Re-grid-connection method and system for transmitting-end converter of offshore wind power flexible direct-current grid-connected system

Abstract

The embodiment of the invention provides a re-grid-connection method and system for a transmitting-end converter of an offshore wind power flexible direct-current grid-connected system. The method comprises the following steps: connecting a current-limiting resistor in series in a direct-current side charging loop of the transmitting-end converter; closing a direct-current switch of the transmitting-end converter, locking all the sub-modules, and carrying out uncontrolled charging on a capacitor of each sub-module; all the sub-modules are put into use, and controllable charging is carried out on the capacitor of each sub-module; and the current-limiting resistor is cut off, a wind power plant voltage is established, and re-grid-connected starting of the sending-end converter is completed. According to the embodiment of the invention, through a pre-charging strategy that the current-limiting resistor is connected in series in the charging loop of the transmitting-end converter, additional equipment investment is not needed, the cost is low, the efficiency is high, rapid re-grid connection of the transmitting-end converter in a passive state is simply and effectively realized, impact generated at the moment when the converter is unlocked is overcome, equipment damage is avoided, and the service life of the transmitting-end converter is prolonged. Therefore, the operation safety and reliability of the offshore wind power flexible direct current system are improved.

Description

Technical Field

[0001] The embodiments of the present disclosure belong to the technical field of flexible direct current grid-connected systems, and specifically relate to a method and system for re-grid-connecting a sending-end converter of an offshore wind power flexible direct current grid-connected system. Background Art

[0002] The development of larger-scale wind farms in the offshore is an inevitable trend in the future development of offshore wind power, and the Flexible High Voltage Direct Current (F-HVDC) technology provides key technical support for the large-scale intensive development and grid connection of offshore wind power. Modular Multilevel Converter (MMC) is a device widely used in flexible high-voltage direct current transmission systems. It has a modular structure, is easy to expand, and can achieve four-quadrant operation. Before the MMC is started, the sub-module capacitors of the converter need to be charged to ensure that in the normal unlocked state, the voltage value superimposed by each conducting sub-module is not much different from the voltage at the DC end, so as to avoid excessive pulse currents that cause equipment damage.

[0003] When charging the converter on the AC side, the capacitor of the submodule can be charged to a higher voltage, so that it can unlock the converter under normal conditions with low current. However, for some special occasions, it is necessary to charge the capacitor of the converter submodule on the DC side. For example, when the sea wind flexible direct current system temporarily loses power due to some fault, in order to quickly reconnect the passive sending-end converter to the grid, the sending-end converter can take power from the DC side and pre-charge the submodule after the receiving-end converter is normally controlled and the DC voltage is established. After the sending-end converter completes the pre-charging, the wind farm AC voltage is established and the wind turbine is started. At this time, the start-up of the sending-end converter from the DC side is divided into two stages - the uncontrolled charging stage is at the beginning of the startup. Since the submodule capacitor voltage is low and the IGBT cannot be triggered, the submodule is in a locked state and can only be uncontrollably charged through the diode; the controllable charging stage is after the uncontrolled charging stage ends, and the number of locked and bypassed submodules in the bridge arm is dynamically changed through the control strategy. Continuing to increase the submodule capacitor voltage to the rated value is the main goal of the controllable charging stage. In this case, it is necessary to solve the overcurrent problem in the unlocked state of the sending-end converter.

[0004] There are two ways to pre-charge the converter capacitor: self-excitation and external excitation. The external excitation pre-charging method requires the use of an auxiliary power supply to charge the submodule capacitor. The auxiliary power supply that matches the rated voltage of the submodule is connected between the positive and negative poles of the DC side. The submodule is controlled to be put in and out so that the rated voltage of the submodule capacitor is charged to near the rated value. However, this method increases additional equipment investment, and can only charge one submodule at a time for the same-phase upper and lower bridge arms. The charging time is long and is not suitable for actual engineering. The self-excitation pre-charging method is currently mainly based on the pre-charging of half-bridge submodules, using a complex closed-loop control structure, and it is difficult to adjust the PI parameters. Summary of the invention

[0005] The embodiments of the present disclosure aim to solve at least one of the technical problems existing in the prior art, and provide a method and system for re-grid-connecting a sending-end converter of an offshore wind power flexible direct current grid-connected system.

[0006] One aspect of the present disclosure provides a method for reconnecting a sending-end converter of an offshore wind power flexible direct current grid-connected system to the grid, wherein the sending-end converter includes a plurality of submodules, and the method includes:

[0007] A current limiting resistor is connected in series in the DC side charging circuit of the sending-end converter;

[0008] Closing the DC switch of the sending-end converter, locking all the submodules, and performing uncontrolled charging on the capacitors of the submodules;

[0009] Putting all the submodules into operation to controllably charge the capacitors of the submodules;

[0010] The current limiting resistor is removed, the wind farm voltage is established, and the re-grid connection start of the sending-end converter is completed.

[0011] Furthermore, the resistance of the current limiting resistor satisfies the following expression:

[0012]

[0013] Among them, R lim is the current limiting resistor value, U dc_nom is the rated voltage of the DC system, I st is the maximum charging current.

[0014] Furthermore, the maximum charging current I st It is 75A.

[0015] Further, the uncontrolled charging of the capacitors of the submodules includes:

[0016] The capacitor voltage of each submodule is charged to:

[0017]

[0018] Among them, U SM is the submodule capacitor voltage, U dc_nom is the rated voltage of the DC system, and N is the number of sub-modules in each bridge arm of the equivalent charging circuit of the sending-end converter.

[0019] Furthermore, the number of submodules in each bridge arm in the equivalent charging circuit of the sending-end converter is:

[0020]

[0021] Where N is the number of submodules in each bridge arm of the equivalent charging circuit of the sending-end converter, U dc_nom is the rated voltage of the DC system, U SM_nom is the rated voltage of the submodule, and k is the redundancy factor, which is generally 1 to 1.1.

[0022] Further, the uncontrolled charging of the capacitors of the submodules includes:

[0023] The capacitor voltage of each submodule is charged to 40% to 50% of the rated voltage value.

[0024] Furthermore, before all the submodules are put into operation, the method further includes:

[0025] Set the number of submodules of the upper and lower bridge arms of the sending-end converter to:

[0026]

[0027] Among them, n is the number of submodules of the upper and lower bridge arms, U dc_nom is the rated voltage of the DC system, U SM_nom is the rated voltage of the submodule.

[0028] Furthermore, the capacitors of the submodules are controlled to be charged, including:

[0029] The capacitor voltage of each submodule is charged to a rated voltage value.

[0030] Furthermore, the current limiting resistor is removed, the wind farm voltage is established, and the grid-connected re-start of the sending-end converter is completed, including:

[0031] The current limiting resistors are cut off step by step, so that the voltage of the wind farm is gradually increased.

[0032] Another aspect of the present disclosure provides a system for reconnecting a sending-end converter of an offshore wind power flexible direct current grid-connected system, wherein the system is used to implement the above-mentioned method for reconnecting a sending-end converter of an offshore wind power flexible direct current grid-connected system.

[0033] A method and system for reconnecting the sending-end converter of an offshore wind power flexible direct current grid-connected system according to an embodiment of the present disclosure adopts a pre-charging strategy of connecting a current-limiting resistor in series in the charging circuit of the sending-end converter. This strategy does not require additional equipment investment and has low cost and high efficiency. It simply and effectively realizes the rapid reconnection of the sending-end converter in a passive state, overcomes the impact generated at the moment of converter unlocking, avoids equipment damage, and thus improves the operational safety and reliability of the offshore wind power flexible direct current system. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 A schematic flow chart of a method for reconnecting a sending-end converter of an offshore wind power flexible direct current grid-connected system to the grid according to an embodiment of the present disclosure;

[0035] Figure 2 This is a schematic diagram of an equivalent charging circuit of a sending-end converter according to another embodiment of the present disclosure;

[0036] Figure 3 This is a schematic diagram of a starting voltage waveform of a sending-end converter according to another embodiment of the present disclosure. DETAILED DESCRIPTION

[0037] The following will be combined with the drawings in the embodiments of the present disclosure to clearly and completely describe the technical solutions in the embodiments of the present disclosure. Obviously, the described embodiments are only part of the embodiments of the present disclosure, not all of the embodiments. Based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present disclosure.

[0038] In addition, the described features, structures or characteristics may be combined in one or more embodiments in any suitable manner. In the following description, many specific details are provided to provide a full understanding of the embodiments of the present disclosure. However, those skilled in the art will appreciate that the technical solutions of the present disclosure may be practiced without one or more of the specific details, or other methods, components, devices, steps, etc. may be adopted. In other cases, known methods, devices, implementations or operations are not shown or described in detail to avoid blurring the various aspects of the present disclosure.

[0039] The flowcharts shown in the accompanying drawings are only exemplary and do not necessarily include all the contents and operations / steps, nor must they be executed in the order described. For example, some operations / steps can be decomposed, and some operations / steps can be combined or partially combined, so the actual execution order may change according to actual conditions.

[0040] It should be understood that although the terms first, second, third, etc. may be used in this disclosure to describe various components, these components should not be limited by these terms. These terms are used to distinguish one component from another component. Therefore, the first component discussed below can be referred to as the second component without departing from the teachings of the concepts of the present disclosure. As used in this disclosure, the term "and / or" includes any one of the associated listed items and all combinations of one or more.

[0041] Those skilled in the art will appreciate that the drawings are merely schematic diagrams of example embodiments, and the modules or processes in the drawings are not necessarily necessary for implementing the present disclosure, and therefore cannot be used to limit the protection scope of the present disclosure.

[0042] like Figure 1 As shown, an embodiment of the present disclosure provides a method for reconnecting a sending-end converter of an offshore wind power flexible direct current grid-connected system to the grid, wherein the sending-end converter includes a plurality of submodules, and the method includes:

[0043] Step S1, connecting a current limiting resistor in series in the DC side charging circuit of the sending-end converter.

[0044] Specifically, at the beginning of the pre-charging process, each bridge arm of the MMC (Modular Multilevel Converter) is approximately in a short-circuit state. In order to prevent the system from generating excessive current during the charging phase and endangering the safety of the equipment, a current-limiting resistor needs to be connected in series in the charging circuit.

[0045] The required current limiting resistor R lim The value of should satisfy the following expression:

[0046]

[0047] Among them, R lim is the current limiting resistor value, U dc_nom is the rated voltage of the DC system, I st is the maximum charging current.

[0048] When charging from the AC side, the actual project generally requires a maximum charging current of 50A per phase, which is 25A evenly divided between the upper and lower bridge arms. Therefore, when charging from the DC side, the maximum charging current I st Take 25×3=75A.

[0049] Step S2: close the DC switch of the sending-end converter, lock all the submodules, and perform uncontrolled charging on the capacitor of each submodule.

[0050] Specifically, for the sending-end converter, since its AC side is connected to the wind farm and is a passive grid, it needs to be powered from the DC side to start. After closing the DC switch of the sending-end converter, all submodules of the sending-end converter are locked at the same time. At this time, the equivalent charging circuit of the sending-end converter is as follows: Figure 2 shown.

[0051] The number of submodules in each bridge arm in the equivalent charging circuit of the sending-end converter is:

[0052]

[0053] Where N is the number of submodules in each bridge arm of the equivalent charging circuit of the sending-end converter, U dc_nom is the rated voltage of the DC system, U SM_nom is the rated voltage of the submodule, and k is the redundancy factor, which is generally 1 to 1.1.

[0054] At this time, all submodule capacitors of the sending-end converter are charged through the DC side without control, and the voltage of each submodule capacitor in the upper and lower bridge arms of the three-phase is charged to about 40-50% of the rated voltage. Specifically, in the passive charging (uncontrolled charging) stage, the voltage of each submodule capacitor will be charged to:

[0055]

[0056] Among them, U dc_nom is the rated voltage of the DC system, U SM_nom is the rated voltage of the submodule, and N is the number of submodules in each bridge arm in the equivalent charging circuit of the sending-end converter.

[0057] Step S3: putting all the submodules into operation to controllably charge the capacitors of the submodules.

[0058] Specifically, the blocking of the submodule is released. At this time, in order to enable the voltage of each submodule capacitor to be charged to the rated voltage value, the number of submodules put into use in the upper and lower bridge arms of the sending-end converter is set to:

[0059]

[0060] Among them, n is the number of submodules of the upper and lower bridge arms, U dc_nom is the rated voltage of the DC system, U SM_nom is the rated voltage of the submodule.

[0061] During the controllable charging stage, the capacitor voltage of each submodule is charged to a value close to the rated voltage.

[0062] Step S4: cutting off the current limiting resistor, establishing the wind farm voltage, and completing the grid-connected re-start of the sending-end converter.

[0063] Specifically, after the pre-charging process described above is completed, the current limiting resistors of the sending-end converter are removed step by step, so that the voltage of the wind farm gradually increases from 0 to the rated voltage, completing the re-grid-connected startup process of the sending-end converter.

[0064] A method for reconnecting a sending-end converter of an offshore wind power flexible direct current grid-connected system according to an embodiment of the present disclosure uses a pre-charging strategy of connecting a current-limiting resistor in series in the charging circuit of the sending-end converter. This method does not require additional equipment investment and has low cost and high efficiency. It simply and effectively realizes the rapid reconnection of the sending-end converter in a passive state, overcomes the impact generated at the moment of converter unlocking, avoids equipment damage, and thus improves the operational safety and reliability of the offshore wind power flexible direct current system.

[0065] like Figure 3 The figure shows a schematic diagram of the starting voltage waveform of the sending-end converter of the offshore wind power flexible direct current grid-connected system using the method for grid-connecting the sending-end converter of the embodiment of the present disclosure. Figure 3 (a) is the voltage waveform of the three-phase upper bridge arm submodule. Figure 3 (b) is the voltage waveform of the three-phase lower bridge arm submodule. Figure 3 (c) is the AC voltage of the wind farm. Figure 3 As can be seen from (a) and (b) in Figure 1, when the DC switch of the sending-end converter is closed, the submodule capacitor voltage will be in an uncontrolled charging state and eventually rise to 1 kV, which is about half of the rated value. Then, at about 9 seconds, the uncontrolled charging ends and the submodule is controlled. At about 12 seconds, the submodule capacitor voltage is charged to about 2 kV rated value. At this time, the pre-charging resistor is removed, as shown in Figure 1. Figure 3 As shown in (c), the sending-end converter starts to build the wind farm voltage. At about 14.2s, the wind farm voltage is built and the sending-end converter is started.

[0066] Another embodiment of the present disclosure provides a re-grid connection system for a sending-end converter of an offshore wind power flexible direct current grid-connected system, the system at least comprising a sending-end converter and a current limiting resistor connected in series in a charging circuit of the sending-end converter, for implementing the re-grid connection method for a sending-end converter of an offshore wind power flexible direct current grid-connected system described in the previous embodiment. The specific implementation process has been described in detail above, and will not be repeated in this article.

[0067] It is to be understood that the above embodiments are merely exemplary embodiments used to illustrate the principles of the present disclosure, but the present disclosure is not limited thereto. For those of ordinary skill in the art, various modifications and improvements can be made without departing from the spirit and substance of the present disclosure, and these modifications and improvements are also considered to be within the scope of protection of the present disclosure.

Claims

1. A method for reconnecting a sending-end converter of an offshore wind power flexible direct current grid-connected system, wherein the sending-end converter comprises a plurality of submodules, characterized in that: The method comprises: A current limiting resistor is connected in series in the DC side charging circuit of the sending-end converter; Closing the DC switch of the sending-end converter, locking all the submodules, and performing uncontrolled charging on the capacitors of the submodules; Putting all the submodules into operation to controllably charge the capacitors of the submodules; The current limiting resistor is removed, the wind farm voltage is established, and the re-grid connection start of the sending-end converter is completed.

2. The method according to claim 1, characterized in that The resistance of the current limiting resistor satisfies the following expression: Among them, R lim is the current limiting resistor value, U dc_nom is the rated voltage of the DC system, I st is the maximum charging current.

3. The method according to claim 2, characterized in that The maximum charging current I st It is 75A.

4. The method according to claim 1, characterized in that: The uncontrolled charging of the capacitors of the submodules includes: The capacitor voltage of each submodule is charged to: Among them, U SM is the submodule capacitor voltage, U dc_nom is the rated voltage of the DC system, and N is the number of sub-modules in each bridge arm of the equivalent charging circuit of the sending-end converter.

5. The method according to claim 4, characterized in that The number of submodules in each bridge arm of the equivalent charging circuit of the sending-end converter is: Where N is the number of submodules in each bridge arm of the equivalent charging circuit of the sending-end converter, U dc_nom is the rated voltage of the DC system, U SM_nom is the rated voltage of the submodule, and k is the redundancy factor, which is generally 1 to 1.

1.

6. The method according to claim 1, characterized in that The uncontrolled charging of the capacitors of the submodules includes: The capacitor voltage of each submodule is charged to 40% to 50% of the rated voltage value.

7. The method according to claim 1, characterized in that Before putting all the submodules into operation, the method further comprises: Set the number of submodules of the upper and lower bridge arms of the sending-end converter to: Among them, n is the number of submodules of the upper and lower bridge arms, U dc_nom is the rated voltage of the DC system, U SM_nom is the rated voltage of the submodule.

8. The method according to claim 1, characterized in that Controllably charging the capacitors of each of the submodules comprises: The capacitor voltage of each submodule is charged to a rated voltage value.

9. The method according to claim 1, characterized in that: The method of removing the current limiting resistor, establishing the wind farm voltage, and completing the grid-connected restart of the sending-end converter includes: The current limiting resistors are cut off step by step, so that the voltage of the wind farm is gradually increased.

10. A sending-end converter re-grid-connected system for an offshore wind power flexible direct current grid-connected system, characterized in that: The system is used to implement the method for reconnecting the sending-end converter of the offshore wind power flexible direct current grid-connected system as described in any one of claims 1 to 9.

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