Flexible DC power transmission system and fault ride-through method and system thereof

Abstract

The invention discloses a flexible direct-current power transmission system and a fault ride-through method and system thereof. The flexible direct-current power transmission system comprises a main control module, and a new energy sending-out system, an alternating-current energy consumption device, a sending-end converter, a direct-current circuit breaker, a receiving-end converter and a receiving-end alternating-current power grid which are connected in sequence, the direct current circuit breaker is deployed on a direct current power transmission line between the sending end converter and the receiving end converter, and the receiving end converter is a modular multi-level converter formed by cascading a plurality of half-bridge sub-modules with damping branches; the main control module is in communication connection with the half-bridge sub-modules, the direct-current circuit breaker and the alternating-current energy consumption device and used for controlling the damping branches of the half-bridge sub-modules in the running state, the direct-current circuit breaker and the alternating-current energy consumption device to act when a direct-current fault occurs so as to restrain direct current and adjust direct-current voltage to be within a set voltage range. Therefore, no-locking fault ride-through is completed. According to the invention, high-reliability and low-cost DC fault ride-through can be realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible direct current transmission, in particular to a flexible direct current transmission system and a fault ride-through method and system thereof. BACKGROUND

[0002] The new energy sending-out system based on voltage source converter high voltage direct current transmission (VSC-HVDC) technology has broad development prospects. Among them, the modular multilevel converter (MMC) is the core equipment of the VSC-HVDC system.

[0003] In the fault ride-through process of the flexible direct current transmission system including the new energy sending-out system, the MMC faces the problem of overcurrent caused by the short-circuit fault of the direct current line, and needs to be disconnected in time. In view of the challenge of system overcurrent disconnection, the existing engineering scheme mainly includes: replacing part of the MMC half-bridge module with a full-bridge module, reducing the current by outputting negative voltage through the full-bridge module, which has high loss and high cost; or directly locking the MMC and disconnecting the alternating current circuit breaker, thereby reducing the short-circuit current, but it is difficult to restart quickly after the MMC is locked. Therefore, the existing direct current fault ride-through scheme has problems such as serious influence on the transient stability of the power system and high cost. SUMMARY

[0004] In order to solve the problems of the prior art, the present application provides a flexible direct current transmission system and a fault ride-through method and system thereof, aiming to realize high-reliability and low-cost direct current fault ride-through.

[0005] The purpose of the present application is achieved by using the following technical solutions:

[0006] On the one hand, the present application provides a flexible direct current transmission system, comprising: a master control module, and a new energy sending-out system, an alternating current energy consumption device, a sending-end converter, a direct current circuit breaker, a receiving-end converter and a receiving-end alternating current power grid connected in sequence;

[0007] The direct current circuit breaker is arranged on the direct current transmission line between the sending-end converter and the receiving-end converter, and the receiving-end converter is a modular multilevel converter composed of a plurality of half-bridge sub-modules with damping branches in cascade;

[0008] The master module is in communication connection with the plurality of half-bridge sub-modules, the DC circuit breaker and the AC energy consumption device respectively, and is configured to control the damping branch of the half-bridge sub-module in the running state, the DC circuit breaker and the AC energy consumption device to act when a DC fault occurs, so as to suppress the DC current and adjust the DC voltage to a set voltage range, so as to complete the non-locking fault ride-through.

[0009] Optionally, the half-bridge sub-module further comprises a first switching device, a second switching device and a capacitor; the first switching device and the second switching device are connected in series and are connected in parallel with the capacitor; the damping branch is connected in parallel across the first switching device; the damping branch comprises a fully-controlled power electronic switch and a resistor connected in series.

[0010] In another aspect, the embodiments of the present application also provide a method for fault ride-through of a flexible DC power transmission system, which is applied to the flexible DC power transmission system described in any of the above embodiments; the method comprises:

[0011] When the master module identifies that a DC fault occurs, the master module controls the damping branch of the half-bridge sub-module in the running state to be put into operation, and controls the DC circuit breaker and the AC energy consumption device to act, so as to suppress the DC current and adjust the DC voltage to be within the set voltage range, so as to complete the non-locking fault ride-through.

[0012] Optionally, the master module controls the damping branch of the half-bridge sub-module in the running state to be put into operation, and controls the DC circuit breaker and the AC energy consumption device to act, so as to suppress the DC current and adjust the DC voltage to be within the set voltage range, so as to complete the non-locking fault ride-through, which comprises:

[0013] The master module controls the damping branch of the half-bridge sub-module in the running state to be put into operation to limit the current, so as to reduce the rising rate of the DC current;

[0014] When the rising rate of the DC current reduces to a set value, the master module controls the damping branch to be cut out and controls the state of the DC circuit breaker to be switched from closed to unclosed, so as to make the DC voltage recover;

[0015] When the DC voltage exceeds a rated voltage value during the recovery of the DC voltage, the master module controls the AC energy consumption device to be repeatedly put into operation and cut out until the DC voltage drops and is maintained within the set voltage range, so as to complete the non-locking fault ride-through.

[0016] Optionally, the master module controls the damping branch of the half-bridge sub-module in the running state to be put into operation to limit the current, so as to reduce the rising rate of the DC current, which comprises:

[0017] The master control module controls the first switching device of the half-bridge submodule to be blocked and controls the fully-controlled power electronic switch of the damping branch to be turned on, so that the fully-controlled power electronic switch adopts the control logic of the first switching device to reduce the rising rate of the direct current;

[0018] Correspondingly, when the rising rate of the direct current decreases to a set value, the master control module controls the damping branch to be cut out and controls the state of the direct current circuit breaker to be switched from closed to open, so that the direct current voltage is restored, comprising:

[0019] When the rising rate of the direct current decreases to the set value, the master control module controls the fully-controlled power electronic switch to be blocked to cut out the damping branch, and controls the state of the direct current circuit breaker to be switched from the closed to the open, so that the direct current voltage is restored.

[0020] Optionally, the master control module controls the alternating current energy consumption device to be repeatedly switched in and out until the direct current voltage decreases and is maintained within the set voltage range, to complete the non-blocking fault ride-through, comprising:

[0021] The master control module controls the alternating current energy consumption device to be repeatedly switched in and out until the direct current voltage decreases and is maintained within the set voltage range;

[0022] When the direct current voltage is maintained within the set voltage range, the master control module controls the state of the direct current circuit breaker to be switched from the open to the closed, and controls the alternating current energy consumption device to be completely cut out, so that the flexible direct current power transmission system restores the power transmission capability, thereby completing the non-blocking fault ride-through.

[0023] Optionally, after the non-blocking fault ride-through is completed, further comprising:

[0024] When the master control module identifies that an alternating current fault occurs, the master control module controls the damping branch of the half-bridge submodule in the running state to act, adjusts the direct current voltage to be within the set voltage range, to complete the non-blocking fault ride-through.

[0025] Optionally, the master control module controls the damping branch of the half-bridge submodule in the running state to act, adjusts the direct current voltage to be within the set voltage range, to complete the non-blocking fault ride-through, comprising:

[0026] The master control module repeatedly controls the fully-controlled power electronic switch of the damping branch to be turned on and turned off, so that the resistance of the damping branch repeatedly consumes energy, thereby reducing the direct current voltage;

[0027] When the direct current voltage decreases to the set voltage range, the damping branch is controlled to be completely cut out, to complete the non-blocking fault ride-through.

[0028] Optionally, after the completion of the non-locking fault ride-through, further comprising:

[0029] When the master module identifies normal operation, the master module keeps the damping branch inaction, controls the state of the DC circuit breaker to be closed and the AC energy consumption device to be in an inactivated state.

[0030] In still another aspect, the embodiments of the present application further provide a fault ride-through system of a flexible DC power transmission system, applied to the flexible DC power transmission system of any one of the above; the system comprises:

[0031] An identification unit is configured to identify whether a DC fault occurs;

[0032] A control unit is configured to, when the DC fault is identified to occur, control the damping branch of the half-bridge submodule in operation to be put into operation, and control the DC circuit breaker and the AC energy consumption device to act, so as to suppress the DC current and adjust the DC voltage to be within a set voltage range, to complete the non-locking fault ride-through.

[0033] Compared with the prior art, the present application has the following beneficial effects:

[0034] The embodiments of the present application provide a flexible DC power transmission system and a fault ride-through method and system thereof, the flexible DC power transmission system comprising: a master module, and a new energy sending-out system, an AC energy consumption device, a sending-end converter, a DC circuit breaker, a receiving-end converter and a receiving-end AC power grid connected in sequence; the DC circuit breaker is arranged on a DC power transmission line between the sending-end converter and the receiving-end converter, the receiving-end converter is a modular multilevel converter composed of a plurality of half-bridge submodules with damping branches; the master module is in communication connection with the plurality of half-bridge submodules, the DC circuit breaker and the AC energy consumption device, and is configured to, when a DC fault occurs, control the damping branch of the half-bridge submodule in operation, the DC circuit breaker and the AC energy consumption device to act, so as to suppress the DC current and adjust the DC voltage to be within a set voltage range, to complete the non-locking fault ride-through. In this way, when the DC fault occurs, the receiving-end converter composed of a plurality of half-bridge submodules with damping branches, i.e., a damping-enhanced modular multilevel converter, is used to put the damping branch into operation by locking the upper tube switching device in the half-bridge submodule in operation, so as to limit the development of the fault current, and the DC circuit breaker and the AC energy consumption device arranged in cooperation are used to break the current, so as to realize the non-locking fault ride-through with low cost and high reliability. In this way, the damping-enhanced modular multilevel converter, the DC circuit breaker and the AC energy consumption device are used in cooperation to suppress the current under the DC fault, so as to realize the non-locking fault ride-through under the DC fault.

[0035] It should be understood that the above general description and the following detailed description are only exemplary and explanatory, but not limiting the technical solutions provided by the embodiments of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings can be obtained from these drawings without creative labor, wherein:

[0037] Figure 1 A composition schematic diagram of a flexible direct current power transmission system provided by the embodiments of the present application;

[0038] Figure 2 A composition schematic diagram of a full and half mixed type damping enhanced MMC for an inverter at a receiving end in a flexible direct current power transmission system provided by the embodiments of the present application;

[0039] Figure 3 A flowchart schematic diagram of a fault ride-through method of a flexible direct current power transmission system provided by the embodiments of the present application;

[0040] Figure 4 A no-locking ride-through process schematic diagram of a fault ride-through method of a flexible direct current power transmission system provided by the embodiments of the present application under a direct current fault;

[0041] Figure 5 A schematic diagram of a principle of direct current fault current suppression realized by a fault ride-through method of a flexible direct current power transmission system provided by the embodiments of the present application;

[0042] Figure 6 A no-locking ride-through process schematic diagram of a fault ride-through method of a flexible direct current power transmission system provided by the embodiments of the present application under an alternating current fault;

[0043] Figure 7 A composition schematic diagram of a fault ride-through system of a flexible direct current power transmission system provided by the embodiments of the present application. DETAILED DESCRIPTION

[0044] The embodiments of the present application will be described below with reference to the drawings and preferred embodiments, and those skilled in the art can easily understand other advantages and effects of the present application from the content disclosed in the present specification. The present application can also be implemented or applied by means of other different specific embodiments, and each detail in the present specification can be modified or changed based on different views and applications without departing from the spirit of the present application. It should be understood that the preferred embodiments are only for illustrating the present application, but not for limiting the protection scope of the present application.

[0045] In the following description, reference is made to "some embodiments", which describe a subset of all possible embodiments, but it is understood that "some embodiments" can be the same or different subsets of all possible embodiments, and can be combined with each other, without conflict.

[0046] In the following description, the terms "first\second\third" are only to distinguish similar objects, and do not represent a specific order of the objects. It is understood that "first\second\third" can be interchanged in a specific order or sequence as allowed, so that the embodiments of the application described herein can be implemented in an order other than that illustrated or described herein.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments of the application belong. The terms used herein are only for the purpose of describing the embodiments of the application, and are not intended to limit the embodiments of the application.

[0048] Embodiment 1:

[0049] Referring to Figure 1 As shown in the figure, a composition schematic diagram of a flexible direct current power transmission system provided by the embodiments of the application is provided; wherein the flexible direct current power transmission system 100 comprises: a master control module (not shown in the figure), and a new energy sending-out system 101, an alternating current energy consumption device 102, a sending-end converter 103, a direct current circuit breaker 104, a receiving-end converter 105 and a receiving-end alternating current power grid 106 connected in sequence; Figure 1 The direct current circuit breaker 104 is arranged on a direct current power transmission line between the sending-end converter 103 and the receiving-end converter 105, and the receiving-end converter 105 is a modular multi-level converter composed of a plurality of half-bridge sub-modules 1051 with damping branches 10511.

[0050] The master control module is in communication connection with the plurality of half-bridge sub-modules 1051, the direct current circuit breaker 104 and the alternating current energy consumption device 102 respectively, and is used for controlling the damping branch 10511 of the half-bridge sub-module 1051 in an operating state, the direct current circuit breaker 104 and the alternating current energy consumption device 102 to act when a direct current fault occurs, to suppress the direct current and adjust the direct current voltage to a set voltage range, to complete the non-blocking fault ride-through.

[0051] The master control module is in communication connection with the plurality of half-bridge sub-modules 1051, the direct current circuit breaker 104 and the alternating current energy consumption device 102 respectively, and is used for controlling the damping branch 10511 of the half-bridge sub-module 1051 in an operating state, the direct current circuit breaker 104 and the alternating current energy consumption device 102 to act when a direct current fault occurs, to suppress the direct current and adjust the direct current voltage to a set voltage range, to complete the non-blocking fault ride-through.

[0052] In some embodiments of the present application, the receiving-end converter 105 is an MMC composed of a plurality of half-bridge sub-modules 1051 with a damping branch 10511; in other words, the receiving-end converter 105 is a Damping Enhanced (DE) MMC. Correspondingly, Figure 1 The half-bridge sub-module 1051 with a damping branch 10511 shown in FIG. 1 is a Damping Enhanced-Sub Module (DE-SM).

[0053] Here, Figure 1 The receiving-end converter 105 shown in FIG. 1 is exemplified by taking a half-bridge DE-MMC as an example; wherein, u A , u B , u C respectively denote the A-phase voltage, the B-phase voltage and the C-phase voltage in the receiving-end converter 105.

[0054] In some embodiments of the present application, continuing to refer to Figure 1 As shown in FIG. 1, the half-bridge sub-module 1051 further comprises a first switching device 10512, a second switching device 10513 and a capacitor 10514; the first switching device 10512 and the second switching device 10513 are connected in series and then connected in parallel with the capacitor 10514; the damping branch 10511 is connected in parallel across the first switching device 10512; the damping branch 10511 comprises a fully-controlled power electronic switch and a resistor connected in series.

[0055] It should be noted that, Figure 1 The fully-controlled power electronic switches in the first switching device 10512, the second switching device 10513 and the damping branch 10511 shown in FIG. 1 are all anti-parallel connected with a diode. The first switching device 10512 and the second switching device 10513 can both be fully-controlled power electronic switches.

[0056] It should be noted that the fully-controlled power electronic switch can be an IGBT, an Integrated Gate-Commutated Thyristor (IGCT), an Injection Enhanced Gate Transistor (IEGT) with a voltage resistance of 4KV or above, etc., and can not be configured with an anti-parallel diode.

[0057] In some embodiments of the present application, the DC circuit breaker 104 can be a low-capacity DC circuit breaker.

[0058] In some embodiments of the application, the damping branch 10511 in the half-bridge submodule 1051 in the operating state in the receiving-end converter 105 does not act when the flexible HVDC power transmission system 100 is in normal operation. Here, continuing to refer to Fig. 1, the resistor does not consume energy, i.e., the all-controllable power electronic switch in the damping branch 10511 is latched. Figure 1

[0059] In some embodiments of the application, if the master module identifies that the DC voltage in the flexible HVDC power transmission system 100 drops rapidly, resulting in a corresponding DC current rise, it is determined that a DC fault (a DC short-circuit fault occurs); at this time, the master module in the flexible HVDC power transmission system 100 first controls the damping branch 10511 in the half-bridge submodule 1051 in the operating state in the receiving-end converter 105 to act, i.e., controls the all-controllable power electronic switch in the damping branch 10511 in the half-bridge submodule 1051 in the receiving-end converter 105 that is put into operation to be turned on, the first switching device 10512 is latched, and at the same time, the all-controllable power electronic switch in the damping branch 10511 uses the original control logic of the first switching device 10512, so that the resistor in the damping branch 10511 is connected in series with the capacitor 10514 to limit the current, so as to greatly reduce the amplitude of the current that needs to be turned off by the DC circuit breaker 104; then, the master module controls the DC circuit breaker 104 to switch from the closed state to the open state to break the current, so as to clear the fault and restore the DC voltage to the rated value, so that the damping branch 10511 exits the operation. Here, since the DC circuit breaker 104 is not closed at this time, the flexible HVDC power transmission system 100 loses the power transmission capability, and because the new energy sending system 101 still transmits energy to the receiving-end AC power grid 106, the master module needs to be further used to control the AC energy consumption device 102 to be activated to be put into operation, so as to maintain the DC voltage in the flexible HVDC power transmission system 100 within a certain range, so as to realize the recovery of the power transmission capability of the flexible HVDC power transmission system 100, i.e., to complete the non-latching fault ride-through of the flexible HVDC power transmission system 100 under the DC fault.

[0060] ​It should be noted that if a short-circuit fault occurs in the receiving end AC power grid 106 of the flexible DC power transmission system 100, the voltage of the capacitor 10514 of the half-bridge submodule 1051 in the operating state in the receiving end converter 105 will rise. Here, if the main control module of the flexible DC power transmission system 100 detects that the voltage of the capacitor 10514 of the half-bridge submodule 1051 in the operating state in the receiving end converter 105 is higher than the set upper limit value, the main control module will control the fully controlled power electronic switch of the damping branch 10511 in the half-bridge submodule 1051 in the operating state in the receiving end converter 105 to be turned on, that is, to make the damping branch 10511 consume energy, so that the voltage of the capacitor 10514 of the half-bridge submodule 1051 is reduced. In addition, if the main control module detects that the voltage of the capacitor 10514 of the half-bridge submodule 1051 is lower than the set lower limit value, the main control module will correspondingly control the fully controlled power electronic switch of the damping branch 10511 to be turned off, that is, to stop consuming energy. The above process is repeated until the energy consumption process is completed.

[0061] In addition, as shown in Figure 2 , a schematic diagram of the receiving end converter 105 in the flexible DC power transmission system 100 provided by the above embodiment is given taking the full-half mixed damping enhanced MMC scheme as an example; wherein the receiving end converter 105 not only involves a plurality of half-bridge submodules 1051 with damping branches 10511, that is, DE-SM, but also involves a plurality of full-bridge submodules (FBSM). Similarly, the Figure 2 u A , u B , and u C respectively denote the A-phase voltage, the B-phase voltage, and the C-phase voltage in the receiving end converter 105.

[0062] It should be noted that for the full-half mixed damping enhanced MMC scheme, when the receiving end AC power grid of the flexible DC power transmission system in which the full-half mixed damping enhanced MMC scheme is located has an AC side fault, the damping branch of the half-bridge submodule in the operating state in the receiving end converter acts, and its working principle is similar to that of the half-bridge damping enhanced MMC, which will not be described here. For a DC fault, the damping branch of the half-bridge submodule in the operating state in the receiving end converter can first act to reduce the DC fault current rise rate, and then the full-bridge submodule in the operating state in the receiving end converter outputs a counter pressure to reduce the DC fault current.

[0063] That is, for the full and half mixed type damping enhanced MMC scheme, the main control module of the flexible DC power transmission system, when identifying that the flexible DC power transmission system 100 occurs DC fault (DC voltage drop and causes DC current to rise), controls the damping branch of the half bridge submodule in the running state in the full and half mixed type damping enhanced MMC to limit the current, so as to reduce the rising rate of the DC current; and in the case that the rising rate of the DC current is reduced to the set value, the damping branch is controlled to be cut out, and the full bridge submodule in the running state in the full and half mixed type damping enhanced MMC is correspondingly controlled to output counter pressure, so as to reduce the DC current of the flexible DC power transmission system. In addition, the composite switch disposed in the flexible DC power transmission system is further controlled to be opened (the composite switch corresponds to the DC circuit breaker in the half bridge type damping enhanced MMC scheme), so that the DC voltage is recovered; finally, in the case that the DC voltage exceeds the rated voltage value during the DC voltage recovery process, the AC energy consumption device is repeatedly controlled to be switched until the DC voltage is reduced and maintained in the set voltage range, so as to complete the complete clearance of the DC fault.

[0064] The flexible DC power transmission system provided by the embodiment of the application, when a DC fault occurs, the receiving converter composed of a plurality of half bridge submodules with damping branches, that is, the damping enhanced modular multilevel converter, limits the development of the fault current by locking the upper tube switch device in the half bridge submodule in the running state of the receiving converter to put in the damping branch, and cooperates with the DC circuit breaker and the AC energy consumption device to realize the non-blocking fault ride-through with low cost and high reliability. In this way, by cooperating the damping enhanced modular multilevel converter with the DC circuit breaker and the AC energy consumption device, the current under the DC fault is suppressed, so that the non-blocking fault ride-through under the DC fault is realized.

[0065] Embodiment 2:

[0066] Based on the same inventive concept, the embodiment of the application also provides a fault ride-through method of a flexible DC power transmission system, which is applied to the flexible DC power transmission system 100 provided by any one of the above embodiments. Figure 3 As shown in the figure, the flowchart of the fault ride-through method of the flexible DC power transmission system provided by the embodiment of the application, the method comprises the following steps:

[0067] Step 301, when the main control module identifies that a DC fault occurs, the main control module controls the damping branch of the half bridge submodule in the running state to be put in, and controls the DC circuit breaker and the AC energy consumption device to act, so as to suppress the DC current and adjust the DC voltage to the set voltage range, so as to complete the non-blocking fault ride-through.

[0068] In some embodiments of the present application, the "the master control module controls the damping branch of the half-bridge submodule in the running state to be put into operation, and controls the DC circuit breaker and the AC energy consumption device to act to suppress the DC current and adjust the DC voltage to be within the set voltage range, to complete the non-locking fault ride through" in the above step 301 can be realized by the following steps 3011 to 3013: Figure 3

[0069] Step 3011, the master control module controls the damping branch of the half-bridge submodule in the running state to be put into operation to limit the current, so as to reduce the rising rate of the DC current.

[0070] Step 3012, when the rising rate of the DC current is reduced to a set value, the master control module controls the damping branch to be cut out and controls the state of the DC circuit breaker to be switched from closed to open, so as to restore the DC voltage.

[0071] Step 3013, when the DC voltage exceeds the rated voltage value during the recovery of the DC voltage, the master control module controls the AC energy consumption device to be repeatedly put into and cut out until the DC voltage is reduced and maintained within the set voltage range, to complete the non-locking fault ride through.

[0072] In some embodiments of the present application, for the above step 3011, it can be specifically realized by the following way:

[0073] The master control module controls the first switching device of the half-bridge submodule to be locked and controls the fully controlled power electronic switch of the damping branch to be turned on, so that the fully controlled power electronic switch adopts the control logic of the first switching device, to reduce the rising rate of the DC current.

[0074] Correspondingly, the above step 3012 can be specifically realized by the following way:

[0075] When the rising rate of the DC current is reduced to the set value, the master control module controls the fully controlled power electronic switch to be locked to cut out the damping branch, and controls the state of the DC circuit breaker to be switched from closed to open, so as to restore the DC voltage.

[0076] And, as for the "the master control module controls the AC energy consumption device to be repeatedly put into and cut out until the DC voltage is reduced and maintained within the set voltage range, to complete the non-locking fault ride through" in the above step 3013, it can be further realized by the following process:

[0077] First, the master control module controls the AC energy consumption device to be repeatedly put into and cut out until the DC voltage is reduced and maintained within the set voltage range. ​

[0078] Then, when the DC voltage is maintained within the set voltage range, the main control module controls the state of the DC circuit breaker to switch from unclosed to reclosed, and controls the AC energy dissipation device to be completely disconnected, so that the flexible DC transmission system can restore its power transmission capability and thus complete the fault ride-through without blocking.

[0079] In some embodiments of the present invention, such as Figure 4 As shown, U dc I represents the DC voltage of the flexible DC transmission system. dc T represents the DC current of the flexible DC transmission system. control I represents the control signal of the fully controlled power electronic switch in the damping branch of a half-bridge submodule in a flexible DC transmission system that is in operation. R T represents the current flowing through the resistor in the damping branch of the half-bridge submodule during operation. ACC This represents the control signal for AC energy dissipation devices in a flexible DC transmission system. Correspondingly, the fault ride-through process of the flexible DC transmission system fault ride-through method provided in this embodiment of the invention under a DC fault is as follows:

[0080] At time t0: The main control module of the flexible DC transmission system detects a fault in the DC line of the flexible DC transmission system, that is, the DC voltage of the flexible DC transmission system drops rapidly and the DC current rises rapidly.

[0081] At time t1: When the main control module detects that the DC current of the flexible DC transmission system has increased and reached the set value, it controls the damping branch of the half-bridge submodule in the flexible DC transmission system to operate. Here, taking a single-phase MMC as an example, the basic principle of the MMC with a damping branch in its internal half-bridge submodule suppressing DC fault short-circuit current is as follows: Figure 5 As shown, in the initial stage of a DC fault, the discharge current of capacitor C in the half-bridge submodule is dominant. The discharge current of capacitor C in the half-bridge submodule can be controlled by adjusting the resistance R connected in series with the damping branch. For the half-bridge submodule in the MMC, there are two states during normal operation. When the half-bridge submodule is engaged, the first switching device T1 of the half-bridge submodule is turned on, and the second switching device T2 is locked. When the half-bridge submodule is disconnected, T2 is turned on, and T1 is locked. Moreover, regardless of the state during normal operation, the fully controlled power electronic switch T3 in the damping branch is locked.

[0082] When the main control module detects a DC fault in the flexible DC transmission system, for the half-bridge submodules that are in operation, T1 is blocked, T3 is turned on, and T2 is blocked; for the disconnected half-bridge submodules, T3 is blocked, T2 is turned on, and T1 is blocked. Through these operations, the rate of increase of the DC short-circuit current is limited to a low level.

[0083] It should be noted that, Figure 5 The MMC is exemplified by one input half-bridge submodule and one cut-off half-bridge submodule.

[0084] At t2, the DC circuit breaker of the flexible DC power transmission system breaks the current, the fault is cleared, the DC voltage returns to the rated value, and the damping circuit in the half-bridge submodule in the running state is out of operation. However, since the DC circuit breaker is not closed at this time, the system loses the power transmission capability, and the new energy sending system side still transmits energy to the receiving end AC power grid, so the capacitor of the half-bridge submodule is charged, U dc continuously rises.

[0085] At t3, U dc reaches the upper limit of the DC voltage, at which time the AC energy consumption device in the flexible DC power transmission system is activated and put into operation. When the AC energy consumption device is put into operation, the DC voltage decreases, and when U dc reaches the lower limit of the DC voltage, the AC energy consumption device is out of operation, and the above stages are repeated thereafter to maintain the DC voltage within a certain range.

[0086] At t4, the DC circuit breaker recloses, the system restores the power transmission capability, and the AC energy consumption device is exited.

[0087] It should be noted that in the actual application of the flexible DC power transmission system, that is, in the system fault ride-through process, the receiving end converter will not only face the problem of system overcurrent caused by DC line short-circuit fault (DC fault), but also face the problem of system overvoltage caused by surplus power after the receiving end AC line fault, that is, AC fault, which needs to be quickly absorbed.

[0088] As a person skilled in the art, it should be known that: in view of the overvoltage suppression challenge of the flexible DC power transmission system, the existing engineering scheme mainly includes two types, one is to additionally configure an energy consumption device outside the MMC to absorb power, and the other is to additionally configure a controllable arrester outside the MMC to suppress overvoltage for a short time. However, the energy consumption device and the controllable arrester are both composed of a large number of power electronic switches and are independently arranged, which limits their further development and application. In addition, the additional installation of DC circuit breakers, energy consumption devices and other equipment outside the MMC will bring a sharp increase in engineering cost and land area. Based on this, the fault ride-through method of the flexible DC power transmission system provided in the embodiment of the present application can perform the following step A after step 301 is executed, that is, after the non-blocking fault ride-through under the DC fault is completed.

[0089] Step A, when the main control module identifies that an AC fault occurs, the main control module controls the damping branch of the half-bridge submodule in the running state to act, adjusts the DC voltage to be within the set voltage range, to complete the non-blocking fault ride-through.

[0090] In some embodiments of the present invention, the step A, "the main control module controls the damping branch of the half-bridge submodule in operation to adjust the DC voltage to the set voltage range in order to complete the non-blocking fault ride-through," can be achieved by the following steps A1 and A2:

[0091] Step A1: The main control module repeatedly controls the fully controlled power electronic switch of the damping branch to turn on and off, so that the resistance of the damping branch repeatedly consumes energy, thereby reducing the DC voltage.

[0092] Step A2: When the DC voltage drops to the set voltage range, control the damping branch to be completely disconnected to complete the non-blocking fault crossing.

[0093] Here you can refer to Figure 6 The diagram illustrates the non-blocking ride-through process of the flexible DC transmission system fault ride-through method provided in this embodiment of the invention under an AC fault; where U dc P represents the DC voltage of the flexible DC transmission system. send P represents the output power of the flexible DC transmission system. out P represents the output power of the flexible DC transmission system. surplus T represents the surplus power of the flexible DC transmission system. outer_loop and T inner_loop For the outer loop control signal and the inner loop control signal of the damping circuit in the half-bridge submodule in operation, respectively, I R This represents the current flowing through the resistance in the damping branch of the half-bridge submodule during operation. Correspondingly, the fault ride-through process of the flexible DC transmission system provided in this embodiment of the invention under AC fault conditions is as follows:

[0094] Before time t0, the flexible DC transmission system is operating normally. The damping branch inside the half-bridge submodule of the flexible DC transmission system is not working, that is, the receiving-end converter of the flexible DC transmission system is operating according to the normal control logic.

[0095] At time t0: The AC side voltage of the flexible DC transmission system drops rapidly, and the output power P of the flexible DC transmission system decreases. send Capacity decline, surplus power P surplus Charging the capacitors in the system causes the DC voltage of the flexible DC transmission system to rise, which in turn leads to an AC fault. When the capacitor voltage of the flexible DC transmission system rises to a set limit (correspondingly, the capacitor voltage of the half-bridge submodule in operation within the flexible DC transmission system is higher than the set upper limit), the damping branch of the half-bridge submodule in operation begins to work.

[0096] t1 moment: the outer loop control signal changes from 0 to 1, the damping branch of the half-bridge submodule in the running state acts, it is gradually put into, thereby causing the capacitor voltage of the half-bridge submodule in the running state to drop, and further the DC voltage of the flexible DC power transmission system also drops, when all the damping branches in the bridge arm of the receiving end converter of the flexible DC power transmission system are put into, the DC voltage of the flexible DC power transmission system drops to the minimum.

[0097] t2 moment: the damping branch gradually exits, and the DC voltage of the flexible DC power transmission system gradually rises.

[0098] t3 moment: the damping branch completely exits.

[0099] After that, the above process is repeated, and the DC voltage of the flexible DC power transmission system will also fluctuate within the rated value range.

[0100] t4 moment: the AC side voltage of the flexible DC power transmission system starts to recover, the surplus power of the flexible DC power transmission system gradually decreases, and a power overshoot will occur at the moment of power recovery of the onshore converter station, causing the DC voltage of the flexible DC power transmission system to drop below the rated value, at this time the outer loop control signal changes to 0, the damping branch no longer acts, the DC voltage of the flexible DC power transmission system gradually recovers to the vicinity of the rated value, and the flexible DC power transmission system operates normally.

[0101] t5 moment: the damping branch exits, the AC fault is recovered, and the flexible DC power transmission system operates normally.

[0102] Based on the above description, the embodiment of the application provides a damping enhanced (internal half-bridge submodule carrying damping branch) MMC scheme, the damping branch is integrated in the MMC half-bridge submodule, the current limiting and energy consumption functions are realized at the same time, and a low-capacity DC circuit breaker is configured to realize the rapid clearing and restarting of the system fault. Here, the damping branch can be composed of a welded IGBT and a resistor and is configured in all submodules of the MMC. In this way, the damping branch can share the energy supply, control and protection systems with the MMC, and the damping branch does not act when the flexible DC power transmission system operates normally, independent control logic is adopted, and the operation logic and characteristics of the flexible DC power transmission system are not affected.

[0103] That is, based on the flexible DC power transmission system provided in the above embodiment, when the main control module detects that there is a DC fault overcurrent (rapid drop of DC voltage) inside the flexible DC power transmission system, the damping branch of the half-bridge submodule in the operating state in the MMC is put into operation to limit the current, and the low-capacity DC circuit breaker is further matched to break the current and the AC energy consumption device, so that the non-blocking fault ride-through under DC fault with low cost and high reliability is realized. At the same time, when the main control module detects that the DC voltage inside the flexible DC power transmission system rises, the damping branch of the half-bridge submodule in the operating state in the MMC is put into operation to limit the voltage, so as to control the fluctuation range of the capacitor voltage of the half-bridge submodule in the operating state in the MMC. In this way, the damping branch realizes the multiplexing of current limiting and energy consumption functions, that is, the fault ride-through method of the flexible DC power transmission system provided in the embodiment can greatly reduce the fault ride-through cost of the new energy sending system, and has high reliability.

[0104] Correspondingly, the fault ride-through of the flexible DC power transmission system provided in the embodiment can further perform the following step B after step 301 is performed, that is, after the non-blocking fault ride-through is completed:

[0105] When the main control module identifies normal operation, the main control module keeps the damping branch inoperative, controls the state of the DC circuit breaker to be closed, and controls the AC energy consumption device to be in an inactive state.

[0106] The fault ride-through method of the flexible DC power transmission system provided in the embodiment, when a DC fault occurs, uses the receiving converter composed of a plurality of half-bridge submodules with damping branches, that is, the damping enhanced modular multilevel converter, to block the upper tube switching device in the half-bridge submodule in the operating state in the receiving converter to put the damping branch into operation, limit the development of the fault current, and cooperate with the deployed DC circuit breaker to break the current and the AC energy consumption device, so that the non-blocking fault ride-through with low cost and high reliability is realized. In this way, by cooperating the damping enhanced modular multilevel converter with the DC circuit breaker and the AC energy consumption device, the current under DC fault is suppressed, so that the non-blocking fault ride-through under DC fault is realized.

[0107] Based on the above description, the embodiments of the present application provide a flexible HVDC transmission system and a corresponding fault ride-through method, that is, a new energy VSC-HVDC system using a damping enhanced MMC, a DC circuit breaker and an AC energy consumption device, which can realize the control method of the AC and DC fault ride-through process; wherein the damping enhanced MMC scheme in the embodiments of the present application integrates a damping branch in the MMC half-bridge sub-module, the damping branch shares the energy supply and water cooling system with the MMC sub-module, and can simultaneously realize the functions of fault current limiting and overvoltage suppression, cooperates with the low-cost DC circuit breaker and the AC energy consumption device, and can provide a low-cost solution for the DC fault ride-through problem. At the same time, only the damping enhanced MMC scheme can be used to provide a low-cost solution for the AC fault ride-through problem.

[0108] In other words, the damping enhanced MMC can realize the suppression of current under DC fault and the absorption of surplus power under AC fault by adding an energy consumption module in the MMC sub-module, and cooperates with the low-cost DC circuit breaker to realize the removal of DC fault, which further illustrates the effectiveness of the scheme provided by the embodiments of the present application.

[0109] Therefore, the embodiments of the present application propose a damping enhanced MMC topology to solve the defects of the existing DC fault ride-through scheme requiring a large-capacity and expensive DC circuit breaker and the AC fault ride-through scheme requiring an independent energy consumption device, installs a damping branch in the original half-bridge sub-module, and realizes the damping of the damping branch under the conditions of system overvoltage and overcurrent, realizes the suppression of fault current and the absorption of surplus power. The technical scheme provided by the embodiments of the present application adopts the damping branch and the sub-module to share the water cooling, energy supply and other systems, and adopts the welding type IGBT and the resistor to constitute the damping branch, which provides an economical and effective solution for the fault ride-through of the new energy transmission system. The damping enhanced MMC absorbs the surplus power through the damping branch and the lower tube IGBT to ensure that the DC voltage meets the system demand during the fault period. The damping enhanced MMC limits the development of the fault current by locking the upper tube IGBT and inputting the damping branch, and cooperates with the low-capacity DC circuit breaker to break the current, which has low cost and high reliability.

[0110] That is, the embodiments of the present application can provide a new AC and DC fault ride-through method and related device, which forms a damping enhanced MMC by adding a damping branch in the MMC half-bridge sub-module, the damping branch shares the water cooling, control and protection system with the MMC, and only needs to configure a low-cost DC circuit breaker and an AC energy consumption device, which is more mature, has higher reliability and lower cost than the existing scheme; wherein the damping branch is input according to the overvoltage and overcurrent characteristics of the system to realize the suppression of fault current and the absorption of surplus power, and does not change the original control logic of the MMC.

[0111] Embodiment 2:

[0112] Based on the same inventive concept, the present application also provides a fault ride-through system 700 of a flexible DC power transmission system, which is applied to the flexible DC power transmission system provided by the above-mentioned embodiments, as shown in Figure 7 The system 700 comprises:

[0113] An identification unit 701 is configured to identify whether a DC fault occurs.

[0114] A control unit 702 is configured to, when it is identified that the DC fault occurs, control the damping branch of the half-bridge sub-module in the running state to be put into operation, and control the DC circuit breaker and the AC energy consumption device to act to suppress the DC current and adjust the DC voltage to be within a set voltage range, so as to complete the non-locking fault ride-through.

[0115] It should be noted that the description of the system is similar to the description of the above-mentioned method embodiments, and has similar beneficial effects as the method embodiments. For technical details not disclosed in the system embodiments of the present application, please refer to the description of the method embodiments of the present application for understanding.

[0116] The basic principles, main features and advantages of the present application are shown and described above. It should be understood by those skilled in the art that the present application is not limited by the above-mentioned embodiments, and the above-mentioned embodiments and descriptions in the specification are only the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection claimed by the present application is defined by the appended claims and their equivalents.

[0117] That is, the above is only an embodiment of the present application and is not used to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application is included in the scope of the claims of the present application.

Claims

1. A flexible direct current power transmission system, characterized by The application relates to a flexible DC power transmission system, which comprises a master control module and sequentially connected new energy sending-out system, AC energy consumption device, sending-end converter, DC circuit breaker, receiving-end converter and receiving-end AC power grid. The DC circuit breaker is arranged on a DC transmission line between the sending-end converter and the receiving-end converter, and the receiving-end converter is a modular multilevel converter composed of a plurality of half-bridge sub-modules with damping branches. The master control module is in communication connection with the plurality of half-bridge sub-modules, the DC circuit breaker and the AC energy consumption device, and is used for controlling the damping branches of the half-bridge sub-modules in operation, the DC circuit breaker and the AC energy consumption device to act when a DC fault occurs, so as to suppress the DC current and adjust the DC voltage to a set voltage range, thereby completing the fault ride-through without blocking. The half-bridge sub-module further comprises a first switching device, a second switching device and a capacitor; the first switching device and the second switching device are connected in series and are connected in parallel with the capacitor; the damping branch is connected in parallel across the first switching device; and the damping branch comprises a fully-controlled power electronic switch and a resistor connected in series.

2. The flexible DC power transmission system of claim 1, wherein, The application is applied to the flexible DC power transmission system as claimed in any one of claims 1 to 2.

3. A method of fault ride-through for a flexible DC power transmission system, characterized in that, When the master control module identifies that a DC fault occurs, the master control module controls the damping branches of the half-bridge sub-modules in operation to be put into operation, and controls the DC circuit breaker and the AC energy consumption device to act, so as to suppress the DC current and adjust the DC voltage to a set voltage range, thereby completing the fault ride-through without blocking. The master control module controls the damping branches of the half-bridge sub-modules in operation to be put into operation, and controls the DC circuit breaker and the AC energy consumption device to act, so as to suppress the DC current and adjust the DC voltage to a set voltage range, thereby completing the fault ride-through without blocking.

4. The method of claim 3, wherein, The master control module controls the damping branches of the half-bridge sub-modules in operation to be put into operation, and controls the DC circuit breaker and the AC energy consumption device to act, so as to suppress the DC current and adjust the DC voltage to a set voltage range, thereby completing the fault ride-through without blocking. The master control module controls the damping branches of the half-bridge sub-modules in operation to be put into operation, and controls the DC circuit breaker and the AC energy consumption device to act, so as to suppress the DC current and adjust the DC voltage to a set voltage range, thereby completing the fault ride-through without blocking. The master control module controls the damping branches of the half-bridge sub-modules in operation to be put into operation, and controls the DC circuit breaker and the AC energy consumption device to act, so as to suppress the DC current and adjust the DC voltage to a set voltage range, thereby completing the fault ride-through without blocking. The master control module controls the damping branches of the half-bridge sub-modules in operation to be put into operation, and controls the DC circuit breaker and the AC energy consumption device to act, so as to suppress the DC current and adjust the DC voltage to a set voltage range, thereby completing the fault ride-through without blocking.

5. The method of claim 4, wherein, The master control module controls the first switching device of the half-bridge sub-module to be blocked and controls the fully-controlled power electronic switch of the damping branch to be turned on, so that the fully-controlled power electronic switch adopts the control logic of the first switching device, thereby reducing the rising rate of the DC current. ​ Correspondingly, when the rising rate of the direct current decreases to the set value, the main control module controls the full-control power electronic switch to cut out the damping branch, and controls the state of the direct current circuit breaker to switch from the closed state to the open state, so that the direct current voltage is recovered. When the rising rate of the direct current decreases to the set value, the main control module controls the full-control power electronic switch to cut out the damping branch, and controls the state of the direct current circuit breaker to switch from the closed state to the open state, so that the direct current voltage is recovered.

6. The method of claim 4, wherein, The main control module controls the AC energy consumption device to be repeatedly switched in and out until the direct current voltage decreases and is maintained within the set voltage range, so as to complete the non-locking fault ride-through. The main control module controls the AC energy consumption device to be repeatedly switched in and out until the direct current voltage decreases and is maintained within the set voltage range. When the direct current voltage is maintained within the set voltage range, the main control module controls the state of the direct current circuit breaker to switch from the open state to the closed state, and controls the AC energy consumption device to be completely cut out, so that the flexible direct current power transmission system restores the power transmission capability, thereby completing the non-locking fault ride-through.

7. The method of claim 3, wherein, After the non-locking fault ride-through is completed, the method further comprises: When the main control module identifies that an AC fault occurs, the main control module controls the damping branch of the half-bridge submodule in the running state to act, and adjusts the direct current voltage to be within the set voltage range, so as to complete the non-locking fault ride-through.

8. The method of claim 7, wherein, The main control module controls the damping branch of the half-bridge submodule in the running state to act, and adjusts the direct current voltage to be within the set voltage range, so as to complete the non-locking fault ride-through, comprising: The main control module repeatedly controls the full-control power electronic switch of the damping branch to be turned on and turned off, so that the resistance of the damping branch repeatedly consumes energy, thereby reducing the direct current voltage; When the direct current voltage decreases to the set voltage range, the damping branch is controlled to be completely cut out, so as to complete the non-locking fault ride-through.

9. The method of claim 3, wherein, After the non-locking fault ride-through is completed, the method further comprises: When the main control module identifies that a normal operation occurs, the main control module keeps the damping branch from acting, controls the state of the direct current circuit breaker to be closed, and controls the AC energy consumption device to be in an inactive state.

10. A fault ride-through system for a flexible DC power transmission system, characterized in that, The method is applied to the flexible direct current power transmission system according to any one of claims 1 to 2, and the system comprises: An identification unit is configured to identify whether a direct current fault occurs; A control unit is configured to, when it is identified that a direct current fault occurs, control the damping branch of the half-bridge submodule in the running state to be put into operation, and control the direct current circuit breaker and the AC energy consumption device to act, so as to suppress the direct current and adjust the direct current voltage to be within a set voltage range, thereby completing the non-locking fault ride-through.

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