Fault-tolerant dc substation for offshore wind power boosting and collecting

Abstract

The application provides a fault-tolerant DC substation for offshore wind power voltage boosting and gathering, comprising multiple power transmission groups, each of which comprises a wind farm, an inverter and a transformer; a positive power supply end of the wind farm is connected with a first connecting end of the inverter, and a negative power supply end of the wind farm is connected with a second connecting end of the inverter; two ends of a main side of the transformer are connected with a third connecting end and a fourth connecting end of the inverter respectively; the secondary sides of the transformers of the multiple power transmission groups are connected in series and then connected with a modular multilevel converter to form a medium-frequency loop; and the modular multilevel converter is connected with a DC power grid. Based on internal power electronic switches and modulation modes of the converter, the application realizes the functions of topology fault reconstruction and fault isolation, has the ability of fault-tolerant operation of wind farm side port short-circuit, and ensures the high-reliability operation ability of the multi-port DC substation when a fault occurs at a certain port.

Description

Technical Field

[0001] This invention relates to the field of renewable energy power generation technology, and more specifically, to a fault-tolerant DC substation for offshore wind power boosting and collection. Background Technology

[0002] The proportion of renewable energy in the power system will continue to increase. Among them, offshore wind power will become an important part of the future power system due to its characteristics of occupying less land resources, causing less pollution during development, being sustainable and stable, and having huge reserves.

[0003] Offshore wind power grid connection methods mainly include high-voltage alternating current (HVAC), high-voltage direct current (HVDC), and frequency division transmission technology. Among them, HVDC technology is more suitable for meeting the long-distance power transmission needs of offshore wind farms due to its low line loss. In recent years, theoretical research on the circuit topology, power characteristics, and control strategies of HVDC has been continuously improving.

[0004] Existing research on HVDC technology is mostly limited to single-entry, single-output wind farm scenarios. Based on inverter and rectification technologies and single-phase intermediate frequency transformers, the power output from a single wind farm can be fed into a DC transmission system to achieve wind power grid connection while reducing the space requirements of offshore platforms. However, when multiple offshore wind farms need to be connected to the grid, this technical solution will bring additional equipment and transmission line costs.

[0005] Reference 1, patent document CN112290527A, discloses an offshore wind power DC collection grid structure based on a current collector, including: n wind turbine groups, n current collectors, an offshore converter station, and an onshore converter station, where n is an integer not less than 1. Each wind turbine group includes k wind turbines, where k is an integer not less than 1. Each wind turbine outputs a medium-low voltage DC voltage VDCij, where i is an integer and 1 ≤ i ≤ n, and j is an integer and 1 ≤ j ≤ k. VDCi1 to VDCik are all transmitted to current collector i. Current collector i outputs a medium-voltage DC voltage to the offshore converter station, which raises the medium-voltage DC bus voltage to a high-voltage DC voltage before transmitting it to the onshore converter station. However, this patent document still suffers from the drawback of high transmission line costs.

[0006] Document 2, H.Liu, MSADahidah, J.Yu, RTNaayagi and M.Armstrong, Design and Control of Unidirectional DC–DC Modular Multilevel Converter for Offshore DCCollection Point: Theoretical Analysis and Experimental Validation[J].IEEETransactions on Power Electronics, 2019, 34(6): 5191-5208. This paper proposes an improved unidirectional DC-DC modular multilevel converter for the application scenario of offshore wind farms being integrated into high-voltage DC transmission systems. The proposed converter includes a single-phase MMC inverter, which is coupled to a series rectifier module through a multi-stage winding intermediate frequency transformer. The modulation characteristics of the converter realize the extension at different voltage levels. In addition to the current isolation characteristics, the transformer also provides step gain for the output voltage. In terms of efficiency, loss and equipment applicability, the converter has advantages over the most competitive unidirectional cascaded DC-DC converters, such as input series-output series and input parallel-output series. In addition, unlike the traditional dq control method which includes many conversion methods, this paper adopts a simple proportional resonance control strategy in the stationary coordinate system, which directly acts on the AC output of the MMC. The analysis, design, simulation and experimental results confirm the excellent performance of the proposed converter. However, this paper uses a transformer with multiple secondary windings to connect the MMC and multiple series-connected diode rectifiers to realize the integration of offshore wind farms into the high-voltage direct current transmission system. However, in a multi-winding transformer, power can be transmitted bidirectionally between any two ports. Due to the characteristics of the multi-winding transformer, the power transmitted between each port is highly coupled, which makes the system power characteristics complex. The manufacturing method of multi-winding high-voltage intermediate frequency transformers is also more complex and costly. At the same time, this paper does not clearly propose a method to deal with failures.

[0007] Document 3, S.Zhao, Y.Chen, S.Cui, BJMortimer and RWDe Doncker, Three-PortBidirectional Operation Scheme of Modular-Multilevel DC–DC ConvertersInterconnecting MVDC and LVDC Grids[J]. IEEE Transactions on PowerElectronics, 2021, 36(7):7342-7348. This paper is based on a modular multilevel DC-DC converter (MMDC), which is an attractive solution for connecting medium-voltage and low-voltage power grids. Through the modular multilevel converter (MMC) on the medium-voltage side and the full-bridge converter (FBs) on the low-voltage side, the MMDC can be flexibly adjusted to different rated power and voltage levels. However, the existing MMDC is limited to two-port operation, and the DC voltage and the output power of the FBs must be kept consistent. This paper proposes a control strategy that enables the three-port MMDC to operate independently in both directions and connect the medium-voltage power grid to two low-voltage power grids with different rated voltages. It also proposes a comprehensive operation strategy, including two different modulation schemes for operation modes. This strategy can optimize the current of the MMC transformer in the full load range. Compared with the most common three-port isolated DC-DC converter: three active bridges, the power transfer relationship between the three ports is simple, the amount of calculation required for bidirectional control is small, and the power circulation current is essentially eliminated. Experimental results verify the reliability and effectiveness of the operation strategy proposed in this paper. However, this paper proposes a three-port MMDC converter topology and control strategy, including two operating modes, but does not propose a fault-tolerant operation method after a fault. At the same time, it does not involve research on expanding the number of circuit ports. The three-port MMDC converter is not suitable for multi-port DC aggregation application scenarios. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the purpose of this invention is to provide a fault-tolerant DC substation for offshore wind power boosting and aggregation.

[0009] According to the present invention, a fault-tolerant DC substation for offshore wind power boosting and aggregation includes multiple transmission groups, each of which includes a wind farm, an inverter, and a transformer.

[0010] The positive power supply terminal of the wind farm is connected to the first connection terminal of the inverter, and the negative power supply terminal of the wind farm is connected to the second connection terminal of the inverter.

[0011] The two ends of the main side of the transformer are respectively connected to the third and fourth connection terminals of the inverter;

[0012] The secondary sides of the transformers of multiple transmission units are connected in series and then connected to a modular multilevel converter to form a medium-frequency circuit; the modular multilevel converter is connected to the DC power grid.

[0013] Preferably, an inductor is connected in series in the intermediate frequency circuit.

[0014] Preferably, each transmission group also includes a current-limiting reactor;

[0015] The positive power supply terminal of the wind farm is connected to one end of the current-limiting reactor, and the other end of the current-limiting reactor is connected to the first connection terminal of the inverter.

[0016] Preferably, the DC port of the modular multilevel converter on the high-voltage side is connected to the DC power grid via a high-voltage cable.

[0017] Preferably, each transmission group also includes a voltage stabilizing capacitor;

[0018] One end of the voltage stabilizing capacitor is connected to the first connection terminal of the inverter, and the other end of the voltage stabilizing capacitor is connected to the second connection terminal of the inverter.

[0019] Preferably, the inverter on the wind farm side is a low-voltage inverter.

[0020] Preferably, the converter on the high-voltage side is a modular multilevel converter.

[0021] Preferably, the output power of each wind farm port is controlled by adjusting the duty cycle of the output voltage of each inverter and the phase shift angle relative to the output voltage of the modular multilevel converter.

[0022] Preferably, the series voltage V on the secondary side of the transformer P With the output voltage V of the modular multilevel converter MMC All are 7-level waveforms with the same waveform, V P Phase relative to V MMC Phase leading phase angle The wind farm transmits power to the power grid.

[0023] Preferably, it is capable of fault-tolerant operation in the event of a wind farm port fault: when a DC fault occurs at a wind farm port, a pulse-locked protection method is used to isolate the faulty port, V P The waveform changes from a 7-level waveform to a 5-level waveform, V MMC This then changes to a 5-level waveform, V P Phase relative to V MMC Phase leading phase angle Isolate faulty ports and allow non-faulty ports to operate with fault tolerance.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. Based on inverter technology and MMC technology, this invention utilizes a medium-frequency transformer to establish connections between multiple offshore wind farms and the onshore DC grid, and achieves electrical isolation between the offshore wind farms and the DC grid.

[0026] 2. This invention is based on the internal switch reconfiguration control method of substation, which can realize flexible disconnection of faulty ports, reduce the fault range, and not affect the normal operation of other wind power DC access ports;

[0027] 3. The topology proposed in this invention can flexibly change the number of ports according to actual application requirements, while still maintaining a relatively simple inter-port power transmission characteristic. Attached Figure Description

[0028] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0029] Figure 1 This is a topology diagram of a fault-tolerant DC substation for offshore wind power boosting and aggregation in one embodiment.

[0030] Figure 2 This is the equivalent circuit diagram of a multi-port DC substation under normal operating conditions.

[0031] Figure 3 This is a phasor diagram of the voltage and current in the intermediate frequency circuit.

[0032] Figure 4 A schematic diagram of a single wind farm and inverter unit;

[0033] Figure 5 This is a control block diagram for the normal operation of a DC substation.

[0034] Figure 6 This is a schematic diagram of fault-tolerant operation of a DC substation;

[0035] Figure 7 This is the equivalent circuit of a substation in fault-tolerant operation mode.

[0036] Figure 8 This is a block diagram of fault-tolerant operation control for a DC substation.

[0037] Figure 9 This is a schematic diagram of a four-port DC substation model;

[0038] Figure 10 A schematic diagram of the voltage waveforms at each port of a four-port DC substation;

[0039] Figure 11A schematic diagram showing the simulation and calculation results of the power transmission at each port of a DC substation under open-loop control;

[0040] Figure 12 This is a schematic diagram of the drive pulses of FB1 switch transistors S1-S4 before and after the fault occurred.

[0041] Figure 13 This is a schematic diagram of the waveform of the sum of the secondary voltages Vp of the transformer at the fault location;

[0042] Figure 14 This is a schematic diagram showing the power transmitted at each port before and after the fault occurred;

[0043] Figure 15 This is a schematic diagram showing the simulated and calculated values ​​of the transmission power of the non-faulty port before and after the fault occurred. Detailed Implementation

[0044] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0045] Example 1:

[0046] like Figure 1 As shown, this embodiment provides a fault-tolerant DC substation for offshore wind power boosting and aggregation, comprising multiple transmission units. Each transmission unit includes a wind farm, an inverter, and a transformer. The positive power supply terminal of the wind farm is connected to the first connection terminal of the inverter, and the negative power supply terminal of the wind farm is connected to the second connection terminal of the inverter. The two ends of the main side of the transformer are respectively connected to the third and fourth connection terminals of the inverter. The secondary sides of the transformers of multiple transmission units are connected in series and then connected to a modular multilevel converter to form an intermediate frequency circuit. The modular multilevel converter is connected to the DC grid. An inductor is connected in series in the intermediate frequency circuit.

[0047] Each transmission group also includes a current-limiting reactor. The positive power supply terminal of the wind farm is connected to one end of the current-limiting reactor, and the other end of the current-limiting reactor is connected to the first connection terminal of the inverter. Each transmission group also includes a voltage-stabilizing capacitor. One end of the voltage-stabilizing capacitor is connected to the first connection terminal of the inverter, and the other end of the voltage-stabilizing capacitor is connected to the second connection terminal of the inverter.

[0048] The output power of each wind farm port is controlled by adjusting the duty cycle of the output voltage of each inverter and the phase shift angle relative to the output voltage of the modular multilevel converter. The ratio of the output power of each wind farm port depends entirely on the ratio of the duty cycles of the output voltages of each inverter.

[0049] The converter on the high-voltage side is a modular multilevel converter. The inverter on the wind farm side is a low-voltage inverter. The DC port of the modular multilevel converter on the high-voltage side is connected to the DC grid via a high-voltage cable to achieve power transmission.

[0050] Transformer secondary series voltage V P With the output voltage V of the modular multilevel converter MMC All are 7-level waveforms with the same waveform, V P Phase relative to V MMC Phase leading phase angle The wind farm transmits power to the grid. It is capable of fault-tolerant operation in the event of a DC fault at the wind farm port: when a DC fault occurs at the wind farm port, a pulse-locked protection method is used to isolate the faulty port. P The waveform changes from a 7-level waveform to a 5-level waveform, V MMC This then changes to a 5-level waveform, V P Phase relative to V MMC Phase leading phase angle Isolate faulty ports and allow non-faulty ports to operate with fault tolerance.

[0051] This embodiment addresses the need for offshore wind farms to connect to onshore DC power grids by proposing a multi-port DC substation topology scheme with port fault isolation capability. Compared with existing technical solutions, this embodiment has the following characteristics:

[0052] It features multi-channel offshore wind power DC infeed / single-channel HVDC transmission power conversion and step-up conversion functions. On the low-voltage side, a power electronic converter inverts the DC current output from the offshore wind farm into two-level or multi-level AC current, which is then converted and stepped up in series via a power transformer. On the high-voltage side, a modular multilevel converter (MMC) is used. The number of sub-modules depends on the number of wind farm ports and the type of power electronic converter. This converter rectifies the AC current transmitted from the wind farm ports into DC current, which is then transmitted to the DC grid via cables.

[0053] The substation utilizes a series connection of transformer secondary windings to achieve high step-up ratio DC power collection. A mid-to-high frequency connection is established between the wind farm transformer port and the MMC port, reducing the size of the AC transformer and lowering the space and weight requirements of the offshore platform.

[0054] Based on the power electronic switches and modulation methods inside the converter, the topology fault reconstruction and fault isolation functions are realized, and the fault-tolerant operation capability of short-circuit faults at the wind farm side port is provided to ensure the high reliability operation capability of the multi-port DC substation when a fault occurs at one port.

[0055] The proposed embodiment is a DC substation system based on multi-input single-output boost transmission. This system can collect the power output from multiple offshore wind farms and transmit it to the DC grid through a single DC cable, saving transmission line costs. It also has fault isolation and fault-tolerant operation capabilities, ensuring that when a single port fails, the non-faulty ports can still transmit power normally.

[0056] Example 2:

[0057] The difference between this embodiment and Embodiment 1 is that the inverter is a midpoint clamping inverter.

[0058] Example 3:

[0059] The difference between this embodiment and Embodiment 1 is that the inverter is a Vienna inverter.

[0060] Example 4:

[0061] Those skilled in the art can understand this embodiment as a more specific description of Embodiment 1.

[0062] like Figure 1 As shown, this embodiment provides a multi-port DC substation topology. This embodiment relates to renewable energy power generation, offshore wind power boosting and aggregation operation, and particularly to the design of a multi-port boost substation topology, fault-tolerant operation, and high-reliability fault reconfiguration. The specific technical implementation scheme is as follows: DC substation topology design:

[0063] In this embodiment, the multi-port DC substation topology includes n wind farms (WF1, WF2…WF…) on the power supply side. n ), voltage stabilizing capacitor and current limiting reactance (C1, C2…C n L1, L2…L n ), and the inverters (Inv1, Inv2…Inv) connected to it. n ) and transformers (Tr1, Tr2…Tr n Each transformer's secondary winding is connected in series, forming an intermediate frequency (IF) circuit with a Modular Multilevel Converter (MMC) to achieve power transfer. The IF circuit contains a series inductor L. t By limiting the loop current, the MMC is connected to the DC grid to transmit the wind farm's output power to the grid, ultimately achieving DC aggregation. The low-voltage side inverter unit can be implemented based on two-level or three-level inverters such as full-bridge inverters, midpoint clamping inverters, and Vienna inverters.

[0064] Multi-port DC substation operation control and power characteristics:

[0065] A. Normal operating mode:

[0066] for Figure 1 The DC substation topology shown in the figure can be used to formulate voltage and current equations according to Kirchhoff's laws, as shown in equation (1). In the equation, the directions of each current are as follows: Figure 9 As shown, V MMC L represents the AC port voltage of the MMC. t I represents the inductance value of the auxiliary inductor connected in series in an AC circuit. MMC This indicates the MMC output current, V. a1u V a2u These represent the total voltage (V) of the upper half of bridge arms a1 and a2 in the MMC. a1L V a2L These represent the total voltage of the lower half of bridge arms a1 and a2 in the MMC, respectively, and L0 represents the inductance value of the MMC bridge arm, in V. MV Indicates the medium-voltage DC grid voltage, i a1u i a2u These represent the upper half-arm currents of bridge arms a1 and a2 in the MMC, respectively. a1l i a2l These represent the lower half-arm currents of bridge arms a1 and a2 in the MMC, respectively.

[0067]

[0068] Simplify, eliminate V MMC Equation (2) can be obtained:

[0069]

[0070] make: L eq =L t +L0, Equation (2) can be simplified to Equation (3):

[0071]

[0072] In the formula, V P Given the series voltages on the secondary sides of each transformer, the equivalent circuit of the DC substation is obtained according to equation (3), as follows: Figure 2 As shown.

[0073] Based on the equivalent circuit, and using Fourier series, the output voltages V1, V2…V of each inverter can be represented. n and MMC output voltage V s It can be expressed as the sum of the fundamental frequency and all harmonics, as shown in equation (4):

[0074]

[0075] In the formula, V P1 V P2…V Pn The fundamental voltage and harmonic voltage amplitudes output by each transformer are functions of the harmonic order n, and their values ​​are determined by the wind farm voltage and the modulation method of each inverter. MMC output voltage V S Compared to V P The phase shift angle.

[0076] For the fundamental and harmonic voltages, respectively according to Figure 3 The vector diagram shown is used to calculate the current expression for the corresponding frequency and its power. Superposition yields the current expression for the intermediate frequency loop as shown in equation (5):

[0077] The power transmitted by each harmonic voltage is superimposed to obtain the intermediate frequency circuit current I. L The expression is shown in equation (5), where ω represents the angular frequency of each voltage in the AC circuit.

[0078]

[0079] According to equations (4) and (5), the instantaneous transmission power of each port is calculated as follows:

[0080]

[0081] Wherein, P1, P2…P n P represents the instantaneous transmission power of each port. S This refers to the instantaneous received power of the MMC port.

[0082] Since the integral of the odd-order harmonic current over one cycle of other harmonic voltages is zero, for each harmonic voltage, only the corresponding harmonic current needs to be considered to calculate the average power transmitted by that harmonic voltage in the intermediate frequency circuit. The average power transmitted at each port is shown in equation (7).

[0083]

[0084] In equation (7), V P1 V P2 …V Pn The power distribution between the ports is determined by the inverter modulation method and the voltage of each wind farm. If the effect of higher harmonics is ignored, the power distribution between the ports is as shown in equation (8):

[0085] P1:P2:...:P n =V p1 :V p2 :...:V pn (8)

[0086] Therefore, the distribution of wind farm output power can be controlled simply by changing the modulation method of the corresponding inverter.

[0087] like Figure 4 As shown, for a single wind farm and inverter unit, when the voltage of the stabilizing capacitor is V... C The inverter output power is P n The wind farm output power is P WFn When the voltage of the stabilizing capacitor changes, it is shown in Equation (9), where C is the capacitance value of the stabilizing capacitor.

[0088]

[0089] Therefore, under normal operating conditions, when the wind farm output power P WFn When changes occur, the port transmission power P is adjusted through the inverter control strategy. n This will allow the voltage V of the stabilizing capacitor to be reduced. C Stablize.

[0090] Under normal operating conditions, the substation control block diagram is as follows: Figure 5 As shown. The voltage of the stabilizing capacitor in each wind farm is sampled, and the fundamental and harmonic components V of the output voltage of each inverter are calculated using the power characteristics shown in equation (7). p1 V p2 …V pn And the phase shift angle of the MMC output voltage relative to the series voltage on the secondary side of the transformer. This determines the power electronic switching and modulation methods inside each inverter, as well as the control methods of each sub-module of the MMC. Through the pulse width modulation (PWM) device, control pulses are sent to the inverter and the MMC to control the output power of each wind farm port, thereby maintaining the voltage stability of each wind farm.

[0091] B. Wind farm-side short-circuit fault-tolerant operation mode:

[0092] Taking the DC grounding short-circuit fault in wind farm WF1 as an example, this paper illustrates the fault-tolerant operation mode of the substation.

[0093] like Figure 6 As shown, when a ground fault occurs in WF1, a pulse-locked protection mode is used to control the inverter Inv1 to output zero voltage, which is equivalent to turning the faulty port transformer T... r1 A short circuit occurs, but the other ports can still function normally.

[0094] After a fault occurs, the equivalent circuit of the substation in fault-tolerant operation mode is as follows: Figure 7 As shown.

[0095] For the average transmission power of each port under normal operation as described in equation (7), it is only necessary to adjust the corresponding port voltage V. p1 Setting it to 0V, the expression for the average port transmission power under fault-tolerant operation is shown in equation (10):

[0096]

[0097] Therefore, in fault-tolerant operation mode, the power transmission of the faulty port is zero, while the other ports can still transmit power normally. At this time, the power distribution between the ports is as shown in equation (11). Compared with equation (8), the power distribution characteristics of the non-faulty ports do not change before and after the fault.

[0098] P2:P3:...:P n =V p2 :V p3 :...:V pn (11)

[0099] After a fault occurs, the substation fault-tolerant operation control block diagram is as follows: Figure 8 As shown, by using pulse blocking to cut off the fault, the transformer corresponding to the faulty wind farm outputs 0V, which is equivalent to short-circuiting the corresponding transformer, thus achieving fault isolation. At this time, the power distribution characteristics of the remaining non-faulty ports do not change and can still be controlled by the voltage balance control method used during normal operation.

[0100] Working principle: Under normal operating conditions, the voltage waveforms at each AC port are as follows: Figure 10 As shown, the three sets of full-bridge inverters output three-level voltages with different duty cycles, and the grid-side modular multilevel converter outputs a seven-level voltage with the same series voltage waveform as the secondary side of the transformer group. The current expression in the AC circuit at this time is shown in equation (5).

[0101] If higher harmonics are ignored, the average transmission power of each port can be calculated as shown in Equation (7), and the power distribution between ports is shown in Equation (8). The power distribution is entirely determined by the duty cycle of the voltage at each port. Therefore, by increasing or decreasing the duty cycle of the output voltage of each full-bridge inverter, the increase or decrease of the output power at the corresponding wind farm port can be controlled.

[0102] Under fault conditions, the average transmission power of each port is shown in Equation (10), and the power distribution between ports is shown in Equation (11). Compared with before the fault, there are no other significant changes except that the power of the faulty port drops to 0. The output power of the corresponding wind farm port can still be changed by controlling the increase or decrease of the duty cycle of the output voltage of each full-bridge inverter.

[0103] like Figure 1 and Figure 10 As shown, the series voltage V on the secondary side of the transformer P With the output voltage V of the modular multilevel converter MMC All are 7-level waveforms with the same waveform, V P Phase relative to V MMC Phase leading phase angle This enables power transfer from the wind farm to the power grid.

[0104] like Figure 1 and Figure 13 As shown, when a DC fault occurs at the wind farm port, a pulse-locked protection method is used to isolate the faulty port. P The waveform changes from a 7-level waveform to a 5-level waveform, V MMC This then changes to a 5-level waveform, V P Phase relative to V MMC Phase leading phase angle This enables the isolation of faulty ports and the fault-tolerant operation of non-faulty ports.

[0105] Example 5:

[0106] Those skilled in the art can understand this embodiment as a more specific description of Embodiment 1.

[0107] To verify the technical solution proposed in this application, this embodiment establishes a simulation environment based on Matlab / Simulink, such as... Figure 7 The system model shown is used in the following scenario: a four-port DC substation.

[0108] Based on the DC substation topology proposed in this application, the design is as follows: Figure 9 The four-port DC substation model shown is used to connect three wind farms to the onshore DC grid. It uses full-bridge inverters FB1, FB2, and FB3 to connect the offshore wind farm and the intermediate frequency transformer.

[0109] The voltage control target of the stabilizing capacitor at the wind farm outlet is 10kV, the rated voltage of the DC grid is 30kV, the rated power of a single wind farm is 10MW, and the system parameters are shown in Table 1.

[0110] Table 1 Basic System Parameters of DC Substation

[0111] <![CDATA[Wind farm voltage stabilizing capacitors C1, C2, C3]]> 1mF Rated capacity of medium frequency transformer 5MW Medium frequency transformer rated frequency 5kHz <![CDATA[Inductance L of intermediate frequency circuit t > 500uH <![CDATA[MMC bridge arm inductor L0]]> 5uH <![CDATA[MMC sub-module capacitor C SM > 0.01F

[0112] A. Verification of normal operating power characteristics

[0113] for Figure 9 The four-port DC substation shown has the following output voltage waveforms at each port: Figure 10 As shown. V1, V2, and V3 are two-level waveforms, V p The waveform is a superposition of V1, V2, and V3, forming a six-level waveform. Vs is also a six-level waveform, but it lags behind Vp by a phase shift angle.

[0114] for Figure 10 The voltage waveform shown, using a Fourier sequence, V in equation (4) P1 VP2 …V Pn As shown in equation (12):

[0115]

[0116] Substituting equation (12) into equation (6), the average transmission power at each port of the substation is obtained as shown in equation (13):

[0117]

[0118] To verify the power characteristics, the verification was carried out for different values ​​of the duty cycle d1, d2, d3 of the full-bridge inverter and different phase shift angles of the MMC output voltage relative to the full-bridge output voltage. The simulated value of the average transmission power of each port was recorded and compared with the calculated value obtained by considering the fundamental wave and the 3rd, 5th, and 7th harmonics according to Equation (12). The results are shown in Table 2.

[0119] Table 2 Simulation and calculation results of power transmission at each port of the DC substation under open-loop control.

[0120]

[0121] Plot the simulation results and calculation results in Table 2 as a line graph, such as... Figure 11 As shown, using formula (14), the accuracy of the average transmission power calculation for each port is calculated, where Value_cal is the calculated power value, and Value_Sim is the simulated power value.

[0122]

[0123] Calculation result: acc P1 =96.18%; acc P2 =96.94%; acc P3 =96.87%, which verifies the correctness of formula (12) for calculating transmission power.

[0124] B. Fault-tolerant operation verification

[0125] For a four-port DC substation model, the fault-tolerant operation mode after a short circuit occurs in wind farm WF1 is simulated. Three wind farms are represented by current sources: WF1, WF2, and WF3, with output powers of 5MW, 2.5MW, and 3.5MW respectively. Each port is independently controlled by a PI controller, with the control variable being the voltage of the wind farm's stabilizing capacitor, and the control target being 10kV. The simulation duration is 0.2s, simulating a short-circuit fault in the wind farm occurring at 0.1s.

[0126] After a short circuit fault occurs in WF1, the drive pulses for each switch in FB1 are as follows: Figure 12As shown, the two IGBTs in the upper half of the bridge arm remain off, while the two IGBTs in the lower half of the bridge arm are on, resulting in a 0V output voltage for FB1. This effectively short-circuits transformer Inv1, isolating the fault point from the normally operating system and preventing it from affecting the normal operation of non-faulty ports.

[0127] The sum of the secondary voltages of the three transformers, V p Waveforms before and after the fault are as follows Figure 13 As shown, comparing the voltage waveform before and after the fault, it changed from a six-level waveform to a four-level waveform. This indicates that the inverter output voltage at the faulty port is 0V, the transformer was successfully short-circuited, and the non-faulty ports can still operate normally.

[0128] Before and after the fault occurred, the transmission power of each port was as follows: Figure 14 As shown, after a fault occurs, the transmission power of the non-faulty port drops briefly, and then the power transmission capability can be restored to its pre-fault state through PI control. Figure 15 The simulated and calculated values ​​of port transmission power before and after the fault were compared. The comparison revealed that the calculation errors before and after the fault were both within 5%. This experimental result shows that the power transmission system has fault-tolerant operation capability; after a fault occurs, the non-faulty ports can still transmit power normally, and the system power characteristics do not change.

[0129] This invention, based on inverter technology and modular multilevel converter technology, utilizes intermediate frequency transformers to establish connections between multiple offshore wind farms and onshore DC power grids. Furthermore, it achieves electrical isolation between the offshore wind farms and the DC power grid.

[0130] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A fault-tolerant DC substation for offshore wind power boosting and collection, characterized in that, It includes multiple transmission units, each of which includes a wind farm, inverters, and transformers; The positive power supply terminal of the wind farm is connected to the first connection terminal of the inverter, and the negative power supply terminal of the wind farm is connected to the second connection terminal of the inverter. The two ends of the main side of the transformer are respectively connected to the third and fourth connection terminals of the inverter; The secondary windings of the transformers from multiple transmission units are connected in series and then connected to a modular multilevel converter to form a medium-frequency circuit; the modular multilevel converter is connected to the DC power grid. Transformer secondary series voltage V P With the output voltage V of the modular multilevel converter MMC All are 7-level waveforms with the same waveform, V P Phase relative to V MMC Phase leading phase angle The wind farm transmits power to the power grid; Capable of fault-tolerant operation in the event of a wind farm port fault: When a DC fault occurs at a wind farm port, pulse-locked protection is used to isolate the faulty port. P The waveform changes from a 7-level waveform to a 5-level waveform, V MMC This then changes to a 5-level waveform, V P Phase relative to V MMC Phase leading phase angle Isolate faulty ports and allow non-faulty ports to operate with fault tolerance; The pulse blocking protection method is as follows: the two IGBTs of the upper half-bridge arm remain closed, and the two IGBTs of the lower half-bridge arm are turned on, so that the output voltage of FB1 is 0V.

2. The fault-tolerant DC substation for offshore wind power boosting and collection according to claim 1, characterized in that, An inductor is connected in series in the intermediate frequency circuit.

3. The fault-tolerant DC substation for offshore wind power boosting and collection according to claim 1, characterized in that, Each transmission group also includes current-limiting reactors; The positive power supply terminal of the wind farm is connected to one end of the current-limiting reactor, and the other end of the current-limiting reactor is connected to the first connection terminal of the inverter.

4. The fault-tolerant DC substation for offshore wind power boosting and collection according to claim 1, characterized in that, The DC port of the modular multilevel converter on the high-voltage side is connected to the DC power grid via a high-voltage cable.

5. The fault-tolerant DC substation for offshore wind power boosting and collection according to claim 3, characterized in that, Each transmission group also includes voltage stabilizing capacitors; One end of the voltage stabilizing capacitor is connected to the first connection terminal of the inverter, and the other end of the voltage stabilizing capacitor is connected to the second connection terminal of the inverter.

6. The fault-tolerant DC substation for offshore wind power boosting and collection according to claim 1, characterized in that, The inverter on the wind farm side is a low-voltage inverter.

7. The fault-tolerant DC substation for offshore wind power boosting and collection according to claim 1, characterized in that, The converter on the high-voltage side is a modular multilevel converter.

8. The fault-tolerant DC substation for offshore wind power boosting and collection according to claim 1, characterized in that, The output power of each wind farm port is controlled by adjusting the duty cycle of the output voltage of each inverter and the phase shift angle relative to the output voltage of the modular multilevel converter.

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