Steady-state operation control method for offshore direct-current wind turbine generator

Abstract

The invention discloses a steady-state operation control method for an offshore direct-current wind turbine generator. The method comprises the steps that in the first stage, medium-voltage direct-current collection voltage is established and maintained through an offshore direct-current wind turbine generator; and in the second stage, after the offshore centralized booster station is started and takes over the control of the medium-voltage direct-current collection voltage, the offshore direct-current wind turbine generator set is switched to a power output mode, and a lead-lag link is added to a power control link of the machine-side converter, so that the power output is stable. According to the method, the problem of coordination control between the offshore direct-current wind turbine generator and the centralized booster station adopting diode uncontrolled rectification is solved, and stable operation of the system under starting and wind speed fluctuation is realized through two-stage coordination and additional control links.

Description

A method for steady-state operation control of offshore DC wind turbines Technical Field

[0001] This invention belongs to the field of converter control technology for offshore DC wind turbines, and particularly relates to a steady-state operation control method for offshore DC wind turbines. Background Technology

[0002] As offshore wind power expands to deeper waters and on a larger scale, medium-voltage DC collection via high-voltage DC transmission has become a promising technology due to its advantages such as long transmission distance, low loss, and no need for reactive power compensation. In this architecture, offshore DC wind turbines and offshore centralized booster stations are key equipment. Currently, research on medium-voltage DC converters and high-voltage DC converters mainly focuses on the functional implementation and performance optimization of individual equipment. In the research process, other parts of the system are often simplified and equivalent, neglecting the coupling effects of control strategies within the unit and between various systems.

[0003] The architecture of the offshore wind power medium-voltage DC collection system is shown in Figure 1. The AC power generated by the wind turbine is rectified by the machine-side converter (MSC) and then the voltage is raised to the medium-voltage DC voltage level by the medium-voltage DC converter. After collection, the voltage is further raised to the high-voltage DC transmission voltage level by the offshore centralized step-up device before long-distance transmission. Among them, the medium-voltage and high-voltage DC converters are the key networking equipment of the system.

[0004] Furthermore, research on the overall startup, steady-state operation, and coordinated control with offshore DC wind turbines at the system level remains limited. Some existing system control strategies either neglect the actual operating characteristics of the turbine in an effort to reduce complexity, or fail to adequately consider the turbine's startup pressurization process and the specific coordination methods with the substation. This leads to oscillations in system power output during actual operation, especially when facing wind speed fluctuations, making it difficult to maintain stable operation.

[0005] Therefore, existing technologies lack a control method that can effectively coordinate offshore DC wind turbines and offshore centralized booster stations, and ensure the stable operation of the system from startup to power transmission. Summary of the Invention

[0006] This invention proposes a steady-state operation control method for offshore DC wind turbines to solve the problems existing in the prior art.

[0007] To achieve the above objectives, the present invention provides a steady-state operation control method for offshore DC wind turbines, comprising:

[0008] In the first phase, the medium-voltage DC collection voltage is established and maintained by offshore DC wind turbines;

[0009] In the second stage, after the offshore centralized booster station is started up and takes over the control of the medium-voltage DC collection voltage, the offshore DC wind turbine will switch to power output mode.

[0010] In the second phase, a lead-lag circuit is added to the power control stage of the generator-side converter of the offshore DC wind turbine to stabilize the power output.

[0011] Optionally, in the first stage, the low-voltage DC link voltage of the wind turbine is maintained stable by the machine-side converter.

[0012] Optionally, the machine-side converter uses DC voltage control to generate an inner-loop current reference value and performs inner-loop current control.

[0013] Optionally, the first stage further includes establishing a stable AC output voltage by a grid-side converter.

[0014] Optionally, the grid-side converter decomposes the output voltage and controls the decomposed voltage components to maintain the stability of the AC output voltage.

[0015] Optionally, the second stage further includes controlling the grid-side converter to switch to a low-voltage DC link voltage control mode.

[0016] Optionally, the switching of the control network-side converter includes: generating an AC voltage reference value based on the low-voltage DC link voltage, and performing constant voltage and constant frequency control.

[0017] Optionally, the offshore centralized booster station generates an AC voltage reference value through a medium-voltage DC voltage outer loop to take over the control of the medium-voltage DC collection voltage.

[0018] Compared with the prior art, the present invention has the following advantages and technical effects:

[0019] The control method provided by this invention, through the design of a coordinated control architecture including two stages—startup and power transmission—and the introduction of lead-lag correction in the power control stage, effectively achieves smooth switching and coordinated operation between offshore DC wind turbines and offshore centralized booster stations. This method ensures that the system stably establishes a medium-voltage DC collection voltage during the startup stage, and during the power transmission stage, even in the face of wind speed fluctuations, it maintains stable turbine power output and suppresses oscillations, thereby guaranteeing the safety and stability of the offshore DC wind power collection system from startup to steady-state operation. Attached Figure Description

[0020] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0021] Figure 1 is a schematic diagram of the topology of the offshore wind power medium-voltage DC collection system in the background technology of this invention;

[0022] Figure 2 is a schematic diagram of the topology of an offshore DC wind turbine according to an embodiment of the present invention;

[0023] Figure 3 is a schematic diagram of the control architecture of an offshore DC wind turbine according to an embodiment of the present invention;

[0024] Figure 4 is a schematic diagram of the machine-side converter control during the startup phase of an embodiment of the present invention;

[0025] Figure 5 is a schematic diagram of grid-side converter control during the startup phase of an embodiment of the present invention;

[0026] Figure 6 is a schematic diagram of the control of the offshore centralized booster station during the power transmission stage according to an embodiment of the present invention;

[0027] Figure 7 is a schematic diagram of the generator-side converter control in the power transmission stage according to an embodiment of the present invention;

[0028] Figure 8 is a schematic diagram of grid-side converter control in the power transmission stage according to an embodiment of the present invention;

[0029] Figure 9 is a schematic diagram of wind speed according to an embodiment of the present invention;

[0030] Figure 10 is a schematic diagram of the simulation results of the conventional method according to an embodiment of the present invention;

[0031] Figure 11 is a schematic diagram of the simulation results of the method according to an embodiment of the present invention. Detailed Implementation

[0032] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0033] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0034] Example 1

[0035] The offshore DC wind turbine topology addressed in this invention is shown in Figure 2. Its structure mainly consists of an impeller, drive train, permanent magnet synchronous generator (PMSG), turbine-side converter, grid-side converter, and a twelve-pulse rectifier bridge. The grid-side converter and the twelve-pulse rectifier bridge together constitute a medium-voltage DC-DC converter. The output port of the twelve-pulse rectifier bridge is connected to an offshore centralized booster station, as shown in Figure 1. When the offshore DC wind turbine is in the startup phase charging the offshore centralized booster station, the offshore centralized booster station does not perform additional control on the medium-voltage DC collection voltage. The offshore DC wind turbine is essentially in an unloaded pressurized state, equivalent to circuit breaker BK1 being open. When the offshore centralized booster station completes charging, it takes over the control of the medium-voltage DC collection voltage, establishing a stable DC voltage, equivalent to a DC voltage source, i.e., BK1 is closed.

[0036] Correspondingly, its control architecture is shown in Figure 3, mainly including pitch angle control, turbine-side converter control, and grid-side converter control. The wind turbine drivetrain and pitch angle control are similar to those of traditional AC wind turbines. The control is achieved by collecting the turbine-side converter voltage u. ms Machine-side converter current i ms Low-voltage DC link voltage V LV The synchronization angle θ is extracted from the same physical quantity through a phase-locked loop (PLL). m The system transforms the three-phase stationary coordinate system (abc coordinate system) to a two-phase rotating coordinate system (dq coordinate system), and then uses sinusoidal pulse width modulation (SPWM) to generate gate on / off signals for the switching devices of the generator-side converter to control the generator-side converter. Similarly, the AC voltage u of the grid-side converter is acquired. o Grid-side converter AC current i o The system is transformed from a three-phase stationary coordinate system (abc coordinate system) to a two-phase rotating coordinate system (dq coordinate system). Its synchronization angle θ0 is obtained by direct integration of the given frequency f0. Then, the gate on / off signals of the grid-side converter switching devices are generated by sinusoidal pulse width modulation (SPWM) to control the grid-side converter.

[0037] As can be seen from the topology of the offshore wind power medium-voltage DC collection system, since both the offshore DC wind turbine and the offshore centralized booster station use diode uncontrolled rectifier devices, without an additional circuit, the onshore power cannot be fed back to the offshore side to provide the charging energy required for the startup of the offshore centralized booster station. Therefore, the startup charging of the offshore centralized booster station needs to be coordinated by the offshore DC wind turbine.

[0038] This embodiment provides a steady-state operation control method for offshore DC wind turbines, including:

[0039] In the first phase, the medium-voltage DC collection voltage is established and maintained by offshore DC wind turbines;

[0040] In the second stage, after the offshore centralized booster station is started up and takes over the control of the medium-voltage DC collection voltage, the offshore DC wind turbine will switch to power output mode.

[0041] In the second phase, a lead-lag circuit is added to the power control stage of the generator-side converter of the offshore DC wind turbine to stabilize the power output.

[0042] In the first stage, the low-voltage DC link voltage of the wind turbine is maintained stable by the machine-side converter. The machine-side converter uses DC voltage control to generate an inner loop current reference value and performs inner loop current control.

[0043] Specifically, the main task of offshore DC wind turbines during the system startup phase is to establish a stable medium-voltage DC voltage to provide a stable charging voltage for the offshore centralized booster station. The turbine-side converter of the offshore DC wind turbine is primarily responsible for maintaining the low-voltage DC link voltage V during startup. LV Stable, therefore, as shown in Figure 4, the outer loop d-axis uses DC voltage control to generate the inner loop current reference value i. msd_ref ,Right now K pLV1 T iLV1 These are the proportional and integral coefficients of the low-voltage DC outer loop of the machine-side converter during the startup phase; i msd_ref With the d-axis component of the machine-side converter current i msd The difference is then input to the PI controller, based on the inductive reactance ω of the machine-side converter. r L ms q-axis component of the machine-side converter current i msq d-axis component of the converter voltage on the machine side msd Obtain the reference value u of the d-axis component of the converter voltage on the machine side. msd_ref ,Right now K pLVd1 T iLVd1 These are the proportional and integral coefficients of the inner loop of the d-axis current of the machine-side converter during the startup phase.

[0044] The q-axis current reference value is set to zero, i.e., i msq_ref =0, and the q-axis component of the generator-side converter current i msq The difference is then input to the PI controller, based on the inductive reactance ω of the machine-side converter. r L ms d-axis component of the current in the machine-side converter msd q-axis component of the machine-side converter voltage u msq Obtain the reference value u of the q-axis component of the converter voltage on the machine side. msq_ref ,Right now K pLVq1 T iLVq1These are the proportional and integral coefficients of the inner loop of the q-axis current of the machine-side converter, respectively. msd_ref u msq_ref The control signal for the machine-side converter is then generated through sinusoidal pulse width modulation (SPWM).

[0045] Furthermore, the first stage involves establishing a stable AC output voltage using a grid-side converter. The grid-side converter decomposes the output voltage and controls the decomposed voltage components to maintain the stability of the AC output voltage.

[0046] Specifically, in V LV Under stable conditions, the grid-side converter only needs to maintain a stable rated output, using V / f control, as shown in Figure 5, to control the output voltage u. o After dq decomposition, the d-axis component u of the grid-side converter output voltage is... od Given a reference value of 1p.u., the q-axis component u oq Given a reference value set to 0, the PI controller generates a reference value i for the dq-axis component of the inner loop current. od_ref i oq_ref The dq-axis component of the grid-side converter output current i od i oq The difference is then input to the PI controller, based on the inductive reactance ω0L of the grid-side converter. f and u od u oq Obtain the reference value u of the dq-axis component of the grid-side converter output voltage. od_ref u oq_ref u od_ref u oq_ref The grid-side converter control signal is then generated through sinusoidal pulse width modulation (SPWM).

[0047] In this embodiment, the offshore centralized booster station generates an AC voltage reference value through the medium-voltage DC voltage outer loop to take over the control of the medium-voltage DC collection voltage.

[0048] Specifically, once the capacitor voltage of the submodule in the offshore centralized booster station stabilizes, the centralized booster system enters the medium-voltage DC collector voltage control mode. Since the MMC converter section in the offshore centralized booster station is connected to a diode uncontrolled rectifier via an intermediate frequency transformer, the diode's forward voltage is provided by the centralized booster system. Furthermore, because the offshore centralized booster station takes over the medium-voltage DC collector voltage control at this time, the d-axis control strategy of the offshore centralized booster station is designed to superimpose a medium-voltage DC voltage control outer loop on top of the traditional AC voltage / frequency control (V / f control). The medium-voltage DC voltage outer loop generates the AC voltage reference value in the V / f control, i.e. K pMVd T iMVdThese are the proportional and integral coefficients of the medium-voltage DC control loop at the offshore centralized substation. They are linearly combined with 1 to improve the stabilization speed of the control loop, and are related to the d-axis component u of the AC output voltage at the offshore centralized substation. osd After subtraction, the PI controller generates the reference value i for the d-axis component of the inner loop current. osd_ref Then, the d-axis component of the output current from the offshore centralized booster station is connected to the central booster station. osd The difference is input to the PI controller, based on the equivalent inductive reactance ω0L of the offshore centralized booster station bridge arm. e dq-axis component of AC output voltage of offshore centralized booster station osd u osq Obtain the d-axis reference value u of the AC side output voltage of the offshore centralized booster station. osd_ref The q-axis component u of the AC side output voltage of the offshore centralized booster station. osq Given a reference value set to 0, and u osq After subtraction, the PI controller generates the reference value i for the q-axis component of the inner loop current. osq_ref Then, the q-axis component of the output current from the offshore centralized booster station is connected to the central booster station. osq The difference is input to the PI controller, based on the equivalent inductive reactance ω0L of the offshore centralized booster station bridge arm. e dq-axis component of AC output voltage of offshore centralized booster station osd u osq Obtain the q-axis reference value u of the AC side output voltage of the offshore centralized booster station. osq_ref u osd_ref u osq_ref The control signal for the offshore centralized booster station is then generated by the closest level approximation modulation, as shown in Figure 6.

[0049] When the offshore centralized booster station switches to medium-voltage DC collector voltage control, the offshore DC wind turbine only needs to ensure stable power output. The turbine-side converter switches to a constant active power / AC voltage control mode similar to that of traditional AC wind turbines. Simultaneously, a gain-loaded lead-lag circuit is added to the power control stage to ensure stable power output during wind speed fluctuations. The active power measurement value P0, after passing through the gain-loaded lead-lag circuit, is corrected to P. 0m At this time, the reference value of the d-axis component of the inner loop current of the machine-side converter is... K p_d T i_d These are the proportional and integral coefficients of the d-axis power control element of the machine-side converter in the power transmission stage. msd_ref With the d-axis component of the machine-side converter current i msd The difference is then input to the PI controller, based on the inductive reactance ω of the machine-side converter. r L ms q-axis component of the current in the machine-side converter msqd-axis component of the converter voltage on the machine side msd Obtain the reference value u of the d-axis component of the converter voltage on the machine side. msd_ref .

[0050] The amplitude of the AC voltage output by the permanent magnet synchronous generator is U ms Given a reference value of U ms_ref The PI controller generates a reference value for the q-axis component of the current in the machine-side converter, i.e. The q-axis component of the current from the machine-side converter, i msq The difference is then input to the PI controller, based on the inductive reactance ω of the machine-side converter. r L ms d-axis component of the current in the machine-side converter msd q-axis component of the machine-side converter voltage u msq Obtain the reference value u of the q-axis component of the converter voltage on the machine side. msq_ref K p_q T i_q These are the proportional and integral coefficients of the outer loop of the q-axis voltage of the machine-side converter during the power transmission stage. msd_ref u msq_ref The control signal for the machine-side converter is then generated through sinusoidal pulse width modulation (SPWM), as shown in Figure 7.

[0051] Furthermore, the second phase includes controlling the switching of the grid-side converter to the low-voltage DC link voltage control mode.

[0052] The switching of the converter on the control network side includes: generating an AC voltage reference value based on the low-voltage DC link voltage, and performing constant voltage and constant frequency control.

[0053] The grid-side converter switches to low-voltage DC link voltage control mode. However, due to the decoupling characteristics of the diode-controlled rectifier and the need to establish a conduction voltage for it, the control strategy design of the grid-side converter is similar to that of an offshore DC booster station. After generating an AC voltage reference value through the low-voltage DC voltage control outer loop, constant V / f control is performed. K pLV2 T iLV2 These represent the proportional and integral coefficients of the low-voltage DC control loop of the grid-side converter during the power transmission stage. They are linearly combined with 1 to improve the stabilization speed of the control loop, and are related to the d-axis component u of the grid-side converter output voltage. od After subtraction, the PI controller generates the reference value i for the d-axis component of the inner loop current. od_ref Then, it is compared with the d-axis component of the grid-side converter output current i. od The difference is then input to the PI controller, based on the inductive reactance ω0L of the grid-side converter. f The output voltage dq-axis component u of the grid-side converter od u oqObtain the d-axis reference value u of the grid-side converter output voltage. od_ref The q-axis component of the grid-side converter output voltage, u. oq Given a reference value set to 0, and u oq After subtraction, the PI controller generates the reference value i for the q-axis component of the inner loop current. oq_ref Then, it is compared with the q-axis component of the grid-side converter output current i. oq The difference is then input to the PI controller, based on the inductive reactance ω0L of the grid-side converter. f dq-axis component of grid-side converter output voltage u od u oq Obtain the q-axis reference value u of the grid-side converter output voltage. oq_ref u od_ref u oq_ref The grid-side converter control signal is then generated by sinusoidal pulse width modulation, as shown in Figure 8.

[0054] In the system startup phase, the method proposed in this invention first establishes a medium-voltage DC collection voltage by the offshore DC wind turbine after self-starting, providing a stable charging voltage for the offshore centralized booster station. After the offshore DC booster station is started, it takes over the control of the medium-voltage DC voltage, and the offshore DC wind turbine is responsible for maintaining a stable power output. Through the coordinated control of the two, the system achieves stable operation and stable transmission of offshore wind power.

[0055] The following experiments were also conducted in this embodiment:

[0056] The control method remains consistent during the startup phase. During the power transmission phase, the method proposed in this invention is compared with the traditional method in the power control stage without an additional lead-lag stage with gain.

[0057] The wind turbine has a rated wind speed of 15 m / s. The wind conditions are set such that the wind speed decreases from 15 m / s to 10 m / s at 2s, further decreases from 10 m / s to 8 m / s at 4s, increases from 8 m / s to 10 m / s at 6s, and increases from 10 m / s to 15 m / s at 8s. The total simulation time is 10s, as shown in Figure 9.

[0058] The simulation results of the traditional method are shown in Figure 10, where (a) is the waveform diagram of the reference value of electromagnetic torque and active power; (b) is the waveform diagram of the generator output active power; (c) is the waveform diagram of the low voltage DC bus voltage; (d) is the waveform diagram of the medium voltage DC collection voltage; (e) is the waveform diagram of the wind turbine AC output voltage; and (f) is the waveform diagram of the wind turbine AC output current.

[0059] One second prior, the offshore DC wind turbine is in the startup phase, responsible for establishing a stable medium-voltage DC voltage, as shown in Figure 10(d). At this time, the power control loop is disabled, with only a small charging current and corresponding charging power, as shown in Figures 10(f) and (b). At 1 second, the power transmission phase begins, and the offshore centralized booster station takes over the control of the medium-voltage DC collection voltage, i.e., circuit breaker BK1 in Figure 2 closes, and the offshore DC wind turbine begins to output power. When the wind speed fluctuates, the wind energy captured by the rotor will also change accordingly. Therefore, the reference power command value P0_ref and the input electromagnetic torque of the permanent magnet synchronous generator (PMSG) generated through the wind turbine drive chain change accordingly, as shown in Figure 10(a). During wind speed fluctuations, as can be seen from Figure 10(b), the active power P0 output by the PMSG oscillates significantly and cannot track the reference command value. The low-voltage DC link voltage, AC output voltage, and AC output current of the wind turbine also oscillate significantly and cannot stabilize at their rated values.

[0060] The method proposed in this invention involves adding a gain-enhanced lead-lag stage to the MSC power control stage during the power transmission phase. The simulation results of the proposed method are shown in Figure 11, where (a) is a waveform diagram of electromagnetic torque and active power reference values; (b) is a waveform diagram of generator output active power; (c) is a waveform diagram of low-voltage DC bus voltage; (d) is a waveform diagram of medium-voltage DC collection voltage; (e) is a waveform diagram of wind turbine AC output voltage; and (f) is a waveform diagram of wind turbine AC output current.

[0061] When wind speed fluctuates, the PMSG reference power command value P0_ref generated by the wind turbine drive train and the input electromagnetic torque change accordingly, as shown in Figure 11(a). During wind speed fluctuations, as can be seen from Figure 11(b), the PMSG output active power P0 can smoothly track the reference command value, and the low-voltage DC link voltage, AC output voltage, and AC output current of the wind turbine remain stable at their rated values, verifying the effectiveness of the method proposed in this invention.

[0062] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A steady-state operation control method for offshore DC wind turbines, characterized in that, include: In the first phase, the medium-voltage DC collection voltage is established and maintained by offshore DC wind turbines; In the second stage, after the offshore centralized booster station is started up and takes over the control of the medium-voltage DC collection voltage, the offshore DC wind turbine switches to the power output mode. In this second stage, a lead-lag circuit is added to the power control circuit of the turbine-side converter of the offshore DC wind turbine to stabilize the power output.

2. The method according to claim 1, characterized in that, In the first stage, the low-voltage DC link voltage of the wind turbine is maintained stable by the machine-side converter.

3. The method according to claim 2, characterized in that, The machine-side converter uses DC voltage control to generate an inner loop current reference value and performs inner loop current control.

4. The method according to claim 1, characterized in that, The first stage further includes establishing a stable AC output voltage by a grid-side converter.

5. The method according to claim 4, characterized in that, The grid-side converter decomposes the output voltage and controls the decomposed voltage components to maintain the stability of the AC output voltage.

6. The method according to claim 1, characterized in that, The second phase further includes controlling the grid-side converter to switch to low-voltage DC link voltage control mode.

7. The method according to claim 6, characterized in that, The switching of the converter on the control network side includes: generating an AC voltage reference value based on the low-voltage DC link voltage, and performing constant voltage and constant frequency control.

8. The method according to claim 1, characterized in that, The offshore centralized booster station generates an AC voltage reference value through a medium-voltage DC voltage outer loop to take over the control of the medium-voltage DC collection voltage.

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