An additional frequency control method and controller for accessing VSC-MTDC

Abstract

The application provides an additional frequency control method and controller of VSC-MTDC, comprising: obtaining frequency deviation and preset DC voltage deviation, calculating obtained AC frequency droop coefficient and DC frequency droop coefficient, connecting and converting the obtained AC frequency droop coefficient and DC frequency droop coefficient by a frequency modulation factor into a dynamic frequency droop coefficient, adjusting reference power increment by coefficient change, obtaining reference power increment after frequency modulation; obtaining voltage recovery gain by voltage recovery gain theoretical value, adjusting input voltage to recovery voltage according to the voltage recovery gain; adjusting output power by the reference power increment after frequency modulation and the recovery voltage, outputting a control signal to the VSC-MTDC, and performing additional frequency control on the MTDC; the application can keep the DC voltage stable when the MTDC provides active power, and can smoothly recover the DC voltage to the rated value after AC system disturbance and after the MTDC provides additional active power, thereby improving the stability of the MTDC.

Description

Technical Field

[0001] This invention relates to the field of control of offshore wind power access to multi-terminal direct current (MTDC) transmission systems, and particularly to an additional frequency control method and controller based on MTDC systems. Background Technology

[0002] Compared to traditional line commutated converter (LCC) HVDC transmission technology, voltage source converter (VSC) or modular multilevel converter (MMC) based HVDC technology offers advantages such as independent control of active and reactive power, no need for reactive power compensation, and no commutation failure issues, making it one of the best solutions for grid connection of new energy sources. Voltage source converter based multi-terminal direct current (VSC-MTDC) transmission systems can operate in different modes, thus playing a significant role in realizing large-scale clean energy transmission and offshore wind power transmission.

[0003] To ensure the stable operation of power transmission systems, it is necessary to improve the ability of hybrid AC / DC systems to maintain AC system frequency stability. Therefore, researching the frequency support capability of VSC-MTDC systems when AC systems are subjected to interference is of great significance. Currently, VSC-MTDC systems mainly employ additional frequency controllers to improve AC system frequency stability. However, when using additional frequency controllers to provide power support for AC systems with large frequency deviations, it may cause the DC voltage to exceed the stable operating range, and the power will fluctuate drastically during the process of the DC voltage recovering to its rated value. Summary of the Invention

[0004] This invention provides an additional frequency control method and controller for connecting to a VSC-MTDC, which can avoid serious deviations in DC voltage when adjusting the AC system frequency and improve the DC voltage recovery capability.

[0005] To address the aforementioned technical problems, this invention provides a dynamic additional frequency control method for VSC-MTDC, comprising:

[0006] The MTDC system is subjected to additional frequency control according to the dynamic additional frequency controller;

[0007] The dynamic additional frequency controller includes a frequency control module, a DC voltage recovery module, and a droop control module.

[0008] The frequency control module is used to obtain the frequency deviation by the difference between the input frequency and the upper and lower limit thresholds of the additional frequency control. The frequency deviation and the preset DC voltage deviation are used to obtain the AC frequency droop coefficient and the DC frequency droop coefficient. The AC frequency droop coefficient and the DC frequency droop coefficient are connected by a preset frequency modulation factor and converted into a dynamic frequency droop coefficient. The dynamic frequency droop coefficient adjusts the reference power increment by changing the coefficient. The frequency-modulated reference power increment is transmitted to the droop control module.

[0009] A DC voltage recovery module is designed based on the requirement of maintaining DC voltage stability in a DC system. The DC voltage recovery module is used to calculate the theoretical value of voltage recovery gain based on the DC voltage droop coefficient, active power deviation value, DC voltage deviation value and power loss of DC grid. The voltage recovery gain is obtained from the theoretical value of voltage recovery gain. The input voltage is adjusted to the recovery voltage according to the voltage recovery gain and the recovery voltage is transmitted to the droop control module.

[0010] The droop control module receives the reference power increment and the recovery voltage, adjusts the output power, and outputs a control signal to the VSC-MTDC to perform additional frequency control on the MTDC system, realizing dynamic additional frequency control function: when the AC system is disturbed and the frequency drops, the additional frequency controller can control the MTDC to provide active power to the AC system to maintain the frequency. However, excessive active power provided by the MTDC will cause the DC voltage to drop. In order to maintain the balance between frequency and DC voltage, the designed dynamic frequency droop coefficient can prevent the DC voltage from exceeding the stable operating range while the MTDC provides active power. At the same time, the designed DC voltage recovery module can smoothly restore the DC voltage to the rated value after the AC system is disturbed and after the MTDC provides additional active power, without causing drastic fluctuations, thus improving the stability of the MTDC system.

[0011] Furthermore, the frequency control module connects the AC frequency droop coefficient and the DC frequency droop coefficient using a preset frequency modulation factor and converts them into a dynamic frequency droop coefficient, specifically:

[0012] The dynamic frequency droop coefficient is calculated using the following formula:

[0013]

[0014] in, γ is the dynamic frequency droop coefficient, and γ is the frequency modulation factor. This refers to the droop factor of the AC frequency. This is the droop factor at DC frequency;

[0015] By calculating the dynamic frequency droop coefficient in the frequency control module, dynamic control of frequency changes can be achieved, making the frequency changes of the AC system more stable.

[0016] Furthermore, the frequency control module is also used to acquire AC frequency data and DC voltage data in the VSC-MTDC system, specifically:

[0017] The frequency control module calculates the AC frequency droop coefficient based on the AC frequency data, the preset maximum AC frequency droop coefficient, and the preset initial AC frequency droop coefficient; it calculates the DC frequency droop coefficient based on the DC voltage data, the preset minimum DC frequency droop coefficient, and the preset initial DC frequency droop coefficient; and it calculates the frequency modulation factor based on the DC voltage data and a preset positive integer value.

[0018] By acquiring AC frequency and DC voltage data from the VSC-MTDC system, the necessary data for calculating the dynamic frequency droop coefficient can be obtained, making frequency modulation operations more accurate.

[0019] Furthermore, based on the AC frequency data, the preset maximum AC frequency droop coefficient, and the preset initial AC frequency droop coefficient, the AC frequency droop coefficient is calculated, specifically as follows:

[0020] The AC frequency droop coefficient is calculated using the following formula:

[0021]

[0022] in, This refers to the droop factor of the AC frequency. and These are the maximum AC frequency droop factor and the initial AC frequency droop factor, Δf and Δf, respectively. max These are the frequency deviation and maximum frequency deviation in the AC frequency data, respectively.

[0023] Furthermore, based on the DC voltage data, the preset minimum DC frequency droop coefficient, and the preset initial DC frequency droop coefficient, the DC frequency droop coefficient is calculated, specifically as follows:

[0024] The DC frequency droop coefficient is calculated using the following formula:

[0025]

[0026] in, This is the droop factor at DC frequency. and These are the minimum DC frequency droop coefficient and the initial DC frequency droop coefficient, ΔU, respectively. dc and ΔU dcmax These represent the DC voltage deviation and the maximum DC voltage deviation in the DC voltage data, respectively.

[0027] Furthermore, based on the DC voltage data and a preset positive integer value, the frequency modulation factor is calculated, specifically as follows:

[0028] The frequency modulation factor is calculated using the following formula:

[0029]

[0030]

[0031] Where γ is the frequency modulation factor, ΔU dcγ1 and ΔU dcγ2 These are the starting DC voltage deviation and the ending DC voltage deviation of γ in the DC voltage data, respectively, where n is a positive integer.

[0032] Furthermore, the voltage recovery gain in the DC voltage recovery module is defined as follows:

[0033]

[0034]

[0035] Where K is the voltage recovery gain, K′ is the voltage recovery gain during the DC voltage recovery process, and K th t0 and t1 are the theoretical values ​​of voltage recovery gain, respectively, and N is a positive integer.

[0036] The theoretical value of the voltage recovery gain in the DC voltage recovery module is calculated as follows:

[0037]

[0038] Among them, K th K is the theoretical value of voltage recovery gain. vi Let be the DC voltage droop factor of the i-th VSC. Let be the initial active power deviation value of the i-th VSC. Let be the active power deviation value after the i-th VSC disturbance. P represents the initial DC voltage deviation value. loss This refers to the power loss of a DC power grid.

[0039] By defining the voltage recovery gain in the DC voltage recovery module and calculating the theoretical value of the voltage recovery gain, the voltage recovery gain can be increased to the theoretical value, achieving smooth and error-free DC voltage recovery after frequency modulation of the MTDC system, thereby improving the operational stability of the DC system.

[0040] Furthermore, the initial active power deviation value is calculated as follows:

[0041]

[0042]

[0043] in, Let be the initial active power deviation value of the i-th VSC. This is the reference value for active power transmitted via VSC. Let i be the initial active power of the i-th VSC. and R represents the initial DC voltage of the i-th and j-th VSCs, respectively. ij To connect two interconnected converters VSC i and VSC j The resistance between;

[0044] The calculation method for the active power deviation value after the disturbance is as follows:

[0045]

[0046] in, Let be the active power deviation value after the i-th VSC disturbance. The steady-state active power of the i-th VSC after the disturbance;

[0047] The initial DC voltage deviation value is calculated as follows:

[0048]

[0049] in, U is the initial DC voltage deviation value. dcref This is the reference value for the VSC DC voltage;

[0050] According to the energy conservation law of MTDC systems:

[0051]

[0052] Available at the initial moment Alternative The power loss of the DC power grid is calculated as follows:

[0053]

[0054] Among them, P loss This refers to the power loss of a DC power grid.

[0055] The present invention also provides a dynamic additional frequency controller, comprising: a frequency control module, a DC voltage recovery module, and a droop control module;

[0056] The frequency control module is used to obtain the frequency deviation by the difference between the input frequency and the upper and lower limit thresholds of the additional frequency control. The frequency deviation and the preset DC voltage deviation are used to obtain the AC frequency droop coefficient and the DC frequency droop coefficient. The AC frequency droop coefficient and the DC frequency droop coefficient are connected by a preset frequency modulation factor and converted into a dynamic frequency droop coefficient. The dynamic frequency droop coefficient adjusts the reference power increment by changing the coefficient. The frequency-modulated reference power increment is transmitted to the droop control module.

[0057] The DC voltage recovery module is used to calculate the theoretical value of voltage recovery gain based on the DC voltage droop coefficient, active power deviation value, DC voltage deviation value and power loss of DC grid, obtain the voltage recovery gain from the theoretical value of voltage recovery gain, adjust the input voltage to the recovery voltage according to the voltage recovery gain, and transmit the recovery voltage to the droop control module;

[0058] The droop control module receives the reference power increment and the recovery voltage, adjusts the output power, and outputs a control signal to the VSC-MTDC system to perform additional frequency control on the MTDC system, realizing a dynamic additional frequency control function: when the AC system is disturbed and the frequency drops, the additional frequency controller can control the MTDC to provide active power to the AC system to maintain the frequency. However, excessive active power provided by the MTDC will cause the DC voltage to drop. In order to maintain the balance between frequency and DC voltage, the designed dynamic frequency droop coefficient can prevent the DC voltage from exceeding the stable operating range while the MTDC provides active power. At the same time, the designed DC voltage recovery module can smoothly restore the DC voltage to the rated value after the AC system is disturbed and after the MTDC provides additional active power, without causing drastic fluctuations, thus improving the stability of the MTDC system. Attached Figure Description

[0059] Figure 1 : A flowchart illustrating the steps of an additional frequency control method for accessing VSC-MTDC provided in an embodiment of the present invention;

[0060] Figure 2 : A schematic diagram of the structure of one embodiment of the dynamic frequency control method provided by the present invention;

[0061] Figure 3: A schematic diagram of the structure of one embodiment of the frequency control module provided by the present invention;

[0062] Figure 4 : A schematic diagram of the structure of an embodiment of the DC voltage recovery module provided by the present invention;

[0063] Figure 5 : A schematic diagram of an embodiment of the dynamic additional frequency controller provided by the present invention;

[0064] Figure 6 : A schematic diagram of the structure of the simulation model of the offshore wind farm connected to the multi-terminal flexible DC transmission system provided by the present invention;

[0065] Figure 7 The simulation model of the offshore wind farm multi-terminal flexible DC transmission system provided by this invention shows the frequency of AC1 and the DC voltage of VSC1 when a disturbance occurs.

[0066] Figure 8 The simulation model of the offshore wind farm multi-terminal flexible DC transmission system provided by this invention includes the frequencies of AC1 and AC3 and the DC voltage of VSC1 when a large disturbance occurs.

[0067] Figure 9 The simulation model of the offshore wind farm multi-terminal flexible DC transmission system provided by this invention shows the DC voltage and actual power during the DC voltage recovery process.

[0068] Figure 10 The main parameters of the simulation system in the simulation model of the offshore wind farm multi-terminal flexible DC transmission system provided by this invention;

[0069] Figure 11 The theoretical values ​​and system simulation results of four VSC DC voltages are compared in the simulation model of the offshore wind farm multi-terminal flexible DC transmission system provided by this invention. Detailed Implementation

[0070] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0071] This invention provides a dynamic additional frequency controller for implementing an additional frequency control method for connecting to a VSC-MTDC. The dynamic additional frequency controller includes a frequency control module M1, a DC voltage recovery module M2, and a droop control module M3.

[0072] The frequency control module M1 is used to obtain the frequency deviation by the difference between the input frequency and the upper and lower limit thresholds of the additional frequency control. The frequency deviation and the preset DC voltage deviation are used to obtain the AC frequency droop coefficient and the DC frequency droop coefficient. The AC frequency droop coefficient and the DC frequency droop coefficient are connected by a preset frequency modulation factor and converted into a dynamic frequency droop coefficient. The dynamic frequency droop coefficient adjusts the reference power increment by changing the coefficient. The frequency-modulated reference power increment is then transmitted to the droop control module.

[0073] The DC voltage recovery module M2 is used to calculate the theoretical value of voltage recovery gain based on the DC voltage droop coefficient, active power deviation value, DC voltage deviation value and power loss of DC grid. The voltage recovery gain is obtained from the theoretical value of voltage recovery gain. The input voltage is adjusted to the recovery voltage according to the voltage recovery gain, and the recovery voltage is transmitted to the droop control module.

[0074] The droop control module M3 is used to receive the reference power increment and the recovery voltage, adjust the output power and output a control signal to the VSC-MTDC to perform additional frequency control on the MTDC system.

[0075] Please refer to Figure 1 The present invention provides a flowchart of an additional frequency control method for accessing a VSC-MTDC; the method includes steps S1 to S4, each step being as follows:

[0076] Step S1: Obtain the frequency deviation and DC voltage deviation, and design the AC frequency droop coefficient and DC frequency droop coefficient.

[0077] The frequency control module M1, which is improved based on the dynamic frequency droop coefficient, is as follows: Figure 3 As shown. In the frequency control module M1, the frequency deviation is obtained by the difference between the input frequency and the upper and lower limit thresholds of the additional frequency control. The AC frequency droop coefficient and the DC frequency droop coefficient are obtained by using the frequency deviation and the preset DC voltage deviation.

[0078] Step S2: Connect the AC frequency droop coefficient and the DC frequency droop coefficient with a preset frequency modulation factor and convert them into a dynamic frequency droop coefficient to participate in power regulation, thereby obtaining a reference power increment.

[0079] The AC frequency droop coefficient and the DC frequency droop coefficient in the frequency control module M1 are connected by a preset frequency modulation factor and converted into a dynamic frequency droop coefficient. The dynamic frequency droop coefficient adjusts the reference power increment by changing the coefficient, and the frequency-modulated reference power increment is transmitted to the droop control module M3.

[0080] Step S3: Calculate the voltage recovery gain based on the theoretical value of the calculated voltage recovery gain, adjust the input voltage to the voltage recovery gain, and obtain the recovered voltage.

[0081] DC voltage recovery module M2, such as Figure 4 As shown. The DC voltage recovery module M2 calculates the theoretical value of voltage recovery gain based on the DC voltage droop coefficient, active power deviation value, DC voltage deviation value, and power loss of the DC grid. The voltage recovery gain is obtained from the theoretical value of voltage recovery gain. The input voltage is adjusted to the recovery voltage according to the voltage recovery gain, and the recovery voltage is transmitted to the droop control module M3.

[0082] Step S4: Adjust the output power according to the reference power increment and the recovery voltage, output a control signal to the VSC-MTDC system, and perform additional frequency control on the MTDC system.

[0083] The frequency control module M1, the DC voltage recovery module M2, and the droop control module M3 are connected to form a dynamic additional frequency controller, such as... Figure 2 As shown, the droop control module M3 is used to receive the reference power increment and the recovery voltage, adjust the output power and output a control signal to the VSC-MTDC system to perform additional frequency control on the MTDC system, thereby realizing the dynamic additional frequency control function.

[0084] Furthermore, the method for obtaining the frequency deviation and DC voltage deviation, and designing the AC frequency droop coefficient and DC frequency droop coefficient includes:

[0085] S1.1: The AC frequency droop coefficient is defined as follows:

[0086]

[0087] in, This refers to the droop factor of the AC frequency. and These are the maximum AC frequency droop factor and the initial AC frequency droop factor, Δf and Δf, respectively. max These are the frequency deviation and maximum frequency deviation in the AC frequency data, respectively.

[0088] S1.2: The definition of the DC frequency droop coefficient is as follows:

[0089]

[0090] in, This is the droop factor at DC frequency. and These are the minimum DC frequency droop coefficient and the initial DC frequency droop coefficient, ΔU, respectively. dc and ΔU dcmax These represent the DC voltage deviation and the maximum DC voltage deviation in the DC voltage data, respectively.

[0091] Furthermore, the method for calculating the preset frequency modulation factor and the dynamic frequency droop coefficient includes:

[0092] S2.1: The frequency modulation factor is defined as follows:

[0093]

[0094]

[0095] Where γ is the frequency modulation factor, ΔU dcγ1 and ΔU dcγ2 These are the beginning DC voltage deviation and the end DC voltage deviation of γ in the DC voltage data, respectively, where n is a positive integer;

[0096] S2.2: The dynamic frequency droop coefficient is defined as follows:

[0097]

[0098] in, γ is the dynamic frequency droop coefficient, and γ is the frequency modulation factor. This refers to the droop factor of the AC frequency. This represents the droop factor at DC frequency.

[0099] Furthermore, the methods for calculating the theoretical value of voltage recovery gain, voltage recovery gain, active power deviation value, DC voltage deviation value, and power loss of the DC grid include:

[0100] S3.1: The active power deviation value includes the initial active power deviation value and the active power deviation value after the disturbance; the initial active power deviation value is calculated as follows:

[0101]

[0102]

[0103] in, Let be the initial active power deviation value of the i-th VSC. This is the reference value for active power transmitted via VSC. Let i be the initial active power of the i-th VSC. and R represents the initial DC voltage of the i-th and j-th VSCs, respectively. ij To connect two interconnected converters VSCi and VSC j The resistance between;

[0104] The calculation method for the active power deviation value after the disturbance is as follows:

[0105]

[0106] in, Let be the active power deviation value after the i-th VSC disturbance. Let be the steady-state active power of the i-th VSC after the disturbance.

[0107] S3.2: The calculation method for the DC voltage deviation value is as follows:

[0108]

[0109] in, U is the initial DC voltage deviation value. dcref This is the reference value for the VSC DC voltage.

[0110] S3.3: According to the energy conservation law of MTDC systems:

[0111]

[0112] Available at the initial moment Alternative The power loss of the DC power grid is calculated as follows:

[0113]

[0114] Among them, P loss This refers to the power loss of a DC power grid.

[0115] S3.4: Assume that the i-th VSC participates in DC voltage recovery, while other converters are under fixed active power control and do not participate in DC voltage recovery. If the voltage recovery gains of the converters participating in DC voltage recovery are equal, then the theoretical value of the voltage recovery gain of the DC voltage recovery module is calculated as follows:

[0116]

[0117] Among them, K th K is the theoretical value of voltage recovery gain. vi Let be the DC voltage droop factor of the i-th VSC. Let be the initial active power deviation value of the i-th VSC. Let be the active power deviation value after the i-th VSC disturbance. P represents the initial DC voltage deviation value. loss This refers to the power loss of a DC power grid. (j=2,...,n) represents the active power of VSC in steady state after the disturbance.

[0118] S3.5: The voltage recovery gain in the DC voltage recovery module is defined as follows:

[0119]

[0120]

[0121] Where K is the voltage recovery gain, K′ is the voltage recovery gain during the DC voltage recovery process, and K th t0 and t1 are the theoretical values ​​of voltage recovery gain, respectively, and N is a positive integer.

[0122] As described above, the frequency control module can take into account both the power requirements of the AC grid and the voltage stability requirements of the DC grid when the VSC-MTDC system is frequency-regulated; and the DC voltage recovery module can achieve smooth and error-free DC voltage recovery after the MTDC system is frequency-regulated, thereby improving the operational stability of the DC system.

[0123] For each step in this control method, and in combination Figures 2-5 The control method will be explained in detail below:

[0124] The flowchart for implementing the dynamic additional frequency control method is as follows: Figure 2 As shown.

[0125] The AC frequency droop coefficient and DC frequency droop coefficient are obtained using equations (1) and (2). Then, a frequency modulation factor is designed using equations (3) and (4). The AC frequency droop coefficient and DC frequency droop coefficient are combined using the frequency modulation factor to improve the frequency droop coefficient, as shown in equation (5). The improved frequency control module is shown below. Figure 3 As shown.

[0126] The initial active power deviation of the i-th VSC in the VSC-MTDC system is calculated using equations (6) and (7). The active power deviation of the i-th VSC after disturbance is calculated using equation (8). The DC voltage deviation in the initial state is calculated using equation (9). The power loss of the DC grid is calculated using equations (10) and (11). Since the initial active power deviation of the i-th VSC can be used to replace the initial active power of the i-th VSC at the initial moment, the calculation method of the power loss of the DC grid in equation (10) can be replaced by equation (11).

[0127] exist Figure 3 Based on the frequency control module, a DC voltage recovery module is added, such as... Figure 4As shown. Using equation (13), the DC voltage recovery gain in three stages is designed. The DC voltage recovery gain during the DC voltage recovery process and the theoretical calculated values ​​after the DC voltage recovery is completed are shown in equation (14) and equation (12), respectively.

[0128] Connecting the frequency control module, the DC voltage recovery module, and the droop control module constitutes a dynamic additional frequency controller, such as... Figure 5 As shown.

[0129] This invention builds a system on the PSCAD / EMTDC platform, such as Figure 6 The simulation model of the offshore wind power connected to a multi-terminal flexible DC transmission system shown in Table 1 verifies the performance of the novel dynamic frequency-addition controller. Key parameters are listed in Table 1. Figure 10 As shown. Figure 7 Let AC1 be the frequency of the AC system and VSC1 be the DC voltage of the AC system under small disturbances. Figure 8 The frequencies of AC1 and AC2 in AC systems under large disturbances, and the DC voltage of VSC1, Figure 9 For the DC voltage and actual power during the DC voltage recovery process, Figure 11 The theoretical and actual DC voltage values ​​for the four VSCs are given.

[0130] When t = 15s, the load on AC1 of AC system suddenly increases by 100MW. The frequency f1 of AC1 and the DC voltage U of VSC1... dc1 Changes such as Figure 7 As shown in the figure, under the original additional frequency control, f1 decreases from 50Hz to 49.903Hz, with a frequency change Δf1 of 0.097Hz. In contrast, the system using the new dynamic additional frequency control has a frequency change Δf1 of 0.066Hz, which is 31.96% smaller than the system using the original additional frequency control. The DC voltage deviation ΔU of the system using the original additional frequency control is shown in Figure 1. dc1 The DC voltage deviation ΔU of the system employing the novel dynamic additional frequency control is 12.16kV. dc1 The voltage is 13.05 kV, an increase of only 7.32% compared to the previous version. Simulation results show that the proposed control strategy effectively improves the frequency deviation of the AC1 system with only a slight increase in DC voltage.

[0131] At t=15s, the load on AC1 suddenly increased by 300MW. The frequencies f1 and f3 of AC1 and AC3, and the DC voltage U of VSC1... dc1 like Figure 8 As shown. The original additional frequency control VSC1 DC voltage U dc1The DC voltage deviation ΔU decreased from 400.19kV to 359.68kV. dc1 The voltage is 40.51kV, and the DC voltage drop ratio is 10.12%. Furthermore, the frequency f3 in the AC system AC3 drops from 50Hz to 49.653Hz. The frequency deviation Δf3 is 0.347Hz, indicating a significant decrease in the frequency stability of the normal AC system. By employing a novel dynamic additional frequency control, the DC voltage deviation ΔU... dc1 The voltage dropped from 40.51kV to 20.98kV, significantly improving the operating conditions of the DC grid. Furthermore, the frequency deviation Δf3 of the AC system AC3 was 0.179Hz, greatly reducing its impact on normal AC systems.

[0132] Figure 11 A comparison of the theoretical values ​​and system simulation results of the DC voltage for four VSCs is presented. The results show that the relative error between the theoretical and actual values ​​of the DC voltage for each VSC in the initial state, after disturbance, and after voltage recovery does not exceed 0.1%, indicating that the theoretical calculation accuracy of the DC voltage recovery module is high.

[0133] When t=30s is set, the DC voltage recovery module will activate after a large frequency disturbance. Figure 9 For the DC voltage and actual power during the DC voltage recovery process, Figure 9 The comparison shows four modes during DC voltage recovery: Mode 1 is when the voltage recovery gain K is equal to the theoretical value K0. th Mode 2 increases the voltage recovery gain K from 0 to a fixed slope; Mode 3 designs the voltage recovery gain K as a function similar to a Butterworth filter, increasing from 0 to the theoretical value K of the voltage recovery gain. th Mode 4 represents other DC voltage recovery strategies. (By...) Figure 9 It can be seen that after applying the new dynamic additional frequency control, the DC voltage U of VSC1... dc1 The voltage recovers from 379.20kV to 400.35kV. Modes 1, 2, and 3 all restore the DC voltage to its initial value. Although mode 4 also achieves DC voltage recovery, when VSC3 is running in a fixed DC voltage control mode, U... dc1 It cannot be restored to the initial value. Furthermore, Mode 1 momentarily disrupts the balance of the MTDC system control structure, causing a DC voltage U... dc1 The active power P3 and P4 oscillate strongly. Mode 4 has a significant impact on P3. Therefore, among the four types of DC voltage recovery processes, mode 3 provides the least power fluctuation and the smoothest DC voltage recovery.

[0134] As can be seen from the above, the accessory frequency control method for accessing the VSC-MTDC system provided by the technical solution of this application can effectively improve the frequency deviation of the AC system, reduce the impact of sudden load increase on the AC system, and enable the DC voltage to recover stably and smoothly after a large frequency disturbance.

[0135] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention for those skilled in the art.

Claims

1. An additional frequency control method for accessing VSC-MTDC, characterized in that, include: The VSC-MTDC system is subjected to additional frequency control according to the dynamic additional frequency controller; The dynamic additional frequency controller includes a frequency control module, a DC voltage recovery module, and a droop control module. The frequency control module is used to obtain the frequency deviation by the difference between the input frequency and the upper and lower limit thresholds of the additional frequency control. The frequency deviation and the preset DC voltage deviation are used to obtain the AC frequency droop coefficient and the DC frequency droop coefficient. The AC frequency droop coefficient and the DC frequency droop coefficient are connected by a preset frequency modulation factor and converted into a dynamic frequency droop coefficient. The dynamic frequency droop coefficient adjusts the reference power increment by changing the coefficient. The frequency-modulated reference power increment is transmitted to the droop control module. The DC voltage recovery module is used to calculate the theoretical value of voltage recovery gain based on the DC voltage droop coefficient, active power deviation value, DC voltage deviation value and power loss of DC grid, obtain the voltage recovery gain from the theoretical value of voltage recovery gain, adjust the input voltage to the recovery voltage according to the voltage recovery gain, and transmit the recovery voltage to the droop control module; The droop control module is used to receive the reference power increment and the recovery voltage, adjust the output power and output a control signal to the VSC-MTDC system to perform additional frequency control on the MTDC system; The voltage recovery gain in the DC voltage recovery module is defined as follows: in, K For voltage recovery gain, This refers to the voltage recovery gain during the DC voltage recovery process. This is the theoretical value of voltage recovery gain. and These are the start and end times of DC voltage recovery, respectively. N It is a positive integer; The theoretical value of the voltage recovery gain in the DC voltage recovery module is calculated as follows: in, This is the theoretical value of voltage recovery gain. Let be the DC voltage droop factor of the i-th VSC. Let be the initial active power deviation value of the i-th VSC. Let be the active power deviation value after the i-th VSC disturbance. This represents the initial DC voltage deviation value. This refers to the power loss of a DC power grid.

2. The additional frequency control method for accessing VSC-MTDC as described in claim 1, characterized in that, The frequency control module connects the AC frequency droop factor and the DC frequency droop factor using a preset frequency modulation factor and converts them into a dynamic frequency droop factor, specifically: The dynamic frequency droop coefficient is calculated using the following formula: in, γ is the dynamic frequency droop coefficient, and γ is the frequency modulation factor. This refers to the droop factor of the AC frequency. This represents the droop factor at DC frequency.

3. The additional frequency control method for accessing VSC-MTDC as described in claim 1, characterized in that, The frequency control module is also used to acquire AC frequency data and DC voltage data in the VSC-MTDC system, specifically: The frequency control module calculates the AC frequency droop coefficient based on the AC frequency data, the preset maximum AC frequency droop coefficient, and the preset initial AC frequency droop coefficient; it calculates the DC frequency droop coefficient based on the DC voltage data, the preset minimum DC frequency droop coefficient, and the preset initial DC frequency droop coefficient; and it calculates the frequency modulation factor based on the DC voltage data and a preset positive integer value.

4. The additional frequency control method for accessing VSC-MTDC as described in claim 3, characterized in that, The AC frequency droop coefficient is calculated based on the AC frequency data, the preset maximum AC frequency droop coefficient, and the preset initial AC frequency droop coefficient, specifically as follows: The AC frequency droop coefficient is calculated using the following formula: in, This refers to the droop factor of the AC frequency. and These are the maximum AC frequency droop factor and the initial AC frequency droop factor, respectively. and These are the frequency deviation and maximum frequency deviation in the AC frequency data, respectively.

5. The additional frequency control method for accessing VSC-MTDC as described in claim 3, characterized in that, The DC frequency droop coefficient is calculated based on the DC voltage data, the preset minimum DC frequency droop coefficient, and the preset initial DC frequency droop coefficient, specifically as follows: The DC frequency droop coefficient is calculated using the following formula: in, This is the droop factor at DC frequency. and These are the minimum DC frequency droop factor and the initial DC frequency droop factor, respectively. and These represent the DC voltage deviation and the maximum DC voltage deviation in the DC voltage data, respectively.

6. The additional frequency control method for accessing VSC-MTDC as described in claim 3, characterized in that, The frequency modulation factor is calculated based on the DC voltage data and a preset positive integer value, specifically as follows: The frequency modulation factor is calculated using the following formula: Where γ is the frequency modulation factor. and These are the initial DC voltage deviation of γ and the final DC voltage deviation of γ in the DC voltage data, respectively. denoted as DC voltage deviation in the DC voltage data, where n is a positive integer.

7. The additional frequency control method for accessing VSC-MTDC as described in claim 1, characterized in that, The initial active power deviation value is calculated as follows: in, Let be the initial active power deviation value of the i-th VSC. This is the reference value for active power transmitted via VSC. Let i be the initial active power of the i-th VSC. and Let be the initial DC voltages of the i-th and j-th VSCs, respectively. To connect two interconnected converters and The resistance between; The calculation method for the active power deviation value after the disturbance is as follows: in, Let be the active power deviation value after the i-th VSC disturbance. The steady-state active power of the i-th VSC after the disturbance; The initial DC voltage deviation value is calculated as follows: in, This represents the initial DC voltage deviation value. This is the reference value for the VSC DC voltage; According to the energy conservation law of MTDC systems: Available at the initial moment Alternative The power loss of the DC power grid is calculated as follows: in, This refers to the power loss of a DC power grid.

8. An additional frequency controller, characterized in that, The additional frequency control method for accessing VSC-MTDC as described in any one of claims 1 to 7 is adopted; The additional frequency controller includes: a frequency control module, a DC voltage recovery module, and a droop control module; The frequency control module is used to obtain the frequency deviation by the difference between the input frequency and the upper and lower limit thresholds of the additional frequency control. The frequency deviation and the preset DC voltage deviation are used to obtain the AC frequency droop coefficient and the DC frequency droop coefficient. The AC frequency droop coefficient and the DC frequency droop coefficient are connected by a preset frequency modulation factor and converted into a dynamic frequency droop coefficient. The dynamic frequency droop coefficient adjusts the reference power increment by changing the coefficient. The frequency-modulated reference power increment is transmitted to the droop control module. The DC voltage recovery module is used to calculate the theoretical value of voltage recovery gain based on the DC voltage droop coefficient, active power deviation value, DC voltage deviation value and power loss of DC grid, obtain the voltage recovery gain from the theoretical value of voltage recovery gain, adjust the input voltage to the recovery voltage according to the voltage recovery gain, and transmit the recovery voltage to the droop control module; The droop control module receives the reference power increment and the recovery voltage, adjusts the output power, and outputs a control signal to the VSC-MTDC to perform additional frequency control on the MTDC system.

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