Starting Method for MMC-Diode Hybrid Offshore Wind Power Bipolar Flexible DC Transmission System

Abstract

The invention discloses a starting method for an MMC-diode hybrid offshore wind power bipolar flexible DC transmission system. For a hybrid offshore wind power bipolar flexible DC transmission system in which the positive terminal of the sending end adopts an MMC converter and the negative terminal adopts a diode rectifier device, through the coordinated control of the MMC converter at the positive terminal of the sending end, the MMC converter at the positive terminal of the receiving end, and the MMC converter at the negative terminal of the receiving end, the stable starting and grid connection of the hybrid bipolar flexible DC system and the offshore wind farm and the bipolar coordinated operation are realized. By adopting the hybrid offshore wind power bipolar flexible DC transmission system and the starting method proposed by the invention, the construction cost of the offshore converter station can be significantly reduced, and the safety and stability of the system during starting and operation can be ensured.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and particularly relates to a starting method for a MMC-diode hybrid offshore wind power bipolar flexible DC transmission system. Background Art

[0002] With the gradual increase of the voltage level and transmission capacity of flexible DC transmission systems, bipolar flexible DC transmission systems have received increasing attention due to their high flexibility and reliability. Flexible DC transmission technology based on the modular multilevel converter (MMC) topology has the advantages of low manufacturing difficulty, low system loss, and high waveform quality, and has very good application prospects in long-distance power collection and transmission. However, in a large-capacity offshore wind farm bipolar flexible DC transmission system, the volume and weight of the offshore converter station adopting the MMC topology are relatively large, and its construction and transportation costs are very high, which is not conducive to the parity grid connection of offshore wind power. Therefore, exploring a lightweight design method for offshore converter stations and improving the economy of bipolar DC transmission schemes is an important research direction in this field.

[0003] In a bipolar flexible DC transmission system, replacing part of the MMC rectifier device with a diode rectifier device can significantly reduce the weight and cost of the offshore conversion platform. A feasible design scheme is as follows: the sending-end positive pole adopts an MMC converter to maintain the stability of the sending-end AC grid, and the sending-end negative pole adopts a diode rectifier device to assist in power transmission. This scheme can optimize the engineering construction cost under the condition of ensuring the safe and stable operation of the system. However, since the diode rectifier belongs to uncontrolled rectification, the starting scheme adopted by the classical offshore wind farm flexible DC transmission system is no longer applicable to this hybrid topology. At present, there is little research on the starting method of the diode scheme for the offshore wind farm bipolar DC transmission system, and it is urgent to propose a starting method for the MMC-diode hybrid offshore wind power bipolar flexible DC transmission system. Summary of the Invention

[0004] The object of the present invention is to provide a starting method for a MMC-diode hybrid offshore wind power bipolar flexible DC transmission system for a hybrid offshore wind power bipolar flexible DC transmission system with an MMC converter at the sending-end positive pole and a diode rectifier device at the negative pole, so as to realize the smooth starting and grid connection of the hybrid bipolar flexible DC system and the offshore wind farm.

[0005] In order to achieve the above invention object, the following technical solutions are adopted in this method:

[0006] A starting method for a MMC-diode hybrid offshore wind power bipolar flexible DC transmission system, the topology of the MMC-diode hybrid offshore wind power bipolar flexible DC transmission system includes: a sending-end positive pole MMC converter, a sending-end negative pole diode converter, a receiving-end positive pole MMC converter, and a receiving-end negative pole MMC converter; characterized in that:

[0007] The system startup process begins. First, the positive starting resistor is connected, and uncontrolled rectification charging is performed on the capacitors of each sub-module of the positive MMC converter.

[0008] The receiving-end positive MMC converter adopts a DC bus voltage and reactive power control mode to adjust the positive DC bus voltage to the rated value.

[0009] The negative starting resistor is connected, and uncontrolled rectification charging is performed on the capacitors of each sub-module of the negative MMC converter.

[0010] The receiving-end negative MMC converter adopts a DC bus voltage and reactive power control mode to adjust the negative DC bus voltage to the first preset voltage value.

[0011] The sending-end positive MMC converter adopts a constant AC voltage control mode to adjust the offshore AC grid voltage to the rated value.

[0012] The offshore wind turbines are connected to the grid one by one. When the grid-connected capacity reaches nearly half or half of the rated capacity, the grid connection is paused. Since the negative diode converter is not conducting when the offshore wind turbines are connected to the grid one by one, all the output power of the wind farm is transmitted from the positive MMC converter. To ensure that the positive MMC converter does not become overloaded, the grid-connected capacity at this time cannot exceed half of the rated capacity. In addition, from the perspective of improving the system startup efficiency, the power connected to the grid first should not be too low at this time. Therefore, it is appropriate to be nearly half or half of the rated capacity.

[0013] The receiving-end negative MMC converter gradually reduces the negative DC bus voltage to make the sending-end negative diode converter conduct.

[0014] By adjusting the negative DC bus voltage through the receiving-end negative MMC converter, the positive and negative converters respectively bear half of the output power of the wind farm.

[0015] The remaining unconnected offshore wind turbines continue to be connected to the grid. The sending-end positive MMC converter maintains the sending-end AC grid voltage at the rated value, and the receiving-end negative MMC converter adjusts the negative DC bus voltage to evenly distribute the output power of the wind farm between the positive and negative poles.

[0016] After all the wind turbines are connected to the grid, the system startup process ends.

[0017] The control system used to implement the startup method includes: the sending-end positive MMC converter control system, the receiving-end positive MMC converter control system, and the receiving-end negative MMC converter control system.

[0018] The sending-end positive MMC converter control system adopts an outer loop of sending-end AC grid voltage control and an inner loop of current control.

[0019] For the control system of the receiving-end positive MMC converter, an outer loop for controlling the positive DC bus voltage and reactive power and an inner loop for current control are adopted.

[0020] For the control system of the receiving-end negative MMC converter, an outer loop for controlling the negative DC bus voltage and reactive power and an inner loop for current control are adopted.

[0021] Furthermore, in the control system of the receiving-end negative MMC converter, the reference value U of the negative DC bus voltage is calculated according to the following method dc2ref :

[0022] Before the time node when the grid-connected capacity of the offshore wind turbine reaches half of the rated capacity and grid connection is suspended, set U dc2ref as the first preset voltage value, and the first preset voltage value needs to be higher than the rated negative DC bus voltage to ensure that when the offshore AC grid voltage is the rated value, the sending-end negative diode rectifier cannot conduct;

[0023] After the time node when the grid-connected capacity of the offshore wind turbine reaches half of the rated capacity and grid connection is suspended, gradually reduce the reference value U of the negative DC bus voltage dc2ref , until the sending-end negative diode rectifier conducts;

[0024] After the sending-end negative diode rectifier conducts, the calculation method of U dc2ref is as follows:

[0025] U dc2ref =U dcn2 -F PIP (s)(0.5P wind -P g2 )

[0026]

[0027] Among them, U dcn2 is the rated value of the negative DC bus voltage, F PIP (s) is the transfer function of the PI controller in this stage, k pp is the proportional coefficient, k ip is the integral coefficient, P wind is the output power of the wind farm, and P g2 is the active power of the receiving-end negative MMC.

[0028] The beneficial effects of the present invention are as follows: By adopting the technical solution of the present invention, the smooth start-up and grid connection of the MMC-diode hybrid bipolar flexible DC system and the offshore wind farm can be realized, and the coordinated distribution of the output power of the offshore wind farm between the positive MMC converter and the negative diode converter can also be realized. Brief Description of the Drawings

[0029] Figure 1 This is a typical topology diagram of the hybrid offshore wind power bipolar VSC-HVDC transmission system of the present invention.

[0030] Figure 2 This is a flow chart of the start-up method for the MMC-diode hybrid offshore wind power bipolar VSC-HVDC transmission system of the present invention.

[0031] Figure 3 This is a schematic diagram of a specific example system for the control method of the MMC converters at the receiving-end positive and negative poles of the present invention. Among them, 1 - receiving-end positive DC bus voltage and reactive power control module, 2 - receiving-end positive current control module, 3 - receiving-end positive Park inverse transformation module, 4 - receiving-end positive internal circulating current control module, 5 - receiving-end positive arm voltage calculation module, 6 - receiving-end negative DC bus voltage reference value calculation module, 7 - receiving-end negative DC bus voltage and reactive power control module, 8 - receiving-end negative current control module, 9 - receiving-end negative Park inverse transformation module, 10 - receiving-end negative internal circulating current control module, 11 - receiving-end negative arm voltage calculation module.

[0032] Figure 4 This is a schematic diagram of a specific example system for the control method of the MMC converter at the sending-end positive pole of the present invention. Among them, 12 - sending-end positive voltage control module, 13 - sending-end positive current control module, 14 - sending-end positive Park inverse transformation module, 15 - sending-end positive internal circulating current control module, 16 - sending-end positive arm voltage calculation module. Specific embodiments

[0033] In order to describe the present invention more specifically, the technical solutions of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] In the start-up method of the MMC-diode hybrid offshore wind power bipolar VSC-HVDC transmission system of the present invention, the implementation of the control methods for the MMC converters at the receiving-end positive and negative poles is as Figure 3 shown, including a receiving-end positive DC bus voltage and reactive power control module 1, a receiving-end positive current control module 2, a receiving-end positive Park inverse transformation module 3, a receiving-end positive internal circulating current control module 4, a receiving-end positive arm voltage calculation module 5, a receiving-end negative DC bus voltage reference value calculation module 6, a receiving-end negative DC bus voltage and reactive power control module 7, a receiving-end negative current control module 8, a receiving-end negative Park inverse transformation module 9, a receiving-end negative internal circulating current control module 10, and a receiving-end negative arm voltage calculation module 11.

[0035] As Figure 3 shown, in the start-up method of the MMC-diode hybrid offshore wind power bipolar VSC-HVDC transmission system of the present invention, the control method for the MMC converter at the receiving-end positive pole includes the following steps:

[0036] Using the rectifier positive DC bus voltage and reactive power control module 1, for the positive DC bus voltage U dc1 It is controlled by a PI controller to make it follow the reference value U dc1ref , and the reference value U dc1ref Is given as the rated voltage of the positive DC bus. After passing through the limiter, the output of the controller is used as the reference value I of the rectifier positive d-axis current gdref1 ; for the reactive power Q g1 It is controlled by a PI controller to make it follow the reference value Q g1ref , and after passing through the limiter, the output of the controller is used as the reference value I of the rectifier positive q-axis current gqref1 .

[0037] Using the rectifier positive current control module 2, for the rectifier positive d and q-axis currents I gd1 and I gq1 A PI controller is used for control to make it follow the reference values I gdref1 and I gqref1 , and the output of the controller is used as the rectifier positive MMC reference differential mode voltage U difdq1 .

[0038] Using the rectifier positive Park inverse transformation module 3, the rectifier positive MMC reference differential mode voltage U difdq1 Is subjected to Park inverse transformation to obtain the reference differential mode voltage U in the stationary three-phase coordinate system difabc1 , and the angle used for Park inverse transformation is the AC grid phase θ g .

[0039] Using the rectifier positive internal circulating current control module 4, for the rectifier positive MMC internal circulating current I cabc1 A resonant controller is used for control, and the output of the rectifier positive internal circulating current controller is used as the rectifier positive MMC reference common mode voltage U comabc1 .

[0040] Using the rectifier positive bridge arm voltage calculation module 5, according to the rectifier positive reference differential mode voltage U difabc1 and the reference common mode voltage U comabc1 , after calculation, the reference voltages U of the upper and lower bridge arms of the rectifier positive MMC are obtained prefabc1 and U nrefabc1 , and the control of the rectifier positive MMC converter is realized through the valve control module.

[0041] As Figure 3 shown, in the starting method of the MMC-diode hybrid offshore wind power bipolar flexible DC transmission system of the present invention, the control method of the rectifier negative MMC converter includes the following steps:

[0042] Using the receiving-end negative DC bus voltage reference value calculation module 6, calculate the negative DC bus voltage reference value U according to the following method dc2ref :

[0043] Before the offshore wind turbine is connected to the grid, set U dc2ref to the first preset voltage value. In this embodiment, the first preset voltage value is set to 1.2 times the rated value of the negative DC bus voltage;

[0044] After the grid-connected capacity of the offshore wind turbine reaches half of the rated capacity and grid connection is suspended, gradually reduce the negative DC bus voltage reference value U dc2ref until the sending-end negative diode rectifier conducts;

[0045] After the sending-end negative diode rectifier conducts, the calculation method of U dc2ref is as follows:

[0046] U dc2ref = U dcn2 - F PIP (s)(0.5P wind - P g2 )

[0047]

[0048] where U dcn2 is the rated value of the negative DC bus voltage, F PIP (s) is the transfer function of the PI controller at this stage, k pp is the proportional coefficient, k ip is the integral coefficient, P wind is the output power of the wind farm, and P g2 is the active power of the receiving-end negative MMC.

[0049] Using the receiving-end negative DC bus voltage and reactive power control module 7, control the negative DC bus voltage U dc2 through a PI controller to make it follow the reference value U dc2ref . After the output of the controller passes through a limiting link, it is used as the reference value I gdref2 of the receiving-end negative d-axis current; control the reactive power Q g2 through a PI controller to make it follow the reference value Q g2ref . After the output of the controller passes through a limiting link, it is used as the reference value I gqref2 of the receiving-end negative q-axis current.

[0050] Using the receiving-end negative current control module 8, control the receiving-end negative d- and q-axis currents I gd2 and I gq2 using a PI controller to make them follow the reference value I gdref2and I gqref2 The output of the controller serves as the reference differential-mode voltage U of the receiving-end negative MMC difdq2 .

[0051] Using the receiving-end negative Park inverse transformation module 9, perform Park inverse transformation on the reference differential-mode voltage U of the receiving-end negative MMC difdq2 to obtain the reference differential-mode voltage U in the stationary three-phase coordinate system difabc2 , and the angle used for the Park inverse transformation is the AC grid phase θ g .

[0052] Using the receiving-end negative internal circulating current control module 10, control the internal circulating current I of the receiving-end negative MMC cabc2 with a resonant controller. The output of the receiving-end negative internal circulating current controller serves as the reference common-mode voltage U of the receiving-end negative MMC comabc2 .

[0053] Using the receiving-end negative arm voltage calculation module 11, based on the reference differential-mode voltage U of the receiving-end negative difabc2 and the reference common-mode voltage U comabc2 , calculate to obtain the reference voltages U of the upper and lower arms of the receiving-end negative MMC prefabc2 and U nrefabc2 , and control the converter of the receiving-end negative MMC through the valve control module

[0054] In the starting method of the MMC-diode hybrid offshore wind power bipolar flexible DC transmission system of the present invention, the control method of the sending-end positive MMC converter is implemented as Figure 4 shown, including the sending-end positive voltage control module 12, the sending-end positive current control module 13, the sending-end positive Park inverse transformation module 14, the sending-end positive internal circulating current control module 15, and the sending-end positive arm voltage calculation module 16

[0055] As Figure 4 shown, in the starting method of the MMC-diode hybrid offshore wind power bipolar flexible DC transmission system of the present invention, the control method of the sending-end positive MMC converter includes the following steps

[0056] Using the sending-end positive voltage control module 12, control the d-axis and q-axis voltages U gdqs through a PI controller so that they respectively follow the given reference values U gdref and U gqref (U gqref is set to 0). After passing through the limiting link, the output of the controller serves as the reference values I gdref3 and I gqref3 of the positive d-axis and q-axis currents

[0057] Using the sending-end positive current control module 13, control the sending-end positive d-axis and q-axis currents Igd3 and I gq3 It is controlled by a PI controller to follow the reference value I gdref3 and I gqref3 , and the output of the controller is used as the reference differential-mode voltage U of the positive terminal of the sending-end MMC difdq3 .

[0058] Using the positive-terminal Park inverse transformation module 14 of the sending end, the reference differential-mode voltage U of the positive-terminal MMC of the sending end difdq3 is subjected to Park inverse transformation to obtain the reference differential-mode voltage U in the stationary three-phase coordinate system difabc3 , and the angle used for Park inverse transformation is the reference phase θ r .

[0059] Using the internal circulating current control module 15 of the positive terminal of the sending end, the internal circulating current I of the positive-terminal MMC of the sending end cabc3 is controlled by a resonant controller, and the output of the internal circulating current controller of the positive terminal of the sending end is used as the reference common-mode voltage U of the positive-terminal MMC of the sending end comabc3 .

[0060] Using the positive-terminal bridge arm voltage calculation module 16, according to the reference differential-mode voltage U of the positive terminal of the sending end difabc3 and the reference common-mode voltage U comabc3 , the reference voltages U prefabc3 and U nrefabc3 of the upper and lower bridge arms of the positive-terminal MMC of the sending end are obtained through calculation, and the control of the converter of the positive-terminal MMC of the sending end is realized through the valve control module.

[0061] Using the above receiving-end and sending-end MMC control systems, combined with Figure 2 , the startup process of the MMC-diode hybrid offshore wind power bipolar flexible DC transmission system when the technical solution of the present invention is adopted is described.

[0062] At the beginning of the system startup process, the positive starting resistor is first connected to perform uncontrolled rectification charging on the capacitors of each sub-module of the positive MMC converter; the receiving-end positive MMC converter adopts a DC bus voltage and reactive power control mode to adjust the positive DC bus voltage to the rated value; then, the negative starting resistor is connected to perform uncontrolled rectification charging on the capacitors of each sub-module of the negative MMC converter; the receiving-end negative MMC converter adopts a DC bus voltage and reactive power control mode to adjust the negative DC bus voltage to the first preset voltage value; next, the sending-end positive MMC converter adopts a constant AC voltage control mode to adjust the offshore AC grid voltage to the rated value; since the negative DC bus voltage is the first preset voltage value, which is higher than the rated voltage, the diode converter cannot conduct. At this time, the offshore wind turbines start to be connected to the grid one by one. In this stage, the output power of the wind farm will all be transmitted through the positive MMC converter. When the grid-connected capacity reaches half of the rated capacity, the grid connection is suspended; then, the negative DC bus voltage is gradually reduced through the receiving-end negative MMC converter to make the sending-end negative diode converter conduct, and the negative DC bus voltage is continuously adjusted to gradually transfer the output power of the wind farm from the positive MMC converter to the negative diode converter for sending; when the output powers of the positive MMC converter and the negative diode converter are equal, the remaining unconnected offshore wind turbines are started to continue the grid connection. In this stage, the sending-end positive MMC converter keeps the sending-end AC grid voltage at the rated value, and the receiving-end negative MMC converter adjusts the negative DC bus voltage to evenly distribute the output power of the wind farm between the positive and negative poles; after all the wind turbines are connected to the grid, the system startup process ends.

[0063] The above description of the embodiments is to enable those of ordinary skill in the art of the present technology to understand and apply the present invention. It is obvious that those who are familiar with the technology in the field can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative labor. Therefore, the present invention is not limited to the above embodiments, and all improvements and modifications made by those skilled in the art according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. Starting method for MMC-diode hybrid offshore wind power bipolar flexible DC transmission system, the topology of the MMC-diode hybrid offshore wind power bipolar flexible DC transmission system includes: The sending-end positive MMC converter, the sending-end negative diode converter, the receiving-end positive MMC converter, and the receiving-end negative MMC converter; characterized in that: At the beginning of the system startup process, the positive startup resistor is first connected to perform uncontrolled rectification charging on the capacitors of each sub-module of the positive MMC converter. The receiving-end positive MMC converter adopts a DC bus voltage and reactive power control mode to adjust the positive DC bus voltage to the rated value. The negative startup resistor is connected to perform uncontrolled rectification charging on the capacitors of each sub-module of the negative MMC converter. The receiving-end negative MMC converter adopts a DC bus voltage and reactive power control mode to adjust the negative DC bus voltage to the first preset voltage value. The sending-end positive MMC converter adopts a constant AC voltage control mode to adjust the offshore AC grid voltage to the rated value. The offshore wind turbines are connected to the grid one by one. When the grid-connected capacity reaches half of the rated capacity, the grid connection is paused. The receiving-end negative MMC converter gradually reduces the negative DC bus voltage to turn on the sending-end negative diode converter. By adjusting the negative DC bus voltage through the receiving-end negative MMC converter, the positive and negative converters respectively bear half of the output power of the wind farm. The remaining unconnected offshore wind turbines continue to be connected to the grid. The sending-end positive MMC converter maintains the sending-end AC grid voltage at the rated value. The receiving-end negative MMC converter adjusts the negative DC bus voltage so that the output power of the wind farm is evenly distributed between the positive and negative poles. After all the wind turbines are connected to the grid, the system startup process ends. The control system adopted to implement the startup method includes: the sending-end positive MMC converter control system, the receiving-end positive MMC converter control system, and the receiving-end negative MMC converter control system. The sending-end positive MMC converter control system adopts an outer loop of sending-end AC grid voltage control and an inner loop of current control. The receiving-end positive MMC converter control system adopts an outer loop of positive DC bus voltage and reactive power control and an inner loop of current control. The receiving-end negative MMC converter control system adopts an outer loop of negative DC bus voltage and reactive power control and an inner loop of current control.

2. The starting method of the MMC-diode hybrid offshore wind power bipolar flexible DC transmission system according to claim 1, characterized in that: In the receiving-end negative MMC converter control system, the reference value U of the negative DC bus voltage is calculated according to the following method dc2ref : Before the time node when the grid-connected capacity of the offshore wind turbine reaches half of the rated capacity and grid connection is suspended, set U dc2ref as the first preset voltage value, which needs to be higher than the rated DC bus voltage of the negative pole to ensure that when the offshore AC grid voltage is at the rated value, the sending-end negative diode rectifier cannot conduct; After the grid-connected capacity of the offshore wind turbine reaches half of the rated capacity and grid connection is suspended, gradually reduce the reference value U of the negative DC bus voltage dc2ref , until the sending-end negative diode rectifier conducts; After the sending-end negative diode rectifier conducts, U dc2ref The calculation method is as follows: U dc2ref = U dcn2 - F PIP (s)(0.5P wind - P g2 ) Among them, U dcn2 is the rated value of the negative DC bus voltage, F PIP (s) is the transfer function of the PI controller in the stage after the sending-end negative diode rectifier is turned on, k pp is the proportionality coefficient, k ip is the integral coefficient, P wind is the output power of the wind farm, P g2 is the active power of the receiving-end negative MMC.

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