WO2023179029A1 - 一种海上风电不控整流直流输电系统的控制方法 - Google Patents
一种海上风电不控整流直流输电系统的控制方法 Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/001—Methods to deal with contingencies, e.g. abnormalities, faults or failures
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
- H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
- H02M7/06—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes without control electrode or semiconductor devices without control electrode
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/70—Wind energy
- Y02E10/76—Power conversion electric or electronic aspects
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
Definitions
- the invention relates to a control method for an offshore wind power uncontrolled rectifier DC transmission system, and belongs to the technical field of flexible DC transmission.
- the DC power transmission scheme is the only engineering implementation plan to realize the integration of offshore wind power into the grid.
- the current DC power transmission scheme has the following problems: 1) The offshore converter station platform is large and expensive; 2) The converter valves generally use flexible DC Inverters have a large number of sub-modules and expensive capacitors. Therefore, making offshore converter valves and offshore converter station platforms lighter and improving the overall economy of DC transmission solutions are important directions currently being actively explored by academia and industry.
- DC transmission solutions based on diode-controlled rectifier units have gradually gained favor due to their technical maturity, equipment reliability and economic excellence.
- the offshore converter station used diode-uncontrolled rectifier valves, and the onshore converter station used modular multi-level converters.
- the offshore converter station was connected to the power grid via submarine AC submarine cables.
- Onshore grid connection When the offshore wind farm is black-started, the onshore power grid charges and starts the offshore wind farms one by one through the submarine AC submarine cable; during normal operation, the submarine AC submarine cable provides the grid-connected voltage of the offshore wind farm and maintains the connection between the offshore wind farm grid and the onshore power grid. Synchronicity.
- this DC power transmission solution is generally only suitable for offshore wind power. When offshore wind power is connected to the grid, submarine AC submarine cables cannot achieve long-distance AC connections due to capacitive currents.
- the modular multi-level converters of onshore converter stations still need There is a problem of high cost.
- the existing published literature proposes a solution of connecting an auxiliary converter in parallel to the DC side of the diode valve of the offshore converter station.
- the auxiliary converter is an offshore wind power generator.
- the field provides grid-connected voltage and black-start power supply, but because the auxiliary converter needs to be directly connected in series on the DC side of the diode valve, its sub-module selection needs to match the DC side voltage, so the number of sub-modules is large.
- some literature proposes to modify and upgrade the topology of the auxiliary converter.
- the auxiliary converter adopts a combination of high and low arms. The topology is complex.
- the purpose of the present invention is to provide a control method for an offshore wind power uncontrolled rectifier DC power transmission system, which can be applied to offshore wind farm grid-connected power transmission situations in two different application scenarios: offshore wind power grid construction and grid following application scenarios. .
- a control method for an offshore wind power uncontrolled rectifier DC transmission system which includes the following steps:
- An offshore wind power uncontrolled rectifier DC transmission system is set up.
- the system includes: offshore wind farms, offshore converter stations, positive and negative submarine DC submarine cables, onshore converter stations and onshore power grids;
- the offshore wind farms include: connected in sequence The wind turbine, the machine-side converter, the grid-side converter and the offshore step-up transformer, and the AC side of the offshore step-up transformer is connected to the AC side of the offshore converter station through the offshore AC busbar, the The DC side of the offshore converter station is connected to the DC side of the onshore converter station via the positive and negative submarine DC submarine cables;
- the AC side of the onshore converter station is connected to the onshore power grid;
- the The offshore converter station includes a full-bridge MMC unit and a diode-uncontrolled rectifier unit.
- the full-bridge MMC unit is used to provide starting power or grid-connected voltage for the offshore wind farm during black start or normal operation. , and the DC side of the full-bridge MMC unit is also provided with a bypass switch; the diode uncontrolled rectifier unit is used to rectify the electric energy output by the offshore wind farm and then transmit it to the onshore converter station;
- the onshore converter station includes a thyristor rectifier unit;
- the onshore converter station is controlled to precharge the full-bridge MMC unit in the offshore converter station, and the offshore wind farm is black-started through the full-bridge MMC unit;
- a grid control strategy or a grid-following control strategy is selected to output the energy generated by the offshore wind farm to the DC side and pass through the positive and negative
- the polar submarine DC submarine cable is sent to the onshore converter station and onshore power grid.
- the method of black starting an offshore wind farm through a full-bridge MMC unit includes the following steps:
- the full-bridge MMC unit After the black start process is completed, the full-bridge MMC unit enters the normal operation mode.
- the method of unlocking the thyristor rectifier unit and precharging the full-bridge MMC unit to unlock the full-bridge MMC unit includes:
- Unlock the thyristor rectifier unit control the arc extinction angle so that the DC side transmission voltage of the thyristor rectifier unit is a negative potential, and uncontrollably precharge the full-bridge MMC unit through the positive and negative submarine DC submarine cables and the diode uncontrolled rectifier unit;
- the full-bridge MMC unit When the sub-module capacitor voltage in the full-bridge MMC unit is charged to the controllable charging threshold, the full-bridge MMC unit enters an orderly and controllable charging stage until the sub-module capacitor voltage rating is charged;
- the thyristor rectifier unit controls the full-bridge MMC unit. After the DC side voltage rises to the preset DC voltage value during the startup stage, the full-bridge MMC unit is unlocked.
- the method of controlling the AC side output voltage of the full-bridge MMC unit and sequentially black-starting all wind turbines in the black-start area of the offshore wind farm includes the following steps:
- the machine-side converter of the wind turbine will be charged.
- the DC side voltage capacitor of the converter and the grid-side converter is charged from zero to the charging threshold of the generator-side converter and the grid-side converter, so that the generator-side converter and the grid-side converter enter the controllable and orderly charging stage.
- the capacity of the full-bridge MMC unit is determined based on the number of wind turbines to be started in the offshore wind farm. The method is as follows:
- the thyristor rectifier unit in the onshore converter station adopts a constant DC voltage control strategy. By controlling the arc extinction angle, the DC side voltage of the thyristor rectifier unit reaches the command reference value;
- the full-bridge MMC unit adopts a constant active power and output AC voltage control strategy, and simultaneously performs low-order harmonic suppression control;
- the offshore wind farm adopts network control, and the energy generated by the offshore wind farm is output to the DC side through the diode uncontrolled rectifier unit, and is sent to the thyristor rectifier unit through the positive and negative submarine DC submarine cables.
- U dI is the DC side voltage of the thyristor rectifier unit
- U dioI is the no-load DC voltage of the thyristor rectifier unit
- ⁇ is the arc extinction angle
- d xI is the relative inductive voltage drop of the thyristor rectifier unit
- d rI is the relative resistance of the thyristor rectifier unit.
- Voltage drop U dioNI is the rated no-load DC voltage of the thyristor rectifier unit
- U T is the voltage drop of the thyristor rectifier unit
- I d is the DC current
- I dN is the rated DC current.
- the full-bridge MMC unit adopts a control strategy of constant active power and output AC voltage, and a method of low-order harmonic suppression control at the same time, including:
- the DC voltage and AC voltage amplitude reference values of the full-bridge MMC unit are calculated;
- the dual-loop power control method is used to dynamically calculate the inner loop current reference value and the voltage reference value in the dq coordinate system;
- the reference voltages of the six bridge arms of the full-bridge MMC unit are calculated and used to modulate and generate switching device pulses.
- the thyristor converter unit in the onshore converter station adopts a constant DC voltage control strategy. By controlling the arc extinction angle, the DC side voltage of the thyristor rectifier unit reaches the command reference value;
- the full-bridge MMC unit adopts a constant AC voltage and AC frequency control strategy
- the offshore wind farm adopts grid-following control, and the energy generated by the offshore wind farm is output to the DC side through the diode uncontrolled rectifier unit, and is sent to the thyristor rectifier unit through the submarine DC submarine cable.
- V mref is the reference value of the three-phase AC voltage of the full-bridge MMC unit
- U MMCref and It is the reference value and measured value of the effective value of the AC voltage of the full-bridge MMC unit
- k p5 and k i5 are the proportional coefficient and the integral coefficient respectively.
- the present invention uses a diode rectifier unit at the sending end and a thyristor rectifying unit at the receiving end. It is cheap, has mature technology, is a lightweight platform, and is extremely technically and economically competitive;
- the sending end of the present invention adopts a diode uncontrolled rectifier unit, which has small harmonics and excellent system performance. It is matched with the full-bridge MMC unit.
- the onshore converter station precharges the full-bridge MMC unit to realize offshore operation. Black start of the wind farm; during normal operation, the full-bridge MMC unit provides grid-connected voltage for the offshore wind farm;
- the present invention does not need to change the existing offshore wind farm control strategy, adopts mature technologies, does not require the development of new technologies, and can quickly realize project implementation, which has important promotion significance. Therefore, the present invention can be widely used. It is used in the field of offshore wind power flexible DC transmission technology.
- Figure 1 is a schematic structural diagram of an offshore wind power uncontrolled rectifier DC transmission system provided by an embodiment of the present invention
- FIG. 2 is a schematic structural diagram of an offshore wind power uncontrolled rectifier DC transmission system provided by another embodiment of the present invention.
- Figure 3 is a schematic diagram of a six-pulsation diode rectifier unit used in the embodiment of the present invention.
- Figure 4 is a schematic diagram of a twelve-pulse diode rectifier unit used in the embodiment of the present invention.
- Figure 5 is a schematic structural diagram of a full-bridge MMC unit based on full-bridge submodules in an embodiment of the present invention
- Figure 6 is a schematic diagram of a six-pulse thyristor unit used in the embodiment of the present invention.
- Figure 7 is a schematic diagram of a twelve-pulse thyristor unit used in the embodiment of the present invention.
- Figure 8 is the electrical equivalent circuit of the black start process of the system shown in Figure 1;
- Figure 9 is the electrical equivalent circuit of the black start process of the system shown in Figure 2;
- Figure 10 is a flow chart of the control method of the offshore wind power uncontrolled rectifier DC transmission system provided by the embodiment of the present invention.
- Figure 11 is an equivalent circuit during steady-state operation of the DC transmission system provided by the embodiment of the present invention.
- Figure 12 is an outer loop power controller provided by an embodiment of the present invention.
- Figure 13 is a constant active power controller provided by an embodiment of the present invention.
- Figure 14 is a block diagram of an inner loop current controller provided by an embodiment of the present invention.
- FIG. 15 is a block diagram of an AC voltage controller provided by an embodiment of the present invention.
- an offshore wind power uncontrolled rectifier DC transmission system is disclosed.
- the offshore converter station i.e., the sending end
- Modular multi-level converter referred to as full-bridge MMC unit
- the DC side is connected to the onshore converter station (inverter side) through a submarine DC submarine cable to transport the energy generated by the offshore wind farm to the onshore AC station
- the onshore converter adopts thyristor rectifier unit.
- the full-bridge MMC unit designed by the present invention can utilize the bidirectional charging characteristics to draw energy from the onshore converter station during the black start process to achieve pre-charging, thereby unlocking and providing starting power for the entire offshore wind farm.
- the AC side full-bridge MMC unit provides the grid-connected voltage and frequency for the offshore wind farm.
- the wind turbines in the offshore wind farm can still use the original conventional control strategy for control; the DC side bypass switch switches the full-bridge MMC unit to the grid.
- the unit is bypassed to reduce the on-state loss caused by the DC side current passing through the full-bridge MMC unit.
- the invention has a simple structure, low cost, good technical and economic efficiency, is suitable for offshore wind farm grid-connected power transmission occasions, and has broad application prospects.
- this embodiment provides an offshore wind power uncontrolled rectifier DC transmission system.
- the system includes: an offshore wind farm 1, an offshore converter station 2, a positive and negative submarine DC cable 3, an onshore Converter station 4 and onshore power grid 5 (receiving end AC power grid).
- the AC side of the offshore wind farm 1 is connected to the AC side of the offshore converter station 2 through the offshore AC bus 15, and the DC side of the offshore converter station 2 is connected to the onshore converter station 4 through the positive and negative submarine DC submarine cables 3.
- the DC side is connected; the AC side of the onshore converter station 4 is connected to the onshore power grid 5.
- the offshore wind farm 1 is composed of multiple direct-drive offshore wind turbines 11, a machine-side converter 12, a grid-side converter 13, an offshore step-up transformer 14, etc.
- the output end of each offshore wind turbine 11 enters the AC side of the machine-side converter 12 through the AC collection cable; the DC side of the machine-side converter 12 is connected to the DC side of the grid-side converter 13, and the grid-side converter 13
- the AC power output by the AC side of the converter 13 is collected by the offshore step-up transformer 14 and then connected to the AC side of the offshore converter station 2 through the offshore AC bus 15 .
- the offshore converter station 2 includes a full-bridge MMC unit 21 , a diode uncontrolled rectifier unit and a first filter 29 .
- the full-bridge MMC unit 21 is used to provide starting power or grid-connected voltage for the offshore wind farm 1 during black start or normal operation;
- the diode uncontrolled rectifier unit is used to conduct electric energy output by the offshore wind farm 1 After rectification, it is transmitted to the onshore converter station 4 through the positive and negative submarine DC submarine cables 3, and the first filter 29 is used to filter out high-frequency characteristic sub-harmonic noise.
- the diode uncontrolled rectification unit can be composed of the first six-pulsation diode rectification unit 22 and the second six-pulsation diode rectification unit 23 or directly use a twelve-pulsation diode rectification unit 24.
- the first six-pulsation diode rectification unit 22 and the second six-pulsation diode rectification unit 23 are respectively disposed on both sides of the full-bridge MMC unit 21. side, and after the AC side of the first six-pulsation diode rectification unit 22 and the second six-pulsation diode rectification unit 23 are connected in parallel with the AC side of the full-bridge MMC unit 21, they are connected to the offshore wind power through the first filter 29 and the AC circuit breaker.
- the offshore AC bus 15 of field 1 is connected; the DC side end of the first six-pulsation diode rectifier unit 22 and the six-pulsation diode uncontrolled rectification unit 23 are respectively connected to the high-voltage end and low-voltage end of the DC side of the full-bridge MMC unit 21.
- the other ends of the DC sides of the first six-pulsation diode rectifier unit 22 and the second six-pulsation diode rectifier unit 23 are connected to the positive and negative submarine DC submarine cables 3 .
- the AC side of the full-bridge MMC unit 21 and the AC side of the twelve-pulse diode rectifier unit 24 are connected in parallel and then passed through the first filter 29 It is connected to the offshore AC bus 15 of the offshore wind farm 1; the low-voltage end of the DC side of the full-bridge MMC unit 21 is connected in series with the high-voltage end of the DC side of the twelve-pulsation diode rectifier unit 24, and the high-voltage end of the DC side of the full-bridge MMC unit 21
- the low-voltage end of the DC side of the twelve-pulsation diode rectifier unit 24 is connected to the positive and negative submarine DC submarine cables 3 .
- a first transformer 25 is also provided between the full-bridge MMC unit 21 and the offshore AC bus 15, and the connection type of the first transformer 25 is Y/D (star/angle type). Prevent zero-sequence harmonic components from feeding into offshore wind farms. More preferably, if the zero-sequence component of the full-bridge MMC unit 21 is well controlled, the first transformer 25 can be omitted or replaced with a line reactor to save investment.
- a bypass is provided between the high-voltage end and the low-voltage end of the DC side of the full-bridge MMC unit 21, and a bypass switch 28 is provided on the bypass.
- a transformer is also provided between the AC side of the diode-controlled rectifier unit and the offshore AC bus 15, and the type of the transformer is optimized and configured according to the capacity characteristics.
- a second six-pulsation diode rectification unit 22 is configured between the first six-pulsation diode rectification unit 22 and the offshore AC bus 15
- Transformer 26 adopts a single-phase two-winding transformer with a connection type of Y/Y.
- a third transformer 27 is configured between the second six-pulse diode rectifier unit 23 and the offshore AC bus 15 and adopts a connection type of Y/D.
- a single-phase two-winding transformer when the transmission capacity of the DC power transmission system is less than the preset value, the AC sides of the first six-pulsation diode uncontrolled rectification unit 22 and the second six-pulsation diode rectification unit are connected in parallel with the offshore AC bus.
- a three-phase three-winding transformer with connection type Y/Y/D is installed at connection 15. Among them, the second transformer 26 and the third transformer 27 are used for voltage conversion and preventing the transmission of zero sequence components.
- a fourth transformer 28 is configured between the twelve-pulsation diode rectification unit 24 and the offshore AC bus 15.
- the connection type is single-phase dual winding; more preferably, two parallel single-phase dual-winding transformers can be configured between the twelve-pulse diode rectifier unit 24 and the offshore AC bus 15 to achieve another transformer after one transformer fails.
- the three-phase three-winding transformer adopts the connection type Y/Y/D (star/star/angle), which can form a twelve-pulsation rectifier bridge with the twelve-pulsation diode rectifier unit 24 to reduce harmonics.
- the full-bridge MMC unit 21 adopts a three-phase six-pulse modular multi-level converter of full-bridge sub-modules.
- the first filter 29 can be an HP3 filter to filter out 23, 25, 35, 37 , 47th, 49th and other high-frequency characteristic harmonics.
- the onshore converter station 4 mainly includes a thyristor rectifier unit and a second filter 44.
- the thyristor rectifier unit is used to convert the electric energy transmitted by the offshore wind farm 1 and send it to the onshore power grid 5, and the second filter 44 is used to filter high-frequency characteristic sub-harmonic noise.
- the thyristor rectifier unit can be composed of a first six-pulse thyristor rectifier unit 41 and a second six-pulse thyristor rectifier unit 42 or directly use a twelve-pulse thyristor rectifier unit 43 , the specific form needs to correspond to the structure of the diode-uncontrolled rectifier unit in offshore converter station 2.
- the DC sides of the first six-pulsation thyristor rectification unit 41 and the second six-pulsation thyristor rectification unit 42 are connected to the positive and negative submarine DC sea.
- the AC sides of the first six-pulse thyristor rectifier unit 41 and the second six-pulse thyristor rectifier unit 42 are connected to the onshore power grid 5 through the second filter 44 and the onshore AC busbar 45 respectively.
- the high-voltage end and low-voltage end of the DC side of the twelve-pulse thyristor rectifier unit 43 are connected to the positive and negative submarine DC submarine cables 3.
- the AC side of the rectifier unit 43 is connected to the onshore power grid 5 through the second filter 44 and the onshore AC bus 45 .
- a transformer is provided between the thyristor rectifier unit and the onshore AC bus 45, and the type of the transformer is optimally configured according to the capacity characteristics.
- the first six-pulse thyristor rectifier unit 41 and the onshore AC bus 45 are configured.
- the fifth transformer 46 adopts a single-phase two-winding transformer with a connection type of Y/Y.
- the sixth transformer 47 is configured between the second six-pulse thyristor rectifier unit 42 and the land AC bus 45 and adopts a connection type of Y/D.
- a single-phase two-winding transformer when the transmission capacity of the DC power transmission system is less than the preset value, the AC sides of the first six-pulse thyristor rectifier unit 41 and the second six-pulse thyristor rectifier unit 42 are connected in parallel with the onshore AC bus.
- a three-phase three-winding transformer with connection type Y/Y/D is configured at the 45 connection.
- a seventh transformer 48 is configured between the twelve-pulse thyristor rectifier unit 43 and the onshore AC bus 45, and its connection
- the type is single-phase double winding; when the DC transmission system capacity is less than the preset threshold (for example, 1500MW), a three-phase three-winding transformer is configured between the twelve-pulse thyristor rectifier unit 43 and the onshore AC bus 45; it is more preferred
- the connection type of the three-phase three-winding transformer is Y/Y/D, which can form a twelve-pulse commutation bridge with the twelve-pulse thyristor rectifier unit to reduce harmonics.
- the second filter 44 selects a double-tuned filter HP1224 or a parallel capacitor SC (a small series inductor), and selects and configures an HP3 filter according to the low-order harmonics of the power grid.
- the electrical equivalent circuit of the DC side of the DC transmission system during the black start process of the offshore DC transmission system shown in Figure 1 is given.
- the offshore converter station 2 Before the black start, the offshore converter station 2 is completely in a power-off state, the sub-module capacitor of the full-bridge MMC unit 21 is zero, and the offshore wind turbine 11 does not start because there is no black start power supply.
- the onshore converter station 4 provides black start power, and the equivalent circuit is a voltage source plus a diode (one-way conduction); the equivalent circuit of the diode uncontrolled rectifier unit in the offshore converter station 2 is a diode, full-bridge MMC
- the unit equivalent circuit is a diode connected in series with a capacitor.
- the electrical equivalent circuit of the DC side of the DC transmission system during the black start process of the offshore DC transmission system shown in Figure 2 is given.
- the offshore converter station 2 Before the black start, the offshore converter station 2 is completely in a power-off state, the sub-module capacitor of the full-bridge MMC unit 21 is zero, and the offshore wind turbine 11 does not start because there is no black start power supply.
- onshore converter station 4 provides black start power, and the equivalent circuit is a voltage source plus a diode (one-way conduction); the equivalent circuit of the diode uncontrolled rectifier unit of offshore converter station 2 is a diode, full-bridge MMC unit
- the equivalent circuit of 21 is a diode connected in series with a capacitor.
- this embodiment provides an offshore wind power uncontrolled rectifier DC power transmission system, which can realize the application scenarios of grid-connected wind turbines and grid-following wind turbines.
- This solution does not require changing the wind turbine control strategy of the offshore wind farm, and the configuration
- the related equipment has mature technology and low cost, which greatly reduces the size of the offshore platform; the designed full-bridge MMC can take advantage of the bidirectional charging characteristics to draw energy from the onshore converter station during the black start process to achieve pre-charging, and then unlock The entire offshore wind farm provides starting power.
- the DC side bypass switch bypasses the full-bridge MMC to reduce the on-state loss caused by the DC side current passing through the MMC.
- the invention has simple structure, low cost and good technical and economic efficiency.
- this embodiment provides a control method for the offshore wind power uncontrolled rectifier DC transmission system, which specifically includes the following steps:
- the onshore converter station 4 is controlled to precharge the full-bridge MMC unit 21 in the offshore converter station 2, and the offshore wind farm 1 is black-started through the full-bridge MMC unit 21;
- step (3) During normal operation, if the offshore wind farm adopts a grid-based control strategy, proceed to step (3); if the offshore wind farm adopts a grid-following control strategy, proceed to step (4);
- the offshore wind farm 1 itself provides the grid-connected voltage and frequency, and the diode uncontrolled rectifier unit outputs the energy generated by the offshore wind farm 1 to the DC side, and sends it to the onshore converter via the positive and negative submarine DC submarine cables 3.
- the full-bridge MMC unit 21 provides the grid-connected voltage and frequency for the offshore wind farm 1, and the diode uncontrolled rectifier unit outputs the energy generated by the offshore wind farm 1 to the DC side and passes it through the positive and negative submarine DC cables. 3 is sent to the onshore converter station 4 and onshore power grid 5.
- the method for black starting the offshore wind farm 1 includes the following steps:
- the method for precharging the full-bridge MMC unit 21 includes the following steps:
- the thyristor rectifier unit controls the full-bridge MMC unit 21. After the DC side voltage slowly rises to the preset DC voltage value during the startup stage, the full-bridge MMC unit 21 is unlocked.
- the method for black starting the offshore wind farm 1 includes the following steps:
- the generator-side converter 12 and the grid-side converter 13 in the black start area charge the DC side capacitor voltage of the generator-side converter 12 and the grid-side converter 13 to After the rated value, the wind turbine 11 is unlocked and the output of the wind turbine 11 is controlled to be zero;
- step (1.2.3) when the full-bridge MMC unit 21 charges the wind turbines 11 in the black start area of the offshore wind farm 1, all the wind turbines 11 are divided into x areas. , there are j wind turbines 11 in the area started each i time. Under special circumstances, only one wind turbine can be started at a time.
- the capacity of the full-bridge MMC unit 21 is determined according to the number of wind turbines 11 to be started. The method is as follows:
- S is the capacity of the full-bridge MMC unit
- k is the margin coefficient, generally 1.1 to 1.3
- ⁇ is the number of wind turbines in the area with the most wind turbines in one start
- P is the power generation of a single wind turbine Machine capacity.
- the offshore wind farm adopts a network control strategy, that is, the machine-side converter connected to the wind turbine adopts constant DC voltage control, and the grid-side converter adopts constant AC voltage and Fixed AC frequency control.
- the equivalent model of the entire offshore wind power uncontrolled rectifier DC transmission system is shown in Figure 11.
- P w1 is the power transmitted by the offshore wind farm
- P w2 is the active power fed into the offshore converter station
- ⁇ P w is the active power loss of offshore wind power between the transmission to the offshore converter station (it can be ignored in simplified calculations )
- P MMC is the active power flowing into the full-bridge MMC unit
- P dc is the DC transmission power.
- P dc is the DC transmission power
- U dcr is the diode valve output DC voltage (offshore station DC voltage)
- I dc is the DC current
- U dci is the DC voltage of the onshore converter station
- U MMC is the full-bridge MMC output. AC voltage phase voltage effective value.
- the active power input to the full-bridge MMC unit needs to be controlled to zero. Since the onshore converter station can achieve constant U dci by controlling the arc extinction angle, in order to achieve the above purpose, it is necessary to control the output AC voltage of the full-bridge MMC unit.
- control method includes the following steps:
- the thyristor rectifier unit in the onshore converter station adopts a constant DC voltage control strategy. By controlling the arc extinction angle, the DC side voltage of the thyristor rectifier unit reaches the command reference value U dc ;
- the full-bridge MMC unit adopts a constant active power and output AC voltage control strategy, and simultaneously performs low-order harmonic (11th and 13th) suppression control;
- the offshore wind farm adopts network control, and the energy generated by the offshore wind farm is output to the DC side through the diode uncontrolled rectifier unit, and is sent to the thyristor rectifier unit through the submarine DC submarine cable.
- a six-pulse thyristor rectifier unit is used as an example for introduction.
- the calculation formula of the DC voltage output by the thyristor rectifier unit is:
- U dI is the DC side voltage of the thyristor rectifier unit
- U dioI is the no-load DC voltage of the thyristor rectifier unit
- ⁇ is the arc extinction angle
- d xI is the relative inductive voltage drop of the thyristor rectifier unit
- d rI is the relative resistance of the thyristor rectifier unit.
- Voltage drop U dioNI is the rated no-load DC voltage of the thyristor rectifier unit
- U T is the voltage drop of the thyristor rectifier unit
- I d is the DC current
- I dN is the rated DC current.
- control strategy of the full-bridge MMC unit includes the following steps:
- Dual-loop power control link According to the obtained DC voltage and AC voltage amplitude reference values, the dual-loop power control method is used to dynamically calculate the inner loop current reference value and the voltage reference value in the dq coordinate system.
- Bridge arm reference value generation link According to the voltage reference value in the dq coordinate system, perform coordinate transformation calculation to obtain the three-phase AC voltage reference value, and then obtain the reference voltage of the six bridge arms of the full-bridge MMC unit, using Modulation generates switching device pulses.
- the calculation formulas for the target value DC voltage and AC voltage amplitude of the full-bridge MMC unit are as follows: :
- P w1 is the transmission power of the offshore wind farm
- U dci is the DC voltage of the onshore converter station
- R line is the DC line resistance
- U MMCref is the reference value of the effective value of the AC voltage of the full-bridge MMC unit.
- the dual-loop power control includes an outer power loop and an inner current loop, and the outer power loop is divided into an active power control part And the AC voltage control part, the inner current loop is a fast control loop.
- the outer loop power loop is implemented using proportional integral control (PI control), which is used to calculate the inner loop current reference value.
- PI control proportional integral control
- the inner current loop is implemented using proportional integral control (PI control).
- PI control proportional integral control
- the DC voltage of the full-bridge MMC unit is controlled to 0, and the calculation formula of the reference voltage of the six bridge arms is:
- U apref , U anref , U bpref , U bnref , U cpref , and U cnref are respectively the a-phase upper bridge arm, a-phase lower bridge arm, b-phase upper bridge arm, and b-phase lower bridge of the full-bridge MMC unit.
- the offshore wind farm adopts a network-following control strategy, that is, when the offshore AC system is in a passive state, the full-bridge MMC unit must be switched to a passive island controller to maintain The frequency and voltage of the isolated grid on the offshore wind farm side are stable.
- the essential difference from step (3) is whether the offshore wind farm turbines have self-networking capabilities.
- the entire system control strategy framework and system equivalent model are similar to the network-type control strategy, but there are some changes in the control strategy.
- the AC side full-bridge MMC unit provides grid-connected voltage and frequency to the offshore wind farm, and the wind turbines in the offshore wind farm can still be controlled using the original conventional control strategy.
- the control methods at this time are constant AC voltage control and constant AC frequency control.
- the MMC converter station controlled by the island on the wind farm side can operate at a given voltage and frequency, so the phase-locked synchronization signal is generated by the converter station and is the input of the valve group level controller.
- the reference frequency f of the converter station will remain constant at 50Hz, and the AC voltage will be composed of outer loop voltage control and inner loop current control. This control can not only quickly track the actual current, but also limit the fault current when an AC fault occurs on the wind farm side.
- the thyristor commutation unit in the onshore converter station adopts a constant DC voltage control strategy. By controlling the arc extinction angle, the DC side voltage of the thyristor rectifier unit reaches the command reference value U dc ;
- the offshore wind farm adopts grid-following control.
- the energy generated by the offshore wind farm is output to the DC side through the diode uncontrolled rectifier unit, and is sent to the thyristor rectifier unit through the submarine DC submarine cable.
- steps (4.1) and (4.2) are the same as the network control strategy, and will not be described again here.
- the full-bridge MMC unit adopts a constant AC voltage and AC frequency control strategy, that is, direct voltage control with direct feedback signal.
- the calculation formula of the AC voltage is:
- V mref is the reference value of the three-phase AC voltage of the full-bridge MMC unit
- U MMCref and It is the reference value and measured value of the effective value of the AC voltage of the full-bridge MMC unit
- k p5 and k i5 are the proportional coefficient and the integral coefficient respectively.
- the reference value V mref of the three-phase AC voltage of the full-bridge MMC unit is the reference value of the AC phase voltage amplitude output by the full-bridge MMC unit.
- U MMCref direct feed signal and U MMCref and AC voltage measurement value
- the negative feedback (feed-back) PI signal is composed of two parts added together; the introduction of the direct feedback signal ensures the rapidity of the voltage response, and the negative feedback PI control can eliminate the steady-state error and improve the stability of the system.
- the reference value U MMCref of the AC phase voltage amplitude on the grid side is set to the rated value (1.0pu).
- the three-phase AC voltage of the full-bridge MMC unit is as follows:
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Abstract
一种海上风电不控整流直流输电系统的控制方法，包括：设置一海上风电不控整流直流输电系统，其包括：依次连接的海上风电场（1）、海上换流站（2）、海底直流海缆（3）、陆上换流站（4）和陆上电网（5）；海上换流站（2）包括全桥型MMC单元（21）和二极管不控整流单元；陆上换流站（4）包括晶闸管整流单元；黑启动过程中，控制陆上换流站（4）对海上换流站（2）中的全桥型MMC单元（21）进行预充电，通过全桥型MMC单元（21）对海上风电场（1）进行黑启动；正常运行时，根据海上风电场（1）内的风力发电机（11）是否具备构网能力，选择构网型控制策略或跟网型控制策略，将海上风电场（1）所发出的能量输出到直流侧，并经正负极海底直流海缆（3）送出至陆上换流站（4）和陆上电网（5）。海上风电不控整流直流输电系统的控制方法可以广泛应用于柔性直流输电技术领域。
Description
本发明涉及一种海上风电不控整流直流输电系统的控制方法，属于柔性直流输电技术领域。
远海海上风电开发是未来构建新型电力系统、实现“碳达峰、碳中和”的重要途径。目前，直流输电方案是实现远海风电并网的唯一工程实施方案，然而现阶段的直流输电方案存在以下问题：1)海上换流站平台体积重大、造价昂贵；2)换流阀一般采用柔性直流换流器，子模块数量众多，电容昂贵。因此，海上换流阀和海上换流站平台轻型化，提升直流输电方案整体的经济性是目前学术界和工业界积极探索的重要方向。
基于二极管不控整流单元的直流输电方案，因其技术成熟度、设备可靠性和经济优异性等特点，逐渐得到青睐。早期的基于二极管不控整流单元的直流输电方案中，海上换流站采用二极管不控整流阀，陆上换流站采用模块化多电平换流器，海上换流站经海底交流海缆与陆上电网相连。海上风电场黑启动时，陆上电网通过海底交流海缆将海上风电场逐一充电启动；正常运行时，海底交流海缆提供海上风电场的并网电压，维持海上风电场电网与陆上电网的同步性。然而，这种直流输电方案一般仅适用于近海风电，远海风电并网时海底交流海缆因容性电流无法实现长距离交流连接，此外陆上换流站的模块化多电平换流器仍存在造价昂贵的问题。
针对二极管不控整流单元所连海上风电场黑启动困难的问题，现有公开文献提出了在海上换流站的二极管阀直流侧并联一个辅助换流器的方案，该辅助换流器为海上风电场提供并网电压和黑启动电源，但是因辅助换流器需要直接串联在二极管阀直流侧，其子模块选取需与直流侧电压相匹配，故子模块数量众多。为降低造价，有文献提出对辅助换流器拓扑结构进行改造升级，辅助换流器采用高低臂结合的方式，拓扑结构复杂，为保持高压臂子模块电容电压均衡，需要进行高频谐波注入调制，控制策略复杂。此外还有文献提出利用在海上风电场配置大容量柴油机解决海上风电场黑启动问题，但是海上风电场需要对风机进行改造使之具备构网型控制。
针对上述问题，本发明的目的是提供一种海上风电不控整流直流输电系统的控制方法，可以适用于在海上风电构网和跟网两种不同应用情景下的海上风电场并网送电场合。
为实现上述目的，本发明采取以下技术方案：
一种海上风电不控整流直流输电系统的控制方法，其包括以下步骤：
设置一海上风电不控整流直流输电系统，该系统包括：海上风电场、海上换流站、正负极海底直流海缆、陆上换流站和陆上电网；所述海上风电场包括依次相连的风力发电机、机端变流器、网端变流器和海上升压变压器，且所述海上升压变压器的交流侧经海上交流母线与所述海上换流站的交流侧相连，所述海上换流站的直流侧经所述正负极海底直流海缆与所述陆上换流站的直流侧相连；所述陆上换流站的交流侧与所述陆上电网相连；所述海上换流站包括全桥型MMC单元和二极管不控整流单元，所述全桥型MMC单元用于在海上风电场黑启动或正常运行时，为所述海上风电场提供启动电源或并网电压，且所述全桥型MMC单 元的直流侧还设置有旁路开关；所述二极管不控整流单元用于对所述海上风电场输出的电能进行整流后传送到所述陆上换流站；所述陆上换流站包括晶闸管整流单元；
黑启动过程中，控制陆上换流站对海上换流站中的全桥型MMC单元进行预充电，通过全桥型MMC单元对海上风电场进行黑启动；
正常运行时，根据海上风电场内的风力发电机是否具备构网能力，选择构网型控制策略或跟网型控制策略，将海上风电场所发出的能量输出到直流侧，并经所述正负极海底直流海缆送出至陆上换流站和陆上电网。
进一步，所述通过全桥型MMC单元对海上风电场进行黑启动的方法，包括以下步骤：
解锁晶闸管整流单元，对全桥型MMC单元进行预充电，使得全桥型MMC单元解锁；
控制全桥型MMC单元的交流侧输出电压，对海上风电场待黑启动区域内的全部风力发电机依次进行黑启动；
黑启动过程结束后，全桥型MMC单元进入正常运行时工作模式。
进一步，所述解锁晶闸管整流单元，对全桥型MMC单元进行预充电，使得全桥型MMC单元解锁的方法，包括：
断开全桥型MMC单元直流侧的旁路开关；
断开全桥型MMC单元、二极管不控整流单元与海上风电场交流侧的连接，合上全桥型MMC单元、二极管不控整流单元与海上风电场直流侧的相关开关，使得直流侧回路畅通；
合上晶闸管整流单元与陆上电网交流侧的相关开关；
解锁晶闸管整流单元，通过控制熄弧角使得晶闸管整流单元直流侧输电电压为负电势，经正负极海底直流海缆和二极管不控整流单元对全桥型MMC单元进行不控预充电；
当全桥型MMC单元内子模块电容电压充电到可控充电阈值后，全桥型MMC单元进入有序可控充电阶段，直至充电到子模块电容电压额定值；
晶闸管整流单元控制全桥型MMC单元直流侧电压上升到启动阶段直流电压预设值后，全桥型MMC单元解锁。
进一步，所述控制全桥型MMC单元的交流侧输出电压，对海上风电场待黑启动区域内的全部风力发电机依次进行黑启动的方法，包括以下步骤：
①合上全桥型MMC单元与海上风电场交流侧的开关；
②合上海上风电场待黑启动区域至全桥型MMC单元之间回路相关开关；
③通过控制全桥型MMC单元交流侧输出电压对海上风电场待黑启动区域内风力发电机的机端变流器和网端变流器的直流侧进行充电，将风力发电机的机端变流器和网端变流器直流侧电压电容从零充电到机端变流器和网端变流器的充电阈值，使机端变流器和网端变流器进入可控有序充电阶段；
④待黑启动区域内的风力发电机的机端变流器和网端变流器通过可控有序充电将风力发电机的机端变流器和网端变流器直流侧电容电压充电到额定值后，风力发电机解锁，控制风力发电机出力为零；
⑤重复步骤②-④，将待黑启动区域内的剩余风力发电机均启动起来；
⑥全桥型MMC单元进入正常运行时工作模式，黑启动过程结束。
进一步，所述全桥型MMC单元容量根据海上风电场内的待启动风力发电机的数量确定，方法如下：
S＝k*λ*P
式中，S为全桥型MMC单元容量；k为裕度系数；λ为一次启动中具有最多风力发电机的区域所含风力发电机的数目；P为单台风力发电机容量。
进一步，当采用构网型控制策略时，包括以下步骤：
陆上换流站中晶闸管整流单元采用定直流电压控制策略，通过控制熄弧角使得晶闸管整 流单元直流侧电压达到指令参考值；
控制海上换流站中全桥型MMC单元直流侧的输出电压为零，并合上全桥型MMC单元直流侧设置的旁路开关；
全桥型MMC单元采用定有功功率和输出交流电压控制策略，同时进行低次谐波抑制控制；
海上风电场采用构网型控制，通过二极管不控整流单元将海上风电场所发出的能量输出到直流侧，并经正负极海底直流海缆送出到晶闸管整流单元。
进一步，所述晶闸管整流单元输出的直流电压的计算公式为：
式中，U
dI为晶闸管整流单元直流侧电压，U
dioI为晶闸管整流单元空载直流电压，γ为熄弧角，d
xI为晶闸管整流单元相对感性压降，d
rI为晶闸管整流单元相对阻性压降，U
dioNI为晶闸管整流单元额定空载直流电压，U
T为晶闸管整流单元压降，I
d为直流电流，I
dN为额定直流电流。
进一步，所述全桥型MMC单元采用定有功功率和输出交流电压控制策略，同时进行低次谐波抑制控制的方法，包括：
根据直流输电系统的实际工况以及假设的全桥型MMC单元直流侧的输出电压，计算得到全桥型MMC单元的直流电压和交流电压幅值参考值；
根据得到的直流电压和交流电压幅值参考值，采用双环功率控制方法动态计算内环电流参考值和dq坐标系下的电压参考值；
根据dq坐标系下的电压参考值，计算得到全桥型MMC单元6个桥臂的参考电压，用于调制生成开关器件脉冲。
进一步，当采用跟网型控制策略时，包括以下步骤：
陆上换流站中晶闸管换流单元采用定直流电压控制策略，通过控制熄弧角使得晶闸管整流单元直流侧电压达到指令参考值；
控制海上换流站中全桥型MMC单元直流侧的输出电压为零，并合上全桥型MMC单元直流侧设置的旁路开关；
全桥型MMC单元采用定交流电压和交流频率控制策略；
海上风电场采用跟网型控制，通过二极管不控整流单元将海上风电场所发出的能量输出到直流侧，并经海底直流海缆送出到晶闸管整流单元。
进一步，所述全桥型MMC单元采用定交流电压和交流频率控制策略时，其三相交流电压参考值的计算公式为：
式中，V
mref为全桥型MMC单元三相交流电压的参考值；U
MMCref和
为全桥型MMC单元交流电压有效值的参考值和测量值；
为扰动分量，即交流电网电压前馈项；k
p5和k
i5分别为比例系数和积分系数。
本发明由于采取以上技术方案，其具有以下优点：
1、本发明送端采用二极管整流单元、受端采用晶闸管整流单元，造价便宜、技术成熟、平台轻便，极具技术经济竞争力；
2、本发明送端采用二极管不控整流单元，谐波小、系统性能优异，与全桥型MMC单元相配合，在启动过程中，陆上换流站通过预充电全桥MMC单元，实现海上风电场黑启动；正常运行时，全桥型MMC单元为海上风电场提供并网电压；
综上所述，本发明无需更改现有海上风电场控制策略，采用的都是成熟的技术，无需太大新技术研发，可以快速实现工程实施，具有重要的推广意义，因此，本发明可以广泛应用于海上风电柔性直流输电技术领域中。
通过阅读下文优选实施方式的详细描述，各种其他的优点和益处对于本领域普通技术人员将变得清楚明了。附图仅用于示出优选实施方式的目的，而并不认为是对本发明的限制。在整个附图中，用相同的附图标记表示相同的部件。在附图中：
图1是本发明一实施例提供的海上风电不控整流直流输电系统结构示意图；
图2是本发明另一实施例提供的海上风电不控整流直流输电系统结构示意图；
图3是本发明实施例中采用的六脉动二极管整流单元示意图；
图4是本发明实施例中采用的十二脉动二极管整流单元示意图；
图5是本发明实施例中采用基于全桥子模块的全桥MMC单元的结构示意图；
图6是本发明实施例中采用的六脉动晶闸管单元示意图；
图7是本发明实施例中采用的十二脉动晶闸管单元示意图；
图8是图1中所示系统黑启动过程的电气等值电路；
图9是图2中所示系统黑启动过程的电气等值电路；
图10是本发明实施例提供的海上风电不控整流直流输电系统的控制方法流程图；
图11是本发明实施例提供的直流输电系统稳态运行时的等值电路；
图12是本发明实施例提供的外环功率控制器；
图13是本发明实施例提供的定有功功率控制器；
图14是本发明实施例提供的内环电流控制器框图；
图15是本发明实施例提供的交流电压控制器框图；
图中各附图标记如下：
1、海上风电场；11、风力发电机；12、机端变流器；13、网端变流器；14、升压变压器；15、海上交流母线；2、海上换流站；21、全桥型MMC单元；22、第一六脉动二极管整流单元；23、第二六脉动二极管整流单元；24、十二脉动二极管整流单元；25、第一变压器；26、第二变压器；27、第三变压器；28、第四变压器；29、第一滤波器；3、正负极海底直流海缆；4、陆上换流站；41、第一六脉动晶闸管整流单元；42、第二六脉动晶闸管整流单元；43、十二脉动晶闸管整流单元；44、第二滤波器；45、陆上交流母线；46、第五变压器；47、第六变压器；48、第七变压器；5、陆上电网。
为使本发明实施例的目的、技术方案和优点更加清楚，下面将结合本发明实施例的附图，对本发明实施例的技术方案进行清楚、完整地描述。显然，所描述的实施例是本发明的一部分实施例，而不是全部的实施例。基于所描述的本发明的实施例，本领域普通技术人员所获得的所有其他实施例，都属于本发明保护的范围。
需要注意的是，这里所使用的术语仅是为了描述具体实施方式，而非意图限制根据本申请的示例性实施方式。如在这里所使用的，除非上下文另外明确指出，否则单数形式也意图 包括复数形式，此外，还应当理解的是，当在本说明书中使用术语“包含”和/或“包括”时，其指明存在特征、步骤、操作、器件、组件和/或它们的组合。
本发明的一些实施例中，公开了一种海上风电不控整流直流输电系统，该系统中海上换流站(即送端)采用二极管不控整流单元和小容量三相六桥臂全桥型模块化多电平换流器(简称全桥MMC单元)，且直流侧通过海底直流海缆与陆上换流站(逆变侧)相连，将海上风电场发出的能量输送到陆上交流站，陆上换流器采用晶闸管整流单元。本发明所设计的全桥型MMC单元可以利用双向充电特点，在黑启动过程中，从陆上换流站中汲取能量实现预充电，进而解锁为整个海上风电场提供启动电源。正常运行时，交流侧全桥型MMC单元为海上风电场提供并网电压和频率，海上风电场风力发电机仍可沿用原有常规控制策略，进行控制；直流侧旁路开关将全桥型MMC单元旁路掉，减少直流侧电流经过全桥型MMC单元导致的通态损耗。本发明结构简单，造价便宜，具有较好的技术经济性，适合海上风场并网送电场合，应用前景广阔。
与之相对应地，本发明的另一些实施例中公开了一种海上风电不控整流直流输电系统的控制方法。
实施例1
如图1、图2所示，本实施例提供一种海上风电不控整流直流输电系统，该系统包括：海上风电场1、海上换流站2、正负极海底直流海缆3、陆上换流站4和陆上电网5(受端交流电网)。其中，海上风电场1的交流侧经海上交流母线15与海上换流站2的交流侧相连，海上换流站2的直流侧经正负极海底直流海缆3与陆上换流站4的直流侧相连；陆上换流站4的交流侧与陆上电网5相连。
作为一个优选的实施例，海上风电场1由多台直驱型海上风力发电机11、机端变流器12、网端变流器13和海上升压变压器14等构成。其中，各海上风力发电机11的输出端经交流汇集电缆进入机端变流器12的交流侧；机端变流器12的直流侧与网端变流器13的直流侧相连，网端变流器13的交流侧输出的交流电由海上升压变压器14汇集后经海上交流母线15与海上换流站2的交流侧相连。
作为一个优选的实施例，海上换流站2包括全桥型MMC单元21、二极管不控整流单元和第一滤波器29。其中，全桥型MMC单元21用于在海上风电场1黑启动或正常运行时，为海上风电场提供启动电源或并网电压；二极管不控整流单元用于对海上风电场1输出的电能进行整流后通过正负海底直流海缆3传送到陆上换流站4，第一滤波器29用于滤除高频特征次谐波噪声。
作为一个优选的实施例，二极管不控整流单元可以采用第一六脉动二极管整流单元22和第二六脉动二极管整流单元23构成或直接采用一个十二脉动二极管整流单元24。
具体地，如图2、图3所示，当采用两个六脉动二极管整流单元时，第一六脉动二极管整流单元22和第二六脉动二极管整流单元23分别设置在全桥型MMC单元21两侧，且第一六脉动二极管整流单元22和第二六脉动二极管整流单元23的交流侧与全桥型MMC单元21的交流侧并联汇集后，经第一滤波器29和交流断路器与海上风电场1的海上交流母线15相连；第一六脉动二极管整流单元22和六脉动二极管不控整流单元23的直流侧一端分别与全桥型MMC单元21的直流侧的高压端和低压端相连，第一六脉动二极管整流单元22和第二六脉动二极管整流单元23直流侧的另一端与正负极海底直流海缆3相连。
如图1、图4所示，当采用一个十二脉动二极管整流单元时，全桥型MMC单元21的交流侧与十二脉动二极管整流单元24的交流侧并联汇集后，经第一滤波器29与海上风电场1的海上交流母线15相连；全桥型MMC单元21直流侧的低压端与十二脉动二极管整流单元24直流侧的高压端相串联，全桥型MMC单元21直流侧的高压端和十二脉动二极管整流单元24直流侧的低压端与正负极海底直流海缆3相连。
作为一个优选的实施例，全桥型MMC单元21与海上交流母线15之间还设置有第一变压器25，且第一变压器25的联结型式为Y/D(星型/角型)，用于防止零序谐波分量馈入海上风电场。更为优选地，如果全桥型MMC单元21的零序分量控制较好，可以省略第一变压器25，或将其替换为线路电抗器以节省投资。
作为一个优选的实施例，全桥型MMC单元21直流侧的高压端和低压端之间还设置有旁路，旁路上设置有旁路开关28。
作为一个优选的实施例，二极管不控整流单元的交流侧与海上交流母线15之间还设置有变压器，且变压器的型式根据容量特点进行优化配置。
具体地，若采用两个六脉动二极管整流单元，则当直流输电系统输送的容量大于预设值(例如1500MW)时，在第一六脉动二极管整流单元22与海上交流母线15之间配置第二变压器26，且其采用联结型式为Y/Y的单相双绕组变压器，在第二六脉动二极管整流单元23与海上交流母线15之间配置第三变压器27，且其采用联结型式为Y/D的单相双绕组变压器；当直流输电系统输送容量小于预设值时，则将第一六脉动二极管不控整流单元22和第二六脉动二极管整流单元的交流侧并联后，在与海上交流母线15连接处配置一台联结型式为Y/Y/D的三相三绕组变压器。其中，第二变压器26和第三变压器27用于电压变换和防止零序分量传递。
若采用一个十二脉动二极管不控整流单元，则当直流输电系统容量超过预设阈值(例如1500MW)时，在十二脉动二极管整流单元24与海上交流母线15之间配置第四变压器28，其联结型式为单相双绕组；更为优选地，还可以在十二脉动二极管整流单元24与海上交流母线15之间配置两台并联的单相双绕组变压器，以实现一台变压器故障退出后另外一台变压器仍可运行的高可靠性设计；当直流输电系统容量小于预设阈值时，在十二脉动二极管整流单元24与海上交流母线15之间配置一台三相三绕组变压器；更为优选地，该三相三绕组变压器采用的联结型式为Y/Y/D(星型/星型/角型),可与十二脉动二极管整流单元24构成十二脉动整流桥，降低谐波。
作为一个优选的实施例，如图5所示，全桥型MMC单元21采用全桥子模块的三相六脉动模块化多电平换流器。
作为一个优选的实施例，由于正常运行时全桥型MMC单元21可协助滤除低次谐波，故第一滤波器29可以采用HP3型滤波器，用于滤除23、25、35、37、47、49次等高频特征次谐波。
作为一个优选的实施例，如图1、图2所示，陆上换流站4主要包括晶闸管整流单元和第二滤波器44。其中，晶闸管整流单元用于对海上风电场1传输的电能进行转换，并发送到陆上电网5，第二滤波器44用于滤波高频特征次谐波噪声。
作为一个优选的实施例，如图6、图7所示，晶闸管整流单元可以采用第一六脉动晶闸管整流单元41和第二六脉动晶闸管整流单元42构成或直接采用一个十二脉动晶闸管整流单元43，具体形式需与海上换流站2中二极管不控整流单元的结构相对应。
具体地，如图2、图6所示，当采用两个六脉动晶闸管整流单元时，第一六脉动晶闸管整流单元41和第二六脉动晶闸管整流单元42的直流侧与正负极海底直流海缆3相连，第一六脉动晶闸管整流单元41和第二六脉动晶闸管整流单元42的交流侧分别经第二滤波器44和陆上交流母线45与陆上电网5相连。
如图1、图7所示，当采用一个十二脉动晶闸管整流单元时，十二脉动晶闸管整流单元43直流侧的高压端和低压端与正负极海底直流海缆3相连，十二脉动晶闸管整流单元43交流侧经第二滤波器44和陆上交流母线45与陆上电网5相连。
作为一个优选的实施例，晶闸管整流单元与陆上交流母线45之间设置有变压器，且变压器的型式根据容量特点进行优化配置。
具体地，若采用两个六脉动晶闸管整流单元，则当直流输电系统输送的容量大于预设值(例如1500MW)时，在第一六脉动晶闸管整流单元41与陆上交流母线45之间配置第五变压器46，其采用联结型式为Y/Y的单相双绕组变压器，在第二六脉动晶闸管整流单元42与陆上交流母线45之间配置第六变压器47，其采用联结型式为Y/D的单相双绕组变压器；当直流输电系统输送容量小于预设值时，则将第一六脉动晶闸管整流单元41和第二六脉动晶闸管整流单元42的交流侧并联后，在与陆上交流母线45连接处配置一台联结型式为Y/Y/D的三相三绕组变压器。
若采用一个十二脉动晶闸管整流单元，则当直流输电系统容量超过预设阈值(例如1500MW)时，在十二脉动晶闸管整流单元43与陆上交流母线45之间配置第七变压器48，其联结型式为单相双绕组；当直流输电系统容量小于预设阈值(例如1500MW)时，在十二脉动晶闸管整流单元43与陆上交流母线45之间配置一台三相三绕组变压器；更为优选地，该三相三绕组变压器变压器的联结型式为Y/Y/D,可与十二脉动晶闸管整流单元构成十二脉动换流桥，降低谐波。
作为一个优选的实施例，第二滤波器44选择双调谐滤波器HP1224或并联电容器SC(串接小电感)，并根据电网低次谐波情况选择配置HP3型滤波器。
如图8所示，给出了图1所示海上直流输电系统黑启动过程中直流输电系统直流侧电气等值电路。黑启动前，海上换流站2完全处于失电状态，全桥型MMC单元21的子模块电容器为零，海上风力发电机11因无黑启动电源也未启动。启动过程中，陆上换流站4提供黑启动电源，等值电路为电压源加二极管(单向导电)；海上换流站2中二极管不控整流单元等值电路为二极管，全桥型MMC单元等值电路为二极管串接电容器。
如图9所示，给出了图2所示海上直流输电系统黑启动过程中直流输电系统直流侧的电气等值电路。黑启动前，海上换流站2完全处于失电状态，全桥型MMC单元21的子模块电容器为零，海上风力发电机11因无黑启动电源也未启动。启动过程中，陆上换流站4提供黑启动电源，等值电路为电压源加二极管(单向导电)；海上换流站2二极管不控整流单元等值电路为二极管，全桥型MMC单元21的等值电路为二极管串接电容器。
综上所述，本实施例给出了一种海上风电不控整流直流输电系统，可以实现构网型风机和跟网型风机并网应用场景该方案无需改变海上风电场风机控制策略，所配置相关设备技术成熟、造价低，大大减少了海上平台的体积；所设计的全桥型MMC可以利用双向充电特点，在黑启动过程中从陆上换流站中汲取能量实现预充电，进而解锁为整个海上风电场提供启动电源。直流侧旁路开关将全桥型MMC旁路掉，减少直流侧电流经过MMC导致的通态损耗。本发明结构简单，造价便宜，具有较好的技术经济性。
实施例2
如图10所示，基于实施例1所提供的海上风电不控整流直流输电系统，本实施例提供了一种海上风电不控整流直流输电系统的控制方法，具体包括以下步骤：
(1)黑启动过程中，控制陆上换流站4对海上换流站2中的全桥型MMC单元21进行预充电，通过全桥型MMC单元21对海上风电场1进行黑启动；
(2)正常运行时，若海上风电场采用构网型控制策略，则进入步骤(3)，若海上风电场采用跟网型控制策略，则进入步骤(4)；
(3)由海上风电场1自身提供并网电压和频率，二极管不控整流单元将海上风电场1所发出的能量输出到直流侧，并经正负极海底直流海缆3送出至陆上换流站4和陆上电网5；
(4)由全桥型MMC单元21为海上风电场1提供并网电压和频率，二极管不控整流单元将海上风电场1所发出的能量输出到直流侧，并经正负极海底直流海缆3送出至陆上换流站4和陆上电网5。
作为一个优选的实施例，上述步骤(1)中，对海上风电场1进行黑启动的方法，包括 以下步骤：
(1.1)解锁晶闸管整流单元，对全桥型MMC单元21进行预充电，使得全桥型MMC单元21解锁；
(1.2)控制全桥型MMC单元21交流侧输出电压，对海上风电场1待黑启动区域内的全部风力发电机11依次进行黑启动；
(1.3)黑启动过程结束后，全桥型MMC单元21进入正常运行时工作模式。
作为一个优选的实施例，上述步骤(1.1)中，对全桥型MMC单元21进行预充电的方法，包括以下步骤：
(1.1.1)断开全桥型MMC单元21直流侧的旁路开关；
(1.1.2)断开全桥型MMC单元21、二极管不控整流单元与海上风电场1交流侧的连接，合上直流侧相关开关，确保直流输电系统直流侧回路畅通；
(1.1.3)合上晶闸管整流单元与陆上电网5连接的交流断路器；
(1.1.4)解锁晶闸管整流单元，通过控制熄弧角使得晶闸管整流单元直流侧输电电压为负电势，经正负极海底直流海缆3和二极管不控整流单元对全桥型MMC单元21进行不控预充电；
(1.1.5)当全桥型MMC单元21内子模块电容电压充电到可控充电阈值后，全桥型MMC单元21进入有序可控充电阶段，直至充电到子模块电容电压额定值；
(1.1.6)晶闸管整流单元控制全桥型MMC单元21直流侧电压缓慢上升到启动阶段直流电压预设值后，全桥型MMC单元21解锁。
作为一个优选的实施例，上述步骤(1.2)中，对海上风电场1进行黑启动的方法，包括以下步骤：
(1.2.1)合上全桥型MMC单元21与海上风电场1连接的交流断路器；
(1.2.2)合上海上风电场1待黑启动区域至全桥型MMC单元21之间回路相关开关；
(1.2.3)通过控制全桥型MMC单元21交流侧输出电压对海上风电场1待黑启动区域内风力发电机11的机端变流器12和网端变流器13的直流侧进行充电，控制预充电电流不超过设备耐受能力，将风力发电机11的机端变流器12和网端变流器13直流侧电压电容从零逐步充电到机端变流器12和网端变流器13的充电阈值，机端变流器12和网端变流器13进入可控有序充电阶段；
(1.2.4)待黑启动区域内的机端变流器12和网端变流器13通过可控有序充电将机端变流器12和网端变流器13直流侧电容电压充电到额定值后，风力发电机11解锁，控制风力发电机11出力为零；
(1.2.5)重复步骤(1.2.2)-(1.2.4)，将待黑启动区域内的剩余风力发电机11均启动起来；
(1.2.6)全桥型MMC单元21进入正常运行时工作模式，黑启动过程结束。
作为一个优选的实施例，上述步骤(1.2.3)中，全桥型MMC单元21对海上风电场1待黑启动区域内风力发电机11进行充电时，将所有风力发电机11分成x个区域，每i次启动的区域共有j台风力发电机11。特殊情况下每次只启动一台风力发电机。
作为一个优选的实施例，上述步骤(1.2.3)中，全桥型MMC单元21的容量根据待启动风力发电机11的数量确定，方法如下：
S＝k*λ*P (1)
式中，S为全桥型MMC单元容量；k为裕度系数，一般可取1.1～1.3；λ为一次启动中具有最多风力发电机的区域所含风力发电机的数目；P为单台风力发电机容量。
作为一个优选的实施例，上述步骤(3)中，海上风电场采用构网型控制策略，即风机所连的机端变流器采用定直流电压控制，网端变流器采用定交流电压和定交流频率控制。正 常运行时，整个海上风电不控整流直流输电系统的等效模型如图11所示。
稳态运行满足以下功率方程约束，忽略二极管阀组损耗：
P
w1＝P
w2+ΔP
w (2)
P
w2-P
MMC＝P
dc (3)
式中，P
w1为海上风电场输送功率；P
w2为馈入到海上换流站的有功功率；ΔP
w为海上风电在输送到海上换流站间的有功功率损耗(在简化计算可考虑忽略)；P
MMC为流入全桥型MMC单元的有功功率；P
dc为直流输电功率。
二极管不控整流单元交直流侧功率及电压满足如下方程约束，忽略二极管阀损耗：
式中，P
dc为直流输电功率；U
dcr为二极管阀输出直流电压(海上站直流电压)；I
dc为直流电流；U
dci为陆上换流站直流电压；U
MMC为全桥型MMC输出交流电压相电压有效值。
为实现全桥型MMC单元子模块电压电容平衡、直流输电系统完全输送新能源功率，需要控制输入全桥型MMC单元的有功功率为零。由于陆上换流站可以通过控制熄弧角实现U
dci恒定，为实现上述目的，需要控制全桥型MMC单元输出交流电压。
具体地，控制方法，包括以下步骤：
(3.1)陆上换流站中晶闸管整流单元采用定直流电压控制策略，通过控制熄弧角使得晶闸管整流单元直流侧电压达到指令参考值U
dc；
(3.2)控制海上换流站中全桥型MMC单元直流侧的输出电压为零，并合上全桥型MMC单元直流侧设置的旁路开关；
(3.3)全桥型MMC单元采用定有功功率和输出交流电压控制策略，同时进行低次谐波(11、13次)抑制控制；
(3.4)海上风电场采用构网型控制，通过二极管不控整流单元将海上风电场所发出的能量输出到直流侧，并经海底直流海缆送出到晶闸管整流单元。
作为一个优选的实施例，上述步骤(3.1)中，以六脉动晶闸管整流单元为例进行介绍，晶闸管整流单元输出的直流电压的计算公式为：
式中，U
dI为晶闸管整流单元直流侧电压，U
dioI为晶闸管整流单元空载直流电压，γ为熄弧角，d
xI为晶闸管整流单元相对感性压降，d
rI为晶闸管整流单元相对阻性压降，U
dioNI为晶闸管整流单元额定空载直流电压，U
T为晶闸管整流单元压降，I
d为直流电流，I
dN为额定直流电流。
作为一个优选的实施例，上述步骤(3.3)中，全桥型MMC单元的控制策略包括以下步骤：
(3.3.1)参考值计算环节：根据直流输电系统的实际工况和假设P
MMC＝0，计算得到全桥型MMC单元的直流电压和交流电压幅值参考值。
(3.3.2)双环功率控制环节：根据得到的直流电压和交流电压幅值参考值，采用双环功率控制方法动态计算内环电流参考值和dq坐标系下的电压参考值。
(3.3.3)桥臂参考值生成环节：根据dq坐标系下的电压参考值，进行坐标变换计算得 到三相交流电压参考值，进而得到全桥型MMC单元6个桥臂的参考电压，用于调制生成开关器件脉冲。
作为一个优选的实施例，上述步骤(3.3.1)中，根据公式(2)-(4)并忽略功率损耗，得到全桥型MMC单元的目标值直流电压和交流电压幅值的计算公式如下：
式中，P
w1为海上风电场输送功率；U
dci为陆上换流站直流电压；R
line为直流线路电阻；U
MMCref为全桥型MMC单元交流电压有效值的参考值。
作为一个优选的实施例，上述步骤(3.3.2)中，如图12～图14所示，双环功率控制包括外环功率环和内环电流环，且外环功率环分为有功功率控制部分和交流电压控制部分，内环电流环为快速控制环。
具体地，外环功率环采用比例积分控制(PI控制)实现，用于计算内环电流参考值，计算公式为：
式中，k
p1和k
p2为比例系数；k
i1和k
i2为积分系数；U
MMCref和
分别为全桥型MMC单元交流电压有效值的参考值(目标值)和测量值；P
MMCref为馈入全桥型MMC的有功功率的参考值和测量值，且P
MMCref＝0。
内环电流环采用比例积分控制(PI控制)实现，电压参考值v
dref和v
qrefd的计算公式为：
式中，i
dref和
分别为dq坐标系下d轴电流值的参考值和测量值；i
qref和
分别为dq坐标系下q轴电流值的参考值和测量值；k
p3和k
p4为比例系数；k
i3和k
i4为积分系数；
和
为扰动分量，即交流电网电压前馈项；L为桥臂电抗器等效电抗；ω为基频角频率。
作为一个优选的实施例，上述步骤(3.3.3)中，全桥型MMC单元直流电压控制为0，6个桥臂参考电压的计算公式为：
式中，U
apref、U
anref、U
bpref、U
bnref、U
cpref、U
cnref分别为全桥型MMC单元的a相上桥臂、a相下桥臂、b相上桥臂、b相下桥臂、c相上桥臂、c相下桥臂电压参考值。
作为一个优选的实施例，上述步骤(4)中，海上风电场采用跟网型控制策略，即海上交流系统处于无源状态时，全桥型MMC单元必须切换为无源孤岛控制器，以维持海上风电 场侧孤网频率和电压稳定。与步骤(3)中本质区别在于海上风电场风机是否具有自构网能力，整个系统控制策略框架和系统等值模型与构网型控制策略是类似的，但存在部分控制策略变化。正常运行时，交流侧全桥型MMC单元为海上风电场提供并网电压和频率，海上风电场风机仍可沿用原有常规控制策略，进行控制。
此时的控制方式为定交流电压控制和定交流频率控制。风场侧孤岛控制的MMC换流站可在给定的电压和频率下运行，因此锁相同步信号由换流站生成，阀组级控制器的输入。稳态下，换流站的参考频率f将保持50Hz恒定，交流电压将由外环电压控制和内环电流控制构成。这种控制不仅能快速的跟踪实际电流，更能在风场侧发生交流故障时限制故障电流。
具体地，包括以下步骤：
(4.1)陆上换流站中晶闸管换流单元采用定直流电压控制策略，通过控制熄弧角使得晶闸管整流单元直流侧电压达到指令参考值U
dc；
(4.2)控制海上换流站中全桥型MMC单元直流侧的输出电压为零，并合上全桥型MMC单元直流侧设置的旁路开关；
(4.3)全桥型MMC单元采用定交流电压和交流频率控制策略；
(4.4)海上风电场采用跟网型控制，通过二极管不控整流单元将海上风电场所发出的能量输出到直流侧，并经海底直流海缆送出到晶闸管整流单元。
作为一个优选的实施例，上述步骤(4.1)和(4.2)与构网型控制策略相同，本发明在此不再赘述。
作为一个优选的实施例，上述步骤(4.3)中，全桥型MMC单元采用定交流电压和交流频率控制策略，也即采用带直接反馈信号的直接电压控制，其交流电压的计算公式为：
式中，V
mref为全桥型MMC单元三相交流电压的参考值；U
MMCref和
为全桥型MMC单元交流电压有效值的参考值和测量值；
为扰动分量，即交流电网电压前馈项；k
p5和k
i5分别为比例系数和积分系数。
如图15所示，为无源逆变的直接电压控制框图，可以看出，全桥型MMC单元三相交流电压的参考值V
mref由全桥型MMC单元输出交流相电压幅值的参考值U
MMCref的直馈信号和U
MMCref与交流电压测量值
的负反馈(feed-back)PI信号两部分相加组成；直馈信号的引入保证了电压响应的快速性，负反馈PI控制可以消除稳态误差，并提高系统的稳定性。正常情况下，网侧交流相电压幅值的参考值U
MMCref设为额定值(1.0pu)。
在确定了换流器输出指令值V
mref后，全桥型MMC单元三相交流电压取值如下：
式中，f
ref为电网参考频率，取50Hz；δ为电压相位(取δ＝0)；t为时间；v
aref、v
bref和v
cref分别为全桥型MMC单元21的交流侧输出的a相电压、b相电压、和c相电压，式(13)保证了海上风电系统频率为额定频率。
上述各实施例仅用于说明本发明，其中各部件的结构、连接方式和制作工艺等都是可以有所变化的，凡是在本发明技术方案的基础上进行的等同变换和改进，均不应排除在本发明的保护范围之外。
Claims (10)
- 一种海上风电不控整流直流输电系统的控制方法，其特征在于，包括以下步骤：设置一海上风电不控整流直流输电系统，该系统包括：海上风电场、海上换流站、正负极海底直流海缆、陆上换流站和陆上电网；所述海上风电场包括依次相连的风力发电机、机端变流器、网端变流器和海上升压变压器，且所述海上升压变压器的交流侧经海上交流母线与所述海上换流站的交流侧相连，所述海上换流站的直流侧经所述正负极海底直流海缆与所述陆上换流站的直流侧相连；所述陆上换流站的交流侧与所述陆上电网相连；所述海上换流站包括全桥型MMC单元和二极管不控整流单元，所述全桥型MMC单元用于在海上风电场黑启动或正常运行时，为所述海上风电场提供启动电源或并网电压，且所述全桥型MMC单元的直流侧还设置有旁路开关；所述二极管不控整流单元用于对所述海上风电场输出的电能进行整流后传送到所述陆上换流站；所述陆上换流站包括晶闸管整流单元；黑启动过程中，控制陆上换流站对海上换流站中的全桥型MMC单元进行预充电，通过全桥型MMC单元对海上风电场进行黑启动；正常运行时，根据海上风电场内的风力发电机是否具备构网能力，选择构网型控制策略或跟网型控制策略，将海上风电场所发出的能量输出到直流侧，并经所述正负极海底直流海缆送出至陆上换流站和陆上电网。
- 如权利要求1所述的一种海上风电不控整流直流输电系统的控制方法，其特征在于，所述通过全桥型MMC单元对海上风电场进行黑启动的方法，包括以下步骤：解锁晶闸管整流单元，对全桥型MMC单元进行预充电，使得全桥型MMC单元解锁；控制全桥型MMC单元的交流侧输出电压，对海上风电场待黑启动区域内的全部风力发电机依次进行黑启动；黑启动过程结束后，全桥型MMC单元进入正常运行时工作模式。
- 如权利要求2所述的一种海上风电不控整流直流输电系统的控制方法，其特征在于，所述解锁晶闸管整流单元，对全桥型MMC单元进行预充电，使得全桥型MMC单元解锁的方法，包括：断开全桥型MMC单元直流侧的旁路开关；断开全桥型MMC单元、二极管不控整流单元与海上风电场交流侧的连接，合上全桥型MMC单元、二极管不控整流单元与海上风电场直流侧的相关开关，使得直流侧回路畅通；合上晶闸管整流单元与陆上电网交流侧的相关开关；解锁晶闸管整流单元，通过控制熄弧角使得晶闸管整流单元直流侧输电电压为负电势，经正负极海底直流海缆和二极管不控整流单元对全桥型MMC单元进行不控预充电；当全桥型MMC单元内子模块电容电压充电到可控充电阈值后，全桥型MMC单元进入有序可控充电阶段，直至充电到子模块电容电压额定值；晶闸管整流单元控制全桥型MMC单元直流侧电压上升到启动阶段直流电压预设值后，全桥型MMC单元解锁。
- 如权利要求2所述的一种海上风电不控整流直流输电系统的控制方法，其特征在于，所述控制全桥型MMC单元的交流侧输出电压，对海上风电场待黑启动区域内的全部风力发电机依次进行黑启动的方法，包括以下步骤：①合上全桥型MMC单元与海上风电场交流侧的开关；②合上海上风电场待黑启动区域至全桥型MMC单元之间回路相关开关；③通过控制全桥型MMC单元交流侧输出电压对海上风电场待黑启动区域内风力发电机的机端变流器和网端变流器的直流侧进行充电，将风力发电机的机端变流器和网端变流器直流侧电压电容从零充电到机端变流器和网端变流器的充电阈值，使机端变流器和网端变流 器进入可控有序充电阶段；④待黑启动区域内的风力发电机的机端变流器和网端变流器通过可控有序充电将风力发电机的机端变流器和网端变流器直流侧电容电压充电到额定值后，风力发电机解锁，控制风力发电机出力为零；⑤重复步骤②-④，将待黑启动区域内的剩余风力发电机均启动起来；⑥全桥型MMC单元进入正常运行时工作模式，黑启动过程结束。
- 如权利要求4所述的一种海上风电不控整流直流输电系统的控制方法，其特征在于，所述全桥型MMC单元容量根据海上风电场内的待启动风力发电机的数量确定，方法如下：S＝k*λ*P式中，S为全桥型MMC单元容量；k为裕度系数；λ为一次启动中具有最多风力发电机的区域所含风力发电机的数目；P为单台风力发电机容量。
- 如权利要求1所述的一种海上风电不控整流直流输电系统的控制方法，其特征在于，当采用构网型控制策略时，包括以下步骤：陆上换流站中晶闸管整流单元采用定直流电压控制策略，通过控制熄弧角使得晶闸管整流单元直流侧电压达到指令参考值；控制海上换流站中全桥型MMC单元直流侧的输出电压为零，并合上全桥型MMC单元直流侧设置的旁路开关；全桥型MMC单元采用定有功功率和输出交流电压控制策略，同时进行低次谐波抑制控制；海上风电场采用构网型控制，通过二极管不控整流单元将海上风电场所发出的能量输出到直流侧，并经正负极海底直流海缆送出到晶闸管整流单元。
- 如权利要求6所述的一种海上风电不控整流直流输电系统的控制方法，其特征在于，所述晶闸管整流单元输出的直流电压的计算公式为：
式中，U dI为晶闸管整流单元直流侧电压，U dioI为晶闸管整流单元空载直流电压，γ为熄弧角，d xI为晶闸管整流单元相对感性压降，d rI为晶闸管整流单元相对阻性压降，U dioNI为晶闸管整流单元额定空载直流电压，U T为晶闸管整流单元压降，I d为直流电流，I dN为额定直流电流。 - 如权利要求6所述的一种海上风电不控整流直流输电系统的控制方法，其特征在于，所述全桥型MMC单元采用定有功功率和输出交流电压控制策略，同时进行低次谐波抑制控制的方法，包括：根据直流输电系统的实际工况以及假设的全桥型MMC单元直流侧的输出电压，计算得到全桥型MMC单元的直流电压和交流电压幅值参考值；根据得到的直流电压和交流电压幅值参考值，采用双环功率控制方法动态计算内环电流参考值和dq坐标系下的电压参考值；根据dq坐标系下的电压参考值，计算得到全桥型MMC单元6个桥臂的参考电压，用于调制生成开关器件脉冲。
- 如权利要求1所述的一种海上风电不控整流直流输电系统的控制方法，其特征在于，当采用跟网型控制策略时，包括以下步骤：陆上换流站中晶闸管换流单元采用定直流电压控制策略，通过控制熄弧角使得晶闸管整流单元直流侧电压达到指令参考值；控制海上换流站中全桥型MMC单元直流侧的输出电压为零，并合上全桥型MMC单元直流侧设置的旁路开关；全桥型MMC单元采用定交流电压和交流频率控制策略；海上风电场采用跟网型控制，通过二极管不控整流单元将海上风电场所发出的能量输出到直流侧，并经海底直流海缆送出到晶闸管整流单元。
- 如权利要求9所述的一种海上风电不控整流直流输电系统的控制方法，其特征在于，所述全桥型MMC单元采用定交流电压和交流频率控制策略时，其三相交流电压参考值的计算公式为：
式中，V mref为全桥型MMC单元三相交流电压的参考值；U MMCref和为全桥型MMC单元交流电压有效值的参考值和测量值；
为扰动分量，即交流电网电压前馈项；k p5和k i5分别为比例系数和积分系数。
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