MMC reactive power supporting capability improving method considering dynamic boundary under power grid fault

Abstract

The invention relates to an MMC reactive power support capability improvement method considering a dynamic boundary under a power grid fault, and belongs to the field of flexible DC power transmission control, and the method comprises the steps: constructing an MMC dynamic boundary calculation unit under the power grid fault; constructing a fitting function of a zero-sequence signal calculation unit for improving the reactive support capability of the MMC; whether the zero-sequence signal calculation unit is started or not is judged on the basis of reactive power Q0. Max and Q1. Max which are output by the constructed dynamic boundary calculation unit and have no zero-sequence signal injection and reactive power Qdet detected in real time in the system; and if the zero-sequence signal calculation unit is started, calculating a zero-sequence signal of an expanded operation domain of the MMC under the current working condition in real time, and injecting the zero-sequence signal into the MMC modulation module, thereby realizing improvement of the reactive support capability of the MMC. According to the method, the reactive power supporting capability of the converter can be improved, the risks of power grid collapse and system oscillation are reduced, the dynamic boundary of the converter under the power grid fault is considered, and the method has flexibility, real-time performance and economical efficiency.

Description

Technical Field

[0001] The invention relates to a method for improving the reactive support capacity of an MMC taking into account a dynamic boundary under a power grid fault, and belongs to the technical field of flexible direct current transmission control. Background Art

[0002] As one of the core devices of the flexible DC transmission system, the Modular Multilevel Converter (MMC) has the advantages of excellent output waveform quality, high modularity and strong scalability, high reliability and flexible control. It is widely used in large-capacity long-distance power transmission, offshore wind power grid connection and power system regulation and optimization. Compared with traditional two-level or three-level converters, MMC has significant differences in topology and control methods, making the system operation more reliable and flexible. However, when a fault occurs on the grid side and causes a voltage drop, the grid-type MMC needs to increase reactive power in a short time to support the grid side voltage. If the MMC lacks sufficient reactive support capacity, the grid voltage will drop severely, which may cause voltage collapse. At the same time, the converter itself may be overloaded or shut down due to the inability to provide reactive support, affecting the energy transmission of the grid and causing large-scale power outages.

[0003] However, the traditional methods of improving the reactive power support capacity of converter stations mainly include increasing the capacity of converters, adding reactive power compensation devices such as static var compensators (SVCs), adjusting control strategies, allowing short-term overload operation, and adding filters or energy storage systems. However, these methods generally have the disadvantages of high cost, increased equipment complexity and maintenance difficulty, compensation capacity limited by rated capacity, difficulty in long-term full-load operation, and insufficient dynamic adaptability, making it difficult to balance economy, reliability, and flexibility. Therefore, it is of great significance to develop a method to economically and flexibly improve the reactive power support capacity of MMCs under power grid faults. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention provides a method for improving the reactive support capacity of MMC taking into account the dynamic boundaries under power grid faults, which can expand the operating domain of MMC, improve the reactive support capacity of the converter, reduce the risk of power grid collapse and system oscillation, and overcome the defects of traditional methods such as high cost, equipment complexity, maintenance difficulty and insufficient dynamic adaptability. It takes into account the dynamic boundaries of the converter under power grid faults and has flexibility, real-time and economy.

[0005] The present invention adopts the following technical solution: A method for improving the reactive power support capability of MMC taking into account the dynamic boundary under power grid faults comprises the following steps: S1: Constructing the MMC dynamic boundary computing unit under power grid fault; S2: Constructing the fitting function of the zero-sequence signal calculation unit to improve the reactive power support capability of MMC; S3: Reactive power with and without zero-sequence signal injection output based on the constructed dynamic boundary calculation unit , The reactive power detected in real time in the system Q det Determine whether to start the zero-sequence signal calculation unit; S4: If the zero-sequence signal calculation unit is started, the zero-sequence signal of the expanded operating domain under the current working condition of the MMC is calculated in real time and injected into the MMC modulation module to improve the reactive power support capacity of the MMC.

[0006] Preferably, the implementation process of step S1 is: S11: Considering the grid voltage drop under grid fault, the minimum allowable voltage of the PCC point is Us.min =0.7* Us.rated , the maximum allowable voltage is Us.max =1.0* Us.rated ,in Us.rated The AC side voltage is divided by the rated voltage of the PPC point U s Calculate the point and calculate the AC side voltage U s ; S12: Calculate the voltages on each AC side when no zero-sequence signal is injected Us i Command modulation signal margin S margin The maximum active power output of MMC when it is 0 P 0 , Maximum reactive power Q 0 , modulation signal margin when there is no zero-sequence signal injection S margin Considered as Us i , P 0 , Q 0 function, each U si and U si The corresponding calculated P 0 , Q 0 Forming a Dataset G no.A3 ; Then calculate the voltage on each AC side when the zero-sequence signal is injected Us i Command modulation signal margin S margin The maximum active power output of MMC when it is 0 P 1 , Maximum reactive power Q 1 , modulation signal margin when zero sequence signal is injected S margin Considered as Us i , P 1 , Q 1 , A 3 , α 3 function, each U si and U si The corresponding calculated P 1 , Q 1 , A 3 , α 3 Forming a Dataset G A3 ,in, A 3 is the amplitude of the third harmonic component, α 3 is the phase angle of the third harmonic component.

[0007] Preferably, the implementation process of step S2 is: S21: The data set obtained in step S12 G no.A3 AC side voltage U s , Maximum active power P 0 As an independent variable, the maximum reactive power Q 0 As the dependent variable, the fitting function A is obtained as follows: ; in, Indicates a maximum reactive power when no zero-sequence signal is injected. Q 0 Can be expressed A series of numbers composed of The data set obtained in step S12 GA3 The AC side voltage U s , Maximum active power P 1 As an independent variable, the maximum reactive power Q 1 As the dependent variable, the fitting function B is obtained as follows: ; in, Indicates a maximum reactive power when a zero-sequence signal is injected, Q 1 Can be expressed A series of numbers composed of Input any AC voltage U s and active power P , we can get the maximum reactive power that the MMC can output when there is no zero-sequence signal injection The maximum reactive power that the MMC can output when the zero-sequence signal is injected ; S22: Divide the AC side voltage U s , Active Power P , reactive power Q The calculation point of each U s , P , Q It constitutes an operating point of MMC operation. Each operating point corresponds to the amplitude of the third harmonic component of a zero-sequence signal. A 3 and phase angle α 3 Maximize the modulation signal margin; Calculate the given U s , P , Q The modulation signal margin is maximized under A 3 and α 3 , and store the data into a dataset G A3α3 ; S23: The data set obtained in step S22 G A3α3 middle U s , P , Q As an independent variable, A 3 and α 3They are used as dependent variables for function fitting, and fitting function C and fitting function D are obtained: Fitting function C: ; Fitting function D: ; in, A 3.Cal and phase angle α 3.Cal are the amplitude and phase angle of the third harmonic component that maximizes the modulation signal margin after fitting; Input any AC voltage U s , Active Power P , reactive power Q The amplitude of the third harmonic component of the zero-sequence signal that can maximize the modulation signal margin can be obtained by MMC at this time. A 3.Cal and phase angle α 3.Cal .

[0008] Preferably, the implementation process of step S3 is: S31: Real-time detection of the AC side voltage in the system U s.det ,MMC output active power P det and reactive power Q det ,Will U s.det , P det Substitute into fitting function A and fitting function B respectively to obtain the maximum reactive power that the MMC can output when there is no zero-sequence signal injection The maximum reactive power that the MMC can output when the zero-sequence signal is injected ; The real-time detection U s.det , P det and Q det Substitute the fitting functions C and D respectively to obtain the amplitude of the third harmonic component of the zero-sequence signal injected into the MMC at this time. A 3.Cal and phase angle α 3.Cal ; S32: Comparison of reactive power Q det , calculated in step S6 and The size of Q det≤ hour, A 3 =0, α 3 =0, no alarm; when <Q det < hour, A 3 = A 3.Cal , α 3 = α 3.Cal , do not call the police; Q det ≥ hour, A 3 = A 3.Cal , α 3 = α 3.Cal , alarm, prompting that the current MMC operating conditions have exceeded the limit conditions; When there is no alarm, the zero sequence signal calculation unit is started and the A 3 and α 3 Inject the MMC modulation module to improve the MMC reactive power support capacity without over-modulation under linear control.

[0009] Preferably, in step S11, ; in, U si Indicates the calculation range of the minimum and maximum allowable voltages at the PPC point. i The AC side voltage of the calculation point is n To calculate the interval.

[0010] Preferably, in step S12, the modulation signal margin is S margin Considered as Us i , P 0 , Q 0 The function is: ; Modulation signal margin when zero sequence signal is injected S margin Considered as Us i , P1 , Q 1 , A 3 , α 3 The function is: ; Taking the upper bridge arm of phase A as an example, the modulation signal margin S margin The calculation formula is as follows: ; ; in, S ap ( t ) is the modulation signal of the upper bridge arm of phase A, A 0 , A 1 , A 2 and A 3 are the amplitudes of DC, 1st, 2nd and 3rd harmonic components respectively, A 3 Also called zero-sequence signal amplitude; α 1 , α 2 , α 3 Indicates the phase angle of the 1st, 2nd and 3rd harmonic components; ω is the fundamental angular frequency.

[0011] Preferably, in step S22, the AC side voltage is divided into U s , Active Power P , reactive power Q The calculation point, where the active power P , reactive power Q The calculation point division is adjusted according to the MMC operation parameters, and its expression is as follows: ; in, i Indicates i calculation points, j Indicates the maximum number of active power calculations. k Indicates the maximum number of reactive power calculations; P max , Q max It is a dataset G no.A3 and GA3 The maximum active power and reactive power recorded in are expressed as follows: ; Indicates that the data set is obtained without zero-sequence signal injection G no.A3 All active power data in Indicates that the data set is obtained without zero-sequence signal injection G no.A3 All reactive power data in Indicates the data set obtained under zero-sequence signal injection G A3 All active power data in Indicates the data set obtained under zero-sequence signal injection G A3 All reactive power data in.

[0012] For any details not provided in the present invention, please refer to the prior art.

[0013] The beneficial effects of the present invention are: The present invention proposes a method for improving the reactive support capacity of MMC taking into account the dynamic boundaries under power grid faults, which can expand the operating domain of MMC, improve the reactive support capacity of the converter, and reduce the risk of power grid collapse and system oscillation. The method proposed in the present invention overcomes the defects of traditional methods such as high cost, large equipment complexity and maintenance difficulty, and insufficient dynamic adaptability. It takes into account the dynamic boundaries of the converter under power grid faults and has flexibility, real-time and economy. The present invention can realize adjustment based on the real-time working conditions of the converter, improve the reactive support capacity of MMC, ensure effective voltage stabilization and improve system operation performance when there is a grid-side fault or voltage fluctuation, and provide solid technical support for the safe and reliable operation of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The drawings in the specification, which constitute a part of the present application, are used to provide further understanding of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute improper limitations on the present application.

[0015] Figure 1 The overall schematic diagram of the method for improving the reactive power support capability of MMC considering the dynamic boundary under power grid faults; Figure 2 A flow chart is constructed for the fitting function of the zero-sequence signal calculation unit to improve the reactive power support capability of MMC; Figure 3 It is the MMC dynamic boundary data and fitting diagram; Figure 4 This is a comparison chart of the reactive power output of the grid-type MMC and the maximum output reactive power when the voltage drops; Figure 5 It is a schematic diagram of the output zero-sequence signal amplitude; Figure 6 It is a schematic diagram of the phase angle of the output zero-sequence signal; Figure 7 This is a modulation signal diagram of the upper bridge arm of phase A after adopting the method proposed by the present invention; Figure 8 This is a modulation signal diagram of the upper bridge arm of phase A without adopting the method proposed by the present invention. DETAILED DESCRIPTION

[0016] In order to enable those skilled in the art to better understand the technical solutions in this specification, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the drawings in the implementation of this specification, but are not limited to this. Anything not fully described in the present invention shall be based on the conventional technology in the art.

[0017] Example 1 A method for improving the reactive power support capability of MMC considering the dynamic boundary under power grid faults, such as Figure 1 As shown, Figure 1 As shown, the following steps are included: S1: Constructing the MMC dynamic boundary computing unit under power grid fault; S2: Constructing the fitting function of the zero-sequence signal calculation unit to improve the reactive power support capability of MMC; S3: Reactive power with and without zero-sequence signal injection output based on the constructed dynamic boundary calculation unit , The reactive power detected in real time in the system Q det Determine whether to start the zero-sequence signal calculation unit; S4: If the zero-sequence signal calculation unit is started, the zero-sequence signal of the expanded operating domain under the current working condition of the MMC is calculated in real time and injected into the MMC modulation module to improve the reactive power support capacity of the MMC.

[0018] Example 2 A method for improving the reactive power support capability of MMC taking into account the dynamic boundary under power grid faults, as shown in Example 1, except that Figure 2 As shown, the implementation process of step S1 is: S11: Considering the grid voltage drop under grid fault, the minimum allowable voltage of the PCC point is Us.min =0.7* Us.rated , the maximum allowable voltage is Us.max =1.0* Us.rated ,in Us.rated The AC side voltage is divided by the rated voltage of the PPC point Us Calculate the point and calculate the AC side voltage U s , as follows: ; in, U si Indicates the calculation range of the minimum and maximum allowable voltages at the PPC point. i The AC side voltage of the calculation point is n For the calculation interval, it can be determined by the required accuracy of the calculation. If n =10, then U si for U s0 ~U s10 There are 11 in total. Us.min arrive Us.max Divided into 11 calculation points.

[0019] S12: Calculate the voltages on each AC side when no zero-sequence signal is injected Us i Command modulation signal margin S margin The maximum active power output of MMC when it is 0 P 0 , Maximum reactive power Q 0 In the construction of the dynamic boundary calculation unit of the present invention, the modulation signal margin is S margin Considered as Us i , P 0 , Q 0 Function: ; In this function, P 0 , Q 0 are all unknowns. MMC is modeled. Us i When determined, the current modulation signal margin can be calculated through the established model S margin 0 P 0 , Q 0 , each Us i The following calculations yield multiple P 0 , Q0 ,For example Us 0 、Us 1 Each corresponds to 20 P 0 , Q 0 ( P 0 , Q 0 Determine a point).

[0020] In this embodiment, the modulation signal of the MMC is obtained by superimposing the output signals of the power controller, circulating current controller and zero-sequence signal controller in the station-level controller, wherein: Us i , P 0 , Q 0 This will affect the modulation signal S ap ( t ) A 1 and α 1 Value, which in turn affects the modulation signal margin S margin , so S margin Can be considered as Us i , P 0 , Q 0 The function of Us i , P 0 , Q 0 Considered as an independent variable, signal margin S margin considered as the dependent variable.

[0021] Each U si and U si The corresponding calculated P 0 , Q 0 Forming a Dataset G no.A3 ; Then calculate the voltage on each AC side when the zero-sequence signal is injected Us i Command modulation signal margin S marginThe maximum active power output of MMC when it is 0 P 1 , Maximum reactive power Q 1 In the construction of the dynamic boundary calculation unit of the present invention, the modulation signal margin when the zero-sequence signal is injected S margin Considered as Us i , P 1 , Q 1 , A 3 , α 3 Function: ; In this function, the modulation signal margin S margin is the dependent variable, Us i , P 1 , Q 1 , A 3 , α 3 is the independent variable, that is, given Us i , P 1 , Q 1 , A 3 , α 3 The value can be obtained S margin By modeling MMC, other parameters and Us i When determined, the modulation signal margin can be calculated through the established model S margin 0 P 1 , Q 1 , A 3 , α 3 The value of .

[0022] Each U si and U si The corresponding calculated P 1 , Q1 , A 3 , α 3 Forming a Dataset G A3 ,in, A 3 is the amplitude of the third harmonic component, α 3 is the phase angle of the third harmonic component.

[0023] Taking the upper bridge arm of phase A as an example, the modulation signal margin S margin The calculation formula is as follows: ; ; in, S ap ( t ) is the modulation signal of the upper bridge arm of phase A, which is generated by the MMC station-level control: power controller, circulating current controller, average capacitor voltage controller, and zero-sequence signal injection controller, such as Figure 1 , S ap Generally refers to the A-phase upper bridge arm modulation signal, S an Indicates the modulation signal of the lower bridge arm of phase A, the modulation signal margin when no zero-sequence signal is injected S margin Modulation signal margin when zero sequence signal is injected S margin This formula can be used to calculate. When no zero-sequence signal is injected, A 3 , α 3 The value of is 0; when the zero-sequence signal is injected, A 3 , α 3 The value of is not 0. A 0 , A 1 , A 2 and A 3 are the amplitudes of DC, 1st, 2nd and 3rd harmonic components respectively, A 3 Also called zero-sequence signal amplitude; α 1 , α 2 , α 3 Indicates the phase angle of the 1st, 2nd and 3rd harmonic components; ω is the fundamental angular frequency. U s , P , Q It will affect the size of each frequency modulation signal through each station-level controller, thereby affecting the size of the modulation signal margin.

[0024] Example 3 A method for improving the reactive power support capability of MMC taking into account the dynamic boundary under power grid fault is shown in Example 2, except that the implementation process of step S2 is as follows: S21: The data set obtained in step S12 G no.A3 AC side voltage U s , Maximum active power P 0 As an independent variable, the maximum reactive power Q 0 As the dependent variable, the fitting function A is obtained as follows: ; in, Indicates a maximum reactive power when no zero-sequence signal is injected. Q 0 Can be expressed A series of numbers composed of f b0 ( ) represents a function, which represents the boundary function when there is no zero-sequence signal. This function can be obtained by fitting the existing neural network. It is impossible to give a specific function expression. By inputting the independent variable into the black box, the dependent variable can be output. Us i express Us No. i The final calculation point is the AC side voltage. U s Fit a function to the independent variable.

[0025] The data set obtained in step S12 G A3 The AC side voltage U s , Maximum active power P 1 As an independent variable, the maximum reactive power Q 1 As the dependent variable, the fitting function B is obtained as follows: ; in, Indicates a maximum reactive power when a zero-sequence signal is injected, Q1 Can be expressed A series of numbers composed of f b1 ( ) represents a function, which represents the boundary function when there is a zero-sequence signal. This function can be obtained by fitting the existing neural network. It is impossible to give a specific function expression. By inputting the independent variable into the black box, the dependent variable can be output.

[0026] Input any AC voltage U s and active power P , we can get the maximum reactive power that the MMC can output when there is no zero-sequence signal injection The maximum reactive power that the MMC can output when the zero-sequence signal is injected ; S22: Divide the AC side voltage U s , Active Power P , reactive power Q The calculation point where the AC side voltage U s The calculation points of the active power are divided as shown in the formula of step S11 above. P , reactive power Q The calculation point division is adjusted according to the MMC operation parameters, and its expression is as follows: ; in, i Indicates i calculation points, j Indicates the maximum number of active power calculations. k Indicates the maximum number of reactive power calculations; P max , Q max It is a dataset G no.A3 and G A3 The maximum active power and reactive power recorded in are expressed as follows: ; Indicates that the data set is obtained without zero-sequence signal injection G no.A3 All active power data in Indicates that the data set is obtained without zero-sequence signal injection G no.A3 All reactive power data in Indicates the data set obtained under zero-sequence signal injection G A3All active power data in Indicates the data set obtained under zero-sequence signal injection G A3 All reactive power data in.

[0027] Each U s , P , Q It constitutes an operating point of MMC operation. P i express P No. i calculation points, Q i express Q No. i calculation points, each operating point corresponds to the amplitude of the third harmonic component of a zero-sequence signal A 3 and phase angle α 3 Maximize the modulation signal margin; By modeling the given U s , P , Q The modulation signal margin is maximized under A 3 and α 3 , and store the data into a dataset G A3α3 ;because U s , P , Q The zero-sequence signal will affect the size of each frequency modulation signal through each station-level controller, thereby affecting the size of the modulation signal margin. Therefore, in the construction of the zero-sequence signal calculation unit of the present invention, the modulation signal margin S margin Can be considered as Us i , P i , Q i , A 3 , α 3 function.

[0028] S23: The data set obtained in step S22 G A3α3 middle U s , P , Q As an independent variable, A3 and α 3 They are loaded into the neural network as dependent variables for training and function fitting, and the trained fitting function C and fitting function D are obtained: Fitting function C: ; Fitting function D: ; in, A 3.Cal and phase angle α 3.Cal are the amplitude and phase angle of the third harmonic component that maximizes the modulation signal margin after fitting; A neural network is a tool that can be used to fit data and generate a fitting function. By using the neural network package that comes with MATLAB and inputting dependent variables and independent variables, the data can be fit trained to obtain a fitting function. This part can be obtained using existing technology.

[0029] The above fitting function C and fitting function D are combined into the zero-sequence signal calculation unit, and any AC side voltage is input into the zero-sequence signal calculation unit. U s , Active Power P , reactive power Q The amplitude of the third harmonic component of the zero-sequence signal that can maximize the modulation signal margin can be obtained by MMC at this time. A 3.Cal and phase angle α 3.Cal .

[0030] Example 4 A method for improving the reactive power support capability of MMC taking into account the dynamic boundary under power grid fault is shown in Example 3, except that the implementation process of step S3 is as follows: S31: Real-time detection of the AC side voltage in the system U s.det ,MMC output active power P det and reactive power Q det ,Will U s.det , P det Substitute into fitting function A and fitting function B respectively to obtain the maximum reactive power that the MMC can output when there is no zero-sequence signal injection The maximum reactive power that the MMC can output when the zero-sequence signal is injected ; The real-time detection U s.det , Pdet and Q det Substitute the fitting functions C and D respectively to obtain the amplitude of the third harmonic component of the zero-sequence signal injected into the MMC at this time. A 3.Cal and phase angle α 3.Cal ; S32: Comparison of reactive power Q det , calculated in step S6 and The size of Q det ≤ hour, A 3 =0, α 3 =0, no alarm; when <Q det < hour, A 3 = A 3.Cal , α 3 = α 3.Cal , do not call the police; Q det ≥ hour, A 3 = A 3.Cal , α 3 = α 3.Cal , alarm, indicating that the current MMC operating conditions have exceeded the limit conditions. If the MMC is to continue to operate, the staff needs to take other measures. See Table 1 for details: Table 1 Zero sequence signal output comparison table

[0031] When there is no alarm, the zero sequence signal calculation unit is started and the A 3 and α 3 Inject the MMC modulation module to improve the MMC reactive power support capacity without over-modulation under linear control.

[0032] The present invention inputs the real-time collected AC side voltage and the active and reactive power output by the converter, and calculates the zero-sequence signal amplitude and phase angle that maximize the modulation signal margin of the MMC under the current working conditions. Injecting the zero-sequence signal can enable the MMC to improve its reactive support capability and work in a linear control area without overmodulation.

[0033] In order to verify the method for improving the reactive power support capability of MMC taking into account the dynamic boundary under power grid fault proposed in this embodiment, the following example is used for verification. The main circuit parameters of the flexible DC converter station MMC in the embodiment are shown in Table 2.

[0034] Table 2 MMC main circuit parameters

[0035] First, a dynamic boundary calculation unit is constructed. U s When a drop occurs, consider the grid voltage margin standard and the data in Table 2 and select Us.min =148.75kV, take n= 12 pairs U s Calculate the voltage of each grid side U s The modulation signal margin is S margin Active power when it is 0 P 0 , reactive power Q 0 , save the data obtained from each calculation to form a data set G no.A3 .

[0036] Then calculate the voltage on each AC side when the zero-sequence signal is injected Us i Command modulation signal margin S margin When the active power output of MMC is 0 P 1 , reactive power Q 1 , save the data obtained from each calculation to form a data set G A3 .

[0037] The dataset G no.A3 The working point in Figure 3 As shown by the blue solid dots in the middle, the data set G A3 The working point in Figure 3 As shown by the red hollow dots in the figure. Gno.A3 The AC side voltage U s , Active Power P 0 As an independent variable, reactive power Q 0 As the dependent variable, function fitting is performed through neural network ; The dataset G A3 The AC side voltage U s , Active Power P 1 As an independent variable, reactive power Q 1 As the dependent variable, function fitting is performed through neural network The two fitting function surfaces are as follows Figure 3 shown.

[0038] The above two fitting functions are combined into the dynamic boundary calculation unit. At this time, the MMC dynamic boundary calculation unit under the MMC main circuit parameters in Table 2 is obtained. When the unit is input U s , P The maximum reactive power that the MMC can output with or without zero-sequence signal injection under the current voltage and active power output can be obtained. and .

[0039] Then, a zero-sequence signal calculation unit is constructed. Calculate the given AC side voltage U s ,MMC output active power P , reactive power Q The zero-sequence signal amplitude that maximizes the modulation signal margin is A 3 and phase angle α 3 , select P max , Q max For the dataset G A3 The maximum active power and reactive power recorded in n= 7, j= twenty two ，k= 11. Solve the zero-sequence signal amplitude and phase angle that maximize the modulation signal margin at each operating point. The solution process is: by modeling the MMC, the maximum modulation signal margin is regarded as the optimization objective function, and the zero-sequence signal amplitude is A 3 and phase angle α 3The zero-sequence signal amplitude and phase angle are solved by treating them as independent variables and adding constraints to ensure that the objective function finally converges. The internal point method and other algorithms that can solve nonlinear optimization problems with constraints can be used to complete the solution of the zero-sequence signal amplitude and phase angle. This is a prior art and will not be described here. This embodiment solves the zero-sequence signal amplitude and phase angle under 1694 operating points in total, and stores the data to form a data set. G A3α3 .

[0040] The dataset G A3α3 The AC side voltage U s , Active Power P , reactive power Q As an independent variable, the zero-sequence signal amplitude A 3 and phase angle α 3 As the dependent variable, we can get A 3.Cal =f A3 (U s , P , Q) , α 3.Cal =f A3 (U s , P , Q) The above two fitting functions are combined into the zero-sequence signal calculation unit, and the zero-sequence signal calculation unit under the MMC main circuit parameters in Table 2 is obtained. U s , P , Q The zero-sequence signal amplitude that maximizes the modulation signal margin under the current voltage and active and reactive power output of MMC can be obtained. A 3.Cal and phase angle α 3.Cal .

[0041] The network-based MMC operates normally in U s =212.5kV, P =1500MW, Q =100MVar working condition, when T=1s, a fault occurs on the grid side causing the grid side voltage U sFor example, the voltage drops to 0.75pu, that is, 159.375kV. Due to the characteristics of grid-type control, in order to maintain the grid-side voltage from dropping, the MMC increases reactive power. At this time, the dynamic boundary calculation unit outputs the current voltage and the maximum reactive power that the MMC can output under the active power output. , And the real-time detection value of MMC output reactive power Q det like Figure 4 As shown. Figure 4 It can be seen that when T<1s, Q det < , then the output A 3 =0, α 3 =0, such as Figure 5 , Figure 6 At T=1s, the MMC generates additional reactive power, <Q det < , at this time, the zero-sequence signal calculation unit is started.

[0042] After the zero-sequence signal calculation unit is started, the grid-side voltage U s , Real-time detection value of MMC output active power P det , Real-time detection value of reactive power Q det Input zero sequence signal calculation unit, the output zero sequence signal amplitude and phase angle are as follows Figure 5 , Figure 6 As shown. The calculated zero-sequence signal is injected into the modulation module, and the modulation signal waveform after the zero-sequence signal is injected using the proposed method is shown in Figure 7 As shown in the figure, it can be seen that the MMC avoids the risk of over-modulation at this time and is still in the linear control area (when the modulation signal is between 0 and 1, it is in the linear control area). At the same time, if the method proposed by the present invention is not adopted, when the voltage drops and the grid-type MMC increases reactive power, the modulation signal waveform is as follows Figure 8 As shown in the figure, it can be seen that the MMC modulation signal is over-modulated at this time. Due to the existence of the limiting link in the modulation module, the modulation signal exceeding 0~1 is clipped, so that the MMC enters the nonlinear control area.

[0043] By adopting the method for improving the reactive support capacity of MMC taking into account the dynamic boundary under power grid faults proposed in the present invention, the reactive support capacity of MMC can be improved in real time according to the current operating conditions in an economical and efficient manner, so that the grid-forming MMC can provide greater reactive support when the voltage drops, significantly improving the voltage stability and anti-interference ability of the system, while avoiding the harmonic distortion and operation risks caused by overmodulation, and improving the overall performance and reliability of the system.

[0044] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for improving the reactive power support capability of MMC taking into account the dynamic boundary under power grid fault, characterized in that: The steps include: S1: Constructing the MMC dynamic boundary computing unit under power grid fault; S2: Constructing the fitting function of the zero-sequence signal calculation unit to improve the reactive power support capability of MMC; S3: Reactive power with and without zero-sequence signal injection output based on the constructed dynamic boundary calculation unit , The reactive power detected in real time in the system Q det Determine whether to start the zero-sequence signal calculation unit; S4: If the zero-sequence signal calculation unit is started, the zero-sequence signal of the expanded operating domain under the current working condition of the MMC is calculated in real time and injected into the MMC modulation module to improve the reactive power support capacity of the MMC.

2. The method for improving the MMC reactive power support capability taking into account the dynamic boundary under power grid fault according to claim 1, characterized in that: The implementation process of step S1 is: S11: Considering the voltage drop on the grid side under the power grid fault, the minimum allowable voltage at the PCC point is Us.min =0.7* Us.rated , the maximum allowable voltage is Us.max =1.0* Us.rated ,in Us.rated The AC side voltage is divided by the rated voltage of the PPC point U s Calculate the point and calculate the AC side voltage U s ; S12: Calculate the voltages on each AC side when no zero-sequence signal is injected Us i Command modulation signal margin S margin The maximum active power output of MMC when it is 0 P 0 , Maximum reactive power Q 0 , modulation signal margin when there is no zero-sequence signal injection S margin Considered as Us i , P 0 , Q 0 function, each U si and U si The corresponding calculated P 0 , Q 0 Forming a Dataset G no.A3 ; Then calculate the voltage on each AC side when the zero-sequence signal is injected Us i Command modulation signal margin S margin The maximum active power output of MMC when it is 0 P 1 , Maximum reactive power Q 1 , modulation signal margin when zero sequence signal is injected S margin Considered as Us i , P 1 , Q 1 , A 3 , α 3 function, each U si and U si The corresponding calculated P 1 , Q 1 , A 3 , α 3 Forming a Dataset G A3 ,in, A 3 is the amplitude of the third harmonic component, α 3 is the phase angle of the third harmonic component.

3. The method for improving the MMC reactive power support capability taking into account the dynamic boundary under power grid fault according to claim 2, characterized in that: The implementation process of step S2 is: S21: The data set obtained in step S12 G no.A3 AC side voltage U s , Maximum active power P 0 As an independent variable, the maximum reactive power Q 0 As the dependent variable, the fitting function A is obtained as follows: ； in, Indicates a maximum reactive power when no zero-sequence signal is injected; The data set obtained in step S12 G A3 The AC side voltage U s , Maximum active power P 1 As an independent variable, the maximum reactive power Q 1 As the dependent variable, the fitting function B is obtained as follows: ； in, Indicates a certain maximum reactive power when a zero-sequence signal is injected; Input any AC voltage U s and active power P , we can get the maximum reactive power that the MMC can output when there is no zero-sequence signal injection The maximum reactive power that the MMC can output when the zero-sequence signal is injected ; S22: Divide the AC side voltage U s , Active Power P , reactive power Q The calculation point of each U s , P , Q It constitutes an operating point of MMC operation. Each operating point corresponds to the amplitude of the third harmonic component of a zero-sequence signal. A 3 and phase angle α 3 Maximize the modulation signal margin; Calculate the given U s , P , Q The modulation signal margin is maximized under A 3 and α 3 , and store the data into a dataset G A3α3 ; S23: The data set obtained in step S22 is G A3α3 middle U s , P , Q As an independent variable, A 3 and α 3 They are used as dependent variables for function fitting, and fitting function C and fitting function D are obtained: Fitting function C: ; Fitting function D: ; in, A 3.Cal and phase angle α 3.Cal are the amplitude and phase angle of the third harmonic component that maximizes the modulation signal margin after fitting; Input any AC voltage U s , Active Power P , reactive power Q The amplitude of the third harmonic component of the zero-sequence signal that can maximize the modulation signal margin can be obtained by MMC at this time. A 3.Cal and phase angle α 3.Cal .

4. The method for improving the MMC reactive power support capability taking into account the dynamic boundary under power grid fault according to claim 3, characterized in that: The implementation process of step S3 is: S31: Real-time detection of the AC side voltage in the system U s.det ,MMC output active power P det and reactive power Q det ,Will U s.det , P det Substitute into fitting function A and fitting function B respectively to obtain the maximum reactive power that the MMC can output when there is no zero-sequence signal injection The maximum reactive power that the MMC can output when the zero-sequence signal is injected ; The real-time detection U s.det , P det and Q det Substitute the fitting functions C and D respectively to obtain the amplitude of the third harmonic component of the zero-sequence signal injected into the MMC at this time. A 3.Cal and phase angle α 3.Cal ; S32: Comparison of reactive power Q det , calculated in step S6 and The size of Q det ≤ hour, A 3 =0, α 3 =0, no alarm; when <Q det < hour, A 3 = A 3.Cal , α 3 = α 3.Cal , do not call the police; when Q det ≥ hour, A 3 = A 3.Cal , α 3 = α 3.Cal , alarm, prompting that the current MMC operating conditions have exceeded the limit conditions; When there is no alarm, the zero sequence signal calculation unit is started and the A 3 and α 3 Inject the MMC modulation module to improve the MMC reactive power support capacity without over-modulation under linear control.

5. The method for improving the MMC reactive power support capability taking into account the dynamic boundary under power grid fault according to claim 2, characterized in that: In step S11, ; in, U si Indicates the calculation range of the minimum and maximum allowable voltages at the PPC point. i The AC side voltage of the calculation point is n To calculate the interval.

6. The method for improving the reactive power support capability of MMC taking into account the dynamic boundary under power grid fault according to claim 2, characterized in that: In step S12, the signal margin is modulated when there is no zero-sequence signal injection. S margin Considered as Us i , P 0 , Q 0 The function is: ； Modulation signal margin when zero sequence signal is injected S margin Considered as Us i , P 1 , Q 1 、A 3 、α 3 The function is: ； Taking the upper bridge arm of phase A as an example, the modulation signal margin S margin The calculation formula is as follows: ； ； in, S ap ( t ) is the modulation signal of the upper bridge arm of phase A, A 0 , A 1 , A 2 and A 3 are the amplitudes of DC, 1st, 2nd and 3rd harmonic components respectively, A 3 Also called zero-sequence signal amplitude; α 1 , α 2 , α 3 Indicates the phase angle of the 1st, 2nd and 3rd harmonic components; ω is the fundamental angular frequency.

7. The method for improving the reactive power support capability of MMC taking into account the dynamic boundary under power grid fault according to claim 3, characterized in that: In step S22, the AC side voltage is divided U s , Active Power P , reactive power Q The calculation point, where the active power P , reactive power Q The calculation point division is adjusted according to the MMC operation parameters, and its expression is as follows: ； in, i Indicates i calculation points, j Indicates the maximum number of active power calculations. k Indicates the maximum number of reactive power calculations; P max , Q max It is a dataset G no.A3 and G A3 The maximum active power and reactive power recorded in are expressed as follows: ； Indicates that the data set is obtained without zero-sequence signal injection G no.A3 All active power data in Indicates that the data set is obtained without zero-sequence signal injection G no.A3 All reactive power data in Indicates the data set obtained under zero-sequence signal injection G A3 All active power data in Indicates the data set obtained under zero-sequence signal injection G A3 All reactive power data in.

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