Flexible direct-current converter with unloading function, and control method therefor

Abstract

Provided are a flexible direct-current converter with an unloading function, and a control method therefor. The flexible direct-current converter comprises a modular multi-level flexible direct-current converter and a direct-current unloading apparatus installed in the modular multi-level flexible direct-current converter, wherein the modular multi-level flexible direct-current converter comprises an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor and a lower bridge arm reactor; the direct-current unloading apparatus comprises an unloading bridge arm and an auxiliary turn-off bridge arm; the unloading bridge arm can dissipate surplus power in a system; and the auxiliary turn-off bridge arm can assist in turning off switching devices in the unloading bridge arm. The flexible direct-current converter with an unloading function has six topologies. The flexible direct-current converter itself has an unloading function, thereby satisfying a fault ride-through process of a flexible direct-current transmission system and ensuring the safe and stable operation of the system.

Description

Flexible direct current converter with unloading function and control method TECHNICAL FIELD

[0001] The present application relates to the field of flexible direct current transmission and distribution and power electronics, in particular to a flexible direct current converter with unloading function and a control method. BACKGROUND

[0002] Flexible direct current transmission technology has the characteristics of independent controllability of active and reactive power, no need for reactive power compensation, power supply to passive network, and unchanged voltage polarity during power flow reversal, and is very suitable for large-scale new energy grid connection.

[0003] Taking a large-scale offshore wind power grid connection system as an example, due to the large inertia of wind turbine generators, the sending power of the offshore converter station (sending end) cannot be suddenly changed. When a fault occurs on the AC grid side of the onshore converter station (receiving end), the grid-connected power of the onshore converter station (receiving end) is limited, and it is unable to fully accommodate the sending power of the offshore converter station (sending end). The surplus power will charge the capacitors in the converter and the parasitic capacitance of the DC submarine cable, resulting in overvoltage. The DC unloading device can consume the surplus power in the DC power grid to avoid overvoltage or large-scale wind turbine tripping.

[0004] The existing DC unloading device can be divided into three types: centralized unloading device, distributed unloading device and hybrid unloading device. Among them, the centralized unloading device has the problem of difficulty in voltage sharing of series connected switching devices, and the voltage fluctuation is large during operation; the voltage fluctuation of the distributed unloading device is small, but a large number of switching devices such as full-controlled switching devices and thyristors are needed, and additional water cooling devices are also needed, which significantly increases the cost; the hybrid unloading device is a combination of centralized and distributed unloading devices, which reduces the device cost to some extent compared with the distributed unloading device, but still needs a large number of switching devices and water cooling devices. SUMMARY

[0005] In view of the defects in the prior art, the purpose of the present application is to provide a flexible direct current converter with unloading function and a control method.

[0006] According to one aspect of the present application, a flexible direct current converter with unloading function is provided, comprising: a modular multilevel flexible direct current converter, and a DC unloading device installed in the modular multilevel flexible direct current converter;

[0007] The modular multilevel flexible direct current converter comprises an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor and a lower bridge arm reactor;

[0008] The DC unloading device comprises an unloading bridge arm and an auxiliary turn-off bridge arm, the unloading bridge arm can consume the surplus power in the system, and the auxiliary turn-off bridge arm can assist in turning off the switching devices in the unloading bridge arm;

[0009] The HVDC converter topology of the unloading function comprises one or more of the following:

[0010] The first topology, the DC unloading device is single-phase, including two identical parts, installed on the upper and lower converter station bridge arms of any phase of the HVDC converter respectively; each part comprises: an unloading bridge arm, an auxiliary turn-off bridge arm; the auxiliary turn-off bridge arm is connected in series with the upper and lower converter station bridge arms and then connected in parallel with the unloading bridge arm;

[0011] The second topology, the DC unloading device is three-phase, each phase is the same as the DC unloading device in the first topology;

[0012] The third topology, the DC unloading device is single-phase, each phase comprises two unloading bridge arms and an auxiliary turn-off bridge arm; the two unloading bridge arms are connected in series and connected in parallel with the upper and lower converter station bridge arms of any phase of the HVDC converter, and the auxiliary turn-off bridge arm is connected between the upper and lower unloading bridge arms and between the upper and lower converter station bridge arms;

[0013] The fourth topology, the DC unloading device is three-phase, each phase is the same as the DC unloading device in the third topology;

[0014] The fifth topology, the DC unloading device is three-phase; each phase comprises an unloading bridge arm and two auxiliary turn-off bridge arms; the two auxiliary turn-off bridge arms are connected between the AC port and the upper converter station bridge arm and between the AC port and the lower converter station bridge arm respectively; the unloading bridge arm is connected between the upper and lower converter station bridge arms;

[0015] The sixth topology, the DC unloading device is three-phase; each phase comprises an unloading bridge arm and an auxiliary turn-off bridge arm; the unloading bridge arm and the auxiliary turn-off bridge arm are connected in series and connected between the upper and lower converter station bridge arms.

[0016] Preferably, each phase of the modular multilevel HVDC converter comprises: an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor;

[0017] One end of the upper bridge arm reactor is star-connected, with the neutral point connected to the positive pole of the DC port, and the other end of the upper bridge arm reactor of each phase is connected to the upper end of the upper converter station bridge arm of the corresponding phase; for each phase of the converter station bridge arm without the DC unloading device, the lower end of the upper converter station bridge arm is connected to the upper end of the lower converter station bridge arm; one end of the lower bridge arm reactor is star-connected, with the neutral point connected to the negative pole of the DC port, and the other end of the lower bridge arm reactor of each phase is connected to the lower end of the lower converter station bridge arm of the corresponding phase.

[0018] Preferably, in the first topology, the current unloading device is only installed on one phase bridge arm of the HVDC converter station;

[0019] The DC unloading device comprises an upper unloading bridge arm, an upper auxiliary blocking bridge arm, a lower unloading bridge arm, and a lower auxiliary blocking bridge arm.

[0020] One end of the upper auxiliary blocking bridge arm is connected to the lower end of the upper bridge arm of the HVDC converter station, and the other end is connected to the AC port; one end of the lower auxiliary blocking bridge arm is connected to the upper end of the lower bridge arm of the HVDC converter station, and the other end is connected to the AC port; one end of the upper unloading bridge arm is connected to the upper end of the upper bridge arm of the HVDC converter station, and the other end is connected to the AC port; one end of the lower unloading bridge arm is connected to the lower end of the lower bridge arm of the HVDC converter station, and the other end is connected to the AC port.

[0021] Alternatively, one end of the upper bridge arm of the HVDC converter station is connected to the lower end of the upper auxiliary blocking bridge arm, and the other end is connected to the AC port;

[0022] One end of the lower bridge arm of the HVDC converter station is connected to the upper end of the lower auxiliary blocking bridge arm, and the other end is connected to the AC port;

[0023] One end of the upper auxiliary blocking bridge arm is connected to the upper end of the upper unloading bridge arm, and the other end is connected to the AC port; one end of the lower auxiliary blocking bridge arm is connected to the lower end of the lower unloading bridge arm, and the other end is connected to the AC port;

[0024] Preferably, in the second topology, the current unloading device is installed on three phase bridge arms of the HVDC converter station; each phase is the same as the first topology.

[0025] Preferably, in the third topology, the DC unloading device is only installed on one phase bridge arm of the HVDC converter station;

[0026] The DC unloading device comprises an upper unloading bridge arm, a lower unloading bridge arm, and an auxiliary blocking bridge arm.

[0027] The upper end of the upper unloading bridge arm is connected to the upper end of the upper bridge arm of the HVDC converter station, the lower end of the upper unloading bridge arm is connected to the upper end of the lower unloading bridge arm, the lower end of the lower unloading bridge arm is connected to the lower end of the lower bridge arm of the HVDC converter station; one end of the auxiliary blocking bridge arm is connected to the lower end of the upper unloading bridge arm, and the other end is connected to the lower end of the upper bridge arm of the HVDC converter station.

[0028] Preferably, in the fourth topology, the current unloading device is installed on three phase bridge arms of the HVDC converter station; each phase is the same as the third topology.

[0029] Preferably, each phase of the HVDC converter station with modular multilevel converter comprises a first upper bridge arm of the HVDC converter station, a second upper bridge arm of the HVDC converter station, a first lower bridge arm of the HVDC converter station, a second lower bridge arm of the HVDC converter station, an upper bridge arm reactor, and a lower bridge arm reactor.

[0030] The upper bridge arm reactor is star-connected at one end, with the neutral point connected to the positive pole of the DC port, and the other end of the upper bridge arm reactor of each phase is connected to the upper end of the first upper converter station bridge arm of the corresponding phase; the upper end of the second upper converter station bridge arm is connected to the lower end of the first upper converter station bridge arm.

[0031] The lower bridge arm reactor is star-connected at one end, with the neutral point connected to the negative pole of the DC port, and the other end of the lower bridge arm reactor of each phase is connected to the lower end of the second lower converter station bridge arm of the corresponding phase; the lower end of the first lower converter station bridge arm is connected to the upper end of the second lower converter station bridge arm.

[0032] Preferably, the fifth topology, the DC unloading device is installed on the three-phase bridge arm of the VSC-HVDC;

[0033] Each phase of the DC unloading device comprises an unloading bridge arm, an upper auxiliary turn-off bridge arm, and a lower auxiliary turn-off bridge arm.

[0034] The upper end of the unloading bridge arm is connected to the lower end of the first upper converter station bridge arm, and the lower end of the unloading bridge arm is connected to the upper end of the second lower converter station bridge arm; one end of the upper auxiliary turn-off bridge arm is connected to the lower end of the second upper converter station bridge arm, and the other end is connected to the AC port; one end of the lower auxiliary turn-off bridge arm is connected to the upper end of the first lower converter station bridge arm, and the other end is connected to the AC port.

[0035] Preferably, the sixth topology, the DC unloading device is installed on the three-phase bridge arm of the VSC-HVDC;

[0036] Each phase of the DC unloading device comprises an unloading bridge arm and an auxiliary turn-off bridge arm; the upper end of the unloading bridge arm is connected to the lower end of the first upper converter station bridge arm, the lower end of the unloading bridge arm is connected to the upper end of the auxiliary turn-off bridge arm, and the lower end of the auxiliary turn-off bridge arm is connected to the upper end of the second lower converter station bridge arm.

[0037] Preferably, the modular multilevel VSC-HVDC is a medium-voltage or high-voltage, three-phase, voltage source converter with a modular multilevel structure, and has the function of AC-DC power conversion, and can realize the connection of medium-high voltage AC power grid and DC system.

[0038] Preferably, the upper converter station bridge arm and the lower converter station bridge arm comprise a plurality of series-connected converter station sub-modules.

[0039] The converter station bridge arm comprises bipolar sub-modules, unipolar sub-modules, or a mixture of bipolar sub-modules and unipolar sub-modules.

[0040] The unloading bridge arm comprises a series-connected unloading resistor R and a series-connected string of semi-controlled switching devices, or a series-connected unloading resistor R and a series-connected capacitor string C.

[0041] The direction of the half-controlled switch device string is consistent with the voltage polarity direction of the converter station bridge arm; and the resistor is a power resistor dissipating surplus power.

[0042] The auxiliary turn-off bridge arm comprises one bipolar sub-module or a plurality of bipolar sub-modules connected in series.

[0043] According to a second aspect of the present application, a control method of a VSC with unloading capability is provided, the VSC with unloading capability has two operation modes, including:

[0044] Mode 1: normal operation mode, in which the AC system is normally operated, the VSC is controlled to perform reactive power control, and the unloading bridge arm is not operated; all the unloading bridge arms are in the turn-off state, and the current only flows from the bridge arm reactor and the converter station bridge arm;

[0045] Mode 2: fault unloading mode, in which the AC system has a short-circuit or ground fault, and the DC power grid has surplus power, the converter station is controlled to perform active power control and unloading power control, including: controlling the AC current of the converter station, and controlling the turn-on and turn-off of the thyristors in the unloading bridge arm;

[0046] The current flows from the converter station bridge arm, the unloading bridge arm and the auxiliary turn-off bridge arm, and the surplus power is consumed.

[0047] Preferably, for the fourth VSC topology with unloading function, the output voltages of the auxiliary turn-off bridge arm and the converter station bridge arm are controlled to realize the turn-off of the thyristors in the unloading bridge arm.

[0048] Preferably, the output voltages of the auxiliary turn-off bridge arm and the converter station bridge arm are controlled to change simultaneously in each power frequency cycle, so as to realize the turn-off of the thyristors in the unloading bridge arm in each power frequency cycle.

[0049] Preferably, when the upper unloading bridge arm of a certain phase is turned off, the voltage of the upper converter station bridge arm of the corresponding phase is controlled to be zero, the voltage of the lower converter station bridge arm of the corresponding phase is controlled to be the DC power grid voltage, and the voltage of the auxiliary turn-off bridge arm is controlled to be a positive polarity voltage not greater than 0.05 pu, so that the thyristors in the upper unloading bridge arm are subjected to reverse voltage and turned off.

[0050] When the lower unloading bridge arm of a certain phase is turned off, the voltage of the lower converter station bridge arm of the corresponding phase is controlled to be zero, the voltage of the upper converter station bridge arm of the corresponding phase is controlled to be the DC power grid voltage, and the voltage of the auxiliary turn-off bridge arm is controlled to be a negative polarity voltage not greater than 0.05 pu, so that the thyristors in the lower unloading bridge arm are subjected to reverse voltage and turned off.

[0051] Preferably, when the unloading bridge arm of a certain phase is turned off, the voltages of the bridge arms of other phases are controlled to follow the change, so as to compensate the change of the AC measuring line voltage of the converter station; assuming that the time when the unloading bridge arm of the A phase is turned off is t0, the voltages of the bridge arms of the converter station at this time are:

[0052] Wherein, u' ap , u' an , u' bp , u' bn , u' cp , u' cn are the modulation voltages of the upper and lower bridge arms of the A phase, the upper and lower bridge arms of the B phase, and the upper and lower bridge arms of the C phase under the fault unloading condition; u ap , u an , u bp , u bn , u cp , u cn are the modulation voltages of the upper and lower bridge arms of the A phase, the upper and lower bridge arms of the B phase, and the upper and lower bridge arms of the C phase under the normal working condition, and U DC is the DC voltage of the DC grid.

[0053] Preferably, the modulation voltages calculated according to equation (1), the DC side output voltage of the converter station and the AC side output voltage are respectively:

[0054] Wherein, U DC and U' DC are the DC voltages of the DC grid before and after the fault occurs; U' AB , U' BC , U' CA and U AB , U BC , U CA are the AC line voltages of the AC grid before and after the fault occurs; changing the bridge arm voltage does not change the DC voltage and the AC line voltage output by the converter station.

[0055] Compared with the prior art, the embodiment of the present application has at least one beneficial effect as follows:

[0056] The VSC of the embodiment of the present application has the unloading capability, and the VSC itself has the unloading function, so as to meet the fault ride-through process of the VSC transmission system and ensure the safe and stable operation of the system.

[0057] Compared with the existing DC unloading device, the VSC of the embodiment of the present application greatly reduces the number of switching devices, uses the half-controlled thyristor switching device instead of the fully-controlled switching device, and has the significant economic advantage of low construction cost.

[0058] Compared with the existing parallel type centralized DC unloading device, the VSC of the embodiment of the application has the unloading capacity, adopts series thyristor to control the unloading bridge arm, is easier to realize the device voltage sharing than the traditional series full-controlled switch device scheme, and can precisely and continuously control the unloading power by controlling the conduction time of the thyristor.

[0059] Compared with the existing parallel type distributed or hybrid DC unloading device, the VSC of the embodiment of the application has the unloading capacity, has fewer switch devices, uses centralized resistance, is convenient for heat dissipation, reduces the heat dissipation pressure of the converter station, and thus significantly reduces the cost of the device.

[0060] The control method of the VSC with the unloading capacity provided by the embodiment of the application controls the active power output of the converter station and the unloading power of the unloading device to cooperate, so that the stability of the DC power grid voltage is realized when the AC side of the receiving end converter station fails.

[0061] The control method of the VSC with the unloading capacity provided by the embodiment of the application controls the output voltage of the bridge arm and the auxiliary turn-off bridge arm of the converter station, realizes the controllable turn-off of the unloading bridge arm, and does not affect the DC output voltage and the AC output line current of the converter station in the control process, and the DC voltage fluctuation is smaller than that of the traditional DC unloading device. BRIEF DESCRIPTION OF DRAWINGS

[0062] Other features, objects and advantages of the application will become more apparent from the following detailed description of non-limiting embodiments with reference to the following drawings:

[0063] Fig. 1 is a schematic diagram of the application scene of the VSC with the unloading capacity according to an embodiment of the application;

[0064] Fig. 2 is a schematic diagram of the first VSC with the unloading capacity according to a preferred embodiment of the application;

[0065] Fig. 3 is a schematic diagram of the second VSC with the unloading capacity according to a preferred embodiment of the application;

[0066] Fig. 4 is a schematic diagram of the third VSC with the unloading capacity according to a preferred embodiment of the application;

[0067] Fig. 5 is a schematic diagram of the fourth VSC with the unloading capacity according to a preferred embodiment of the application;

[0068] Fig. 6 is a schematic diagram of the fifth VSC with the unloading capacity according to a preferred embodiment of the application;

[0069] Fig. 7 is a schematic diagram of a sixth HVDC converter topology with unloading capability in a preferred embodiment of the present application;

[0070] Fig. 8 is a schematic diagram of a typical topology of an unloading bridge arm, an auxiliary turn-off bridge arm, a converter station bridge arm and a sub-module in a preferred embodiment of the present application;

[0071] Fig. 9 is a simulation waveform diagram of AC side grid voltage and current of the HVDC converter with unloading capability in a specific embodiment of the present application;

[0072] Fig. 10 is a simulation waveform diagram of DC side grid voltage and current of the HVDC converter with unloading capability in a specific embodiment of the present application;

[0073] Fig. 11 is a simulation waveform diagram of the converter station bridge arm modulation voltage of the HVDC converter with unloading capability in a specific embodiment of the present application.

[0074] Fig. 12 is a simulation waveform diagram of the unloading bridge arm switching signal, the A-phase auxiliary turn-off bridge arm voltage and the unloading bridge arm current of the HVDC converter with unloading capability in a specific embodiment of the present application. DETAILED DESCRIPTION

[0075] The present application will be described in detail below with specific embodiments. The following embodiments will help those skilled in the art to further understand the present application, but do not limit the present application in any form. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present application. These all belong to the protection scope of the present application.

[0076] In an embodiment of the present application, a HVDC converter with unloading function is provided, as shown in Fig. 1, comprising: a modular multilevel HVDC converter, and a DC unloading device installed in the modular multilevel HVDC converter;

[0077] The modular multilevel HVDC converter comprises an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor and a lower bridge arm reactor;

[0078] The DC unloading device comprises an unloading bridge arm and an auxiliary turn-off bridge arm, the unloading bridge arm can consume surplus power in the system, and the auxiliary turn-off bridge arm can assist in turning off the switching device in the unloading bridge arm;

[0079] The HVDC converter topology with unloading function comprises one or more of the following:

[0080] In the first topology, the DC unloading device is single-phase, comprising two identical parts, which are installed on the upper converter station bridge arm and the lower converter station bridge arm respectively; each part comprises: an unloading bridge arm and an auxiliary turn-off bridge arm; the auxiliary turn-off bridge arm is connected in series with the upper converter station bridge arm and the lower converter station bridge arm, and then connected in parallel with the unloading bridge arm;

[0081] The second topology is a three-phase DC unloading device, each phase being the same as the DC unloading device in the first topology;

[0082] The third topology is a single-phase DC unloading device, each phase including two unloading bridge arms and one auxiliary turn-off bridge arm; the two unloading bridge arms are connected in series and are connected in parallel with the upper converter station bridge arm and the lower converter station bridge arm, and the auxiliary turn-off bridge arm is connected between the upper and lower unloading bridge arms and between the upper and lower converter station bridge arms;

[0083] The fourth topology is a three-phase DC unloading device, each phase being the same as the DC unloading device in the third topology;

[0084] The fifth topology is a three-phase DC unloading device; each phase includes one unloading bridge arm and two auxiliary turn-off bridge arms; the two auxiliary turn-off bridge arms are connected between the AC port and the upper converter station bridge arm and between the AC port and the lower converter station bridge arm, respectively; and the unloading bridge arm is connected between the upper converter station bridge arm and the lower converter station bridge arm.

[0085] The sixth topology is a three-phase DC unloading device; each phase includes one unloading bridge arm and one auxiliary turn-off bridge arm; the unloading bridge arm and the auxiliary turn-off bridge arm are connected in series and are connected between the upper converter station bridge arm and the lower converter station bridge arm.

[0086] The flexible DC converter in the above embodiments has an unloading function itself, meets the fault ride-through process of the flexible DC transmission system, and ensures safe and stable operation of the system.

[0087] In a preferred embodiment, the first topology of the flexible DC converter with an unloading function is further provided, as shown in FIG. 2, each phase of the flexible DC converter with a modular multilevel flexible DC converter includes an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor. The upper bridge arm reactor is connected in star, the neutral point of which is connected to the positive pole of the DC port, and the other end of each phase upper bridge arm reactor is connected to the upper end of the corresponding phase upper converter station bridge arm. For each phase converter station bridge arm without a DC unloading device, the lower end of the upper converter station bridge arm is connected to the upper end of the lower converter station bridge arm. The lower bridge arm reactor is connected in star, the neutral point of which is connected to the negative pole of the DC port, and the other end of each phase lower bridge arm reactor is connected to the lower end of the corresponding phase lower converter station bridge arm.

[0088] The DC unloading device is installed only on one phase bridge arm of the flexible DC converter. The DC unloading device includes an upper unloading bridge arm, an upper auxiliary turn-off bridge arm, a lower unloading bridge arm, and a lower auxiliary turn-off bridge arm. One end of the upper auxiliary turn-off bridge arm is connected to the lower end of the upper converter station bridge arm, and the other end is connected to the AC port. One end of the lower auxiliary turn-off bridge arm is connected to the upper end of the lower converter station bridge arm, and the other end is connected to the AC port. One end of the upper unloading bridge arm is connected to the upper end of the upper converter station bridge arm, and the other end is connected to the AC port.

[0089] It should be noted that the positions of the converter station bridge arms and the auxiliary blocking bridge arms can also be transposed to achieve the same function. That is, their connection topologies can also be as follows: one end of the upper converter station bridge arm is connected to the lower end of the upper auxiliary blocking bridge arm, and the other end is connected to the AC port; one end of the lower converter station bridge arm is connected to the upper end of the lower auxiliary blocking bridge arm, and the other end is connected to the AC port; one end of the upper unloading bridge arm is connected to the upper end of the upper auxiliary blocking bridge arm, and the other end is connected to the AC port; and one end of the lower unloading bridge arm is connected to the lower end of the lower auxiliary blocking bridge arm, and the other end is connected to the AC port.

[0090] In a preferred embodiment, a second topology of the VSC with unloading function is further provided, as shown in FIG. 3, each phase of the VSC with modular multilevel converter includes: an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor. One end of the upper bridge arm reactor is connected in star, and the neutral point is connected to the positive pole of the DC port; the other end of each phase of the upper bridge arm reactor is connected to the upper end of the corresponding phase of the upper converter station bridge arm. One end of the lower bridge arm reactor is connected in star, and the neutral point is connected to the negative pole of the DC port; the other end of each phase of the lower bridge arm reactor is connected to the lower end of the corresponding phase of the lower converter station bridge arm.

[0091] The DC unloading device is installed on the three-phase bridge arm of the VSC. Each phase of the DC unloading device includes: an upper unloading bridge arm, an upper auxiliary blocking bridge arm, a lower unloading bridge arm, and a lower auxiliary blocking bridge arm. One end of the upper auxiliary blocking bridge arm is connected to the lower end of the upper converter station bridge arm, and the other end is connected to the AC port. One end of the lower auxiliary blocking bridge arm is connected to the upper end of the lower converter station bridge arm, and the other end is connected to the AC port. One end of the upper unloading bridge arm is connected to the upper end of the upper converter station bridge arm, and the other end is connected to the AC port. One end of the lower unloading bridge arm is connected to the lower end of the lower converter station bridge arm, and the other end is connected to the AC port.

[0092] Similarly, the positions of the converter station bridge arms and the auxiliary blocking bridge arms can also be transposed. One end of the upper converter station bridge arm is connected to the lower end of the upper auxiliary blocking bridge arm, and the other end is connected to the AC port; one end of the lower converter station bridge arm is connected to the upper end of the lower auxiliary blocking bridge arm, and the other end is connected to the AC port; one end of the upper unloading bridge arm is connected to the upper end of the upper auxiliary blocking bridge arm, and the other end is connected to the AC port; and one end of the lower unloading bridge arm is connected to the lower end of the lower auxiliary blocking bridge arm, and the other end is connected to the AC port.

[0093] In a preferred embodiment, a third topology of the VSC with DC unloading function is further provided, as shown in FIG. 4, each phase of the MMC-VSC includes: an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor. The upper bridge arm reactor is connected in star, and the neutral point is connected to the positive pole of the DC port, and the other end of each phase upper bridge arm reactor is connected to the upper end of the corresponding phase upper converter station bridge arm. The lower end of the upper converter station bridge arm is connected to the upper end of the lower converter station bridge arm. The lower bridge arm reactor is connected in star, and the neutral point is connected to the negative pole of the DC port, and the other end of each phase lower bridge arm reactor is connected to the lower end of the corresponding phase lower converter station bridge arm.

[0094] The DC unloading device is installed on only one phase bridge arm of the VSC. The DC unloading device includes: an upper unloading bridge arm, a lower unloading bridge arm, and an auxiliary turn-off bridge arm. The upper end of the upper unloading bridge arm is connected to the upper end of the upper converter station bridge arm, the lower end of the upper unloading bridge arm is connected to the upper end of the lower unloading bridge arm, and the lower end of the lower unloading bridge arm is connected to the lower end of the lower converter station bridge arm. One end of the auxiliary turn-off bridge arm is connected to the lower end of the upper unloading bridge arm, and the other end is connected to the lower end of the upper converter station bridge arm.

[0095] In a preferred embodiment, a fourth topology of the VSC with DC unloading function is further provided, as shown in FIG. 5, each phase of the MMC-VSC includes: an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor. The upper bridge arm reactor is connected in star, and the neutral point is connected to the positive pole of the DC port, and the other end of each phase upper bridge arm reactor is connected to the upper end of the corresponding phase upper converter station bridge arm. The lower end of the upper converter station bridge arm is connected to the upper end of the lower converter station bridge arm. The lower bridge arm reactor is connected in star, and the neutral point is connected to the negative pole of the DC port, and the other end of each phase lower bridge arm reactor is connected to the lower end of the corresponding phase lower converter station bridge arm.

[0096] The DC unloading device is installed on only one phase bridge arm of the VSC. The DC unloading device includes: an upper unloading bridge arm, a lower unloading bridge arm, and an auxiliary turn-off bridge arm. The upper end of the upper unloading bridge arm is connected to the upper end of the upper converter station bridge arm, the lower end of the upper unloading bridge arm is connected to the upper end of the lower unloading bridge arm, and the lower end of the lower unloading bridge arm is connected to the lower end of the lower converter station bridge arm. One end of the auxiliary turn-off bridge arm is connected to the lower end of the upper unloading bridge arm, and the other end is connected to the lower end of the upper converter station bridge arm.

[0097] In a preferred embodiment, a fifth topology of the VSC with DC unloading function is further provided, as shown in FIG. 6, each phase of the MMC-VSC includes: a first upper converter station bridge arm, a second upper converter station bridge arm, a first lower converter station bridge arm, a second lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor. The upper bridge arm reactor is connected in star, and the neutral point is connected to the positive pole of the DC port. The other end of each phase of the upper bridge arm reactor is connected to the upper end of the first upper converter station bridge arm of the corresponding phase. The upper end of the second upper converter station bridge arm is connected to the lower end of the first upper converter station bridge arm. The lower bridge arm reactor is connected in star, and the neutral point is connected to the negative pole of the DC port. The other end of each phase of the lower bridge arm reactor is connected to the lower end of the second lower converter station bridge arm of the corresponding phase. The lower end of the first lower converter station bridge arm is connected to the upper end of the second lower converter station bridge arm.

[0098] The DC unloading device is installed on the three-phase bridge arm of the VSC. Each phase of the DC unloading device includes: an unloading bridge arm, an upper auxiliary turn-off bridge arm, and a lower auxiliary turn-off bridge arm. The upper end of the unloading bridge arm is connected to the lower end of the first upper converter station bridge arm, and the lower end of the unloading bridge arm is connected to the upper end of the second lower converter station bridge arm. One end of the upper auxiliary turn-off bridge arm is connected to the lower end of the second upper converter station bridge arm, and the other end is connected to the AC port. One end of the lower auxiliary turn-off bridge arm is connected to the upper end of the first lower converter station bridge arm, and the other end is connected to the AC port.

[0099] In a preferred embodiment, a sixth topology of the VSC with DC unloading function is further provided, as shown in FIG. 7, each phase of the MMC-VSC includes: a first upper converter station bridge arm, a second upper converter station bridge arm, a first lower converter station bridge arm, a second lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor. The upper bridge arm reactor is connected in star, and the neutral point is connected to the positive pole of the DC port. The other end of each phase of the upper bridge arm reactor is connected to the upper end of the first upper converter station bridge arm of the corresponding phase. The upper end of the second upper converter station bridge arm is connected to the lower end of the first upper converter station bridge arm, and the lower end of the second upper converter station bridge arm is connected to the AC port. The lower bridge arm reactor is connected in star, and the neutral point is connected to the negative pole of the DC port. The other end of each phase of the lower bridge arm reactor is connected to the lower end of the second lower converter station bridge arm of the corresponding phase. The lower end of the first lower converter station bridge arm is connected to the upper end of the second lower converter station bridge arm, and the upper end of the first lower converter station bridge arm is connected to the AC port.

[0100] The DC unloading device is installed on the three-phase bridge arm of the VSC. Each phase of the DC unloading device includes: an unloading bridge arm and an auxiliary turn-off bridge arm. The upper end of the unloading bridge arm is connected to the lower end of the first upper converter station bridge arm, the lower end of the unloading bridge arm is connected to the upper end of the auxiliary turn-off bridge arm, and the lower end of the auxiliary turn-off bridge arm is connected to the upper end of the second lower converter station bridge arm.

[0101] In some preferred embodiments, as shown in Fig. 8, the converter station bridge arm comprises a plurality of converter station sub-modules connected in series. The converter station bridge arm can be composed of bipolar sub-modules, can be composed of unipolar sub-modules, or can be composed of a mixture of bipolar sub-modules and unipolar sub-modules.

[0102] The unloading bridge arm is used to consume surplus power in the DC system in the event of a fault in the AC system. The unloading bridge arm can be composed of a unloading resistor R and a half-controlled switching device string T in series, or can be composed of a unloading resistor R and a capacitor string C in series. The direction of the thyristor string is consistent with the voltage polarity direction of the converter station bridge arm. The resistor is a power resistor for dissipating surplus power.

[0103] The auxiliary turn-off bridge arm is composed of one or more bipolar sub-modules connected in series.

[0104] Compared with existing DC unloading devices, the VSC of the embodiment of the present application has a significant economic advantage of low construction cost, as the number of switching devices is greatly reduced, and half-controlled thyristor switching devices are used instead of fully-controlled switching devices.

[0105] Compared with existing parallel centralized DC unloading devices, the VSC of the present application uses series thyristors to control the unloading bridge arm, which is easier to achieve device voltage equalization than the traditional series fully-controlled switching device scheme, and can also precisely and continuously control the unloading power by controlling the conduction time of the thyristor.

[0106] Compared with existing parallel distributed or hybrid DC unloading devices, the VSC of the present application has fewer switching devices, uses centralized resistors, is easy to dissipate heat, and reduces the heat dissipation pressure of the converter station, thus significantly reducing the cost of the device.

[0107] Based on the same inventive concept, in other embodiments of the present application, a control method for a VSC with unloading capability is provided, and the VSC with unloading capability has two operating modes, including:

[0108] Mode 1: normal operating mode, in which the AC system is normally operated, the converter is controlled for reactive power control, and the unloading bridge arm is not operated. The reactive power control is the interactive reactive power control between the converter station and the AC system. All unloading bridge arms are in the off state, and the current only flows from the bridge reactor and the converter station bridge arm;

[0109] Mode 2: fault unloading mode, in this mode, the AC system has a short circuit or ground fault, the DC power grid has surplus power, the control converter station to carry out active power control and unloading power control, including: control of the converter station AC current, control of the opening and closing of the thyristor in the unloading bridge arm. The current flows from the converter station bridge arm, also from the unloading bridge arm and auxiliary blocking bridge arm, consuming surplus power.

[0110] In the above embodiment, the active power output of the converter station and the unloading power of the unloading device are cooperated to realize the stability of the DC power grid voltage when the AC side of the receiving end converter station fails.

[0111] Further, in a preferred embodiment, for the fourth kind of HVDC converter topology with unloading function, the output voltages of the auxiliary blocking bridge arm and the converter station bridge arm are controlled to realize the blocking of the thyristor in the unloading bridge arm.

[0112] Further, in a preferred embodiment, the output voltages of the auxiliary blocking bridge arm and the converter station bridge arm are controlled to change simultaneously in each power frequency cycle, realizing the blocking of the thyristor in the unloading bridge arm in each power frequency cycle.

[0113] Further, in a preferred embodiment, when the upper unloading bridge arm of a phase is blocked, the voltage of the corresponding converter station bridge arm is controlled to be zero, the voltage of the corresponding lower converter station bridge arm is controlled to be the DC power grid voltage, and the voltage of the auxiliary blocking bridge arm is controlled to be a positive voltage not greater than 0.05pu, so that the thyristor in the upper unloading bridge arm is blocked under reverse voltage; when the lower unloading bridge arm of a phase is blocked, the voltage of the corresponding lower converter station bridge arm is controlled to be zero, the voltage of the corresponding upper converter station bridge arm is controlled to be the DC power grid voltage, and the voltage of the auxiliary blocking bridge arm is controlled to be a negative voltage not greater than 0.05pu, so that the thyristor in the lower unloading bridge arm is blocked under reverse voltage.

[0114] Further, in a preferred embodiment, in order to avoid affecting the AC side line current of the converter station, when the unloading bridge arm of a phase is blocked, the voltages of the converter station bridge arms of other phases are controlled to change accordingly to compensate for the change of the AC line voltage of the converter station; assuming that the time when the upper unloading bridge arm of phase A is blocked is t0, at this time the voltages of the converter station bridge arms are controlled as follows:

[0115] Wherein, u ap , u an , u bp , u bn , u cp , u cn are the modulation voltages of the upper and lower converter station bridge arms of phase A, the upper and lower converter station bridge arms of phase B, and the upper and lower converter station bridge arms of phase C under fault unloading; u ap , u an , u bp , u bn, u cp , u cn are the modulation voltages of the upper and lower converter station bridge arms of phase A, phase B and phase C respectively under normal working condition, U DC is the DC grid voltage.

[0116] The modulation voltages calculated according to equation (1), the DC side output voltage of the converter station and the AC side output voltage are respectively:

[0117] wherein, U DC and U' DC are the DC grid voltages before and after the fault occurs respectively; U' AB , U' BC , U' CA and U AB , U BC , U CA are the AC grid line voltages before and after the fault occurs respectively. Therefore, changing the converter station bridge arm voltage does not change the DC voltage and AC line voltage output by the converter station.

[0118] Further, in a preferred embodiment, the opening time of the thyristor in the unloading bridge arm in each power frequency cycle is controlled, and the power consumed by the unloading bridge arm is controlled.

[0119] The application of the flexible DC converter with unloading function and the control method in the above embodiments will be further described below in combination with specific simulation examples.

[0120] In combination with the above embodiments, the following uses MATLAB / Simulink software to simulate and verify the system, and the simulation parameters are shown in Table 1.

[0121] Table 1 Simulation verification parameters

[0122] Simulation Example One:

[0123] The typical application scenario of the flexible DC converter with unloading function in the DC system is shown in FIG. 1 of the specification, the sending end converter station collects power and sends it to the DC grid, and the receiving end converter station converts the DC power into AC power.

[0124] In order to verify the feasibility and rationality of the flexible DC converter with unloading function and the control method thereof, two stages are simulated, and the specific working condition design is as follows:

[0125] Stage One (0-1.5s, normal operation mode):

[0126] In stage one, the HVDC system is in normal operation, the power of the sending converter station and the receiving converter station is equal, the voltage and the current of the DC power grid are rated values, the HVDC converter with power unloading function operates in mode 1 and does not unload power.

[0127] Stage two (power unloading mode in 1.5s-4s):

[0128] In the simulation, it is assumed that the AC side of the receiving converter station is faulty, the AC voltage drops to 0.2pu at the 1.5s of the simulation, the power of the receiving converter station is reduced to 0.2 times of the rated value, and the output power of the sending converter station is unchanged. Excess power appears in the DC system.

[0129] Then, the control method of the HVDC converter with power unloading function provided by the application is used, the DC power unloading device is transitioned to mode 2 and starts to consume the excess power. The sum of the total power consumed by the unloading bridge arm and the AC active power output by the converter station is the DC power grid power, so that overvoltage accidents are avoided.

[0130] Fig. 9 is a diagram of the AC grid voltage and current simulation waveforms of the HVDC converter with power unloading function in the implementation process, which totally contains 6 waveforms, from top to bottom, the AC grid voltage waveform and the AC grid current waveform in the whole simulation process, the AC grid voltage waveform and the AC grid current waveform in the normal operation mode, and the AC grid voltage waveform and the AC grid current waveform in the fault power unloading mode. In the normal operation mode, the AC grid voltage and the AC grid current are rated values, and in the fault power unloading mode, the AC grid voltage is reduced to 0.2pu and the AC grid current amplitude remains unchanged. In the fault power unloading mode, the line voltage and the phase current of the AC grid remain sinusoidal, which proves that the control method of the HVDC converter with power unloading function provided by the embodiment of the application can change the bridge arm voltage of the converter station to unload power while ensuring that the line voltage and the phase current waveforms output on the AC side do not be obviously distorted.

[0131] Fig. 10 is a diagram of the DC grid voltage and current simulation waveforms of the HVDC converter with power unloading function in the implementation process, which totally contains 2 waveforms, from top to bottom, the DC grid voltage waveform and the DC grid current waveform in the whole simulation process. Before the fault occurs, the DC grid voltage and the DC grid current are rated values, which are 500kV and 3000A respectively, and after the fault occurs, the DC grid voltage is kept at 500kV by using the control method of the HVDC converter with power unloading function provided by the application, so that the DC grid current is always kept at the rated value of 3000A after the fault.

[0132] Figure 11 is a diagram of simulation waveforms of the modulation voltage of the internal converter station bridge arm and the unloading bridge arm current of the flexible direct current converter with unloading capacity in the implementation process, and totally contains 6 waveforms, from top to bottom, they are: the modulation voltage per unit value waveforms of the upper and lower converter station bridge arms of phase A, phase B and phase C before the fault occurs; the modulation voltage per unit value waveforms of the upper and lower converter station bridge arms of phase A, phase B and phase C after the fault occurs. Before the fault occurs, the voltage of the converter station bridge arm is a three-phase sinusoidal wave. After the fault occurs, the control method of the flexible direct current converter with unloading capacity provided by the embodiment of the present application superimposes a three-phase symmetrical pulse in the voltage of the converter station bridge arm to turn off each unloading bridge arm. When the unloading bridge arm of phase A is turned off, the voltage of the upper converter station bridge arm of phase A is zero, the voltage of the lower converter station bridge arm of phase A is the DC power grid voltage, the upper converter station bridge arms of phase B and phase C subtract the modulation voltage of the upper converter station bridge arm of phase A before the fault, and the lower converter station bridge arms of phase B and phase C add the modulation voltage of the upper converter station bridge arm of phase A before the fault. Similarly, when other unloading bridge arms are turned off, the same change amount is symmetrically added and subtracted in the converter station bridge arms of phase A, phase B and phase C.

[0133] Figure 12 contains 3 waveforms, which are respectively the waveforms of the A-phase unloading bridge arm drive signal, the A-phase auxiliary turn-off bridge arm voltage and the current simulation waveforms of the A-phase upper unloading bridge arm of the flexible direct current converter with unloading capacity in the process of implementing the control method of the above embodiment after the fault occurs. The current in the unloading bridge arm is intermittent, which proves that the control method provided by the present application can effectively turn off the unloading bridge arm and accurately control the unloading power.

[0134] The simulation waveform results show that the flexible direct current converter with unloading capacity provided by the embodiment of the present application can effectively consume the surplus power and maintain the stability of the DC power grid voltage when the fault occurs at the receiving end converter station AC side. The control method of the flexible direct current converter with unloading capacity provided by the embodiment of the present application can coordinate the control of the active power of the converter station and the dissipation power of the unloading bridge arm when the fault occurs, and can accurately and smoothly control the unloading power. The control method of the flexible direct current converter with unloading capacity provided by the embodiment of the present application controls the output voltage of the converter station bridge arm and the auxiliary turn-off bridge arm, realizes the controllable turn-off of the unloading bridge arm, and does not affect the DC output voltage and AC output line current waveform quality of the converter station in the control process. Compared with the existing parallel centralized DC unloading device, the flexible direct current converter with unloading capacity provided by the embodiment of the present application produces smaller DC voltage ripple when unloading; compared with the existing parallel distributed DC unloading device, the unloading device provided by the embodiment of the present application reduces the number of switching devices, uses thyristors instead of fully controlled switching devices, and reduces the requirement for heat dissipation devices of the centralized unloading resistor, so the cost is lower.

[0135] The specific embodiments of the present application are described above. It needs to be understood that the present application is not limited to the specific embodiments described above, and various modifications or changes can be made by those skilled in the art within the scope of the claims, which does not affect the essential content of the present application. The above preferred features can be combined in any manner without conflict, and used.

Claims

1. A flexible direct current converter with a de-loading function, characterized in that, The application relates to a modular multilevel flexible direct current converter and a direct current unloading device installed in the modular multilevel flexible direct current converter. The modular multilevel flexible direct current converter comprises an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor and a lower bridge arm reactor. The direct current unloading device comprises an unloading bridge arm and an auxiliary turn-off bridge arm, the unloading bridge arm can consume surplus power in the system, and the auxiliary turn-off bridge arm can assist in turning off the switching device in the unloading bridge arm. The flexible direct current converter topology of the unloading function comprises one or more of the following: In a first topology, the direct current unloading device is single-phase, comprises two identical parts arranged on the upper converter station bridge arm and the lower converter station bridge arm of any phase of the flexible direct current converter, each part comprises an unloading bridge arm and an auxiliary turn-off bridge arm, and the auxiliary turn-off bridge arm is connected in series with the upper converter station bridge arm and the lower converter station bridge arm and then connected in parallel with the unloading bridge arm. In a second topology, the direct current unloading device is three-phase, and each phase is the same as the direct current unloading device in the first topology. In a third topology, the direct current unloading device is single-phase, each phase comprises two unloading bridge arms and an auxiliary turn-off bridge arm, the two unloading bridge arms are connected in series and connected in parallel with the upper converter station bridge arm and the lower converter station bridge arm of any phase of the flexible direct current converter, and the auxiliary turn-off bridge arm is connected between the upper and lower unloading bridge arms and between the upper and lower converter station bridge arms. In a fourth topology, the direct current unloading device is three-phase, and each phase is the same as the direct current unloading device in the third topology. In a fifth topology, the direct current unloading device is three-phase, each phase comprises an unloading bridge arm and two auxiliary turn-off bridge arms, the two auxiliary turn-off bridge arms are connected between the AC port and the upper converter station bridge arm and between the AC port and the lower converter station bridge arm respectively, and the unloading bridge arm is connected between the upper converter station bridge arm and the lower converter station bridge arm. In a sixth topology, the direct current unloading device is three-phase, each phase comprises an unloading bridge arm and an auxiliary turn-off bridge arm, and the unloading bridge arm and the auxiliary turn-off bridge arm are connected in series and connected between the upper converter station bridge arm and the lower converter station bridge arm. Each phase of the modular multilevel flexible direct current converter comprises an upper converter station bridge arm, a lower converter station bridge arm, an upper bridge arm reactor and a lower bridge arm reactor.

2. The HVDC converter with unloading function according to claim 1, characterized in that, One end of the upper bridge arm reactor is star-connected, the neutral point of which is connected to the positive pole of the direct current port, the other end of the upper bridge arm reactor of each phase is connected to the upper end of the upper converter station bridge arm of the corresponding phase, for each phase converter station bridge arm without the direct current unloading device, the lower end of the upper converter station bridge arm is connected to the upper end of the lower converter station bridge arm, and one end of the lower bridge arm reactor is star-connected, the neutral point of which is connected to the negative pole of the direct current port, the other end of the lower bridge arm reactor of each phase is connected to the lower end of the lower converter station bridge arm of the corresponding phase. In the first topology, 3. [Amended according to Rule 26 12.02.2025] A flexible rectifier with unloading function according to claim 2, characterized in that, The direct current unloading device is only arranged on the bridge arm of one phase of the flexible direct current converter. The direct current unloading device comprises an upper unloading bridge arm, an upper auxiliary turn-off bridge arm, a lower unloading bridge arm and a lower auxiliary turn-off bridge arm. ​ The one end of the upper auxiliary turn-off bridge arm is connected with the lower end of the upper converter station bridge arm, and the other end is connected with the AC port; the one end of the lower auxiliary turn-off bridge arm is connected with the upper end of the lower converter station bridge arm, and the other end is connected with the AC port; the one end of the upper unloading bridge arm is connected with the upper end of the upper converter station bridge arm, and the other end is connected with the AC port; the one end of the lower unloading bridge arm is connected with the lower end of the lower converter station bridge arm, and the other end is connected with the AC port. Alternatively, the one end of the upper converter station bridge arm is connected with the lower end of the upper auxiliary turn-off bridge arm, and the other end is connected with the AC port; the one end of the lower converter station bridge arm is connected with the upper end of the lower auxiliary turn-off bridge arm, and the other end is connected with the AC port; the one end of the upper unloading bridge arm is connected with the upper end of the upper auxiliary turn-off bridge arm, and the other end is connected with the AC port; the one end of the lower unloading bridge arm is connected with the lower end of the lower auxiliary turn-off bridge arm, and the other end is connected with the AC port.

4. The HVDC converter with unloading function according to claim 3, characterized in that, In the second topology, the flow unloading device is installed on the three-phase bridge arm of the HVDC converter; each phase is the same as the first topology.

5. The HVDC converter with unloading function according to claim 2, characterized in that, In the third topology, The DC unloading device is only installed on one phase bridge arm of the HVDC converter; The DC unloading device comprises an upper unloading bridge arm, a lower unloading bridge arm, and an auxiliary turn-off bridge arm. The upper end of the upper unloading bridge arm is connected with the upper end of the upper converter station bridge arm, the lower end of the upper unloading bridge arm is connected with the upper end of the lower unloading bridge arm, and the lower end of the lower unloading bridge arm is connected with the lower end of the lower converter station bridge arm; one end of the auxiliary turn-off bridge arm is connected with the lower end of the upper unloading bridge arm, and the other end is connected with the lower end of the upper converter station bridge arm.

6. The HVDC converter according to claim 5, characterized in that In the fourth topology, the flow unloading device is installed on the three-phase bridge arm of the HVDC converter; each phase is the same as the third topology.

7. The HVDC converter with unloading function according to claim 1, characterized in that, Each phase of the HVDC converter with the modular multilevel converter comprises a first upper converter station bridge arm, a second upper converter station bridge arm, a first lower converter station bridge arm, a second lower converter station bridge arm, an upper bridge arm reactor, and a lower bridge arm reactor. One end of the upper bridge arm reactor is connected in star, and the neutral point is connected with the positive pole of the DC port; the other end of the upper bridge arm reactor of each phase is connected with the upper end of the first upper converter station bridge arm of the corresponding phase; the upper end of the second upper converter station bridge arm is connected with the lower end of the first upper converter station bridge arm. One end of the lower bridge arm reactor is connected in star, and the neutral point is connected with the negative pole of the DC port; the other end of the lower bridge arm reactor of each phase is connected with the lower end of the second lower converter station bridge arm of the corresponding phase; the lower end of the first lower converter station bridge arm is connected with the upper end of the second lower converter station bridge arm.

8. The HVDC converter according to claim 7, characterized in that In the fifth topology, the DC unloading device is installed on the three-phase bridge arm of the HVDC converter; Each phase of the DC unloading device comprises an unloading bridge arm, an upper auxiliary turn-off bridge arm, and a lower auxiliary turn-off bridge arm. The upper end of the unloading bridge arm is connected with the lower end of the first upper converter station bridge arm, and the lower end of the unloading bridge arm is connected with the upper end of the second lower converter station bridge arm; one end of the upper auxiliary turn-off bridge arm is connected with the lower end of the second upper converter station bridge arm, and the other end is connected with the AC port; one end of the lower auxiliary turn-off bridge arm is connected with the upper end of the first lower converter station bridge arm, and the other end is connected with the AC port.

9. The HVDC converter according to claim 1, wherein, In the sixth topology, the DC unloading device is installed on the three-phase bridge arm of the HVDC converter; The direct current unloading device comprises an unloading bridge arm and an auxiliary off bridge arm.

10. The flexible HVDC converter with unloading function according to any one of claims 1-9, characterized in that, The modular multilevel flexible direct current converter is a three-phase voltage source converter with a modular multilevel structure, and has the functions of AC-DC power conversion and connection between a medium-high voltage AC power grid and a DC system.

11. A flexible HVDC converter with a stress relief function according to any one of claims 1-9, characterized in that, The upper converter bridge arm and the lower converter bridge arm comprise a plurality of series-connected converter sub-modules. The converter bridge arm comprises bipolar sub-modules, unipolar sub-modules, or a combination of bipolar sub-modules and unipolar sub-modules. The unloading bridge arm comprises a series connection of an unloading resistor R and a string of semi-controlled switching devices, or a series connection of an unloading resistor R and a string of capacitors C. The direction of the string of semi-controlled switching devices is consistent with the voltage polarity direction of the converter bridge arm. The auxiliary off bridge arm comprises one bipolar sub-module or a plurality of series-connected bipolar sub-modules.

12. A control method of a flexible direct current converter having a de-loading capability, characterized by, The flexible direct current converter with unloading capability has two operation modes, including: Mode 1: normal operation mode, in which the AC system is normally operated, the converter is controlled to perform reactive power control, and the unloading bridge arm is not operated; all the unloading bridge arms are in an off state, and current only flows through the bridge reactor and the converter bridge arm; Mode 2: fault unloading mode, in which the AC system is short-circuited or grounded, the DC power grid has surplus power, the converter is controlled to perform active power control and unloading power control, including: controlling the AC current of the converter, and controlling the turn-on and turn-off of the thyristors in the unloading bridge arm; current flows through the converter bridge arm, the unloading bridge arm and the auxiliary off bridge arm, and surplus power is consumed. For the fourth flexible direct current converter topology with unloading function, the output voltages of the auxiliary off bridge arm and the converter bridge arm are controlled to turn off the thyristors in the unloading bridge arm.

13. The control method of a flexible direct current converter having an unloading capability according to claim 12, characterized by, The output voltages of the auxiliary off bridge arm and the converter bridge arm are controlled to change simultaneously in each power frequency cycle, so as to turn off the thyristors in the unloading bridge arm in each power frequency cycle.

14. The control method of a flexible direct current converter having an unloading capability according to claim 13, characterized by, When turning off the upper unloading bridge arm of a phase, the voltage of the upper converter bridge arm of the corresponding phase is controlled to be zero, the voltage of the lower converter bridge arm of the corresponding phase is controlled to be the DC power grid voltage, and the voltage of the auxiliary off bridge arm is controlled to be a positive polarity voltage not greater than 0.05 pu, so that the thyristors in the upper unloading bridge arm are subjected to reverse voltage and turned off.

15. The control method of a flexible direct current converter having an unloading capability according to claim 14, characterized by, When turning off the lower unloading bridge arm of a phase, the voltage of the lower converter bridge arm of the corresponding phase is controlled to be zero, the voltage of the upper converter bridge arm of the corresponding phase is controlled to be the DC power grid voltage, and the voltage of the auxiliary off bridge arm is controlled to be a negative polarity voltage not greater than 0.05 pu, so that the thyristors in the lower unloading bridge arm are subjected to reverse voltage and turned off. Changing the converter bridge arm voltage does not change the DC voltage and AC line voltage output by the converter, specifically, 16. The control method of a flexible direct current converter having an unloading capability according to claim 15, characterized by, When the unloading bridge arm of a phase is turned off, the voltages of the bridge arms of other phases are controlled to follow the change, so as to compensate the change of the AC measuring line voltage of the converter station; assuming that the time when the unloading bridge arm of phase A is turned off is t0, at this time, the voltages of the bridge arms of the converter station are: wherein u ap , u an , u bp , u bn , u cp , u cn are the modulation voltages of the upper and lower converter station bridge arms of phase A, phase B and phase C respectively under the fault unloading condition; u ap , u an , u bp , u bn , u cp , u cn are the modulation voltages of the upper and lower converter station bridge arms of phase A, phase B and phase C respectively under the normal working condition, and U DC is the DC power grid voltage.

17. The control method of a flexible direct current converter having unloading capability according to claim 16, characterized by, ​ The modulation voltage calculated according to equation (1), the output voltage on the DC side of the converter station and the output voltage on the AC side are respectively: wherein U DC and U' DC are the DC grid voltages before and after the fault, respectively; U' AB , U' BC , U' CA and U AB , U BC , U CA are the AC grid line voltages before and after the fault, respectively.

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