CN116937649A - Starting method of offshore wind power flexible low-frequency sending-out system - Google Patents

Starting method of offshore wind power flexible low-frequency sending-out system Download PDF

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Publication number
CN116937649A
CN116937649A CN202310706798.0A CN202310706798A CN116937649A CN 116937649 A CN116937649 A CN 116937649A CN 202310706798 A CN202310706798 A CN 202310706798A CN 116937649 A CN116937649 A CN 116937649A
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China
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grid
converter
voltage
reference value
wind turbine
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CN202310706798.0A
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Inventor
黄伟煌
彭发喜
刘涛
蔡东晓
李岩
黄小威
肖晃庆
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South China University of Technology SCUT
CSG Electric Power Research Institute
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South China University of Technology SCUT
CSG Electric Power Research Institute
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Priority to CN202310706798.0A priority Critical patent/CN116937649A/en
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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
    • 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
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • 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
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
    • 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
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/28—The renewable source being wind energy
    • 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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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Control Of Eletrric Generators (AREA)

Abstract

The application discloses a starting method of a flexible low-frequency sending-out system of offshore wind power, which comprises the following steps: starting an auxiliary converter, calculating a phase reference value of the auxiliary converter, controlling the d-axis voltage of the low-frequency alternating current bus to rise from zero slope to a voltage initial value, and setting the q-axis voltage reference value of the low-frequency alternating current bus to be 0; calculating a d-axis voltage reference value of the low-frequency alternating current bus, and improving the voltage of the low-frequency alternating current bus to be close to a rated value; starting the wind turbine generator one by one; when the active power output by the auxiliary converter is reduced to zero, calculating a d-axis voltage reference value of the low-frequency alternating current bus, and controlling the active power output by the auxiliary converter to be zero. The flexible low-frequency sending-out system of the offshore wind power adopting the starting method can establish the alternating voltage of the offshore low-frequency alternating current system through the auxiliary converter to provide starting energy for the wind turbine, and the wind turbine adopting the starting method can realize smooth starting.

Description

Starting method of offshore wind power flexible low-frequency sending-out system
Technical Field
The application relates to the technical field of starting of wind turbines, in particular to a starting method of a flexible low-frequency sending-out system for offshore wind power.
Background
The offshore wind power flexible low-frequency sending-out system based on DRU (diode rectification unit, diode Rectifier Unit) can increase the distance of offshore wind power sending-out, reduce the difficulty of system construction and maintenance, and promote the economical efficiency of the system.
In a DRU-based offshore wind power flexible low-frequency delivery system, the alternating frequency of an offshore wind farm is lower than that of a land grid, and starting energy cannot be directly obtained from the land grid through an alternating current line. The DRU has unidirectional trend, and cannot transmit active power to the offshore wind farm. Thus, the offshore wind farm requires an additional black start power supply to provide the energy required for starting the wind turbines. The existing black start power supply of the wind farm can be divided into the following three types: diesel generator, energy storage device and auxiliary converter. The existing starting method taking the diesel generator as the black starting power supply does not consider the condition of accessing the DRU, but the cost of the energy storage device is higher, and the system construction cost can be greatly increased. In the existing starting method using the auxiliary converter as the black starting power supply, the auxiliary converter is required to transmit part of wind power under normal working conditions besides providing starting energy for the offshore wind turbine, so that the capacity of the auxiliary converter is large, and the system cost is not facilitated to be reduced.
Disclosure of Invention
The application aims to solve the defects in the prior art and provides a starting method of an offshore wind power flexible low-frequency sending-out system. According to the starting method, a small-capacity auxiliary converter connected with the DRU in parallel is arranged in a land converter station and used as a black starting power supply to establish alternating current voltage of an offshore wind power plant, so that the wind turbine can acquire initial energy, and meanwhile, the system construction cost is reduced.
The aim of the application can be achieved by adopting the following technical scheme:
a starting method of a flexible low-frequency offshore wind power transmission system comprises the following steps: the system comprises an offshore wind farm, an alternating current cable, a land converter station and an alternating current power grid, wherein the offshore wind farm comprises a wind turbine generator set and a step-up transformer; the land convertor station comprises a low-frequency alternating current bus, a diode rectifying unit (diode rectifier unit, DRU), a modularized multi-level converter (Modular Multilevel Converter, MMC), an auxiliary converter, an alternating current filter and a shunt reactor, wherein the auxiliary converter adopts a two-level voltage source type converter (voltage source converter, VSC) topology. The wind turbine generator in the offshore wind farm is connected with an alternating current cable after passing through a step-up transformer, the alternating current cable is connected with a low-frequency alternating current bus in a land convertor station, a DRU alternating current side, an auxiliary convertor alternating current side, an alternating current filter and a shunt reactor are connected in parallel at the low-frequency alternating current bus, a DRU direct current side is connected with an MMC direct current side, and an MMC alternating current side is connected with an alternating current power grid;
the starting method comprises the following steps:
s1, starting an auxiliary converter, calculating a phase reference value of the auxiliary converter, and controlling the grid-connected point d-axis voltage of the auxiliary converter to rise from a zero slope to a first initial value u of the d-axis voltage gd0 Controlling the voltage of the q-axis of the parallel grid point of the auxiliary converter to be 0;
s2, calculating a first reference value of the grid-connected point d axis voltage of the auxiliary converter, and improving the voltage of the low-frequency alternating current bus to be close to a rated value;
s3, starting the wind turbine generators one by one, and adopting power reduction control after starting;
the starting process of the wind turbine generator is as follows:
s31, closing an alternating current filter at the grid side of the wind turbine generator system, and precharging a direct current capacitor of the converter;
s32, unlocking a grid-side converter phase-locked loop, and observing the voltage amplitude and the phase of grid-connected points of the wind turbine generator;
s33, starting a fixed direct current voltage control mode of the grid-side converter, unlocking the grid-side converter, calculating a phase reference value of the grid-side converter and a first reference value of grid-connected point d axis voltage of the wind turbine, and setting a reference value of grid-connected point q axis voltage of the wind turbine to be 0;
s34, starting a constant direct current voltage control mode of the side converter, unlocking the side converter, starting an active power control mode of the grid side converter, and calculating a grid-connected point d axis voltage second reference value of the wind turbine;
s4, when the active power output by the auxiliary converter is reduced to zero, calculating a second reference value of the grid-connected point d axis voltage of the auxiliary converter, and controlling the active power output by the auxiliary converter to be zero;
s5, all the wind turbine generators boost the output active power and enter a stable running state.
Further, in the step S1, the phase reference value θ of the auxiliary converter a * The calculation formula of (2) is as follows:
where s is the Laplacian, ω base Is the reference value of the voltage frequency omega 0 For initial value of voltage frequency, K G And K T The ratio parameter and the time parameter of the first-order inertial controller are respectively Q a To assist the reactive power actual value of the converter, Q a * Is a reactive power reference value for the auxiliary converter. And calculating a phase reference value by the auxiliary converter through reactive power, so that the auxiliary converter and the wind turbine generator set synchronously run.
Further, in the step S2, a first reference value of the auxiliary converter grid-connected point d-axis voltageThe calculation formula of (2) is as follows:
where s is the Laplacian, u gd0 A first initial value of d-axis voltage; p (P) DRU * And P DRU The active power reference value and the actual value of the DRU respectively; k (k) pD And k iD The proportional and integral parameters of the first active power controller, respectively. By setting lower auxiliary convertersThe DRU is controlled to flow a small amount of active power, so that the calculated +.>Near 1p.u., the low frequency ac bus voltage is raised to near nominal.
Further, in the step S4, a calculation formula of the second reference value of the auxiliary converter grid-connected point d-axis voltage is as follows:
wherein s is the Laplacian,is the second initial value of d-axis voltage, P a Is the active power of the auxiliary converter; k (k) pa And k ia The proportional and integral parameters of the second active power controller, respectively. The auxiliary converter controls the active power of the auxiliary converter to be 0, so that all active power emitted by the offshore wind farm is sent out through the DRU, the auxiliary converter does not transmit active power, the capacity of the auxiliary converter is reduced, and the system construction cost is reduced.
Further, in the step S33, the phase reference value θ of the grid-side converter * The calculation formula is as follows:
where s is the Laplacian, ω base Is the reference value of the voltage frequency omega 0 Is the initial value of the voltage frequency, K G And K T The ratio parameter and the time parameter of the first-order inertial controller are respectively, Q is the actual reactive power value of the wind turbine generator, and Q is the actual reactive power value of the wind turbine generator * Is the reactive power reference value delta of the wind turbine generator 0 The voltage phase of the fan grid-connected point when the grid-side converter is unlocked; when the grid-side converter is unlocked, the phase reference value of the grid-side converter is delta 0 ，δ 0 The phase reference value of the grid-side converter is the same as the voltage phase of the grid-connected point of the fan.
Grid-connected point d axis voltage first reference value of wind turbine generatorThe calculation formula of (2) is as follows:
where s is Laplacian, U dc AndU dc * respectively a direct-current voltage reference value and an actual value, k of the wind turbine generator pd And k id Proportional and integral parameters, u, of the DC voltage controller, respectively fd0 And (5) the voltage amplitude of the grid-connected point of the fan when the grid-side converter is unlocked. When the grid-side converter is unlocked, the d-axis voltage of the grid-side converter is a first reference value u fd0 ，u fd0 And the d-axis voltage first reference value of the grid-side converter is the same as the voltage amplitude of the grid-connected point of the fan.
Further, in step S34, a second reference value is obtained for the grid-connected point d-axis voltage of the wind turbineThe calculation formula is as follows:
where s is Laplacian, P s * And P s Respectively an active power reference value and an actual value of the wind turbine generator; k (k) pp And k ip The proportional parameter and the integral parameter of the active power controller of the fan are respectively, u fd0 And (5) the voltage amplitude of the grid-connected point of the fan when the grid-side converter is unlocked. The net side converter controls the output active power through the d-axis voltage.
Compared with the prior art, the application has the following advantages and effects:
(1) In the application, the auxiliary converter is used for establishing alternating voltage of the offshore low-frequency alternating current system and providing starting energy for the wind turbine. Because the wind turbine generator is started one by one, the auxiliary converter only needs to provide active power required by controlling alternating voltage and active power required by starting one wind turbine generator set in the whole starting process, and after the wind turbine starts to output active power, the auxiliary converter controls the active power to be zero and controls all wind power to be transmitted through the DRU. Therefore, the capacity required by the auxiliary converter is smaller, and the system construction cost can be reduced.
(2) According to the application, the wind turbine generator can control the direct-current voltage of the converter through the alternating-current voltage amplitude. By observing the voltage amplitude and the phase of the grid-connected point of the fan before the grid-side converter is unlocked, the voltage output by the grid-side converter during unlocking is identical to the voltage of the grid-side converter during unlocking, and power fluctuation during unlocking of the grid-side converter can be reduced.
(3) According to the method, the phase reference value is calculated according to reactive power by the auxiliary converter and the wind turbine generator system network side converter, so that the auxiliary converter and the wind turbine generator system can keep synchronous operation.
Drawings
The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and together with the description serve to explain the application and do not constitute a limitation on the application. In the drawings:
FIG. 1 is a block diagram of an offshore wind turbine low frequency AC delivery system of the present disclosure;
FIG. 2 is a flow chart of a method of starting an offshore wind turbine low frequency ac delivery system of the present disclosure;
FIG. 3 is a flow chart of a wind turbine start-up method of the offshore wind turbine low frequency AC delivery system disclosed by the application;
FIG. 4 is a waveform diagram of simulation of voltage amplitude of a common connection point of a fan in a test system according to the starting method of the offshore wind power low-frequency alternating current transmission system disclosed by the application;
FIG. 5 is a waveform diagram of the auxiliary converter active power simulation in a test system for the method for starting the offshore wind power low-frequency AC delivery system disclosed by the application;
FIG. 6 is a simulated waveform diagram of DC voltage of a fan converter in a test system according to the method for starting the offshore wind power low-frequency AC transmission system disclosed by the application;
FIG. 7 is a waveform diagram of simulation of active power of a wind turbine in a test system according to the method for starting an offshore wind power low-frequency AC delivery system disclosed by the application;
FIG. 8 is a waveform diagram of simulation of voltage amplitude of a common connection point of a fan in a first test system by using the starting method of the offshore wind power low-frequency alternating current transmission system disclosed by the application;
FIG. 9 is a waveform diagram of the active power simulation of the auxiliary converter in the second test system of the starting method of the offshore wind power low-frequency alternating current transmission system disclosed by the application;
FIG. 10 is a simulated waveform diagram of DC voltage of a fan converter in a second test system of the method for starting the offshore wind power low-frequency AC transmission system disclosed by the application;
FIG. 11 is a waveform diagram of the active power simulation of a fan in a second test system of the method for starting the offshore wind power low-frequency alternating current transmission system.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present application more apparent, the technical solutions of the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application, and it is apparent that the described embodiments are some embodiments of the present application, but not all embodiments of the present application. All other embodiments, which can be made by those skilled in the art based on the embodiments of the application without making any inventive effort, are intended to be within the scope of the application.
Example 1
The starting method of the offshore wind power flexible low-frequency sending-out system provided by the embodiment of the application is based on the offshore wind power flexible low-frequency sending-out system shown in fig. 1, and comprises the following steps: the system comprises an offshore wind farm, an alternating current cable, a land converter station and an alternating current power grid, wherein the offshore wind farm comprises a wind turbine generator set and a step-up transformer; the land converter station comprises a low-frequency alternating current bus, diode rectifying units (diode rectifier unit, DRUs), a modularized multi-level converter (Modular Multilevel Converter, MMC), an auxiliary converter, an alternating current filter and a shunt reactor, wherein the auxiliary converter is responsible for establishing alternating current voltage of an offshore low-frequency alternating current system, therefore, VSC is needed, and because the auxiliary converter has smaller capacity, the auxiliary converter adopts two-level VSC topology in order to reduce the number of switching devices and the system construction cost. The wind turbine generator in the offshore wind farm is connected with an alternating current cable after passing through a step-up transformer, the alternating current cable is connected with a low-frequency alternating current bus in a land convertor station, a DRU alternating current side, an auxiliary convertor alternating current side, an alternating current filter and a shunt reactor are connected in parallel at the low-frequency alternating current bus, a DRU direct current side is connected with an MMC direct current side, and an MMC alternating current side is connected with an alternating current power grid;
the starting method of the offshore wind power flexible low-frequency sending-out system provided by the embodiment of the application, as shown in fig. 3, comprises the following steps:
s1, starting an auxiliary converter, calculating a phase reference value of the auxiliary converter, and controlling the grid-connected point d-axis voltage of the auxiliary converter to rise from a zero slope to a first initial value u of the d-axis voltage gd0 Controlling the voltage of the q-axis of the parallel grid point of the auxiliary converter to be 0;
in the embodiment of the application, the phase reference value theta of the auxiliary converter a * The calculation formula of (2) is as follows:
where s is the Laplacian, ω base Is the reference value of the voltage frequency omega 0 For initial value of voltage frequency, K G And K T The ratio parameter and the time parameter of the first-order inertial controller are respectively Q a To assist the reactive power actual value of the converter, Q a * Is a reactive power reference value for the auxiliary converter.
In an embodiment of the application, u gd0 The conduction voltage is lower than that of the DRU, so that the DRU is not conducted and no active power flows.
S2, calculating a first reference value of the grid-connected point d axis voltage of the auxiliary converter, and improving the voltage of the low-frequency alternating current bus to be close to a rated value;
first reference value of auxiliary converter grid-connected point d axis voltageThe calculation formula of (2) is as follows:
where s is the Laplacian, u gd0 A first initial value of d-axis voltage; p (P) DRU * And P DRU The active power reference value and the actual value of the DRU respectively; k (k) pD And k iD The proportional and integral parameters of the first active power controller, respectively. The auxiliary converter controls the DRU to flow a small amount of active power to raise the low-frequency alternating current bus voltage to be close to the rated value.
In the embodiment of the application, the relation between the voltage amplitude of the low-frequency alternating-current bus and the DRU active power is as follows:
wherein T is DRU For converting transformer transformation ratio，X T For leakage reactance of converter transformer, V dcDRU Is DRU direct current voltage, U g Is the DRU alternating current bus voltage amplitude. It can be seen that the auxiliary converter only needs to control DRU conduction and flow a small amount of active power to maintain the low frequency ac bus voltage amplitude near the nominal value. By choosing a smaller P DRU * The active power output by the auxiliary converter can be smaller, and the calculated u gd * Can approach the rated value and meet the requirement of system pressure building.
S3, starting the wind turbine generators one by one, and adopting power reduction control after starting;
the starting step of the wind turbine generator comprises the following steps:
s31, closing an alternating current filter at the grid side of the wind turbine generator system, and precharging a direct current capacitor of the converter;
in the embodiment of the application, in order to reduce the impact on the auxiliary converter in the pre-charging process, the grid-side alternating current circuit breaker of the first started wind turbine generator is closed before the auxiliary converter starts to build voltage, and pre-charging is performed while the auxiliary converter builds voltage. And after the starting of the last fan is completed, the other fans close the network side alternating current breaker when the fan starts to start.
S32, unlocking a grid-side converter phase-locked loop, and observing the voltage amplitude and the phase of grid-connected points of the wind turbine generator;
s33, starting a fixed direct current voltage control mode of the grid-side converter, unlocking the grid-side converter, calculating a phase reference value of the grid-side converter and a first reference value of grid-connected point d axis voltage of the wind turbine, and setting a reference value of grid-connected point q axis voltage of the wind turbine to be 0;
phase reference value θ of grid-side converter * The calculation formula is as follows:
where s is the Laplacian, ω base Is the reference value of the voltage frequency omega 0 Is the initial value of the voltage frequency, K G And K T Respectively, first-order inertial controlThe proportion parameter and the time parameter of the generator are Q is the actual reactive power value of the wind turbine generator, Q * Is the reactive power reference value delta of the wind turbine generator 0 The voltage phase of the fan grid-connected point when the grid-side converter is unlocked;
grid-connected point d axis voltage first reference value of wind turbine generatorThe calculation formula of (2) is as follows:
where s is Laplacian, U dc AndU dc * respectively a direct-current voltage reference value and an actual value, k of the wind turbine generator pd And k id Proportional and integral parameters, u, of the DC voltage controller, respectively fd0 And (5) the voltage amplitude of the grid-connected point of the fan when the grid-side converter is unlocked.
According to the phase reference value theta of the network-side converter * Calculation formula and wind turbine generator grid-connected point d axis voltage first reference valueWhen the network side converter is unlocked, the phase reference value theta of the network side converter * The voltage phase of the grid-connected point of the wind turbine generator is identical to the voltage phase of the grid-connected point of the fan, and the d axis voltage of the grid-connected point of the wind turbine generator is a first reference value +.>And the voltage amplitude of the grid-connected point of the wind turbine generator is the same as that of the wind turbine generator, and the q-axis voltage reference value of the grid-connected point of the wind turbine generator is 0. Therefore, the voltage of the grid-side converter is the same as that of the grid-connected point of the fan, and the power fluctuation of the grid-side converter during unlocking can be reduced.
S34, starting a constant direct current voltage control mode of the side converter, unlocking the side converter, starting an active power control mode of the grid side converter, and calculating a grid-connected point d axis voltage second reference value of the wind turbine;
grid-connected point d axis voltage second reference value of wind turbine generatorThe calculation formula is as follows:
where s is Laplacian, P s * And P s Respectively an active power reference value and an actual value of the wind turbine generator; k (k) pp And k ip The proportional parameter and the integral parameter of the active power controller of the fan are respectively, u fd0 And (5) the voltage amplitude of the grid-connected point of the fan when the grid-side converter is unlocked.
In the embodiment of the application, after the grid-side converter obtains the d-axis voltage reference value and the q-axis voltage reference value of the grid-connected point of the wind turbine, the d-axis voltage reference value and the q-axis voltage reference value of the modulation voltage of the grid-side converter are obtained through double inner loop control calculation, and the phase reference value theta of the grid-side converter is calculated according to the phase reference value theta of the grid-side converter * After park inverse transformation is carried out to obtain an a-axis voltage reference value, a b-axis voltage reference value and a c-axis voltage reference value of the modulation voltage under an abc static coordinate system, control pulses corresponding to all IGBTs of the grid-side converter can be generated through a common pulse width modulation theory, and therefore the grid-side converter of the offshore wind turbine generator can be controlled.
S4, when the active power output by the auxiliary converter is reduced to zero, calculating a second reference value of the grid-connected point d axis voltage of the auxiliary converter, and controlling the active power output by the auxiliary converter to be zero;
the calculation formula of the second reference value of the auxiliary converter grid-connected point d axis voltage is as follows:
wherein s is the Laplacian,is the second initial value of d-axis voltage, P a Is the active power of the auxiliary converter; k (k) pa And k ia The proportional and integral parameters of the second active power controller, respectively.
In the embodiment of the application, according to active power balance, the active power emitted by the auxiliary converter is that
P a ＝P DRU -P WT
Wherein P is wt Is the active power generated by the offshore power generation field. As the wind turbines are started up one by one, the active power emitted by the offshore wind farm increases, and the active power emitted by the auxiliary converter gradually decreases. After the active power sent by the auxiliary converter is reduced to 0, the auxiliary converter controls the active power sent by the auxiliary converter to be 0, so that all active power sent by the offshore wind farm is sent out through the DRU, the auxiliary converter does not transmit the active power, the capacity of the auxiliary converter is reduced, and the system construction cost is reduced.
S5, all the wind turbine generators boost the output active power and enter a stable running state.
In the embodiment of the application, after the auxiliary converter obtains the d-axis voltage reference value and the q-axis voltage reference value of the auxiliary converter grid-connected point, the auxiliary converter obtains the d-axis voltage reference value and the q-axis voltage reference value of the modulation voltage of the auxiliary converter through double-inner-loop control calculation, and the modulation voltage d-axis voltage reference value and the q-axis voltage reference value of the auxiliary converter are calculated according to the phase reference value theta of the auxiliary converter a * After park inverse transformation is carried out to obtain an a-axis voltage reference value, a b-axis voltage reference value and a c-axis voltage reference value of the modulation voltage under an abc static coordinate system, a common nearest level approximation modulation theory can be adopted to generate control pulses corresponding to all sub-modules of the auxiliary converter, and therefore the auxiliary converter can be controlled.
Example 2
Based on the starting method of the flexible low-frequency sending-out system for offshore wind power disclosed in the embodiment 1, in the embodiment, simulation verification is carried out by adopting a test system comprising 4 offshore wind turbines, the capacity of each offshore wind turbine is 100MW, the capacity of an auxiliary converter is 20MW, and the rated frequency of the system is 20Hz.
First, the auxiliary converter establishes an ac voltage of the low frequency ac system. The auxiliary converter starts to build up an ac voltage at time t=1.0 s, controlling the offshore ac voltage to increase from zero to 0.9p.u. at a rate of 0.45p.u./s, during which time the DRU is not conducting. At time t=3.0 s the auxiliary converter controls the DRU to transmit active power at 5MW. In this process, the grid-side ac circuit breaker of the blower 1 is kept closed and precharged while the auxiliary converter builds up voltage.
After that, the fans start to start up one by one. When time t=3.5 s, the fan 1 unlocks the phase-locked loop of the grid-side converter, and the voltage amplitude and the phase of the grid-connected point of the fan 1 are observed. At time t=4.0 s the fan 1 grid side inverter is unlocked, controlling the direct voltage of the fan 1 to 1p.u.. And when the time t=5.0 s, the fan 1 side converter is unlocked, the net side converter converts the active power of the control output, and the reference value of the active power is increased from 0 to 10MW. At time t=5.5s, t=7.5s and t=9.5s, fans 2, 3 and 4 are started in sequence, and the output active power is controlled to be 10MW.
Finally, at time t=12 s all fans increase the emitted active power linearly to 100MW with a rise speed of 30MW/s. To compensate for DRU absorption reactive power, a set of ac filters is put into operation at time t=12.6 s, and a set of shunt reactors is withdrawn at times t=13.2 s, t=13.8 s, and t=14.4 s, respectively.
Fig. 4 shows a variation of the voltage amplitude of the common connection point of the fans in the test system one. After time t=3.0 s, the alternating voltage amplitude is increased to 0.97p.u. along with the active power transmitted to 5MW by the auxiliary converter control DRU, the effect that the control voltage amplitude is close to the rated value is achieved, the voltage amplitude of the common connection point of the fan is maintained near the rated value in the starting process, and the voltage fluctuation is small in the starting process.
Fig. 5 shows the active power variation of the auxiliary converter in a test system. It can be seen that when the fan 1 grid-side converter is unlocked to perform controllable charging at time t=4.0s, the active power emitted by the auxiliary converter is maximum and is about 9MW. Along with the rising of the active power of the fan, when the time t=5.2 s, the active power emitted by the auxiliary converter is reduced to 0, and then the auxiliary converter controls the active power emitted by the auxiliary converter to be 0, so that all the active power emitted by the fan is emitted through the DRU. The active power of the auxiliary converter changes smoothly during the starting process.
Fig. 6 shows a variation curve of the dc voltage of the fan converter in the test system one. The voltage of the d axis of the grid-connected grid of the fan can be controlled to charge by the grid-side current converter, and the direct current voltage of the fan current converter is controlled to be a rated value.
Fig. 7 shows a variation curve of active power of the fan in the first test system. Through the observation of the voltage amplitude and the phase, the power change of the fan is smoother in the starting process, and the phenomena of large impact and instability are avoided.
Example 3
Based on the starting method of the flexible low-frequency sending-out system of the offshore wind power disclosed in the embodiment 1, simulation verification is carried out by adopting a test system II comprising 5 offshore wind turbines, the capacity of each offshore wind turbine is 200MW, the capacity of an auxiliary converter is 35MW, and the rated frequency of the system is 20Hz.
At time t=1.0 s, the auxiliary converter is unlocked, the ac voltage starts to build up, the ac voltage at sea increases linearly from zero to 0.9p.u., the rate of rise is 0.45p.u./s. The auxiliary converter controls the DRU to transmit active power at 10MW at time t=3.0 s. In this stage the fan 1 grid side ac circuit breaker remains closed and precharges. At time t=4.0 s the fan 1 grid side inverter is unlocked, controlling the direct voltage of the fan 1 to 1p.u.. At time t=5.0 s the fan 1 side converter is unlocked and the grid side converter controls the fan active power reference to rise from 0 to 10MW. At time t=5.5s, t=7.5s, t=9.5s, and t=11.5s, fans 2, 3, 4 and 5 are started in sequence, and the output active power is controlled to be 20MW. At time t=14s all fans increase the active power emitted linearly to 200MW with a rise speed of 60MW/s. In order to compensate reactive power absorbed by the DRU, switching of the alternating current filter and the shunt reactor is performed while active power of the fan is increased. The capacity of the auxiliary converter in the embodiment is only 3.5% of the total capacity of the system, which is beneficial to reducing the construction cost of the system.
Fig. 8 shows a variation of the voltage amplitude at the common connection point of the two fans in the test system. After time t=3.0 s, the auxiliary converter transmits a small amount of active power by controlling the DRU, and controls the amplitude of the alternating voltage to be increased to 0.976p.u., so that the effect that the amplitude of the control voltage is close to the rated value is achieved, and the voltage amplitude of the common connection point of the fan is maintained near the rated value in the starting process, and the voltage fluctuation is small in the starting process.
Fig. 9 shows the active power variation of the auxiliary converter in the test system two. It can be seen that as the active power emitted by the fan 1 increases, the active power emitted by the auxiliary converter decreases to 0 at time t=5.2 s, and then the auxiliary converter controls the active power emitted by itself to be 0, so that all the active power emitted by the fan is emitted through the DRU. The active power of the auxiliary converter changes smoothly in the whole process, and the maximum value of the active power is about 18MW which is smaller than the capacity of the auxiliary converter.
Fig. 10 shows a graph of the dc voltage of the inverter during the test system. The direct current voltage of the control fan converter can be controlled to be rated value through the control fan grid-connected point d-axis voltage of the visible grid-side converter.
Fig. 11 shows a variation curve of the active power of the fan in the second test system. The power change of the fan is smooth in the starting process, and the phenomena of large impact and instability are avoided.
The above examples are preferred embodiments of the present application, but the embodiments of the present application are not limited to the above examples, and any other changes, modifications, substitutions, combinations, and simplifications that do not depart from the spirit and principle of the present application should be made in the equivalent manner, and the embodiments are included in the protection scope of the present application.

Claims (6)

1. A starting method of an offshore wind power flexible low-frequency sending-out system comprises the following steps: the system comprises an offshore wind farm, an alternating current cable, a land converter station and an alternating current power grid, wherein the offshore wind farm comprises a wind turbine generator set and a step-up transformer; the land converter station comprises a low-frequency alternating current bus, a diode rectifying unit, a modularized multi-level converter, an auxiliary converter, an alternating current filter and a parallel reactor, wherein the diode rectifying unit is called DRU for short, and the modularized multi-level converter is called MMC for short; the wind turbine generator in the offshore wind farm is connected with an alternating current cable after passing through a step-up transformer, the alternating current cable is connected with a low-frequency alternating current bus in a land convertor station, a DRU alternating current side, an auxiliary convertor alternating current side, an alternating current filter and a shunt reactor are connected in parallel at the low-frequency alternating current bus, a DRU direct current side is connected with an MMC direct current side, and an MMC alternating current side is connected with an alternating current power grid; the starting method is characterized by comprising the following steps of:
s1, starting an auxiliary converter, calculating a phase reference value of the auxiliary converter, and controlling the grid-connected point d-axis voltage of the auxiliary converter to rise from a zero slope to a first initial value u of the d-axis voltage gd0 Controlling the voltage of the q-axis of the parallel grid point of the auxiliary converter to be 0;
s2, calculating a first reference value of the grid-connected point d axis voltage of the auxiliary converter, and improving the voltage of the low-frequency alternating current bus to be close to a rated value;
s3, starting the wind turbine generators one by one, and adopting power reduction control after starting;
the starting process of the wind turbine generator is as follows:
s31, closing an alternating current filter at the grid side of the wind turbine generator system, and precharging a direct current capacitor of the converter;
s32, unlocking a grid-side converter phase-locked loop, and observing the voltage amplitude and the phase of grid-connected points of the wind turbine generator;
s33, starting a fixed direct current voltage control mode of the grid-side converter, unlocking the grid-side converter, calculating a phase reference value of the grid-side converter and a first reference value of grid-connected point d axis voltage of the wind turbine, and setting a reference value of grid-connected point q axis voltage of the wind turbine to be 0;
s34, starting a constant direct current voltage control mode of the side converter, unlocking the side converter, starting an active power control mode of the grid side converter, and calculating a grid-connected point d axis voltage second reference value of the wind turbine;
s4, when the active power output by the auxiliary converter is reduced to zero, calculating a second reference value of the grid-connected point d axis voltage of the auxiliary converter, and controlling the active power output by the auxiliary converter to be zero;
s5, all the wind turbine generators boost the output active power and enter a stable running state.
2. The method according to claim 1, wherein in step S1, the phase reference value θ of the auxiliary converter a * The calculation formula of (2) is as follows:
where s is the Laplacian, ω base Is the reference value of the voltage frequency omega 0 For initial value of voltage frequency, K G And K T The ratio parameter and the time parameter of the first-order inertial controller are respectively Q a To assist the reactive power actual value of the converter, Q a * Is a reactive power reference value for the auxiliary converter.
3. The method for starting a flexible low-frequency transmission system for offshore wind power according to claim 1, wherein in the step S2, the auxiliary converter is connected to a first reference value of the d-axis voltageThe calculation formula of (2) is as follows:
where s is the Laplacian, u gd0 A first initial value of d-axis voltage; p (P) DRU * And P DRU The active power reference value and the actual value of the DRU respectively; k (k) pD And k iD The proportional and integral parameters of the first active power controller, respectively.
4. The method for starting the offshore wind power flexible low-frequency transmission system according to claim 1, wherein in the step S4, a second reference value calculation formula of the auxiliary converter grid-connected point d-axis voltage is as follows:
wherein s is the Laplacian,is the second initial value of d-axis voltage, P a Is the active power of the auxiliary converter; k (k) pa And k ia The proportional and integral parameters of the second active power controller, respectively.
5. The method according to claim 1, wherein in step S33, the phase reference value θ of the grid-side inverter * The calculation formula is as follows:
where s is the Laplacian, ω base Is the reference value of the voltage frequency omega 0 Is the initial value of the voltage frequency, K G And K T The ratio parameter and the time parameter of the first-order inertial controller are respectively, Q is the actual reactive power value of the wind turbine generator, and Q is the actual reactive power value of the wind turbine generator * Is the reactive power reference value delta of the wind turbine generator 0 The voltage phase of the fan grid-connected point when the grid-side converter is unlocked;
grid-connected point d axis voltage first reference value of wind turbine generatorThe calculation formula of (2) is as follows:
where s is Laplacian, U dc AndU dc * respectively a direct-current voltage reference value and an actual value, k of the wind turbine generator pd And k id Proportional and integral parameters, u, of the DC voltage controller, respectively fd0 And (5) the voltage amplitude of the grid-connected point of the fan when the grid-side converter is unlocked.
6. The method for starting the offshore wind power flexible low-frequency delivery system according to claim 1, wherein in the step S34, the grid-connected point d-axis voltage of the wind turbine generator is a second reference valueThe calculation formula is as follows:
where s is Laplacian, P s * And P s Respectively an active power reference value and an actual value of the wind turbine generator; k (k) pp And k ip The proportional parameter and the integral parameter of the active power controller of the fan are respectively, u fd0 And (5) the voltage amplitude of the grid-connected point of the fan when the grid-side converter is unlocked.
CN202310706798.0A 2023-06-14 2023-06-14 Starting method of offshore wind power flexible low-frequency sending-out system Pending CN116937649A (en)

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