC converters of the wind turbines, and the second phase shift duty cycle is adjusted. The second phase shift duty cycle is the phase shift duty cycle between the switch drive signals of the DC / DC converters of adjacent wind turbines; When the second phase shift duty cycle is greater than 0, the unbalanced power between the DC / DC converters is the output power ΔP of the power balancing unit. i The expression is: When the second phase shift duty cycle is less than 0, the unbalanced power between the DC / DC converters is the output power ΔP of the power balancing unit. i The expression is: Among them, V HV is the DC bus voltage, T s is the switching period of the wind turbine DC / DC converter, L B,i is the energy storage inductance value of the balancing unit, d B,i is the second phase shift duty cycle, and N is the number of wind turbine DC / DC converters connected in series.

5. The offshore wind power DC collection circuit according to claim 1, characterized in that: The energy storage device includes a full-bridge switch structure. When the power grid is normal, only the secondary side rear half bridge arm switch tube works to achieve voltage balancing control and DC collection system energy delivery; when the grid voltage drops, the primary side switch tube is unlocked and switched to a charging state to absorb the unbalanced power between the DC collection bus and the onshore power grid.

6. A low voltage fault ride-through method for an offshore wind power DC collection circuit according to any one of claims 1 to 5, characterized in that: The method includes continuously detecting a voltage drop in the onshore power grid. When the DC bus voltage rises and the output power of the wind power generation system is greater than the power received by the power grid, the voltage detection module triggers a low voltage fault ride-through control mode; starting the wind turbine to adopt an overspeed load reduction control strategy, and at the same time, the working mode of the energy storage device is switched to a charging state to absorb the excess active power generated by the wind turbine; during the fault period, the DC bus voltage state is continuously monitored until the bus voltage returns to the rated value and remains stable. When the power grid voltage recovers, the wind turbine control system slowly increases the active power output to restore the MPPT working state, and at the same time, the energy storage device stops charging and returns to normal mode, thereby realizing the transition of the wind farm from low voltage fault ride-through to steady-state operation.

7. A low voltage fault ride-through method for an offshore wind power DC collection circuit according to claim 6, characterized in that: If the fan speed reaches the maximum speed during the fault period, the rotor acceleration process will be stopped and the fan will maintain the maximum speed.

8. The low voltage fault ride-through method of an offshore wind power DC collection circuit according to claim 6, characterized in that: The overspeed load reduction control strategy is to adjust the fan operating state, increase the fan speed, make the fan run at the overspeed point under the current corresponding wind speed, reduce the fan output power, and assist the energy storage system to achieve dynamic power balance.

9. A low voltage fault ride-through method for an offshore wind power DC collection circuit according to claim 8, characterized in that: Overspeed and load reduction operation power P of the fan d for: Among them, P m is the fan capacity in stable working state, ρ is air density, A is fan impeller area, v is fan wind speed, λ is blade tip speed ratio, β is blade pitch angle, C d is the fan coefficient, d% is the fan load reduction rate, and the expression is: Where ΔP m is the load reduction capacity of the fan, P2 is the capacity of the fan under overspeed load reduction state, P n It is the fan capacity before overspeed load reduction state.

10. A low voltage fault ride-through method for an offshore wind power DC collection circuit according to claim 6, characterized in that: The energy storage device is in charging state, and the error between the bus voltage reference value and the actual value is input into the voltage proportional integral controller, whose output is used as the input current reference value of the energy storage device converter, and is input into the current proportional integrator with the actual value of the converter input current to generate the phase shift angle of the primary side of the converter, thereby realizing tracking control of the DC bus voltage.

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