Island starting method and device for extra-high voltage flexible direct current power transmission system

Abstract

The invention provides an island starting method and device for an extra-high voltage flexible direct current power transmission system, and belongs to the technical field of flexible direct current power transmission. The method includes closing the first switch; executing a preset operation on the second switch; closing the third switch according to a preset first sequence, and carrying out direct current side charging on the island terminal voltage source converter to a set value; obtaining a direct-current voltage reference value of the island terminal voltage source converter according to a preset target direct-current voltage; according to the direct-current voltage reference value, controlling a networking end voltage source converter to be synchronously unlocked, and establishing a networking end direct-current voltage and an island end direct-current voltage; and controlling all island end voltage source converters to be unlocked synchronously, and establishing a first alternating voltage and a first frequency at a primary side of a converter transformer of each island end voltage source converter, so that an island end alternating current bus establishes a second alternating voltage and a second frequency, thereby completing island starting. Therefore, stable starting of the island is realized, and reliable transmission of large-scale new energy power through the extra-high voltage flexible direct current power transmission system is ensured.

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 the fields of wind and solar renewable energy power transmission, island power supply, urban power supply, grid interconnection, etc. due to its flexible operation, power decoupling and independence from grid phase commutation.

[0003] Since most of the new energy concentration areas are located in remote areas and at the end of the power grid, the AC grid in the new energy concentration areas is usually weak or even not connected to the AC grid. Therefore, when using flexible DC transmission technology to send new energy power in new energy concentration areas, it is often necessary to adopt an island access method, that is, the new energy station is not connected to the AC grid, and only the island-end voltage source converter of the flexible DC transmission system provides the AC voltage required for the new energy station to work. At present, the DC poles of the new energy transmission projects put into operation at home and abroad through flexible DC island transmission all adopt a single voltage source converter topology.

[0004] On this basis, the UHV flexible DC transmission system adopts multiple voltage source converter series technology to increase the transmission capacity exponentially. With the continuous increase in the scale of new energy sites, the demand for using UHV flexible DC to deliver new energy power is becoming more and more urgent. To this end, it is necessary to smoothly start the UHV flexible DC transmission system island to ensure the reliable delivery of large-scale new energy power through the flexible DC transmission system. However, there is no method in the prior art that can achieve smooth island startup of the UHV flexible DC transmission system. 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 of an ultra-high voltage flexible direct current transmission system cannot be started smoothly; another purpose 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] The embodiment of the present application provides an island starting method for an ultra-high voltage flexible direct current transmission system, wherein the ultra-high voltage flexible direct current transmission system comprises at least one direct current pole, wherein the direct current pole comprises an island end connected to a new energy station and a networking end connected to a large alternating current power grid, wherein the island end and the networking end are connected via a direct current line, wherein the island end comprises a plurality of island end voltage source converters connected in series, and the networking end comprises a plurality of networking end voltage source converters connected in series, wherein the island starting method comprises:

[0007] 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;

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

[0009] The third switch is closed in a preset first order, and the DC side of the isolated island voltage source converter is charged, so that the submodule capacitor voltage in the isolated island voltage source converter is charged to a set value; the third switch is an AC switch of the networked 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 a control target of the island-end DC voltage;

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

[0012] Control all island-end voltage source converters to unlock synchronously, and 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, so that the island-end AC busbar establishes a set second AC voltage and a second frequency, and completes the island startup.

[0013] In some embodiments, it also 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 an open state, the step before establishing the set second AC voltage and second frequency at 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 comprises:

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

[0019] The local pole DC voltage reference value is evenly distributed 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 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-pole 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 the island-end voltage source converters to be unlocked synchronously, and 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 bus, and completing the island startup includes:

[0024] 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;

[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 the AC voltage phase reference value of the island end according to the AC frequency setting value of 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 the master active current reference value and the 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 the 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 the 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 bus, and completing the island startup include:

[0029] 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;

[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 the AC voltage phase reference value of the island end according to the AC frequency setting value of 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 the master active current reference value and the 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 the self-control of the master converter;

[0032] The non-master converter adopts the AC voltage phase reference value, calculates 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.

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

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

[0035] Closing the non-master converter AC switch according to a preset third sequence;

[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] Correspondingly, the present application also provides 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 direct current pole, the direct current pole comprising an island end connected to a new energy station and a networking end connected to a large alternating current power grid, the island end and the networking end being connected via a direct current line, the island end comprising a plurality of island end voltage source converters connected in series, the networking end comprising a plurality of networking end voltage source converters connected in series, the island starting device being used to execute the island starting method for an ultra-high voltage flexible direct current transmission system in any of the above items, the island starting device comprising:

[0038] An island sequence control unit, a charging control unit and an unlocking control unit, wherein the island sequence control unit, the charging control unit and the unlocking control unit are electrically connected to the UHV flexible direct current power transmission system.

[0039] In some embodiments, it also 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 used 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;

[0042] The unlocking control unit is used to control the synchronous unlocking of the network-end voltage source converter according to the DC voltage reference value, establish the network-end DC voltage and the island-end DC voltage; control all island-end voltage source converters to be unlocked synchronously, and 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, so that the island-end AC bus establishes a set second AC voltage and a second frequency, thereby completing the island startup.

[0043] In some embodiments, it also 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 solution in the above technical solution has the following advantages or beneficial effects:

[0048] Compared with the prior art, the present invention provides an island starting method for a UHV flexible DC transmission system, comprising: 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 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 the third switch according to a preset first sequence, and charging the DC side of the voltage source converter at the island end, so that the capacitor voltage of the submodule in the voltage source converter at the island end is charged to a set value; the third switch is a voltage source at the networking end. The AC switch of the converter is controlled; the DC voltage reference value of the voltage source converter at the island end is obtained according to the preset target DC voltage; the target DC voltage is the DC voltage control target at the island end; according to the DC voltage reference value, the voltage source converter at the network end is controlled to be unlocked synchronously, and the DC voltage at the network end and the DC voltage at the island end are established; all the voltage source converters at the island end are controlled to be unlocked synchronously, and the set first AC voltage and first frequency are established 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 the set second AC voltage and second frequency, and the island start-up is completed. In this way, the smooth start-up of the island of the UHV flexible DC transmission system is achieved, and the reliable transmission of large-scale new energy power through the UHV flexible DC transmission system is guaranteed.

[0049] Compared with the prior art, the island starting device of a UHV flexible DC transmission system of the present application executes the above-mentioned island starting method of the UHV flexible DC transmission system. It can be understood that the island starting device of the UHV flexible DC transmission system can have the beneficial effects of the above-mentioned island starting method of the UHV flexible DC transmission system, which will not be repeated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The technical solution and other beneficial effects of the present application will be made apparent by describing in detail the specific implementation methods of the present application in conjunction with the accompanying drawings.

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

[0053] Figure 3 A schematic diagram of the control strategy principle of the isolated 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;

[0054] Figure 4 A schematic diagram of the structure of an island starting device for an ultra-high voltage flexible direct current 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 technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present 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 structural schematic diagram of the ultra-high voltage flexible direct current transmission system provided by the embodiment of the present application is illustrated; in the embodiment of the present application, the ultra-high voltage flexible direct current transmission system includes at least one DC pole, the DC pole includes an island end connected to a new energy site and a networking end connected to a large AC power grid, the island end and the networking end are connected through a DC line, the island end includes a plurality of island end voltage source converters connected in series, and the networking end includes a plurality of networking end voltage source converters connected in series. Among them, the island end is only connected to the new energy site, not to the large AC power grid, and the networking end is connected to the large AC power grid.

[0059] Based on this, a method for island starting of a UHV flexible DC transmission system is provided in the first embodiment of the present application, 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, closing the third switch according to the preset first sequence, and charging the DC side of the isolated island voltage source converter, so that the submodule capacitor voltage in the isolated island voltage source converter is charged to a set value; the third switch is the AC switch of the interconnected 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, according to the DC voltage reference value, control the network-end voltage source converter to be unlocked synchronously, and establish the network-end DC voltage and the island-end DC voltage;

[0065] Step 106, control all island-end voltage source converters to unlock synchronously, and 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, so that the island-end AC bus establishes a set second AC voltage and a second frequency, and completes the island startup.

[0066] Specifically, in the embodiment of the present application, the operation control characteristics of the isolated 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 includes two parts: the DC transformer quality and the AC voltage reference wave.

[0069] In one implementation, 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 of the island-end AC bus in step 106 also include:

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

[0072] In one implementation, 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 DC pole currently located.

[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 an ultra-high voltage flexible DC transmission system provided in an embodiment of the present application is shown; Figure 3 The schematic diagram of the control strategy principle of the isolated island end voltage source converter connected in series in the DC pole of the island start 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 isolated 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 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.

[0078] In one embodiment, when the preset operation is to close the second switch in a preset second sequence, step 106 controls all the island-end voltage source converters to unlock synchronously, and 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, so that the island-end AC bus establishes a set second AC voltage and a second frequency, and completing the island startup includes:

[0079] Step 601, 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 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 of.

[0082] Step 603, the master converter calculates the AC voltage phase reference value at the island end according to the AC frequency setting value at the island end, so as to control the primary side of the converter transformer of the master converter to establish the target AC voltage, and calculates the master active current reference value and the 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 the self-control of the master converter;

[0083] Specifically, the main control 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 main control 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 side 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 and AC voltage reference wave of the voltage source converter are used to control the bridge arm voltage, so as to realize the control of this voltage source converter.

[0084] Step 604: 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.

[0085] Specifically, the non-master converter uses the AC voltage phase reference value θ ref , Main control 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 the island-end voltage source converters to unlock synchronously, and 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, so that the island-end AC bus establishes a set second AC voltage and a second frequency, and completing the island startup includes:

[0087] Step 605, 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 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 of.

[0090] Step 607, the master converter calculates the AC voltage phase reference value at the island end according to the AC frequency setting value at the island end, so as to control the primary side of the converter transformer of the master converter to establish the target AC voltage, and calculates the master active current reference value and the 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 the self-control of the master converter;

[0091] Specifically, the main control 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 main control 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, calculates 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 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 closes the third switch according to a preset second sequence so that the island-end AC busbar establishes a set second AC voltage and a set second frequency, including:

[0095] Step 701, closing the AC switch of the main control converter to establish a second AC voltage and a second frequency on the AC bus at the island end;

[0096] Step 702, closing the non-master converter AC switch according to a preset third sequence;

[0097] Step 703: 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.

[0098] Correspondingly, the second embodiment of the present application provides an island starting device for an ultra-high voltage flexible direct current transmission system, which is used to execute any island starting method for an ultra-high voltage flexible direct current transmission system as described in the first embodiment.

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

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

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

[0102] The island sequence control unit 100 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;

[0103] The charging control unit 200 is used 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 network-end voltage source converter according to the DC voltage reference value, establish the network-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, thereby completing the island startup.

[0105] In one embodiment, it further includes:

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

[0107] It can be understood that the island starting device for the ultra-high voltage flexible direct current transmission system can have all the beneficial effects of the above-mentioned island starting method for 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 illustration and not for limitation. 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 foregoing claims and the full scope of equivalents to which such claims are entitled. For the purpose of comprehensiveness, 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 in the foregoing claims is not intended to be a waiver of such subject matter, nor should it be considered that the applicant has not considered such subject matter to be part of the disclosed application subject matter.

Claims

1. A method for island starting of a UHV flexible direct current transmission system, wherein the UHV flexible direct current transmission system comprises at least one DC pole, wherein the DC pole comprises an island end connected to a new energy station and a networking end connected to a large AC power grid, wherein the island end and the networking end are connected via a DC line, wherein the island end comprises a plurality of island end voltage source converters connected in series, and the networking end comprises a plurality of networking end voltage source converters connected in series, wherein: The island startup method comprises: 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; Performing a preset operation on a second switch; the second switch is an AC switch of the island-end voltage source converter; The third switch is closed in a preset first order, and the DC side of the isolated island voltage source converter is charged, so that the submodule capacitor voltage in the isolated island voltage source converter is charged to a set value; the third switch is an AC switch of the networked 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 a control target of the island-end DC voltage; According to the DC voltage reference value, controlling the network-end voltage source converter to be unlocked synchronously to establish a network-end DC voltage and an island-end DC voltage; Control all island-end voltage source converters to unlock synchronously, and 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, so that the island-end AC busbar establishes a set second AC voltage and a second frequency, and completes the island startup.

2. The island starting method of the UHV flexible DC power transmission system according to claim 1, characterized in that: 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 of the UHV flexible DC 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 at the island-end AC busbar also include: The third switch is closed according to a preset second sequence.

4. The island starting method of the UHV flexible DC transmission system according to claim 1, characterized in that: The step of obtaining a DC voltage reference value of the island-end voltage source converter according to a preset target DC voltage comprises: Acquire a local DC voltage reference value of the island end according to the target DC voltage; The local pole DC voltage reference value is evenly distributed 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 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-pole series voltage source converters.

5. The island starting method of the UHV flexible DC transmission system according to claim 2, characterized in that: In the case where the preset operation is to close the second switch in a preset second sequence, the steps of controlling all the 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 bus, and completing the island startup include: 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; 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 the AC voltage phase reference value of the island end according to the AC frequency setting value of 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 the master active current reference value and the 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 the 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 of the UHV flexible DC power transmission system according to claim 2, characterized in that: In the case where the preset operation is to keep the second switch in the open state, the steps of controlling all the 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 bus, and completing the island startup include: 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; 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 the AC voltage phase reference value of the island end according to the AC frequency setting value of 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 the master active current reference value and the 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 the self-control of the master converter; The non-master converter adopts the AC voltage phase reference value, calculates 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.

7. The island starting method of the UHV flexible DC 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 a set second AC voltage and a set second frequency on the island-end AC bus comprises: Closing the AC switch of the main control converter to establish the second AC voltage and the second frequency on the island-end AC bus; Closing the non-master converter AC switch according to a preset third sequence; 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 end connected to a new energy station and a networking end connected to a large AC power grid, the island end and the networking end being connected via a DC line, the island end comprising a plurality of island end voltage source converters connected in series, the networking end comprising a plurality of networking end voltage source converters connected in series, characterized in that: The island starting device is used to execute the island starting method of the ultra-high voltage flexible direct current transmission system according to any one of claims 1 to 7, and the island starting device comprises: An island sequence control unit, a charging control unit and an unlocking control unit, wherein the island sequence control unit, the charging control unit and the unlocking control unit are electrically connected to the UHV flexible direct current power transmission system.

9. The island starting device for the ultra-high voltage flexible direct current 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 used 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; The unlocking control unit is used to control the synchronous unlocking of the network-end voltage source converter according to the DC voltage reference value, establish the network-end DC voltage and the island-end DC voltage; control all island-end voltage source converters to be unlocked synchronously, and 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, so that the island-end AC bus establishes a set second AC voltage and a second frequency, thereby completing the island startup.

10. The island starting device for the ultra-high voltage flexible direct current 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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