A power grid frequency modulation method and device based on MMC bridge arm capacitor reuse

Abstract

The application provides a power grid frequency modulation method and device based on MMC bridge arm capacitor multiplexing, and relates to the technical field of power grid frequency control. In view of the function conflict problem between the participation of the MMC bridge arm capacitor in power grid frequency control and voltage stabilization and filtering, the MMC bridge arm sub-module is transformed in a full-bridge type multiplexing mode, the MMC power throughput capacity is enhanced in a parallel super capacitor mode, the regulation and control of the bridge arm capacitor on voltage fluctuation is enhanced, and the support capacity on the power grid frequency is simultaneously improved; the principle and method of the participation of the MMC in the power grid frequency support are analyzed by simulating the frequency characteristics of a synchronous machine, the power grid frequency modulation model of the participation of the MMC in the MTDC system is constructed, the power grid frequency modulation method of the participation of the MMC in the MTDC system is optimized, the frequency deviation generated when the active power of the power grid and the load demand are unbalanced is taken as the input of the frequency modulation model, and corresponding frequency modulation strategies are adopted based on the frequency modulation demand of the MTDC system. The application can realize the reasonable distribution of the power required for frequency modulation, and improve the comprehensive stability characteristics of the new power system.

Description

Technical Field

[0001] This invention relates to the field of power grid frequency control technology, specifically to a power grid frequency regulation method and apparatus based on MMC bridge arm capacitor reuse. Background Technology

[0002] Against the backdrop of large-scale development of clean energy, multi-terminal direct current transmission (MTDC) systems have been widely used in long-distance grid connection of large-scale clean energy projects due to their advantages such as low energy loss, fast control speed, flexible operation, and high power supply reliability. However, the high penetration rate of new energy sources through MTDC grid connection has increasingly complicated the frequency characteristics of the power system, resulting in weak grid characteristics on the AC side. Traditional thermal power units suffer from insufficient reserve capacity and long response time lags and low ramp rates, leading to poor frequency regulation performance in response to system frequency changes. To meet the grid's requirements for frequency stability, power compensation using wind power and energy storage devices is currently mainly used to improve the performance of traditional units. However, wind power output is affected by uncertain factors such as wind speed, resulting in complex control methods and poor control timeliness. Energy storage construction and electricity costs are also high, making these less than ideal auxiliary frequency regulation methods. Therefore, there is an urgent need for an auxiliary frequency regulation technology that can fully utilize existing resources, respond quickly to system frequency changes, and make frequency regulation more economical to address the shortcomings of thermal power units.

[0003] In flexible DC transmission, modular multilevel converters (MMCs), characterized by large capacity, high voltage withstand capability, stable output waveform, and simple manufacturing, are widely used. To buffer the impact of power surges on the system, maintain voltage stability, and improve the power quality of flexible DC transmission, MMC submodules typically incorporate multiple modular bridge arm capacitors for voltage regulation and filtering. Capacitors have energy storage capabilities, and extracting their energy can enable the MMC to provide frequency regulation power. However, when the MMC participates in frequency regulation, it is prone to conflicts with its own filtering and voltage equalization functions. Therefore, in the process of using bridge arm capacitor energy extraction to assist grid frequency regulation, it is necessary to enhance the active power output capability of the bridge arm capacitors during frequency regulation to ensure their energy requirements during multiplexing. On the other hand, it is crucial to avoid conflicts between application requirements. The control of bridge arm capacitor energy extraction needs to consider the coordination between multiple terminals, enhancing the response speed, response time, and response effect of grid frequency regulation for different frequency regulation needs. Summary of the Invention

[0004] The purpose of this invention is to provide a power grid frequency regulation method and apparatus based on MMC bridge arm capacitor reuse, which solves problems such as how to achieve MMC participation in frequency regulation and voltage stabilization and reduce filtering conflicts through bridge arm capacitor reuse, and can realize the reasonable allocation of power required for frequency regulation and improve the overall stability characteristics of the new power system.

[0005] To achieve the above objectives, in a first aspect, the present invention provides a power grid frequency regulation method based on MMC bridge arm capacitor reuse, comprising:

[0006] The bridge arm sub-modules of the MMC are reused in the full-bridge structure to form a reused sub-module. Then, a supercapacitor is connected in parallel to the bridge arm sub-module, thereby improving the bridge arm structure of the MMC; the grid frequency is transmitted to the MMC; the MMC is a modular multilevel converter.

[0007] By simulating the frequency characteristics of a synchronous machine, the principle and method of MMC participating in grid frequency support are analyzed. A grid frequency regulation model of MMC participating in the MTDC system is constructed, and the grid frequency regulation method of MMC participating in the MTDC system is optimized. The MTDC system is a multi-terminal flexible DC transmission system.

[0008] The frequency deviation generated when the active power of the power grid and the active power of the load are unbalanced is used as the input of the power grid frequency regulation model;

[0009] Based on the degree of disturbance to the power grid caused by the frequency deviation, a corresponding frequency regulation strategy is adopted according to the frequency regulation requirements of the MTDC system.

[0010] According to the present invention, a power grid frequency regulation method based on MMC arm capacitor reuse is provided, wherein the step of adopting a corresponding frequency regulation strategy based on the frequency regulation requirements of the MTDC system according to the degree of disturbance to the power grid caused by the frequency deviation includes:

[0011] If the absolute value of the frequency deviation is less than the first threshold, the grid frequency regulation dead zone is set to the first threshold, and grid frequency regulation is not performed.

[0012] According to the present invention, a power grid frequency regulation method based on MMC arm capacitor reuse is provided, wherein the step of adopting a corresponding frequency regulation strategy based on the frequency regulation requirements of the MTDC system according to the degree of disturbance to the power grid caused by the frequency deviation includes:

[0013] If the absolute value of the frequency deviation is greater than or equal to the first threshold and less than the second threshold, the affected side MMC uses virtual inertial control to assist the thermal power unit in responding to the frequency change, and converts the frequency change signal into DC voltage fluctuation and multiplexing submodule voltage fluctuation through the droop control loop.

[0014] According to the present invention, a power grid frequency regulation method based on MMC arm capacitor reuse is provided, wherein the step of adopting a corresponding frequency regulation strategy based on the frequency regulation requirements of the MTDC system according to the degree of disturbance to the power grid caused by the frequency deviation includes:

[0015] If the absolute value of the frequency deviation is greater than or equal to the second threshold and less than the third threshold, the unaffected MMC provides active power support to the affected power grid, and adjusts the number of multiplexing sub-modules for power grid frequency regulation according to the power margin of the unaffected MMC and the relative degree of frequency deviation, so as to achieve reasonable allocation of frequency regulation power.

[0016] According to the present invention, a power grid frequency regulation method based on MMC bridge arm capacitor reuse is provided, wherein the first threshold is 0.033Hz, the second threshold is 0.2Hz, and the third threshold is 0.5Hz.

[0017] According to the present invention, a power grid frequency regulation method based on MMC arm capacitor reuse is provided, which analyzes the principle and method of MMC participating in power grid frequency support based on the number of energy storage sub-modules in the MMC arm, the capacity of the arm capacitor, and the voltage of the reused sub-module.

[0018] According to the present invention, a power grid frequency regulation method based on MMC arm capacitor reuse is provided, wherein the principle and method of analyzing the participation of MMC in power grid frequency support by simulating the frequency characteristics of a synchronous machine include:

[0019] The changes in the capacitance of the multiplexed submodule and the changes in the output energy of the MMC are expressed as follows:

[0020] (1)

[0021] In the formula, Δ E SM and Δ E MMC These are the changes in the capacitor of the multiplexed submodule and the changes in the output energy of the MMC, respectively. S SM and S MMC These are the capacities of the reused submodule and the MMC, respectively. C SM and C eq These are the reused submodule capacitor and the MMC equivalent capacitor, respectively. U SM and U SM,ref These are the voltage value and rated voltage value of the multiplexed submodule, respectively; U dc and U dc,ref These are the real-time and reference values ​​of the DC voltage, respectively.

[0022] Assuming each arm of the MMC has N multiplexed submodules, and all are in operation, differentiating equation (1) yields the dynamic equation of the MMC when it outputs active power as follows:

[0023] (2)

[0024] In the formula, N The number of reused submodules in a single bridge arm; P AC and P DC These represent the AC-side power and DC-side power of the MMC, respectively.

[0025] The relationship between the voltage of the multiplexed submodule and the DC voltage is as follows:

[0026] (3)

[0027] Substituting equation (3) into equation (2), the equivalent capacitance of the MMC is obtained as follows:

[0028] (4)

[0029] The active-load frequency equation for a synchronous machine is:

[0030] (5)

[0031] In the formula, H The equivalent inertia constant of the synchronizing machine; P M and P E These are the mechanical power and electromagnetic power output by the prime mover, respectively. f and f ref These are the grid frequency value and the frequency reference value, respectively; the inertial time constant of the simulated synchronous machine is calculated to obtain the virtual inertial coefficient of the MMC. H MMC The formula for calculation is:

[0032] (6)

[0033] Using a phase-locked loop to transmit the frequency change of the power grid to the MMC, and combining equations (2) and (6), the relationship between the MMC and the power grid frequency change is obtained as follows:

[0034] (7)

[0035] Where, Δ P MMC Let MMC power change be the variable; integrating equation (7), we get:

[0036] (8)

[0037] Will U dc =Δ U dc + U dc,ref Substituting into equation (8) and simplifying, we get:

[0038] (9)

[0039] In the formula, Δ U dc Δ U SM and Δ f Let represent the DC voltage change, the multiplexing submodule voltage change, and the grid frequency deviation, respectively. Since the DC voltage change is small, its quadratic term can be ignored, resulting in:

[0040] (10)

[0041] Therefore, the relationship between the grid frequency deviation and the voltage change of the multiplexing submodule and the DC voltage change is as follows:

[0042] (11).

[0043] According to the present invention, a power grid frequency regulation method based on MMC arm capacitor reuse is provided. In the power grid frequency regulation model, the power of the thermal power unit includes the governor and the reheat turbine, as shown below:

[0044] (12)

[0045] In the formula, Δ P SG This represents the power change of the synchronous machine; R This is the adjustment coefficient; T g , T t , T r These are the time constant of the governor, the reheat time constant, and the gas capacity time constant of the thermal power unit, respectively. K r is the reheat coefficient of the thermal power unit; s is the Laplace operator;

[0046] Therefore, the frequency deviation of the power grid in each region is:

[0047] (13)

[0048] In the formula, D Δ is the load damping coefficient. PL This represents the load disturbance power value.

[0049] According to the present invention, a power grid frequency regulation method based on MMC bridge arm capacitor reuse is provided, wherein the optimization of the power grid frequency regulation method for MMC participation in MTDC systems includes:

[0050] The energy change in MMC response to frequency and voltage changes is:

[0051] (14)

[0052] In the formula, the subscript i Indicates the first i Regional power grid; Δ W MMC,f,i Indicates the first i The energy change in the frequency of the MMC response of a regional power grid; Δ W MMC,v,i Indicates the first i The energy change in the MMC response voltage of a regional power grid; Δ f i For the first i Regional power grid frequency deviation; H MMC,i For the first i The virtual inertia coefficient of the MMC of a regional power grid;

[0053] Define a criterion for judging the frequency-voltage variation of the power grid. J i for:

[0054] (15)

[0055] Judgment Indicators J i When applied to the design of the virtual inertia coefficient of MMC, the new virtual inertia coefficient of MMC is obtained as follows:

[0056] (16)

[0057] Set the power grid frequency deviation to reach the second threshold. η 2. When the MMC is activated to participate in the frequency support of other regional power grids to the disturbed power grid in the MTDC system, the corresponding DC voltage change is:

[0058] (17)

[0059] The power margin of each MMC represents the frequency modulation capability. The power margin of each MMC is as follows:

[0060] (18)

[0061] In the formula, Δ P MMC,margin,i For the first i The power margin of the MMC in each regional power grid; P MMC,max,i and P MMC,min,i The first i Maximum and minimum power values ​​of MMC for each regional power grid; P MMC,i For the first i The power of the MMC in each regional power grid;

[0062] The number of multiplexed submodules participating in frequency modulation is adjusted by the power margin ratio of the MMC:

[0063] (19)

[0064] In the formula, round is the floor function; N FV,i For the first i The number of multiplexing submodules involved when a regional power grid responds to DC voltage;

[0065] At this point, the equivalent capacitance of MMC can be described as:

[0066] Substituting equation (19) into equation (4), the equivalent capacitance of the MMC after changing the multiplexed submodule is:

[0067] (20)

[0068] At this time, the output power of the MMC on the undisturbed side is:

[0069] (twenty one)

[0070] In the formula, N0 is the initially set number of reused submodules.

[0071] Secondly, the present invention provides a power grid frequency regulation device based on MMC bridge arm capacitor reuse, comprising:

[0072] An improvement unit is used to reuse the bridge arm sub-modules of the MMC in the full-bridge structure to form a reused sub-module, and then connect the bridge arm sub-modules in parallel with a supercapacitor, thereby improving the bridge arm structure of the MMC; the grid frequency is transmitted to the MMC; the MMC is a modular multilevel converter;

[0073] The analysis and optimization unit is used to analyze the principle and method of MMC participating in grid frequency support by simulating the frequency characteristics of the synchronous machine, construct the grid frequency regulation model of MMC participating in the MTDC system, and optimize the grid frequency regulation method of MMC participating in the MTDC system; the MTDC system is a multi-terminal flexible DC transmission system.

[0074] The input unit is used to take the frequency deviation generated when the active power of the power grid and the active power of the load are unbalanced as the input of the power grid frequency regulation model;

[0075] The frequency modulation unit is used to adopt corresponding frequency modulation strategies based on the frequency modulation requirements of the MTDC system, according to the degree of disturbance to the power grid caused by the frequency deviation.

[0076] Compared with the prior art, the present invention has at least the following technical effects:

[0077] This invention provides a power grid frequency regulation method and apparatus based on MMC arm capacitor reuse, which is an MMC arm capacitor reuse and control technology for power grid frequency regulation. To address the challenges of new power systems with low inertia and poor frequency stability, the invention enables a multi-terminal flexible DC system to respond to frequency changes in the connected weak AC power grid by reusing MMC arm capacitors. To resolve the functional conflict between MMC arm capacitor reuse in power grid frequency control, voltage stabilization, and filtering, the structure of each sub-module of the MMC arm is modified to expand the capacitor power throughput. Furthermore, based on the frequency deviation of each AC power grid and actual frequency regulation requirements, a capacitor power extraction and multi-terminal collaborative control method are designed to achieve a reasonable allocation of power required for frequency regulation and improve the overall stability characteristics of the new power system. Attached Figure Description

[0078] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0079] In the attached diagram:

[0080] Figure 1 This is a schematic diagram of the MMC bridge arm capacitor reuse structure of the present invention;

[0081] Figure 2 This is a schematic diagram of the power grid frequency regulation model structure of the MMC-MTDC system of the present invention;

[0082] Figure 3 This is a flowchart of the power grid frequency regulation method based on MMC bridge arm capacitor reuse according to the present invention;

[0083] Figure 4 This is a voltage deviation comparison curve of an embodiment of the present invention when a disturbance occurs;

[0084] Figure 5a , Figure 5b , Figure 5c These are frequency deviation comparison curves for regions 1, 2, and 3 when a disturbance occurs, according to an embodiment of the present invention.

[0085] Figure 6a , Figure 6b , Figure 6c These are power deviation comparison curves for regions 1, 2, and 3 when a disturbance occurs, according to an embodiment of the present invention.

[0086] Figure 7a , Figure 7b , Figure 7c These are energy output comparison curves for regions 1, 2, and 3 when a disturbance occurs, according to an embodiment of the present invention.

[0087] Figure 8a , Figure 8b This is a graph comparing the number of MMC reused submodules in unaffected regions 2 and 3 of this invention.

[0088] Figure 9a , Figure 9b These are comparison curves of the MMC equivalent capacitance of regions 2 and 3 on the unaffected side in an embodiment of the present invention.

[0089] Figure 10 This is a comparison curve of the virtual inertia coefficient of the disturbed power grid during a disturbance, according to an embodiment of the present invention. Detailed Implementation

[0090] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0091] The following detailed description of some embodiments of the present invention will be provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0092] Please see Figure 3This invention provides a power grid frequency regulation method based on MMC bridge arm capacitor reuse. Addressing the functional conflict between MMC bridge arm capacitors participating in power grid frequency control and voltage regulation / filtering, the method modifies the MMC bridge arm submodules with full-bridge reuse. This is achieved by connecting supercapacitors in parallel to enhance the MMC power throughput, improve the bridge arm capacitors' ability to regulate voltage fluctuations, and simultaneously enhance their support for power grid frequency. The reuse of bridge arm capacitors reduces the conflict between MMC participation in frequency regulation and voltage regulation / filtering, including:

[0093] Step 1: Reuse the bridge arm sub-modules of the MMC in the full-bridge structure to form a reused sub-module, and then connect the bridge arm sub-module in parallel with a supercapacitor to improve the bridge arm structure of the MMC; transmit the grid frequency to the MMC;

[0094] Specifically, to address the issue of poor energy storage capacity of MMC bridge arm capacitors, the bridge arm structure of the MMC is improved. By using a full-bridge structure to reuse MMC bridge arm sub-modules, the impact of the switching of Insulated Gate Bipolar Transistors (IGBTs) on the capacitor's withstand voltage is avoided, thus enhancing the withstand voltage of the bridge arm capacitors. This is achieved by connecting a supercapacitor C in parallel. ultra This enhances the energy throughput capability of the submodules; a phase-locked loop (PLL) is used to transmit the grid frequency to the MMC, enabling the bridge arm capacitors to respond to changes in the grid frequency. Specifically, such as... Figure 1 As shown, to improve the energy throughput capability of the bridge arm capacitors, the traditional half-bridge submodule composed of filter capacitors in the MMC is improved to... Figure 1 The yellow box shows the energy storage full-bridge submodule capacitor composed of supercapacitors, which enables the bridge arm capacitors to be reused. The green box indicates the reuse improvement proposed to enhance the voltage withstand capability of the bridge arm capacitors, and the red box indicates the supercapacitor part added to enhance the energy storage capability of the submodule.

[0095] Step 2: By simulating the frequency characteristics of synchronous machines (synchronous generators, synchronous motors), and based on factors such as the number of energy storage submodules in the MMC bridge arm, the bridge arm capacitor capacity, and the voltage of the reused submodules, analyze the principle and method of MMC participating in grid frequency support, such as... Figure 2 As shown, a power grid frequency regulation model for MMC-MTDC system is constructed, and the power grid frequency regulation method for MMC-MTDC system is optimized.

[0096] Specifically, the inertial energy of the synchronous generator and the charging and discharging energy of the multiplexed submodule capacitor in the MMC can be expressed as:

[0097] (1)

[0098] In the formula, Δ E SM and Δ E MMC These are the changes in the capacitor of the multiplexed submodule and the changes in the output energy of the MMC, respectively. S SM and S MMC To reuse the capacity of submodules and MMC; C SM and C eq These are the submodule capacitance value and the MMC equivalent capacitance, respectively. U SM and U SM,ref These are the voltage value and rated voltage value of the multiplexed submodule, respectively; U dc and U dc,ref These are the real-time and reference values ​​of the DC voltage, respectively.

[0099] Assuming each arm of the MMC has N multiplexed submodules, and all are operational, then differentiating equation (1) yields the dynamic equation of the MMC when it outputs active power as follows:

[0100] (2)

[0101] In the formula, N The number of reused submodules in a single bridge arm; P AC and P DC These represent the AC-side power and DC-side power of the MMC, respectively.

[0102] Depend on Figure 1 As shown in the MMC structure diagram based on multiplexed submodules, there is a linear relationship between the voltage of the multiplexed submodules and the DC voltage in the MMC, as shown in equation (3):

[0103] (3)

[0104] Substituting equation (3) into equation (2), the equivalent capacitance of the MMC can be calculated as follows:

[0105] (4)

[0106] The active-load frequency equation for a synchronous generator is:

[0107] (5)

[0108] In the formula H The equivalent inertia constant of the synchronous generator;P M and P E These are the mechanical power and electromagnetic power output by the prime mover, respectively. f and f ref These are the grid frequency value and the frequency reference value, respectively. The method for calculating the inertial time constant of a simulated synchronous generator, and the virtual inertial coefficient of the MMC. H MMC It can be calculated as:

[0109] (6)

[0110] The design utilizes a phase-locked loop to transmit grid frequency changes to the MMC, leveraging the rapid charging and discharging capabilities of the MMC's capacitors to quickly respond to grid frequency changes. Combining equations (2) and (6), the relationship between the MMC and grid frequency changes can be obtained as follows:

[0111] (7)

[0112] Where, Δ P MMC Let be the change in MMC power. Integrating equation (7), we obtain equation (8):

[0113] (8)

[0114] Will U dc =Δ U dc + U dc,ref Substituting into equation (8), we can simplify to equation (9).

[0115] (9)

[0116] In the formula, Δ U dc Δ U SM and Δ f These represent the DC voltage change, the multiplexing submodule voltage change, and the grid frequency deviation, respectively. Since the DC voltage change is small, its quadratic term can be ignored; therefore, equation (9) can be rewritten as:

[0117] (10)

[0118] Therefore, the relationship between the grid frequency deviation and the voltage change of the multiplexing submodule and the DC voltage change can be expressed as:

[0119] (11)

[0120] Specifically, considering the coupling relationship between the capacitor voltage and DC line voltage of the MMC multiplexing submodule, as well as the conflict between participating in grid frequency regulation and stabilizing DC voltage and filtering functions, the control method of the MMC needs to be optimized, that is, the grid frequency regulation method of the MMC participating in the MTDC system needs to be optimized:

[0121] When the system is disturbed, the MMC will first determine the frequency deviation of the power grid and take targeted control measures according to the relevant requirements of power grid frequency regulation. If the frequency deviation is within the dead zone, frequency regulation will not be initiated. If it exceeds the dead zone but is less than 0.2Hz, the MMC in this area will participate in frequency regulation. If it is greater than 0.2Hz, the MMCs in other areas will be activated to participate in frequency regulation in coordination.

[0122] As shown in equation (9), the output power of the MMC at this time is affected not only by the frequency deviation but also by the change in DC voltage. The frequency support capability of the MMC is mainly related to the virtual inertia coefficient of the MMC, while the DC voltage response capability is mainly affected by the number of multiplexed submodules. As shown in equation (14):

[0123] (14)

[0124] In the formula, the subscript i Indicates the first i Regional power grid; Δ W MMC,f,i Indicates the first i The energy change in the frequency of the MMC response of a regional power grid; Δ W MMC,v,i Indicates the first i The energy change in the MMC response voltage of a regional power grid; Δ f i For the first i Regional power grid frequency deviation; H MMC,i For the first i The virtual inertia coefficient of the MMC of a regional power grid.

[0125] To improve the frequency response of the MMC while avoiding DC voltage overshooting, a criterion for judging grid frequency-voltage changes is defined. J i As shown in equation (15):

[0126] (15)

[0127] when J i A value greater than 1 indicates that the frequency deviation is greater than the voltage deviation, therefore the frequency modulation requirement is greater than the voltage stability requirement; when J iA value less than 1 indicates a large voltage deviation caused by frequency modulation, and the investment in frequency modulation should be reduced. If this criterion is applied to the design of the virtual inertia coefficient of the MMC as shown in equation (16), the magnitude of the virtual inertia coefficient can be adjusted to simultaneously achieve the dual goals of reducing frequency deviation and DC voltage deviation, resulting in the new virtual inertia coefficient of the MMC:

[0128] (16)

[0129] To prevent the MMC from frequently changing its operating state, a threshold should be set for the MMC to participate in DC voltage deviation. According to power grid regulations, the MMC can only operate briefly when the power grid frequency deviation is greater than 0.2Hz, and cannot operate in this state for a long period of time. Therefore, a second threshold is set when the power grid frequency deviation reaches a certain level. η When 2=0.2Hz, the frequency support of other regional power grids in the MMC-MTDC system for the disturbed power grid is initiated. The corresponding DC voltage change at this time is:

[0130] (17)

[0131] When responding to changes in DC voltage, the frequency modulation power undertaken by each MMC should be allocated according to its own frequency modulation capability. If the frequency modulation capability is strong, it undertakes more frequency modulation power, and vice versa if the frequency modulation capability is weak. The power margin of each MMC can be used to represent its frequency modulation capability. The power margin of each MMC can be expressed as:

[0132] (18)

[0133] In the formula, Δ P MMC,margin,i For the first i The power margin of the MMC in each regional power grid; P MMC,max,i and P MMC,min,i The first i Maximum and minimum power values ​​of MMC for each regional power grid; P MMC,i For the first i The power of the MMC in the regional power grid.

[0134] Considering the correlation between frequency stability and frequency modulation capability in each region, the number of multiplexed submodules participating in frequency modulation for each phase's individual bridge arm is designed so that the number of multiplexed submodules participating in frequency modulation can be adjusted by the MMC power margin ratio, as shown in equation (19):

[0135] (19)

[0136] In the formula, round is the floor function; NFV,i For the first i The number of multiplexing submodules participating when a regional power grid responds to DC voltage, and the equivalent capacitance of the MMC can be described as:

[0137] Substituting equation (19) into equation (4), we can solve for the equivalent capacitance of the MMC after the submodule is changed:

[0138] (20)

[0139] At this time, the output energy of the unaffected MMC can be expressed as:

[0140] (twenty one)

[0141] In the formula, N0 is the initially set number of reused submodules.

[0142] With the above design, the MMC can increase the equivalent capacitance and handle more frequency modulation power by reducing the number of multiplexed submodules when the frequency modulation margin is large compared to other areas. Conversely, it can reduce the power distribution by increasing the number of multiplexed submodules, making the power distribution in the frequency modulation process more reasonable.

[0143] To avoid the impact of dynamic changes in frequency modulation parameters on voltage, the affected MMC only adjusts the virtual inertia coefficient, and all bridge arm capacitors are engaged in frequency modulation; the unaffected MMC only adjusts the number of multiplexed submodules, and the virtual inertia coefficient remains unchanged.

[0144] Step 3: Calculate the frequency deviation Δ caused by the imbalance between the active power of the power grid and the active power of the load. f As input to the power grid frequency regulation model.

[0145] The power of a thermal power unit mainly consists of a governor and a reheat turbine, which can be expressed as equation (12):

[0146] (12)

[0147] In the formula, Δ P SG This represents the power change of the synchronous motor; R This is the adjustment coefficient; T g , T t , T r These are the time constant of the governor, the reheat time constant, and the gas capacity time constant of the thermal power unit, respectively. K r is the reheat coefficient of the thermal power unit; s is the Laplace operator.

[0148] exist Figure 2The expression for the frequency deviation of the power grid in each region is as follows:

[0149] (13)

[0150] in, D Δ is the load damping coefficient. P L This represents the load disturbance power value.

[0151] Through the above design, the MMC-MTDC system can provide active power support to the disturbed power grid based on the frequency deviation of the regional power grid, so as to mitigate the impact of power grid disturbance.

[0152] Step 4: Based on the degree of disturbance to the power grid caused by the frequency deviation, adopt the corresponding frequency regulation strategy according to the frequency regulation requirements of the MTDC system;

[0153] Specifically, if the absolute value of the frequency deviation is less than the first threshold, the grid frequency regulation dead zone is set to the first threshold, and grid frequency regulation is not performed.

[0154] If the absolute value of the frequency deviation is greater than or equal to the first threshold and less than the second threshold, the affected side MMC uses virtual inertial control to assist the thermal power unit in responding to the frequency change, and converts the frequency change signal into DC voltage fluctuation and multiplexing submodule voltage fluctuation through the droop control loop.

[0155] If the absolute value of the frequency deviation is greater than or equal to the second threshold and less than the third threshold, the unaffected side MMC provides active power support to the affected power grid, and adjusts the number of multiplexing sub-modules for power grid frequency regulation according to the power margin of the unaffected side MMC and the relative degree of frequency deviation, so as to achieve reasonable allocation of frequency regulation power.

[0156] In some embodiments, the first threshold is 0.033 Hz, the second threshold is 0.2 Hz, and the third threshold is 0.5 Hz. Step 4 may further include:

[0157] Based on the magnitude of the frequency deviation, the degree of disturbance to the power grid is determined, and different frequency regulation strategies are adopted according to the frequency regulation requirements of the MTDC system:

[0158] If |Δ f If the frequency deviation is less than 0.033 Hz, the grid frequency regulation dead zone is set to 0.033 Hz. When the frequency deviation is less than 0.033 Hz, all generating units will not start frequency regulation.

[0159] If 0.2Hz > |Δ f|≥0.033Hz, when the frequency fluctuation is less than 0.2Hz and does not reach the boundary of the normal operating frequency deviation of the power grid, the frequency regulation resources of the local power grid are used to support the disturbed power grid, and the power grids of other regions do not participate in the frequency regulation of the disturbed power grid. In this scenario, the MMC on the disturbed side uses virtual inertial control to assist the thermal power unit in responding to the frequency change. In order to avoid the impact of frequency regulation on DC voltage, the DC voltage change value is introduced to participate in the setting of the virtual inertial coefficient. Since the DC voltage fluctuation does not reach the frequency regulation start threshold of the MMC on other sides, the frequency regulation resources of other regions do not respond to the frequency regulation demand of the disturbed side.

[0160] If 0.5Hz > |Δ f |≥0.2Hz, according to national standards, when |Δ f At 0.2Hz, the power grid can operate briefly but not continuously, triggering the coordinated frequency regulation control of the MMC-MTDC system. At this time, the unaffected MMC senses the distress signal from the affected side through the DC voltage deviation and responds to the DC voltage change by extracting energy from the bridge arm capacitors, providing active power support to the affected power grid. Furthermore, it adjusts the number of multiplexed submodules engaged in frequency regulation based on the power margin and relative degree of frequency deviation of each unaffected MMC, thereby achieving a reasonable allocation of frequency regulation power.

[0161] Based on the same inventive concept, another embodiment of the present invention provides a power grid frequency regulation device based on MMC bridge arm capacitor reuse. This device corresponds to the method of the aforementioned embodiment and includes:

[0162] The improved unit is used to reuse the bridge arm sub-modules of the MMC in the full-bridge structure to form a reused sub-module, and then connect the bridge arm sub-modules in parallel with supercapacitors, thereby improving the bridge arm structure of the MMC; it transmits the grid frequency to the MMC; the MMC is a modular multilevel converter;

[0163] The analysis and optimization unit is used to analyze the principles and methods of MMC participating in grid frequency support by simulating the frequency characteristics of the synchronous machine, construct the grid frequency regulation model of MMC participating in the MTDC system, and optimize the grid frequency regulation method of MMC participating in the MTDC system; the MTDC system is a multi-terminal flexible DC transmission system.

[0164] The input unit is used to take the frequency deviation caused by the imbalance between the active power of the power grid and the active power of the load as the input of the power grid frequency regulation model;

[0165] The frequency modulation unit is used to adopt corresponding frequency modulation strategies based on the frequency modulation requirements of the MTDC system, according to the degree of disturbance to the power grid caused by the frequency deviation.

[0166] The following is a specific embodiment of the present invention.

[0167] Frequency regulation design is carried out based on the power grid frequency regulation model of an MMC-MTDC system with a rated voltage of 500kV. Figure 2 The control simulation scenario with a 0.1 pu step disturbance applied to region 1 is used to compare and analyze the method proposed in the aforementioned embodiments. Table 1 shows the parameters appearing in this embodiment, and Table 2 shows the system variable parameter table for this embodiment.

[0168] Table 1. Parameter definitions in this embodiment

[0169]

[0170] Table 2 System Variable Parameter Table

[0171]

[0172] in, Figure 4 This is a voltage deviation comparison curve of an embodiment of the present invention when a disturbance occurs. Curve 1 represents the method of the present invention, and curve 2 represents the traditional unit frequency regulation method. Figure 5a , Figure 5b , Figure 5c The graphs shown are frequency deviation comparison curves for regions 1, 2, and 3 when disturbances occur, according to embodiments of the present invention. Curve 1 represents the method of the present invention, and curve 2 represents the traditional unit frequency regulation method. Figure 6a , Figure 6b , Figure 6c The figures are power deviation comparison curves for regions 1, 2, and 3 when disturbances occur, according to embodiments of the present invention. Curve 1 represents the method of the present invention, and curve 2 represents the traditional unit frequency regulation method. Figure 7a , Figure 7b , Figure 7c The graphs shown are energy output comparison curves for regions 1, 2, and 3 when disturbances occur, according to embodiments of the present invention. Curve 1 represents the method of the present invention, and curve 2 represents the traditional unit frequency regulation method. Figure 8a , Figure 8b This is a graph comparing the number of MMC multiplexing submodules in unaffected regions 2 and 3 of this invention. Curve 1 represents the method of this invention, and curve 2 represents the traditional unit frequency regulation method. Figure 9a , Figure 9b The graphs show the MMC equivalent capacitance comparison curves for regions 2 and 3 on the unaffected side of the present invention, respectively. Curve 1 represents the method of the present invention, and curve 2 represents the traditional unit frequency regulation method. Figure 10 This is a comparison curve of the virtual inertia coefficient of the disturbed power grid during a disturbance according to an embodiment of the present invention. Curve 1 represents the method of the present invention, and curve 2 represents the traditional unit frequency regulation method.

[0173] from Figures 4-10It can be seen that the method of this invention can effectively enhance the energy throughput capacity of the bridge arm capacitors by reusing the MMC. Compared with the control method with fixed parameters, the method of this invention can adjust the MMC output according to the actual frequency regulation requirements of the MMC-MTDC system, reducing the change of DC voltage without affecting frequency regulation, and effectively solving the functional conflict between frequency regulation, voltage regulation, and filtering caused by reusing the bridge arm capacitors. From an economic perspective, compared with relying solely on traditional power generation, the coordinated control of multi-terminal frequency regulation resources can reduce the coal consumption increased by power generation and reflect the economic benefits of frequency regulation. In the future, more economical and efficient optimized operation strategies need to be developed based on regional real-time electricity prices under the spot market environment to fully realize its huge economic potential.

[0174] This invention can determine the actual frequency and voltage regulation needs based on the frequency deviation and DC voltage changes of the disturbed power grid, reduce the impact on DC voltage deviation during auxiliary frequency regulation, and adjust the input of multiplexing submodules by determining the frequency regulation power margin of the MMC on the undisturbed side, making the allocation of frequency regulation power more reasonable and more suitable for today's new power systems with low inertia and poor frequency stability.

[0175] The MMC arm capacitor reuse technology proposed in this invention can effectively reduce frequency changes caused by system disturbances and quickly enter a steady state, resulting in better control performance. This method mainly includes: modifying the arm capacitor structure of the MMC for reuse; designing a frequency support method for the MMC based on grid frequency changes and the frequency regulation requirements of multi-terminal flexible DC systems; and effectively enhancing the safe operation capability of the MMC-MTDC system when participating in frequency regulation through the coordinated cooperation between the MMC and thermal power units.

[0176] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the embodiments disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. It should be understood that the invention is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A power grid frequency regulation method based on MMC bridge arm capacitor reuse, characterized in that, include: The bridge arm sub-modules of the MMC are reused in the full-bridge structure to form a reused sub-module. Then, a supercapacitor is connected in parallel to the bridge arm sub-module, thereby improving the bridge arm structure of the MMC; the grid frequency is transmitted to the MMC; the MMC is a modular multilevel converter. By simulating the frequency characteristics of a synchronous machine, the principle and method of MMC participating in grid frequency support are analyzed. A grid frequency regulation model of MMC participating in the MTDC system is constructed, and the grid frequency regulation method of MMC participating in the MTDC system is optimized. The MTDC system is a multi-terminal flexible DC transmission system. The frequency deviation generated when the active power of the power grid and the active power of the load are unbalanced is used as the input of the power grid frequency regulation model; Based on the degree of disturbance to the power grid caused by the frequency deviation, a corresponding frequency regulation strategy is adopted according to the frequency regulation requirements of the MTDC system. The step of adopting a corresponding frequency regulation strategy based on the frequency deviation's impact on the power grid and the frequency regulation requirements of the MTDC system includes: If the absolute value of the frequency deviation is less than the first threshold, the power grid frequency regulation dead zone is set to the first threshold, and no power grid frequency regulation is performed. The step of adopting a corresponding frequency regulation strategy based on the frequency deviation's impact on the power grid and the frequency regulation requirements of the MTDC system includes: If the absolute value of the frequency deviation is greater than or equal to the first threshold and less than the second threshold, the affected side MMC uses virtual inertial control to assist the thermal power unit in responding to the frequency change, and converts the frequency change signal into DC voltage fluctuation and multiplexing submodule voltage fluctuation through the droop control loop. The step of adopting a corresponding frequency regulation strategy based on the frequency deviation's impact on the power grid and the frequency regulation requirements of the MTDC system includes: If the absolute value of the frequency deviation is greater than or equal to the second threshold and less than the third threshold, the unaffected side MMC provides active power support to the affected power grid, and adjusts the number of multiplexing sub-modules for power grid frequency regulation according to the power margin of the unaffected side MMC and the relative degree of frequency deviation, so as to achieve reasonable allocation of frequency regulation power. The number of multiplexed submodules participating in frequency modulation is adjusted by the power margin ratio of the MMC: (19) In the formula, round is the floor function; N FV,i For the first i The number of multiplexing submodules participating when the regional power grid responds to DC voltage; N0 is the initially set number of multiplexing submodules; Δ P MMC,margin,i For the first i The power margin of the MMC in each regional power grid; Δ f i For the first i Regional power grid frequency deviation.

2. The power grid frequency regulation method based on MMC bridge arm capacitor reuse according to claim 1, characterized in that, The first threshold is 0.033Hz, the second threshold is 0.2Hz, and the third threshold is 0.5Hz.

3. The power grid frequency regulation method based on MMC bridge arm capacitor reuse according to claim 1, characterized in that, Based on the number of energy storage submodules in the MMC bridge arm, the bridge arm capacitor capacity, and the voltage of the reused submodules, this paper analyzes the principle and method of MMC's participation in grid frequency support.

4. The power grid frequency regulation method based on MMC bridge arm capacitor reuse according to claim 3, characterized in that, The principle and method of MMC's participation in grid frequency support, analyzed by simulating the frequency characteristics of a synchronous machine, include: The changes in the capacitance of the multiplexed submodule and the changes in the output energy of the MMC are expressed as follows: (1) In the formula, Δ E SM and Δ E MMC These are the changes in the capacitor of the multiplexed submodule and the changes in the output energy of the MMC, respectively. S SM and S MMC These are the capacities of the reused submodule and the MMC, respectively. C SM and C eq These are the reused submodule capacitor and the MMC equivalent capacitor, respectively. U SM and U SM,ref These are the voltage value and rated voltage value of the multiplexed submodule, respectively; U dc and U dc,ref These are the real-time and reference values ​​of the DC voltage, respectively. Assuming each arm of the MMC has N multiplexed submodules, and all are in operation, differentiating equation (1) yields the dynamic equation of the MMC when it outputs active power as follows: (2) In the formula, N The number of reused submodules in a single bridge arm; P AC and P DC These represent the AC-side power and DC-side power of the MMC, respectively. The relationship between the voltage of the multiplexed submodule and the DC voltage is as follows: (3) Substituting equation (3) into equation (2), the equivalent capacitance of the MMC is obtained as follows: (4) The active-load frequency equation for a synchronous machine is: (5) In the formula, H The equivalent inertia constant of the synchronizing machine; P M and P E These are the mechanical power and electromagnetic power output by the prime mover, respectively. f and f ref These are the grid frequency value and the frequency reference value, respectively; the inertial time constant of the simulated synchronous machine is calculated to obtain the virtual inertial coefficient of the MMC. H MMC The formula for calculation is: (6) Using a phase-locked loop to transmit the frequency change of the power grid to the MMC, and combining equations (2) and (6), the relationship between the MMC and the power grid frequency change is obtained as follows: (7) Where, Δ P MMC Let MMC power change be the variable; integrating equation (7), we get: (8) Will U dc =Δ U dc + U dc,ref Substituting into equation (8) and simplifying, we get: (9) In the formula, Δ U dc Δ U SM and Δ f Let these represent the DC voltage change, the multiplexing submodule voltage change, and the grid frequency deviation, respectively. Since the DC voltage change is relatively small, its quadratic term can be ignored, resulting in: (10) Therefore, the relationship between the grid frequency deviation and the voltage change of the multiplexing submodule and the DC voltage change is as follows: (11)。 5. The power grid frequency regulation method based on MMC bridge arm capacitor reuse according to claim 4, characterized in that, In the power grid frequency regulation model, the power of the thermal power unit includes the governor and the reheat turbine, as shown below: (12) In the formula, Δ P SG This represents the power change of the synchronous machine; R This is the adjustment coefficient; T g , T t , T r These are the time constant of the governor, the reheat time constant, and the gas capacity time constant of the thermal power unit, respectively. K r is the reheat coefficient of the thermal power unit; s is the Laplace operator; Therefore, the frequency deviation of the power grid in each region is: (13) In the formula, D Δ is the load damping coefficient. P L This represents the load disturbance power value.

6. The power grid frequency regulation method based on MMC bridge arm capacitor reuse according to claim 5, characterized in that, The optimization of the grid frequency regulation method for MMC participation in MTDC systems includes: The energy change in MMC response to frequency and voltage changes is: (14) In the formula, the subscript i Indicates the first i Regional power grid; Δ W MMC,f,i Indicates the first i The energy change in the frequency of the MMC response of a regional power grid; Δ W MMC,v,i Indicates the first i The energy change in the MMC response voltage of a regional power grid; Δ f i For the first i Regional power grid frequency deviation; H MMC,i For the first i The virtual inertia coefficient of the MMC of a regional power grid; Define a criterion for judging the frequency-voltage variation of the power grid. J i for: (15) Judgment Indicators J i When applied to the design of the virtual inertia coefficient of MMC, the new virtual inertia coefficient of MMC is obtained as follows: (16) Set the power grid frequency deviation to reach the second threshold. η 2. When the MMC is activated to participate in the frequency support of other regional power grids to the disturbed power grid in the MTDC system, the corresponding DC voltage change is: (17) The power margin of each MMC represents the frequency modulation capability. The power margin of each MMC is as follows: (18) In the formula, Δ P MMC,margin,i For the first i The power margin of the MMC in each regional power grid; P MMC,max,i and P MMC,min,i The first i Maximum and minimum power values ​​of MMC for each regional power grid; P MMC,i For the first i The power of the MMC in each regional power grid; At this point, the equivalent capacitance of MMC can be described as: Substituting equation (19) into equation (4), the equivalent capacitance of the MMC after changing the multiplexed submodule is: (20) At this time, the output power of the MMC on the undisturbed side is: (21) In the formula, N0 is the initially set number of reused submodules.

7. A power grid frequency regulation device based on MMC bridge arm capacitor reuse, characterized in that, The apparatus for implementing the power grid frequency regulation method based on MMC bridge arm capacitor reuse as described in claim 1 includes: An improvement unit is used to reuse the bridge arm sub-modules of the MMC in the full-bridge structure to form a reused sub-module, and then connect the bridge arm sub-modules in parallel with a supercapacitor, thereby improving the bridge arm structure of the MMC; the grid frequency is transmitted to the MMC; the MMC is a modular multilevel converter; The analysis and optimization unit is used to analyze the principle and method of MMC participating in grid frequency support by simulating the frequency characteristics of the synchronous machine, construct the grid frequency regulation model of MMC participating in the MTDC system, and optimize the grid frequency regulation method of MMC participating in the MTDC system; the MTDC system is a multi-terminal flexible DC transmission system. The input unit is used to take the frequency deviation generated when the active power of the power grid and the active power of the load are unbalanced as the input of the power grid frequency regulation model; The frequency modulation unit is used to adopt corresponding frequency modulation strategies based on the frequency modulation requirements of the MTDC system, according to the degree of disturbance to the power grid caused by the frequency deviation.

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