DRU system based on blocking type auxiliary converter and starting method thereof

Abstract

The invention provides a DRU system based on a blocking type auxiliary current converter and a starting method of the DRU system. The DRU system comprises a shore current converter, an offshore rectifier and an offshore auxiliary current converter. The starting method comprises four stages. A V / f control strategy is adopted for activating a shore converter control system and a blocking type auxiliary converter, and an offshore alternating-current power grid is established; the chopper circuit is triggered, and the circuit breaker is safely disconnected by using a zero-current switching technology; the offshore wind plant quits the low-voltage ride-through mode, and starting of the DRU system is completed; according to the invention, by introducing the blocking type auxiliary current converter and combining the DRU and the main current converter, black start and stable operation of the wind field are realized; moreover, the system has the characteristics of quick response and high reliability, can quickly establish AC voltage in the starting stage, and provides required starting energy for the wind turbine generator. The auxiliary converter can provide reactive power support and harmonic suppression during normal operation of a wind field, and the stability and efficient operation of the system are ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of offshore wind power direct current transmission, and in particular to a DRU system based on a blocking auxiliary converter and a starting method thereof. Background Art

[0002] With the rapid development of offshore wind power technology, the construction and operation of offshore wind farms face many challenges, especially in power transmission and system startup.

[0003] Although the traditional flexible direct current transmission (VSC-HVDC) technology based on modular multilevel converter (MMC) performs well in long-distance and large-capacity power transmission, it has the disadvantages of high cost, complex structure and large size, which limits its widespread application in offshore wind farms.

[0004] The diode rectifier unit (DRU) has attracted extensive attention in the field of offshore wind power transmission due to its low cost, compact size and high reliability. However, the diode rectifier unit (DRU) has the following inherent defects: the diode rectifier unit (DRU) cannot actively establish the voltage of the internal power grid of the wind farm, and does not have an inverter function. It cannot provide the return power required for starting the wind turbine, and is therefore not suitable for the black start process of the wind farm.

[0005] In order to solve the above problems, the existing technology introduces auxiliary converters to make up for the shortcomings of diode rectifier units (DRUs). The auxiliary converter can establish AC voltage during the startup phase of the wind farm and provide the necessary startup energy for the wind turbine. In addition, the auxiliary converter can also provide reactive power support and harmonic suppression during normal operation of the wind farm, thereby improving the stability and efficiency of the system. Although the full-power MMC auxiliary converter can provide comprehensive functions. However, its high cost and large size increase the complexity and economic burden of the system. Some solutions that use small-capacity auxiliary converters still need to be optimized in terms of control strategy and system stability. Summary of the invention

[0006] In view of the deficiencies in the prior art, the present invention provides a DRU system based on a blocking auxiliary converter and a starting method thereof. The present invention introduces a new type of blocking auxiliary converter, combines the DRU and the main converter, to achieve the black start and stable operation of the wind farm. The present invention has the characteristics of fast response and high reliability, can quickly establish AC voltage during the starting phase, and provide the required starting energy for the wind turbine. At the same time, the present invention can also provide reactive power support and harmonic suppression during the normal operation of the wind farm, to ensure the stability and efficient operation of the system.

[0007] The technical solution of the present invention is: a DRU system based on a blocking auxiliary converter, the system is used to solve the problem that the DRU cannot actively build the internal network voltage of the offshore wind farm and cannot meet the black start of the offshore wind farm when the offshore wind power is connected to the industrial frequency power grid; the DRU system includes:

[0008] An onshore converter FB-MMC, wherein the AC side of the onshore converter FB-MMC is connected to the power frequency power grid;

[0009] An offshore rectifier, wherein the DC port of the offshore rectifier is connected to the DC port of the onshore converter FB-MMC via a DC transmission line, and the AC end of the offshore rectifier is connected to the offshore AC power grid of offshore wind power;

[0010] Offshore auxiliary converter Blocking MMC, the DC side of the offshore auxiliary converter Blocking MMC is connected to the DC transmission line through a circuit breaker, and the AC side of the offshore auxiliary converter Blocking MMC is connected to the offshore AC power grid of offshore wind power.

[0011] Preferably, the DRU system further comprises an offshore wind farm, which is used to capture wind energy and convert it into electrical energy, and then transmit the electrical energy to an offshore AC power grid of offshore wind power.

[0012] Preferably, the DRU system further comprises an offshore station three-winding connection transformer, an offshore station double-winding transformer, and an onshore station double-winding transformer; the AC side of the offshore rectifier is connected to the offshore AC power grid of the offshore wind power through the offshore station three-winding connection transformer;

[0013] The AC side of the offshore auxiliary converter Blocking MMC is connected to the offshore AC power grid of offshore wind power through an offshore station double-winding transformer;

[0014] The AC side of the onshore converter FB-MMC is connected to the industrial frequency power grid through a double-winding transformer of the onshore station.

[0015] Preferably, the onshore converter FB-MMC adopts a full-bridge MMC based on a full-bridge submodule FBSM.

[0016] Preferably, the offshore rectifier adopts a diode rectifier DRU.

[0017] Preferably, the diode rectifier DRU includes six bridge arms, each bridge arm is composed of multiple diodes, and a 12-pulse rectifier circuit is formed by connecting two 6-pulse rectifier bridges in series with a 30° phase shift; the two 6-pulse rectifier bridges are combined by connecting the DC side in series and the AC side in parallel, and the phase difference of the AC side input voltage is 30°.

[0018] Preferably, the offshore auxiliary converter Blocking MMC adopts a blocking MMC converter valve constructed by mixing a blocking series twin submodule D-SDSM and a half-bridge submodule HBSM.

[0019] Preferably, the offshore auxiliary converter Blocking MMC comprises three phase units; each phase unit comprises an upper bridge arm and a lower bridge arm; each upper and lower bridge arm is constructed by mixing a blocking series dual submodule D-SDSM and a half-bridge submodule HBSM configured in a certain proportion; and each upper and lower bridge arm is also connected to a corresponding bridge arm inductor L0 and bridge arm resistor R0;

[0020] Each bridge arm includes N capacitors, wherein N=2N1+N2, N1 is the number of D-SDSMs, and N2 is the number of HBSMs.

[0021] Preferably, the blocking type series twin module D-SDSM is based on the topology of the series twin module SDSM, with IGCT replacing IGBT for T5 on the internal series branch of the series twin module SDSM, and a damping resistor module R is added to the internal branch of the series twin module SDSM where D6 is located.

[0022] Preferably, the half-bridge submodule HBSM includes two switching devices connected in series, one of which is a controllable switch and its anti-parallel diode, and the other is a controllable switch. An output midpoint is formed between the two switching devices connected in series, and the output voltage is adjusted by a control signal.

[0023] Preferably, the present invention further provides a method for starting a DRU system based on a blocking auxiliary converter, comprising the following steps:

[0024] T1), activate the control system of the onshore converter FB-MMC to achieve controllable charging and enter the low-voltage operation state; close the circuit breakers S3 and S4 of the offshore auxiliary converter Blocking MMC to provide it with a low-voltage controlled charging path; finally unlock the offshore auxiliary converter Blocking MMC and enable its control system to switch to the controllable charging mode;

[0025] T2), closing the circuit breaker S2, the offshore auxiliary converter Blocking MMC adopts a V / f control strategy to establish an offshore AC power grid and control the offshore wind farm to operate in a limited power mode;

[0026] T3), the chopper circuit is triggered because the offshore wind farm enters the low voltage ride-through LVRT, and the output power is controlled to be 0. The generated power is dissipated in the chopper circuit, and finally the zero current switching technology is used to safely disconnect the circuit breakers S3 and S4;

[0027] T4), after disconnecting circuit breakers S3 and S4, closing circuit breaker S1, the offshore wind farm exits the low voltage ride-through LVRT mode, the offshore rectifier starts to conduct active power, the DRU system starts up and enters stable operation, and the offshore auxiliary converter Blocking MMC works in STATCOM mode.

[0028] Preferably, in step T1), the charging process of the control system of the onshore converter FB-MMC includes two stages: uncontrollable pre-charging and controllable pre-charging. In the uncontrollable pre-charging stage, the offshore auxiliary converter Blocking MMC is in a blocking mode; when the offshore auxiliary converter Blocking MMC is charged to a triggerable level, it enters the controllable pre-charging stage;

[0029] In the controllable pre-charging stage, the offshore auxiliary converter Blocking MMC divides all bridge arms into two groups, one group consists of half-bridge sub-modules HBSM, and the other group consists of blocking series twin sub-modules D-SDSM; the blocking series twin sub-modules D-SDSM and the half-bridge sub-modules HBSM are charged in a certain order, and finally all capacitor voltages in the offshore auxiliary converter Blocking MMC are charged to the rated value.

[0030] Preferably, within a cycle T, the capacitors in the blocking series dual submodule D-SDSM are charged throughout the entire process; and the capacitors in the half-bridge submodule HBSM are charged only within half a cycle; that is:

[0031] U c_D-SDSM =2U c_HBSM ;

[0032] Where U c_D-SDSM is the charging voltage of each capacitor in the blocking series dual module D-SDSM during the uncontrollable pre-charging stage; U c_HBSM It is the charging voltage of each capacitor in the half-bridge submodule HBSM during the uncontrollable pre-charging stage.

[0033] Preferably, when the ratio of the blocking series twin submodule D-SDSM to the half-bridge submodule HBSM of the offshore auxiliary converter Blocking MMC is greater than or equal to the critical value kN1, the offshore auxiliary converter Blocking MMC has a DC fault clearing capability and can quickly clear the fault current by locking when a DC fault occurs, thereby ensuring that the DC fault arc does not rekindle; after locking, the reverse voltage provided by the cascaded capacitor of the bridge arm will block the path of the AC source feeding the fault current to the fault point, while forcing the fault current to quickly drop to zero or less than a very small limit value, thereby achieving complete locking of the offshore auxiliary converter Blocking MMC and preventing the fault arc from rekindling.

[0034] Preferably, the critical value kN1 is expressed as:

[0035]

[0036] Among them, N1 is the number of blocking series dual modules D-SDSM, N2 is the number of half-bridge modules HBSM, U m is the voltage of any phase among a, b and c; U dc is the rated voltage of the DC side.

[0037] The beneficial effects of the present invention are:

[0038] 1. The present invention realizes the black start and stable operation of the wind farm by introducing a new type of blocking auxiliary converter, combining the DRU and the main converter;

[0039] 2. The blocking auxiliary converter of the present invention has the characteristics of fast response and high reliability, can quickly establish AC voltage during the startup phase, and provide the required startup energy for the wind turbine;

[0040] 3. The auxiliary converter of the present invention can also provide reactive power support and harmonic suppression during normal operation of the wind farm, ensuring the stability and efficient operation of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 The circuit structure diagram of the DRU system based on the blocking auxiliary converter of the present invention;

[0042] Figure 2 This is a diagram showing the internal circuit structure of the blocking auxiliary converter of the present invention;

[0043] Figure 3 This is a circuit structure diagram of a blocking type series twin module D-SDSM of the present invention;

[0044] Figure 4 1 is a circuit structure diagram of a half-bridge submodule HBSM of the present invention;

[0045] Figure 5 It is a schematic diagram of the process of starting the DRU system based on the blocking auxiliary converter of the present invention;

[0046] Figure 6 The power flow diagram of the DRU system based on the blocking auxiliary converter in the startup phase of the present invention. DETAILED DESCRIPTION

[0047] The specific implementation of the present invention will be further described below in conjunction with the accompanying drawings:

[0048] Example 1

[0049] like Figure 1 As shown, this embodiment provides a DRU system based on a blocking auxiliary converter, which is used to solve the problem that the DRU cannot actively build the internal network voltage of the offshore wind farm and cannot meet the black start of the offshore wind farm when the offshore wind power is connected to the industrial frequency power grid; the DRU system includes:

[0050] An onshore converter FB-MMC, wherein the AC side of the onshore converter FB-MMC is connected to the power frequency power grid;

[0051] An offshore rectifier, wherein the DC port of the offshore rectifier is connected to the DC port of the onshore converter FB-MMC via a DC transmission line, and the AC end of the offshore rectifier is connected to the offshore AC power grid of offshore wind power; the offshore rectifier includes DRU1 and DRU2;

[0052] Offshore auxiliary converter Blocking MMC, the DC side of the offshore auxiliary converter Blocking MMC is connected to the DC transmission line through a circuit breaker, and the AC side of the offshore auxiliary converter Blocking MMC is connected to the offshore AC power grid of offshore wind power;

[0053] An offshore wind farm is used to capture wind energy and convert it into electrical energy, and then transmit the electrical energy to an offshore AC power grid for offshore wind power. The offshore wind farm adopts a grid-following control structure and is equipped with a chopper circuit to achieve precise control of output voltage and current.

[0054] The three-winding connection transformer of the offshore station, the AC side of the offshore rectifier is connected to the offshore AC power grid of the offshore wind power through the three-winding connection transformer of the offshore station;

[0055] Offshore station double-winding transformer, the AC side of the offshore auxiliary converter Blocking MMC is connected to the offshore AC power grid of offshore wind power through the offshore station double-winding transformer;

[0056] The onshore station double-winding transformer, the AC side of the onshore converter FB-MMC is connected to the industrial frequency power grid through the onshore station double-winding transformer.

[0057] As a preferred embodiment of this embodiment, the onshore converter FB-MMC adopts a full-bridge MMC based on a full-bridge submodule FBSM.

[0058] As a preferred embodiment of the present invention, the offshore rectifier adopts a diode rectifier DRU, which includes six bridge arms, each of which is composed of multiple diodes, and a 12-pulse rectifier circuit is formed by connecting two 6-pulse rectifier bridges in series with a 30° phase shift; the two 6-pulse rectifier bridges are combined in a manner of series connection on the DC side and parallel connection on the AC side, and the phase difference of the input voltage on the AC side is 30°.

[0059] As preferred in this embodiment, Figure 2 As shown, the offshore auxiliary converter Blocking MMC includes three phase units; each phase unit includes an upper bridge arm and a lower bridge arm; this embodiment has a total of 6 bridge arms. And each upper and lower bridge arm is constructed by mixing a blocking series dual submodule D-SDSM and a half-bridge submodule HBSM configured in a certain proportion; and each upper and lower bridge arm is also connected to a corresponding bridge arm inductor L0 and bridge arm resistor R0;

[0060] Each bridge arm includes N capacitors, wherein N=2N1+N2, N1 is the number of D-SDSMs, and N2 is the number of HBSMs.

[0061] Moreover, the parameters and reactance values ​​of each bridge arm are the same, and the six bridge arms are symmetrical. Under normal operating conditions, the output DC voltage is adjusted by controlling the number of blocking series dual submodules D-SDSM and half-bridge submodules HBSM connected in series in the upper and lower bridge arms, and the total number of submodules turned on in each phase unit at any time is equal to N.

[0062] As preferred in this embodiment, Figure 3 As shown, the blocking type series twin module D-SDSM is based on the topology of the series twin module SDSM. Since the voltage withstood by T5 on the internal branch of the series twin module SDSM is twice that of T1-T4, in this embodiment, IGCT is used instead of IGBT for T5 on the internal series branch of the series twin module SDSM; and a damping resistor module R is added to the internal branch of the series twin module SDSM where D6 is located.

[0063] As preferred in this embodiment, Figure 4As shown, the half-bridge submodule HBSM includes two switching devices connected in series, one of which is a controllable switch and its anti-parallel diode, and the other is a controllable switch. An output midpoint is formed between the two switching devices connected in series, and the output voltage is adjusted by a control signal.

[0064] The charging process of the offshore auxiliary converter Blocking MMC includes two stages: uncontrollable pre-charging and controllable pre-charging. In the uncontrollable pre-charging stage, all submodules in the offshore auxiliary converter Blocking MMC are in a locked mode. When all submodules are charged to a triggerable level, the controllable pre-charging stage is entered. In the controllable pre-charging stage, the bridge arms of the offshore auxiliary converter Blocking MMC are divided into two groups according to the submodule type, one group is composed of the HBSM, and the other group is composed of the D-SDSM. Each submodule group is charged in a certain order, and finally the capacitor voltage of all submodules in the offshore auxiliary converter is charged to the rated value.

[0065] Moreover, since the IGBT trigger control needs to obtain energy from the capacitor of the blocking type series twin module D-SDSM, in the initial state before pre-charging, the capacitor voltage of the blocking type series twin module D-SDSM is 0, that is, there is no stored energy, and the IGBT cannot be triggered normally. At this time, the blocking type series twin module D-SDSM can only be in the locking mode; so the first stage of pre-charging is uncontrollable. Moreover, regardless of the positive or negative direction of the bridge arm current, the capacitor of the D-SDSM can be charged through the bridge uncontrolled rectifier circuit composed of anti-parallel diodes; but for the half-bridge submodule HBSM, the capacitor of the half-bridge submodule HBSM can only be charged when the bridge arm current is in the positive direction. Therefore, within a cycle T, the capacitor in the blocking type series twin module D-SDSM is charged throughout the process; the capacitor in the half-bridge submodule HBSM is only charged within half a cycle; that is:

[0066] U c_D-SDSM =2U c_HBSM ;

[0067] Where U c_D-SDSM is the charging voltage of each capacitor in the blocking series dual module D-SDSM during the uncontrollable pre-charging stage; U c_HBSM It is the charging voltage of each capacitor in the half-bridge submodule HBSM during the uncontrollable pre-charging stage.

[0068] In this embodiment, in the uncontrollable pre-charging stage of the offshore auxiliary converter Blocking MMC, each bridge arm can be equivalent to a structure of a capacitor and four diodes, and further analysis can yield the following:

[0069]

[0070] Where: m is the modulation ratio in steady-state operation under rated conditions, N1 is the number of D-SDSMs, N is the number of capacitors in each bridge arm, and U dc is the rated voltage of the DC side.

[0071] In this embodiment, the offshore auxiliary converter Blocking MMC has the ability to clear DC faults. During the startup process of the DRU system, DC faults or transient current abnormalities may occur. The offshore auxiliary converter Blocking MMC can adjust the relationship between the sub-module ratio and its critical value to achieve DC fault clearing.

[0072] When the ratio of the blocking series twin submodule D-SDSM and the half-bridge submodule HBSM of the offshore auxiliary converter Blocking MMC is greater than or equal to the critical value kN1, the offshore auxiliary converter Blocking MMC has the ability to clear DC faults, and can quickly clear the fault current by locking when a DC fault occurs, ensuring that the DC fault arc does not reignite; after locking, the reverse voltage provided by the cascaded capacitor of the bridge arm will block the path of the AC source feeding the fault current to the fault point, and at the same time force the fault current to drop rapidly to zero or less than a very small limit value, so as to achieve complete locking of the offshore auxiliary converter Blocking MMC and prevent the fault arc from reigniting. Wherein, the critical value kN1 is expressed as:

[0073]

[0074] Among them, N1 is the number of blocking series dual modules D-SDSM, N2 is the number of half-bridge modules HBSM, U m is the voltage of any phase among a, b and c; U dc is the rated voltage of the DC side.

[0075] Example 2

[0076] like Figure 5 and 6 As shown, this embodiment provides a method for starting a DRU system based on a blocking auxiliary converter, comprising the following steps:

[0077] T1), activate the control system of the onshore converter FB-MMC to realize controllable charging and enter the low-voltage operation state; close the circuit breakers S3 and S4 of the offshore auxiliary converter Blocking MMC to provide it with a low-voltage controlled charging path; finally unlock the offshore auxiliary converter Blocking MMC and enable its control system to switch to the controllable charging mode; Figure 6 (a)

[0078] T2), close the circuit breaker S2, the offshore auxiliary converter Blocking MMC adopts V / f control strategy to establish an offshore AC power grid and control the offshore wind farm to operate in a limited power mode; Figure 6 (b)

[0079] T3), the chopper circuit is triggered because the offshore wind farm enters the low voltage ride-through LVRT, and the output power is controlled to be 0. The generated power is dissipated in the chopper circuit, and finally the zero current switching technology is used to safely disconnect the circuit breakers S3 and S4; Figure 6 (c)

[0080] T4), after disconnecting circuit breakers S3 and S4, closing circuit breaker S1, the offshore wind farm exits the low voltage ride-through LVRT mode, the offshore rectifier begins to conduct active power, the DRU system is started up and enters stable operation, and the offshore auxiliary converter Blocking MMC works in STATCOM mode, such as Figure 6 (d) as shown.

[0081] As a preferred embodiment of the present invention, in step T1), the charging process of the control system of the onshore converter FB-MMC includes two stages: uncontrollable pre-charging and controllable pre-charging. In the uncontrollable pre-charging stage, the offshore auxiliary converter Blocking MMC is in a blocking mode; when the offshore auxiliary converter Blocking MMC is charged to a triggerable level, it enters the controllable pre-charging stage;

[0082] In the controllable pre-charging stage, the offshore auxiliary converter Blocking MMC divides all bridge arms into two groups, one group consists of half-bridge sub-modules HBSM, and the other group consists of blocking series twin sub-modules D-SDSM; the blocking series twin sub-modules D-SDSM and the half-bridge sub-modules HBSM are charged in a certain order, and finally all capacitor voltages in the offshore auxiliary converter Blocking MMC are charged to the rated value.

[0083] As a preferred embodiment of the present invention, within a cycle T, the capacitors in the blocking series dual submodule D-SDSM are charged throughout the entire process; and the capacitors in the half-bridge submodule HBSM are charged only within half a cycle; that is:

[0084] U c_D-SDSM =2U c_HBSM ;

[0085] Where U c_D-SDSM is the charging voltage of each capacitor in the blocking series dual module D-SDSM during the uncontrollable pre-charging stage; U c_HBSMIt is the charging voltage of each capacitor in the half-bridge submodule HBSM during the uncontrollable pre-charging stage.

[0086] In this embodiment, in the uncontrollable pre-charging stage of the offshore auxiliary converter Blocking MMC, each bridge arm can be equivalent to a structure of a capacitor and four diodes, and further analysis can yield the following:

[0087]

[0088] Where: m is the modulation ratio in steady-state operation under rated conditions, N1 is the number of D-SDSMs, N is the number of capacitors in each bridge arm, and U dc is the rated voltage of the DC side.

[0089] As a preferred embodiment of this embodiment, when the ratio of the blocking series twin submodule D-SDSM and the half-bridge submodule HBSM of the offshore auxiliary converter Blocking MMC is greater than or equal to the critical value kN1, the offshore auxiliary converter Blocking MMC has the ability to clear DC faults, and can quickly clear the fault current by locking when a DC fault occurs, ensuring that the DC fault arc does not reignite; after locking, the reverse voltage provided by the cascaded capacitor of the bridge arm will block the path of the AC source feeding the fault current to the fault point, and at the same time force the fault current to drop rapidly to zero or less than a very small limit value, so as to achieve complete locking of the offshore auxiliary converter Blocking MMC and prevent the fault arc from reigniting. The critical value kN1 is expressed as:

[0090]

[0091] Among them, N1 is the number of blocking series dual modules D-SDSM, N2 is the number of half-bridge modules HBSM, U m is the voltage of any phase among a, b and c; U dc is the rated voltage of the DC side.

[0092] The above embodiments and descriptions are only for illustrating the principles and best embodiments of the present invention. Without departing from the spirit and scope of the present invention, the present invention may be subject to various changes and improvements, all of which fall within the scope of the present invention to be protected.

Claims

1. A DRU system based on a blocking auxiliary converter, the system is used to solve the problem that the DRU cannot actively build the internal network voltage of the offshore wind farm and cannot meet the black start of the offshore wind farm when the offshore wind power is integrated into the industrial frequency power grid; it is characterized by: The DRU system comprises: An onshore converter FB-MMC, wherein the AC side of the onshore converter FB-MMC is connected to the power frequency power grid; An offshore rectifier, wherein the DC port of the offshore rectifier is connected to the DC port of the onshore converter FB-MMC via a DC transmission line, and the AC end of the offshore rectifier is connected to the offshore AC power grid of offshore wind power; Offshore auxiliary converter Blocking MMC, the DC side of the offshore auxiliary converter Blocking MMC is connected to the DC transmission line through a circuit breaker, and the AC side of the offshore auxiliary converter Blocking MMC is connected to the offshore AC power grid of offshore wind power.

2. A DRU system based on a blocking auxiliary converter according to claim 1, characterized in that: The DRU system also includes an offshore wind farm, an offshore station three-winding connection transformer, an offshore station double-winding transformer, and an onshore station double-winding transformer; the offshore wind farm is used to capture wind energy and convert it into electrical energy, and then transmit the electrical energy to the offshore AC power grid of offshore wind power; The DRU system also includes the AC side of the offshore rectifier being connected to the offshore AC power grid of the offshore wind power through the offshore station three-winding connection transformer; The AC side of the offshore auxiliary converter Blocking MMC is connected to the offshore AC power grid of offshore wind power through an offshore station double-winding transformer; The AC side of the onshore converter FB-MMC is connected to the industrial frequency power grid through a double-winding transformer of the onshore station.

3. The DRU system based on a blocking auxiliary converter according to claim 1, characterized in that: The onshore converter FB-MMC adopts a full-bridge MMC based on a full-bridge submodule FBSM.

4. The DRU system based on a blocking auxiliary converter according to claim 1, characterized in that: The offshore rectifier adopts a diode rectifier DRU; the diode rectifier DRU includes six bridge arms, each bridge arm is composed of multiple diodes, and two 6-pulse rectifier bridges are connected in series with a phase shift of 30° to form a 12-pulse rectifier circuit; the two 6-pulse rectifier bridges are combined in a manner of series connection on the DC side and parallel connection on the AC side, and the phase difference of the AC side input voltage is 30°.

5. The DRU system based on a blocking auxiliary converter according to claim 1, characterized in that: The offshore auxiliary converter Blocking MMC adopts a blocking MMC converter valve constructed by mixing a blocking series twin submodule D-SDSM and a half-bridge submodule HBSM.

6. A DRU system based on a blocking auxiliary converter according to claim 5, characterized in that: The offshore auxiliary converter Blocking MMC includes three phase units; each phase unit includes an upper bridge arm and a lower bridge arm; each upper and lower bridge arm is constructed by mixing a blocking series dual submodule D-SDSM and a half-bridge submodule HBSM configured in a certain proportion; and each upper and lower bridge arm is also connected to a corresponding bridge arm inductor L0 and bridge arm resistor R0; Each bridge arm includes N capacitors, wherein N=2N1+N2, N1 is the number of D-SDSMs, and N2 is the number of HBSMs.

7. A DRU system based on a blocking auxiliary converter according to claim 6, characterized in that: The blocking type series twin module D-SDSM is based on the topology of the series twin module SDSM, in which T5 on the internal series branch of the series twin module SDSM is replaced by IGCT instead of IGBT, and a damping resistor module R is added to the internal branch of the series twin module SDSM where D6 is located; The half-bridge submodule HBSM includes two switching devices connected in series, one of which is a controllable switch and its anti-parallel diode, and the other is a controllable switch. An output midpoint is formed between the two switching devices connected in series, and the output voltage is adjusted by a control signal.

8. A method for starting a DRU system based on a blocking auxiliary converter, characterized in that: The startup method is used to start the DRU system described in claims 1-8, and the startup method comprises the following steps: T1), activate the control system of the onshore converter FB-MMC to achieve controllable charging and enter the low-voltage operation state; close the circuit breakers S3 and S4 of the offshore auxiliary converter Blocking MMC to provide it with a low-voltage controlled charging path; finally unlock the offshore auxiliary converter Blocking MMC and enable its control system to switch to the controllable charging mode; T2), closing the circuit breaker S2, the offshore auxiliary converter Blocking MMC adopts a V / f control strategy to establish an offshore AC power grid and control the offshore wind farm to operate in a limited power mode; T3), the chopper circuit is triggered because the offshore wind farm enters the low voltage ride-through LVRT, and the output power is controlled to be 0. The generated power is dissipated in the chopper circuit, and finally the zero current switching technology is used to safely disconnect the circuit breakers S3 and S4; T4), after disconnecting circuit breakers S3 and S4, closing circuit breaker S1, the offshore wind farm exits the low voltage ride-through LVRT mode, the offshore rectifier starts to conduct active power, the DRU system starts up and enters stable operation, and the offshore auxiliary converter Blocking MMC works in STATCOM mode.

9. The method for starting a DRU system based on a blocking auxiliary converter according to claim 8, characterized in that: In step T1), the control system charging process of the onshore converter FB-MMC includes two stages: uncontrollable pre-charging and controllable pre-charging. In the uncontrollable pre-charging stage, the offshore auxiliary converter Blocking MMC is in a locking mode; when the offshore auxiliary converter Blocking MMC is charged to a triggerable level, it enters the controllable pre-charging stage.

10. The method for starting a DRU system based on a blocking auxiliary converter according to claim 9, characterized in that: When the ratio of the blocking series twin submodule D-SDSM to the half-bridge submodule HBSM of the offshore auxiliary converter Blocking MMC is greater than or equal to the critical value kN1, the offshore auxiliary converter Blocking MMC has a DC fault clearing capability, and can quickly clear the fault current by blocking when a DC fault occurs, ensuring that the DC fault arc does not reignite; The critical value kN1 is expressed as: Among them, N1 is the number of blocking series dual modules D-SDSM, N2 is the number of half-bridge modules HBSM, U m is the voltage of any phase among a, b and c; U dc is the rated voltage of the DC side.

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