Active frequency support control method for offshore wind power through MMC-HVDC grid-connected system

Abstract

The invention discloses an active frequency support control method for offshore wind power through an MMC-HVDC grid-connected system, and relates to the technical field of control of offshore wind power through a flexible direct-current power transmission grid-connected system. The problem that the active supporting capability of existing offshore wind power on a shore power grid through a flexible direct-current grid-connected system is weak is solved. According to the invention, the shore converter station is controlled by a virtual synchronous machine; the offshore wind turbine generator set transmits power to the shore converter station through the offshore converter station via flexible direct current to supply power to the shore power grid; the offshore converter station adopts a proportional-integral controller to control direct-current voltage to be kept at a rated value; the virtual synchronous machine control adopts a mode of simulating a synchronous machine rotor motion equation and an excitation equation and is combined with frequency modulation to control an offshore converter, so that the offshore wind turbine generator increases or decreases the power to actively support an on-shore power grid. The method is suitable for active frequency support control of offshore wind power through the MMC-HVDC grid-connected system.

Description

Technical Field

[0001] The present invention relates to the technical field of offshore wind power grid-connected system control via flexible direct current transmission, and in particular to active support control of offshore wind power grid-connected system via flexible direct current transmission. Background Art

[0002] Offshore wind power has developed rapidly due to its abundant wind resources, no need to occupy land resources, suitable for large-scale development, proximity to load centers and many other advantages. It has become an important measure of my country's energy strategy. The flexible direct current transmission (Voltage Source Converter Based HVDC, VSC-HVDC) based on modular multilevel converter (MMC) has become the preferred solution for offshore wind power transmission due to its flexible control and low long-distance transmission loss.

[0003] In order to solve the problem of reducing the inertia of the new power system caused by the massive access of new energy sources and the gradual withdrawal of traditional synchronous generators, the offshore wind power system needs to support the frequency of the onshore power grid through flexible direct current grid connection to enhance the safe and stable operation capability of the new power system. However, in traditional control, the offshore MMC generally establishes the offshore AC power grid in a constant frequency and constant voltage manner, and the onshore MMC adopts DC voltage control to maintain the stability of the DC voltage. After the onshore power grid frequency changes, the receiving end converter station follows the grid frequency change, and because the flexible direct current transmission decouples the onshore and offshore AC power grid frequencies, the offshore wind farm cannot perceive the changes in the onshore power grid frequency. Therefore, the traditional offshore wind power solution through flexible direct current grid connection does not have the function of supporting the onshore power grid frequency.

[0004] In order to enable the offshore wind power flexible direct current grid-connected system to have the ability to support the frequency of the onshore power grid, the existing solutions can be divided into two types: with communication and without communication. The former transmits the frequency change information of the onshore power grid to the offshore wind farm through communication, but considering factors such as communication delay and communication failure, the applicability and reliability of this communication-dependent method are unclear. The non-communication control method adds frequency-DC voltage droop control to the receiving-end converter station, so that the frequency of the offshore AC power grid is changed by the sending-end converter station through the HVDC voltage change, so as to adjust the power generated by the wind turbine, so that the offshore wind farm has a certain frequency support effect on the onshore power grid. This non-communication method uses HVDC voltage to transmit the onshore frequency change information to the offshore wind farm, without communication delay, and avoids the problem of communication failure. The onshore converter station adopts frequency-DC voltage droop control to adjust the HVDC voltage when the onshore frequency is disturbed. After the sending-end converter station senses the DC voltage offset, the offset signal is converted into the change of the offshore AC power grid frequency, thereby realizing the frequency support of the wind farm to the onshore power grid. In addition to using offshore wind farms to provide frequency support, this non-communication method also uses DC side capacitors to release or absorb energy to provide a certain amount of inertia. However, this method belongs to the grid-following type control, and its receiving end converter station follows the changes in the frequency of the onshore power grid, and its support capacity for the onshore power grid is limited. In addition, this control method requires changing the DC voltage to transmit the frequency disturbance information of the onshore power grid. If it deviates from the rated value for a long time, it will affect the system operation performance and increase the difficulty of system design.

[0005] In addition to using wind turbines to provide inertia, additional energy storage can also be used to support the onshore power grid. An existing solution for using battery energy storage to enhance system inertia is as follows: Figure 1 As shown, by adjusting the HVDC voltage, battery energy storage and offshore wind turbines can jointly participate in onshore frequency regulation to achieve frequency support for the onshore power grid. Nowadays, supercapacitors are integrated into MMCs, and supercapacitor energy storage is used to achieve inertia compensation control and support for the onshore power grid. There are also supercapacitors connected to the DC side of each wind turbine, and the HVDC voltage is changed as the onshore frequency changes so that the wind turbine and supercapacitor can work together to provide the active power required for inertia response. The use of energy storage can improve the support capacity of the system, but it will increase costs, and there is also the problem of changing the HVDC voltage.

[0006] The above frequency support strategy belongs to grid-following control, that is, the system follows the changes in the frequency of the onshore power grid. It has a better effect in the case of a strong power grid, but in a weak power grid or even a passive network scenario, this control method is less effective and can only passively track the changes in the frequency of the onshore power grid. It has weak support capabilities for the onshore power grid and cannot actively support the power grid. Summary of the invention

[0007] The present invention aims to solve the problem that the existing offshore wind power grid-connected system has weak active support capability for the onshore power grid, and provides an active frequency support control method for the offshore wind power grid-connected system via MMC-HVDC.

[0008] In the method for actively supporting and controlling the frequency of the offshore wind power grid-connected system via MMC-HVDC of the present invention, the onshore converter station adopts virtual synchronous machine control; the offshore wind turbine generator set transmits power to the onshore converter station via flexible direct current through the offshore converter station to supply power to the onshore power grid;

[0009] The offshore converter station uses a proportional-integral controller to keep the DC voltage at the rated value;

[0010] Virtual synchronous machine control uses a method of simulating the synchronous machine rotor motion equation and excitation equation and combines frequency modulation to control the offshore converter, so that the offshore wind turbines can increase or reduce power to actively support the onshore power grid.

[0011] Furthermore, in the present invention, the formula for controlling the offshore converter by simulating the synchronous machine rotor motion equation and the excitation equation combined with frequency modulation control is:

[0012] The equation of motion of the rotor is:

[0013] Frequency modulation formula: P ref =P ref0 +K ω1 (ω 0 -ω 1 )

[0014] Excitation equation: E=∫[(Q ref -Q)+K v (U ref -U rms )]

[0015] Among them, J 1 is the virtual inertia, D 1 is the virtual damping; ω 0 is the rated angular frequency; ω 1 is the actual angular frequency of the onshore power grid; P e1 is the actual active power measured; P ref is the power reference value, K ω1 is the frequency modulation coefficient of the onshore converter station; P ref0 is the given power reference value, E is the voltage amplitude signal generated by QV control; Q ref and Q are the reactive power reference value and measured value respectively; U ref , U rms are the line voltage reference value and measured value respectively; K v is the droop coefficient between reactive power and voltage.

[0016] Furthermore, in the present invention, after the offshore wind turbine increases or decreases its power to actively support the onshore power grid, the power and frequency of the onshore converter station are:

[0017]

[0018] Among them, ω 10 , P e10 are the initial angular frequency of the onshore power grid and the initial active power of the onshore converter respectively; Δω 1 , ΔP e1 They are respectively the change in angular frequency of the onshore power grid and the change in active power of the onshore converter station after reaching steady state.

[0019] Furthermore, in the present invention, the frequency change of the onshore power grid after reaching a steady state is Δω 1 and the active power change of the onshore converter station ΔP e1 for:

[0020]

[0021] ΔP e1 =ΔP e2

[0022]

[0023] D 2 is the damping of the wind turbine, K ω2 is the frequency modulation coefficient of the wind turbine, ω 20 is the initial angular frequency of the offshore power grid, ΔP e2 To achieve the steady state, the active power change of the offshore converter station, Δω 2 It is the change of angular frequency of offshore power grid after reaching steady state.

[0024] Furthermore, the present invention also includes a Pf control link bypass. After detecting a short circuit fault in the onshore AC power grid, the Pf control link bypass generates the phase angle information required by the controller in a constant frequency manner at the rated frequency, and sends the phase angle information to the onshore converter station.

[0025] The method for actively supporting the frequency of offshore wind power grid-connected system via MMC-HVDC described in the present invention is based on grid-type control. The onshore MMC adopts virtual synchronous generator (VSG) control to provide active support to the grid by simulating synchronous generators. At the same time, to cooperate with the onshore MMC virtual synchronous generator control, the offshore MMC adopts constant DC voltage control to maintain DC voltage stability, and adopts grid-type wind turbines at sea to establish the offshore AC grid voltage. On this basis, a new onshore grid zero voltage ride-through operation strategy with active support control is designed to reduce the overvoltage on the HVDC side and the power impact after fault removal. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a block diagram of the principle of a typical frequency support method for existing offshore wind power flexible direct current transmission and grid-connected systems;

[0027] Figure 2 This is a principle block diagram of the method proposed in the present invention;

[0028] Figure 3 A comparison diagram of the dynamic response of a typical control method system when the load suddenly increases and the dynamic response of the method proposed by the present invention;

[0029] In the figure, (a) is a comparison of load power change curves, (b) is the power of the onshore converter station, (c) is the power of the synchronous generator, (d) is the onshore grid frequency, (e) is the onshore grid frequency change rate, (f) is the HVDC voltage, (g) is the offshore AC grid frequency, and (h) is the offshore wind farm power;

[0030] Figure 4 A comparison chart of the dynamic response of a typical control method system and the dynamic response of the method proposed in the present invention;

[0031] In the figure, (a) is the load power, (b) is the onshore converter station power, (c) is the synchronous generator power, (d) is the onshore grid frequency, (e) is the onshore grid frequency change rate, (f) is the HVDC voltage, (g) is the offshore AC grid frequency, and (h) is the offshore wind farm power;

[0032] Figure 5 It is a comparison diagram of the dynamic response of the typical control method system during zero voltage crossing of the onshore power grid and the dynamic response of the method proposed by the present invention;

[0033] In the figure, (a) is the onshore AC grid voltage, (b) is the onshore converter station AC current, (c) is the onshore converter station power, (d) is the HVDC voltage, (e) is the onshore converter station submodule capacitor voltage, (f) is the offshore AC voltage, (g) is the offshore wind turbine AC current, and (h) is the wind turbine power. DETAILED DESCRIPTION

[0034] The following will be combined with the accompanying 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, rather than 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 belong to the scope of protection of the present invention. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0035] Specific implementation method 1: refer to Figure 2 Specifically describe this embodiment, the frequency active support control method of the offshore wind power grid-connected system via MMC-HVDC described in this embodiment, the onshore converter station adopts virtual synchronous machine control; the offshore wind turbine unit transmits power to the onshore converter station via flexible direct current through the offshore converter station to supply power to the onshore power grid;

[0036] The offshore converter station uses a proportional-integral controller to keep the DC voltage at the rated value;

[0037] Virtual synchronous machine control uses a method of simulating the synchronous machine rotor motion equation and excitation equation and combines frequency modulation to control the offshore converter, so that the offshore wind turbines can increase or reduce power to actively support the onshore power grid.

[0038] Furthermore, in the present invention, the formula for controlling the grid-side converter of the offshore wind turbine by simulating the synchronous machine rotor motion equation and the excitation equation combined with frequency modulation control is:

[0039] The equation of motion of the rotor is:

[0040] Frequency modulation formula: P ref =P ref0 +K ω1 (ω 0 -ω 1 )

[0041] Excitation equation: E=∫[(Q ref -Q)+K v (U ref -U rms )]

[0042] Among them, J 1 is the virtual inertia, D 1 is the virtual damping; ω 0 is the rated angular frequency; ω 1 is the actual angular frequency of the onshore power grid; P e1 is the actual active power measured; P ref is the power reference value, K ω1is the frequency modulation coefficient of the onshore converter station; P ref0 is the given power reference value, E is the voltage amplitude signal generated by QV control; Q ref and Q are the reactive power reference value and measured value respectively; U ref , U rms are the line voltage reference value and measured value respectively; K v is the droop coefficient between reactive power and voltage.

[0043] Furthermore, in the present invention, after the offshore wind turbine increases or decreases its power to actively support the onshore power grid, the power and frequency of the onshore converter station are:

[0044]

[0045] Among them, ω 10 , P e10 are the initial angular frequency of the onshore power grid and the initial active power of the onshore converter respectively; Δω 1 , ΔP e1 They are respectively the change in angular frequency of the onshore power grid and the change in active power of the onshore converter station after reaching steady state.

[0046] Furthermore, in the present invention, the frequency change of the onshore power grid after reaching a steady state is Δω 1 and the active power change of the onshore converter station ΔP e1 for:

[0047]

[0048] ΔP e1 =ΔP e2

[0049]

[0050] D2 is the damping of the wind turbine, Kω2 is the frequency modulation coefficient of the wind turbine, ω 20 is the initial angular frequency of the offshore power grid, ΔP e2 To achieve the steady state, the active power change of the offshore converter station, Δω 2 It is the change of angular frequency of offshore power grid after reaching steady state.

[0051] Furthermore, the present invention also includes a Pf control link bypass. After detecting a short circuit fault in the onshore AC power grid, the Pf control link bypass generates the phase angle information required by the controller in a constant frequency manner at the rated frequency, and sends the phase angle information to the onshore converter station.

[0052] In view of the weak frequency support capability of typical strategies and their unsuitability for passive working conditions, this paper proposes a new active support control strategy based on network control, such as Figure 2The receiving-end converter station adopts virtual synchronous machine control, the sending-end converter station controls the HVDC voltage, and the offshore wind turbine works in the grid-building mode to establish the offshore AC voltage and adjust the power output of the wind turbine, so that the offshore converter station can achieve stable control of the DC voltage and actively support the frequency of the onshore power grid. Figure 2 This is a schematic diagram of the offshore wind power grid-connected system via MMC-HVDC and its supporting strategy according to the present invention, and the parameters in the figure are shown in Table 1.

[0053] Table 1

[0054]

[0055]

[0056] Virtual synchronous machine control uses the rotor motion equation and excitation equation of the synchronous generator to simulate the inertia and damping of the synchronous machine. It is a grid-type control strategy. The onshore converter station adopts VSG control to achieve active support for the onshore power grid. VSG control mainly consists of Pf control that simulates the synchronous machine rotor equation and QV control that simulates the excitation equation.

[0057] The Pf control part mainly simulates the rotor motion equation when the synchronous machine is running, and realizes Pf control by adding the frequency modulation link. The rotor motion equation of the synchronous machine is as follows:

[0058]

[0059] Where J g is the rotor inertia of the synchronous machine; D g is the damping coefficient; ω 0 is the rated angular velocity; ω is the actual angular velocity; θ is the phase of the synchronous machine; T m 、T e are the mechanical torque and electromagnetic torque of the synchronous machine respectively; P m , P e are mechanical power and electromagnetic power respectively. It can be seen from formula (1) that when the mechanical power and electromagnetic power are unbalanced, the frequency of the synchronous machine changes. By changing the electromagnetic power to adjust the frequency, the synchronous machine can be restored to a balanced state.

[0060] In order to introduce inertia and damping, the Pf control link can be designed according to the above rotor motion equation, that is:

[0061]

[0062] Where J 1 is the virtual inertia; D 1 is the virtual damping; ω 0 is the rated angular frequency; ω 1 is the actual angular frequency; Pe1 is the actual active power measured; P ref is the power reference value. In addition, the analog synchronous machine speed regulator adds a frequency modulation link in the control to adjust the frequency. The additional frequency modulation link expression is as follows:

[0063] P ref =P ref0 +K ω1 (ω 0 -ω 1 ) (3)

[0064] In the formula, K ω1 is the frequency modulation coefficient; P ref0 is the given power reference value. From equations (2) and (3), we can get the Pf control link of VSG control, as Figure 2 shown.

[0065] QV control, such as Figure 2 As shown in the figure, the QV link in the virtual synchronous machine control simulates the excitation equation of the synchronous machine, thereby adjusting the reactive power and supporting the voltage. The expression is:

[0066] E=∫[(Q ref -Q)+K v (U ref -U rms )] (4)

[0067] Where E is the voltage amplitude signal generated by QV control; Q ref and Q are the reactive power reference value and measured value respectively; U ref , U rms are the line voltage reference value and measured value respectively; K v is the droop coefficient between reactive power and voltage. The amplitude and phase of the reference voltage can be obtained from equations (2)-(4), and the reference value of the AC voltage loop can be obtained through coordinate transformation, thereby realizing network control.

[0068] When a three-phase short circuit fault occurs in the receiving AC power grid, the typical grid-type control will accumulate a large frequency error during the fault period. After the fault is cleared, the frequency error will cause a large phase difference between the output voltage of the receiving converter station and the grid voltage, causing an impact during the fault recovery process, which is not conducive to the recovery of the system after the fault. To this end, this paper adds a Pf control bypass link to the proposed control, such as Figure 2 As shown, after detecting the short circuit fault of the onshore AC power grid, the Pf control link is bypassed, and the phase angle information required by the controller is generated in a constant frequency manner at the rated frequency. After the fault is cleared, the Pf control part is restored. This scheme effectively avoids the problem of a large impact on the power grid when the fault is cleared, and accelerates the fault recovery process of the system.

[0069] Offshore converter station control: The onshore converter station adopts the control method of virtual synchronous machine, while the HVDC voltage is controlled by the offshore converter station. The DC voltage is controlled at the rated value by using a proportional integral controller. Its control structure is as follows: Figure 2 When the frequency of the onshore grid changes, the offshore converter station increases or decreases active power to support the onshore grid, causing DC voltage disturbances; in order to control the DC voltage back to the rated value, the sending-end converter station absorbs more / less energy from the offshore AC grid to balance the power changes of the onshore converter station, thereby maintaining the stability of the HVDC voltage and eliminating the DC voltage offset problem during the frequency support process, thus avoiding adverse effects on the design and operation of the DC transmission system.

[0070] The converter on the offshore wind turbine side maintains the DC voltage stable, while the converter on the grid side works in the grid-building mode to establish the voltage of the offshore AC grid and control its output power. Its control strategy is as follows: Figure 2 As shown. In order to enable offshore wind turbines to respond to changes in the frequency of the onshore power grid, in addition to using the rotational inertia of the wind turbine, load reduction control can also be used to reserve sufficient standby power. Load reduction control is usually achieved by increasing the pitch angle or overspeed control. When additional power is needed to support the frequency, the standby power can be used to increase power to support the frequency of the onshore power grid by reducing the wind turbine speed or the pitch angle. Under load reduction control, the actual power output of the wind turbine P WTref It can be expressed as:

[0071]

[0072] Where ρ is air density; R is blade radius; V is wind speed; β is pitch angle; λ is tip speed ratio; C p (β, λ) is the wind energy utilization coefficient; P MPPT is the wind turbine power under maximum power tracking; d% is the load reduction factor, which is generally 10% to 20%.

[0073] The frequency of the onshore power grid is coupled with the frequency of the offshore power grid. This section derives the coupling relationship to guide the design of control parameters. Since the wind turbine adopts a grid-type control similar to that of the receiving-end converter station, the onshore power grid frequency ω can be obtained from equations (2) and (3): 1 and the frequency of the offshore grid ω 2 Relationship with power:

[0074]

[0075] Where J 2 , D 2 , K ω2 are the virtual inertia, damping and frequency modulation coefficient of the wind turbine respectively; P WTref Give active power reference value to wind turbine; Pe2 is the active power generated by the wind turbine. When the frequency of the onshore AC power grid changes, it can be seen from formula (10) that after the active support is completed and the system is stabilized again, the power and frequency changes of the onshore converter station can be expressed as:

[0076]

[0077] Where ω 10 , P e10 are the initial angular frequency and active power respectively; Δω 1 , ΔP e1 are the frequency change of the onshore power grid and the active power change of the onshore converter station after reaching steady state. Substituting equation (7) into equation (6) and ignoring the second-order quantity, the relationship between the frequency change of the onshore power grid and the active power change can be obtained:

[0078]

[0079] In addition, after the system stabilizes again, the following relationships exist:

[0080]

[0081] The relationship between the frequency change and power change of the receiving converter station can be obtained from equations (8) and (9), as shown in equation (10). Similarly, the relationship between the frequency change and active power change of the offshore wind turbine after the system is stabilized can be obtained:

[0082]

[0083] In the proposed control strategy, the offshore converter station controls the DC voltage and the onshore converter station increases / decreases the power ΔP e1 After that, in order to maintain the HVDC voltage unchanged, the wind turbine needs to increase / decrease the same power. Therefore, after the frequency support is completed, there is ΔP e1 =ΔP e2 , from which the relationship between the frequency change of the onshore power grid and the frequency change of the offshore power grid can be obtained:

[0084]

[0085] By using formula (11) and combining the frequency regulation capability of wind turbines, the frequency regulation coefficient and damping of the receiving-end converter station and wind turbines can be designed to optimize the active frequency support capability of the system.

[0086] In order to verify the effectiveness of the active support control method proposed in this paper, a PSCAD / EMTDC platform was established. Figure 2The simulation model of the offshore wind power system connected to the flexible direct current grid is shown in Figure 1. The rated power of the MMC converter station is 1200MW. The offshore wind farm is simulated by three wind turbines with lumped parameters. Each wind turbine has a rated power of 400MW and is connected to the offshore converter station through submarine cables of 4km, 5km and 6km in length. Since the wind turbine side converter adopts constant DC voltage control, it is simplified to a DC voltage source in the simulation to improve the simulation efficiency. In the initial stage of the simulation, the offshore wind power system operates at a rated power of 0.5pu. In order to verify the supporting effect of the proposed strategy on the grid frequency, a synchronous machine with a rated power of 1200MW is used to simulate the weak grid on the shore. The synchronous machine model includes excitation and rotor control parts. The excitation module in the PSCAD model library is used to form the excitation part to establish the internal magnetic field of the synchronous machine, thereby determining the terminal voltage; the rotor control part is composed of a steam speed governor and a steam turbine, and the angular frequency of the synchronous machine is maintained near the reference frequency. Other main parameters are shown in Table 2.

[0087] Table 2

[0088]

[0089] The dynamic characteristics of the proposed method in the present invention are verified under onshore power grid frequency disturbance and three-phase short circuit fault, and compared with the typical frequency support strategy.

[0090] Load surge, firstly, load surge simulation is carried out. After the system runs stably, the load is increased by 0.1pu at t=20s. The dynamic response characteristics of the system are as follows Figure 3 shown.

[0091] After the load suddenly increases by 0.1pu, the frequency of the onshore power grid decreases. Under the proposed active support strategy, the receiving-end converter station actively generates 0.1pu active power to support the onshore power grid frequency. The load sudden increase power is almost entirely provided by the offshore wind power flexible direct current grid-connected system, and there is no need for the synchronous machine to increase power. Figure 3 (a), (b), and (c) are shown; however, the additional power of the wind power system under typical control is only 0.05 pu, and the rest must be provided by the synchronous machine. After the system reaches steady state again, the frequency difference of the onshore grid is 0.033 Hz, which is much smaller than the frequency difference of 0.163 Hz under typical control mode. Figure 3 (d) Figure 3 As shown in (e), during the load surge, the proposed strategy can reduce the maximum frequency change rate from -0.9 Hz / s to -0.8 Hz / s, and the system can reach a steady state faster.

[0092] When the onshore converter station increases power to support the onshore grid frequency, since the HVDC voltage in the proposed control scheme is regulated by the offshore converter station using PI control, the converter station absorbs more energy from the offshore AC grid to maintain the stability of the HVDC voltage. After the disturbance, the HVDC voltage returns to the rated value, effectively avoiding the DC voltage offset problem caused by the frequency-DC voltage droop control in the typical scheme, such as Figure 3 (f) as shown.

[0093] As the sending-end converter station absorbs more energy from the offshore AC grid, the wind turbine generates an additional 0.1pu power to balance the additional 0.1pu power generated by the onshore converter station. At the same time, the frequency of the offshore AC grid drops slightly to 49.92Hz, which is higher than the 49.83Hz in typical control. Figure 3 As shown in (g) and (h), it can be seen that when the proposed control actively supports the onshore power grid, the frequency deviation of the offshore AC power grid is smaller and the performance is better.

[0094] Load suddenly decreases. After the system runs stably, the load suddenly decreases by 0.1pu at t=20s. The dynamic response characteristics of the system are as follows: Figure 4 As shown. After the load suddenly decreases by 0.1pu, the grid frequency rises. By adopting the proposed active support control, the receiving-end converter station actively reduces the active power by 0.1pu to support the onshore grid frequency. The power reduction of the offshore wind power flexible direct current grid-connected system is almost equal to the sudden load reduction power. Therefore, there is no need for the synchronous machine to reduce the power, as shown in Figure 4(a), (b), and (c). Under typical control, the wind power system reduces the power by only 0.05pu, and the rest must be reduced by the synchronous machine. After the system reaches steady state again, the frequency difference of the onshore grid is 0.033Hz, which is much smaller than the frequency difference of 0.163Hz under the typical control method. Figure 4 (d) Figure 4 As shown in (e), during the load sudden reduction process, the proposed strategy can reduce the maximum frequency change rate from 1.0 Hz / s to 0.4 Hz / s, and the system can reach a steady state faster.

[0095] Similar to the load surge situation, when the onshore converter station reduces power generation to support the onshore grid frequency, the offshore converter station reduces the energy absorbed from the offshore AC grid to maintain the stability of the HVDC voltage and controls the HVDC voltage back to the rated value after the disturbance, effectively avoiding the DC voltage offset problem caused by droop control in the typical solution, such as Figure 4 (f) as shown.

[0096] As the sending-end converter station reduces the energy absorbed from the offshore AC grid, the wind turbine reduces the power by 0.1pu to balance the 0.1pu power reduction of the onshore converter station. At the same time, the frequency of the offshore AC grid rises slightly to 50.08Hz, which is lower than the 50.17Hz in the typical control. Figure 4As shown in (g) and (h), the frequency deviation of the offshore AC power grid is reduced.

[0097] In order to verify the zero voltage ride-through capability of the onshore power grid of the offshore wind power connected to the flexible direct current grid system under the proposed control strategy, a triple short-circuit fault occurred at the grid connection point at t=2s, and the fault was cleared at t=2.25s.

[0098] After a three-phase short circuit occurs in the onshore AC power grid, the AC voltage at the receiving converter station connection point drops to 0. Due to the limiting effect of the inner current loop, the AC current at the receiving converter station is limited to 1.2 pu, and the output power drops to 0. Figure 5 As shown in (a), (b), and (c), since the wind turbine still generates power, this power surplus causes the HVDC voltage and the MMC submodule capacitor voltage to rise, as shown in Figure 5 As shown in (d) and (e), compared with the typical control, the proposed control strategy can reduce the HVDC voltage rise from 11.6% to 6.1%, effectively reducing the overvoltage on the HVDC side during the fault period.

[0099] In order to control the HVDC voltage back to the rated value, the power of the offshore converter station and wind turbines is rapidly reduced to zero, such as Figure 5 (g) and (h), and avoid DC overvoltage inside the wind turbine by starting DC energy dissipation resistors and slurry bias control. In addition, the proposed control can achieve stable control of the offshore AC grid voltage during faults, such as Figure 5 (f) as shown.

[0100] After t=2s of the fault, the Pf control link is bypassed and re-enabled after the fault is removed, which effectively reduces the impact caused by the inconsistency between the onshore converter station and the grid after the fault is removed. The system can resume stable operation within 0.4s after the fault is removed, which is faster than the typical control. Figure 5 As shown in the above simulation results, it can be seen that the active support control strategy proposed in the present invention has better zero voltage ride-through capability.

[0101] The new active support control scheme for offshore wind power through flexible direct current grid-connected system proposed in the present invention adopts grid-type control for both onshore converter station and offshore wind turbine, while HVDC voltage is adjusted by PI controller at offshore converter station. The proposed control can realize active support for onshore power grid. The frequency deviation and frequency change rate of onshore power grid after power disturbance are both smaller than those of typical frequency support scheme. Moreover, the proposed control does not need to change HVDC voltage, thus ensuring the stability of DC voltage and reducing the frequency deviation of offshore AC power grid. In addition, the proposed control realizes zero voltage crossing of onshore power grid, effectively reduces overvoltage on HVDC side, and solves the problem of large system impact after fault recovery. The proposed active support control scheme can enhance the support capability of offshore wind power to onshore power grid through MMC-HVDC grid-connected system, and has good zero voltage crossing capability, providing a feasible solution for realizing grid-friendly offshore wind power grid connection.

[0102] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that the features described in conjunction with a single embodiment may be used in other described embodiments.

Claims

1. A frequency active support control method for offshore wind power grid-connected system via MMC-HVDC, characterized in that: The method includes: The onshore converter station is controlled by a virtual synchronous machine; the offshore wind turbines are transmitted to the onshore converter station via flexible direct current through the offshore converter station to supply power to the onshore power grid; The offshore converter station uses a proportional-integral controller to keep the DC voltage at the rated value; Virtual synchronous machine control uses a method of simulating the synchronous machine rotor motion equation and excitation equation and combines frequency modulation to control the offshore converter, so that the offshore wind turbines can increase or reduce power to actively support the onshore power grid.

2. The method for active frequency support control of offshore wind power grid-connected system via MMC-HVDC according to claim 1 is characterized in that: The formula for controlling the offshore converter by simulating the synchronous machine rotor motion equation and the excitation equation combined with frequency modulation control is: The equation of motion of the rotor is: Frequency modulation formula: P ref =P ref0 +K ω1 (ω0-ω1) Excitation equation: E=∫[(Q ref -Q)+K v (U ref -U rms )] Where J1 is the virtual inertia, D1 is the virtual damping; ω0 is the rated angular frequency; ω1 is the actual angular frequency of the onshore power grid; P e1 is the actual active power measured; P ref is the power reference value, K ω1 is the frequency modulation coefficient; P ref0 is the given power reference value, E is the voltage amplitude signal generated by QV control; Q ref and Q are the reactive power reference value and measured value respectively; U ref , U rms are the line voltage reference value and measured value respectively; K v is the droop coefficient between reactive power and voltage.

3. The method for active frequency support control of offshore wind power grid-connected system via MMC-HVDC according to claim 2 is characterized in that: After the offshore wind turbine increases or decreases its power to actively support the onshore power grid, the onshore converter station power and angular frequency are: Among them, ω 10 , P e10 are the initial angular frequency of the onshore power grid and the initial active power of the onshore converter respectively; Δω1, ΔP e1 They are respectively the change in angular frequency of the onshore power grid and the change in active power of the onshore converter station after reaching steady state.

4. The method for active frequency support control of offshore wind power grid-connected system via MMC-HVDC according to claim 3 is characterized in that: After reaching steady state, the frequency change of the onshore power grid Δω1 and the active power change of the onshore converter station ΔP e1 for: ΔP e1 =ΔP e2 D2 is the damping of the wind turbine, Kω2 is the frequency modulation coefficient of the wind turbine, ω 20 is the initial angular frequency of the offshore power grid, ΔP e2 is the change in active power of the offshore converter station after reaching steady state, and Δω2 is the change in angular frequency of the offshore power grid after reaching steady state.

5. The method for active frequency support control of offshore wind power grid-connected system via MMC-HVDC according to claim 1, characterized in that: It also includes a Pf control link bypass. After detecting a short circuit fault in the onshore AC power grid, the Pf control link bypass generates the phase angle information required by the controller in a constant frequency manner at the rated frequency, and sends the phase angle information to the onshore converter station.

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