A method for calculating the optimal zero-sequence signal of MMC for overload operation requirements

Abstract

The present invention relates to an MMC optimal zero-sequence signal calculation method for overload operation requirements, belonging to the field of power electronic converter signal modulation, comprising: constructing an optimal zero-sequence signal calculation operation domain; establishing an optimal zero-sequence signal solution mathematical model; continuously iterating active power and reactive power, and inputting the optimal zero-sequence signal solution mathematical model to solve and obtain multiple groups of maximum modulation signal margin values. March , the corresponding amplitude and phase angle of the zero-sequence signal are the amplitude and phase angle of the optimal zero-sequence signal, and the output records are formed respectively OPA 3 ( P , Q ) table and OPAlf 3 ( P , Q ) table; the table is fitted by a function fitting method to obtain a final fitting function formula, which is used for the subsequent rapid calculation of the amplitude and phase angle of the optimal zero-sequence signal. The present invention can significantly reduce the operation time of the modulation signal overmodulation caused by overload operation of the MMC, and reduce the problems such as the reduction of the life of the switching device caused by the overmodulation of the modulation signal.

Description

Technical Field

[0001] The invention relates to an MMC optimal zero-sequence signal calculation method oriented to overload operation requirements, and belongs to the technical field of power electronic converter signal modulation. Background Art

[0002] As an important technological innovation in the field of modern power electronics, modular multilevel converter (MMC) has shown excellent performance in high-voltage direct current transmission systems, especially in large-capacity and long-distance transmission. Compared with traditional converters, modular multilevel converters have the advantages of high modularity and scalability, no need for AC filter groups and excellent dynamic performance, and have become the preferred converter topology for high-voltage direct current transmission systems. However, during operation, the power grid may have faults such as short circuits, causing MMC to overload. The modulation signal of MMC may exceed its limit, resulting in overmodulation of the modulation signal. If it is not effectively controlled, the output voltage of MMC will not match the grid voltage, causing grid voltage fluctuations and other problems. It will also cause uneven distribution of current and thermal stress of switching devices, reduce the life of switching devices, and affect the stable operation of the system. In order to avoid overmodulation of the modulation signal when the MMC is overloaded and improve system stability, it is necessary to inject zero-sequence signals into the control link of MMC to improve the modulation signal margin.

[0003] However, the traditional zero-sequence signal solution method has problems such as low solution accuracy, low calculation efficiency, and long solution time. It cannot meet the requirements of real-time control systems and will cause the MMC to operate in an overmodulated state for a long time, making its current and thermal stress distribution characteristics uneven, leading to overvoltage and overcurrent problems in the converter, reducing the life of the switching device, and affecting the stable operation of the MMC. These problems have become the bottleneck that restricts the MMC from overmodulating the modulation signal when it is overloaded in practical applications, limiting the further application of MMC in power systems. Therefore, developing advanced zero-sequence signal calculation methods to optimize the response of the modulation signal and improve the modulation signal margin is of great significance to improving the stable operation capability of the MMC under overload conditions and promoting the application of MMC in power systems. Summary of the invention

[0004] In order to solve the above problems, the present invention provides an MMC optimal zero-sequence signal calculation method for overload operation requirements, which can significantly reduce the operation time of the MMC due to overmodulation of the modulation signal caused by overload operation, and reduce the problem of reduced life of switching devices caused by overmodulation of the modulation signal.

[0005] The technical solution of the present invention is as follows:

[0006] A method for calculating an optimal zero-sequence signal of an MMC for overload operation requirements comprises the following steps:

[0007] S1: Construct the optimal zero-sequence signal calculation operation domain;

[0008] S2: Establishing the optimal zero-sequence signal solving mathematical model;

[0009] S3: Continuously iterate the active power and reactive power, input each set of active power and reactive power into the optimal zero-sequence signal solution mathematical model established in step S2, and solve to obtain multiple sets of maximum modulation signal margin values Mar , the corresponding amplitude and phase angle of the zero-sequence signal are the amplitude and phase angle of the optimal zero-sequence signal, and the output records are formed respectively OPA 3 ( P , Q ) table and OPAlf 3 ( P , Q )sheet, OPA 3 ( P , Q ) table records each group of active power and reactive power and its corresponding optimal zero-sequence signal amplitude, OPAlf 3 ( P , Q ) table records each group of active power and reactive power and its corresponding optimal zero-sequence signal phase angle;

[0010] S4: For the formation OPA 3 ( P , Q ) table and OPAlf 3 ( P , Q ) table, and use function fitting to OPA 3 ( P , Q ) table and OPAlf 3 ( P , Q ) table to obtain the final fitting function, which is used for the subsequent rapid calculation of the amplitude and phase angle of the optimal zero-sequence signal.

[0011] Preferably, in step S1, when the MMC applies the zero-sequence signal control strategy, its modulation signal will not be overmodulated, and the active power that can be stably operated P and reactive power Q The combined operation area is the MMC limit operation area, and theP - Q Coordinate system, horizontal axis is P The axis represents active power, expressed as active power P is the variable, and the vertical axis is Q The axis represents reactive power, Q Reactive power is a variable, and the origin of the coordinate axis represents active power P Zero and reactive power Q The point where the MMC limit operation domain is zero is selected. P Axis positive direction and Q The intersection points in the positive direction of the axis are the maximum active power of the optimal zero-sequence signal calculation operation domain. P max and maximum reactive power Q max , take its opposite number, which are the minimum active power - P max , minimum reactive power - Q max , P max , Q max 、- P max 、- Q max The optimal zero-sequence signal calculation operation domain is the actual converter operating area.

[0012] Preferably, in step S2, the optimal zero-sequence signal solving mathematical model is:

[0013] (1);

[0014] In the formula, Mar Indicates the maximum modulation signal margin value;

[0015] f mar ( A 3, α 3 ) represents the modulation signal margin function, which is expressed as follows:

[0016] (2);

[0017] D s For a dynamically modulated signal:

[0018] (3);

[0019] S ap (t ) is the modulation signal of the upper bridge arm of phase A:

[0020] (4);

[0021] in, A 0 represents the DC component of the modulating signal; A 1 Indicates the amplitude of the fundamental component; A 2 Indicates the amplitude of the double frequency harmonic component; A 3 Indicates the amplitude of the triple frequency harmonic component; ω represents the angular frequency; t represents the time variable; α 1 Indicates the initial phase angle of the fundamental component; α 2 Indicates the initial phase angle of the double frequency harmonic component; α 3 Indicates the initial phase angle of the tripled frequency harmonic component.

[0022] Preferably, in step S3, the iteration process of active power and reactive power is as follows:

[0023] S31: Set initial active power P 0 , initial reactive power Q 0 , and the iteration step size P step , Q step ;

[0024] S32: Input the active power and reactive power into the optimal zero-sequence signal solution mathematical model established in step S2 to obtain the maximum modulation signal margin value. Mar and the amplitude and phase angle of the optimal zero-sequence signal;

[0025] S33: Iterate the active power. The iteration method is as shown in formula (5). The initial value of n is 0, and n increases by 1 each time iterates:

[0026] (5);

[0027] S34: Determine whether the P n ≥ P max If not, continue to iterate in the manner of formula (5). During the active power iteration process, the reactive power remains unchanged, and return to step S32 for solution; if yes, end the iteration of active power;

[0028] S35: Iterate reactive power. The iteration method is as shown in formula (6). The initial value of m is 0, and m increases by 1 each time iterates:

[0029] (6);

[0030] S36: Determine whether the Q m ≥ Q max If not, continue to iterate in the manner of formula (6) and change the active power to the initial active power P 0 , return to steps S32 to S35 and continue until the Q m ≥ Q max , ending the iteration.

[0031] In the iterative process, the active power is first iterated P n , at this time the reactive power remains unchanged, when the active power P n ≥ P max When , the reactive power is iterated in the manner of formula (6), that is, m+1 and n becomes 0 again, that is, P n Becomes the initial active power P 0 .

[0032] The iterative process can be simply described as:

[0033] First input P 0 and Q 0 , second input P 1 and Q 0 , third input P 2 and Q 0 ...until you enter P n and Q 0 ,when P n ≥ P max When , the reactive power starts to be iterated, and the active power is changed to the initial active power P 0 , enter P 0 and Q 1, P 1 and Q 1 , P 2 and Q 1 ...until again P n ≥ P max , then the reactive power becomes Q 2 , the active power becomes the initial active power P 0 , enter P 0 and Q 2 , P 1 and Q 2 , P 2 and Q 2 ...until again P n ≥ P max , repeat the cycle until the Q m ≥ Q max , ending the iteration.

[0034] Preferably, the final fitting function in step S3 is as follows:

[0035] (7);

[0036] (8);

[0037] in, Z 0 , B 1 , B 2 , B 3 , B 4 , C 1 , C 2 , C 3 , C 4 , C 5 Both A 3( P,Q ) The coefficients of the fitting function are calculated based on the optimal zero-sequence signal amplitude data calculated under different powers, and the values ​​are obtained by fitting the rational function;

[0038] Z 1 , D 1 , D 2 , D 3 , E 1 , E 2 , E 3 , E 4 Both OPAlf 3 ( P , Q ) is a coefficient of the fitting function, the coefficient of which is calculated based on the optimal zero-sequence signal phase angle data calculated under different powers, and its value is obtained by fitting the rational function.

[0039] Preferably, the initial active power P 0 , initial reactive power Q 0 Select a point on the boundary of the optimal zero-sequence signal calculation operation domain, P step =100 MW, Q step =100 MW.

[0040] When using the MMC optimal zero-sequence signal solution method for overload operation requirements, the zero-sequence signal that maximizes the MMC modulation signal margin will be calculated based on the system parameters such as the main circuit parameters of the MMC, so that it can be superimposed on the original modulation signal to reduce the signal peak value, so that the modulation signal is within the maximum allowable value of the MMC modulation signal and the modulation signal margin is maximized. If a short circuit or other fault occurs in the power grid, causing a sudden increase in the MMC output power, the MMC will operate under overload conditions. Formulas (7) and (8) can accurately and quickly calculate the amplitude and phase angle of the optimal zero-sequence signal, inject the desired optimal zero-sequence signal into the control system, reduce the modulation signal peak value, and obtain the maximum modulation signal margin when the MMC is overloaded, so as to meet the MMC overload operation requirements and improve the stability of the MMC system.

[0041] For any details not provided in the present invention, please refer to the prior art.

[0042] The beneficial effects of the present invention are:

[0043] The present invention solves the zero-sequence signal that maximizes the modulation signal margin when the MMC is in overload operation, and injects the optimal zero-sequence signal into the power control system to improve the modulation signal margin when the MMC is in overload operation, so that the MMC has a stronger overload capacity when facing power grid failures or other transient events, which helps to improve the stability of the MMC under various power grid conditions. In the implementation process, the method constructs an optimal zero-sequence signal calculation operation domain, continuously iterates active power and reactive power, inputs the established optimal zero-sequence signal solution model, solves the amplitude and phase angle of the optimal zero-sequence signal under different powers, forms an optimal zero-sequence signal amplitude and phase angle table, and then uses a function fitting method to fit it, and accurately describes the amplitude through a rational fitting function. A 3 Phase Angle α 3 With active power P and reactive power Q The fitting function can quickly solve the optimal zero-sequence signal amplitude and phase angle under each power, and then inject it into the MMC control system, which greatly reduces the operation time of the modulation signal overmodulation caused by overload operation of the MMC, and reduces the life of the switching device caused by overmodulation of the modulation signal. This method can be applied to real-time control systems to quickly calculate the amplitude and phase angle of the optimal zero-sequence signal to meet the overload operation requirements of the MMC. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] 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.

[0045] Figure 1 It is the overall framework diagram of the MMC power control system of the present invention;

[0046] Figure 2 It is a flow chart of the MMC optimal zero-sequence signal calculation method for overload operation requirements of the present invention;

[0047] Figure 3 It is a schematic diagram of the optimal zero-sequence signal calculation operation domain;

[0048] Figure 4 for OPA 3 ( P , Q )Table diagram;

[0049] Figure 5 for OPAlf 3 ( P , Q )Table diagram;

[0050] Figure 6 is the optimal zero-sequence signal amplitude A 3 ( P,Q )’s fitting surface;

[0051] Figure 7 is the optimal zero-sequence signal phase angle α 3 ( P,Q )’s fitting surface;

[0052] Figure 8 For P =2000 MW, Q =400 MW power condition without using the method proposed by the present invention to obtain the optimal zero-sequence signal A phase upper bridge arm modulation signal S ap ( t ) waveform diagram;

[0053] Fig. 9 For P =2000 MW, Q =400 MW power condition using the method proposed in the present invention to obtain the optimal zero-sequence signal A phase upper bridge arm modulation signal S ap ( t ) waveform diagram. DETAILED DESCRIPTION

[0054] 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.

[0055] The overall framework diagram of the MMC power control system is as follows: Figure 1 As shown, the MMC bridge arm modulation signal is composed of the signals output by the average capacitor voltage controller, the output power controller, the circulating current controller, and the zero-sequence signal controller. The average capacitor voltage controller can control the DC component of the modulation signal; the output power controller can control the output power of the MMC by controlling the fundamental component of the modulation signal; the circulating current controller can control the second harmonic component of the modulation signal; and the zero-sequence signal controller can control the third harmonic component of the modulation signal.

[0056] S ap , S anThey respectively represent the A-phase upper bridge arm modulation signal and the A-phase lower bridge arm modulation signal; the A-phase upper bridge arm modulation signal and the A-phase lower bridge arm modulation signal are respectively composed of signals output by the average capacitor voltage controller, the output power controller, the circulating current controller, and the zero-sequence signal controller.

[0057] Example 1

[0058] A method for calculating the optimal zero-sequence signal of MMC for overload operation requirements, such as Figure 2 As shown, the following steps are included:

[0059] S1: Construct the optimal zero-sequence signal calculation operation domain;

[0060] When MMC applies the zero-sequence signal control strategy, its modulation signal will not be over-modulated, and the active power that can operate stably P and reactive power Q The combined operation area is the MMC limit operation area, and the P - Q Coordinate system, horizontal axis is P The axis represents active power, expressed as active power P is the variable, and the vertical axis is Q The axis represents reactive power, Q Reactive power is a variable, and the origin of the coordinate axis represents active power P Zero and reactive power Q The point where the MMC limit operation domain is zero is selected. P Axis positive direction and Q The intersection points in the positive direction of the axis are the maximum active power of the optimal zero-sequence signal calculation operation domain. P max and maximum reactive power Q max , take its opposite number, which are the minimum active power - P max , minimum reactive power - Q max , P max , Q max 、- P max 、- Q max The optimal zero-sequence signal calculation operation domain is formed, which is the actual converter operation area, such as Figure 3 The rectangular area shown.

[0061] S2: Establishing the optimal zero-sequence signal solving mathematical model;

[0062] The optimal zero-sequence signal solution mathematical model is:

[0063] (1);

[0064] In the formula, Mar Indicates the maximum modulation signal margin value; st It means that under the following constraints, 0 ≤ A 3 ≤0.5 and 0 ≤ α 3 ≤0.5;

[0065] f mar ( A 3, α 3 ) represents the modulation signal margin function, which is expressed as follows:

[0066] (2);

[0067] D s For a dynamically modulated signal:

[0068] (3);

[0069] S ap ( t ) is the modulation signal of the upper bridge arm of phase A:

[0070] (4);

[0071] in, A 0 Represents the DC component of the modulating signal; A 1 Indicates the amplitude of the fundamental component; A 2 Indicates the amplitude of the double frequency harmonic component; A 3 Indicates the amplitude of the triple frequency harmonic component; ω represents the angular frequency; t represents the time variable; α 1 Represents the initial phase angle of the fundamental component; α 2 Indicates the initial phase angle of the double frequency harmonic component; α 3 Indicates the initial phase angle of the tripled frequency harmonic component.

[0072] A 0 , A1 , A 2 , A 3 , α 1 , α 2 , α 3 It is obtained by average capacitor voltage controller, output power controller, circulating current controller, zero sequence signal controller, different main circuit parameters and different control targets. A 0 , A 1 , A 2 , A 3 , α 1 , α 2 , α 3 Are different. ω Represents the angular frequency, and its value is 100π.

[0073] The modulation signal through the upper bridge arm of phase A S ap ( t ) can be used to find the modulation signal margin function f mar ( A 3, α 3 ), and then find the maximum modulation signal margin value Mar .

[0074] S3: Continuously iterate the active power and reactive power, input each set of active power and reactive power into the optimal zero-sequence signal solution mathematical model established in step S2, and solve to obtain multiple sets of maximum modulation signal margin values Mar , the corresponding amplitude and phase angle of the zero-sequence signal are the amplitude and phase angle of the optimal zero-sequence signal, and the output records are formed respectively OPA 3 ( P , Q ) table and OPAlf 3 ( P , Q )sheet, OPA 3 ( P , Q ) table records each group of active power and reactive power and its corresponding optimal zero-sequence signal amplitude, such as Figure 4 As shown; OPAlf3 ( P , Q ) table records each group of active power and reactive power and its corresponding optimal zero-sequence signal phase angle, such as Figure 5 shown.

[0075] Based on the established modulation signal margin function f mar ( A 3, α 3 ) It can be seen that different amplitudes and phase angles of zero-sequence signals can be input to obtain different modulation signal margins. When the amplitude and phase angle of a zero-sequence signal are input to make the modulation signal margin f mar When the modulated signal margin value is the smallest, the modulated signal margin value is the largest at this time, and the zero-sequence signal input at this time is defined as the optimal zero-sequence signal. In this embodiment, by iterating the active power and reactive power respectively, inputting each group of active power and reactive power, the maximum modulated signal margin value of all points in the optimal zero-sequence signal calculation operation domain is obtained. Mar , and the corresponding amplitude and phase angle of the optimal zero-sequence signal to form a table. OPA 3 ( P , Q ) table represents the optimal zero-sequence signal amplitude A 3 Follow P and Q The value of the change; OPAlf 3 ( P , Q ) table represents the optimal zero-sequence signal phase angle α 3 Follow P and Q The value of the change.

[0076] S4: For the formation OPA 3 ( P , Q ) table and OPAlf 3 ( P , Q ) table, and use the rational function fitting method to respectively OPA 3 ( P , Q ) table and OPAlf 3 ( P , Q ) table to fit and get the final fitting function. A3 ( P, Q ) function is shown in formula (7), and the fitted OPAlf 3 ( P , Q ) function is shown in formula (8). This fitting function can quickly and accurately describe the amplitude A 3 Phase Angle α 3 With active power P and reactive power Q The change relationship can be applied to real-time control systems to quickly calculate the amplitude and phase angle of the optimal zero-sequence signal.

[0077] (7);

[0078] (8);

[0079] in, Z 0 , B 1 , B 2 , B 3 , B 4 , C 1 , C 2 , C 3 , C 4 , C 5 Both A 3 ( P,Q ) fitting function coefficients, the coefficients are calculated based on the optimal zero-sequence signal amplitude data calculated under different powers, and their values ​​are obtained by rational function fitting; the amplitude of the optimal zero-sequence signal obtained is different for different MMC main circuit parameters. A 3 If the parameters are different, the parameter values ​​obtained by fitting the rational function will also be different.

[0080] Z 1 , D 1 , D 2 , D 3 , E 1 , E2 , E 3 , E 4 Both OPAlf 3 ( P , Q ) The coefficients of the fitting function are calculated based on the optimal zero-sequence signal phase angle data calculated under different powers, and the values ​​are obtained by rational function fitting. The parameters of the MMC main circuit are different, and the phase angle of the optimal zero-sequence signal is α 3 If the parameters are different, the parameter values ​​obtained by fitting the rational function will also be different.

[0081] Example 2

[0082] A method for calculating an optimal zero-sequence signal of an MMC for overload operation requirements is as described in Example 1, except that in step S3, the iterative process of active power and reactive power is as follows:

[0083] S31: Set initial active power P 0 , initial reactive power Q 0 , and the iteration step size P step , Q step ;

[0084] Initial active power P 0 , initial reactive power Q 0 Select a point on the boundary of the optimal zero-sequence signal calculation operation domain, P step =100 MW, Q step =100 MW;

[0085] S32: Input the active power and reactive power into the optimal zero-sequence signal solution mathematical model established in step S2 to obtain the maximum modulation signal margin value. Mar and the amplitude and phase angle of the optimal zero-sequence signal;

[0086] S33: Iterate the active power. The iteration method is as shown in formula (5). The initial value of n is 0, and n increases by 1 each time iterates:

[0087] (5);

[0088] S34: Determine whether the P n ≥ Pmax If not, continue to iterate in the manner of formula (5). During the active power iteration process, the reactive power remains unchanged, and return to step S32 for solution; if yes, end the iteration of active power;

[0089] S35: Iterate reactive power. The iteration method is as shown in formula (6). The initial value of m is 0, and m increases by 1 each time iterates:

[0090] (6);

[0091] S36: Determine whether the Q m ≥ Q max If not, continue to iterate in the manner of formula (6) and change the active power to the initial active power P 0 , return to steps S32 to S35 and continue until the Q m ≥ Q max , ending the iteration.

[0092] The iterative process can be described as:

[0093] First input P 0 and Q 0 , second input P 1 and Q 0 , third input P 2 and Q 0 ...until you enter P n and Q 0 ,when P n ≥ P max When , the reactive power starts to be iterated, and the active power is changed to the initial active power P 0 , enter P 0 and Q 1 , P 1 and Q 1 , P 2 and Q 1 ...until again Pn ≥ P max , then the reactive power becomes Q 2 , the active power becomes the initial active power P 0 , enter P 0 and Q 2 , P 1 and Q 2 , P 2 and Q 2 ...until again P n ≥ P max , repeat the cycle until the Q m ≥ Q max , ending the iteration.

[0094] Example 3

[0095] A method for calculating the optimal zero-sequence signal of an MMC for overload operation requirements is now verified and explained in conjunction with an example. The main circuit parameters of the MMC in this embodiment are shown in Table 1:

[0096] Table 1 Main circuit parameters of MMC

[0097]

[0098] When MMC applies the zero-sequence signal control strategy, its modulation signal will not be over-modulated, and the active power that can operate stably P and reactive power Q The combined operation area is the MMC extreme operation domain. Select the boundary of the MMC extreme operation domain and P Axis positive direction and Q The intersection points in the positive direction of the axis are the maximum active power in the optimal zero-sequence signal calculation operation domain. P max =3059 MW and maximum reactive power Q max =1153 MW, the opposite of which is the minimum active power P min =-3059 MW and minimum reactive power Q min =-1153 MW, construct the optimal zero-sequence signal calculation operation domain as follows Figure 3As shown in FIG. 1 , the constructed optimal zero-sequence signal calculation operation domain is the actual converter operation area.

[0099] In order to reduce the amount of calculation and facilitate the formation of OPA 3 ( P , Q )and OPAlf 3 ( P , Q ) table, the constructed optimal zero-sequence signal calculation operation domain is divided into the transmission power area (active power P>0 ) and the receiving power area (active power P<0 ), through the established optimal zero-sequence signal solving mathematical model, it is found that the active power of the transmission power area and the receiving power area P On the contrary, reactive power Q When the same, the amplitude of the optimal zero-sequence signal to be injected is the same, and the sum of the phase angles is 2π. Therefore, by calculating the transmission power area (active power P>0 ), the optimal zero-sequence signal amplitude and phase angle of the constructed computational operation domain can be solved.

[0100] Set the initial value of iteration active power P 0 =100 MW and reactive power Q 0 =-1153 MW and iteration step P step =100 MW and Q step =100 MW, input it into the optimal zero-sequence signal solution mathematical model established in step 2, and modulate the signal margin f mar ( A 3, α 3 ) function as the objective function, the amplitude and phase angle of the optimal zero-sequence signal are continuously iterated through the particle swarm optimization algorithm to solve the maximum modulation signal margin value Mar =0.1815, the optimal zero-sequence signal amplitude is obtained A 3 =0.0633 and phase angle α 3 =3.0424 and record it.

[0101] In this embodiment, a particle swarm optimization algorithm can be used to optimize the established optimal zero-sequence signal solution mathematical model to iterate the amplitude and phase angle of the optimal zero-sequence signal. The particle swarm optimization algorithm is an existing conventional technology. The optimization process is: first initialize a group of particles, each particle represents a possible solution; the position of each particle represents the current solution, and the speed of each particle is set to represent its moving direction and step size in the search space; continuously update the position of the particle so that the particle moves in the search space in the direction of the optimal solution; each particle records the best position (i.e., the optimal value) found in previous iterations; the position with the highest fitness value in the entire particle swarm (i.e., the optimal value) is recorded as the global optimal solution. The optimal zero-sequence signal solution mathematical model uses a particle swarm optimization algorithm to continuously iterate the amplitude and phase angle of the optimal zero-sequence signal to solve the optimal zero-sequence signal amplitude and phase angle under the input power. When the calculated maximum modulation signal margin value Mar When the change is less than 0.001 in several consecutive iterations, it is considered that the optimal zero-sequence signal amplitude and phase angle have converged to the optimal solution, the iteration ends, and the iterated optimal zero-sequence signal amplitude and phase angle are output.

[0102] After recording, continue to iterate the active power P n Reactive Power Q m The steps for calculating the optimal zero-sequence signal amplitude and phase angle are as above and recorded until the iteration reaches P max = 3059 MW and Q max = 1153 MW, solving for P max , Q max The corresponding maximum modulation signal margin value Mar =-0.1157, and the optimal zero-sequence signal amplitude is obtained A 3 =0.1415 and phase angle α 3 =4.857, and finally the optimal zero-sequence signal amplitude and phase angle of different powers under the optimal zero-sequence signal calculation operation domain can be obtained, forming OPA 3 ( P , Q )and OPAlf 3 ( P , Q ) table, using rational function fitting OPA 3 ( P , Q )and OPAlf 3 (P , Q ) for fitting.

[0103] In this embodiment, the zero-sequence signal amplitude A 3 With active power P and reactive power Q Variation of the fitting function A 3 ( P , Q ) is as follows:

[0104] ;

[0105] In this embodiment, the zero-sequence signal phase angle α 3 With active power P and reactive power Q Variation of the fitting function α 3 ( P,Q ) is as follows:

[0106] ;

[0107] Figure 6 and Figure 7 The optimal zero-sequence signal amplitude is shown respectively. A 3 ( P,Q ) and phase angle α 3 ( P,Q ) function surface, the surface is the result of rational function fitting, the rational fitting function can accurately describe the amplitude A 3 Phase Angle α 3 With active power P and reactive power Q The fitting function can quickly solve the optimal zero-sequence signal amplitude and phase angle under each power, which can be applied to real-time control systems to quickly calculate the optimal zero-sequence signal amplitude and phase angle.

[0108] In order to further verify the effectiveness of the calculation method proposed in the present invention, the modulation signal waveforms of the overload demand MMC without and with the method proposed in the present invention are provided. Figure 8 and Fig. 9 They are P =2000 MW, Q =400 MW The optimal zero-sequence signal A phase upper bridge arm modulation signal obtained by the method proposed by the present invention and without using it S ap ( t) waveform diagram.

[0109] If a short circuit or other fault occurs in the power grid, the MMC will operate under overload conditions. P =2000 MW And reactive power is Q =400 MW When, from Figure 8 It can be seen that the modulation signal of the upper bridge arm of phase A S ap ( t )The maximum and minimum values ​​are 0.937 and -0.105 respectively. According to formula (3), the dynamic modulation signal can be obtained D s is 1.21, and the maximum modulation signal margin value is obtained by solving Mar It is -0.105. At this time, the modulation signal has been over-modulated. If it is not controlled at this time, the stability of MMC will be greatly affected.

[0110] Fig. 9 The modulation signal of the upper bridge arm of phase A when applying the optimal zero-sequence signal injection control strategy for MMC with overload demand is shown. S ap ( t ), it can be seen that when the output active power is P =2000 MW Reactive power is Q =400 MW When the optimal zero-sequence signal amplitude fitted by the present invention is A 3 ( P,Q ) and phase angle α 3 ( P,Q ) function to find the optimal zero-sequence signal amplitude A 3 =0.0958 and phase angle α 3 =4.64, injected into the control link, through Fig. 9 You can see the modulation signal of the upper bridge arm of phase A S ap ( t ) has a maximum and minimum value of 0.943 and 0.0357 respectively. According to formula (3), the dynamic modulation signal can be obtained D s is 0.9286, and the maximum modulation signal margin value is obtained by solving Mar When the optimal zero-sequence signal calculation method proposed in the present invention is not used, the MMC will be over-modulated and the MMC cannot operate stably. When the optimal zero-sequence signal calculation method proposed in the present invention is used, the maximum modulation signal margin value is Mar The value was improved by 0.1407, which enabled the MMC to operate stably and retained a certain modulation signal margin, proving the effectiveness of the method proposed in the present invention.

[0111] 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 calculating the optimal zero-sequence signal of MMC for overload operation requirements, characterized in that: The steps include: S1: Construct the optimal zero-sequence signal calculation operation domain; S2: Establishing the optimal zero-sequence signal solving mathematical model; S3: continuously iterate the active power and reactive power, input each group of active power and reactive power into the optimal zero-sequence signal solution mathematical model established in step S2, solve and obtain multiple groups of maximum modulation signal margin values ​​Mar, and the corresponding zero-sequence signal amplitude and phase angle, i.e., the optimal zero-sequence signal amplitude and phase angle, and output and record them to form OPA3(P,Q) table and OPAlf3(P,Q) table respectively. The OPA3(P,Q) table records each group of active power and reactive power and its corresponding optimal zero-sequence signal amplitude, and the OPAlf3(P,Q) table records each group of active power and reactive power and its corresponding optimal zero-sequence signal phase angle; S4: Use function fitting to fit the OPA3(P,Q) table and the OPAlf3(P,Q) table respectively to obtain the final fitting function formula, which is used for the subsequent rapid calculation of the amplitude and phase angle of the optimal zero-sequence signal.

2. The MMC optimal zero-sequence signal calculation method for overload operation requirements according to claim 1 is characterized in that: In step S1, the intersection of the boundary of the MMC limit operation domain and the positive direction of the P axis and the positive direction of the Q axis is selected to calculate the maximum active power P of the operation domain of the constructed optimal zero-sequence signal. max and the maximum reactive power Q max , P max , Q max , -P max , -Q max The optimal zero-sequence signal calculation operation domain is the actual converter operating area.

3. The MMC optimal zero-sequence signal calculation method for overload operation requirements according to claim 1 is characterized in that: In step S2, the optimal zero-sequence signal solving mathematical model is: Where, Mar represents the maximum modulation signal margin value; f mar (A 3, α3) represents the modulation signal margin function, which is expressed as follows: D s For a dynamically modulated signal: D s =max{|2S ap (t)-1|}(3) S ap (t) is the modulation signal of the upper bridge arm of phase A: S ap (t)=A0-A1cos(ωt+α1)-A2cos(2ωt+α2)-A3cos(3ωt+α3)(4) Among them, A0 represents the DC component of the modulated signal; A1 represents the amplitude of the fundamental component; A2 represents the amplitude of the double harmonic component; A3 represents the amplitude of the triple harmonic component; ω represents the angular frequency; t represents the time variable; α1 represents the initial phase angle of the fundamental component; α2 represents the initial phase angle of the double harmonic component; α3 represents the initial phase angle of the triple harmonic component.

4. The MMC optimal zero-sequence signal calculation method for overload operation requirements according to claim 1 is characterized in that: In step S3, the iteration process of active power and reactive power is as follows: S31: Set the initial active power P0, initial reactive power Q0, and iteration step size P step , Q step ; S32: inputting the active power and reactive power into the optimal zero-sequence signal solving mathematical model established in step S2 respectively to solve the maximum modulation signal margin value Mar and the amplitude and phase angle of the optimal zero-sequence signal; S33: Iterate the active power. The iteration method is as shown in formula (5). The initial value of n is 0, and n increases by 1 each time iterates: P n+1 =P n +P step (5) S34: Determine whether P is satisfied n ≥P max If not, continue to iterate in the manner of formula (5). During the iteration process of active power, the reactive power remains unchanged, and return to step S32 for solution; if yes, end the iteration of active power; S35: Iterate reactive power. The iteration method is as shown in formula (6). The initial value of m is 0, and m increases by 1 each time iterates: Q m+1 =Q m +Q step (6) S36: Determine whether Q is satisfied m ≥Q max If not, continue to iterate in the manner of formula (6), and change the active power to the initial active power P0, and return to steps S32 to S35 to continue executing until Q is satisfied. m ≥Q max , ending the iteration.

5. The MMC optimal zero-sequence signal calculation method for overload operation requirements according to claim 4 is characterized in that: The final fitting function in step S3 is as shown in formulas (7) and (8): Among them, Z0, B1, B2, B3, B4, C1, C2, C3, C4, and C5 are the coefficients of the A3(P,Q) fitting function; Z1, D1, D2, D3, E1, E2, E3, and E4 are coefficients of the α3(P,Q) fitting function.

6. The MMC optimal zero-sequence signal calculation method for overload operation requirements according to claim 4 is characterized in that: The initial active power P0 and initial reactive power Q0 are calculated by selecting a point on the boundary of the operation domain with the optimal zero-sequence signal, P step =100MW, Q step =100MW.

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