Method for calculating harmonic content of dynamic operation of psm high voltage power supply and storage medium

Abstract

The application discloses a PSM high-voltage power supply dynamic operation harmonic content calculation method and a storage medium, and comprises the following steps: S1, determining the number N of PSM high-voltage power supply sub-modules to be put into operation according to the DC side voltage demand; S2, updating the initial key parameters of the PSM high-voltage power supply harmonic calculation model constructed in advance according to the number of sub-modules to be put into operation, and further calculating the sub-module harmonic current waveform; S3, superimposing and phase-shifting the harmonic current waveform according to the number and position of the sub-modules to be put into operation to obtain the PSM high-voltage power supply harmonic generation amount; and S4, repeating S1-S3 to obtain the dynamic operation harmonic content when the PSM DC side voltage changes. The application takes into account the transformer impedance and can accurately calculate the influence of the commutation overlap foot on the energy storage element. The application comprehensively considers the topological structure of the PSM high-voltage power supply and the harmonic superposition of each fusion module, quickly obtains the harmonic generation amount injected by the PSM high-voltage power supply into the power grid during dynamic operation, and can be applied to the determination of key electrical parameters during the later equipment upgrading and reconstruction.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of each step of nuclear fusion type pulse power supply, and particularly relates to a calculation method of dynamic operation harmonic content of a PSM high-voltage power supply and a storage medium. BACKGROUND

[0002] The PSM high-voltage power supply is a key component of an EAST fusion device electron cyclotron resonance heating system (ECRH) and plays an important role in the process of nuclear fusion of a Tokamak device. The PSM high-voltage power supply adopts a three-phase six-pulse rectifier bridge as a basic unit for step-by-step power supply operation. The primary windings of two sets of transformers on the network side are respectively extended by 7.5 degrees, and the secondary windings of the three phases of 32 sets are connected in a star-odd angle and even alternating manner to form a 24-pulse rectifier power supply structure.

[0003] The PSM high-voltage power supply meets the DC side voltage requirement value through the star-shaped and triangular cross-supply of the secondary windings of the sub-modules in a step-by-step operation mode. The operating condition changes frequently, and the calculation of the harmonic content is complex. The existing harmonic content calculation method cannot meet the requirements of the nuclear fusion step-by-step pulse power supply.

[0004] The existing rectifier harmonic content calculation method has disadvantages and cannot be directly applied to the harmonic calculation of the PSM high-voltage power supply of the fusion:

[0005] ① The transformer winding connection mode is single. The traditional research is usually based on the research under the condition of star connection of the secondary windings of the transformer. The secondary windings of the transformer of a single PSM high-voltage power supply are 32 sets of star connection and 32 sets of triangular connection.

[0006] ② The error caused by ignoring the system impedance. The impedance of the PSM fusion power supply transformer is large, and the influence of the commutation overlapping feet on the energy storage elements such as capacitors cannot be ignored, otherwise the waveform calculated by the network side current has a large error from the true result.

[0007] ③ The research volume is limited. The research on the rectifier includes 12-pulse, 18-pulse, and even 44-pulse single multi-pulse rectifiers. There are 64 sets of rectifiers in the secondary side of a PSM fusion power supply, and the operating condition is flexible. The calculation of the 10kV network side harmonic content is complex. SUMMARY

[0008] The calculation method of the dynamic operation harmonic content of the PSM high-voltage power supply provided by the present application can solve the above problems, and the current waveform diagram of the transformer winding is drawn under the conditions of star connection and triangular connection of the transformer, respectively, to obtain high-precision harmonic current spectrum analysis results.

[0009] To achieve the above object, the present application adopts the following technical scheme:

[0010] A method for calculating the harmonic content of a PSM high-voltage power supply in dynamic operation, comprising the following steps,

[0011] S1, determining the number N of PSM high-voltage power supply submodules in operation according to the voltage requirement on the DC side;

[0012] S2, updating the initial key parameters of the PSM high-voltage power supply harmonic calculation model constructed in advance according to the number of submodules in operation, and further calculating the harmonic current waveform of the submodules;

[0013] S3, superimposing and phase-shifting the harmonic current waveforms according to the number and position of the submodules in operation to obtain the harmonic generation amount of the PSM high-voltage power supply;

[0014] S4, repeating S1-S3 to obtain the dynamic operation harmonic content when the DC side voltage of the PSM changes.

[0015] Further, the S2 updates the initial key parameters of the PSM high-voltage power supply harmonic calculation model constructed in advance according to the number of submodules in operation, and further calculates the harmonic current waveform of the submodules, and the steps are as follows,

[0016] S21, determining the number N of PSM high-voltage power supply submodules in operation according to the voltage requirement on the DC side, and calculating the number N of Y-type and Δ connection of the transformer secondary winding Y and N △ , and the position of the transformer connected thereto;

[0017] S22, updating the known calculation model parameters according to the number N of submodules in operation, and setting the unknown parameters of the harmonic calculation model, i.e., the commutation overlap angles of the star connection and the delta connection are respectively Y and γ Δ , the initial phase angles of the commutation voltage are respectively Y +π and θ Δ , the DC voltage values at the commutation time are respectively u Y and u Δ , and the grid-side current values at the commutation time are respectively i Y and i Δ ;

[0018] S23, listing the circuit equivalent expressions in the commutation overlap state and the state after the commutation according to the electrical engineering basis and Kirchhoff's law, determining the equivalent equation set combining the commutation conditions, and solving the unknown parameters set in the calculation model by using matlab programming;

[0019] S24, drawing the current waveforms of the single winding Y-type and Δ connection of the transformer secondary winding according to the solved model parameters;

[0020] S25, according to the sub-module number of Y-shaped and delta connection of the secondary winding, and the transformer position, the harmonic current is superimposed and the phase is shifted, and the harmonic current content of the 10kV side is obtained.

[0021] Further, the step S2 further comprises:

[0022] S26, when the DC side voltage demand changes, repeating steps S21-S25 to calculate the harmonic current content of the dynamic operation of the PSM high voltage power supply.

[0023] Further, the PSM high voltage power supply harmonic calculation model is constructed as follows,

[0024] S201, set the commutation initial parameters;

[0025] The commutation overlap angles under star and delta connection are γ Y and γ Δ , respectively, the initial phase angles of commutation voltage A under star and delta connection are π+θ Y and θ Δ , respectively; the DC voltage values at the commutation time under star and delta connection are u Y and u Δ , respectively; the current values flowing through the diode under the conduction state at the commutation time on the AC side under star and delta connection are i Y and i Δ , respectively.

[0026] S202, construct the mathematical model during the commutation overlap period;

[0027] Star connection:

[0028] Taking the example of D5, D6 and D1 being turned on at the same time and D5 being turned off, the mathematical model of the star connection commutation overlap state of the transformer winding is analyzed, the commutation overlap angle is set as γ Y , the capacitor voltage at the beginning of commutation u Y0cap (0)=u Y , the initial value of commutation i Yc (0)=i Y , i Ya (0)=0, i Yb (0)=-i Y , the transformer is y5 connection mode, and the initial phase of A phase at the beginning of commutation is π+θ Y ;

[0029] According to Kirchhoff's law,

[0030]

[0031] Where u Y0capis the instantaneous voltage of the capacitor during the commutation overlap process when the transformer winding is connected in star.

[0032] Delta connection:

[0033] The mathematical model of the commutation overlap state of the transformer winding connected in delta is analyzed by taking the case that D5, D6 and D1 are simultaneously turned on and D5 is about to be turned off as an example. Let the commutation overlap foot be γ Δ , the capacitor voltage u Δ0cap (0) = u Δ at the initial moment of commutation, the current i Δx (0) = 0, i Δy (0) = -i Δ , i Δz (0) = i Δ , and the initial phase of the A phase at the initial moment of commutation is θ Δ .

[0034]

[0035] In the formula, u Δ0cap is the instantaneous voltage of the capacitor during the commutation overlap process when the transformer winding is connected in delta.

[0036] S203, a mathematical model after commutation is constructed, wherein the voltage and current parameters at the end of the commutation overlap state are the initial values of the post-commutation process;

[0037] Star connection:

[0038] When D1 and D6 are turned on and D5 is turned off, according to Kirchhoff's law,

[0039]

[0040] In the formula, u Y1cap is the instantaneous voltage of the capacitor after commutation when the transformer winding is connected in star.

[0041] Delta connection:

[0042] When D1 and D6 are turned on and D5 is turned off, according to Kirchhoff's law,

[0043]

[0044] In the formula, there is When the transformer winding is connected in delta, u Δ1cap is the instantaneous voltage of the capacitor in the commutation end state.

[0045] S204, a commutation condition is constructed:

[0046] Star connection:

[0047] i YC (γ Y )=0

[0048]

[0049]

[0050]

[0051] Triangle connection:

[0052] i Δz (γ Δ )=0

[0053]

[0054]

[0055]

[0056] In another aspect, the application also discloses a computer readable storage medium, which stores a computer program, and the computer program, when executed by a processor, causes the processor to execute steps of the above method.

[0057] From the above technical solution, the PSM high-voltage power harmonic generation amount calculation method has the following beneficial effects:

[0058] (1) The application considers the star connection and the triangle connection of the secondary winding of the transformer and the impedance thereof, and can accurately calculate the influence of the commutation overlapping foot on the energy storage element;

[0059] (2) The application takes the star transformer and the triangle transformer as the power supply unit respectively, studies the harmonic generation process of the rectifier, and further analyzes the harmonic content of the PSM high-voltage power when in operation;

[0060] (3) The application can flexibly calculate the harmonic content under different operating conditions according to the number of PSM high-voltage power sub-modules in operation;

[0061] (4) The application is established on the basis of the circuit topology structure, and does not use an approximate algorithm in the calculation and solving process, so that the accuracy requirement is guaranteed;

[0062] (5) The application takes the mathematical model of the PSM high-voltage power sub-module in operation as the research basis, and can quickly calculate the harmonic content of the PSM high-voltage power according to the operation condition of the actual PSM fusion high-voltage power;

[0063] (6) The PSM high-voltage power supply topology and the harmonic superposition of each fusion module are comprehensively considered, the harmonic generation amount of the PSM high-voltage power supply dynamic operation injected into the power grid is quickly obtained, and the method can be applied to the determination of key electrical parameters in the later equipment upgrading. BRIEF DESCRIPTION OF DRAWINGS

[0064] Figure 1 is a schematic diagram of a LHCD 4.6GHz cathode high-voltage power supply;

[0065] Figure 2 is an equivalent circuit of a star connection of a transformer secondary winding;

[0066] Figure 3 is an equivalent circuit of a delta connection of a transformer secondary winding;

[0067] Figure 4 is a flowchart of the method of the present application. DETAILED DESCRIPTION

[0068] To make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments.

[0069] Electron cyclotron resonance heating (ECRH) plays an important role in the inductive current heating and driving of a tokamak, and a pulsed step modulation (PSM) high-voltage power supply is a key component of an ECRH system and provides energy for the normal operation of the system.

[0070] As shown in Figure 1 , the PSM high-voltage power supply is a collection of 64 three-phase six-pulse rectifier units. The transformer secondary windings are connected in a delta and star connection to supply power to the rectifiers. Because the transformer has a large leakage inductance, it will form a commutation overlap foot during the commutation of the rectifier. Because the PSM high-voltage power supply has energy storage elements such as large inductance (system impedance) and large capacitance (rectifier DC side), the calculation accuracy of the commutation overlap foot is very high.

[0071] In view of the above problems, the following is a calculation method of the harmonic generation amount of the PSM high-voltage power supply according to the present embodiment, comprising the following steps,

[0072] According to the DC voltage requirement, the number N of PSM high-voltage power supply submodules in operation is determined;

[0073] The initial key parameters of the PSM high-voltage power supply harmonic calculation model constructed in advance are updated according to the number of submodules in operation, and the submodule harmonic current waveform is further calculated;

[0074] According to the number and position of the sub-modules, the harmonic current waveform is superimposed and phase-shifted to obtain the harmonic generation amount of the PSM high-voltage power supply.

[0075] When the voltage at the DC side of the PSM changes, the above steps are repeated to obtain the dynamic operation harmonic content.

[0076] The initial key parameters of the PSM high-voltage power supply harmonic calculation model are updated according to the number of sub-modules in operation, and further operation is performed to obtain the sub-module harmonic current waveform.

[0077] 1) According to the DC side voltage demand, the number of PSM high-voltage power supply sub-modules in operation N is determined, and the number of Y-type and Δ-connected transformer secondary winding N Y and N △ and the position of the transformer connected thereto are calculated.

[0078] 2) According to the number of sub-modules in operation, the known calculation model parameters (phase voltage of transformer winding, load resistance R, DC capacitor C, transformer leakage inductance L Y and L △ ) are updated, and the unknown parameters of the harmonic calculation model are set, i.e. the commutation overlap angles of star connection and delta connection are γ Y and γ Δ , the initial phase angles of commutation voltage are θ Y + π and θ Δ , the DC voltage values at the commutation time are u Y and u Δ , and the grid-side current values at the commutation time are i Y and i Δ ), wherein the A-phase voltage u Δa = U Δ sin(ωt+θ Δ ), u Ya = U Y sin(ωt+θ Y + π), the B-phase and C-phase voltages satisfy the electrical basic law.

[0079] 3) According to the electrical basis and Kirchhoff's law, the equivalent expressions of the circuit in the commutation overlap state and the state after the commutation are listed, the equivalent equation set is determined in combination with the commutation condition, and the unknown parameters set in the calculation model are solved by using matlab programming;

[0080] 4) According to the solved model parameters, the current waveforms flowing through the Y-shaped and Δ-connected single windings of the transformer secondary winding are drawn respectively.

[0081] 5) According to the sub-winding Y-shaped and delta connection input sub-module quantity, and its position in the transformer, the harmonic current superposition and phase shift are carried out, and the 10kV side harmonic current content is obtained.

[0082] 6) When the DC side voltage demand changes, the harmonic current content after the change is calculated by repeating steps 1) to 5).

[0083] The construction steps of the calculation model of the PSM high-voltage power supply harmonic generation amount are as follows:

[0084] S201, set the commutation initial parameters;

[0085] The commutation overlap angles under star and delta connection are γ Y and γ Δ , respectively, the commutation voltage A initial phase angles under star and delta connection are π+θ Y and θ Δ , respectively; the DC voltage values at the commutation time under star and delta connection are u Y and u Δ , respectively; the current values flowing through the diode under the conduction state at the commutation time on the AC side under star and delta connection are i Y and i Δ , respectively.

[0086] Wherein, Figure 2 is the transformer secondary winding star connection equivalent circuit and Figure 3 is the transformer secondary winding delta connection equivalent circuit.

[0087] As shown in Figure 1 , the PSM high-voltage power supply adopts two kinds of transformers D(+7.5°)y5d0 and D(-7.5°)y7d2, the transformer capacity is 2000kVA, the primary side realizes phase shift ±7.5°, the secondary side has 32 groups of windings, which are connected in star and delta connection to supply power to the rectifier, and in the running process, the star and delta cross power supply makes the sub-modules run step by step to realize the requirement of DC voltage amplitude. In order to simplify the calculation, the components with less influence on the circuit (diode voltage drop, current limiting inductance, stray inductance, etc.) are ignored in the equivalent circuit. The following takes the secondary winding y5d0 as an example for analysis.

[0088] As shown in Figure 2 and Figure 3As shown, the sub-side winding y5 and d0 connection mode are used to supply alternating current for the rectifier. The rectifier adopts a three-phase bridge rectifier structure, and D1-D6 are power diodes. Three diodes (D1, D3, and D5) with cathodes connected together are a common cathode group, and three diodes (D4, D6, and D2) with anodes connected together are a common anode group. The conduction sequence in normal operation is D1-D2-D3-D4-D5-D6. C is a power capacitor on the DC side of the rectifier. R is a resistor loaded on the sub-module. L Y and L △ are the equivalent leakage inductances of the transformer. Figure 2 The transformer sub-side adopts Y-shaped connection (y5), and the common connection point is N. The other ends of phases A, B, and C are connected to the three-phase bridge arm midpoints, respectively. Figure 3 The transformer sub-side adopts Δ connection (d0), and phases A, B, and C are connected to the three-phase bridge arm midpoints, respectively.

[0089] S202, a mathematical model during commutation overlap is constructed.

[0090] Star connection:

[0091] The mathematical model of the star connection of the transformer winding during commutation overlap is analyzed by taking the simultaneous conduction of D5, D6, and D1 and the imminent turn-off of D5 as an example. Let the commutation overlap foot be γ Y , the capacitor voltage u Y0cap (0) at the beginning of commutation be u Y , the initial value i Yc (0) of the current be i Y , i Ya (0) = 0, i Yb (0) = -i Y , the transformer be in y5 connection mode, and the initial phase of phase A at the beginning of commutation be π+θ Y .

[0092] According to Kirchhoff's law, we have

[0093]

[0094] where u Y0cap is the instantaneous voltage of the capacitor during the star connection of the transformer winding during commutation overlap.

[0095] Delta connection:

[0096] The mathematical model of the delta connection of the transformer winding during commutation overlap is analyzed by taking the simultaneous conduction of D5, D6, and D1 and the imminent turn-off of D5 as an example. Let the commutation overlap foot be γ Δ , the capacitor voltage u Δ0cap (0) at the beginning of commutation be u Δ , and the initial value i Δx(0) = 0, i Δy (0) = -i Δ , i Δz (0) = i Δ The initial phase of phase A at the commutation initial moment is θ Δ ;

[0097]

[0098] wherein u Δ0cap is the instantaneous voltage of the capacitor at the commutation end process when the transformer winding is connected in delta.

[0099] S203, constructing a mathematical model after commutation, wherein the voltage and current parameters after commutation are initial values of the post-commutation process;

[0100] Star connection:

[0101] When D1 and D6 are turned on and D5 is turned off, according to Kirchhoff's law, there is

[0102]

[0103] wherein u Y1cap is the instantaneous voltage of the capacitor at the commutation end process when the transformer winding is connected in star.

[0104] Delta connection:

[0105] When D1 and D6 are turned on and D5 is turned off, according to Kirchhoff's law, there is

[0106]

[0107] wherein there is When the transformer winding is connected in delta, u Δ1cap is the instantaneous voltage of the capacitor at the commutation end process.

[0108] S204, constructing a commutation condition

[0109] Star connection:

[0110] i YC (γ Y ) = 0 (1)

[0111]

[0112]

[0113]

[0114] Delta connection:

[0115] iΔz (γ Δ )=0 (5)

[0116]

[0117]

[0118]

[0119] First, the mathematical model during commutation overlap and after commutation is calculated, and constant coefficient non-homogeneous differential equations of capacitor instantaneous voltage (u Δ0cap ,u Y0cap ,u Δ1cap and u Y1cap ) are obtained; then, the capacitor instantaneous voltage is brought into the mathematical model during commutation overlap and after commutation; finally, by using (1)-(4) as the commutation condition of star connection of the secondary winding of the transformer, (5)-(8) as the commutation condition of delta connection of the secondary winding of the transformer, the program can be used to obtain the key unknown parameters of the mathematical model set in advance, which is brought into the capacitor instantaneous voltage and other mathematical models, and then the current waveforms during commutation overlap and after commutation are drawn respectively.

[0120] The above mathematical model is analyzed only in the case of commutation overlap (D5, D6 and D1 are turned on at the same time, and D5 is about to be turned off) and after commutation (D1 and D6 are turned on, and D5 is turned off), and the set unknown parameters can be obtained. In the balanced state of three-phase voltage, the transformer winding current waveforms obtained by using power diodes in the remaining commutation overlap and after commutation are the same. By drawing the current waveforms during the remaining commutation overlap and after commutation, the sub-module current waveforms can be obtained, and the periodic characteristics are presented.

[0121] According to the number and position of the running sub-modules, the harmonic current waveforms are superimposed and phase-shifted to obtain the harmonic generation amount of the PSM high-voltage power supply.

[0122] When the voltage at the DC side of the PSM changes, S1-S3 are repeated to obtain the dynamic running harmonic content. As shown in Fig. Figure 1 In the dynamic running of the fusion PSM high-voltage power supply, the load is resistive and basically constant. The number of transformer secondary sub-modules to be put into operation is flexibly put into operation according to the actual working requirements of the DC side voltage, so that the values of the key electrical elements of the fusion PSM high-voltage power supply unit change frequently, and cannot be simply linearly superimposed. By using Figure 4 for calculation, the dynamic running harmonic generation amount of the fusion PSM high-voltage power supply can be quickly and accurately obtained.

[0123] In summary, the embodiment of the present application has the following characteristics:

[0124] (1) The present application considers star connection and triangle connection of secondary winding of transformer and its impedance, and can accurately calculate the influence of commutation overlap foot on energy storage element;

[0125] (2) The present application takes star and triangle transformer as power supply unit respectively, studies the harmonic generation process of rectifier, and further analyzes the harmonic content of PSM high voltage power supply when it is put into operation;

[0126] (3) The present application can flexibly calculate the harmonic content under different operating conditions according to the number of PSM high voltage power supply sub-modules put into operation;

[0127] (4) The present application is based on the circuit topology structure, and does not use approximate algorithm in the calculation process, which guarantees the accuracy requirement;

[0128] (5) The present application takes the mathematical model of PSM high voltage power supply sub-module put into operation as the research basis, and can quickly calculate the harmonic content of PSM high voltage power supply according to the actual PSM fusion high voltage power supply operation condition;

[0129] (6) The present application comprehensively considers the topology structure of PSM high voltage power supply and the harmonic superposition of each fusion module, quickly obtains the harmonic generation amount of PSM high voltage power supply dynamic operation injected into the power grid, and can be applied to the determination of key electrical parameters in the later equipment upgrading and reconstruction.

[0130] The above examples are only used to illustrate the technical solutions of the present application, but not to limit it; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solution deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for calculating the dynamic harmonic content of a PSM high-voltage power supply, characterized by the following steps: S1. Determine the number N of PSM high-voltage power supply sub-modules to be put into operation according to the voltage requirement on the DC side; S2. Update the initial key parameters of the PSM high-voltage power supply harmonic calculation model constructed in advance according to the number of sub-modules put into operation, and further calculate the sub-module harmonic current waveform, as follows: S21, according to the DC side voltage demand, determine the PSM high-voltage power supply sub-module operation quantity N, and calculate the number N of Y type and delta connection of transformer secondary winding Y and N △ , and the position of the transformer connected thereto; S22, updating the known calculation model parameters according to the number N of sub-modules in operation, and setting the unknown parameters of the harmonic calculation model, i.e. the commutation overlap angles under star connection and delta connection, respectively and , the initial phase angles of the A-phase commutation voltage are respectively and ; the DC voltage values at the commutation time are respectively and ; and the grid-side current values at the commutation time on the AC side are respectively and ; S23. According to the electrical basis and Kirchhoff's law, list the equivalent circuit expressions in the commutation overlap state and the state after commutation, determine the equivalent equation set in combination with the commutation condition, and solve the unknown parameters of the calculation model set by using matlab programming; S24. According to the model parameters solved, draw the current waveform flowing through the Y-shaped and delta-connected single winding of the transformer secondary side; S25. According to the number of sub-modules put into operation in Y-shaped and delta-connected secondary winding and the position of the transformer, superimpose and phase-shift the harmonic currents to obtain the harmonic current content on the 10kV side; S3. According to the number and position of the sub-modules put into operation, superimpose and phase-shift the harmonic current waveform to obtain the harmonic generation amount of the PSM high-voltage power supply; S4. When the DC voltage of the PSM changes, repeat S1-S3 to obtain the dynamic harmonic content thereof.

2. The method of claim 1, wherein: The step S2 further comprises: S26. When the DC voltage requirement changes, repeat steps S21-S25 to calculate the harmonic current content of the PSM high-voltage power supply in dynamic operation.

3. The method of claim 1, wherein the PSM high voltage power supply dynamic operating harmonic content is calculated by: ###0001### where: V = the voltage of the PSM high voltage power supply; f = the frequency of the PSM high voltage power supply; and n = the harmonic number. The steps for constructing the PSM high-voltage power supply harmonic calculation model are as follows, S201. Set the commutation initial parameters; The commutation overlap angle under star connection and delta connection is respectively and The initial phase angle of commutation voltage A under star connection and delta connection is respectively and ; The DC voltage values at the commutation time under star connection and delta connection are respectively and ; The current values flowing through the diodes in the on state at the commutation time of the AC side under star connection and delta connection are respectively and ; the rectifier used adopts a three-phase bridge rectifier structure, D1 to D6 are power diodes, three diodes D1, D3 and D5 with cathodes connected together are a common cathode group, three diodes D4, D6 and D2 with anodes connected together are a common anode group, the conduction sequence in normal operation is D1, D2, D3, D4, D5, D6; the cathode of diode D4 is connected with the anode of diode D1, the cathode of diode D6 is connected with the anode of diode D3, and the cathode of diode D2 is connected with the anode of diode D5; S202. Construct the mathematical model during the commutation overlap period; Star connection: Taking D5, D6 and D1 simultaneously conducting and D5 being about to be turned off as an example, a mathematical model of the transformer winding star connection commutation overlap state is analyzed, the commutation overlap angle is set as , the capacitor voltage at the commutation start time is , the commutation initial value is , , , the transformer is in y5 connection mode, the initial phase of the A phase at the commutation initial time is ; According to Kirchhoff's law, wherein is the instantaneous voltage of the transformer winding star-connected commutation overlap process capacitor; C is the power capacitor on the DC side of the rectifier; R is the resistance loaded on the sub-module; L Y is the equivalent leakage inductance of the transformer winding star connection; Delta connection: Taking D5, D6 and D1 simultaneously conducting and D5 being about to be turned off as an example, a mathematical model of the commutation overlap state of the transformer winding in delta connection is analyzed, the commutation overlap angle is set as , the capacitor voltage at the initial commutation moment is , the current is , , , and the initial phase of the A phase at the initial commutation moment is . wherein , is the instantaneous voltage of the phase-change overlap process capacitor when the transformer winding is connected in delta, is the equivalent leakage inductance of the transformer winding connected in delta. S203. Construct the mathematical model after commutation, wherein the voltage and current parameters at the end of the commutation overlap state are the initial values of the process after commutation; Star connection: When D1 and D6 are turned on and D5 is turned off, according to Kirchhoff's law, wherein is the instantaneous voltage of the capacitor at the end of the commutation when the transformer winding is connected star. Delta connection: When D1 and D6 are turned on and D5 is turned off, according to Kirchhoff's law, wherein there is When the transformer winding is connected in delta, Vc is the instantaneous voltage of the capacitor at the end of commutation. S204. Construct the commutation condition: Star connection: Delta connection: 。 4. A computer readable storage medium storing a computer program, wherein the computer program is executed by a processor to make the processor execute the steps of the method according to any one of claims 1 to 3.

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