Modulation control method based on modular multilevel converter

Abstract

The invention provides a control modulation method based on a modular multilevel converter, which is mainly applied to a ship medium-voltage power system and comprises the following steps of: S1, establishing a modular multilevel converter model, and deducing bridge arm voltage and circulating current so as to couple dead-beat control and SVPWM (Space Vector Pulse Width Modulation); s2, discretization derivation is carried out on the established modular multilevel converter model, deadbeat control is improved, and a space vector waveform required by modulation is generated through the improved deadbeat control; s3, improving classical space vector modulation, and modulating a voltage space vector generated by the improved deadbeat control to obtain an optimized switching state; and S4, analyzing the improved dead-beat control and space vector modulation to obtain an optimal switching state and switching time so as to improve the robustness of the closed-loop modular multilevel system.

Description

Technical Field

[0001] The present invention relates to the technical field of medium voltage power system converters for ships, and in particular to a control modulation method based on a modular multi-level converter. Background Art

[0002] Historically, there have been three great changes in the ship power system. Researchers call these three most important ship power revolutions the three revolutions of the ship power system. In the early 19th century, with the rise of the industrial revolution, the ship power system experienced the first revolution, which was marked by the steam engine replacing the traditional wind power and manpower as a new power source for ships. In the mid-20th century, the power system of the ship power system gradually evolved from steam engines to internal combustion engines, diesel engines and gas turbines, which made the power system have a qualitative leap again. This revolution is called the second revolution. The times are developing rapidly, science and technology are also advancing rapidly, and human beings are increasingly developing the ocean, forcing the electric energy required by ships to increase significantly. As a result, higher requirements are put forward for the performance of ship power, and increasingly stringent requirements are put forward for the maneuverability, reliability, fuel economy, etc. of ships.

[0003] In the design of traditional ship propulsion systems, the mechanical propulsion system and the power system are independent of each other, which easily leads to problems such as low energy utilization and low system power density. Modern ship integrated power systems combine the ship propulsion system with the power system in the form of electrical energy. This improvement significantly improves the power density and greatly saves fuel usage. It can flexibly control the access method of renewable energy, easily achieve precise control of the entire ship, and is conducive to the comprehensive utilization of resources throughout the ship. Summary of the invention

[0004] According to the technical problems raised above, a control modulation method based on modular multi-level converter is provided. The present invention constructs a delay-compensated deadbeat control system, obtains a modulation voltage vector with a small error, performs point optimization on the high-level space vector, uses a direct modulation method to control and find the optimal switch state and switch time, performs bubble sorting on the sub-module capacitor voltage, determines the direction of the bridge arm current in the previous cycle, distributes the signal, improves the stability of the sub-module voltage, and significantly reduces the calculation complexity.

[0005] The technical means adopted by the present invention are as follows:

[0006] A control modulation method based on a modular multilevel converter, comprising:

[0007] S1. Establish a modular multilevel converter model and derive the bridge arm voltage and circulating current to couple the deadbeat control and SVPWM modulation;

[0008] S2. Discretize and derive the established modular multilevel converter model, improve the deadbeat control, and generate the space vector waveform required for modulation through the improved deadbeat control;

[0009] S3, improving the classical space vector modulation, and modulating the voltage space vector generated by the improved deadbeat control to obtain an optimized switching state;

[0010] S4. Analyze the improved deadbeat control and space vector modulation to obtain the optimal switching state and switching time to improve the robustness of the closed-loop modular multi-level system.

[0011] Furthermore, step S1 specifically includes:

[0012] S11. In order to maintain the stability of the DC busbar on the output side of the MMC rectifier, the total number of sub-modules put into operation in the upper and lower bridge arms of each phase at any time remains unchanged; the sum of the sub-modules put into operation in the upper and lower bridge arms is calculated using the following formula:

[0013] N=n jp +n jn

[0014] Among them, n jp 、n jn It indicates the number of submodules put into operation in the upper and lower bridge arms of phase j at a certain moment;

[0015] S12. According to the principle of the half-bridge submodule, the working state of the submodule is expressed as:

[0016]

[0017] Wherein, j = a, b, c, a, b, c represent three-phase bridge arms; x = p, n, p, n represent upper and lower bridge arms; y = 1, 2...N, N represents the number of submodules;

[0018] S13, calculate the capacitor voltage of the yth submodule of the j-phase x-bridge arm, and the calculation formula is as follows:

[0019]

[0020] Among them, C sm Indicates the capacitance value of the submodule; i jx It represents the current value flowing through the x-bridge arm of phase j;

[0021] S14, calculate the output voltage of the yth submodule of the j-phase x-bridge arm, and the calculation formula is as follows:

[0022] u jxy =S jxy ·u sm

[0023] S15, calculate the output voltage of the submodule of the upper and lower bridge arms of phase j, and the calculation formula is as follows:

[0024]

[0025] Among them, u jpy represents the voltage value of the yth submodule of the upper bridge arm of phase j; u jny Indicates the voltage value of the yth submodule of the lower bridge arm of phase j;

[0026] S16. According to Kirchhoff's voltage law, the relationship between the upper and lower bridge arm voltages and the grid side voltage is expressed as:

[0027]

[0028] Among them, the above formula is the expression of the bridge arm voltage to the j-phase voltage; U dc Indicates the DC bus output voltage; u jp ,u jn Represents the upper and lower bridge arm voltages of phase j; i jp ,i jn represents the upper and lower bridge arm currents of phase j; R represents the bridge arm equivalent resistance; L represents the bridge arm equivalent inductance;

[0029] S17, adding the two formulas in step S16 to express the relationship between the AC voltage and the upper and lower bridge arm voltages and bridge arm currents, as follows:

[0030]

[0031] S18, subtract the two formulas in step S16 to express the DC side voltage of the MMC rectifier, as follows:

[0032]

[0033] S19. In the MMC rectifier, the inductance of the upper and lower bridge arms of each phase is equal, and the AC current of each phase is evenly distributed between the upper and lower bridge arms. According to Kirchhoff's current law, the relationship between the upper and lower bridge arm currents and the circulating current is obtained as follows:

[0034]

[0035] S110, subtract the two formulas in step S19 to obtain the grid-side current i j ,as follows:

[0036] i j =i jp -i jn

[0037] S111, add the two formulas in step S19 to obtain the circulation icirj ,as follows:

[0038]

[0039] Among them, i jp Represents the upper bridge arm current; i jn Represents the lower arm current.

[0040] Further, step S2 specifically includes:

[0041] S21, MMC three-phase symmetry, taking phase A as an example, the modular multilevel converter model established in step S1 is Euler discretized to obtain the discrete equation as follows:

[0042]

[0043] Among them, u ap (k) and u an (k) represents the sum of all module voltages that should be turned on in the upper and lower bridge arms in the kth control cycle; i ap (k), i an (k) represents the upper and lower bridge arm currents that should be turned on in the kth control cycle; u a (k) represents the AC side voltage in the kth control cycle; U dc (k) represents the DC side voltage of the kth control cycle; T represents the system control cycle;

[0044] S22. According to the classic deadbeat control theory, predict the current value at the next moment as follows:

[0045]

[0046] Among them, i anref It indicates the reference value given by the outer loop, and different control targets can be achieved by coordinating different outer loop control methods;

[0047] S23. Calculate the reference value of circulating current. The calculation formula is as follows:

[0048]

[0049] Among them, K p Indicates the proportionality coefficient; K i represents the integral coefficient; s represents the Laplace domain complex variable; U cref Indicates the submodule capacitor voltage reference value; U cave Indicates the average value of the capacitor voltage of phase A at the current moment;

[0050] S24. Based on the above steps, a modular multi-level classic deadbeat control model is constructed as follows:

[0051]

[0052] S25, analyzing the source of error, predicting the current at time k+2 through an improved algorithm, controlling the k+1 cycle pulse to be generated before the k+1 cycle, thereby avoiding the influence of control delay, and pushing the discrete equation obtained in step S21 back one step, as follows:

[0053]

[0054] S26, Under normal steady state, the DC side voltage of MMC rectifier is U dc With a small fluctuation near the steady-state value, the DC side voltage U of the MMC rectifier in the k+1th control cycle is obtained. dc (k+1), as follows:

[0055] U dc (k+1)=U dcref

[0056] Among them, U dcref Indicates the DC side voltage reference value;

[0057] S27, to obtain accurate prediction of u a (k+1), using the interpolation prediction method to estimate, given that T is small, it is usually believed that u a The increment in the k-1 period is equal to the increment in the k period, that is:

[0058] u a (k+1)-u a (k) = u a (k)-u a (k-1)

[0059] So we get u a The expression for (k+1) is:

[0060] u a (k+1)=2u a (k)-u a (k-1)

[0061] S28, replace i in the formula of step S25 ap (k+1) and i an (k) Derived from the formula in step S21:

[0062]

[0063] S29, substitute i ap (k+1), i an (k+1),u a (k+1), U dc(k+1), the improved deadbeat control equation is as follows:

[0064]

[0065] S210, based on the upper and lower bridge arm voltages obtained in step S29, the AC a-phase reference voltage is solved by the discrete equation in step S21 as follows:

[0066]

[0067] S211, get the AC measurement a phase reference voltage, b phase, c phase and so on, perform Clark transformation to get the space vector voltage U α , U β ,as follows:

[0068]

[0069] Among them, U a , U b , U c They represent the reference voltages of the three phases a, b, and c on the AC side respectively.

[0070] Further, step S3 specifically includes:

[0071] S31, calculate the level of the reference voltage vector, the calculation formula is as follows:

[0072]

[0073] Where N represents the number of levels of modular multi-level; v ref represents the reference voltage vector;

[0074] S32. Find the center of the sub-hexagon:

[0075] After determining the layer and section, determine the candidate center; for N-level MMC, for example, the candidate reference center vector of sector 1 will be (L-1,0,0), (L-1,1,0) to (L-1,L-1,0); calculate the absolute distance value between the reference vector and the candidate center; the minimum distance gives the nearest center, which is the reference center vector. The coordinates of the reference center vector are (S a ,S b ,S c );

[0076] S33, perform vector mapping in the 0-60 degree sector, after determining the optimal starting point, move the origin O of the reference vector to the calculated new origin N, and calculate the mapping vector as follows:

[0077]

[0078] S34, the reference vector, the mapping vector, and the simplified reference vector form a vector triangle, and the simplified reference vector is calculated as follows:

[0079]

[0080] S35. Calculate the time allocated by the vector. The simplified reference vector has the same action time as the initial reference vector. The new reference vector is obtained according to the volt-second balance principle and the nearest three vector method as follows:

[0081]

[0082] Among them, T s Represents the sampling period, T x , T y They represent the action time for synthesizing the simplified reference vector, T z represents the action time of the zero vector;

[0083] S36. Let the angle between the simplified reference vector and the α axis be θ. According to the sine theorem, we can get:

[0084]

[0085] S37, calculate and obtain the action time of each vector, as follows:

[0086]

[0087] S38, calculate the time allocated for the zero vector, as follows:

[0088]

[0089] S39, if T x +T y >T s When , overmodulation processing is required, so:

[0090]

[0091] S310, after determining the vector switching sequence and switching time, seven-segment SVPWM modulation is adopted. In order to reduce the switching frequency of the switching device, the selection of the switching sequence follows the following rules:

[0092] Rule 1: In the vector diagram, when the reference vector is transferred from one sector to another, there is no or only a minimum number of switches.

[0093] Rule 2: In a certain sector, when the seven-segment switch sequentially switches from one state to another, only one phase changes, and multiple phases cannot change crosswise;

[0094] S311. If the reference vector falls on the second sector of the small hexagon, use V 0 、V 2 、V 3 、V 7 Go to Synthesize V ref (k+1), the starting vector is V 0 (530), the switching sequence is as follows:

[0095] 530→540→640→641→640→540→530

[0096] The vector durations of the corresponding states are:

[0097] 530→540→640→641→640→540→530

[0098]

[0099] S312, calculating the switching time of the upper bridge arm of phase A, as follows:

[0100]

[0101] S313, calculating the switching time of the upper bridge arm of phase B, as follows:

[0102]

[0103] S314, calculating the switching time of the upper bridge arm of phase C, as follows:

[0104]

[0105] S315, determining a specific switch switching time according to different query graphs of the vectors, and generating a switch signal.

[0106] Further, step S4 specifically includes:

[0107] S41, in order to maintain the DC voltage and reduce the interphase circulating current, set the reference value of the submodule voltage to maintain the DC voltage and reduce the interphase circulating current;

[0108] S42, collecting the submodule capacitor voltages at the end of the switch state through a collector, and performing bubble sorting, sorting them from small to large, and saving them;

[0109] S43, collecting the bridge arm current state and judging, if the bridge arm current flows in, selecting the submodule with the smallest capacitor voltage when opening the submodule, charging the capacitor to make it approach the reference value set in step S41, and vice versa;

[0110] S44, combining step S41, step S42 and step S43, obtain the optimal switching signal, apply the optimal switching signal to the converter submodule, so that the submodule is normally turned on and off to track the closed-loop voltage reference value, and keep the submodule voltage stable.

[0111] Compared with the prior art, the present invention has the following advantages:

[0112] 1. The control modulation method based on modular multi-level converter provided by the present invention extrapolates the original single-step deadbeat control system by one step, thereby better compensating for the error caused by delay in the system.

[0113] 2. The present invention provides a control modulation method based on a modular multi-level converter, which optimizes the high-level SVPWM direct modulation algorithm and greatly reduces the amount of calculation in the process of calculating the switching state and the switching time.

[0114] Based on the above reasons, the present invention can be widely promoted in the fields of medium voltage power system converters of ships and the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0115] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0116] Figure 1 The figure is a flow chart of the method of the present invention.

[0117] Figure 2 A modular multi-level topology diagram provided by an embodiment of the present invention.

[0118] Figure 3 An improved deadbeat control diagram provided by an embodiment of the present invention.

[0119] Figure 4 This is a SVPWM point finding mapping diagram provided by an embodiment of the present invention.

[0120] Figure 5 A seven-stage switch switching diagram provided for an embodiment of the present invention. DETAILED DESCRIPTION

[0121] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0122] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0123] like Figure 1 As shown, the present invention provides a control modulation method based on a modular multi-level converter, comprising:

[0124] S1. Establish a modular multilevel converter model and derive the bridge arm voltage and circulating current to couple the deadbeat control and SVPWM modulation;

[0125] S2. Discretize and derive the established modular multilevel converter model, improve the deadbeat control, and generate the space vector waveform required for modulation through the improved deadbeat control;

[0126] S3, improving the classical space vector modulation, and modulating the voltage space vector generated by the improved deadbeat control to obtain an optimized switching state;

[0127] S4. Analyze the improved deadbeat control and space vector modulation to obtain the optimal switching state and switching time to improve the robustness of the closed-loop modular multi-level system.

[0128] In specific implementation, as a preferred embodiment of the present invention, step S1 specifically includes:

[0129] S11. In order to maintain the stability of the DC busbar on the output side of the MMC rectifier, the total number of sub-modules put into operation in the upper and lower bridge arms of each phase at any time remains unchanged; the sum of the sub-modules put into operation in the upper and lower bridge arms is calculated using the following formula:

[0130] N=n jp +n jn

[0131] Among them, n jp 、n jn It indicates the number of submodules put into operation in the upper and lower bridge arms of phase j at a certain moment;

[0132] S12. According to the principle of the half-bridge submodule, the working state of the submodule is expressed as:

[0133]

[0134] Wherein, j = a, b, c, a, b, c represent three-phase bridge arms; x = p, n, p, n represent upper and lower bridge arms; y = 1, 2...N, N represents the number of submodules;

[0135] S13, calculate the capacitor voltage of the yth submodule of the j-phase x-bridge arm, and the calculation formula is as follows:

[0136]

[0137] Among them, C sm Indicates the capacitance value of the submodule; i jx Indicates the current value flowing through the j-phase x (upper / lower) bridge arm;

[0138] S14, calculate the output voltage of the yth submodule of the j-phase x-bridge arm, and the calculation formula is as follows:

[0139] u jxy =S jxy ·u sm

[0140] S15, calculate the output voltage of the submodule of the upper and lower bridge arms of phase j, and the calculation formula is as follows:

[0141]

[0142] Among them, u jpy represents the voltage value of the yth submodule of the upper bridge arm of phase j; u jny Indicates the voltage value of the yth submodule of the lower bridge arm of phase j;

[0143] S16. According to Kirchhoff's voltage law, the relationship between the upper and lower bridge arm voltages and the grid side voltage is expressed as:

[0144]

[0145] Among them, the above formula is the expression of the bridge arm voltage to the j-phase voltage; U dc Indicates the DC bus output voltage; u jp ,u jnRepresents the upper and lower bridge arm voltages of phase j; i jp ,i jn represents the upper and lower bridge arm currents of phase j; R represents the bridge arm equivalent resistance; L represents the bridge arm equivalent inductance;

[0146] S17, adding the two formulas in step S16 to express the relationship between the AC voltage and the upper and lower bridge arm voltages and bridge arm currents, as follows:

[0147]

[0148] S18, subtract the two formulas in step S16 to express the DC side voltage of the MMC rectifier, as follows:

[0149]

[0150] S19. In the MMC rectifier, the inductance of the upper and lower bridge arms of each phase is equal, and the AC current of each phase is evenly distributed between the upper and lower bridge arms. According to Kirchhoff's current law, the relationship between the upper and lower bridge arm currents and the circulating current is obtained as follows:

[0151]

[0152] S110, subtract the two formulas in step S19 to obtain the grid-side current i j ,as follows:

[0153] i j =i jp -i jn

[0154] S111, add the two formulas in step S19 to obtain the circulation i cirj ,as follows:

[0155]

[0156] Among them, i jp Represents the upper bridge arm current; i jn Represents the lower arm current.

[0157] In this embodiment, the typical topology of the MMC rectifier is a new topology in which multiple submodules are connected in series. Each phase of the MMC rectifier consists of an upper and lower bridge arm, and there are six bridge arms in total for three phases. Each bridge arm includes N submodules SM connected in series. N And the bridge arm inductance to suppress the inter-phase circulating current.

[0158] In specific implementation, as a preferred embodiment of the present invention, the improved deadbeat control used in step S2 is used to compensate for the system error caused by the traditional deadbeat control. The control flow chart is as follows: Figure 2 As shown, specifically including:

[0159] S21, MMC three-phase symmetry, taking phase A as an example, the modular multilevel converter model established in step S1 is Euler discretized to obtain the discrete equation as follows:

[0160]

[0161] Among them, u ap (k),u an (k) represents the sum of all module voltages that should be turned on in the upper and lower bridge arms in the kth control cycle; i ap (k), i an (k) represents the upper and lower bridge arm currents that should be turned on in the kth control cycle; u a (k) represents the AC side voltage in the kth control cycle; U dc (k) represents the DC side voltage of the kth control cycle; T represents the system control cycle;

[0162] S22. According to the classic deadbeat control theory, the current value at the next moment is predicted to achieve fast and accurate tracking of the output current, reduce the harmonics on the AC side, and effectively reduce the PI adjustment link, simplifying the complexity of the control system, as follows:

[0163]

[0164] Among them, i anref It indicates the reference value given by the outer loop, and different control targets can be achieved by coordinating different outer loop control methods;

[0165] S23. Calculate the reference value of circulating current. The calculation formula is as follows:

[0166]

[0167] Among them, K p Indicates the proportionality coefficient; K i represents the integral coefficient; s represents the Laplace domain complex variable; U cref Indicates the submodule capacitor voltage reference value; U cave Indicates the average value of the capacitor voltage of phase A at the current moment;

[0168] S24. Based on the above steps, a modular multi-level classic deadbeat control model is constructed as follows:

[0169]

[0170] S25. Analyze the source of error. In traditional deadbeat control, it is believed that the sampling, algorithm operation and pulse output of the controller are carried out simultaneously. However, there is always a time delay from current sampling to the generation of the control signal; it is mainly composed of A / D conversion delay, calculation delay and zero-order holder (ZOH) delay in the modulation process; therefore, the output is often delayed, resulting in the arrival of the k+1 cycle, and the pulse is still the pulse of the k cycle, so that the control produces hysteresis and affects the control effect; therefore, the present invention predicts the current at the k+2 moment by improving the algorithm, and controls the k+1 cycle pulse to be generated before the k+1 cycle, thereby avoiding the influence of control delay, and pushing the discrete equation obtained in step S21 back one step, as follows:

[0171]

[0172] S26, Under normal steady state, the DC side voltage of MMC rectifier is U dc With a small fluctuation near the steady-state value, the DC side voltage U of the MMC rectifier in the k+1th control cycle is obtained. dc (k+1), as follows:

[0173] U dc (k+1)=U dcref

[0174] Among them, U dcref Indicates the DC side voltage reference value;

[0175] S27, to obtain accurate prediction of u a (k+1), using the interpolation prediction method to estimate, given that T is small, it is usually believed that u a The increment in the k-1 period is equal to the increment in the k period, that is:

[0176] u a (k+1)-u a (k) = u a (k)-u a (k-1)

[0177] So we get u a The expression for (k+1) is:

[0178] u a (k+1)=2u a (k)-u a (k-1)

[0179] S28, replace i in the formula of step S25 ap (k+1) and i an (k) Derived from the formula in step S21:

[0180]

[0181] S29, substitute i ap (k+1), i an (k+1),u a (k+1), U dc (k+1), the improved deadbeat control equation is as follows:

[0182]

[0183] In this embodiment, the improved deadbeat control equation is divided into two parts: the traditional MMC deadbeat control equation and the error correction term. The traditional MMC deadbeat control equation realizes the main control function, tracks the bridge arm current, and realizes stable operation of the system. The introduction of the error correction term corrects the generated modulation wave in real time to reduce the control error. The error correction term should be 0 in an ideal situation, but due to the influence of sampling error, controller delay, etc., the traditional MMC deadbeat control produces hysteresis, which makes the error correction term not 0, so that it is corrected in real time to increase the control accuracy.

[0184] S210, based on the upper and lower bridge arm voltages obtained in step S29, the AC a-phase reference voltage is solved by the discrete equation in step S21 as follows:

[0185]

[0186] S211, get the AC measurement a phase reference voltage, b phase, c phase and so on, perform Clark transformation to get the space vector voltage U α , U β ,as follows:

[0187]

[0188] Among them, U a , U b , U c Represents the reference voltage of the three phases a, b, and c on the AC side respectively.

[0189] In specific implementation, as a preferred embodiment of the present invention, the improved SVPWM modulation used in step S3 is used to reduce the amount of calculation when the traditional SVPWM calculates the switch state. The vector modulation diagram is as follows: Figure 4 As shown, specifically including:

[0190] S31. By analyzing the multi-level SVPWM law, it can be known that the minimum switching state corresponding to the reference center vector is related to the sector where the modulation vector is located and the number of level layers. The level of the reference voltage vector is calculated, and the calculation formula is as follows:

[0191]

[0192] Where N represents the number of levels of modular multi-level; v ref Represents the reference voltage vector.

[0193] S32. Find the center of the sub-hexagon:

[0194] After determining the layer and section, determine the candidate center; for N-level MMC, for example, the candidate reference center vector of sector 1 will be (L-1,0,0), (L-1,1,0) to (L-1,L-1,0); calculate the absolute distance value between the reference vector and the candidate center; the minimum distance gives the nearest center, which is the reference center vector. The coordinates of the reference center vector are (S a ,S b ,S c );

[0195] S33, perform vector mapping in the 0-60 degree sector, after determining the optimal starting point, move the origin O of the reference vector to the calculated new origin N, and calculate the mapping vector as follows:

[0196]

[0197] S34, the reference vector, the mapping vector, and the simplified reference vector form a vector triangle, and the simplified reference vector is calculated as follows:

[0198]

[0199] S35. Calculate the time allocated by the vector. The simplified reference vector has the same action time as the initial reference vector. The new reference vector is obtained according to the volt-second balance principle and the nearest three vector method as follows:

[0200]

[0201] Among them, T s Represents the sampling period, T x , T y They represent the action time for synthesizing the simplified reference vector, T z represents the action time of the zero vector;

[0202] S36. Let the angle between the simplified reference vector and the α axis be θ. According to the sine theorem, we can get:

[0203]

[0204] S37, calculate and obtain the action time of each vector, as follows:

[0205]

[0206] S38, calculate the time allocated for the zero vector, as follows:

[0207]

[0208] S39, if T x +T y >T s When , overmodulation processing is required, so:

[0209]

[0210] S310, after determining the vector switching sequence and switching time, seven-segment SVPWM modulation is adopted. In order to reduce the switching frequency of the switching device, the selection of the switching sequence follows the following rules:

[0211] Rule 1: In the vector diagram, when the reference vector is transferred from one sector to another, there is no or only a minimum number of switches.

[0212] Rule 2: In a certain sector, when the seven-segment switch sequentially switches from one state to another, only one phase changes, and multiple phases cannot change crosswise;

[0213] S311, detailed synthesis process as follows Figure 4 As shown, if the reference vector falls on the second sector of the small hexagon, V 0 、V 2 、V 3 、V 7 Go to Synthesize V ref (k+1), the starting vector is V 0 (530), the switching sequence is as follows:

[0214] 530→540→640→641→640→540→530

[0215] The vector durations of the corresponding states are:

[0216] 530→540→640→641→640→540→530

[0217]

[0218] S312, calculating the switching time of the upper bridge arm of phase A, as follows:

[0219]

[0220] S313, calculating the switching time of the upper bridge arm of phase B, as follows:

[0221]

[0222] S314, calculating the switching time of the upper bridge arm of phase C, as follows:

[0223]

[0224] S315, query according to different vectors Figure 5 Determine the specific switching time and generate a switching signal.

[0225] In this embodiment, the classic SVPWM modulation of the modular multilevel converter is analyzed and how to reduce the complexity of the modulation process is analyzed, so as to simplify the calculation of the optimal switching state and find the action time of the corresponding switching state:

[0226] In the deadbeat control system, the required space voltage vector is obtained, and the required switching state and its duty cycle can be obtained by modulating it. Because it is almost impossible to directly process voltage vector modulation above three levels, the classic multi-level SVPWM modulation is equivalent to reducing the number of levels. For example, the seven-point flat space voltage vector SVPWM modulation must be reduced to five-point flat and then from five levels to three levels or further reduced to two levels for modulation. In three-level and two-level space vector modulation, the present invention has a relatively mature modulation calculation method. Direct modulation methods can also be used. For example, the seven-point flat can be directly equivalently mapped to two levels for modulation, but this method is full of a large number of formula calculations. The present invention will simplify this process and use space vector sampling modulation that is better understood and better calculated. For the improved SVPWM mentioned in step S2, the key to simplifying the modulation process is how to find a suitable reference center vector to form a two-level sector vector mapping around the center vector. Find the sector of the reference voltage vector, similar to the two-level SVPWM, the space vector area is divided into six sectors, and the measurement angle of each sector is 3 / pi radian intervals.

[0227] In specific implementation, as a preferred embodiment of the present invention, the switch signal is distributed in step S4, and the voltage of each submodule is sorted according to the direction of the bridge arm current to ensure that the capacitor voltage of all submodules remains stable, which specifically includes:

[0228] S41, in order to maintain the DC voltage and reduce the interphase circulating current, set the reference value of the submodule voltage to maintain the DC voltage and reduce the interphase circulating current;

[0229] In this embodiment, by analyzing step S1, it can be known that the number of submodules of the modular multilevel converter is constant at the same time under normal working conditions, which is also to maintain the DC voltage and reduce the interphase circulating current. For example, in a five-level 480V rectifier system, the number of submodules turned on at the same time is six, so the submodule capacitor voltage setting value should be 480V / 4. In this embodiment, it is hoped that all submodules fluctuate around this voltage setting value.

[0230] S42, collecting the submodule capacitor voltages at the end of the switch state through a collector, and performing bubble sorting, sorting them from small to large, and saving them;

[0231] S43. In the current principle, if you want to increase a capacitor voltage, you need to charge the capacitor, that is, current flows in. If you want to reduce the capacitor voltage, you need to discharge the capacitor. Therefore, it is necessary to collect the bridge arm current state and judge. If the bridge arm current flows in, when turning on the submodule, select the submodule with the smallest capacitor voltage, charge the capacitor, and make it close to the reference value set in step S41, and vice versa.

[0232] S44, combining step S41, step S42 and step S43, obtain the optimal switching signal, apply the optimal switching signal to the converter submodule, so that the submodule is normally turned on and off to track the closed-loop voltage reference value, and keep the submodule voltage stable.

[0233] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A control modulation method based on a modular multilevel converter, characterized in that: include: S1. Establish a modular multilevel converter model and derive the bridge arm voltage and circulating current to couple the deadbeat control and SVPWM modulation; S2. Discretize and derive the established modular multilevel converter model, improve the deadbeat control, and generate the space vector waveform required for modulation through the improved deadbeat control; S3, improving the classical space vector modulation, and modulating the voltage space vector generated by the improved deadbeat control to obtain an optimized switching state; S4. Analyze the improved deadbeat control and space vector modulation to obtain the optimal switching state and switching time to improve the robustness of the closed-loop modular multi-level system.

2. A control modulation method based on a modular multilevel converter according to claim 1, characterized in that: Step S1 specifically includes: S11. In order to maintain the stability of the DC busbar on the output side of the MMC rectifier, the total number of sub-modules put into operation in the upper and lower bridge arms of each phase at any time remains unchanged; the sum of the sub-modules put into operation in the upper and lower bridge arms is calculated using the following formula: N=n jp +n jn Among them, n jp 、n jn It indicates the number of submodules put into operation in the upper and lower bridge arms of phase j at a certain moment; S12. According to the principle of the half-bridge submodule, the working state of the submodule is expressed as: Wherein, j = a, b, c, a, b, c represent three-phase bridge arms; x = p, n, p, n represent upper and lower bridge arms; y = 1, 2...N, N represents the number of submodules; S13, calculate the capacitor voltage of the yth submodule of the j-phase x-bridge arm, and the calculation formula is as follows: Among them, C sm Indicates the capacitance value of the submodule; i jx It represents the current value flowing through the x-bridge arm of phase j; S14, calculate the output voltage of the yth submodule of the j-phase x-bridge arm, and the calculation formula is as follows: in jxy =S jxy ·in sm S15, calculate the output voltage of the submodule of the upper and lower bridge arms of phase j, and the calculation formula is as follows: Among them, u jpy represents the voltage value of the yth submodule of the upper bridge arm of phase j; u jny Indicates the voltage value of the yth submodule of the lower bridge arm of phase j; S16. According to Kirchhoff's voltage law, the relationship between the upper and lower bridge arm voltages and the grid side voltage is expressed as: Among them, the above formula is the expression of the bridge arm voltage to the j-phase voltage; U dc Indicates the DC bus output voltage; u jp ,u jn Represents the upper and lower bridge arm voltages of phase j; i jp ,i jn represents the upper and lower bridge arm currents of phase j; R represents the bridge arm equivalent resistance; L represents the bridge arm equivalent inductance; S17, adding the two formulas in step S16 to express the relationship between the AC voltage and the upper and lower bridge arm voltages and bridge arm currents, as follows: S18, subtract the two formulas in step S16 to express the DC side voltage of the MMC rectifier, as follows: S19. In the MMC rectifier, the inductance of the upper and lower bridge arms of each phase is equal, and the AC current of each phase is evenly distributed between the upper and lower bridge arms. According to Kirchhoff's current law, the relationship between the upper and lower bridge arm currents and the circulating current is obtained as follows: S110, subtract the two formulas in step S19 to obtain the grid-side current i j ,as follows: i j =i jp -i jn S111, add the two formulas in step S19 to obtain the circulation i cirj ,as follows: Among them, i jp Represents the upper bridge arm current; i jn Represents the lower arm current.

3. The control modulation method based on modular multilevel converter according to claim 1, characterized in that: Step S2 specifically includes: S21, MMC three-phase symmetry, taking phase A as an example, the modular multilevel converter model established in step S1 is Euler discretized to obtain the discrete equation as follows: Among them, u ap (k) and u an (k) represents the sum of all module voltages that should be turned on in the upper and lower bridge arms in the kth control cycle; i ap (k), i an (k) represents the upper and lower bridge arm currents that should be turned on in the kth control cycle; u a (k) represents the AC side voltage in the kth control cycle; U dc (k) represents the DC side voltage of the kth control cycle; T represents the system control cycle; S22. According to the classic deadbeat control theory, predict the current value at the next moment as follows: Among them, i anref It indicates the reference value given by the outer loop, and different control targets can be achieved by coordinating different outer loop control methods; S23. Calculate the reference value of circulating current. The calculation formula is as follows: Among them, K p Indicates the proportionality coefficient; K i represents the integral coefficient; s represents the Laplace domain complex variable; U cref Indicates the submodule capacitor voltage reference value; U cave Indicates the average value of the capacitor voltage of phase A at the current moment; S24. Based on the above steps, a modular multi-level classic deadbeat control model is constructed as follows: S25, analyzing the source of error, predicting the current at time k+2 through an improved algorithm, controlling the k+1 cycle pulse to be generated before the k+1 cycle, thereby avoiding the influence of control delay, and pushing the discrete equation obtained in step S21 back one step, as follows: S26, Under normal steady state, the DC side voltage of MMC rectifier is U dc With a small fluctuation near the steady-state value, the DC side voltage U of the MMC rectifier in the k+1th control cycle is obtained. dc (k+1), as follows: IN dc (k+1)=U dcref Among them, U dcref Indicates the DC side voltage reference value; S27, to obtain accurate prediction of u a (k+1), using the interpolation prediction method to estimate, given that T is small, it is usually believed that u a The increment in the k-1 period is equal to the increment in the k period, that is: in a (k+1)-th a (k)=u a (k)-u a (k-1) So we get u a The expression for (k+1) is: u a (k+1)=2u a (k)-u a (k-1) S28, replace i in the formula of step S25 ap (k+1) and i an (k) Derived from the formula in step S21: S29, substitute i ap (k+1), i an (k+1),u a (k+1), U dc (k+1), the improved deadbeat control equation is as follows: S210, based on the upper and lower bridge arm voltages obtained in step S29, the AC a-phase reference voltage is solved by the discrete equation in step S21 as follows: S211, get the AC measurement a phase reference voltage, b phase, c phase and so on, perform Clark transformation to get the space vector voltage U α , U β ,as follows: Among them, U a , U b , U c They represent the reference voltages of the three phases a, b, and c on the AC side respectively.

4. The control modulation method based on modular multilevel converter according to claim 1, characterized in that: Step S3 specifically includes: S31, calculate the level of the reference voltage vector, the calculation formula is as follows: Where N represents the number of levels of modular multi-level; v ref represents the reference voltage vector; S32. Find the center of the sub-hexagon: After determining the layer and section, determine the candidate center; for N-level MMC, for example, the candidate reference center vector of sector 1 will be (L-1,0,0), (L-1,1,0) to (L-1,L-1,0); calculate the absolute distance value between the reference vector and the candidate center; the minimum distance gives the nearest center, which is the reference center vector. The coordinates of the reference center vector are (S a ,S b ,S c ); S33, perform vector mapping in the 0-60 degree sector, after determining the optimal starting point, move the origin O of the reference vector to the calculated new origin N, and calculate the mapping vector as follows: S34, the reference vector, the mapping vector, and the simplified reference vector form a vector triangle, and the simplified reference vector is calculated as follows: S35. Calculate the time allocated by the vector. The simplified reference vector has the same action time as the initial reference vector. The new reference vector is obtained according to the volt-second balance principle and the nearest three vector method as follows: Among them, T s Represents the sampling period, T x 、T y They represent the action time for synthesizing the simplified reference vector, T z represents the action time of the zero vector; S36. Let the angle between the simplified reference vector and the α axis be θ. According to the sine theorem, we can get: S37, calculate and obtain the action time of each vector, as follows: S38, calculate the time allocated for the zero vector, as follows: S39, if T x +T y >T s When , overmodulation processing is required, so: S310, after determining the vector switching sequence and switching time, seven-segment SVPWM modulation is adopted. In order to reduce the switching frequency of the switching device, the selection of the switching sequence follows the following rules: Rule 1: In the vector diagram, when the reference vector is transferred from one sector to another, there is no or only a minimum number of switches. Rule 2: In a certain sector, when the seven-segment switch sequentially switches from one state to another, only one phase changes, and multiple phases cannot change crosswise; S311, if the reference vector falls on the second sector of the small hexagon, use V0, V2, V3, V7 to synthesize V ref (k+1), the starting vector is V0(530), and the switching sequence is as follows: 530→540→640→641→640→540→530 The vector durations of the corresponding states are: 530→540→640→641→640→540→530 S312, calculating the switching time of the upper bridge arm of phase A, as follows: S313, calculating the switching time of the upper bridge arm of phase B, as follows: S314, calculating the switching time of the upper bridge arm of phase C, as follows: S315, determining a specific switch switching time according to different query graphs of the vectors, and generating a switch signal.

5. The control modulation method based on modular multilevel converter according to claim 1, characterized in that: Step S4 specifically includes: S41, in order to maintain the DC voltage and reduce the interphase circulating current, set the reference value of the submodule voltage to maintain the DC voltage and reduce the interphase circulating current; S42, collecting the submodule capacitor voltages at the end of the switch state through a collector, and performing bubble sorting, sorting them from small to large, and saving them; S43, collecting the bridge arm current state and judging, if the bridge arm current flows in, selecting the submodule with the smallest capacitor voltage when opening the submodule, charging the capacitor to make it approach the reference value set in step S41, and vice versa; S44, combining step S41, step S42 and step S43, obtain the optimal switching signal, apply the optimal switching signal to the converter submodule, so that the submodule is normally turned on and off to track the closed-loop voltage reference value, and keep the submodule voltage stable.

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