Fault ride-through method and device for offshore wind power through hybrid commutation delivery system

Abstract

The invention discloses a fault ride-through method and device for an offshore wind power output system through a hybrid commutation, and belongs to the field of offshore wind power transmission, and the method comprises the steps: recognizing a fault type according to a voltage fluctuation abnormal event; when it is identified that the fault type is a shore AC power grid fault, starting a DC energy consumption device preset at a receiving end MMC converter to consume active power; when the fault type is identified as an offshore alternating current submarine cable fault, disconnecting an alternating current circuit breaker connected with the failed offshore alternating current submarine cable in the offshore wind power plant, and controlling the switching state of a diode rectifier and an auxiliary current converter; when the fault type is identified as the high-voltage direct-current bus fault, locking the auxiliary converter; and starting a direct-current circuit breaker preset at the receiving-end MMC current converter, so that the direct-current circuit breaker cuts off the connection between the receiving-end MMC current converter and the high-voltage direct-current bus. Therefore, by implementing the method, the technical problem that the fault ride-through method in the prior art is extensive and single can be solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of offshore wind power transmission, in particular to a fault ride-through method and device for offshore wind power through a hybrid converter transmission system. BACKGROUND

[0002] At present, offshore wind power is developing rapidly in the direction of large-scale and long-distance transmission. The hybrid transmission system based on diode rectifier and auxiliary converter (hereinafter referred to as "hybrid converter transmission system") has great advantages in cost and reliability.

[0003] However, in the prior art, the fault ride-through method for such hybrid converter transmission system is usually rough and single. No matter the system encounters shore AC grid fault, offshore AC cable fault, or high-voltage DC bus fault, the existing method often adopts a unified and conservative control strategy, such as immediately locking all converter devices. However, blindly locking the converter device will greatly reduce the power supply reliability and utilization rate of offshore wind power, and cannot meet the actual needs of safe and stable transmission of large-scale offshore wind power. SUMMARY

[0004] The present application provides a fault ride-through method and device for offshore wind power through a hybrid converter transmission system, which can solve the technical problems of rough and single fault ride-through method in the prior art.

[0005] In order to solve the above technical problems, an embodiment of the present application provides a fault ride-through method based on offshore wind power through a hybrid converter transmission system, comprising: When it is determined that a voltage fluctuation abnormal event occurs in the hybrid converter transmission system, the fault type is identified according to the voltage fluctuation abnormal event; wherein the hybrid converter transmission system comprises a diode rectifier, an auxiliary converter, a high-voltage DC bus and a receiving-end MMC converter; When it is identified that the fault type is a shore AC grid fault, a pre-set DC energy dissipation device at the receiving-end MMC converter is enabled, so that the DC energy dissipation device consumes the active power generated by the offshore wind farm during the voltage fluctuation abnormal event; When it is identified that the fault type is an offshore AC cable fault, the AC circuit breaker connected with the offshore AC cable where the fault occurs in the offshore wind farm is disconnected, and the on-off state of the diode rectifier and the auxiliary converter is controlled; When it is identified that the fault type is a high-voltage DC bus fault, the auxiliary converter is locked; a pre-set DC circuit breaker at the receiving-end MMC converter is enabled, so that the DC circuit breaker cuts off the connection between the receiving-end MMC converter and the high-voltage DC bus.

[0006] Further, the determination process of the voltage fluctuation abnormal event comprises: When the AC voltage at the PCC point is detected to be out of the first normal fluctuation range, it is determined that a voltage fluctuation abnormal event occurs in the hybrid current transmission system; The PCC point is a common connection point of the offshore wind farm, the diode rectifier, and the auxiliary converter.

[0007] Further, the identifying the fault type according to the voltage fluctuation abnormal event comprises: When the AC voltage at the PCC point is detected to be out of the upper limit of the first normal fluctuation range, and the HVDC bus voltage is detected to be out of the upper limit of the second normal fluctuation range, it is identified that the fault type is an onshore AC grid fault; When the AC voltage at the PCC point is detected to be out of the lower limit of the first normal fluctuation range, and the HVDC bus voltage is detected to be out of the lower limit of the second normal fluctuation range, it is identified that the fault type is an HVDC bus fault; When the AC voltage at the PCC point is detected to be out of the lower limit of the first normal fluctuation range, and the HVDC bus voltage is detected to be out of the lower limit of the second normal fluctuation range, it is identified that the fault type is an onshore AC grid fault;

[0008] Further, the offshore AC submarine cable comprises branch submarine cables; the offshore wind farm is connected to the diode rectifier and the auxiliary converter through the branch submarine cables and corresponding AC breakers, respectively; The method further comprises: When the fault type is identified as the offshore AC submarine cable fault, and it is determined that the branch submarine cable is faulty, the branch submarine cable with the fault point is taken as a target branch submarine cable; the AC breaker connected to the target branch submarine cable is disconnected, and the diode rectifier and the auxiliary converter are maintained in the on state.

[0009] Further, the offshore AC submarine cable further comprises a collection cable; all the branch submarine cables are connected to the collection cable; The method further comprises: When the fault type is identified as the offshore AC submarine cable fault, and it is determined that the collection cable is faulty, all the AC breakers are disconnected, and the diode rectifier and the auxiliary converter are switched from the on state to the blocking state.

[0010] An embodiment of the present application further provides a fault ride-through device based on a hybrid current transmission system of offshore wind power, comprising: a fault type identification module, configured to identify a fault type according to the voltage fluctuation abnormal event when it is determined that the voltage fluctuation abnormal event occurs in the hybrid converter transmission system; wherein the hybrid converter transmission system comprises a diode rectifier, an auxiliary converter, a high-voltage direct-current bus and a receiving-end MMC converter; a first fault ride-through module, configured to enable a direct-current energy dissipation device preset at the receiving-end MMC converter when it is identified that the fault type is an onshore alternating-current power grid fault, so that the direct-current energy dissipation device consumes active power generated by the offshore wind farm during the voltage fluctuation abnormal event; a second fault ride-through module, configured to disconnect an alternating-current circuit breaker connected to a faulty offshore alternating-current submarine cable in the offshore wind farm, and control the switching state of the diode rectifier and the auxiliary converter when it is identified that the fault type is an offshore alternating-current submarine cable fault; a third fault ride-through module, configured to lock the auxiliary converter, and enable a direct-current circuit breaker preset at the receiving-end MMC converter when it is identified that the fault type is a high-voltage direct-current bus fault, so that the direct-current circuit breaker cuts off the connection between the receiving-end MMC converter and the high-voltage direct-current bus.

[0011] Further, the determination process of the voltage fluctuation abnormal event comprises: determining that the voltage fluctuation abnormal event occurs in the hybrid converter transmission system when it is detected that the alternating-current voltage at the PCC point exceeds the first normal fluctuation range; wherein the PCC point is a common connection point of the offshore wind farm, the diode rectifier and the auxiliary converter.

[0012] Further, the identification of the fault type according to the voltage fluctuation abnormal event comprises: identifying that the fault type is an onshore alternating-current power grid fault when it is detected that the alternating-current voltage at the PCC point exceeds the upper limit of the first normal fluctuation range, and that the high-voltage direct-current bus voltage exceeds the upper limit of the second normal fluctuation range; identifying that the fault type is a high-voltage direct-current bus fault when it is detected that the alternating-current voltage at the PCC point exceeds the lower limit of the first normal fluctuation range, and that the high-voltage direct-current bus voltage exceeds the lower limit of the second normal fluctuation range; identifying that the fault type is an offshore alternating-current submarine cable fault when it is detected that the alternating-current voltage at the PCC point exceeds the lower limit of the first normal fluctuation range, and that the high-voltage direct-current bus voltage does not exceed the second normal fluctuation range.

[0013] Further, the offshore alternating-current submarine cable comprises branch submarine cables; and the offshore wind farm is connected to the diode rectifier and the auxiliary converter through the branch submarine cables and corresponding alternating-current circuit breakers, respectively. the AC circuit breaker connected with the offshore AC submarine cable where the fault occurs is disconnected, and the on-off state of the diode rectifier and the auxiliary converter is controlled, when the fault type is identified as the offshore AC submarine cable fault, comprising: when the fault type is identified as the offshore AC submarine cable fault, and it is determined that the branch submarine cable where the fault point exists is the target branch submarine cable, the AC circuit breaker connected with the target branch submarine cable is disconnected, and the on state of the diode rectifier and the auxiliary converter is maintained.

[0014] Further, the offshore AC submarine cable further comprises a collection cable; all the branch submarine cables are connected to the collection cable. the AC circuit breaker connected with the offshore AC submarine cable where the fault occurs is disconnected, and the on-off state of the diode rectifier and the auxiliary converter is controlled, when the fault type is identified as the offshore AC submarine cable fault, comprising: when the fault type is identified as the offshore AC submarine cable fault, and it is determined that the branch submarine cable where the fault point exists is the target branch submarine cable, the AC circuit breaker connected with the target branch submarine cable is disconnected, and the on state of the diode rectifier and the auxiliary converter is maintained.

[0015] By implementing the present application, the following beneficial effects are achieved: The present application provides a kind of offshore wind power hybrid converter transmission system fault ride-through method and device, the method is when determining that hybrid converter transmission system occurs voltage fluctuation abnormal event, according to the voltage fluctuation abnormal event, identify fault type;When the fault type is identified as the onshore AC power grid fault, enable the DC energy consumption device preset at the receiving end MMC converter, to make the DC energy consumption device consume the active power generated in the process of voltage fluctuation abnormal event of offshore wind farm;Therefore, by immediately enabling the receiving end DC energy consumption device, surplus active power is consumed in time, effectively suppresses the DC bus overvoltage, protects diode rectifier and DC submarine cable from electrical stress impact;When the fault type is identified as offshore AC submarine cable fault, the AC circuit breaker connected with the offshore AC submarine cable where the fault occurs in offshore wind farm is disconnected, and the on-off state of the diode rectifier and the auxiliary converter is controlled;When the fault type is identified as high-voltage DC bus fault, the auxiliary converter is blocked;Enable the DC circuit breaker preset at the receiving end MMC converter, to make the DC circuit breaker cut off the connection between the receiving end MMC converter and the high-voltage DC bus;Therefore, by blocking the auxiliary converter and quickly cutting off the DC circuit breaker, the fault is quickly removed, and the accident is prevented from expanding; Thus, by classifying the fault types of the hybrid converter transmission system, corresponding fault ride-through methods are set for the three typical fault types of the onshore AC power grid fault, the offshore AC submarine cable fault and the high-voltage DC bus fault, precise control and fault isolation are realized, the fault influence range is minimized, and the grid-connected power generation time of the wind farm and the availability of the entire transmission system are significantly improved. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the present application, the drawings needed 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 other drawings can also be obtained by those skilled in the art without creative effort on the basis of these drawings.

[0017] Figure 1 is a flowchart of the fault ride-through method based on the offshore wind power through the hybrid converter transmission system provided by an embodiment of the present application; Figure 2 is a structural schematic diagram of the hybrid converter transmission system provided by an embodiment of the present application; Figure 3 is a voltage monitoring and control schematic diagram provided by an embodiment of the present application; Figure 4 is a schematic diagram of the onshore AC power grid fault provided by an embodiment of the present application; Figure 5 is a schematic diagram of the offshore AC submarine cable fault provided by an embodiment of the present application; Figure 6 is a schematic diagram of the high-voltage DC bus fault provided by an embodiment of the present application; Figure 7 is a schematic diagram of the fault ride-through simulation result of the onshore AC power grid fault provided by an embodiment of the present application; Figure 8 is a schematic diagram of the fault ride-through simulation result of the offshore AC submarine cable fault provided by an embodiment of the present application; Figure 9 is a structural schematic diagram of the fault ride-through device based on the offshore wind power through the hybrid converter transmission system provided by an embodiment of the present application; In the drawings of the specification, the reference signs are as follows: Diode rectifier 1, auxiliary converter 2, high-voltage DC bus 3, receiving end MMC converter 4. DETAILED DESCRIPTION

[0018] In order to make the objects, technical solutions and advantages of the present application clearer, the following will clearly and completely describe the technical solutions in the present application with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0019] 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 this application belongs; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this application; the terms "comprising," "comprises" and "including" and "has" and any variations thereof used herein are intended to cover a non-exclusive inclusion.

[0020] In the description of the embodiments of the present application, the technical terms "first", "second" and the like are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "a plurality of" is two or more, unless otherwise explicitly and specifically limited.

[0021] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the application. The appearance of the phrase in various places in the specification does not necessarily all refer to the same embodiment, or necessarily alternatives to other embodiments. It will be explicitly and implicitly appreciated by a person of ordinary skill in the art that the embodiments described herein can be combined with other embodiments.

[0022] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone. In addition, the character " / " in the present application generally represents an "or" relationship between the front and rear associated objects.

[0023] In the description of the embodiments of the present application, the term "a plurality of" refers to two or more (including two), and similarly, "a plurality of groups" refers to two or more groups (including two groups), and "a plurality of pieces" refers to two or more pieces (including two pieces).

[0024] In the description of the embodiments of the present application, unless otherwise explicitly specified and limited, the technical terms "mounting", "connection", "connecting", "fixing" and the like should be understood in a broad sense, for example, can be fixed connection, can also be detachable connection, or integral; can be mechanical connection, can also be electrical connection; can be direct connection, can also be indirect connection through an intermediate medium, can be internal communication of two elements or interaction relationship between two elements. For those skilled in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0025] Referring to Figure 1 To solve the technical problems of the prior art fault ride-through method, an embodiment of the present application provides a fault ride-through method based on a hybrid converter transmission system for offshore wind power, comprising: S1, when determining that a voltage fluctuation abnormal event occurs in the hybrid converter transmission system, identifying the fault type according to the voltage fluctuation abnormal event; wherein the hybrid converter transmission system comprises a diode rectifier 1, an auxiliary converter 2, a high-voltage DC bus 3 and a receiving-end MMC converter 4; Illustratively, referring to Figure 2 is a specific structure schematic diagram of the hybrid converter transmission system described in the embodiment; the hybrid converter transmission system connects an offshore wind farm and an onshore AC power grid, and the purpose is to transmit the power generated by the offshore wind farm to the onshore AC power grid through the hybrid converter transmission system, so as to realize the grid connection of the generated wind power; one end of the diode rectifier 1 is connected with the offshore wind farm, and the other end of the diode rectifier 1 is connected with one end of the receiving-end MMC converter 4 through the high-voltage DC bus 3; the auxiliary converter 2 is connected with the offshore wind farm, and the other end of the receiving-end MMC converter 4 is connected with the onshore AC power grid.

[0026] Illustratively, referring to Figure 3 In the embodiment, the AC voltage of the point of common coupling (PCC point) of the hybrid converter transmission system is detected and the high-voltage DC bus 3 voltage to determine whether a voltage fluctuation abnormal event occurs in the hybrid converter transmission system: In one preferred embodiment, the voltage fluctuation abnormal event determination process comprises: When the AC voltage of the PCC point is detected to be out of the first normal fluctuation range, it is determined that a voltage fluctuation abnormal event occurs in the hybrid converter transmission system; Wherein, the PCC point is the common connection point of the offshore wind farm, the diode rectifier 1 and the auxiliary converter 2; Specifically, the first normal fluctuation range can be designed according to actual conditions, such as 0.9 times to 1.1 times of the rated AC voltage of the PCC point; similarly, the second normal fluctuation range can be designed according to actual conditions, such as 0.9 times to 1.1 times of the high-voltage DC bus voltage.

[0027] The application realizes the preliminary screening of the voltage fluctuation abnormal event by introducing the AC voltage of the PCC point and the high-voltage DC bus 3 voltage, and provides a data basis for fault type identification; and when only the AC voltage of the PCC point is abnormal and the high-voltage DC bus 3 voltage is normal, the criterion can effectively preliminarily lock the fault range on the AC side, thereby excluding the interference of the DC side fault or the receiving end power grid fault.

[0028] In a preferred embodiment, the identifying the fault type according to the voltage fluctuation abnormal event comprises: when it is detected that the AC voltage of the PCC point exceeds the upper limit of the first normal fluctuation range and it is detected that the high-voltage DC bus 3 voltage exceeds the upper limit of the second normal fluctuation range, the fault type is identified as the onshore AC power grid fault; when it is detected that the AC voltage of the PCC point exceeds the lower limit of the first normal fluctuation range and it is detected that the high-voltage DC bus 3 voltage exceeds the lower limit of the second normal fluctuation range, the fault type is identified as the high-voltage DC bus 3 fault; when it is detected that the AC voltage of the PCC point exceeds the lower limit of the first normal fluctuation range and it is detected that the high-voltage DC bus 3 voltage does not exceed the second normal fluctuation range, the fault type is identified as the offshore AC submarine cable fault; Specifically, when it is detected that the AC voltage of the PCC point exceeds the upper limit of the first normal fluctuation range and it is detected that the high-voltage DC bus 3 voltage exceeds the upper limit of the second normal fluctuation range, it indicates that there is an active power surplus in the system, and the receiving end power grid cannot effectively consume all the power transmitted from the offshore, resulting in energy accumulation in the system and voltage rise, and therefore the fault type is identified as the onshore AC power grid fault; Specifically, when it is detected that the AC voltage of the PCC point exceeds the lower limit of the first normal fluctuation range and it is detected that the high-voltage DC bus 3 voltage exceeds the lower limit of the second normal fluctuation range, it indicates that there is a serious power transmission interruption in the DC power transmission link of the system, and the most typical case is a short-circuit fault of the DC bus, resulting in voltage collapse, and therefore the fault type is identified as the high-voltage DC bus 3 fault; Specifically, when it is detected that the AC voltage of the PCC point exceeds the lower limit of the first normal fluctuation range and it is detected that the high-voltage DC bus 3 voltage does not exceed the second normal fluctuation range, it indicates that the fault point is located on the offshore AC side, resulting in that part of the power cannot be converged, but the stability of the DC backbone is not destroyed, and therefore the fault type is identified as the offshore AC submarine cable fault.

[0029] By constructing the coordination criterion of AC voltage based on PCC point and HVDC bus 3 voltage, the fault type of hybrid converter transmission system is identified, and the fault source is divided into three areas: onshore AC power grid, HVDC bus 3 and offshore AC submarine cable, which greatly improves the speed and reliability of dealing with complex faults.

[0030] S2, when it is identified that the fault type is onshore AC power grid fault, the DC energy dissipation device preset at the receiving end MMC converter 4 is enabled, so that the DC energy dissipation device consumes the active power generated by the offshore wind farm during the voltage fluctuation abnormal event; Illustratively, when a transient fault occurs in the onshore AC power grid, as shown in Figure 4 , the active power cannot be fully transmitted to the AC power grid by the onshore receiving end MMC converter 4, and will accumulate on the HVDC bus 3, causing the HVDC bus 3 voltage to rise, and the peak value of the HVDC bus 3 voltage needs to be limited during the fault; wherein the relationship between the HVDC bus 3 voltage and the active power is: In the formula, represents the equivalent capacitance of the HVDC bus 3 and the receiving end MMC converter 4; represents the active power transmitted by the offshore wind farm to the HVDC bus 3 through the hybrid converter transmission system; represents the active power transmitted by the receiving end MMC converter 4 to the onshore AC power grid; In order to limit the peak value of the HVDC bus 3 voltage, the DC energy dissipation device preset at the receiving end MMC converter 4 is enabled, and when the HVDC bus 3 voltage is higher than 1.08 p.u. during the onshore AC power grid fault, the DC energy dissipation device is put into operation; when the HVDC bus 3 voltage is lower than 0.95 p.u., the DC energy dissipation device is taken out of operation; thus, by putting the DC energy dissipation device into operation, the excess active power generated by the offshore wind farm during the onshore AC power grid fault is released, so as to maintain the stability of the HVDC bus 3 voltage. And based on the hybrid converter transmission system of the diode rectifier 1, when the onshore AC power grid fails, the fault characteristics will be passively transmitted to the offshore wind farm through the action between various electrical quantities; therefore, when the wind turbine in the offshore wind farm detects that the offshore AC voltage rises, the output active power of the wind turbine needs to be reduced at the same time, so as to reduce the HVDC bus 3 voltage.

[0031] S3, when it is identified that the fault type is offshore AC submarine cable fault, the AC circuit breaker connected with the offshore AC submarine cable where the fault occurs in the offshore wind farm is disconnected, and the on-off state of the diode rectifier 1 and the auxiliary converter 2 is controlled; ​Illustratively, for offshore AC grid fault, the fault usually occurs on the offshore AC submarine cable of the offshore wind farm, and the offshore AC submarine cable is generally a permanent fault, at this time, different strategies should be adopted according to different fault points; In a preferred embodiment, the offshore AC submarine cable comprises branch submarine cables; the offshore wind farm is connected with the diode rectifier 1 and the auxiliary converter 2 through the branch submarine cables and corresponding AC circuit breakers respectively; The step of disconnecting the AC circuit breaker connected with the offshore AC submarine cable where the fault occurs and controlling the on-off state of the diode rectifier 1 and the auxiliary converter 2 when it is identified that the fault type is offshore AC submarine cable fault, comprises: When it is identified that the fault type is offshore AC submarine cable fault and it is determined that the fault occurs on the branch submarine cable, the branch submarine cable where the fault point exists is taken as the target branch submarine cable; the AC circuit breaker connected with the target branch submarine cable is disconnected, and the on state of the diode rectifier 1 and the auxiliary converter 2 is maintained; Specifically, referring to Figure 5 For the fault at F1 point in Figure 5 the branch submarine cable of the offshore wind farm, at this time, the voltage of the hybrid converter transmission system will decrease quickly, and the corresponding AC circuit breaker B1 needs to be disconnected as soon as possible to remove the fault branch submarine cable.

[0032] In a preferred embodiment, the offshore AC submarine cable further comprises a collection cable; all the branch submarine cables are connected to the collection cable; The step of disconnecting the AC circuit breaker connected with the offshore AC submarine cable where the fault occurs and controlling the on-off state of the diode rectifier 1 and the auxiliary converter 2 when it is identified that the fault type is offshore AC submarine cable fault, comprises: When it is identified that the fault type is offshore AC submarine cable fault and it is determined that the fault occurs on the collection cable, all the AC circuit breakers are disconnected, and the on state of the diode rectifier 1 and the auxiliary converter 2 is switched to the blocking state; Specifically, referring to Figure 5 For the fault at F2 point in Figure 5 the collection cable of the offshore wind farm, at this time, all the AC circuit breakers must be disconnected, and the auxiliary converter 2 must also be blocked and shut down, and then restarted after the subsequent hybrid converter transmission system is repaired and recovered; because the action time of the AC circuit breaker is usually between 60ms and 100ms, during which the remaining branch wind turbine will enter the low voltage ride through mode, the grid side converter reduces the output of active power, and the excess active power generated by the machine side converter is consumed by the unloading circuit inside the wind turbine; For the diode rectifier 1, after the AC side fault, the AC side voltage drops rapidly, and since the DC bus voltage is constant, the diode rectifier 1 is quickly locked out and no longer transmits active power; for the auxiliary converter 2, since it is mounted on the PCC point (point of common coupling) as an APF (active power filter), it must be quickly locked out to prevent the energy of the sub-modules in the auxiliary converter 2 from being lost; after 100 ms, if the fault branch of the wind farm has been cut off, the AC voltage in the hybrid current transmission and DC transmission system can be re-established to resume normal operation, and if overcurrent still occurs, indicating that the fault has not been cut off, the auxiliary converter 2 is permanently locked out, and the hybrid current transmission and DC transmission system is shut down for maintenance.

[0033] S4, when the fault type is identified as a high-voltage DC bus 3 fault, the auxiliary converter 2 is locked out; a pre-set DC circuit breaker at the receiving end MMC converter 4 is enabled to disconnect the receiving end MMC converter 4 from the high-voltage DC bus 3; Specifically, referring to Figure 6 , the high-voltage DC bus 3 fault can be divided into two types of faults: Figure 6 The fault at point F3 is a single-stage short-circuit fault; the hybrid current transmission and DC transmission system of the embodiment is a pseudo-bipolar system, so the non-fault pole will bear twice the DC voltage during normal operation, which will affect the insulation of the electrical equipment in the hybrid current transmission and DC transmission system; if each electrical equipment in the hybrid current transmission and DC transmission system can withstand twice the insulation voltage during normal operation, active power can still be transmitted; but in this embodiment, the same fault protection measures as for inter-stage short-circuit faults are taken to prevent device overvoltage breakdown; Figure 6 The fault at point F4 is an inter-stage short-circuit fault, which is more serious than a single-stage short-circuit fault, and the DC bus voltage will decrease rapidly; for the receiving end MMC converter 4, the DC inductance is needed to limit the DC short-circuit current at this time, therefore, the DC circuit breaker pre-set at the receiving end MMC converter 4 is used to disconnect the receiving end MMC converter 4 from the high-voltage DC bus 3 to prevent equipment damage; for the offshore AC power grid, due to the characteristics of the diode rectifier 1, the voltage of the offshore AC power grid will also decrease rapidly at this time, so the wind turbine generators in the offshore wind farm will soon enter a low-wearing state; for the auxiliary converter 2, the auxiliary converter 2 must be quickly locked out to prevent the energy stored in the sub-module capacitors from being quickly lost.

[0034] In order to better illustrate the present application, the simulation system is used to simulate each fault type proposed in the embodiment, and the corresponding protection scheme is verified. Specifically, the steady-state simulation model of the offshore wind power through the hybrid converter DC transmission system is built by using the simulation platform, and the corresponding crossing scheme of the two fault types of the onshore AC power grid fault and the offshore AC submarine cable fault is verified respectively. Thus, the simulation results are analyzed to determine whether the crossing scheme realizes the stable crossing of the possible faults in the operation of the offshore wind power through the hybrid converter DC transmission system, and further to provide stable protection.

[0035] Referring to Figure 7 , the simulation results after using the corresponding crossing scheme for the onshore AC power grid fault; it can be seen from Figure 7 (e) that before the fault occurs, the offshore wind farm output power reaches the rated value, and the system is in a stable operating state; as shown in Figure 7 (a), at t=10s, a three-phase ground fault occurs in the onshore AC power grid, which lasts for 500ms, at this time, the fault crossing is performed through the DC energy consumption device, and the current limiting of the receiving converter is put into operation, as shown in Figure 7 (b), during the fault, when the high-voltage DC bus 3 voltage is higher than 1.08p.u, the DC energy consumption device is put into operation; when the high-voltage DC bus 3 voltage is lower than 0.95p.u, the DC energy consumption device is taken out of operation, as shown in Figure 7 (d); throughout the process, the offshore wind farm output active power is constant; it can be seen that the DC bus voltage is always limited within 1.2p.u, as shown in Figure 7 (c); after the fault is over, the DC energy consumption device is taken out, and the system returns to the normal operating state.

[0036] Referring to Figure 8 , the simulation results after using the corresponding crossing scheme for the offshore AC submarine cable fault; before the fault occurs, the system is in a stable operating state, and the offshore wind farm outputs rated power, as shown in Figure 8 (f). At t=10s, a serious three-phase ground fault occurs in the collection cable of the offshore wind farm, which lasts for 100ms. At t=10.1s, the corresponding AC circuit breaker is disconnected to remove the fault branch; it can be seen that after the fault occurs, the AC grid voltage drops rapidly, as shown in Figure 8 (a), at this time, the wind turbine in the offshore wind farm enters the low crossing mode, and the offshore wind farm current is reduced to zero, as shown in Figure 8 (b), the active power is also reduced to zero, as shown in Figure 8 (f). After detecting the AC side fault, the auxiliary converter 2 is quickly locked, so the output current is also reduced to zero, as shown in Figure 8 (e). Since the DC bus voltage is still controlled by the onshore receiving MMC converter 4 to be constant, after the fault occurs, the diode rectifier 1 is quickly cut off, and the conduction current is reduced to zero, as shown inFigure 8 (c) (d) shown; when the AC circuit breaker acts, the fault branch is cut off, the blocking state of the auxiliary converter 2 is released, the AC voltage of the PCC point is re-established, the offshore wind farm restores active power output, and enters a new stable operation state.

[0037] The analysis result shows that the offshore wind power transmission system through hybrid conversion of the fault ride-through method can effectively realize stable ride-through of offshore and onshore AC / DC faults, effectively protect power equipment and efficient transmission of wind power to the grid.

[0038] Referring to Figure 9 , an offshore wind power transmission system through hybrid conversion based on a fault ride-through device is provided, comprising: A fault type identification module is configured to identify a fault type according to a voltage fluctuation abnormal event when determining that the hybrid conversion transmission system has the voltage fluctuation abnormal event; wherein the hybrid conversion transmission system comprises a diode rectifier 1, an auxiliary converter 2, a high-voltage DC bus 3 and a receiving end MMC converter 4. A first fault ride-through module is configured to enable a DC energy consumption device preset at the receiving end MMC converter 4 when identifying that the fault type is an onshore AC grid fault, so that the DC energy consumption device consumes active power generated by the offshore wind farm during the voltage fluctuation abnormal event. A second fault ride-through module is configured to disconnect an AC circuit breaker connected to the offshore AC cable fault in the offshore wind farm, and control the switching state of the diode rectifier 1 and the auxiliary converter 2 when identifying that the fault type is an offshore AC cable fault. A third fault ride-through module is configured to block the auxiliary converter 2 when identifying that the fault type is a high-voltage DC bus 3 fault; and enable a DC circuit breaker preset at the receiving end MMC converter 4, so that the DC circuit breaker cuts off the connection between the receiving end MMC converter 4 and the high-voltage DC bus 3.

[0039] In a preferred embodiment, the determination process of the voltage fluctuation abnormal event comprises: When detecting that the AC voltage of the PCC point exceeds the first normal fluctuation range, it is determined that the hybrid conversion transmission system has a voltage fluctuation abnormal event. Wherein, the PCC point is a common connection point of the offshore wind farm, the diode rectifier 1 and the auxiliary converter 2.

[0040] In a preferred embodiment, the identification of the fault type according to the voltage fluctuation abnormal event comprises: when the AC voltage at the PCC point is detected to exceed the upper limit of the first normal fluctuation range and the HVDC bus 3 voltage is detected to exceed the upper limit of the second normal fluctuation range, the fault type is identified as an onshore AC grid fault; when the AC voltage at the PCC point is detected to exceed the lower limit of the first normal fluctuation range and the HVDC bus 3 voltage is detected to exceed the lower limit of the second normal fluctuation range, the fault type is identified as a HVDC bus 3 fault; when the AC voltage at the PCC point is detected to exceed the lower limit of the first normal fluctuation range and the HVDC bus 3 voltage is detected not to exceed the second normal fluctuation range, the fault type is identified as an offshore AC sea cable fault.

[0041] In a preferred embodiment, the offshore AC sea cable comprises branch sea cables; the offshore wind farm is connected to the diode rectifier 1 and the auxiliary converter 2 through the branch sea cables and corresponding AC breakers, respectively; The AC breaker connected to the offshore AC sea cable where the fault occurs in the offshore wind farm is disconnected, and the on-off state of the diode rectifier 1 and the auxiliary converter 2 is controlled when the fault type is identified as an offshore AC sea cable fault, comprising: When the fault type is identified as an offshore AC sea cable fault and it is determined that a branch sea cable is faulty, the branch sea cable where the fault point exists is taken as a target branch sea cable; the AC breaker connected to the target branch sea cable is disconnected, and the on-off state of the diode rectifier 1 and the auxiliary converter 2 is maintained.

[0042] In a preferred embodiment, the offshore AC sea cable further comprises a collection cable; all branch sea cables are connected to the collection cable; The AC breaker connected to the offshore AC sea cable where the fault occurs in the offshore wind farm is disconnected, and the on-off state of the diode rectifier 1 and the auxiliary converter 2 is controlled when the fault type is identified as an offshore AC sea cable fault, comprising: When the fault type is identified as an offshore AC sea cable fault and it is determined that the collection cable is faulty, all AC breakers are disconnected, and the on-off state of the diode rectifier 1 and the auxiliary converter 2 is switched to a blocking state.

[0043] The above is the preferred embodiment of the present application, it should be noted that for those skilled in the art, without departing from the principles of the present application, can make a number of improvements and refinements, these improvements and refinements are also considered to be within the scope of the present application.

Claims

1. A fault ride-through method for offshore wind power transmitted via a hybrid converter system, characterized in that, include: When determining that a voltage fluctuation abnormal event has occurred in the hybrid converter power transmission system, the fault type is identified based on the voltage fluctuation abnormal event; wherein, the hybrid converter power transmission system includes a diode rectifier, an auxiliary converter, a high-voltage DC bus, and a receiving-end MMC converter; When the fault type is identified as an onshore AC grid fault, the DC power consumption device preset at the receiving end MMC converter is activated so that the DC power consumption device consumes the active power generated by the offshore wind farm during the voltage fluctuation abnormal event. When the fault type is identified as a submarine AC cable fault, the AC circuit breaker in the offshore wind farm connected to the faulty submarine AC cable is disconnected, and the switching on / off status of the diode rectifier and the auxiliary converter is controlled. When a fault type of high-voltage DC bus fault is detected, the auxiliary converter is locked out; the DC circuit breaker preset at the receiving-end MMC converter is activated so that the DC circuit breaker disconnects the receiving-end MMC converter from the high-voltage DC bus.

2. The fault ride-through method for offshore wind power transmitted via a hybrid converter system as described in claim 1, characterized in that, The process for determining abnormal voltage fluctuation events includes: When the AC voltage at the PCC point exceeds the first normal fluctuation range, it is determined that a voltage fluctuation abnormality event has occurred in the hybrid converter output system. The PCC point is the common connection point of the offshore wind farm, the diode rectifier, and the auxiliary converter.

3. The fault ride-through method for offshore wind power transmitted via a hybrid converter system as described in claim 2, characterized in that, The step of identifying the fault type based on the voltage fluctuation anomaly includes: When the AC voltage at the PCC point exceeds the upper limit of the first normal fluctuation range and the voltage at the high-voltage DC bus exceeds the upper limit of the second normal fluctuation range, the fault type is identified as an onshore AC power grid fault. When the AC voltage at the PCC point exceeds the lower limit of the first normal fluctuation range and the voltage at the high-voltage DC bus exceeds the lower limit of the second normal fluctuation range, the fault type is identified as a high-voltage DC bus fault. When the AC voltage at the PCC point exceeds the lower limit of the first normal fluctuation range, and the voltage of the high-voltage DC bus does not exceed the second normal fluctuation range, the fault type is identified as a marine AC submarine cable fault.

4. The fault ride-through method for offshore wind power transmitted via a hybrid converter system as described in claim 1, characterized in that, The offshore AC cable includes branch cables; the offshore wind farm is connected to the diode rectifier and the auxiliary converter through the branch cables and corresponding AC circuit breakers, respectively. When a fault type of offshore AC cable is identified as a fault, disconnecting the AC circuit breaker in the offshore wind farm connected to the faulty offshore AC cable and controlling the on / off status of the diode rectifier and the auxiliary converter includes: When the fault type is identified as a submarine AC cable fault, and it is determined that the fault is in a branch submarine cable, the branch submarine cable with the fault point is designated as the target branch submarine cable; the AC circuit breaker connected to the target branch submarine cable is disconnected, and the diode rectifier and the auxiliary converter are kept in operation.

5. The fault ride-through method for offshore wind power transmitted via a hybrid converter system as described in claim 4, characterized in that, The submarine cable for communication also includes a hub cable; all branch cables are connected to the hub cable. When a fault type of offshore AC cable is identified as a fault, disconnecting the AC circuit breaker in the offshore wind farm connected to the faulty offshore AC cable and controlling the on / off status of the diode rectifier and the auxiliary converter includes: When the fault type is identified as a submarine AC cable fault, and it is determined that the fault is in the collection cable, all AC circuit breakers are disconnected, and the diode rectifier and the auxiliary converter are switched from the engaged state to the locked state.

6. A fault ride-through device for an offshore wind power transmission system via a hybrid converter, characterized in that, include: The fault type identification module is used to identify the fault type based on the voltage fluctuation abnormal event when a voltage fluctuation abnormal event occurs in the hybrid converter power transmission system; wherein, the hybrid converter power transmission system includes a diode rectifier, an auxiliary converter, a high-voltage DC bus, and a receiving-end MMC converter; The first fault ride-through module is used to activate the DC power consumption device preset at the receiving end MMC converter when the fault type is identified as an onshore AC grid fault, so that the DC power consumption device consumes the active power generated by the offshore wind farm during the voltage fluctuation abnormal event. The second fault ride-through module is used to disconnect the AC circuit breaker in the offshore wind farm that is connected to the faulty offshore AC cable when the fault type is detected as an offshore AC cable fault, and to control the on / off status of the diode rectifier and the auxiliary converter. The third fault ride-through module is used to lock out the auxiliary converter when the fault type is identified as a high-voltage DC bus fault; and to activate the DC circuit breaker preset at the receiving-end MMC converter so that the DC circuit breaker disconnects the connection between the receiving-end MMC converter and the high-voltage DC bus.

7. The fault ride-through device for an offshore wind power transmission system via hybrid converter as described in claim 6, characterized in that, The process for determining abnormal voltage fluctuation events includes: When the AC voltage at the PCC point exceeds the first normal fluctuation range, it is determined that a voltage fluctuation abnormality event has occurred in the hybrid converter output system. The PCC point is the common connection point of the offshore wind farm, the diode rectifier, and the auxiliary converter.

8. The fault ride-through device for an offshore wind power transmission system via hybrid converter as described in claim 7, characterized in that, The step of identifying the fault type based on the voltage fluctuation anomaly includes: When the AC voltage at the PCC point exceeds the upper limit of the first normal fluctuation range and the voltage at the high-voltage DC bus exceeds the upper limit of the second normal fluctuation range, the fault type is identified as an onshore AC power grid fault. When the AC voltage at the PCC point exceeds the lower limit of the first normal fluctuation range and the voltage at the high-voltage DC bus exceeds the lower limit of the second normal fluctuation range, the fault type is identified as a high-voltage DC bus fault. When the AC voltage at the PCC point exceeds the lower limit of the first normal fluctuation range, and the voltage of the high-voltage DC bus does not exceed the second normal fluctuation range, the fault type is identified as a marine AC submarine cable fault.

9. The fault ride-through device for an offshore wind power transmission system via hybrid converter as described in claim 6, characterized in that, The offshore AC cable includes branch cables; the offshore wind farm is connected to the diode rectifier and the auxiliary converter through the branch cables and corresponding AC circuit breakers, respectively. When a fault type of offshore AC cable is identified as a fault, disconnecting the AC circuit breaker in the offshore wind farm connected to the faulty offshore AC cable and controlling the on / off status of the diode rectifier and the auxiliary converter includes: When the fault type is identified as a submarine AC cable fault, and it is determined that the fault is in a branch submarine cable, the branch submarine cable with the fault point is designated as the target branch submarine cable; the AC circuit breaker connected to the target branch submarine cable is disconnected, and the diode rectifier and the auxiliary converter are kept in operation.

10. The fault ride-through device for an offshore wind power transmission system via hybrid converter as described in claim 9, characterized in that, The submarine cable for communication also includes a hub cable; all branch cables are connected to the hub cable. When a fault type of offshore AC cable is identified as a fault, disconnecting the AC circuit breaker in the offshore wind farm connected to the faulty offshore AC cable and controlling the on / off status of the diode rectifier and the auxiliary converter includes: When the fault type is identified as a submarine AC cable fault, and it is determined that the fault is in the collection cable, all AC circuit breakers are disconnected, and the diode rectifier and the auxiliary converter are switched from the engaged state to the locked state.

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