A method and device for island starting of ultra-high voltage flexible direct current transmission system

Abstract

The present application provides a method and device for island startup of a UHV flexible direct current transmission system, which belongs to the field of flexible direct current transmission technology. The method includes closing the first switch; performing a preset operation on the second switch; closing the third switch according to a preset first sequence, and charging the DC side of the voltage source converter at the island end to a set value; obtaining the DC voltage reference value of the voltage source converter at the island end according to the preset target DC voltage; according to the DC voltage reference value, controlling the synchronous unlocking of the voltage source converter at the network end to establish the DC voltage at the network end and the DC voltage at the island end; controlling all the voltage source converters at the island end to be unlocked synchronously, and establishing a first AC voltage and a first frequency on the primary side of the converter transformer of the voltage source converter at the island end, so that the AC bus at the island end establishes a second AC voltage and a second frequency, and the island startup is completed. In this way, the smooth startup of the island is achieved, and the reliable transmission of large-scale new energy power through the UHV flexible direct current transmission system is guaranteed.

Description

Technical Field

[0001] The present application belongs to the field of flexible direct current transmission technology, and specifically relates to an island starting method and device for an ultra-high voltage flexible direct current transmission system. Background Art

[0002] Flexible direct current transmission technology based on voltage source converter is widely used in wind and solar renewable energy power transmission, island power supply, urban power supply, grid interconnection and other fields due to its flexible operation, power decoupling and independence from grid commutation.

[0003] Because most renewable energy concentrated areas are geographically remote and located at the end of the power grid, the AC grid in these areas is often weak or even disconnected from the AC grid. Therefore, when using flexible DC transmission technology to transmit renewable energy power from these concentrated areas, an island access approach is often required. This means that the renewable energy station is not connected to the AC grid, and the AC voltage required for operation is provided solely by the voltage source converter at the island end of the flexible DC transmission system. Currently, the DC poles of renewable energy transmission projects currently in operation both domestically and internationally via flexible DC islanding all utilize a single voltage source converter topology.

[0004] On this basis, the UHV Flexible DC transmission system utilizes multiple voltage source converters in series, which can exponentially increase transmission capacity. With the continuous expansion of new energy stations, the demand for using UHV Flexible DC to transmit renewable energy power is becoming increasingly urgent. To achieve this, it is necessary to smoothly start the isolated UHV Flexible DC transmission system to ensure the reliable transmission of large-scale renewable energy power through the Flexible DC transmission system. However, existing technologies do not yet have a method for achieving smooth island startup of UHV Flexible DC transmission systems. Summary of the Invention

[0005] An embodiment of the present application provides an island starting method for an ultra-high voltage flexible direct current transmission system, which is used to solve the problem in the prior art that an island ultra-high voltage flexible direct current transmission system cannot be started smoothly; another object of the present application is to provide an island starting device for an ultra-high voltage flexible direct current transmission system, which is used to execute the above-mentioned island starting method for an ultra-high voltage flexible direct current transmission system.

[0006] An embodiment of the present application provides an island starting method for an ultra-high voltage flexible direct current transmission system. The ultra-high voltage flexible direct current transmission system includes at least one DC pole, the DC pole including an island end connected to a new energy station and a network end connected to a large AC power grid. The island end and the network end are connected via a DC line. The island end includes a plurality of island end voltage source converters connected in series, and the network end includes a plurality of network end voltage source converters connected in series. The island starting method includes:

[0007] When the AC busbar at the island end is not energized, closing the first switch; the first switch is a starting resistor bypass switch or a knife switch of the voltage source converter at the island end;

[0008] Performing a preset operation on a second switch; the second switch is an AC switch of the island-end voltage source converter;

[0009] Closing a third switch in a preset first order and charging the DC side of the island-end voltage source converter to charge the submodule capacitor voltage in the island-end voltage source converter to a set value; the third switch is the AC switch of the network-end voltage source converter;

[0010] Obtaining a DC voltage reference value of the island-end voltage source converter according to a preset target DC voltage; the target DC voltage is the island-end DC voltage control target;

[0011] According to the DC voltage reference value, controlling the network-end voltage source converter to be unlocked synchronously to establish the network-end DC voltage and the island-end DC voltage;

[0012] Control all island-end voltage source converters to unlock synchronously, establish a set first AC voltage and first frequency on the primary side of the converter transformer of the island-end voltage source converter, and establish a set second AC voltage and second frequency on the island-end AC bus to complete island startup.

[0013] In some embodiments, it further includes:

[0014] The preset operation is to close the second switch according to a preset second sequence, or to keep the second switch in an open state.

[0015] In some embodiments, when the preset operation is to keep the second switch in the open state, the step before establishing the set second AC voltage and second frequency on the island-end AC bus further includes:

[0016] The third switch is closed according to a preset second sequence.

[0017] In some embodiments, the step of obtaining a DC voltage reference value of the island-end voltage source converter according to a preset target DC voltage includes:

[0018] Obtaining a local DC voltage reference value of the island end according to the target DC voltage;

[0019] Evenly distributing the local pole DC voltage reference value according to the total number of local pole series flow voltage source converters to obtain the DC voltage reference value of each island end voltage source converter;

[0020] The DC voltage reference value of each island-end voltage source converter is obtained according to the following formula:

[0021] N is a positive integer,

[0022] Among them, U dcref is the local DC voltage reference value at the island end, U dcref-i is the DC voltage reference value of each island-end voltage source converter, and N is the total number of local series voltage source converters.

[0023] In some embodiments, when the preset operation is to close the second switch in a preset second order, the step of controlling all island-end voltage source converters to be unlocked synchronously, establishing a preset first AC voltage and a first frequency on the primary side of the converter transformer of the island-end voltage source converter, and establishing a preset second AC voltage and a second frequency on the island-end AC busbar to complete the island startup includes:

[0024] Determining a master converter and a non-master converter; the master converter is any one of all island-end voltage source converters, and the non-master converter is any one of all island-end voltage source converters except the master converter;

[0025] Determining a DC bias value of a bridge arm voltage of the island-end voltage source converter according to the DC voltage reference value;

[0026] The master converter calculates an AC voltage phase reference value at the island end according to the AC frequency set value at the island end, so as to control the primary side of the converter transformer of the master converter to establish a target AC voltage, and calculates a master active current reference value and a master reactive current reference value according to the target AC voltage; the master active current reference value and the master reactive current reference value are used for self-control of the master converter;

[0027] The non-master converter uses the AC voltage phase reference value, the master active current reference value, and the master reactive current reference value to control itself.

[0028] In some embodiments, when the preset operation is to keep the second switch in the open state, the steps of controlling all island-end voltage source converters to be unlocked synchronously, establishing a set first AC voltage and a first frequency on the primary side of the converter transformer of the island-end voltage source converter, and establishing a set second AC voltage and a second frequency on the island-end AC busbar to complete the island startup include:

[0029] Determining a master converter and a non-master converter; the master converter is any one of all island-end voltage source converters, and the non-master converter is any one of all island-end voltage source converters except the master converter;

[0030] Determining a DC bias value of a bridge arm voltage of the island-end voltage source converter according to the DC voltage reference value;

[0031] The master converter calculates an AC voltage phase reference value at the island end according to the AC frequency set value at the island end, so as to control the primary side of the converter transformer of the master converter to establish a target AC voltage, and calculates a master active current reference value and a master reactive current reference value according to the target AC voltage; the master active current reference value and the master reactive current reference value are used for self-control of the master converter;

[0032] The non-master converter adopts the AC voltage phase reference value, calculates a non-master active current reference value and a non-master reactive current reference value according to the target AC voltage, and controls itself according to the AC voltage phase reference value, the non-master active current reference value, and the non-master reactive current reference value.

[0033] In some embodiments, the step of closing the third switch in a preset second sequence to establish the set second AC voltage and second frequency on the island-end AC bus includes:

[0034] Closing the AC switch of the master converter to establish the second AC voltage and the second frequency on the island-end AC bus;

[0035] closing the non-master converter AC switches in a preset third order;

[0036] The non-master converter uses the AC voltage phase reference value, the master active current reference value, and the master reactive current reference value to control itself.

[0037] Accordingly, the present application also provides an island starting device for an ultra-high voltage flexible direct current transmission system, wherein the ultra-high voltage flexible direct current transmission system includes at least one DC pole, the DC pole including an island end connected to a new energy station and a grid end connected to a large AC power grid, the island end and the grid end are connected via a DC line, the island end includes a plurality of island end voltage source converters connected in series, the grid end includes a plurality of grid end voltage source converters connected in series, the island starting device is used to perform the island starting method for the ultra-high voltage flexible direct current transmission system in any of the above items, and the island starting device includes:

[0038] An islanding sequence control unit, a charging control unit and an unlocking control unit, wherein the islanding sequence control unit, the charging control unit and the unlocking control unit are electrically connected to the ultra-high voltage flexible direct current transmission system.

[0039] In some embodiments, it further includes:

[0040] The island sequence control unit is used to close the first switch when the AC busbar at the island end is not energized, and to close the third switch according to a preset first sequence, and to close the second switch according to a preset second sequence;

[0041] The charging control unit is configured to charge the DC side of the island-end voltage source converter after closing the third switch in a preset first order, so that the capacitor voltage of the submodule in the island-end voltage source converter is charged to a set value;

[0042] The unlocking control unit is used to control the synchronous unlocking of the grid-end voltage source converter according to the DC voltage reference value, establish the grid-end DC voltage and the island-end DC voltage; control the synchronous unlocking of all island-end voltage source converters, and establish the set first AC voltage and first frequency on the primary side of the converter transformer of the island-end voltage source converter, so that the island-end AC bus establishes the set second AC voltage and second frequency, completing the island startup.

[0043] In some embodiments, it further includes:

[0044] The first switch is a starting resistor bypass switch or a knife switch of the island-end voltage source converter;

[0045] The second switch is an AC switch of the island-end voltage source converter;

[0046] The third switch is an AC switch of the network-end voltage source converter.

[0047] The technical solutions in the above technical solutions have the following advantages or beneficial effects:

[0048] Compared with the prior art, the present invention provides an island start method for a UHV flexible DC transmission system, comprising: closing a first switch when the AC busbar at the island end is not energized; the first switch is a starting resistor bypass switch or knife switch of the voltage source converter at the island end; performing a preset operation on the second switch; the second switch is an AC switch of the voltage source converter at the island end; closing a third switch in a preset first order, and charging the DC side of the voltage source converter at the island end, so that the submodule capacitor voltage in the voltage source converter at the island end is charged to a set value; the third switch is a voltage source at the network end. The system controls the AC switches of the converters; obtains the DC voltage reference value of the island-side voltage source converter based on the preset target DC voltage; the target DC voltage serves as the island-side DC voltage control target; based on the DC voltage reference value, controls the synchronous unlocking of the grid-side voltage source converters, establishes the grid-side DC voltage and the island-side DC voltage; controls the synchronous unlocking of all island-side voltage source converters, establishes the preset first AC voltage and first frequency on the primary side of the converter transformer of the island-side voltage source converter, and establishes the preset second AC voltage and second frequency on the island-side AC busbar, completing island startup. This achieves a smooth startup of the island UHV Flexible DC transmission system and ensures the reliable transmission of large-scale renewable energy power through the UHV Flexible DC transmission system.

[0049] Compared to the prior art, the present invention provides a UHV Flexible DC transmission system island starting device that implements the aforementioned UHV Flexible DC transmission system island starting method. It is understood that the UHV Flexible DC transmission system island starting device can also have the beneficial effects of the aforementioned UHV Flexible DC transmission system island starting method, and the details will not be repeated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The following detailed description of the specific embodiments of the present application in conjunction with the accompanying drawings will make the technical solutions and other beneficial effects of the present application apparent.

[0051] Figure 1 A schematic diagram of the structure of the ultra-high voltage flexible direct current transmission system provided in an embodiment of the present application;

[0052] Figure 2 A schematic diagram of the control strategy principle of a single island-end voltage source converter in a DC pole of an island starting method for a UHVDC flexible power transmission system provided in an embodiment of the present application;

[0053] Figure 3 A schematic diagram of the control strategy principle of the DC pole-connected island-end voltage source converter in the island starting method of the ultra-high voltage flexible DC transmission system provided in an embodiment of the present application;

[0054] Figure 4 A schematic diagram of the structure of an island starting device for a UHV flexible DC transmission system provided in an embodiment of the present application;

[0055] Reference numerals include: 100 - island sequence control unit, 200 - charging control unit, 300 - unlocking control unit. DETAILED DESCRIPTION

[0056] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the embodiments described are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative efforts are within the scope of protection of this application.

[0057] The following first introduces the ultra-high voltage flexible direct current transmission system provided by the embodiment of the present application.

[0058] See also Figure 1 , Figure 1 The schematic diagram of the structure of the ultra-high voltage flexible direct current transmission system provided in an embodiment of the present application is shown. In this embodiment of the present application, the ultra-high voltage flexible direct current transmission system includes at least one DC pole, which includes an island terminal connected to a new energy station and a network terminal connected to a large AC power grid. The island terminal and the network terminal are connected by a DC line. The island terminal includes multiple island terminal voltage source converters connected in series, and the network terminal includes multiple network terminal voltage source converters connected in series. The island terminal is connected only to the new energy station and not to the large AC power grid, while the network terminal is connected to the large AC power grid.

[0059] Based on this, the first embodiment of the present application provides a method for island starting of a UHV flexible DC transmission system, comprising:

[0060] Step 101: When the AC busbar at the island end is not energized, close the first switch; the first switch is a starting resistor bypass switch or a knife switch of the voltage source converter at the island end;

[0061] Step 102: performing a preset operation on a second switch; the second switch is an AC switch of the island-end voltage source converter;

[0062] Step 103: Close the third switch in a preset first order and charge the DC side of the island-end voltage source converter to charge the submodule capacitor voltage in the island-end voltage source converter to a set value; the third switch is the AC switch of the network-end voltage source converter.

[0063] Step 104: obtaining a DC voltage reference value of the island-end voltage source converter according to a preset target DC voltage; the target DC voltage is a control target of the island-end DC voltage;

[0064] Step 105: Control the grid-connected voltage source converter to be unlocked synchronously based on the DC voltage reference value, and establish the grid-connected DC voltage and the island-connected DC voltage.

[0065] Step 106: Control all island-end voltage source converters to be unlocked synchronously, establish a set first AC voltage and a first frequency on the primary side of the converter transformer of the island-end voltage source converter, and establish a set second AC voltage and a second frequency on the island-end AC bus, thereby completing island startup.

[0066] Specifically, in the embodiment of the present application, the operation control characteristics of the island-end voltage source converter or the grid-end voltage source converter (collectively referred to as voltage source converter) are as shown in formula (1):

[0067]

[0068] Among them, u pj and u nj are the upper and lower arm voltages of phase j (j = a, b, c) of the voltage source converter, is the DC bias of the bridge arm voltage, u vjref is the AC voltage reference wave of phase j. It can be seen that the control of the voltage source converter is achieved by controlling the bridge arm voltage of the upper and lower bridge arms of each phase. It can also be seen from formula (1) that the bridge arm voltage consists of two parts: the DC transformer mass and the AC voltage reference wave.

[0069] In one embodiment, the preset operation is to close the second switch according to a preset second sequence, or to keep the second switch in an open state.

[0070] In one embodiment, when the preset operation is to keep the second switch in the open state, the steps before establishing the set second AC voltage and second frequency on the island-end AC bus in step 106 further include:

[0071] Step 107: Close the third switch according to the preset second sequence.

[0072] In one embodiment, step 104 of obtaining a DC voltage reference value of the island-end voltage source converter according to a preset target DC voltage includes:

[0073] Step 401 , obtaining a DC voltage reference value of a local pole of an island end according to a target DC voltage; the local pole refers to the current DC pole.

[0074] Step 402 : Evenly distribute the local pole DC voltage reference value according to the total number of the local pole series flow voltage source converters to obtain the DC voltage reference value of each island-end voltage source converter.

[0075] Specifically, see Figure 2 and Figure 3 , Figure 2 A schematic diagram illustrating the control strategy principle of a single island-end voltage source converter in a DC pole of an island starting method for a UHVDC flexible power transmission system provided in an embodiment of the present application is provided; Figure 3 The schematic diagram of the control strategy principle of the island-end voltage source converter connected in series in the DC pole of the island starting method of the ultra-high voltage flexible DC transmission system provided in the embodiment of the present application is illustrated; in the embodiment of the present application, the DC voltage reference value of each island-end voltage source converter is obtained according to the following formula (2):

[0076] N is a positive integer, (2)

[0077] Among them, U dcref is the DC voltage reference value of the island end, U dcref-i is the DC voltage reference value of each island-end voltage source converter, and N is the total number of local series voltage source converters.

[0078] In one embodiment, when the preset operation is to close the second switch in a preset second order, step 106 controls all island-end voltage source converters to be unlocked synchronously, establishes a preset first AC voltage and a first frequency on the primary side of the converter transformer of the island-end voltage source converter, and establishes a preset second AC voltage and a second frequency on the island-end AC bus. Completing island startup includes:

[0079] Step 601: Determine a master converter and a non-master converter; the master converter is any one of all island-end voltage source converters, and the non-master converter is any one of all island-end voltage source converters except the master converter;

[0080] Step 602: Determine the DC bias of the bridge arm voltage of the island-end voltage source converter according to the DC voltage reference value.

[0081] Specifically, the DC bias of the bridge arm voltage of the island end voltage source converter is the DC voltage reference value U at the island end. dVref-i 1 / 2.

[0082] In step 603, the master converter calculates an AC voltage phase reference value at the island end based on the AC frequency set value at the island end, thereby controlling the primary side of the converter transformer of the master converter to establish a target AC voltage. The master active current reference value and the master reactive current reference value are calculated based on the target AC voltage. The master active current reference value and the master reactive current reference value are used for self-control of the master converter.

[0083] Specifically, the master converter sets the AC frequency F at the island end. ref Calculate the AC voltage phase reference value θ at the island end ref, to control the primary side of the converter transformer of the master converter to establish the target AC voltage, and calculate the active current reference value I according to the target AC voltage dref and reactive current reference value I qref ; Active current reference value I dref and reactive current reference value I qref In the embodiment of the present application, the d-axis component Usd of the primary AC voltage Us of the converter transformer of the voltage source converter is controlled to be the AC voltage setting value Usref, the q-axis component Usq of Us is 0, and the active current reference value I is calculated. dref and reactive current reference value I qref The inner loop current control of this voltage source converter is input, and the inner loop current control outputs the AC voltage reference wave of this voltage source converter; the bridge arm voltage DC bias of the voltage source converter and the AC voltage reference wave are used to control the bridge arm voltage, thereby realizing the control of this voltage source converter.

[0084] In step 604 , the non-master converter controls itself using the AC voltage phase reference value, the master active current reference value, and the master reactive current reference value.

[0085] Specifically, the non-master converter uses the AC voltage phase reference value θ ref , Master active current reference value I dref And the main control reactive current reference value I qref Take control of yourself.

[0086] In one embodiment, when the preset operation is to keep the second switch in the open state, step 106 controls all island-end voltage source converters to be unlocked synchronously, establishes a set first AC voltage and a first frequency on the primary side of the converter transformer of the island-end voltage source converter, and establishes a set second AC voltage and a second frequency on the island-end AC bus. Completing the island startup includes:

[0087] Step 605: Determine a master converter and a non-master converter; the master converter is any one of all island-end voltage source converters, and the non-master converter is any one of all island-end voltage source converters except the master converter;

[0088] Step 606, determining a DC bias value of a bridge arm voltage of the island-end voltage source converter according to a DC voltage reference value;

[0089] Specifically, the DC bias of the bridge arm voltage of the island end voltage source converter is the DC voltage reference value U at the island end. dcref 1 / 2.

[0090] Step 607: The master converter calculates an AC voltage phase reference value at the island end based on the AC frequency set value at the island end to control the primary side of the converter transformer of the master converter to establish a target AC voltage. The master active current reference value and the master reactive current reference value are calculated based on the target AC voltage. The master active current reference value and the master reactive current reference value are used for self-control of the master converter.

[0091] Specifically, the master converter sets the AC frequency F at the island end. ref Calculate the AC voltage phase reference value θ at the island end ref , to control the primary side of the converter transformer of the master converter to establish the target AC voltage, and calculate the active current reference value I according to the target AC voltage dref and reactive current reference value I qref ; Active current reference value I dref and reactive current reference value I qref Used for the self-control of the master converter;

[0092] In step 608, the non-master converter uses the AC voltage phase reference value to calculate the non-master active current reference value and the non-master reactive current reference value according to the target AC voltage, and controls itself according to the AC voltage phase reference value, the non-master active current reference value and the non-master reactive current reference value.

[0093] Specifically, the non-master converter uses the AC voltage phase reference value θ ref , calculate the non-master active current reference value and non-master reactive current reference value according to the target AC voltage, and calculate the non-master active current reference value and non-master reactive current reference value according to the AC voltage phase reference value θ ref , non-master active current reference value and non-master reactive current reference value for self-control.

[0094] In one embodiment, step 107 of closing the third switch in a preset second sequence to establish the set second AC voltage and second frequency on the island-end AC bus includes:

[0095] Step 701: Close the AC switch of the master converter to establish a second AC voltage and a second frequency on the AC bus at the island end.

[0096] Step 702: closing the AC switches of the non-master converters according to a preset third sequence;

[0097] In step 703 , the non-master converter controls itself using the AC voltage phase reference value, the master active current reference value, and the master reactive current reference value.

[0098] Accordingly, the second embodiment of the present application provides an island starting device for a UHV flexible DC transmission system, which is used to execute any one of the island starting methods for a UHV flexible DC transmission system in the first embodiment.

[0099] See also Figure 4 , Figure 4 The schematic diagram of the structure of the island start-up device of the ultra-high voltage flexible direct current transmission system provided in the embodiment of the present application is shown. In the embodiment of the present application, the island start-up device includes an island sequence control unit 100, a charging control unit 200 and an unlocking control unit 300.

[0100] Specifically, the islanding sequence control unit 100 , the charging control unit 200 and the unlocking control unit 300 are electrically connected to the UHVDC flexible power transmission system.

[0101] In one embodiment, the island starting device further includes:

[0102] The island sequence control unit 100 is used to close the first switch, close the third switch according to a preset first sequence, and close the second switch according to a preset second sequence when the AC busbar at the island end is not energized;

[0103] The charging control unit 200 is configured to charge the DC side of the island-end voltage source converter after closing the third switch in a preset first order, so that the submodule capacitor voltage in the island-end voltage source converter is charged to a set value;

[0104] The unlocking control unit 300 is used to control the synchronous unlocking of the grid-end voltage source converter according to the DC voltage reference value, establish the grid-end DC voltage and the island-end DC voltage; control the synchronous unlocking of all island-end voltage source converters, and establish the set first AC voltage and first frequency on the primary side of the converter transformer of the island-end voltage source converter, so that the island-end AC bus establishes the set second AC voltage and second frequency, completing the island startup.

[0105] In one embodiment, it further includes:

[0106] The first switch is the starting resistor bypass switch or knife switch of the island end voltage source converter; the second switch is the AC switch of the island end voltage source converter; and the third switch is the AC switch of the network end voltage source converter.

[0107] It can be understood that the island starting device of the ultra-high voltage flexible direct current transmission system can have all the beneficial effects of the above-mentioned island starting method of the ultra-high voltage flexible direct current transmission system, which will not be repeated here.

[0108] It should be understood that the above description is for illustrative purposes only and is not intended to be limiting. Many embodiments and many applications beyond the examples provided will be apparent to those skilled in the art upon reading the above description. Therefore, the scope of the present teachings should not be determined with reference to the above description, but rather with reference to the preceding claims and the full scope of equivalents to which such claims are entitled. For the purpose of completeness, all articles and references, including disclosures of patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein from the preceding claims is not intended to be a disclaimer of such subject matter, nor should it be considered that the applicants did not consider such subject matter to be part of the disclosed subject matter.

Claims

1. A method for island starting of an ultra-high voltage flexible direct current transmission system, wherein the ultra-high voltage flexible direct current transmission system comprises at least one DC pole, wherein the DC pole comprises an island terminal connected to a new energy station and a network terminal connected to a large AC power grid, wherein the island terminal and the network terminal are connected via a DC line, wherein the island terminal comprises a plurality of island terminal voltage source converters connected in series, and the network terminal comprises a plurality of network terminal voltage source converters connected in series, wherein: The island startup method includes: When the AC busbar at the island end is not energized, closing the first switch; the first switch is a starting resistor bypass switch or a knife switch of the voltage source converter at the island end; Performing a preset operation on a second switch; the second switch is an AC switch of the island-end voltage source converter; Closing a third switch in a preset first order and charging the DC side of the island-end voltage source converter to charge the submodule capacitor voltage in the island-end voltage source converter to a set value; the third switch is the AC switch of the network-end voltage source converter; Obtaining a DC voltage reference value of the island-end voltage source converter according to a preset target DC voltage; the target DC voltage is the island-end DC voltage control target; According to the DC voltage reference value, controlling the network-end voltage source converter to be unlocked synchronously to establish the network-end DC voltage and the island-end DC voltage; Control all island-end voltage source converters to unlock synchronously, establish a set first AC voltage and first frequency on the primary side of the converter transformer of the island-end voltage source converter, and establish a set second AC voltage and second frequency on the island-end AC bus to complete island startup.

2. The island starting method for a UHVDC flexible power transmission system according to claim 1, wherein: Also includes: The preset operation is to close the second switch according to a preset second sequence, or to keep the second switch in an open state.

3. The island starting method for a UHVDC flexible power transmission system according to claim 2, characterized in that: When the preset operation is to keep the second switch in the open state, the steps before establishing the set second AC voltage and second frequency on the island-end AC bus further include: The third switch is closed according to a preset second sequence.

4. The island starting method for a UHVDC flexible power transmission system according to claim 1, wherein: The step of obtaining a DC voltage reference value of the island-end voltage source converter according to a preset target DC voltage comprises: Obtaining a local DC voltage reference value of the island end according to the target DC voltage; Evenly distributing the local pole DC voltage reference value according to the total number of local pole series flow voltage source converters to obtain the DC voltage reference value of each island end voltage source converter; The DC voltage reference value of each island-end voltage source converter is obtained according to the following formula: N is a positive integer, Among them, U dcref is the local DC voltage reference value at the island end, U dcref-i is the DC voltage reference value of each island-end voltage source converter, and N is the total number of local series voltage source converters.

5. The island starting method for a UHVDC flexible power transmission system according to claim 2, characterized in that: When the preset operation is to close the second switch in a preset second sequence, the steps of controlling all island-end voltage source converters to be unlocked synchronously, establishing a preset first AC voltage and a first frequency on the primary side of the converter transformer of the island-end voltage source converter, and establishing a preset second AC voltage and a second frequency on the island-end AC busbar to complete the island startup include: Determining a master converter and a non-master converter; the master converter is any one of all island-end voltage source converters, and the non-master converter is any one of all island-end voltage source converters except the master converter; Determining a DC bias value of a bridge arm voltage of the island-end voltage source converter according to the DC voltage reference value; The master converter calculates an AC voltage phase reference value at the island end according to the AC frequency set value at the island end, so as to control the primary side of the converter transformer of the master converter to establish a target AC voltage, and calculates a master active current reference value and a master reactive current reference value according to the target AC voltage; the master active current reference value and the master reactive current reference value are used for self-control of the master converter; The non-master converter uses the AC voltage phase reference value, the master active current reference value, and the master reactive current reference value to control itself.

6. The island starting method for a UHVDC flexible power transmission system according to claim 2, wherein: When the preset operation is to keep the second switch in the open state, the steps of controlling all island-end voltage source converters to be unlocked synchronously, establishing a set first AC voltage and a first frequency on the primary side of the converter transformer of the island-end voltage source converter, and establishing a set second AC voltage and a second frequency on the island-end AC busbar to complete the island startup include: Determining a master converter and a non-master converter; the master converter is any one of all island-end voltage source converters, and the non-master converter is any one of all island-end voltage source converters except the master converter; Determining a DC bias value of a bridge arm voltage of the island-end voltage source converter according to the DC voltage reference value; The master converter calculates an AC voltage phase reference value at the island end according to the AC frequency set value at the island end, so as to control the primary side of the converter transformer of the master converter to establish a target AC voltage, and calculates a master active current reference value and a master reactive current reference value according to the target AC voltage; the master active current reference value and the master reactive current reference value are used for self-control of the master converter; The non-master converter adopts the AC voltage phase reference value, calculates a non-master active current reference value and a non-master reactive current reference value according to the target AC voltage, and controls itself according to the AC voltage phase reference value, the non-master active current reference value, and the non-master reactive current reference value.

7. The island starting method for a UHVDC flexible power transmission system according to claim 3 or 6, characterized in that: The step of closing the third switch in a preset second sequence to establish the set second AC voltage and second frequency on the island-end AC bus comprises: Closing the AC switch of the master converter to establish the second AC voltage and the second frequency on the island-end AC bus; closing the non-master converter AC switches in a preset third order; The non-master converter uses the AC voltage phase reference value, the master active current reference value, and the master reactive current reference value to control itself.

8. An island starting device for an ultra-high voltage flexible direct current transmission system, the ultra-high voltage flexible direct current transmission system comprising at least one DC pole, the DC pole comprising an island terminal connected to a new energy station and a network terminal connected to a large AC power grid, the island terminal and the network terminal being connected via a DC line, the island terminal comprising a plurality of island terminal voltage source converters connected in series, the network terminal comprising a plurality of network terminal voltage source converters connected in series, characterized in that: The island starting device is used to execute the island starting method for the ultra-high voltage flexible direct current transmission system according to any one of claims 1 to 7, and the island starting device includes: An islanding sequence control unit, a charging control unit and an unlocking control unit, wherein the islanding sequence control unit, the charging control unit and the unlocking control unit are electrically connected to the ultra-high voltage flexible direct current transmission system.

9. The island starting device for a UHVDC flexible power transmission system according to claim 8, characterized in that: Also includes: The island sequence control unit is used to close the first switch when the AC busbar at the island end is not energized, and to close the third switch according to a preset first sequence, and to close the second switch according to a preset second sequence; The charging control unit is configured to charge the DC side of the island-end voltage source converter after closing the third switch in a preset first order, so that the capacitor voltage of the submodule in the island-end voltage source converter is charged to a set value; The unlocking control unit is used to control the synchronous unlocking of the grid-end voltage source converter according to the DC voltage reference value, establish the grid-end DC voltage and the island-end DC voltage; control the synchronous unlocking of all island-end voltage source converters, and establish the set first AC voltage and first frequency on the primary side of the converter transformer of the island-end voltage source converter, so that the island-end AC bus establishes the set second AC voltage and second frequency, completing the island startup.

10. The island starting device for a UHVDC flexible power transmission system according to claim 9, characterized in that: Also includes: The first switch is a starting resistor bypass switch or a knife switch of the island-end voltage source converter; The second switch is an AC switch of the island-end voltage source converter; The third switch is an AC switch of the network-end voltage source converter.

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