Nine-switch converter flexible interconnection system and switch loss control method and device thereof

Abstract

The application discloses a nine-switch converter flexible interconnection system and a switching loss control method and device thereof. αβ The method comprises the following steps: firstly, 27 voltage space vectors are obtained by constructing a nine-switch converter mathematical model; secondly, a system Coordinate system mathematical model is established, a prediction model is obtained through forward Euler method discretization processing, and a weight coefficient value function is designed to accurately track the current; then, voltage vectors are selected and inserted into zero vectors, and a fixed switching frequency switching sequence is designed; finally, the action time of the voltage vector is determined through duty cycle modulation, and the optimal voltage vector is periodically and alternately selected. The application effectively solves the problems of the existing system, such as unfixed switching frequency, uneven loss, large current harmonic and heavy calculation burden, balances the power device loss, prolongs the service life of the device and improves the system stability. The application can realize the bidirectional power flexible interconnection between two three-phase power grids and guarantee the system operation stability and power quality.

Description

Technical Field

[0001] This invention belongs to the field of power electronic conversion and flexible interconnection technology of power systems, and particularly relates to a flexible interconnection system of nine-switch converters and its switching loss control method and device. Background Technology

[0002] As power systems evolve towards greater flexibility and intelligence, flexible interconnection has become a key technology for efficient power transmission and grid operation optimization, with broad application prospects in areas such as renewable energy grid integration and regional grid interconnection. The nine-switch converter, as a novel power electronic conversion device, offers advantages over traditional converters, including compact structure, fewer components, high power density, and the ability to simultaneously perform rectification and inversion. However, current research in flexible interconnection systems lacks mature application schemes and control strategies.

[0003] In the field of flexible interconnection, current implementation schemes mostly adopt a dual independent converter topology, that is, an independent converter is configured on the rectifier side and an independent converter on the inverter side, and grid interconnection and power conversion are achieved through the coordinated control of the two converters. However, one of the development trends in power converter design is to reduce power switches and optimize the topology, thereby reducing equipment costs, size and increasing power density. Therefore, in order to further optimize the topology of flexible interconnection systems and give full play to the advantages of the nine-switch converter, such as compact structure, small number of devices and simultaneous rectification and inversion functions, it is urgent to carry out research on adaptive flexible interconnection system design and loss equalization control strategies. Existing control schemes still have many limitations: some technologies only use a fixed switching frequency to control the midpoint fluctuation of single-port converters, without addressing power loss balancing control for dual-port switch-multiplexed topologies such as nine-switch converters; some technologies only control nine-switch converters simply as grid-connected inverters, failing to realize their dual functions of rectification and inversion, and thus unable to achieve bidirectional power transmission between two grids; and some technologies only reduce switching losses through voltage space vector modulation, without solving the core problems of uneven distribution of power switching losses and uneven heat generation in nine-switch converters, which can easily lead to local overheating and aging of devices, reducing system stability and device lifespan. Summary of the Invention

[0004] The purpose of this invention is to provide a flexible interconnection system for nine-switch converters and its switching loss control method and device, which solves the problems of unstable switching frequency, uneven distribution of switching losses, and large current harmonics in the nine-switch converter interconnection system, realizes bidirectional flexible power interconnection between two three-phase power grids, and at the same time ensures system operation stability and power quality.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] In a first aspect, the present invention provides a flexible interconnection system for a nine-switch converter, comprising a nine-switch converter, a rectifier-side power grid, an inverter-side power grid, a control circuit, and filter resistors. R Filter inductor L ; The nine-switch converter includes a DC-side capacitor, three phase bridge arms, and an insulated gate bipolar transistor (IGBT) drive circuit. Each phase bridge arm is connected in series with three power switches, and the three phase bridge arms are connected in parallel and then connected across the positive and negative terminals of the DC-side capacitor. The rectifier side port of the nine-switch converter is filtered by a resistor. R and filter inductor L Connected to the rectifier-side grid, the inverter-side port is connected to a filter resistor. R and filter inductor L Connect to the inverter-side power grid to achieve bidirectional flexible power interconnection between two three-phase power grids; The input terminal of the control circuit is connected to the detection signals of the three-phase current on the rectifier side, the three-phase current on the inverter side, the DC side voltage, the grid voltage on the rectifier side, and the grid voltage on the inverter side of the nine-switch converter. The control circuit incorporates a rectifier-side closed-loop control circuit and an inverter-side closed-loop control circuit. The rectifier-side closed-loop control circuit, based on the detection signals of DC-side voltage, rectifier-side three-phase current, and rectifier-side grid voltage, controls the stability of the DC-side voltage and the power quality of the output three-phase AC power. The inverter-side closed-loop control circuit, based on the detection signals of inverter-side three-phase current and inverter-side grid voltage, tracks the given active and reactive power commands and adjusts the active and reactive power transmitted between the two grids.

[0007] In one embodiment, the drive signal of the intermediate power switch of each phase bridge arm of the nine-switch converter is generated by the logical XOR operation of the drive signals of the two power switches above and below the same bridge arm. Only two power switches are turned on at the same time for each phase bridge arm. Each phase bridge arm corresponds to 3 conduction states. The three phase bridge arms form a total of 27 conduction states, corresponding to 27 voltage space vectors.

[0008] In one embodiment, the rectifier-side closed-loop control loop is a dual closed-loop control structure consisting of a voltage outer loop and a current inner loop. The input of the voltage outer loop is the difference between the DC-side voltage setpoint and the detected value, and the input of the current inner loop is the difference between the output of the voltage outer loop and the sampled value of the three-phase current on the rectifier side. The inverter-side closed-loop control loop is also a dual closed-loop control structure consisting of a power outer loop and a current inner loop. The input of the power outer loop is the difference between the active and reactive power setpoints and the detected values, and the input of the current inner loop is the difference between the output of the power outer loop and the sampled value of the three-phase current on the inverter side.

[0009] In one embodiment, the control circuit is used to generate drive pulse signals for each IGBT device of the nine-switch converter. By performing a symmetrical switching sequence design with a fixed switching frequency, voltage vector duty cycle modulation, and periodic alternation of the optimal voltage vector, the switching loss and conduction loss of the nine IGBT devices are evenly distributed.

[0010] Secondly, the present invention provides a switching loss control method for a nine-switch converter flexible interconnection system, applied to the aforementioned nine-switch converter flexible interconnection system, the method comprising: A mathematical model of a nine-switch converter is constructed using the switching function method. Based on the 27 effective conduction states of the nine-switch converter, 27 voltage space vectors are derived, and the sector distribution of each vector is clarified. Based on the mathematical model of a nine-switch converter, the circuit equations of the rectifier and inverter sides of the power grid are established. αβ The output voltage equation in the stationary coordinate system is obtained by transformation, and the system current prediction model is obtained by discretization using the forward Euler method. Output based on current prediction model Rectifier side and inverter side αβ A value function without weighting coefficients is constructed using the given and predicted current values ​​on the axis. Based on the sector distribution and value function of voltage space vector, an effective voltage vector is selected and a zero vector is inserted to design symmetrical switching sequences with fixed switching frequencies on the rectifier side and inverter side respectively. By using duty cycle modulation technology to determine the duration of each voltage vector within the control cycle, and combining the periodic alternation of the optimal voltage vectors on the rectifier and inverter sides, the switching losses of the nine IGBT devices are balanced.

[0011] In one implementation, the expression for the current prediction model is: , , In the formula, , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβ Voltage on the shaft; , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβ Current on the shaft; , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβ Predicted current on the shaft; , They are rectifier side power grid αβ shaft voltage, , They are respectively Inverter side power grid αβ shaft voltage, Indicates the sampling period. L For filtering inductors, R This is the filter resistor.

[0012] In one implementation, the value function with unweighted coefficients is: , In the formula, , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβ Current setpoint on the shaft; , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβ Predicted current value on the shaft.

[0013] In one embodiment, the symmetrical switching sequence with a fixed switching frequency is designed as follows: Rectifier-side switch sequence design: First, select two effective voltage vectors and one zero vector within the same sector on the rectifier side. Then, based on the selected sector on the rectifier side, select two effective voltage vectors and one zero vector from adjacent sectors on the inverter side to obtain a combined vector. Insert corresponding zero vectors on both sides and in the middle of the combined vector's action sequence to form a 9-segment symmetrical switch sequence on the rectifier side. Inverter-side switching sequence design: First, select two effective voltage vectors and one zero vector within the same sector on the inverter side. Then, based on the selected sector on the inverter side, select two effective voltage vectors and one zero vector from adjacent sectors on the rectifier side to obtain a combined vector. Insert corresponding zero vectors on both sides and in the middle of the combined vector's action sequence to form a 9-segment symmetrical switching sequence on the inverter side.

[0014] In one embodiment, the specific method for determining the voltage vector action time using duty cycle modulation technology is as follows: The partial derivative of the value function with no weight coefficient is obtained to solve for the conduction time of each voltage vector segment within a control cycle; half of the conduction time of each segment is allocated to the symmetrically arranged voltage vectors to obtain the action time of each voltage vector segment in the 9-segment symmetrical switching sequence. The specific method for periodically alternating the selection of the optimal voltage vectors on the rectifier side and the inverter side is as follows: In the first cycle, the optimal voltage vector on the rectifier side of the nine switches is selected, and in the second cycle, the optimal voltage vector on the inverter side of the nine switches is selected. By periodically alternating the selection of the equivalent voltage switching state, this cycle is repeated so that the nine power devices have the same number of switching times in a statistical sense.

[0015] Thirdly, the present invention provides a switching loss control device for a nine-switch converter flexible interconnection system, used to execute the above-described switching loss control method for a nine-switch converter flexible interconnection system, the device comprising: The model building module is used to construct a mathematical model of a nine-switch converter using the switching function method. Based on the 27 effective conduction states of the nine-switch converter, 27 voltage space vectors are derived, and the sector distribution of each vector is clarified. The prediction model generation module is used to establish the circuit equations of the rectifier and inverter side power grids based on the mathematical model of the nine-switch converter. αβ The output voltage equation in the stationary coordinate system is obtained by transformation, and the system current prediction model is obtained by discretization using the forward Euler method. The value function construction module is used for the output of the current prediction model. Rectifier side and inverter side αβ A value function without weighting coefficients is constructed using the given and predicted current values ​​on the axis. The switching sequence design module is used to select the effective voltage vector and insert the zero vector based on the sector distribution and value function of the voltage space vector, and design symmetrical switching sequences with fixed switching frequencies on the rectifier side and inverter side respectively. The loss balancing control module is used to determine the duration of each voltage vector within the control cycle through duty cycle modulation technology. By combining the periodic alternation of the optimal voltage vectors on the rectifier side and the inverter side, the switching losses of the nine IGBT devices are balanced.

[0016] The beneficial effects of this invention are as follows:

[0017] This invention first constructs a complete mathematical model of a nine-switch converter, clarifying the voltage space vectors corresponding to 27 conduction states, providing a precise theoretical foundation for the design of subsequent control strategies. Second, by designing a value function without weighting coefficients, it avoids the performance degradation caused by improper selection of weighting coefficients in traditional value functions, simplifying the control logic. Third, by selecting appropriate voltage vectors and inserting zero vectors, a fixed-frequency switching sequence is designed, solving the problems of inconsistent switching frequencies and large harmonics in traditional control methods, ensuring system power quality. Finally, through duty cycle modulation technology and periodic alternation of voltage vector selection, the switching losses of the nine power devices are balanced, extending device lifespan and improving system stability and reliability. Furthermore, this invention achieves DC-side voltage stabilization and power command tracking through dual closed-loop control circuits on the rectifier and inverter sides, further optimizing system control performance and enabling efficient bidirectional flexible power interconnection between two three-phase power grids. Attached Figure Description

[0018] The accompanying drawings, as part of this invention, are provided to further illustrate the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention, but do not constitute an undue limitation thereof. Clearly, the drawings described below are merely some embodiments, and those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0019] Figure 1 This is a control block diagram of a nine-switch converter flexible interconnection system provided in an embodiment of the present invention;

[0020] Figure 2 A flowchart of a switching loss control method for a nine-switch converter flexible interconnect system provided in an embodiment of the present invention;

[0021] Figure 3 This is a voltage space vector diagram of a nine-switch converter provided in one embodiment of the present invention;

[0022] Figure 4 This is a schematic diagram of voltage vector selection and switch sequence equalization design for a nine-switch converter provided in one embodiment of the present invention, wherein... Figure 4 (a) is a schematic diagram of the switching sequence and voltage vector selection at time k. Figure 4 (b) is a schematic diagram of the switching sequence and voltage vector selection at time k+1. Figure 4 (c) is a schematic diagram of the switching sequence and voltage vector selection at time k+2;

[0023] Figure 5 This is a DC-side output voltage waveform diagram of a nine-switch converter flexible interconnect system provided in one embodiment of the present invention;

[0024] Figure 6 This is a waveform diagram of the three-phase output current on the rectifier and inverter sides of a flexible interconnected nine-switch converter system provided in one embodiment of the present invention. Figure 6 (a) shows the three-phase output current waveform on the rectifier side. Figure 6 (b) shows the three-phase output current waveforms on the inverter side;

[0025] Figure 7 This is a thermal simulation diagram of the nine power switches in a nine-switch converter flexible interconnect system provided in one embodiment of the present invention. Figure 7 (a) is the power switch S a1 S a2 S a3 Thermal simulation diagram, Figure 7 (b) is the power switch S b1 S b2 S b3 Thermal simulation diagram, Figure 7 (c) represents the power switch S. c1 S c2 S c3 Thermal simulation diagram;

[0026] It should be noted that these accompanying drawings and textual descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art by referring to specific embodiments. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below with reference to the accompanying drawings. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0028] like Figure 1 As shown, this invention proposes a flexible interconnection system for a nine-switch converter, which includes a nine-switch converter, a rectifier-side power grid, an inverter-side power grid, a control circuit, and filter resistors. R Filter inductor L The nine-switch converter includes a DC-side capacitor, three phase bridge arms, and an IGBT (Insulated Gate Bipolar Transistor) driver circuit. Each phase bridge arm has three power switches connected in series, and the three phase bridge arms are connected in parallel across the positive and negative terminals of the DC-side capacitor. The rectifier side port of the nine-switch converter is connected to the rectifier-side power grid through a filter resistor R and a filter inductor L, and the inverter side port is connected through a filter resistor... R and filter inductor LConnected to the inverter-side grid, it enables bidirectional flexible power interconnection between two three-phase grids. The input of the control circuit receives detection signals of the three-phase current on the rectifier side, the three-phase current on the inverter side, the DC-side voltage, the rectifier-side grid voltage, and the inverter-side grid voltage. The control circuit has built-in closed-loop control loops on the rectifier side and the inverter side. The rectifier-side closed-loop control loop, based on the detection signals of the DC-side voltage, the three-phase current on the rectifier side, and the rectifier-side grid voltage, controls the stability of the DC-side voltage and the power quality of the output three-phase AC power. The inverter-side closed-loop control loop, based on the detection signals of the three-phase current on the inverter side and the inverter-side grid voltage, tracks the given active and reactive power commands and adjusts the active and reactive power transmitted between the two grids.

[0029] Specifically, the nine-switch converter includes a DC-side capacitor C dc The circuit consists of three phase bridge arms and IGBT (Insulated Gate Bipolar Transistor) drive circuits. Each phase bridge arm is connected in series with three power switches, namely S... a1 S a2 S a3 S b1 S b2 S b3 S c1 S c2 S c3 The three phase bridge arms are connected in parallel and then bridging the DC-side capacitor C. dc Between the positive and negative terminals; the rectifier side port of the nine-switch converter is connected to the rectifier-side grid through a filter resistor R and a filter inductor L, and the inverter side port is connected to the inverter-side grid through a filter resistor R and a filter inductor L, realizing bidirectional flexible power interconnection between the two three-phase grids; the input terminal of the control circuit is connected to the three-phase current of the rectifier side of the nine-switch converter. , , The three-phase current on the inverter side is , , DC side voltage V dc , Rectifier side grid voltage u sR Inverter-side grid voltage u sI The detection signal.

[0030] The control circuit incorporates both a rectifier-side closed-loop control loop and an inverter-side closed-loop control loop. The rectifier-side closed-loop control loop is based on a given DC-side voltage. Output DC side voltage Given rectifier side β Current Three-phase current on the rectifier side , , pass αβ Coordinate transformation to obtain current and rectifier-side power grid , , pass αβ Detection signal obtained by coordinate transformation and The inverter side closed-loop control circuit controls the stability of the DC-side voltage and the power quality of the output three-phase AC power; based on the given active power... and reactive power Inverter side three-phase current , , pass αβ Coordinate transformation to obtain current and Inverter-side power grid , , pass αβ Detection signal obtained by coordinate transformation and This involves regulating the active and reactive power transmitted between the two power grids. Finally, based on the obtained rectifier-side given... αβ Current , and rectifier side given αβ Current , The power switches S are obtained through predictive control using a fixed switching frequency model. a1 S a2 S a3 S b1 S b2 S b3 S c1 S c2 S c3 The pulse signal.

[0031] Furthermore, the drive signal of the intermediate power switch of each phase bridge arm of the nine-switch converter is generated by the logical XOR operation of the drive signals of the two power switches above and below the same bridge arm. Only two power switches are turned on at the same time for each phase bridge arm. Each phase bridge arm corresponds to 3 conduction states. The three phase bridge arms form a total of 27 conduction states, corresponding to 27 voltage space vectors.

[0032] Specifically, each phase bridge arm of the nine-switch converter is equipped with three power switches. The gate drive signal of the middle power switch is generated by the logical XOR operation of the two power switches above and below it in the same bridge arm. Each power switch has two different switching conditions, and the switching state of each power switch is represented by a binary logic function.

[0033] Furthermore, the rectifier-side closed-loop control loop is a dual closed-loop control structure consisting of a voltage outer loop and a current inner loop. The input of the voltage outer loop is the difference between the DC-side voltage setpoint and the detected value, and the input of the current inner loop is the difference between the output of the voltage outer loop and the sampled value of the three-phase current on the rectifier side. The inverter-side closed-loop control loop is also a dual closed-loop control structure consisting of a power outer loop and a current inner loop. The input of the power outer loop is the difference between the active and reactive power setpoints and the detected values, and the input of the current inner loop is the difference between the output of the power outer loop and the sampled value of the three-phase current on the inverter side.

[0034] Furthermore, the control circuit is used to generate drive pulse signals for each IGBT device in the nine-switch converter. By executing a symmetrical switching sequence design with a fixed switching frequency, voltage vector duty cycle modulation, and periodic alternation of the optimal voltage vector, the switching loss and conduction loss of the nine IGBT devices are evenly distributed.

[0035] Reference Figure 2 As shown in the illustration, one embodiment provides a method for controlling switching losses in a nine-switch converter flexible interconnection system. This method is applicable to the aforementioned nine-switch converter flexible interconnection system. Specifically, it includes the following steps:

[0036] Step S100: The mathematical model of the nine-switch converter is constructed using the switching function method. Based on the 27 effective conduction states of the nine-switch converter, 27 voltage space vectors are derived, and the sector distribution of each vector is clarified.

[0037] Each power switch has two different switching conditions. The switching state of each power switch can be represented by a binary logic function, as described below:

[0038] Define the switching state of the IGBT as follows:

[0039] (1)

[0040] In the formula, S abc,123 These represent the nine IGBT devices of the nine-switch converter, namely S a1 S a2 S a3 S b1 S b2 S b3 S c1 S c2 S c3 Because the DC bus cannot be short-circuited and the output port cannot float, the nine-switch converter requires two power switches to be turned on simultaneously in each phase arm. Each phase arm has only 3 on-states, for a total of 27 on-states across the three phases. The relationship between the output voltage equation and the on-states of the nine-switch converter is as follows:

[0041] (2)

[0042] in, , , These are the rectifier-side output voltages of the nine-switch converter, respectively, determined by the power switch S of the nine-switch converter. a1 S a2 S a3 The conduction state and DC side voltage V dc Joint decision; , , These are the inverter-side output voltages of the nine-switch converter, respectively, and are determined by the power switch S of the nine-switch converter. c1 S c2 S c3 The conduction state is determined by the NOT gate and the DC side voltage V. dc A joint decision.

[0043] Based on the 27 conduction states of the nine-switch converter, the voltage space expression corresponding to the rectifier-side output voltage of the nine-switch converter is calculated. The voltage space expression corresponding to the inverter-side output voltage of the nine-switch converter. This yields 27 voltage space vectors; the expressions for these voltage space vectors are as follows:

[0044] Voltage space vector expression corresponding to the rectifier-side output voltage of a nine-switch converter as follows:

[0045] (3)

[0046] in, , , These are the rectifier-side output voltages of the nine-switch converter. To rotate a vector counterclockwise , To rotate a vector counterclockwise , It is the imaginary unit.

[0047] Voltage space vector expression corresponding to the inverter-side output voltage of a nine-switch converter as follows:

[0048] (4)

[0049] in, , , These are the output voltages on the inverter side of the nine-switch converter. To rotate a vector counterclockwise , To rotate a vector counterclockwise .

[0050] Substituting the output voltage equation and conduction state relationship (2) of the nine-switch converter into equations (3) and (4), we can obtain 27 voltage space vectors, such as... Figure 3 As shown in the figure, the output vector of rectifier V1 is 111, indicating that the output voltage vectors of the rectifier side are V1, V2, V3, V4, V5, V6, V7, V8, V9, V1 ... dc V dc V dc The circle corresponding to the rectifier side indicates the number of output vectors on the rectifier side, and the circle corresponding to the inverter side indicates the number of output vectors on the inverter side, which clearly shows the voltage spatial distribution corresponding to each conduction state.

[0051] Step S200: Based on the mathematical model of the nine-switch converter, establish the circuit equations of the rectifier and inverter side power grids. αβ The output voltage equation in the stationary coordinate system is obtained by transformation, and the forward Euler method is used for discretization to obtain the system current prediction model.

[0052] First, clarify the relationship between the two three-phase power grids and the nine-switch converter with resistance and inductance, and establish the circuit equations for the rectifier side and inverter side power grids.

[0053] The rectifier side of the nine-switch converter is connected to the rectifier-side power grid via a filter inductor L and a resistor R. Given a fixed grid voltage direction, the rectifier-side power grid can be described by the following circuit equations:

[0054] (5)

[0055] in, , , The three-phase currents are output from the nine-switch converter on the rectifier side, respectively. , , and , , These are the output voltage of the nine-switch converter on the rectifier side and the output voltage of the power grid on the rectifier side, respectively.

[0056] The inverter side of the nine-switch converter is filtered by an inductor. L and resistance R Connected to the inverter-side power grid, the inverter-side power grid can be described by the following circuit equations, given a fixed grid voltage direction:

[0057] (6)

[0058] in, , , The three-phase currents are output from the nine-switch converter on the inverter side, respectively. , , and , , These are the output voltage of the inverter-side nine-switch converter and the grid output voltage on the inverter side, respectively.

[0059] Based on the three-phase voltage expressions (5) and (6) of the rectifier and inverter sides of the nine-switch converter, the following is performed: αβ Transformation, to obtain αβ The output voltages of the rectifier and inverter sides of the nine-switch converter in the coordinate system are shown in the following formula:

[0060] (7)

[0061] (8)

[0062] in, , and , These are the rectifier side and inverter side of the nine-switch converter. αβ Voltage on the shaft; , and , These are the rectifier side and inverter side of the nine-switch converter. αβ Current on the shaft;

[0063] , Rectifier-side power grid αβ shaft voltage, , Inverter-side power grid αβ Shaft voltage.

[0064] The forward Euler method is used to discretize formulas (7) and (8) to obtain the prediction model of the flexible interconnection system of the nine-switch converter.

[0065] Furthermore, the expression for the current prediction model is:

[0066] (9)

[0067] (10)

[0068] in, , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβVoltage on the shaft; , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβ Current on the shaft; , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβ Predicted current on the shaft; , They are rectifier side power grid αβ shaft voltage, , They are respectively Inverter side power grid αβ shaft voltage, Indicates the sampling period. L For filtering inductors, R This is the filter resistor.

[0069] Step S300: Based on the output of the current prediction model The given and predicted current values ​​on the αβ axis of the rectifier and inverter sides at any given time are used to construct a value function without weighting coefficients.

[0070] To achieve accurate tracking of the rectifier and inverter currents and reduce current errors, a value function is designed to meet the current control requirements of the rectifier and inverter sides of the nine-switch converter.

[0071] Furthermore, the value function without weighted coefficients is:

[0072] (11)

[0073] In the formula, , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβ Current setpoint on the shaft; , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβ Predicted current value on the shaft.

[0074] This value function does not require weighting coefficients, which can effectively avoid the problem of decreased control performance caused by improper selection of weighting coefficients.

[0075] Step S400: Based on the sector distribution and value function of the voltage space vector, select the effective voltage vector and insert the zero vector, and design symmetrical switching sequences with fixed switching frequencies on the rectifier side and inverter side respectively.

[0076] In this application embodiment, the design method of a symmetrical switching sequence with a fixed switching frequency includes:

[0077] Step S410: Rectifier-side switch sequence design: First, select two effective voltage vectors and one zero vector in the same sector on the rectifier side. Then, based on the selected sector on the rectifier side, select two effective voltage vectors and one zero vector in the adjacent sector on the inverter side to obtain a combined vector. Insert the corresponding zero vectors on both sides and in the middle of the action sequence of the combined vector to form a 9-segment symmetrical switch sequence on the rectifier side.

[0078] Step S420: Inverter-side switch sequence design: First, select two effective voltage vectors and one zero vector in the same sector on the inverter side. Then, based on the selected sector on the inverter side, select two effective voltage vectors and one zero vector in the adjacent sector on the rectifier side to obtain a combined vector. Insert the corresponding zero vectors on both sides and in the middle of the action sequence of the combined vector to form a 9-segment symmetrical switch sequence on the inverter side.

[0079] Specifically, by selecting appropriate voltage vectors and inserting zero vectors, a switching sequence with a fixed switching frequency is designed, divided into two cases: the rectifier side and the inverter side. Rectifier side switching sequence design: First, select two voltage vectors V2 and V3 within the same sector on the rectifier side of the nine-switch converter, and one zero vector V1; then, based on the selected sector on the rectifier side of the nine-switch converter, select two voltage vectors V3 and V4 from adjacent sectors on the inverter side of the nine-switch converter, and one zero vector V8, to obtain the combined vector V of the nine-switch converter. 13 V 34 and V 28 ; and insert the zero vector V corresponding to the nine-switch converter on both sides and in the middle of the voltage vector action sequence. 11 and V 88 To achieve a fixed switching frequency, the switching sequence on the rectifier side of the nine-switch converter is shown in Table 1, where VV1~VV 12 This indicates 12 selection vector modes, with the switch sequence vector as shown in "V". 17 " indicates that the output voltage vector on the rectifier side is V1 and the output voltage vector on the inverter side is V7.

[0080] Table 1. Switching Sequence Table for Rectifier Side of Switching Converter

[0081]

[0082] Inverter-side switching sequence design: First, select two voltage vectors V3 and V4 within the same sector on the inverter side of the nine-switch converter, and one zero vector V8; then, based on the selected sector on the inverter side of the nine-switch converter, select two voltage vectors V2 and V3 from adjacent sectors on the rectifier side of the nine-switch converter, and one zero vector V1, to obtain the combined vector V of the nine-switch converter. 13 V 34 and V 28 ; and insert the zero vector V corresponding to the nine-switch converter on both sides and in the middle of the voltage vector action sequence. 88 and V 11 To achieve the goal of fixing the switching frequency, the switching sequence on the inverter side of the nine-switch converter is shown in Table 2.

[0083] Table 2. Switching Sequence Table for Inverter Side of Switching Converter

[0084]

[0085] Step S500: The duty cycle modulation technique is used to determine the duration of each voltage vector within the control cycle. Combined with the periodic alternation of the optimal voltage vectors on the rectifier side and the inverter side, the switching losses of the nine IGBT devices are balanced.

[0086] Furthermore, the duration of each voltage vector is determined using duty cycle modulation technology. The specific method is as follows:

[0087] By taking the partial derivative of the value function with no weighting coefficients, the conduction time of each voltage vector segment within a control cycle is obtained. Half of the conduction time of each segment is allocated to the symmetrically arranged voltage vectors to obtain the action time of each voltage vector segment in the 9-segment symmetrical switching sequence.

[0088] Specifically, the duration of each voltage vector segment is determined by taking the partial derivative of the value function of the voltage space vector:

[0089] (12)

[0090] In the formula, The current tracking value function is without weighting coefficients. The sign of the partial derivative. , The outputs of the rectifier-side nine-switch converter are respectively αβ shaft current, , The outputs of the inverter-side nine-switch converter are respectively αβ Axis current.

[0091] Taking the partial derivatives of the value functions of the rectifier and inverter currents of the nine-switch converter, we obtain the duration of action of each voltage vector segment:

[0092] (13)

[0093] In the formula, , Rectifier side α axis, β Shaft current deviation term, , Inverter side α axis, β Shaft current deviation term, , They are respectively The output of the rectifier-side nine-switch converter αβ shaft current, , They are respectively Inverter-side nine-switch converter output αβ shaft current, , Nine-switch converters on the rectifier side αβ The axial voltage vector corresponds to the 5th segment of the 9-segment voltage vector. , Nine-switch converters on the inverter side αβ The axial voltage vector corresponds to the 5th segment of the 9-segment voltage vector. , They are respectively The output of the rectifier-side nine-switch converter αβ Shaft given current, , They are respectively Inverter-side nine-switch converter output α β Shaft given current.

[0094] (14)

[0095] In the formula, , , , Rectifier side α Axis voltage vector difference and filter inductance L The ratio represents the voltage vectors of segments 1-4 relative to the voltage vector of segment 5 relative to the rectifier side. α The influence coefficient of shaft current, , , , These represent the ratio of the rectifier-side β-axis voltage vector difference to the filter inductance L, indicating the voltage vector difference between segments 1-4 relative to segment 5 relative to the rectifier-side voltage vector. β The influence coefficient of shaft current, , , , Inverter side α Axis voltage vector difference and filter inductance L The ratio represents the voltage vectors of segments 1-4 relative to the voltage vector of segment 5 relative to the inverter side. α The influence coefficient of shaft current, , , Inverter side β The ratio of the axis voltage vector difference to the filter inductance L represents the voltage vector difference between segments 1-4 relative to the voltage vector difference between segment 5 and the inverter side. β The influence coefficient of shaft current, , and , and , and , Nine-switch converters on the rectifier side αβ The axis voltage vectors correspond to the 1st, 2nd, 3rd, and 4th segments of the 9-segment voltage vector. , and , and , and , Nine-switch converters on the inverter side αβ The axis voltage vectors correspond to the first segment of the 9-segment voltage vector. Based on the above formula, the 4th order matrix is ​​solved by combining the results.

[0096] (15)

[0097] (16)

[0098] In the formula, (m,n=1,2,3,4) are the elements of a 4th-order coefficient matrix, derived from the current influence coefficients of each segment in equation (14). ~ The summation of pairwise products gives the value, which characterizes the coupling effect of the voltage vectors of the m-th and n-th segments on the current deviation, where the diagonal element... , , , These are the self-influence coefficients of the voltage vector on the current deviation for each segment, and the off-diagonal elements. (m≠n) are the mutual influence coefficients of the voltage vectors of different segments on the current deviation.

[0099] Invert the 4th-order matrix and combine it with the intermediate variable. , , , Performing matrix operations and taking the negative yields a corresponding 9-segment formula, with the first 4 segments having twice the duration of action.

[0100] (17)

[0101] In the formula, , , , , These represent the conduction times of the 1st, 2nd, 3rd, 4th, and 5th voltage vectors within the period Ts of a switching sequence with a fixed switching frequency. Half of each conduction time is allocated to the symmetrical voltage vectors, resulting in the action times of the 9 voltage vector segments: t1 / 2, t2 / 2, t3 / 2, t4 / 2, t5, t4 / 2, t3 / 2, t2 / 2, and t1 / 2.

[0102] Furthermore, the optimal voltage vectors on the rectifier side and inverter side are selected periodically and alternately, specifically in the following manner:

[0103] In the first cycle, the optimal voltage vector on the rectifier side of the nine switches is selected, and in the second cycle, the optimal voltage vector on the inverter side of the nine switches is selected. By periodically alternating the selection of the equivalent voltage switching state, this cycle is repeated so that the nine power devices have the same number of switching times in a statistical sense.

[0104] Specifically, based on the selected voltage vector and operating time, the optimal voltage vector on the rectifier side of the nine switches in Table 1 is selected in the first cycle, and the optimal voltage vector on the inverter side of the nine switches in Table 2 is selected in the second cycle. By periodically alternating the selection of the equivalent voltage switching state, the nine power devices have the same number of switching times in a statistical sense.

[0105] Furthermore, the specific principle for achieving loss equalization among the nine IGBT devices is as follows:

[0106] The average switching loss of a single IGBT device is determined by the switching frequency, single turn-on loss, and single turn-off loss. Since the number of switching operations and the single switching voltage stress are consistent for the nine IGBT devices, their switching losses are equal.

[0107] The average conduction loss of a single IGBT device is determined by the device's on-resistance and the effective value of the current flowing through it. By periodically and alternately selecting the optimal voltage vector, the value functions of the rectifier side and the inverter side are symmetrical, and the duty cycle is symmetrical. This makes the average current values ​​of the nine power switches approximately equal, thus achieving equal average conduction losses.

[0108] Overall voltage vector selection and switching sequence equalization design, such as Figure 4 As shown in Figure 4. Among them, Figure 4(a) is a schematic diagram of the selection of the switching sequence and voltage vector at time k, Figure 4(b) is a schematic diagram of the selection of the switching sequence and voltage vector at time k+1, and Figure 4(c) is a schematic diagram of the selection of the switching sequence and voltage vector at time k+2. Figure 4 In the diagram, 1 indicates that the power switch is in the ON state, and 0 indicates that the power switch is in the OFF state. The "8 times" marked in Figure 4(c) indicates that after three consecutive control cycles (k, k+1, k+2), the cumulative number of level changes for the nine power switches is 8. The voltage vector V is selected. 11 V 13 V 34 V 28 V 88 As the voltage vector at time k that is dominated by the rectifier side output, the power switch S a1 S a2 S a3 and inverter-side power switch S c1 S c2 S c3 Both are changed twice, with intermediate power switch S b1 S b2 After 4 transformations, S b3 No transformation; then select voltage vector V 88 V 48 V 34 V 15 V 11 As the voltage vector at time k+1 that is dominated by the inverter side output, the power switch S a1 S a2 S a3 and inverter-side power switch S c1 S c2 S c3 All changes were performed 3 times, with the intermediate power switch S b1 S b3 After 4 transformations, S b2 No transformation; then select voltage vector V 11 V 17 V 72 V 68 V 88 As the voltage vector at time k+2 that is dominated by the rectifier side output, the power switch S a1 S a2 S a3 and inverter-side power switch S c1 S c2 S c3 All changes were performed 3 times, with the intermediate power switch S b2 Sb3 After 4 transformations, S b1 No change; in summary, after three cycles, the power switch S a1 S a2 S a3 S b1 S b2 S b3 S c1 S c2 S c3 Each was transformed 8 times.

[0109] Power switching losses are mainly determined by switching losses and conduction losses, and the specific analysis process is as follows:

[0110] The average switching loss of a single power switch can be expressed as:

[0111] (18)

[0112] Among them, f s Indicates the switching frequency. and These represent the voltage stress experienced by each switch. During each state transition, the voltage amplitude experienced by the nine power switches is consistent. The average switching loss of a single power switch is mainly determined by the number of switching cycles per unit time. Since the nine power devices have the same number of switching cycles in a statistical sense, the switching losses of the nine power devices are the same.

[0113] The average conduction loss of a single power switch can be approximated as:

[0114] (19)

[0115] in, This represents the average current flowing through the device; Indicates the on-resistance of the device; the three single-phase conduction modes of the nine-switch converter flexible interconnect system correspond to:

[0116] Mode 1: Power Switch S a1 and power switch S a2 On, power switch S a3 Off, power switch S a1 The conduction current is i RA -i IX Power switch S a2 The conduction current is -i IX Power switch S a3 The conduction current is 0;

[0117] Mode 2: Power switch S a1 and power switch S a3 On, power switch Sa2 Off, power switch S a1 The conduction current is i RA Power switch S a2 The conduction current is 0, and the power switch S a3 The conduction current is i IX ;

[0118] Mode 3: Power Switch S a2 and power switch S a3 On, power switch S a1 Off, power switch S a1 The conduction current is 0, and the power switch S a2 The conduction current is -i RA Power switch S a3 The conduction current is i IX -i RA .

[0119] Therefore, the power switch S is obtained. a1 S a2 S a3 The average current is:

[0120] (20)

[0121] in, , , These represent power switches S and S, respectively. a1 S a2 S a3 The average current flowing through the circuit, D1, D2, and D3 are the duty cycles corresponding to conduction modes 1, 2, and 3, respectively; and due to the DC-side power balance in the three modes, the output currents on the rectifier and inverter sides of the nine-switch converter flexible interconnection system satisfy the following:

[0122] (twenty one)

[0123] in, Corresponding to the A-phase current on the rectifier side, This corresponds to the X-phase current on the inverter side.

[0124] Since the optimal voltage vector on the rectifier side of the nine-switch converter is selected in the first cycle and the optimal voltage vector on the inverter side in the second cycle, the equivalent voltage switching states are selected periodically and alternately. Furthermore, the output value functions on the rectifier and inverter sides of the nine-switch converter are symmetrical, therefore the duty cycle is symmetrical: D1 = D3. .

[0125] Each sampling period T s The three inner modes must have full time coverage, therefore the duty cycle satisfies the following equation:

[0126] (twenty two)

[0127] Substituting equations (21) and (22) into (20), we obtain the simplified power switch S. a1 S a2 S a3 Average current:

[0128] (twenty three)

[0129] in, , , These represent power switches S and S, respectively. a1 S a2 S a3 The average current flowing through, The corresponding current of phase A on the rectifier side.

[0130] Mode 2, the single-current mode, is a "reference current term" shared by all devices. However, only modes 1 and 3 truly cause differences between devices. The frequency and weight of these two modes on the three power switches are completely symmetrical. Therefore, the power switch S... a1 S a2 S a3 The average current values ​​of the two phases are approximately equal, and the analysis of the other two phase power switches is similar. Therefore, the average conduction loss of a single power switch is approximately equal.

[0131] Based on power switching losses Calculation formula:

[0132] (twenty four)

[0133] in, The average switching loss of a single power switch. This represents the average conduction loss of a single power switch.

[0134] Since switching losses and conduction losses are approximately equal, a balanced distribution of switching losses is achieved. To verify the effectiveness of the nine-switch converter flexible interconnect system and the balanced switching loss control method, simulations were performed using MATLAB / Simulink. Resistance and inductance parameters: resistor 0.01Ω, inductor 0.001H; switching frequency 20kHz; grid frequency 50Hz; DC-side voltage 1200V. The DC-side output voltage is as follows... Figure 5 As shown, the DC-side voltage stabilizes at 1200V in 0.8s. The three-phase output current on the rectifier side and the three-phase output current on the inverter side are as follows: Figure 6 As shown, where, Figure 6 In figure (a), the waveform of the three-phase output current on the rectifier side is shown. The vertical axis of the figure represents the three-phase current i on the rectifier side.R,abc The three-phase output current on the rectifier side is expressed as i Ra i Rb i Rc Both are 500A 。 Figure 6 Figure (b) shows the waveform of the three-phase output current on the inverter side. In the figure, the vertical axis represents the three-phase current i on the inverter side. I,xyz The three-phase output current on the inverter side is expressed as i Ix i Iy i Iz All are 500A. Thermal simulations of the nine power switches were obtained using PLECS simulation. Figure 7 As shown, where, Figure 7 (a) is the power switch S a1 S a2 Thermal simulation diagram of Sa3 Figure 7 (b) is the power switch S b1 S b2 S b3 Thermal simulation diagram, Figure 7 (c) represents the power switch S. c1 S c2 S c3 Thermal simulation diagram. Power switch S a1 S a2 S a3 Switching losses fluctuate around 100W, power switch S b1 S b2 S b3 Switching losses fluctuate around 120W, power switch S c1 S c2 S c3 Switching losses fluctuate around 95W, and the total losses of each power device are relatively similar, achieving loss balance.

[0135] The following is an embodiment of the switching loss control device for the flexible interconnect system of the nine-switch converter of the present invention, which can be used to execute the embodiment of the switching loss control method for the flexible interconnect system of the nine-switch converter of the present invention. For details not disclosed in the embodiment of the switching loss control device for the flexible interconnect system of the nine-switch converter of the present invention, please refer to the embodiment of the switching loss control method for the flexible interconnect system of the nine-switch converter of the present invention.

[0136] In one embodiment, a switching loss control device for a nine-switch converter flexible interconnect system is provided, for executing the above-described switching loss control method for the nine-switch converter flexible interconnect system. The device includes:

[0137] The model building module is used to construct a mathematical model of a nine-switch converter using the switching function method. Based on the 27 effective conduction states of the nine-switch converter, 27 voltage space vectors are derived, and the sector distribution of each vector is clarified.

[0138] The prediction model generation module is used to establish the circuit equations of the rectifier and inverter side power grids based on the mathematical model of the nine-switch converter. αβ The output voltage equation in the stationary coordinate system is obtained by transformation, and the system current prediction model is obtained by discretization using the forward Euler method.

[0139] The value function construction module is used for the output of the current prediction model. Rectifier side and inverter side αβ A value function without weighting coefficients is constructed using the given and predicted current values ​​on the axis.

[0140] The switching sequence design module is used to select the effective voltage vector and insert the zero vector based on the sector distribution and value function of the voltage space vector, and design symmetrical switching sequences with fixed switching frequencies on the rectifier side and inverter side respectively.

[0141] The loss balancing control module is used to determine the duration of each voltage vector within the control cycle through duty cycle modulation technology. By combining the periodic alternation of the optimal voltage vectors on the rectifier side and the inverter side, the switching losses of the nine IGBT devices are balanced.

[0142] It should be noted that the various functional modules in the embodiments of the present invention can be integrated into one processing module, or each unit can exist as a separate physical entity, or two or more units can be integrated into one module. The integrated module can be implemented in hardware or as a software functional module.

[0143] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made to it in form and detail without departing from the scope defined by the claims of the present invention.

Claims

1. A method for controlling switching losses in a flexible interconnection system with nine switch converters, characterized in that: The flexible interconnection system of the nine-switch converter includes a nine-switch converter, a rectifier-side power grid, an inverter-side power grid, a control circuit, and filter resistors. R Filter inductor L ; The nine-switch converter includes a DC-side capacitor, three phase bridge arms, and an insulated gate bipolar transistor (IGBT) drive circuit. Each phase bridge arm is connected in series with three power switches, and the three phase bridge arms are connected in parallel and then connected across the positive and negative terminals of the DC-side capacitor. The rectifier side port of the nine-switch converter is filtered by a resistor. R and filter inductor L Connected to the rectifier-side grid, the inverter-side port is connected to a filter resistor. R and filter inductor L Connect to the inverter-side power grid to achieve bidirectional flexible power interconnection between two three-phase power grids; The input terminal of the control circuit is connected to the detection signals of the three-phase current on the rectifier side, the three-phase current on the inverter side, the DC side voltage, the grid voltage on the rectifier side, and the grid voltage on the inverter side of the nine-switch converter. The control circuit incorporates a rectifier-side closed-loop control circuit and an inverter-side closed-loop control circuit. The rectifier-side closed-loop control circuit, based on the detection signals of DC-side voltage, rectifier-side three-phase current, and rectifier-side grid voltage, controls the stability of the DC-side voltage and the power quality of the output three-phase AC power. The inverter-side closed-loop control circuit, based on the detection signals of inverter-side three-phase current and inverter-side grid voltage, tracks the given active and reactive power commands and adjusts the active and reactive power transmitted between the two grids. The switching loss control method includes: A mathematical model of a nine-switch converter is constructed using the switching function method. Based on the 27 effective conduction states of the nine-switch converter, 27 voltage space vectors are derived, and the sector distribution of each vector is clarified. Based on the mathematical model of a nine-switch converter, the circuit equations of the rectifier and inverter sides of the power grid are established. αβ The output voltage equation in the stationary coordinate system is obtained by transformation, and the system current prediction model is obtained by discretization using the forward Euler method. Output based on current prediction model Rectifier side and inverter side αβ A value function without weighting coefficients is constructed using the given and predicted current values ​​on the axis. Based on the sector distribution and value function of voltage space vector, an effective voltage vector is selected and a zero vector is inserted to design symmetrical switching sequences with fixed switching frequencies on the rectifier side and inverter side respectively. By using duty cycle modulation technology to determine the duration of each voltage vector within the control cycle, and combining the periodic alternation of the optimal voltage vectors on the rectifier and inverter sides, the switching losses of the nine IGBT devices are balanced.

2. The switching loss control method for a nine-switch converter flexible interconnection system according to claim 1, characterized in that: The expression for the current prediction model is: ， ， In the formula, , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβ Voltage on the shaft; , and , They are Current on the αβ axis of the rectifier and inverter sides of the time-nine switching converter; , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβ Predicted current on the shaft; , They are rectifier side power grid αβ shaft voltage, , They are respectively Inverter side power grid αβ shaft voltage, Indicates the sampling period. L For filtering inductors, R This is the filter resistor.

3. The switching loss control method for a nine-switch converter flexible interconnection system according to claim 1, characterized in that: The value function with unweighted coefficients is: ， In the formula, , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβ Current setpoint on the shaft; , and , They are Time-Nine Switching Converter Rectifier Side and Inverter Side αβ Predicted current value on the shaft.

4. The switching loss control method for a nine-switch converter flexible interconnection system according to claim 1, characterized in that: The symmetrical switching sequence with a fixed switching frequency is designed as follows: Rectifier-side switch sequence design: First, select two effective voltage vectors and one zero vector within the same sector on the rectifier side. Then, based on the selected sector on the rectifier side, select two effective voltage vectors and one zero vector from adjacent sectors on the inverter side to obtain a combined vector. Insert corresponding zero vectors on both sides and in the middle of the combined vector's action sequence to form a 9-segment symmetrical switch sequence on the rectifier side. Inverter-side switching sequence design: First, select two effective voltage vectors and one zero vector within the same sector on the inverter side. Then, based on the selected sector on the inverter side, select two effective voltage vectors and one zero vector from adjacent sectors on the rectifier side to obtain a combined vector. Insert corresponding zero vectors on both sides and in the middle of the combined vector's action sequence to form a 9-segment symmetrical switching sequence on the inverter side.

5. The switching loss control method for a nine-switch converter flexible interconnection system according to claim 1, characterized in that: The specific method for determining the voltage vector action time using duty cycle modulation technology is as follows: The partial derivative of the value function with no weight coefficient is obtained to solve for the conduction time of each voltage vector segment within a control cycle; half of the conduction time of each segment is allocated to the symmetrically arranged voltage vectors to obtain the action time of each voltage vector segment in the 9-segment symmetrical switching sequence. The specific method for periodically alternating the selection of the optimal voltage vectors on the rectifier side and the inverter side is as follows: In the first cycle, the optimal voltage vector on the rectifier side of the nine switches is selected, and in the second cycle, the optimal voltage vector on the inverter side of the nine switches is selected. By periodically alternating the selection of the equivalent voltage switching state, this cycle is repeated so that the nine power devices have the same number of switching times in a statistical sense.

6. The switching loss control method for a nine-switch converter flexible interconnection system according to claim 1, characterized in that: The drive signal of the intermediate power switch of each phase bridge arm of the nine-switch converter is generated by the logical XOR operation of the drive signals of the two power switches above and below the same bridge arm. Only two power switches are turned on at the same time for each phase bridge arm. Each phase bridge arm corresponds to 3 conduction states. The three phase bridge arms form a total of 27 conduction states, corresponding to 27 voltage space vectors.

7. The switching loss control method for a nine-switch converter flexible interconnection system according to claim 1, characterized in that: The rectifier-side closed-loop control circuit is a dual-loop control structure consisting of a voltage outer loop and a current inner loop. The input of the voltage outer loop is the difference between the DC-side voltage setpoint and the detected value, and the input of the current inner loop is the difference between the output of the voltage outer loop and the sampled value of the three-phase current on the rectifier side. The inverter-side closed-loop control circuit is also a dual-loop control structure consisting of a power outer loop and a current inner loop. The input of the power outer loop is the difference between the active and reactive power setpoints and the detected values, and the input of the current inner loop is the difference between the output of the power outer loop and the sampled value of the three-phase current on the inverter side.

8. The switching loss control method for a nine-switch converter flexible interconnection system according to claim 1, characterized in that: The control circuit is used to generate drive pulse signals for each IGBT device in the nine-switch converter. By executing a symmetrical switching sequence design with a fixed switching frequency, voltage vector duty cycle modulation, and periodic alternation of the optimal voltage vector, the switching loss and conduction loss of the nine IGBT devices are evenly distributed.

9. A switching loss control device for a nine-switch converter flexible interconnection system, characterized in that: The apparatus for performing the method according to any one of claims 1 to 8 comprises: The model building module is used to construct a mathematical model of a nine-switch converter using the switching function method. Based on the 27 effective conduction states of the nine-switch converter, 27 voltage space vectors are derived, and the sector distribution of each vector is clarified. The prediction model generation module is used to establish the circuit equations of the rectifier and inverter side power grids based on the mathematical model of the nine-switch converter. αβ The output voltage equation in the stationary coordinate system is obtained by transformation, and the system current prediction model is obtained by discretization using the forward Euler method. The value function construction module is used for the output of the current prediction model. Rectifier side and inverter side αβ A value function without weighting coefficients is constructed using the given and predicted current values ​​on the axis. The switching sequence design module is used to select the effective voltage vector and insert the zero vector based on the sector distribution and value function of the voltage space vector, and design symmetrical switching sequences with fixed switching frequencies on the rectifier side and inverter side respectively. The loss balancing control module is used to determine the duration of each voltage vector within the control cycle through duty cycle modulation technology. By combining the periodic alternation of the optimal voltage vectors on the rectifier side and the inverter side, the switching losses of the nine IGBT devices are balanced.

Images