Offshore wind power multi-terminal DC transmission system and DC fault active control method thereof

Abstract

The invention discloses an offshore wind power multi-terminal DC transmission system, including an offshore wind farm, a flexible DC multi-terminal transmission system based on a half-bridge modular multi-level converter, and an onshore power grid; the flexible DC multi-terminal transmission system includes N offshore converters, N DC busbars A, K DC busbars B, and N onshore converters, K≤N; the offshore wind farm includes M wind turbines, each wind turbine is connected to at least one offshore converter, and each wind turbine is connected to the AC side of the offshore converter through a transformer and an AC circuit breaker in turn; the DC busbar A, one end of which is connected to the DC side of the offshore converter through a DC isolating switch, and the other end of which is connected to the DC side of the onshore converter through a DC circuit breaker; the two ends of the DC busbar B are each connected to the DC side of two onshore converters through a DC circuit breaker. The invention realizes seamless, autonomous and controlled conversion between the grid-connected control mode and the networking control mode.

Description

Technical Field

[0001] The present invention relates to the field of new energy flexible direct current grid connection, and in particular to an offshore wind power multi-terminal direct current transmission system and a direct current fault active control method thereof. Background Art

[0002] At present, the continuous depletion of fossil energy and the increasingly severe environmental pollution have made wind power generation widely concerned. Offshore wind power is becoming an important direction for the development of wind power due to its advantages such as more stable wind speed, wider space and less impact on the environment. European countries, represented by the United Kingdom, have vigorously developed offshore wind power since 2000. By the end of 2020, offshore wind power can provide 10% of the electricity consumption in the United Kingdom. Although the development of offshore wind power in my country started late, it has developed rapidly in recent years. At present, the development of offshore wind power is developing towards scale, offshore and large-scale wind turbines. The transmission methods of offshore wind power mainly include high-voltage AC transmission and flexible DC transmission. Compared with high-voltage AC transmission, high-voltage DC transmission has the advantages of small cable charging current and no transmission distance limit, and is favored in the field of large-scale offshore wind power transmission.

[0003] With the continuous large-scale development of offshore wind energy, the demand for offshore wind power multi-terminal DC transmission systems is increasing. Compared with point-to-point DC transmission, multi-terminal DC transmission has the advantages of high flexibility, high transmission reliability and low cost. In order to ensure the reliable operation of multi-terminal DC systems after DC faults, multi-terminal systems with DC circuit breakers are becoming the focus of attention and research at home and abroad. However, offshore wind power multi-terminal DC transmission with DC circuit breakers still faces many challenges in the harsh offshore environment of humidity and salt spray: (1) DC circuit breakers and offshore converters are large in size and heavy in weight, making offshore platform construction difficult; (2) The investment cost of DC circuit breakers and offshore converters is high; (3) The construction time is relatively long. Therefore, while meeting the demand for offshore wind power transmission, how to improve the economic efficiency of offshore wind power multi-terminal DC transmission systems is an important issue that needs to be solved urgently. Among them, the type of offshore HVDC converter and the DC circuit breaker on the offshore HVDC converter side are the key factors determining the economic efficiency of multi-terminal DC transmission systems. In summary, it is urgent to explore the economic feasibility of multi-terminal DC transmission systems under different offshore HVDC converter topologies (excluding offshore DC circuit breakers) and to study DC fault problems. Summary of the invention

[0004] The present invention provides an offshore wind power multi-terminal DC transmission system and a DC fault active control method thereof to solve the technical problems existing in the known technology.

[0005] The technical solution adopted by the present invention to solve the technical problems existing in the known technology is: an offshore wind power multi-terminal DC transmission system, including an offshore wind farm, a flexible DC multi-terminal transmission system based on a half-bridge modular multi-level converter and an onshore power grid; the flexible DC multi-terminal transmission system includes N offshore converters, N DC busbars A, K DC busbars B and N onshore converters, K≤N; the offshore converter is used to convert the AC power generated by the offshore wind farm into DC power, and the onshore converter is used to convert the DC power into AC power; onshore The DC sides of the onshore converter and the offshore converter are both provided with DC reactors; the offshore wind farm comprises M wind turbines, each of which is connected to at least one offshore converter, and each of which is connected to the AC side of the offshore converter in turn through a transformer and an AC circuit breaker; a DC bus A, one end of which is connected to the DC side of the offshore converter through a DC disconnector, and the other end of which is connected to the DC side of the onshore converter through a DC circuit breaker; both ends of the DC bus B are connected to the DC sides of the two onshore converters through DC circuit breakers.

[0006] Furthermore, the DC sides of two adjacent onshore converters are connected in sequence through a DC bus bar B.

[0007] Furthermore, the controller of the offshore converter includes a dual closed-loop controller, which includes an inner loop current control module and an outer loop control module; the outer loop control module includes an active power control module, a DC voltage control module, an AC voltage control module and a reactive power control module, the active power control module is used to control the output power of the offshore converter; the DC voltage control module is used to control the DC side voltage of the offshore converter; the AC voltage control module is used to control the AC side voltage of the offshore converter; the reactive power control module includes a frequency control module, and the frequency control module is used to control the AC side frequency of the offshore converter; the inner loop current control module includes an active power current module and a reactive power current module; the active power control module, the DC voltage control module and the AC voltage control module output active current reference signals to the active power current module; the reactive power control module outputs reactive current reference signals to the reactive power current module.

[0008] Furthermore, the active power control module or the DC voltage control module is provided with a current limiter A and a current limiter B connected in series in sequence, and the reactive power control module is provided with a current limiter C; the AC voltage control module inputs the difference between the maximum AC voltage on the AC side of the offshore converter and the corresponding actual voltage, and outputs a signal to the current limiter B.

[0009] The present invention also provides a DC fault active control method for an offshore wind power multi-terminal DC transmission system, which includes an offshore wind farm, a flexible DC multi-terminal transmission system based on a half-bridge modular multi-level converter, and an onshore power grid; the flexible DC multi-terminal transmission system includes N offshore converters, N DC busbars A, K DC busbars B, and N onshore converters, K≤N; the offshore converter is used to convert the AC power generated by the offshore wind farm into DC power, and the onshore converter is used to convert the DC power into AC power; DC reactors are provided on the DC sides of the onshore converter and the offshore converter; the offshore wind farm includes M wind The wind turbine generator set is configured to connect each wind turbine generator set to at least one offshore converter, and each wind turbine generator set is connected to the AC side of the offshore converter through a transformer and an AC circuit breaker in turn; one end of the DC bus A is connected to the DC side of the offshore converter through a DC isolating switch, and the other end of the DC bus A is connected to the DC side of the onshore converter through a DC circuit breaker; both ends of the DC bus B are connected to the DC sides of the two onshore converters through DC circuit breakers; according to the fault detection result, the DC transmission line of the flexible DC multi-terminal transmission system is divided into a normal line and a fault line, and the fault line is isolated.

[0010] Furthermore, the fault of the DC transmission line is determined by one of the following methods or a combination of the following methods: the DC reactor voltage of the onshore converter and the offshore converter is detected, and if the voltage change rate of the DC reactor exceeds the corresponding set value range, it is determined that the DC transmission line connected to the DC reactor is faulty; the current of the DC disconnect switch on the DC side of the offshore converter is detected, and if the current of the DC disconnect switch is less than the corresponding set threshold value, it is determined that the DC transmission line connected to the DC disconnect switch is faulty.

[0011] Furthermore, when a fault is detected in a DC transmission line, the DC circuit breaker on the DC side of the onshore converter and the DC isolating switch on the DC side of the offshore converter corresponding to the faulty line are disconnected; 20ms after the fault is detected, if the detected DC voltage is still lower than 50% of the rated value, the AC circuit breaker on the AC side of the offshore converter connected to the faulty line is disconnected, so that the current input by the wind turbine to the faulty line drops to zero; when the fault is removed, the corresponding AC circuit breaker is kept closed first, and then the DC circuit breaker on the DC side of the corresponding onshore converter and the DC isolating switch on the DC side of the offshore converter are closed.

[0012] Furthermore, the controller of the offshore converter is provided with a dual closed-loop controller, which includes an inner loop current control module and an outer loop control module; the outer loop control module includes an active power control module, a DC voltage control module, an AC voltage control module and a reactive power control module, the active power control module is used to control the output power of the offshore converter; the DC voltage control module is used to control the DC side voltage of the offshore converter; the AC voltage control module is used to control the AC side voltage of the offshore converter; the reactive power control module includes a frequency control module, the frequency control module is used to control the AC side frequency of the offshore converter; the inner loop current ... The module includes an active power current module and a reactive power current module; the active power control module and the DC voltage control module work alternately; the active power control module, the DC voltage control module and the AC voltage control module output active current reference signals to the active power current module; the reactive power control module outputs reactive current reference signals to the reactive power current module; the active power control module or the DC voltage control module is provided with a current limiter A and a current limiter B connected in series in sequence, and the reactive power control module is provided with a current limiter C; the AC voltage control module inputs the difference between the maximum AC voltage on the AC side of the offshore converter and the corresponding actual voltage, and outputs a signal to the current limiter B;

[0013] When the detection voltage input by the AC voltage control module is 1.1 times the rated voltage, the control output of the current limiter D is I max , output current I max As the upper limit value of current limiter B;

[0014] During normal operation, the offshore converter operates in the grid-connected control mode, the active power control module or the DC voltage control module in its outer loop control module is in an activated state, and the frequency control module and the AC voltage control module are in an inactivated state; the output current upper limit value of the current limiter C is set to 0, and the inner loop reactive current reference signal is a constant determined by the reactive power reference value and the rated voltage; the phase-locked loop corresponding to the offshore converter operates normally, providing the phase angle required for coordinate transformation for the grid-connected control mode.

[0015] Furthermore, the offshore converter is provided with a freewheeling diode and a load unloading circuit on the DC side; when a DC transmission line is detected to have a fault, the working state of the offshore converter corresponding to the faulty line is as follows:

[0016] During the period when the offshore converter is locked but its AC circuit breaker remains closed, the electric energy generated by the wind turbine is fed into the fault line through the freewheeling diode of the offshore converter; the offshore converter operates in the current limiting control mode, its active power control module is in the activated state, and the AC voltage control module is in the inactivated state; its DC side unloading circuit is activated to consume the active power that cannot be delivered in the offshore converter; when the AC side voltage of the offshore converter is lower than the preset threshold, the reactive power control module operates in the networking frequency control mode, its frequency control module is in the activated state, and the output current upper limit value of the current limiter C is set to I max , while the active power control module is in saturation state, the active current reference signal I output by the current limiter B dref =I dmax1 , Among them I dref is the active current reference signal; I qref is the reactive current reference signal; I dmax1 is the output upper limit of the current limiter A;

[0017] During the period when the offshore converter is locked and its AC circuit breaker is disconnected, as the AC voltage on the AC side of the offshore converter increases, the active power control module exits the saturation state and its output current I dmax1 Starts to decrease; at this time, the AC voltage control module is in an activated state; the active current reference signal I output by the current limiter B dref =I dmax2 , I dmax2 is the output upper limit of the current limiter D; after stabilization, I dref =I dmax2 ≈0; the phase-locked loop still works normally to synchronize the wind turbines; each wind turbine operates in a network control mode based on the phase-locked loop.

[0018] Furthermore, after the fault of the faulty line is cleared, the steps of resuming operation of the equipment of the corresponding line include the following steps:

[0019] Step A1, the AC circuit breaker of the offshore converter is closed again, and the DC disconnector of the offshore converter is closed;

[0020] Step A2, the offshore converter is unlocked, its DC voltage control module is in an activated state, and its active power control module is in an inactivated state; its DC voltage control module operates in a fixed DC voltage control mode; and is synchronized with the wind turbine generator set through a phase-locked loop;

[0021] Step A3, in a constant DC voltage control mode, when the DC voltage on the DC side of the offshore converter is equal to the DC voltage of other normal lines, closing the DC circuit breaker on the DC side of the onshore converter;

[0022] Step A4, the offshore converter switches from the fixed DC voltage control mode to the networking voltage control mode, that is, its AC voltage control module is in an activated state, and its active power control module or DC voltage control module is in an inactivated state; when the AC voltage control module is saturated again, the offshore converter switches to the grid-connected control mode, that is, the active power control module or DC voltage control module is in an activated state, and its AC voltage control module is in an inactivated state.

[0023] The advantages and positive effects of the present invention are:

[0024] The method proposed in the present invention effectively realizes the DC fault isolation of the multi-terminal DC power transmission system without configuring a DC circuit breaker on the onshore converter side or the offshore converter side. The offshore DC isolating switch used has the advantages of low cost and small footprint.

[0025] The seamless switching strategy between the offshore converter grid-connected control mode and the networking control mode proposed in the present invention can achieve effective fault circuit limitation after a DC fault, during the period when the inverter station is locked but its AC circuit breaker is not disconnected; after the offshore AC circuit breaker of the inverter station is disconnected, a seamless, autonomous and controlled conversion between the grid-connected control mode and the networking control mode can be achieved based on local measurements.

[0026] The DC fault recovery strategy for offshore wind power sent through an inverter station proposed in the present invention can restart the offshore inverter station after the DC fault line is isolated, and restore power delivery from the offshore wind farm to a healthy multi-terminal DC transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is the topological structure of the offshore wind power multi-terminal DC transmission system of the present invention.

[0028] Figure 2 This is the schematic diagram of the network voltage control during the fault recovery process of the flexible DC multi-terminal transmission system.

[0029] Figure 3 This is the schematic diagram of the network frequency control during the fault recovery process of the flexible DC multi-terminal transmission system.

[0030] Figure 4 The phase angle schematic diagram required for coordinate transformation is provided for the phase-locked loop in the fault recovery process of the flexible DC multi-terminal transmission system.

[0031] Figure 5 It is a flowchart of the networking control strategy of the flexible DC multi-terminal transmission system and the seamless switching of fixed AC voltage control during recovery.

[0032] Figure 6 Flowchart of DC fault isolation and fault recovery strategy for flexible DC multi-terminal transmission system.

[0033] In the figure:

[0034] The numbers indicate the device number.

[0035] I max Indicates the output upper limit of the outer loop control module.

[0036] I qref Indicates the active current reference signal.

[0037] I dref Indicates the active current reference signal.

[0038] I q0 Represents the reactive current constant signal.

[0039] I dmax1 Indicates the output upper limit of current limiter A.

[0040] I dmax2 Indicates the output upper limit of the current limiter D.

[0041] I dmin1 Indicates the output lower limit of current limiter A.

[0042] I dmin2 Indicates the output lower limit of the current limiter D.

[0043] I qmax Indicates the output upper limit of the current limiter C.

[0044] I qmin Indicates the output lower limit of the current limiter C.

[0045] P ref It indicates the reference value of the output active power of the offshore converter.

[0046] V dcref Indicates the reference value of the offshore converter output DC voltage.

[0047] V acmax Indicates the maximum AC voltage on the AC side of the offshore converter.

[0048] Q ref Indicates the reactive power reference value.

[0049] Q represents the actual value of reactive power.

[0050] k q Represents the ratio coefficient of the multiplier.

[0051] V q Indicates reactive voltage;

[0052] ω 0represents the rated synchronous angular speed of the wind turbine on the AC side of the offshore converter;

[0053] ω ref represents the reference synchronous angular velocity of the wind turbine on the AC side of the offshore converter;

[0054] ω represents the actual synchronous angular velocity of the wind turbine on the AC side of the offshore converter;

[0055] S represents the Laplace shift factor.

[0056] θ represents the phase angle required for coordinate transformation. DETAILED DESCRIPTION

[0057] In order to further understand the content, features and effects of the present invention, the following embodiments are listed and described in detail with reference to the accompanying drawings:

[0058] The Chinese meanings of the following English words and English abbreviations in this application are as follows:

[0059] OWF: offshore wind farm.

[0060] HB-MMC: Half-Bridge Modular Multilevel Converter

[0061] MMC-MTDC: Flexible multi-terminal direct current transmission system.

[0062] MMC: Converter station.

[0063] AC GRID: Onshore alternating current grid.

[0064] HVDC: High Voltage Direct Current.

[0065] CABLE: DC bus.

[0066] ACCB: Alternating Current Circuit Breaker.

[0067] DCCB: Direct current circuit breaker.

[0068] DCS: DC isolating switch.

[0069] PLL: Phase-locked loop.

[0070] T: Transformer.

[0071] Mode: mode.

[0072] See also Figures 1 to 6A multi-terminal DC transmission system for offshore wind power comprises an offshore wind farm, a flexible DC multi-terminal transmission system based on a half-bridge modular multilevel converter and an onshore power grid; the flexible DC multi-terminal transmission system comprises N offshore converters, N DC busbars A, K DC busbars B and N onshore converters, K≤N; the offshore converter is used to convert the AC power generated by the offshore wind farm into DC power, and the onshore converter is used to convert the DC power into AC power; DC reactors are provided on the DC sides of the onshore converter and the offshore converter; the offshore wind farm comprises M wind turbines, each wind turbine is connected to at least one offshore converter, and each wind turbine is connected to the AC side of the offshore converter in turn through a transformer and an AC circuit breaker; the DC busbar A, one end of which is connected to the DC side of the offshore converter through a DC isolating switch, and the other end of which is connected to the DC side of the onshore converter through a DC circuit breaker; both ends of the DC busbar B are connected to the DC sides of two onshore converters through DC circuit breakers.

[0073] Preferably, the DC sides of two adjacent onshore converters can be connected in sequence through a DC bus B.

[0074] Preferably, the controller of the offshore converter may include a dual closed-loop controller, which may include an inner-loop current control module and an outer-loop control module; the outer-loop control module may include an active power control module, a DC voltage control module, an AC voltage control module and a reactive power control module, the active power control module is used to control the output power of the offshore converter; the DC voltage control module is used to control the DC side voltage of the offshore converter; the AC voltage control module is used to control the AC side voltage of the offshore converter; the reactive power control module includes a frequency control module, and the frequency control module is used to control the AC side frequency of the offshore converter; the inner-loop current control module may include an active power current module and a reactive power current module; the active power control module, the DC voltage control module and the AC voltage control module may output an active current reference signal to the active power current module; the reactive power control module may output a reactive current reference signal to the reactive power current module.

[0075] Preferably, the active power control module or the DC voltage control module may be provided with a current limiter A and a current limiter B connected in series in sequence, and the reactive power control module may be provided with a current limiter C; the AC voltage control module may input the difference between the maximum AC voltage on the AC side of the offshore converter and the corresponding actual voltage, and output a signal to the current limiter B.

[0076] The present invention also provides a DC fault active control method for an offshore wind power multi-terminal DC transmission system, which includes an offshore wind farm, a flexible DC multi-terminal transmission system based on a half-bridge modular multi-level converter, and an onshore power grid; the flexible DC multi-terminal transmission system includes N offshore converters, N DC busbars A, K DC busbars B, and N onshore converters, K≤N; the offshore converter is used to convert the AC power generated by the offshore wind farm into DC power, and the onshore converter is used to convert the DC power into AC power; DC reactors are provided on the DC sides of the onshore converter and the offshore converter; the offshore wind farm includes M wind The wind turbine generator set is configured to connect each wind turbine generator set to at least one offshore converter, and each wind turbine generator set is connected to the AC side of the offshore converter through a transformer and an AC circuit breaker in turn; one end of the DC bus A is connected to the DC side of the offshore converter through a DC isolating switch, and the other end of the DC bus A is connected to the DC side of the onshore converter through a DC circuit breaker; both ends of the DC bus B are connected to the DC sides of the two onshore converters through DC circuit breakers; according to the fault detection result, the DC transmission line of the flexible DC multi-terminal transmission system is divided into a normal line and a fault line, and the fault line is isolated.

[0077] Preferably, the fault of the DC transmission line can be determined by one of the following methods or a combination of several methods: detecting the DC reactor voltage of the onshore converter and the offshore converter, and if the voltage change rate of the DC reactor exceeds the corresponding set value range, determining that the DC transmission line connected to the DC reactor is faulty; or detecting the current of the DC disconnector on the DC side of the offshore converter, and if the current of the DC disconnector is less than the corresponding set threshold value, determining that the DC transmission line connected to the DC disconnector is faulty.

[0078] Preferably, when a fault is detected in a DC transmission line, the DC circuit breaker on the DC side of the onshore converter and the DC isolating switch on the DC side of the offshore converter corresponding to the faulty line can be disconnected; 20ms after the fault is detected, if the detected DC voltage is still lower than 50% of the rated value, the AC circuit breaker on the AC side of the offshore converter connected to the faulty line can be disconnected, so that the current input by the wind turbine to the faulty line drops to zero; when the fault is cleared, the corresponding AC circuit breaker can be kept closed first, and then the DC circuit breaker on the DC side of the corresponding onshore converter and the DC isolating switch on the DC side of the offshore converter can be closed.

[0079] Preferably, the controller of the offshore converter may be provided with a dual closed-loop controller, which may include an inner-loop current control module and an outer-loop control module; the outer-loop control module may include an active power control module, a DC voltage control module, an AC voltage control module and a reactive power control module, the active power control module being used to control the output power of the offshore converter; the DC voltage control module being used to control the DC side voltage of the offshore converter; the AC voltage control module being used to control the AC side voltage of the offshore converter; the reactive power control module may include a frequency control module, the frequency control module being used to control the AC side frequency of the offshore converter; the inner-loop current control module being used to control the AC side frequency of the offshore converter; the AC voltage control module being used to control the AC side frequency of the offshore converter; the AC voltage control module being used to control the AC side voltage of the offshore converter; the reactive power ... voltage of the offshore converter; the reactive power control module being used to control the AC side frequency of the offshore converter; the reactive power control module being used to control the AC voltage of the offshore converter; the reactive power control module being used to control the AC side frequency of the offshore converter; the reactive power control module being used to control the AC voltage of the offshore converter; the reactive power control module being used to control the AC side frequency of the offshore converter; the reactive power control module being used to control the AC voltage of the offshore converter; the reactive power control module being used to control the AC voltage of the offshore converter; the reactive power control module being used to control the AC voltage of the offshore converter; the reactive power control module being used to control the AC side frequency of the offshore converter; the reactive power control module being used to control the AC voltage of the offshore converter; the reactive power control module The block includes an active power current module and a reactive power current module; the active power control module and the DC voltage control module work alternately; the active power control module, the DC voltage control module and the AC voltage control module can output an active current reference signal to the active power current module; the reactive power control module can output a reactive current reference signal to the reactive power current module; the active power control module or the DC voltage control module can be provided with a current limiter A and a current limiter B connected in series in sequence, and the reactive power control module is provided with a current limiter C; the AC voltage control module can input the difference between the maximum AC voltage on the AC side of the offshore converter and the corresponding actual voltage, and output a signal to the current limiter B.

[0080] When the detection voltage input by the AC voltage control module is 1.1 times the rated voltage, the control output of the current limiter D can be I max , output current I max Can be used as the upper limit value of current limiter B.

[0081] During normal operation, the offshore converter operates in the grid-connected control mode, the active power control module or the DC voltage control module in its outer loop control module is in an activated state, and the frequency control module and the AC voltage control module are in an inactivated state; the output current upper limit value of the current limiter C is set to 0, and the inner loop reactive current reference signal is a constant determined by the reactive power reference value and the rated voltage; the phase-locked loop corresponding to the offshore converter operates normally, providing the phase angle required for coordinate transformation for the grid-connected control mode.

[0082] See also Figures 2 to 4 , in the figure:

[0083] I max Indicates the output upper limit of the outer loop control module. qref Represents the active current reference signal. dref Represents the active current reference signal. q0 Represents the reactive current constant signal. dmax1 Indicates the output upper limit of the current limiter A. I dmax2 Indicates the output upper limit of the current limiter D. dmin1Indicates the output lower limit of current limiter A. I dmin2 Indicates the output lower limit of the current limiter D. qmax Indicates the output upper limit of the current limiter C. qmin Indicates the output lower limit of the current limiter C.

[0084] Current limiter A, current limiter C, and current limiter D can all be PI regulators with limiters.

[0085] The active power control module inputs the offshore converter output active power reference value P ref The difference between the actual active power and the corresponding active power is then output to the active power current module after passing through a PI regulator with a limiter.

[0086] The DC voltage control module inputs the offshore converter output DC voltage reference value V dcref The difference between the actual DC voltage and the corresponding DC voltage is then output to the active power current module after passing through a PI regulator with a limiter.

[0087] The reactive power control module may include a frequency control module and a fourth adder. The frequency control module may include a first adder, a multiplier, a second adder, a third adder and a current limiter C connected in sequence, the first and third adders are both inverting adders; the second and fourth adders are both in-phase adders.

[0088] The first adder has a positive input terminal which can input a reactive power reference value Q ref The negative input terminal can input the actual value of reactive power Q, and the output terminal of the first adder is connected to the input terminal of the multiplier. The ratio coefficient of the multiplier can be k q .

[0089] The second adder has one positive input terminal connected to the output terminal of the multiplier, and the second positive input terminal can be input with the voltage angular velocity rated value ω. 0 The second adder outputs the reference synchronous angular velocity ω of the wind turbine on the AC side of the offshore converter ref .

[0090] The third adder has a positive input terminal connected to the output terminal of the second adder, a negative input terminal inputting the voltage angular velocity detection value ω, and an output terminal connected to the input terminal of the current limiter C.

[0091] The fourth adder has one positive input terminal connected to the output terminal of the current limiter C, and the second positive input terminal can input the reactive current reference signal constant I q0 I q0 It can be a constant determined by the reactive power reference value and the rated voltage.

[0092] The phase-locked loop includes a fifth adder, a PI regulator, a sixth adder, and an integrator which are connected in sequence.

[0093] The fifth adder may be an inverting adder; the sixth may be a non-inverting adder.

[0094] The fifth adder can input 0 at its positive input terminal and the reactive voltage signal V at its negative input terminal. q , the output end of the fifth adder is connected to the input end of the PI regulator.

[0095] The sixth adder has one positive input terminal connected to the output terminal of the PI regulator, and the second positive input terminal can input the voltage angular velocity rated value ω 0 .

[0096] The 1 / s integrator outputs the phase angle θ required for coordinate transformation.

[0097] Preferably, the offshore converter is provided with a freewheeling diode and a load unloading circuit on the DC side; when a DC transmission line is detected to have a fault, the working state of the offshore converter corresponding to the faulty line is as follows:

[0098] During the period when the offshore converter is locked but its AC circuit breaker remains closed, the electric energy generated by the wind turbine is fed into the fault line through the freewheeling diode of the offshore converter; the offshore converter operates in the current limiting control mode, its active power control module is in the activated state, and the AC voltage control module is in the inactivated state; its DC side unloading circuit is activated to consume the active power that cannot be delivered in the offshore converter; when the AC side voltage of the offshore converter is lower than the preset threshold, the reactive power control module operates in the networking frequency control mode, its frequency control module is in the activated state, and the output current upper limit value of the current limiter C is set to I max , while the active power control module is in saturation state, the active current reference signal I output by the current limiter B dref =I dmax1 , Among them I dref is the active current reference signal; I qref is the reactive current reference signal; I dmax1 is the output upper limit of current limiter A.

[0099] During the period when the offshore converter is locked and its AC circuit breaker is disconnected, as the AC voltage on the AC side of the offshore converter increases, the active power control module exits the saturation state and its output current I dmax1 Starts to decrease; at this time, the AC voltage control module is in an activated state; the active current reference signal I output by the current limiter B dref =I dmax2 , I dmax2 is the output upper limit of the current limiter D; after stabilization, Idref =I dmax2 ≈0; the phase-locked loop still works normally to synchronize the wind turbines; each wind turbine operates in a network control mode based on the phase-locked loop.

[0100] Preferably, after the fault of the faulty line is cleared, the step of resuming operation of the equipment of the corresponding line includes the following steps:

[0101] Step A1, the AC circuit breaker of the offshore converter is closed again, and the DC disconnector of the offshore converter is closed;

[0102] Step A2, the offshore converter is unlocked, its DC voltage control module is in an activated state, and its active power control module is in an inactivated state; its DC voltage control module operates in a fixed DC voltage control mode; and is synchronized with the wind turbine generator set through a phase-locked loop;

[0103] Step A3, in a constant DC voltage control mode, when the DC voltage on the DC side of the offshore converter is equal to the DC voltage of other normal lines, closing the DC circuit breaker on the DC side of the onshore converter;

[0104] Step A4, the offshore converter switches from the fixed DC voltage control mode to the networking voltage control mode, that is, its AC voltage control module is in an activated state, and its active power control module or DC voltage control module is in an inactivated state; when the AC voltage control module is saturated again, the offshore converter switches to the grid-connected control mode, that is, the active power control module or DC voltage control module is in an activated state, and its AC voltage control module is in an inactivated state.

[0105] The above-mentioned offshore wind farm, half-bridge modular multilevel converter, onshore power grid, offshore converter, DC bus A, DC bus B, onshore converter, wind turbine, dual closed-loop controller, inner loop current control module, outer loop control module, active power control module, DC voltage control module, AC voltage control module, reactive power control module, frequency control module, current limiter A, current limiter B, current limiter C, first to sixth adders, multipliers, integrators, PI regulators, PI regulators with limiters and other components and modules can all be applicable components and modules in the prior art, or components and modules constructed using software and hardware in the prior art and conventional technical means.

[0106] The following is a preferred embodiment of the present invention to further illustrate the working process and working principle of the present invention:

[0107] An offshore wind power multi-terminal DC transmission system, such as Figure 1As shown in the circuit diagram, the system consists of two wind turbines (the 1st to the 2nd wind turbines), a flexible DC transmission system based on a half-bridge modular multilevel converter, and an onshore power grid (the 3rd to the 5th onshore power grid). The electric energy generated by the wind turbine is connected to the offshore converter of the flexible DC transmission system after transformation, and then connected to each onshore power grid through the DC bus A. Each offshore converter is equipped with the following control modules: DC voltage / active power / AC voltage, AC current, circulating current suppression, and voltage-based vertical and horizontal energy balance control. An AC circuit breaker (the 1st to the 2nd AC circuit breaker) is configured between the wind turbine and the offshore converter, an AC circuit breaker (the 3rd to the 5th AC circuit breaker) is configured between the onshore power grid and the onshore converter, a DC circuit breaker (the 1st to the 7th DC circuit breaker) is configured on the onshore converter side of the DC bus, and a DC disconnector (the 1st to the 3rd DC disconnector) is used on the offshore converter side. Taking the DC fault of the 14th DC bus A, i.e. Cable 14 in the figure, as an example, the present invention proposes an enhanced passive AC voltage Vac control module for an offshore converter to achieve active control after a DC fault occurs in an offshore wind power multi-terminal DC transmission system.

[0108] A method for actively controlling DC faults in an offshore wind power multi-terminal DC transmission system is provided. The method is implemented based on the above-mentioned offshore wind power multi-terminal DC transmission system. The method is specifically developed as follows:

[0109] DC fault detection and isolation: When a DC fault occurs in the 14th DC bus A in the flexible DC multi-terminal transmission system, the 4th DC circuit breaker is configured on the onshore converter side of the line, and the 1st DC circuit breaker, the 1st offshore converter and its DC disconnector, and the 1st wind turbine set are configured on the onshore converter side of the 13th DC bus A, and the multi-terminal DC transmission system is divided into a normal line and a fault line, and the fault line is isolated.

[0110] Step (1-1): The present invention detects DC faults and locates lines based on the voltage change rate of the DC reactor of the onshore converter or the offshore converter, and quickly and accurately selects and disconnects the fourth DC circuit breaker configured on the onshore converter side of the faulty line and the first DC circuit breaker configured on the onshore converter side of the 13th DC bus connected to the first offshore converter, so that the first offshore converter is not affected.

[0111] Step (1-2): After detecting the DC fault line, the first offshore converter is locked. 20ms after the fault detection, if the detected DC voltage is still lower than 50% of the rated value, the first AC circuit breaker is opened to reduce the current fed into the DC fault point by the wind turbine to zero. Otherwise, if the DC voltage is restored, the first AC circuit breaker remains closed.

[0112] Step (1-3): When the current of the second DC isolating switch is less than 50A, the second DC isolating switch is turned off, thereby isolating the 14th DC bus from the first offshore converter.

[0113] Seamless switching strategy between grid-connected control mode and networking control mode of offshore converter: In normal mode, the offshore converter operates in grid-connected control mode, and the outer loop control module adopts active power control module or DC voltage control module (hereinafter referred to as P / Vdc control module) and reactive power control module Q. At this time, the voltage and frequency of the offshore AC power grid are provided by the first offshore converter. After the first offshore converter is locked after a DC fault, the first wind turbine loses voltage and frequency control. At this time, the offshore converter needs to operate in networking control to collaboratively establish the voltage and frequency of the offshore AC power grid. The offshore converter in the present invention relies on local measurement to automatically realize the conversion of the offshore converter between grid-connected control mode and networking control mode, thereby preventing the wind turbine from exiting operation after the first offshore converter is locked and tripped. The present invention proposes a seamless switching strategy between grid-connected control mode and networking control mode of offshore converters, and its control block diagram is shown as follows. Figures 2 to 5 shown.

[0114] Step (2-1): During normal operation, the first offshore converter operates in the grid-connected control mode to control the voltage and frequency of the offshore AC power grid. Its networking control mode is in an inactive state. Specifically: a. Inner loop active current I dref It is determined by the active power control module, and the voltage control of the network control has a reference value of 1.1 times the rated voltage (greater than the measured voltage value), and the control output is I max , this output current I max As the upper limit value of the dynamic limiter of the output current of the P / Vdc control module; during normal operation, the output current of the P / Vdc control module is less than I max , so the normal operation of P / Vdc control is not affected. b. During normal operation, the reactive power control module works in the network frequency control mode, and its output current limit I qmax is set to 0, and the reactive power control module is inactive. At this time, the inner loop reactive current I qref Equal to I q0 , I q0 is a constant, determined by the reactive power reference value and the rated voltage. c. Under normal operating conditions, the offshore converter phase-locked loop works normally and provides the phase angle required for coordinate transformation for the grid-connected control mode.

[0115] Step (2-2): When the DC fault causes the first offshore converter to lock but the first AC circuit breaker remains closed, the offshore wind power is fed to the fault point through the freewheeling diode of the first offshore converter. The DC fault causes the AC voltage of the offshore power grid to drop, and the offshore converter operates in the current limiting mode. The DC side unloading circuit of the offshore converter is activated to consume the active power that cannot be delivered in the offshore converter. When the offshore AC voltage V ac When the output current of the reactive power control module in the networking control mode is lower than the preset threshold, the output current of the reactive power control module in the networking control mode is limited to I qmax Increase to I max , to ensure the frequency stability during this period. The active power control module of the offshore converter is in saturation state. dref =I dmax1 ,in That is, reactive current is given priority, and the AC voltage control module in the networking control mode remains inactive.

[0116] Step (2-3): During the period when the DC fault causes the first offshore converter to be locked and the first AC circuit breaker to be disconnected, the offshore AC voltage increases. At this time, the active power control module of the networking control mode exits the saturation state, and its output current I dmax1 Started to decline, no longer I max At this time, the AC voltage control module is in the active state, I dref =I dmax2 , which is completely determined by the AC voltage control module of the networking control mode, that is, it runs in the AC voltage control mode. After stabilization, I dref =I dmax2 ≈0. Although the offshore converters operate in the networking control mode, the phase-locked loop still works normally to synchronize the offshore converters. The offshore converters operate in the networking control mode based on the phase-locked loop.

[0117] In the above manner, the offshore converter achieves seamless, autonomous and controlled conversion between the grid-connected control mode and the networking control mode after a DC fault and during the period when the first offshore converter is locked and its first AC circuit breaker is disconnected.

[0118] DC fault recovery strategy for offshore wind power delivered via the 1st offshore converter: The goal is to restart the 1st offshore converter after the DC fault line is isolated and restore power delivery from the offshore wind farm to a healthy multi-terminal DC transmission system. Figure 5 A flow chart showing the DC fault isolation and fault recovery strategy of the flexible DC multi-terminal transmission system. Fault ride-through refers to the ability to maintain operation without disconnecting from the large power grid after a fault occurs until a stable operating state is reached.

[0119] Step (3-1): With the opening of the second offshore DC disconnector, the faulty line is isolated. The first AC circuit breaker of the first offshore converter is closed again to interconnect with the offshore wind turbine.

[0120] Step (3-2): The first offshore converter is unlocked and operates in a constant DC voltage control mode, and is synchronized with the offshore wind turbine through a phase-locked loop. After power is restored, the half-bridge modular multilevel converter is activated and operates in a DC voltage control mode with PLL activated.

[0121] Step (3-3): Based on the DC voltage control method of the first offshore converter, adjust its DC voltage to be equal to the DC voltage of the normal line, and then close the first DC circuit breaker configured on the onshore converter side to achieve DC flexible interconnection with the healthy multi-terminal DC transmission system.

[0122] Step (3-4): The first offshore converter switches from the fixed DC voltage control back to the networking control mode to control the voltage and frequency of the offshore wind turbine. This makes the networking control of the offshore converter saturated again, and the offshore converter returns to the grid-connected control mode.

[0123] DC voltage control method, that is Figure 5 Mode=1, AC voltage control mode is Figure 5 Mode=0 in .

[0124] The overall timing of the DC fault isolation and restoration strategy for offshore wind power delivered to a multi-terminal system via a half-bridge modular multilevel converter, Figure 6 The following is the overall flow chart. The steps are as follows:

[0125] Step (4-1): The fault isolation part includes locking the corresponding offshore converter for 20ms after detecting a DC fault, detecting whether the DC voltage is restored, and if not, tripping the AC circuit breaker, and then identifying the DC disconnector of the faulty line. When the current of the DC disconnector is less than 50A, cutting off the corresponding DC disconnector.

[0126] Step (4-2): The fault recovery part includes resetting and closing the AC circuit breaker, re-sending power to the first offshore converter, unlocking the first offshore converter, making it run under constant DC voltage control, matching the DC voltage, and finally resetting and closing the first DC circuit breaker configured on the onshore converter side. The first offshore converter is then switched back to constant AC voltage control, and the wind turbine converter is switched back to grid-connected control mode, thereby restoring power transmission.

[0127] The embodiments described above are only used to illustrate the technical ideas and features of the present invention, and their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. The patent scope of the present invention cannot be limited only by these embodiments, that is, any equivalent changes or modifications made to the spirit disclosed by the present invention still fall within the patent scope of the present invention.

Claims

1. An offshore wind power multi-terminal DC transmission system, It is characterized in that It includes an offshore wind farm, a flexible DC multi-terminal transmission system based on a half-bridge modular multi-level converter, and an onshore power grid; the flexible DC multi-terminal transmission system includes N offshore converters, N DC busbars A, K DC busbars B, and N onshore converters, K≤N; the offshore converter is used to convert the AC power generated by the offshore wind farm into DC power, and the onshore converter is used to convert the DC power into AC power; DC reactors are provided on the DC sides of the onshore converter and the offshore converter; the offshore wind farm includes M wind turbines, each wind turbine is connected to at least one offshore converter, and each wind turbine is connected to the AC side of the offshore converter in turn through a transformer and an AC circuit breaker; the DC busbar A, one end of which is connected to the DC side of the offshore converter through a DC isolating switch, and the other end of which is connected to the DC side of the onshore converter through a DC circuit breaker; both ends of the DC busbar B are connected to the DC sides of two onshore converters through DC circuit breakers; The controller of the offshore converter includes a double closed-loop controller, which includes an inner loop current control module and an outer loop control module; the outer loop control module includes an active power control module, a DC voltage control module, an AC voltage control module and a reactive power control module, and the inner loop current control module includes an active power current module and a reactive power current module; the active power control module, the DC voltage control module and the AC voltage control module output active current reference signals to the active power current module; the reactive power control module outputs reactive current reference signals to the reactive power current module; The active power control module or the DC voltage control module is provided with a current limiter A and a current limiter B connected in series in sequence, and the reactive power control module is provided with a current limiter C; the AC voltage control module inputs the difference between the maximum AC voltage on the AC side of the offshore converter and the corresponding actual voltage, and outputs a signal to the current limiter B.

2. The offshore wind power multi-terminal DC transmission system according to claim 1, It is characterized in that The DC sides of two adjacent onshore converters are connected in sequence through a DC bus bar B.

3. The offshore wind power multi-terminal DC transmission system according to claim 1, It is characterized in that The active power control module is used to control the output power of the offshore converter; the DC voltage control module is used to control the DC side voltage of the offshore converter; the AC voltage control module is used to control the AC side voltage of the offshore converter; the reactive power control module includes a frequency control module, and the frequency control module is used to control the AC side frequency of the offshore converter.

4. A DC fault active control method using the offshore wind power multi-terminal DC transmission system according to any one of claims 1 to 3, It is characterized in that An offshore wind farm, a flexible DC multi-terminal transmission system based on a half-bridge modular multi-level converter and an onshore power grid are set up; the flexible DC multi-terminal transmission system is equipped with N offshore converters, N DC busbars A, K DC busbars B and N onshore converters, K≤N; the offshore converter is used to convert the AC power generated by the offshore wind farm into DC power, and the onshore converter is used to convert the DC power into AC power; DC reactors are set up on the DC sides of the onshore converter and the offshore converter; M wind turbines are set up in the offshore wind farm, so that each wind turbine is connected to at least one The offshore converter is connected, and each wind turbine is connected to the AC side of the offshore converter through a transformer and an AC circuit breaker in turn; one end of the DC bus A is connected to the DC side of the offshore converter through a DC disconnector, and the other end of the DC bus A is connected to the DC side of the onshore converter through a DC circuit breaker; both ends of the DC bus B are connected to the DC sides of the two onshore converters through DC circuit breakers; according to the fault detection result, the DC transmission line of the flexible DC multi-terminal transmission system is divided into a normal line and a fault line, and the fault line is isolated.

5. The DC fault active control method of the offshore wind power multi-terminal DC transmission system according to claim 4, It is characterized in that The fault of the DC transmission line is determined by one of the following methods or a combination of the following methods: the DC reactor voltage of the onshore converter and the offshore converter is detected. If the voltage change rate of the DC reactor exceeds the corresponding set value range, it is determined that the DC transmission line connected to the DC reactor is faulty; the current of the DC disconnector on the DC side of the offshore converter is detected. If the current of the DC disconnector is less than the corresponding set threshold value, it is determined that the DC transmission line connected to the DC disconnector is faulty.

6. The DC fault active control method of the offshore wind power multi-terminal DC transmission system according to claim 5, It is characterized in that When a fault is detected in a DC transmission line, the DC circuit breaker on the DC side of the onshore converter and the DC isolating switch on the DC side of the offshore converter corresponding to the faulty line are disconnected; 20ms after the fault is detected, if the detected DC voltage is still lower than 50% of the rated value, the AC circuit breaker on the AC side of the offshore converter connected to the faulty line is disconnected to reduce the current input from the wind turbine to the faulty line to zero; when the fault is removed, the corresponding AC circuit breaker is kept closed first, and then the DC circuit breaker on the DC side of the onshore converter and the DC isolating switch on the DC side of the offshore converter are closed.

7. The DC fault active control method for an offshore wind power multi-terminal DC transmission system according to claim 4, It is characterized in that The controller of the offshore converter is provided with a double closed-loop controller, which includes an inner loop current control module and an outer loop control module; the outer loop control module includes an active power control module, a DC voltage control module, an AC voltage control module and a reactive power control module, the active power control module is used to control the output power of the offshore converter; the DC voltage control module is used to control the DC side voltage of the offshore converter; the AC voltage control module is used to control the AC side voltage of the offshore converter; the reactive power control module includes a frequency control module, the frequency control module is used to control the AC side frequency of the offshore converter; the inner loop current ... The active power current module and the reactive power current module are included; the active power control module and the DC voltage control module work alternately; the active power control module, the DC voltage control module and the AC voltage control module output active current reference signals to the active power current module; the reactive power control module outputs reactive current reference signals to the reactive power current module; the active power control module or the DC voltage control module is provided with a current limiter A and a current limiter B connected in series in sequence, and the reactive power control module is provided with a current limiter C; the AC voltage control module inputs the difference between the maximum AC voltage and the corresponding actual voltage on the AC side of the offshore converter, and outputs a signal to the current limiter B; When the detection voltage input by the AC voltage control module is 1.1 times the rated voltage, the control output of the current limiter D is I max , output current I max As the upper limit value of current limiter B; During normal operation, the offshore converter operates in the grid-connected control mode, the active power control module or the DC voltage control module in its outer loop control module is in an activated state, and the frequency control module and the AC voltage control module are in an inactivated state; the output current upper limit value of the current limiter C is set to 0, and the inner loop reactive current reference signal is a constant determined by the reactive power reference value and the rated voltage; the phase-locked loop corresponding to the offshore converter operates normally, providing the phase angle required for coordinate transformation for the grid-connected control mode.

8. The DC fault active control method for the offshore wind power multi-terminal DC transmission system according to claim 7, It is characterized in that The offshore converter is provided with a freewheeling diode and a load unloading circuit on the DC side; when a DC transmission line is detected to have a fault, the working state of the offshore converter corresponding to the faulty line is as follows: During the period when the offshore converter is locked but its AC circuit breaker remains closed, the electric energy generated by the wind turbine is fed into the fault line through the freewheeling diode of the offshore converter; the offshore converter operates in the current limiting control mode, its active power control module is in the activated state, and the AC voltage control module is in the inactivated state; Its DC side unloading circuit is activated to consume the active power that cannot be delivered in the offshore converter; when the AC side voltage of the offshore converter is lower than the preset threshold, the reactive power control module works in the network frequency control mode, its frequency control module is in an activated state, and the output current upper limit value of the current limiter C is set to I max , while the active power control module is in saturation state, the active current reference signal I output by the current limiter B dref =I dmax1 , Among them I dref is the active current reference signal; I qref is the reactive current reference signal; I dmax1 is the output upper limit of the current limiter A; During the period when the offshore converter is locked and its AC circuit breaker is disconnected, as the AC voltage on the AC side of the offshore converter increases, the active power control module exits the saturation state and its output current I dmax1 Starts to decrease; at this time, the AC voltage control module is in an activated state; the active current reference signal I output by the current limiter B dref =I dmax2 , I dmax2 is the output upper limit of the current limiter D; after stabilization, I dref =I dmax2 ≈0; the phase-locked loop still works normally to synchronize the wind turbines; each wind turbine operates in a network control mode based on the phase-locked loop.

9. The DC fault active control method for an offshore wind power multi-terminal DC transmission system according to claim 7, It is characterized in that After the fault of the faulty line is cleared, the steps for resuming operation of the equipment on the corresponding line include the following steps: Step A1, the AC circuit breaker of the offshore converter is closed again, and the DC disconnector of the offshore converter is closed; Step A2, the offshore converter is unlocked, its DC voltage control module is in an activated state, and its active power control module is in an inactivated state; Its DC voltage control module operates in a constant DC voltage control mode; and maintains synchronization with the wind turbine through a phase-locked loop; Step A3, in a constant DC voltage control mode, when the DC voltage on the DC side of the offshore converter is equal to the DC voltage of other normal lines, closing the DC circuit breaker on the DC side of the onshore converter; Step A4, the offshore converter switches from the fixed DC voltage control mode to the networking voltage control mode, that is, its AC voltage control module is in an activated state, and its active power control module or DC voltage control module is in an inactivated state; when the AC voltage control module is saturated again, the offshore converter switches to the grid-connected control mode, that is, the active power control module or DC voltage control module is in an activated state, and its AC voltage control module is in an inactivated state.

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