Offshore wind farm bipolar converter station coordinated control method and device, storage medium

Abstract

The present invention discloses a coordinated control method and device for a bipolar converter station at an offshore wind farm, and a storage medium, comprising the following steps: step S1, establishing an AC voltage with constant amplitude and frequency for the wind farm when the system is in steady-state operation through WFMMC1 for full fault ride-through control, and adopting a fixed AC voltage amplitude / fixed frequency control to stabilize the voltage at the wind farm grid connection point; step S2, adopting active power balancing control through WFMMC2 for reactive power support to balance the power distribution between the positive and negative poles; step S3, adopting a fixed DC voltage-fixed reactive power control through GSVSC for active power priority; step S4, adopting a fixed active power-AC voltage control through WTVSC for power coordination. The technical solution of the present invention can improve the safe and stable operation capability and fault ride-through capability of the offshore wind power through the flexible direct current transmission system.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular relates to a coordinated control method and device for a bipolar converter station in an offshore wind farm, and a storage medium. Background Art

[0002] Today's society is changing with each passing day, and behind this lies the contradiction between the ever-expanding demand for energy and the decreasing reserves of traditional non-renewable energy. Not only that, the large-scale use of fossil energy also puts tremendous pressure on our ecological environment.

[0003] MMC-HVDC (MMC Based High Voltage Direct Current, MMC-based flexible direct current transmission, MMC, Modular Multilevel Converter) has the advantages of low manufacturing difficulty, low switching loss, good waveform quality, and low harmonic content. It has good application prospects in long-distance offshore wind power transmission. The wiring methods of the MMC-HVDC system are mainly pseudo-bipolar and true bipolar. Compared with the pseudo-bipolar wiring method, the positive and negative converters of the true bipolar MMC-HVDC system are independently controlled, and it has the advantages of large transmission capacity, high reliability, strong fault tolerance and flexible control. As offshore wind power will develop in the direction of large capacity, high voltage level, and long distance in the future, the true bipolar MMC-HVDC system will become the preferred solution for offshore wind power transmission projects.

[0004] At present, the research mainly focuses on the fault characteristics of the offshore AC side under the symmetrical unipolar connection topology and its corresponding fault ride-through strategy. There are relatively few studies on the offshore AC side fault of the true bipolar system. Unlike the traditional rotating generator grid-connected system, the offshore wind power true bipolar MMC-HVDC grid-connected system uses power electronic converters to access the grid, which has the characteristics of many controllable devices, complex control methods, weak system inertia and low impedance. When a fault occurs on the offshore AC side, overvoltage, overcurrent and other phenomena will occur, which will pose a great threat to related devices and cause the system to fail to ride through the fault. In addition, under inappropriate fault ride-through control strategies, the system fault characteristics are weak and the fault detection is difficult, which may cause the corresponding relay protection device to refuse to operate, thus bringing great challenges to the safe and stable operation of the system. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a coordinated control method and device for a bipolar converter station in an offshore wind farm, as well as a storage medium, which can improve the safe and stable operation capability and fault ride-through capability of offshore wind power through a flexible direct current transmission system.

[0006] To achieve the above object, the present invention adopts the following technical solution:

[0007] A coordinated control method for a bipolar converter station in an offshore wind farm comprises the following steps:

[0008] Step S1, establishing an AC voltage with constant amplitude and frequency for the wind farm when the system is in steady-state operation through WFMMC1 for full fault ride-through control, and adopting constant AC voltage amplitude / constant frequency control to stabilize the voltage at the wind farm grid connection point;

[0009] Step S2, using active power balancing control through WFMMC2 for reactive power support to balance the power distribution between the positive and negative electrodes;

[0010] Step S3, using a GSVSC for active power priority to adopt a constant DC voltage-constant reactive power control;

[0011] Step S4: adopting fixed active power-AC voltage control through WTVSC for power coordination.

[0012] Preferably, in step S3, d-axis control is used to achieve voltage stability on the DC side of the wind turbine generator set; and q-axis control is used to achieve zero reactive power of the wind farm.

[0013] Preferably, in step S4, the d-axis is used to control the active power to track the power reference value of the maximum power operating point, and power is delivered to the grid-side converter; the q-axis is used to control the voltage amplitude on the AC side; wherein, when a fault occurs on the AC side of the system, power coordinated control is adopted, and when the wind turbine is operating at maximum power delivery, the active power reference value on the machine side is reduced according to the voltage level at the grid-connected point.

[0014] Preferably, in step S1, when a fault occurs on the offshore AC side, a full fault ride-through strategy is adopted to achieve offshore AC side fault ride-through.

[0015] Preferably, in step S2, when a fault occurs on the offshore AC side, reactive support is provided to meet the reactive power demand of the system, and negative sequence current is provided through negative sequence voltage droop control.

[0016] The present invention also provides a coordinated control device for a bipolar converter station at an offshore wind farm, comprising: a WFMMC1 for full fault ride-through control, a WFMMC2 for reactive support, a GSVSC for active power priority, and a WTVSC for power coordination, wherein the WFMMC1 for full fault ride-through control, the WFMMC2 for reactive support, the GSVSC for active power priority, and the WTVSC for power coordination are connected to wind turbines respectively; wherein the WFMMC1 establishes an AC voltage with constant amplitude and frequency for the wind farm when the system is in steady-state operation, and adopts a fixed AC voltage amplitude / fixed frequency control to stabilize the voltage at the wind farm grid connection point; the WFMMC2 adopts active power balancing control to balance the power distribution between the positive and negative poles; the GSVSC adopts a fixed DC voltage-fixed reactive power control; and the WTVSC adopts a fixed active power-AC voltage control.

[0017] The present invention also provides a storage medium, on which a computer program is stored, and when the computer program is run, the method for coordinated control of bipolar converter stations in an offshore wind farm is executed.

[0018] The present invention has the following beneficial effects:

[0019] 1. Improve the economy and adaptability of the system: In the bipolar MMC-HVDC system of the present invention, the positive and negative poles of the offshore converter station adopt two different control strategies, one pole adopts a fixed V / f control, and the other pole adopts an active balance control strategy, which reduces the power loss on the DC side and improves the power transmission margin, and can adapt to various operating conditions of offshore wind power;

[0020] 2. Enhance the system's ability to ride through faults on the offshore AC side: The offshore wind farm-bipolar converter station coordinated control strategy achieves effective support for the AC side voltage during faults, improves power transmission efficiency and wind energy utilization, increases the system's ability to withstand fault resistance, and enhances the system's ability to ride through faults, which is of great significance to promoting the development and utilization of offshore wind power technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0022] Figure 1 This is a flow chart of the inter-pole coordinated control method of the offshore wind power true bipolar MMC-HVDC system of the present invention;

[0023] Figure 2It is a schematic diagram of the simulation results of rated power operation under coordinated control in the inter-pole coordinated control method of the offshore wind power true bipolar MMC-HVDC system of the present invention; wherein 2-a is the simulation result of the offshore AC bus voltage; 2-b is the simulation result of the DC voltage; 2-c is the simulation result of the active power; 2-d is the simulation result of the reactive power; 2-e is the simulation result of the metal return line current; 2-f is the simulation result of the wind farm power;

[0024] Figure 3 It is a schematic diagram of the simulation results of the flexible DC system when a two-phase grounding fault occurs under coordinated control in the inter-pole coordinated control method of the offshore wind power true bipolar MMC-HVDC system of the present invention; wherein, 3-a is the simulation result of the AC bus voltage on the WFMMC side; 3-b is the true result of the metal loop current; 3-c is the simulation result of the WFMMC1 current; 3-d is the simulation result of the WFMMC2 current; 3-e is the active power simulation result; 3-f is the reactive power simulation result. DETAILED DESCRIPTION

[0025] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] Embodiment 1:

[0028] like Figure 1 As shown, an embodiment of the present invention provides a coordinated control method for a bipolar converter station in an offshore wind farm, comprising the following steps:

[0029] Step S1: By using the WFMMC1 (Wind Farm Modular Multilevel Converter) for full fault ride-through control, an AC voltage with constant amplitude and frequency is established for the wind farm when the system is in steady-state operation, and V / f control (constant AC voltage amplitude / constant frequency control) is adopted to control the voltage stability of the wind farm grid connection point; wherein, when a fault occurs on the offshore AC side, a full fault ride-through strategy is adopted to achieve fault ride-through on the offshore AC side;

[0030] Step S2, using active power balancing control through WFMMC2 for reactive power support to balance the power distribution between the positive and negative poles and reduce the power transmission loss on the DC side of the system; wherein, when a fault occurs on the AC side at sea, reactive power support is provided to meet the reactive power demand of the system, and negative sequence current is provided through negative sequence voltage droop control to reduce the overvoltage of the non-fault phase;

[0031] Step S3, using a GSVSC (grid side VSC) for active power priority to adopt a constant DC voltage-constant reactive power control, wherein the d-axis control realizes the DC side voltage stability of the wind turbine set to ensure the normal delivery of active power; the q-axis control realizes the reactive power of the wind farm to be 0, so that the wind farm operates in a unit power factor mode, thereby improving the utilization rate of wind power; wherein, when a fault occurs on the AC side of the system, the active power priority control mode is used instead, and the negative sequence current is controlled to be 0 to prevent overcurrent;

[0032] Step S4, using fixed active power-AC voltage control through WTVSC (wind turbine side VSC) for power coordination, the d-axis controls the active power to track the power reference value of the maximum power operating point, and transmits power to the grid-side converter; the q-axis controls the AC side voltage amplitude to ensure the voltage stability of the wind turbine end; wherein, when a fault occurs on the AC side of the system, power coordination control is adopted, and while ensuring that the wind turbine set operates in the maximum power delivery state, the machine-side active power reference value is reduced according to the voltage level at the grid-connected location to avoid overvoltage in the system after the fault ends.

[0033] As an implementation method of the present invention, the control of WFMMC1 is as follows: Figure 1 As shown, the superscripts + and - respectively represent the positive and negative sequence components; the subscripts d and q respectively represent the d-axis and q-axis voltage components; the subscript ref represents the reference value; and They represent the positive and negative sequence components of the dq axis voltage on the WFMMC grid side respectively; and They represent the reference values ​​of the positive and negative sequence components of the dq axis voltage on the WFMMC grid side, respectively, and their values ​​are shown in formula (1.1); S f Indicates a fault signal. When a fault is detected, S fThe value of changes from 0 to 1, thereby locking the integrator; after the fault is cleared, its value changes from 1 to 0, so that the integrator is put into operation and the actual voltage is controlled to be consistent with the voltage reference value. In order to ensure the basic stability of the AC voltage phase during the fault, the positive sequence q axis voltage outer loop is not locked. The locking control is to lock the integrator in the voltage outer loop when the system fault is detected so that it no longer works during the fault. The reasons for this are: first, to prevent the integrator from continuing to work during the fault and reaching saturation, so as to lose control of the system AC voltage; second, after the fault is over, the output value of the integrator is near the original steady-state value, which can speed up the system recovery. K od_w 、T od_w , K oq_w and T oq_w Represent the parameters of d-axis and q-axis proportional controller and integral controller respectively; and They represent the maximum allowable positive and negative sequence currents of the d and q axes of the WFMMC respectively; and The reference values ​​of the d-axis and q-axis currents of WFMMC are respectively expressed as follows:

[0034]

[0035] in, Indicates the negative sequence voltage amplitude.

[0036] As an implementation method of the present invention, the control of WFMMC2 is as follows: Figure 1 As shown, I dcn Indicates the metal loop current; I nref Indicates the reference value of the metal loop current, which is taken as 0; Q w2 represents the reactive power of WFMMC2; Q ref_w2 Indicates the reactive power reference value of WFMMC2, which is taken as 0; and They represent the maximum allowable positive and negative sequence currents of the d and q axes of WFMMC2 respectively; Indicates the positive and negative sequence output voltage reference value of WFMMC2; the q-axis current reference value during fault As shown in formula (2):

[0037]

[0038] Among them, K w is the reactive current regulation coefficient of WFMMC2, which is taken as 1.5; U sw_pu is the per unit value of the voltage amplitude on the WFMMC grid side; I wN is the rated current of WFMMC2.

[0039] As an implementation method of the present invention, the control of GSVSC is as follows: Figure 1 As shown, V dc Indicates the DC side voltage of the fan; V dc_ref Indicates the DC side voltage reference value, which is taken as 1; Q g Indicates the reactive power of the wind turbine grid-side converter; Q ref_w2 Represents the reactive power reference value of the wind turbine grid-side converter, which is taken as 0; I dmax_g ,I qmax_g They represent the maximum allowable currents of the d-axis and q-axis of the wind turbine grid-side converter respectively; Indicates the positive and negative sequence output voltage reference value of the wind turbine grid-side converter; the q-axis current reference value during a fault As formula (3)

[0040]

[0041] Among them, K is the reactive current regulation coefficient of the wind turbine grid-side converter, which is taken as 1.5; U sg_pu is the per unit value of the grid-side voltage amplitude of the wind turbine grid-side converter; I gN is the rated current of the wind turbine grid-side converter.

[0042] As an implementation method of the present invention, the control of WTVSC is as follows: Figure 1 As shown, P m Indicates the active power on the machine side; U m Indicates the effective value of the generator side voltage, U ref_m Indicates the reference value of the effective value of the generator side voltage, which is taken as 1; u cjref_m , represents the reference value of the machine side output voltage; the machine side active power P ref_m The reference value is as shown in formula (4)

[0043]

[0044] In PSCAD / EMTDC, Figure 1 The offshore wind power true bipolar MMC-HVDC system shown in the figure has system parameters listed in Table 1.

[0045] Table 1

[0046]

[0047] A) Steady-state operation

[0048] When the system is running at rated power in steady state, the system simulation results are as follows: Figure 2As shown in the figure, the offshore AC bus voltage is controlled at 66kV by WFMMC1, and will slightly increase to around 69kV during the offshore wind power ramp-up stage (2s-3s); the metal loop current is maintained at around 0.3kA during the offshore wind power ramp-up stage (2s-3s), mainly because there is a difference with the metal loop current reference value. The d-axis current reference value of WFMMC2 is generated by the PI integrator, and the metal loop current is controlled to zero after the system stabilizes; during the period of 2s-3s, due to the existence of the metal loop current, there is a power difference between WFMMC1 and WFMMC2, and the active power transmitted by the two poles is equal after the system reaches a steady state; the reactive power of WFMMC2 is controlled to zero after the system is in a steady state, and WFMMC1 provides 100MVar reactive power to meet the reactive power demand of the system.

[0049] B) Fault ride-through

[0050] At 6.0s, a two-phase grounding fault occurred on phases B and C on the offshore AC side, which lasted for 0.625s. The simulation results of the flexible DC system during the two-phase grounding fault are as follows: Figure 3 As shown. Figure 3 It can be seen that under the coordinated control of the offshore wind farm-bipolar converter station, when a two-phase grounding fault occurs on the offshore AC side, the flexible DC system quickly reaches the fault steady-state stage, the AC bus voltage amplitude on the WFMMC side drops to about 0.52pu, and the non-fault phase voltage is around 0.7pu; the metal loop current is also controlled near zero during the fault steady-state period; the current characteristics of WFMMC1 and WFMMC2 are basically the same, the positive sequence d-axis current is the same, and the positive sequence q-axis current of the latter is larger; the active power transmitted by WFMMC1 and WFMMC2 is basically the same, both about 0.2pu; WFMMC2 injects more reactive power into the system, about 0.2pu, to meet the reactive power demand during the system fault. During the entire fault ride-through period, the flexible DC system did not experience overcurrent, recovery overvoltage, and non-fault phase overvoltage, and was able to achieve two-phase grounding fault ride-through.

[0051] Embodiment 2:

[0052] An embodiment of the present invention also provides a coordinated control device for a bipolar converter station in an offshore wind farm, comprising: WFMMC1 for full fault ride-through control, WFMMC2 for reactive support, GSVSC for active power priority, and WTVSC for power coordination, wherein the WFMMC1 for full fault ride-through control, the WFMMC2 for reactive support, the GSVSC for active power priority, and the WTVSC for power coordination are respectively connected to wind turbines; wherein WFMMC1 establishes an AC voltage with constant amplitude and frequency for the wind farm when the system is in steady-state operation, and adopts fixed AC voltage amplitude / fixed frequency control to stabilize the voltage at the wind farm grid connection point; WFMMC2 adopts active power balancing control to balance the power distribution between the positive and negative poles; GSVSC adopts fixed DC voltage-fixed reactive power control; and WTVSC adopts fixed active power-AC voltage control.

[0053] Embodiment 3:

[0054] An embodiment of the present invention further provides a storage medium, on which a computer program is stored, and when the computer program is run, a coordinated control method for bipolar converter stations in an offshore wind farm is executed.

[0055] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A coordinated control method for a bipolar converter station in an offshore wind farm, characterized in that: The following steps are involved: Step S1, establishing an AC voltage with constant amplitude and frequency for the wind farm when the system is in steady-state operation through WFMMC1 for full fault ride-through control, and adopting constant AC voltage amplitude / constant frequency control to stabilize the voltage at the wind farm grid connection point; Step S2, using active power balancing control through WFMMC2 for reactive power support to balance the power distribution between the positive and negative electrodes; Step S3, using a GSVSC for active power priority to adopt a constant DC voltage-constant reactive power control; Step S4, using fixed active power-AC voltage control through WTVSC for power coordination; In step S3, d-axis control is used to achieve voltage stability on the DC side of the wind turbine generator set; q-axis control is used to achieve zero reactive power of the wind farm; In step S4, the d-axis is used to control the active power to track the power reference value of the maximum power operation point, and power is delivered to the grid-side converter; the q-axis is used to control the voltage amplitude of the AC side; wherein, when a fault occurs on the AC side of the system, power coordinated control is used, and when the wind turbine generator set is running at the maximum power delivery state, the active power reference value of the machine side is reduced according to the voltage level at the grid connection point; In step S1, when a fault occurs on the offshore AC side, a full fault ride-through strategy is adopted to achieve fault ride-through on the offshore AC side; In step S2, when a fault occurs on the offshore AC side, reactive power support is provided to meet the reactive power demand of the system, and negative sequence current is provided to the fault point through negative sequence voltage droop control.

2. A coordinated control device for a bipolar converter station in an offshore wind farm that implements the coordinated control method for a bipolar converter station in an offshore wind farm according to claim 1, characterized in that: include: WFMMC1 for full fault ride-through control, WFMMC2 for reactive power support, GSVSC for active power priority and WTVSC for power coordination, wherein the WFMMC1 for full fault ride-through control, WFMMC2 for reactive power support, GSVSC for active power priority and WTVSC for power coordination are connected to wind turbines respectively; wherein WFMMC1 establishes an AC voltage with constant amplitude and frequency for the wind farm when the system is in steady-state operation, and adopts fixed AC voltage amplitude / fixed frequency control to stabilize the voltage at the wind farm grid connection point; WFMMC2 adopts active power balancing control to balance the power distribution between the positive and negative poles; GSVSC adopts fixed DC voltage-fixed reactive power control; and WTVSC adopts fixed active power-AC voltage control.

3. A storage medium having a computer program stored thereon, wherein the computer program executes the coordinated control method for bipolar converter stations in an offshore wind farm according to claim 1 when running.

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