Frequency support method for offshore wind power medium frequency uncontrolled rectifier DC transmission system

Abstract

The present invention discloses a frequency support method for an offshore wind power medium frequency uncontrolled rectifier DC transmission system, comprising: enabling the offshore wind turbine to adopt a grid-type control strategy, transmitting the onshore power grid frequency to the offshore wind turbine through additional harmonics, and then utilizing multiple energy sources to provide frequency support for the onshore power grid. The present invention does not require a dedicated communication channel from the onshore power grid to the offshore wind turbine, and fully utilizes multiple energy sources in the system to provide frequency support for the onshore power grid, including high-voltage direct current transmission system capacitors, offshore wind turbines, and energy storage devices configured in the offshore wind turbines. The method of the present invention is simple to implement, highly reliable, and economical, so that the offshore wind power medium frequency uncontrolled rectifier DC transmission system plays a guiding role in the frequency support of the onshore power grid, and has great application value in actual engineering.

Description

Technical Field

[0001] The present invention belongs to the technical field of power transmission and distribution of electric power systems, and in particular relates to a frequency support method for an offshore wind power medium-frequency uncontrolled rectifier DC transmission system. Background Art

[0002] Offshore wind energy has enormous development potential. In recent years, the focus of wind power development both domestically and internationally has gradually shifted from onshore to offshore. Compared to nearshore wind farms, offshore wind farms offer more abundant and stable wind energy resources. Economically and reliably connecting long-distance offshore wind power to the grid is a key technology that urgently requires research. Currently, operational offshore wind farms primarily utilize flexible HVDC transmission systems based on MMC (Modular Multilevel Converters). However, these systems require the construction of large offshore converter platforms, which carries high investment and maintenance costs.

[0003] Compared to modular multilevel converters, diode rectifier units (DRUs) offer lower investment costs, lower power losses, and higher operational reliability, improving the economics and reliability of offshore wind power grid-connected systems. However, DRUs require an external voltage source to provide commutation voltage for operation. Therefore, wind turbines require a grid-based control strategy to establish the AC voltage amplitude and frequency of the offshore AC system.

[0004] With the large-scale integration of offshore wind power, power system frequency stability issues are becoming increasingly prominent. Providing frequency support ancillary services from offshore wind power DC transmission systems is an effective measure to alleviate frequency stability issues in onshore power grids. However, due to the inability of diode rectifier units to control frequency, existing communication-free frequency support methods for offshore wind power flexible DC transmission systems are not suitable for offshore wind power DC transmission systems based on diode rectifier units.

[0005] To date, published literature lacks research on frequency support technologies for offshore wind turbine DC transmission systems based on diode rectifiers. Because DC transmission systems decouple the onshore grid and offshore wind turbines, offshore wind turbines cannot naturally sense changes in the onshore grid frequency. Therefore, additional measures are needed to transmit the onshore grid frequency to the offshore wind turbines. The literature [Saborio-Romano, Bidadfar, Sakamuri, et al. Primary Frequency Regulation Response of Offshore Wind Farms Connected to HVDC Transmission Systems via Diode Rectifiers [C]. IEEE Milan Power Technology Conference, 2019: 1-6] proposes transmitting the onshore grid frequency to the offshore wind turbines via long-distance communication channels; the literature [Xiao H, Huang X, Huang Y, et al. Self-synchronization Control and Frequency Response of Offshore Wind Farms Connected to HVDC Transmission Systems Based on Diode Rectifiers [J]. IEEE Transactions on Sustainable Energy, 2022, Vol. 13(Issue 3): 1681–1692. [The IEEE Transactions on Sustainable Energy, 2022, Vol. 13(Issue 3): 1681–1692] proposes transmitting the onshore grid frequency to offshore wind turbines via changes in the DC voltage amplitude of the DC transmission system. This eliminates the communication channel from the onshore grid to the offshore rectifier station. However, these methods still require a communication channel from the offshore rectifier station to each offshore wind turbine. Existing technologies generally rely on communication channels to transmit onshore grid frequency. However, the hundreds of kilometers of communication channels in long-distance, large-capacity offshore wind power transmission systems are detrimental to the timeliness, reliability, and cost-effectiveness of frequency support.

[0006] In order to avoid communication from the onshore power grid to the offshore rectifier station, it is necessary to study the frequency support method of the onshore power grid from the DC transmission system of offshore wind power through the diode rectifier unit that does not require communication. Summary of the Invention

[0007] In view of the above, the present invention provides a frequency support method for an offshore wind power medium-frequency uncontrolled rectifier DC transmission system. The method is simple to implement and has high reliability. It is suitable for offshore wind power transmission scenarios based on diode rectifier units. It can improve the frequency friendliness of the offshore wind power DC transmission system connected to the grid and has great application value in actual engineering.

[0008] A frequency support method for an offshore wind power medium-frequency uncontrolled rectifier DC transmission system, comprising:

[0009] The offshore wind turbines adopt a grid-type control strategy to transmit the onshore power grid frequency to the offshore wind turbines through additional harmonics, and then use multiple energy sources to provide frequency support for the onshore power grid.

[0010] Furthermore, the offshore wind power medium-frequency uncontrolled rectifier DC transmission system includes a grid-type offshore wind farm, a DRU-based offshore rectifier station, and an MMC-based onshore inverter station, wherein the offshore wind farm includes multiple offshore wind turbines, the offshore wind turbines are connected to the AC bus through a medium-frequency AC collection cable, the AC side of the offshore rectifier station is connected to the AC bus through a converter transformer, the DC side of the offshore rectifier station is connected to the DC side of the onshore inverter station through a high-voltage DC cable, and the AC side of the onshore inverter station is connected to the onshore power grid through a converter transformer; the offshore wind farm, the medium-frequency AC collection cable, and the converter transformer of the offshore rectifier station constitute an offshore AC system; the DC side of the offshore rectifier station, the high-voltage DC cable, and the DC side of the onshore inverter station constitute a high-voltage DC transmission system.

[0011] Furthermore, the energy source includes a high-voltage direct current transmission system capacitor, an offshore wind turbine and an energy storage device configured in the offshore wind turbine. The high-voltage direct current transmission system capacitor includes the equivalent capacitance of the high-voltage direct current submarine cable and the sub-module capacitance of the MMC.

[0012] Furthermore, the offshore wind turbines adopt a grid-type control strategy, the specific implementation of which includes:

[0013] The active power-AC voltage amplitude controller is used to control the output active power of the offshore wind turbine to track the active power command value without error, thereby controlling the AC voltage amplitude at the AC outlet of the offshore wind turbine to change with the change of the output active power of the offshore wind turbine;

[0014] The reactive power-frequency controller is used to control the reactive power output of offshore wind turbines to dynamically balance the reactive load of the offshore AC system, thereby controlling the frequency at the AC outlet of the offshore wind turbines and maintaining synchronous operation and uniform reactive power distribution among all wind turbines.

[0015] Furthermore, the active power command value is generated and provided by a maximum power point tracking strategy.

[0016] Furthermore, the specific implementation method of transmitting the onshore grid frequency to the offshore wind turbine through additional harmonics is as follows:

[0017] Detecting the frequency of the onshore power grid using the phase-locked loop of the MMC in the onshore inverter station f on , and then control the MMC to make the DC component of the DC voltage of the HVDC transmission system U dc is the command value, and the DC component U dc Additional DC side harmonic voltage u dch , the DC side harmonic voltage u dchThe frequency is K · f on , K is the frequency transfer proportional coefficient;

[0018] Additional DC side harmonic voltage u dch Generation of DC side harmonic current in HVDC transmission system i dch , and its frequency is K · f on ;

[0019] The DRU in the offshore rectifier station converts the DC side harmonic current in the HVDC transmission system i dch Transmitted to the offshore AC system, generating two-frequency AC side harmonic currents i ach The two frequencies are K · f on + f off and K · f on - f off , f off It is the frequency of the maritime AC system and is in the medium frequency range;

[0020] The AC side harmonic current i ach Propagation in the offshore AC system enables offshore wind turbines to detect the AC side harmonic voltages of the above two frequencies at their AC outlets. u ach , and then use the phase-locked loop of the grid-side converter of the offshore wind turbine to obtain the harmonic voltage from the AC side u ach Extracting onshore grid frequency f on .

[0021] Furthermore, the specific implementation method of using multiple energy sources to provide frequency support for the onshore power grid is as follows:

[0022] When the onshore grid frequency deviates from its rated value by more than 0.03 Hz, the MMC in the onshore inverter station is controlled to make the DC component of the HVDC system DC voltage U dc Change, the amount of change is equal to k 1·Δ f , thereby utilizing the HVDC system capacitors to store energy and provide inertia support for the onshore power grid;

[0023] When the onshore grid frequency deviates from its rated value by more than 0.06 Hz, the offshore wind turbine changes the active power instruction value of the grid-side converter in the unit according to the onshore grid frequency deviation, and the change is equal to k 2·Δ f + k 3·Δ f' , thereby utilizing the energy stored in the wind turbine rotors in offshore wind turbines to provide inertia support for the onshore power grid;

[0024] When the onshore grid frequency deviates from its rated value by more than 0.09 Hz, the offshore wind turbine changes the active power instruction value of the energy storage device in the unit according to the onshore grid frequency deviation, and the change is equal to k 4·Δ f + k 5·Δ f' , thereby using energy storage devices to store energy and provide inertia support and primary frequency regulation for the onshore power grid;

[0025] Where: Δ f is the frequency offset, Δ f' is the differential of the frequency offset, k 1~ k 5 is the given proportional coefficient.

[0026] A computer device includes a memory and a processor, wherein the memory stores a computer program, and the processor is used to execute the computer program to implement the frequency support method of the above-mentioned offshore wind power medium-frequency uncontrolled rectifier DC transmission system.

[0027] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the frequency support method for the above-mentioned offshore wind power medium-frequency uncontrolled rectifier DC transmission system.

[0028] Based on the above technical solution, the present invention has the following beneficial technical effects:

[0029] 1. The frequency transfer method proposed in this invention can transmit the onshore grid frequency to the offshore wind turbine by adding harmonics. This eliminates the need for a dedicated communication channel from the onshore grid to the offshore wind turbine, allowing the offshore wind turbine to obtain local information on onshore grid frequency changes from its AC outlet. This method is simple, highly reliable, and economical, and has great application value in practical engineering projects.

[0030] 2. The frequency support method proposed in this invention can fully utilize the various energy sources with frequency support capabilities in the offshore wind power DC transmission system, including HVDC system capacitors, offshore wind turbines, and energy storage devices configured in offshore wind turbines, and plays a guiding role in the frequency support of the offshore wind power transmission system to the onshore power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a structural diagram of an offshore wind power medium-frequency uncontrolled rectifier DC transmission system in an embodiment of the present invention.

[0032] Figure 2 Schematic diagram of a DC voltage simulation waveform of a high-voltage DC transmission system in an embodiment of the present invention.

[0033] Figure 3 Schematic diagram of a DC current simulation waveform of a high-voltage DC transmission system in an embodiment of the present invention.

[0034] Figure 4 Schematic diagram of the simulated waveform of the active power output of the offshore wind farm in an embodiment of the present invention.

[0035] Figure 5 Schematic diagram of a simulated waveform of the onshore power grid frequency in an embodiment of the present invention. DETAILED DESCRIPTION

[0036] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0037] like Figure 1 As shown in FIG, the offshore wind power medium-frequency uncontrolled rectifier DC transmission system in this embodiment includes an offshore wind turbine, a medium-frequency AC collection submarine cable, a DRU-based offshore rectifier station, a high-voltage DC submarine cable, and an MMC-based onshore inverter station. The offshore wind turbine adopts a grid-type control strategy, which includes the following two aspects:

[0038] (1) The active power-AC voltage amplitude controller is used to control the wind turbine output active power to track the active power command value without difference. The active power command value is generated by the maximum power point tracking strategy, and the AC voltage amplitude at the AC outlet of the wind turbine is controlled to change with the change of the wind turbine output active power.

[0039] (2) Use the reactive power-frequency controller to control the reactive power output of the wind turbines to dynamically balance the reactive load of the offshore AC system, control the frequency at the AC outlet of the wind turbines, and maintain synchronous operation and uniform distribution of reactive power among all wind turbines.

[0040] In this embodiment, the method for supporting the frequency of the onshore power grid by the DC transmission system first transmits the onshore power grid frequency to the offshore wind turbine through additional harmonics, without the need for a dedicated communication channel from the onshore power grid to the offshore wind turbine. The method includes the following steps:

[0041] (1) The modular multilevel converter in the onshore inverter station detects the onshore grid frequency through a phase-locked loop f on, while controlling the DC component of the HVDC system DC voltage U dc is the command value, and the DC component U dc Additional DC side harmonic voltage u dch . Additional DC side harmonic voltage u dch The frequency is K · f on , K It is the frequency transfer proportional coefficient and is between 0 and 1.

[0042] (2) Additional DC side harmonic voltage u dch Generating additional DC side harmonic currents in HVDC transmission systems i dch , additional DC side harmonic current i dch The frequency is K · f on .

[0043] (3) The diode rectifier unit in the offshore rectifier station converts the additional DC side harmonic current in the HVDC transmission system into i dch Transmitted to the offshore AC system, generating additional AC side harmonic current i ach , additional AC side harmonic current i ach The frequency is K · f on ± f off , f off is the frequency of the maritime communication system, f off In the medium frequency range (100~200Hz), the size and weight of the transformer and AC filter in the offshore AC system can be reduced, thereby improving the economic efficiency of the offshore AC system. There are two main reasons for this: On the one hand, since the AC transformer works according to the principle of electromagnetic induction, the induced electromotive force can be expressed as E =4.44 fn Φ =4.44 f ,in E is the effective value of the induced electromotive force, f is the AC frequency, n is the number of winding turns, Φ is the magnetic flux amplitude, S is the cross-sectional area of ​​the core,B is the maximum value of magnetic induction intensity; E Under the same conditions, if we keep B and N unchanged, then f is a fixed value, which means f rise, S Therefore, after adopting the medium frequency, the cross-sectional area of ​​the transformer core can be reduced, thereby reducing the volume and weight of the transformer. On the other hand, the reactive power emitted by the AC filter is calculated as Q =2π f 2 C ,in Q 、 U 、 C They are the reactive power, AC voltage, and equivalent capacitance of the AC filter. Q Under the same conditions, if we keep U If unchanged, f rise, C Therefore, after adopting the intermediate frequency, the capacitance value of the AC filter is reduced, thereby reducing the volume and weight of the AC filter.

[0044] (4) Additional AC side harmonic current i ach Propagation in the offshore AC system enables offshore wind turbines to detect frequencies at their AC outlets. K · f on ± f off AC side harmonic voltage u ach , according to the phase-locked loop from the AC side harmonic voltage u ach Extracting onshore grid frequency f on .

[0045] After the offshore wind turbines obtain the onshore grid frequency, they provide frequency support to the onshore grid through the HVDC system capacitors, the offshore wind turbines, and the energy storage devices configured within the offshore wind turbines. The specific implementation includes:

[0046] (1) When the onshore grid frequency deviates from its rated value by more than 0.02 Hz, the DC component of the DC voltage of the HVDC system controlled by the modular multilevel converter in the onshore inverter station U dc Change, thereby using the HVDC system capacitors to store energy and provide inertia support for the onshore power grid.

[0047] (2) When the onshore grid frequency deviates from its rated value by more than 0.04 Hz, the offshore wind turbine changes the output active power command value according to the onshore grid frequency deviation, thereby utilizing the energy stored in the wind turbine rotor in the offshore wind turbine to provide inertia support for the onshore grid.

[0048] (3) When the frequency deviation of the onshore power grid exceeds 0.08 Hz from its rated value, the energy storage device configured in the offshore wind turbine changes the output active power command value according to the onshore power grid frequency deviation, making full use of the energy stored in the energy storage device to provide inertia support and primary frequency regulation for the onshore power grid.

[0049] The parameters of the offshore wind power medium frequency uncontrolled rectifier DC transmission system in this embodiment are shown in Table 1:

[0050] Table 1

[0051]

[0052] A corresponding simulation platform is built in the electromagnetic transient simulation software PSCAD / EMTDC to simulate the sudden increase in load on the onshore power grid. t = 20s ago, the offshore wind power transmission system and the onshore power grid are both operating stably; assuming that t =20s, the active load of the onshore power grid suddenly increases by 350MW; Figures 2 to 5 The simulation results of key electrical quantities are given. Figure 2 It can be seen that when the frequency deviation of the onshore power grid exceeds the set threshold, the MMC can control the DC component of the HVDC transmission system to decrease, thereby releasing the energy stored in the capacitor of the HVDC transmission system. At the same time, the MMC can add DC side harmonic voltage to the HVDC transmission system, thereby transmitting the onshore power grid frequency through the additional harmonics. Figure 3 It can be seen that the additional DC side harmonic voltage can generate DC side harmonic current in the HVDC transmission system; Figure 4 It can be seen that the output active power of offshore wind turbines and the energy storage devices configured in offshore wind turbines increases, making up for the active power shortage of the onshore power grid; Figure 5 It can be seen that after using the frequency support method of the present invention, the change rate of the onshore power grid frequency is reduced and the minimum value of the onshore power grid frequency is increased, which proves that the present invention can effectively suppress the onshore power grid frequency fluctuation and realize onshore power grid frequency support.

[0053] The above description of the embodiments is intended to facilitate understanding and application of the present invention by those skilled in the art. It is apparent that those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without requiring creative effort. Therefore, the present invention is not limited to the above embodiments. Any improvements or modifications made by those skilled in the art based on the disclosure of the present invention should fall within the scope of protection of the present invention.

Claims

1. A frequency support method for an offshore wind power medium-frequency uncontrolled rectifier DC transmission system, characterized by: The offshore wind turbines adopt a grid-type control strategy, which transmits the onshore grid frequency to the offshore wind turbines through additional harmonics, and then uses multiple energy sources to provide frequency support for the onshore grid; The specific implementation method of transmitting the onshore grid frequency to the offshore wind turbine through additional harmonics is as follows: The onshore power grid frequency f is detected by using the phase-locked loop of the MMC in the onshore inverter station. on , and then control the MMC to make the DC component of the DC voltage U dc is the command value, and in the DC component U dc Additional DC side harmonic voltage u dch , the DC side harmonic voltage u dch The frequency is K·f on , K is the frequency transfer proportional coefficient; Additional DC side harmonic voltage u dch In the HVDC transmission system, the DC side harmonic current i is generated dch , whose frequency is also K·f on ; The DRU in the offshore rectifier station converts the DC side harmonic current i in the HVDC transmission system dch Transmitted to the offshore AC system, generating two-frequency AC side harmonic current i ach , the two frequencies are K·f on +f off and K·f on -f off , f off It is the frequency of the maritime AC system and is in the medium frequency range; The AC side harmonic current i ach Propagation in the offshore AC system enables the offshore wind turbine to detect the AC side harmonic voltage u of the above two frequencies at its AC outlet. ach , and then use the phase-locked loop of the grid-side converter in the offshore wind turbine to obtain the AC side harmonic voltage u ach Extract the onshore grid frequency f on .

2. The frequency support method for an offshore wind power medium frequency uncontrolled rectifier DC transmission system according to claim 1, characterized in that: The energy source includes a high-voltage direct current transmission system capacitor, an offshore wind turbine and an energy storage device configured in the offshore wind turbine. The high-voltage direct current transmission system capacitor includes the equivalent capacitance of the high-voltage direct current submarine cable and the sub-module capacitance of the MMC.

3. The frequency support method for an offshore wind power medium frequency uncontrolled rectifier DC transmission system according to claim 1, characterized in that: The offshore wind turbine generator system adopts a grid-type control strategy, the specific implementation of which includes: The active power-AC voltage amplitude controller is used to control the output active power of the offshore wind turbine to track the active power command value without error, thereby controlling the AC voltage amplitude at the AC outlet of the offshore wind turbine to change with the change of the output active power of the offshore wind turbine; The reactive power-frequency controller is used to control the reactive power output of offshore wind turbines to dynamically balance the reactive load of the offshore AC system, thereby controlling the frequency at the AC outlet of the offshore wind turbines and maintaining synchronous operation and uniform reactive power distribution among all wind turbines.

4. The frequency support method for an offshore wind power medium frequency uncontrolled rectifier DC transmission system according to claim 3, characterized in that: The active power command value is generated and provided by a maximum power point tracking strategy.

5. The frequency support method for an offshore wind power medium frequency uncontrolled rectifier DC transmission system according to claim 2, characterized in that: The specific implementation methods of using multiple energy sources to provide frequency support for the onshore power grid are as follows: When the onshore grid frequency deviates from its rated value by more than 0.03Hz, the MMC in the onshore inverter station is controlled to make the DC component of the DC voltage U dc Change, the amount of change is equal to k1·Δf, so that the energy stored in the capacitor of the HVDC transmission system is used to provide inertia support for the onshore power grid; When the onshore grid frequency deviates from its rated value by more than 0.06Hz, the offshore wind turbine will change the active power command value of the grid-side converter in the turbine according to the onshore grid frequency deviation. The change is equal to k2·Δf+k3·Δf', thereby utilizing the energy stored in the wind turbine rotor in the offshore wind turbine to provide inertia support for the onshore grid. When the onshore grid frequency deviates from its rated value by more than 0.09 Hz, the offshore wind turbine changes the active power command value of the energy storage device in the unit according to the onshore grid frequency deviation. The change is equal to k4·Δf+k5·Δf', thereby using the energy storage device to store energy and provide inertia support and primary frequency regulation for the onshore grid. Wherein: Δf is the frequency offset, Δf' is the differential of the frequency offset, and k1 to k5 are given proportional coefficients.

6. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: The processor is used to execute the computer program to implement the frequency support method for an offshore wind power medium-frequency uncontrolled rectifier DC transmission system according to any one of claims 1 to 5.

7. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, it implements the frequency support method for an offshore wind power medium-frequency uncontrolled rectifier DC transmission system as described in any one of claims 1 to 5.

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