Neutral-point potential control method based on diode-clamped three-level grid-connected inverter and related device

Abstract

The embodiment of the invention discloses a midpoint potential control method based on a diode clamping type three-level grid-connected inverter and a related device. The method comprises the following steps: generating an initial three-phase modulation wave through a current loop closed-loop control method according to a collected three-phase current value and a three-phase voltage value of a power grid side of the diode-clamped three-level grid-connected inverter; according to an initial modulation waveform generated by the initial three-phase modulation wave, a three-phase current value and an acquired voltage value of a voltage dividing capacitor at the direct current side of the diode clamping type three-level grid-connected inverter, calculating a zero-sequence component required by neutral-point potential balance; superposing a zero-sequence component required by neutral-point potential balance with the initial modulation waveform to obtain a final modulation wave; and generating a driving signal matched with each phase of bridge arm in the diode clamping type three-level grid-connected inverter according to the final modulation wave. By adopting the invention, the neutral-point voltage can be balanced, the distortion of the output waveform at the alternating current side is reduced, the voltage stress of the device is reduced, and the service life of the power device is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic systems, and in particular to a midpoint potential control method and related devices based on a diode clamped three-level grid-connected inverter. Background Art

[0002] With the continuous development of power electronics technology, people's demand for high-voltage and high-power power electronic equipment is becoming more and more urgent. Three-level inverters have been widely used because of their advantages such as lower output voltage harmonic content and lower device withstand voltage requirements compared to two-level inverters. However, it also brings many problems, especially the imbalance of midpoint potential. In the diode-clamped three-level grid-connected inverter, since the midpoint of the two voltage-dividing capacitors in series on the DC side is connected to the midpoint of the clamping diode on the bridge arm, the two voltage-dividing capacitors will be charged and discharged during the operation of the system, which will cause the midpoint voltage to fluctuate; at the same time, the difference in the parameters of the voltage-dividing capacitors will also cause the imbalance of the midpoint voltage. These will cause the distortion of the output waveform on the AC side, increase the voltage stress of the device, and affect the service life of the power device. Summary of the invention

[0003] The embodiment of the present application provides a midpoint potential control method and related device based on a diode-clamped three-level grid-connected inverter, which is beneficial to balancing the voltage between the midpoint of two voltage-dividing capacitors connected in series on the DC side and the midpoint of the clamping diode on the bridge arm, thereby reducing the distortion of the output waveform on the AC side, reducing the voltage stress of the device, and improving the service life of the power device.

[0004] A first aspect of an embodiment of the present application provides a method for controlling a midpoint potential of a diode-clamped three-level grid-connected inverter, the method comprising: Generate an initial three-phase modulation wave according to the collected three-phase current value and three-phase voltage value on the grid side of the diode-clamped three-level grid-connected inverter by a current loop closed-loop control method; Calculating the zero-sequence component required for midpoint potential balance according to the initial modulation waveform generated by the initial three-phase modulation wave, the three-phase current value and the collected voltage value of the voltage-dividing capacitor on the DC side of the diode-clamped three-level grid-connected inverter; Superimposing the zero-sequence component required for the midpoint potential balance with the initial modulation waveform to obtain a final modulation wave; A driving signal matching each phase bridge arm in the diode-clamped three-level grid-connected inverter is generated according to the final modulation wave.

[0005] Optionally, the generating of an initial three-phase modulation wave according to the collected three-phase current value and three-phase voltage value on the grid side of the diode-clamped three-level grid-connected inverter by the current loop closed-loop control method includes: Collecting three-phase current values ​​and three-phase voltage values ​​on the grid side of the diode-clamped three-level grid-connected inverter, wherein the three-phase current values ​​and three-phase voltage values ​​are static alternating current quantities; The three-phase current value and the three-phase voltage value are transformed to obtain a two-phase current value and a two-phase voltage value, wherein the two-phase current value and the two-phase voltage value are rotating direct current values; Determine the two-phase voltage control signal according to the two-phase current value and the two-phase voltage value and the given two-phase current reference value by a current loop closed-loop control method; The two-phase voltage control signal is inversely transformed to obtain an initial three-phase modulation wave.

[0006] Optionally, the transforming the three-phase current value and the three-phase voltage value to obtain the two-phase current value and the two-phase voltage value includes: Acquire phase information according to the three-phase voltage values ​​through a phase-locked loop to obtain a phase angle; Clarke transformation and Park transformation are performed on the three-phase current values ​​and the three-phase voltage values ​​according to the phase angle to obtain two-phase current values ​​and two-phase voltage values.

[0007] Optionally, the determining of the two-phase voltage control signal according to the two-phase current value and the two-phase voltage value and a given two-phase current reference value by the current loop closed-loop control method includes: Calculating the two-phase voltage reference value according to the difference between the two-phase current value and the given two-phase current reference value through the proportional-integral controller of the current loop; The two-phase voltage reference value is used as feedforward compensation, and the two-phase rotating coordinate axis control equation is decoupled to obtain a two-phase voltage control signal.

[0008] Optionally, before calculating the zero-sequence component required for midpoint potential balance according to the initial modulation waveform generated by the initial three-phase modulation wave, the three-phase current value and the collected voltage value of the voltage-dividing capacitor on the DC side of the diode-clamped three-level grid-connected inverter, the method further includes: Determine the maximum voltage value and the minimum voltage value of the three-phase voltage values; Determine the zero-sequence component of discontinuous pulse width modulation according to the distribution factor of the given total action time, the maximum voltage value and the minimum voltage value; The zero-sequence component of the discontinuous pulse width modulation is superimposed on the initial three-phase modulation wave to generate an initial modulation waveform.

[0009] Optionally, the calculating of the zero-sequence component required for midpoint potential balance according to the initial modulation waveform generated by the initial three-phase modulation wave, the three-phase current value and the collected voltage value of the voltage divider capacitor on the DC side of the diode-clamped three-level grid-connected inverter includes: Acquiring the capacitance value of the voltage-dividing capacitor on the DC side of the diode-clamped three-level grid-connected inverter and collecting the voltage value of the voltage-dividing capacitor; Determining the difference in initial charge amounts of the voltage-dividing capacitors according to the difference in voltage values ​​of the voltage-dividing capacitors and the capacitance value of the voltage-dividing capacitors; Determine the current flowing into the midpoint according to the three-phase current value and the switching function of the bridge arm switch tube in the diode clamped three-level grid-connected inverter; According to the current flowing to the midpoint, respectively calculate the average charge flowing into the midpoint before and after the zero-sequence component is injected; The zero-sequence component required for midpoint potential balance is calculated based on the difference between the average charge flowing into the midpoint before and after the zero-sequence component is injected, and the difference in the initial charge amount of the voltage-dividing capacitor.

[0010] Optionally, generating a driving signal matching each phase bridge arm in the diode-clamped three-level grid-connected inverter according to the final modulation wave comprises: Comparing the final modulated wave with two carrier waves of the same phase in the diode-clamped three-level grid-connected inverter; If the final modulated wave is greater than the upper carrier of the two carriers, a first drive signal is generated, and the first drive signal is used to drive one of the switch tubes of the upper bridge arm corresponding to the upper carrier to turn on; if the final modulated wave is less than or equal to the upper carrier of the two carriers, a second drive signal is generated, and the second drive signal is used to drive one of the switch tubes of the upper bridge arm corresponding to the upper carrier to turn off; If the final modulated wave is greater than the lower carrier of the two carriers, a third drive signal is generated, and the third drive signal is used to drive the other switch tube of the upper bridge arm corresponding to the upper carrier to turn on; if the final modulated wave is less than or equal to the lower carrier of the two carriers, a fourth drive signal is generated, and the fourth drive signal is used to drive the other switch tube of the upper bridge arm corresponding to the upper carrier to turn off; The switch tube of the lower bridge arm corresponding to the lower carrier and the switch tube of the upper bridge arm corresponding to the upper carrier are complementarily turned on.

[0011] A second aspect of an embodiment of the present application provides a midpoint potential control device based on a diode clamped three-level grid-connected inverter, the device comprising: An initial modulation wave generating unit, used for generating an initial three-phase modulation wave according to the collected three-phase current value and three-phase voltage value of the grid side of the diode clamped three-level grid-connected inverter through a current loop closed-loop control method; A zero-sequence component calculation unit, used to calculate the zero-sequence component required for midpoint potential balance according to the initial modulation waveform generated by the initial three-phase modulation wave, the three-phase current value and the collected voltage value of the voltage-dividing capacitor on the DC side of the diode-clamped three-level grid-connected inverter; A final modulation wave generating unit, used for superimposing the zero-sequence component required for the midpoint potential balance with the initial modulation waveform to obtain a final modulation wave; A drive signal generating unit is used to generate a drive signal matching each phase bridge arm in the diode clamped three-level grid-connected inverter according to the final modulation wave.

[0012] A third aspect of the embodiments of the present application provides an electronic device, including: a processor and a memory; The processor is connected to the memory, wherein the memory is used to store a computer program, and the processor is used to call the computer program to execute the method in the first aspect of the embodiment of the present application.

[0013] A fourth aspect of an embodiment of the present application provides a computer-readable storage medium, which stores a computer program. The computer program includes program instructions. When the program instructions are executed by a processor, the method in the first aspect of the embodiment of the present application is executed.

[0014] It can be seen that in the embodiment of the present application, first, an initial three-phase modulation wave is generated according to the three-phase current value and three-phase voltage value collected on the grid side of the diode clamped three-level grid-connected inverter through a current loop closed-loop control method; then, the zero-sequence component required for the midpoint potential balance is calculated according to the initial modulation waveform generated by the initial three-phase modulation wave, the three-phase current value and the voltage value of the voltage divider capacitor on the DC side of the diode clamped three-level grid-connected inverter collected; finally, the zero-sequence component required for the midpoint potential balance is superimposed on the initial modulation waveform to obtain the final modulation wave; and a driving signal matching each phase bridge arm in the diode clamped three-level grid-connected inverter is generated according to the final modulation wave. The zero-sequence component required for the midpoint potential balance can balance the midpoint voltage imbalance caused by the charging and discharging of the two voltage divider capacitors, as well as the midpoint voltage imbalance caused by the difference in the voltage divider capacitor parameters, thereby reducing the distortion of the output waveform on the AC side, reducing the voltage stress of the device, and improving the service life of the power device. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the embodiments of the present application 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 only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0016] Figure 1 A topological structure diagram of a switching model of a diode-clamped three-level inverter provided in one embodiment of the present application is shown; Figure 2 A schematic diagram of the structure of a new energy grid-connected inverter system provided by an embodiment of the present application is shown; Figure 3 A schematic flow chart of a method for controlling a midpoint potential of a diode-clamped three-level grid-connected inverter according to an embodiment of the present application is shown; Figure 4 A control block diagram after decoupling of dq axes of vector control provided by an embodiment of the present application is shown; Figure 5 The diagram shows a zero-sequence component distribution diagram corresponding to the voltage vector of the DPWM1 strategy provided by one embodiment of the present application when it is located in different sectors; Figure 6 A diagram showing the magnitude relationship of a three-phase sine wave in each sector provided by an embodiment of the present application is shown; Figure 7 A topological structure diagram of a three-level DC side circuit provided by an embodiment of the present application is shown; Figure 8 A schematic diagram of co-directional carrier modulation provided by an embodiment of the present application is shown; FIG. 9 ( a ) shows a schematic diagram of upper and lower capacitor voltage waveforms using a traditional DPWM strategy provided by an embodiment of the present application; FIG9 ( b ) shows a schematic diagram of voltage waveforms of upper and lower capacitors using the modulation strategy provided by the present application according to an embodiment of the present application; FIG. 10 ( a ) shows a schematic diagram of upper and lower capacitor voltage waveforms using a traditional DPWM strategy provided by another embodiment of the present application; FIG. 10 ( b ) shows a schematic diagram of voltage waveforms of upper and lower capacitors using the modulation strategy provided by the present application, provided in another embodiment of the present application; FIG. 11 ( a ) shows a schematic diagram of Fourier analysis of an AC current waveform using a traditional DPWM strategy provided by another embodiment of the present application; FIG. 11 ( b ) shows a Fourier analysis schematic diagram of an AC current waveform using the modulation strategy provided by the present application, provided by another embodiment of the present application; Fig.12 A schematic diagram showing a comparison of the total loss of the system under the SPWM, SVPWM, DPWM and modulation strategies provided by the present application according to an embodiment of the present application is shown; Fig.13 A schematic structural diagram of a midpoint potential control device based on a diode clamped three-level grid-connected inverter provided in one embodiment of the present application is shown; Fig.14 A schematic diagram of the structure of a computer device provided in one embodiment of the present application is shown. DETAILED DESCRIPTION The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0017] Please refer to Figure 1 , which shows a topological structure diagram of a switch model of a diode clamped three-level inverter provided in an embodiment of the present application. As shown in the figure, the AC weak grid is connected to the diode clamped three-level inverter through a filter. Compared with the two-level inverter, the diode clamped three-level inverter has two additional voltage-dividing capacitors on the DC side. and , each phase has two switches in the upper and lower bridge arms, and the midpoints of the two switches are connected to the midpoint of the capacitor through a clamping diode. The filter includes the filter inductor L on the grid side and the line equivalent resistor R. , , is the three-phase current flowing through the grid side, , , is the three-phase voltage flowing through the grid side. Ignoring the voltage fluctuation on the DC side of the diode clamped three-level inverter, an ideal DC source is used. Replace diode clamped three-level inverter power supply.

[0018] Each phase of the three-level inverter has two identical switches in the upper and lower bridge arms. The three-phase structure is the same. Taking phase A as an example, three different working states are formed according to the different on-off modes, which are defined as P state, O state, and N state. Among them, P state means that the switch tube and Conductivity, and Off; O state means the switch tube or Conductivity, and Off; N state means the switch tube and Conductivity, and The above also applies to phase B and phase C. From the above analysis, we can see that the four switch tubes output three levels according to their switch state combination: ,0, .

[0019] Please refer to Figure 2 , which shows a schematic diagram of the structure of a new energy grid-connected inverter system provided by an embodiment of the present application. The structure of the new energy grid-connected inverter system is divided into two parts: a main circuit and a controller. The topological structure diagram of the main circuit is shown in FIG. Figure 2 The main circuit is described in the upper part. Figure 1 The topological structure of the controller is shown in the figure below. Figure 2 The point of common coupling (PCC) between the main circuit and the controller is located at And between the AC weak power grid. The specific control process is as follows: First, the three-phase voltage is collected by the sensor , , and three-phase current , , , then , , Input to the phase-locked loop (PLL), PLL can obtain phase information in real time and output phase angle Then according to , three-phase current , , Can be transformed into two-phase current by Clark and Park , , three-phase voltage , , Can be transformed into two-phase voltage by Clark and Park , Among them, the three phases are the three axes abc of the stationary coordinate system, and the two phases are the two coordinate axes dq of the rotating coordinate system.

[0020] Then, and a given reference current value After comparison, the proportional-integral (PI) controller is used to introduce the two-phase voltage into the PI control link. As a feedforward compensation, the same operation is performed on the q-axis component. Finally, the coupling between the d-axis and the q-axis is decoupled to generate a two-phase voltage control signal and . Two-phase voltage control signal and It can be converted into three-phase voltage by Clark and Park's inverse transformation , , , three-phase voltage , , Then the initial modulation waveform is generated by the Discontinuous Pulse Width Modulation (DPWM) module , and .

[0021] Secondly, collect two voltage-dividing capacitors and Voltage value and ,Will , , , , , , and It is sent to the zero-sequence component calculation module to calculate the zero-sequence component required for the midpoint potential balance strategy.

[0022] Finally, the zero-sequence component is superimposed on the initial modulation waveform to obtain the final modulation waveform, which is sent to the PWM module to generate a drive signal matching each phase bridge arm, which controls the opening and closing of the power device.

[0023] Please refer to Figure 3 , which shows a flow chart of a method for controlling the midpoint potential of a diode-clamped three-level grid-connected inverter according to an embodiment of the present application. The method can be applied to Figure 1 The switching model of the diode clamped three-level inverter shown in Figure 2 The new energy grid-connected inverter system shown can include the following steps: Step 301: Generate an initial three-phase modulation wave according to the collected three-phase current values ​​and three-phase voltage values ​​on the grid side of the diode-clamped three-level grid-connected inverter by a current loop closed-loop control method.

[0024] Among them, the current loop closed-loop control method is a technology used to ensure that the system output current can accurately follow the given reference current. It enables the system to maintain high accuracy and high stability by detecting and adjusting the current in real time. For example, the three-phase current value and the three-phase voltage value are collected by sensors.

[0025] Among them, the modulation wave refers to the reference waveform used to generate the pulse width modulation (PWM) signal in the power electronic converter. PWM is a technology widely used in the field of power electronics and motor control, which controls the output voltage or current by changing the width of the pulse. PWM technology can be divided into two main types: continuous pulse width modulation (CPWM) and discontinuous pulse width modulation (DPWM). In CPWM, the pulse width of the PWM signal is adjusted according to the comparison result of the reference signal and the carrier signal in each switching cycle. In each switching cycle, the switching element is frequently turned on and off to track the changes of the reference signal. In DPWM, the pulse width of the PWM signal remains unchanged during certain time periods, and the switching element remains in a fixed state (either fully open or fully closed). By reducing the number of switching times, switching losses and electromagnetic interference are reduced.

[0026] Specifically, the current loop closed-loop control method generates an initial three-phase modulation wave according to the collected three-phase current value and three-phase voltage value on the grid side of the diode-clamped three-level grid-connected inverter, including: Collecting three-phase current values ​​and three-phase voltage values ​​on the grid side of the diode-clamped three-level grid-connected inverter, wherein the three-phase current values ​​and three-phase voltage values ​​are static alternating current quantities; The three-phase current value and the three-phase voltage value are transformed to obtain a two-phase current value and a two-phase voltage value, wherein the two-phase current value and the two-phase voltage value are rotating direct current values; Determine the two-phase voltage control signal according to the two-phase current value and the two-phase voltage value and the given two-phase current reference value by a current loop closed-loop control method; The two-phase voltage control signal is inversely transformed to obtain an initial three-phase modulation wave.

[0027] Specifically, the three-phase current value and the three-phase voltage value are transformed to obtain the two-phase current value and the two-phase voltage value, including: Acquire the phase information of the q-axis in the two-phase voltage value through a phase-locked loop to obtain a phase angle; Clarke transformation and Park transformation are performed on the three-phase current values ​​according to the phase angle to obtain two-phase current values.

[0028] For example, the three-phase voltage value is collected by the sensor , , And three-phase current value , , , respectively, by Clark transformation, and then by Park transformation into two-phase voltage , and two-phase current , The Clark transformation is as follows:

[0029] Clark transformation is used to transform the three-phase stationary coordinate system into a two-phase vertical stationary coordinate system, where Represents the corresponding physical quantity current or voltage, are the components projected onto the α and β axes respectively. The Park transformation is as follows:

[0030] in are projected onto The weight of the axis, yes Axis and The angle of the axis, in the case of grid voltage orientation, It is also the output angle of PLL, which is calculated based on the three-phase voltage value. , , Get.

[0031] Specifically, the current loop closed-loop control method determines the two-phase voltage control signal according to the two-phase current value and the two-phase voltage value and a given two-phase current reference value, including: Calculating the two-phase voltage reference value according to the difference between the two-phase current value and the given two-phase current reference value through the proportional-integral controller of the current loop; The two-phase voltage reference value is used as feedforward compensation, and the two-phase rotating coordinate axis control equation is decoupled to obtain a two-phase voltage control signal.

[0032] Among them, the proportional-integral (PI) controller is a commonly used control algorithm, which is widely used in industrial automation and control systems. The PI controller combines the advantages of proportional control and integral control, and can achieve a good balance between steady-state error and dynamic response.

[0033] For example, please refer to Figure 4 , which shows a control block diagram of the vector control dq axis decoupling provided by an embodiment of the present application. The given two-phase current reference value is and , then the differences between the two-phase current values ​​and the given two-phase current reference values ​​are and According to the mathematical formula of the proportional-integral controller of the current loop, the two-phase voltage reference value can be calculated and :

[0034]

[0035] in, is the current loop proportional adjustment coefficient, is the current loop integral adjustment coefficient, is the Laplace operator.

[0036] Then the two-phase voltage reference value and As feedforward compensation, the two-phase rotating coordinate axis control equation is decoupled to obtain the two-phase voltage control signal and :

[0037] For example, the two-phase voltage control signal and By performing the inverse transformation of Park and Clark transformation, the initial three-phase modulated wave can be obtained.

[0038] Step 302: Calculate the zero-sequence component required for midpoint potential balance according to the initial modulation waveform generated by the initial three-phase modulation wave, the three-phase current value and the collected voltage value of the voltage divider capacitor on the DC side of the diode-clamped three-level grid-connected inverter.

[0039] Further, before calculating the zero-sequence component required for midpoint potential balance according to the initial modulation waveform generated by the initial three-phase modulation wave, the three-phase current value and the collected voltage value of the voltage-dividing capacitor on the DC side of the diode-clamped three-level grid-connected inverter, the method further includes: Determine the maximum voltage value and the minimum voltage value of the three-phase voltage values; Determine the zero-sequence component of discontinuous pulse width modulation according to the distribution factor of the given total action time, the maximum voltage value and the minimum voltage value; The zero-sequence component of the discontinuous pulse width modulation is superimposed on the initial three-phase modulation wave to generate an initial modulation waveform.

[0040] For example, taking DPWM1 as an example, Figure 5 The figure shows the zero-sequence component distribution diagram corresponding to the voltage vector of the DPWM1 strategy provided by one embodiment of the present application when it is located in different sectors. Figure 6 The following figure shows the relationship between the magnitudes of the three-phase sine waves in various sectors provided by an embodiment of the present application. 、 、 The maximum and minimum voltage values ​​are and , based on which we can use and The distribution of the zero-sequence component of the DPWM1 strategy is shown in Table 1 below: Table 1 Distribution of zero-sequence components of DPWM1 strategy

[0041] The same is true for other DPWM strategies, so there are the following zero-sequence components :

[0042] in, is the distribution factor of the total action time of positive and negative small vectors, In a fundamental wave cycle, according to different intervals, Or 1, the zero-sequence voltage that needs to be injected into the corresponding DPWM strategy can be obtained. The zero-sequence voltage is superimposed on the initial three-phase modulation wave to obtain the initial modulation waveform of the DPWM strategy. , and .

[0043] Specifically, the calculating of the zero-sequence component required for midpoint potential balance according to the initial modulation waveform generated by the initial three-phase modulation wave, the three-phase current value and the collected voltage value of the voltage divider capacitor on the DC side of the diode-clamped three-level grid-connected inverter includes: Acquiring the capacitance value of the voltage-dividing capacitor on the DC side of the diode-clamped three-level grid-connected inverter and collecting the voltage value of the voltage-dividing capacitor; Determining the difference in initial charge amounts of the voltage-dividing capacitors according to the difference in voltage values ​​of the voltage-dividing capacitors and the capacitance value of the voltage-dividing capacitors; Determine the current flowing into the midpoint according to the three-phase current value and the switching function of the bridge arm switch tube in the diode clamped three-level grid-connected inverter; According to the current flowing to the midpoint, respectively calculate the average charge flowing into the midpoint before and after the zero-sequence component is injected; The zero-sequence component required for midpoint potential balance is calculated based on the difference between the average charge flowing into the midpoint before and after the zero-sequence component is injected, and the difference in the initial charge amount of the voltage-dividing capacitor.

[0044] For example, see Figure 7 , which shows a topological structure diagram of a three-level DC side circuit provided by an embodiment of the present application. The capacitance values ​​of the voltage divider capacitors are all C, and the initial voltage values ​​of the voltage divider capacitors are respectively 、 , then according to the charge calculation formula, the difference in the initial charge between the two can be obtained: :

[0045] If the midpoint current flows out of the midpoint as the positive direction, the current at the midpoint is analyzed as follows:

[0046] in, is the current flowing out of the midpoint, is the current flowing through the upper capacitor, is the current flowing through the lower capacitor, and is the voltage value of the voltage divider capacitor at the current moment.

[0047] According to the above formula, the incremental difference of the upper and lower capacitor charges caused by the midpoint current in time t can be deduced as:

[0048] in, and It reflects the change of the upper and lower capacitor voltages in the time period of 0 to t.

[0049] It can be seen that the midpoint current This will cause the charge of the upper and lower capacitors to fluctuate, because a midpoint current will be generated when the output is at an 0 level. , which means that when the output state of each phase of the three-level inverter is 0 level, it will affect the charge of the upper and lower capacitors, so the midpoint current The expression of can also be obtained through the three-phase load current:

[0050] in, (x=a,b,c) is the switching function of the bridge arm switch in the diode clamped three-level grid-connected inverter, and its expression is:

[0051] See also Figure 8 , which shows a schematic diagram of co-directional carrier modulation provided by an embodiment of the present application. Figure 8 It can be seen that in one switching cycle, there are two changes in the switching function. No matter which one, according to the principle of similar triangles, the following relationship can be obtained:

[0052] in, Indicates the duration of the O state in a switching cycle, is a symbolic function.

[0053] Exemplarily, according to the above formula, the average charge flowing into the midpoint before injecting the zero-sequence component is:

[0054] The average charge flowing into the midpoint after the zero-sequence component is injected is:

[0055] in, is the switching cycle, , and is the initial modulation waveform, and is , and The final modulated waveform after injecting zero-sequence components is , is the average current in a switching cycle.

[0056] Exemplarily, the difference between the average charge flowing into the midpoint before and after the zero-sequence component is injected is:

[0057] in, is the injected zero sequence component, which brings The average current required for the charge change , is a symbolic function.

[0058] Assuming that after the zero-sequence component is injected, the average charge of the upper and lower capacitors in each switching cycle is always equal, that is, the midpoint charge increment brought by the zero-sequence component Make up for and :

[0059] The difference in the average charge flowing into the midpoint before and after the zero-sequence component is injected can be calculated , and the difference in the initial charge of the voltage divider capacitor Calculate the zero-sequence component required for midpoint potential balance :

[0060] in, , is the switching frequency of the grid-connected inverter system.

[0061] Step 303: superimpose the zero-sequence component required for the midpoint potential balance with the initial modulation waveform to obtain a final modulation wave.

[0062] Step 304: Generate a driving signal matching each phase bridge arm in the diode-clamped three-level grid-connected inverter according to the final modulation wave.

[0063] Specifically, the generating, according to the final modulation wave, a driving signal matching each phase bridge arm in the diode clamped three-level grid-connected inverter comprises: Comparing the final modulated wave with two carrier waves of the same phase in the diode-clamped three-level grid-connected inverter; If the final modulated wave is greater than the upper carrier of the two carriers, a first drive signal is generated, and the first drive signal is used to drive one of the switch tubes of the upper bridge arm corresponding to the upper carrier to turn on; if the final modulated wave is less than or equal to the upper carrier of the two carriers, a second drive signal is generated, and the second drive signal is used to drive one of the switch tubes of the upper bridge arm corresponding to the upper carrier to turn off; If the final modulated wave is greater than the lower carrier of the two carriers, a third drive signal is generated, and the third drive signal is used to drive the other switch tube of the upper bridge arm corresponding to the upper carrier to turn on; if the final modulated wave is less than or equal to the lower carrier of the two carriers, a fourth drive signal is generated, and the fourth drive signal is used to drive the other switch tube of the upper bridge arm corresponding to the upper carrier to turn off; The switch tube of the lower bridge arm corresponding to the lower carrier and the switch tube of the upper bridge arm corresponding to the upper carrier are complementarily turned on.

[0064] Exemplarily, one of the switch tubes of the upper bridge arm corresponding to the added carrier is , the other switch is , one of the switch tubes of the lower bridge arm corresponding to the lower carrier is , the other switch is , Indicates three phases a, b, and c.

[0065] In order to verify the feasibility of this application, a neutral point clamped three-level inverter grid-connected system was built in PLECS. The parameters are shown in Table 2. The specific verification process is as follows: Table 2 Simulation system parameters

[0066] When there is no initial potential difference between the DC side capacitors, the traditional DPWM strategy and the modulation strategy provided by the present invention are used respectively, and the corresponding upper and lower capacitor voltage waveforms are shown in Figures 9(a) and 9(b). It can be seen that the solution provided by the present application can significantly reduce the fluctuation of the midpoint potential.

[0067] When there is an initial potential difference between the DC side capacitors, the conventional DPWM strategy and the modulation strategy provided by the present invention are used respectively, and the corresponding upper and lower capacitor voltage waveforms are shown in FIG10(a) and FIG10(b). It can be seen that in FIG10(a), although the conventional DPWM strategy also has a certain midpoint potential balancing capability, the midpoint potential after balancing fluctuates greatly, and the balancing speed is also slow. The solution provided by the present application can quickly achieve midpoint potential balance and has a strong midpoint voltage regulation capability.

[0068] Figure 11(a) and 11(b) The Fourier analysis of the AC current waveform under the traditional DPWM strategy and the modulation strategy provided by the present invention is shown. It can be seen from the figure that the harmonic content of the AC current waveform under the modulation strategy provided by the present invention is slightly reduced compared with the traditional DPWM strategy.

[0069] Fig.12 The total loss comparison of the system under SPWM, SVPWM, DPWM and the modulation strategy provided by the present invention is shown. It can be seen from the figure that although the switching loss of the modulation strategy of the present invention is higher than that of the traditional DPWM strategy, compared with the SPWM and SVPWM strategies, the switching loss is reduced, and the mid-point potential balance under low switching loss is achieved.

[0070] In summary, it can be seen that in the embodiment of the present application, first, an initial three-phase modulation wave is generated according to the three-phase current value and three-phase voltage value of the grid side of the diode clamped three-level grid-connected inverter collected by the current loop closed-loop control method; then, the zero-sequence component required for the midpoint potential balance is calculated according to the initial modulation waveform, the three-phase current value and the voltage value of the voltage divider capacitor on the DC side of the diode clamped three-level grid-connected inverter collected by the initial three-phase modulation wave; finally, the zero-sequence component required for the midpoint potential balance is superimposed on the initial modulation waveform to obtain the final modulation wave; and a driving signal matching each phase bridge arm in the diode clamped three-level grid-connected inverter is generated according to the final modulation wave. The zero-sequence component required for the midpoint potential balance can balance the midpoint voltage imbalance caused by the charging and discharging of the two voltage divider capacitors, and the midpoint voltage imbalance caused by the difference in the voltage divider capacitor parameters, thereby reducing the distortion of the output waveform on the AC side, reducing the voltage stress of the device, and improving the service life of the power device.

[0071] Fig.13The schematic diagram of the structure of a midpoint potential control device based on a diode clamped three-level grid-connected inverter provided in one embodiment of the present application is shown, which is applied to a new energy grid-connected AC system. The device includes: An initial modulation wave generating unit 1301 is used to generate an initial three-phase modulation wave according to the collected three-phase current value and three-phase voltage value of the grid side of the diode clamped three-level grid-connected inverter through a current loop closed-loop control method; A zero-sequence component calculation unit 1302 is used to calculate the zero-sequence component required for midpoint potential balance according to the initial modulation waveform generated by the initial three-phase modulation wave, the three-phase current value and the collected voltage value of the voltage-dividing capacitor on the DC side of the diode-clamped three-level grid-connected inverter; A final modulation wave generating unit 1303 is used to superimpose the zero-sequence component required for the midpoint potential balance with the initial modulation waveform to obtain a final modulation wave; The driving signal generating unit 1304 is used to generate a driving signal matching each phase bridge arm in the diode clamped three-level grid-connected inverter according to the final modulation wave.

[0072] Fig.14 A structural schematic diagram of a computer device provided by an embodiment of the present application is shown, including a memory and a processor, the memory stores a computer program, and when the processor executes the computer program, the functions of the computer system of the midpoint potential control method of the diode-clamped three-level grid-connected inverter in any of the above-mentioned embodiments are implemented.

[0073] The embodiment of the present application also provides a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a computer, the computer executes the functions of the computer system of the midpoint potential control method of a diode-clamped three-level grid-connected inverter in any of the above embodiments.

[0074] The embodiments of the present application also provide a computer program product including instructions, which, when executed by a computer, enables the computer to execute the functions of a computer system of the midpoint potential control method of a diode-clamped three-level grid-connected inverter in any of the above embodiments.

[0075] It should be understood that the specific examples in this application are only intended to help those skilled in the art to better understand the embodiments of the present application, rather than to limit the scope of the present invention.

[0076] It can be understood that in the various implementations of the present application, the size of the serial number of each process does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the implementation methods of the present application.

[0077] It can be understood that the various embodiments described in this application can be implemented individually or in combination, and the embodiments of this application are not limited to this.

[0078] Unless otherwise stated, all technical and scientific terms used in the embodiments of the present application have the same meaning as those generally understood by those skilled in the art of the technical field of the present application. The terms used in the present application are only for the purpose of describing specific embodiments and are not intended to limit the scope of the present application. The term "and / or" used in the present application includes any and all combinations of one or more related listed items. The singular forms of "a kind of", "above" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0079] It can be understood that the processor of the embodiment of the present application can be an integrated circuit chip with signal processing capabilities. In the implementation process, each step of the above method implementation can be completed by the hardware integrated logic circuit or software instructions in the processor. The above processor can be a general processor, a digital signal processor (DigitalSignal Processor, DSP), an application-specific integrated circuit (Application Specific Integrated Circuit, ASIC), a field programmable gate array (Field Programmable Gate Array, FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components. The methods, steps and logic block diagrams disclosed in the embodiment of the present application can be implemented or executed. The general processor can be a microprocessor or the processor can also be any conventional processor, etc. The steps of the method disclosed in the embodiment of the present application can be directly embodied as a hardware decoding processor to perform, or the hardware and software modules in the decoding processor are combined and executed. The software module can be located in a mature storage medium in the field such as a random access memory, a flash memory, a read-only memory, a programmable read-only memory or an electrically erasable programmable memory, a register, etc. The storage medium is located in the memory, and the processor reads the information in the memory and completes the steps of the above method in combination with its hardware.

[0080] It is understood that the memory in the embodiments of the present application may be a volatile memory or a non-volatile memory, or may include both volatile and non-volatile memory. Among them, the non-volatile memory may be a read-only memory (ROM), a programmable read-only memory (programmable ROM, PROM), an erasable programmable read-only memory (erasable PROM, EPROM), an electrically erasable programmable read-only memory (EEPROM) or a flash memory. The volatile memory may be a random access memory (RAM). It should be noted that the memory of the systems and methods described herein is intended to include, but is not limited to, these and any other suitable types of memory.

[0081] Those of ordinary skill in the art will appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0082] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working processes of the systems, devices and units described above can refer to the corresponding processes in the aforementioned method implementation methods and will not be repeated here.

[0083] In the several embodiments provided in the present application, it should be understood that the disclosed systems, devices and methods can be implemented in other ways. For example, the device implementation described above is only schematic. For example, the division of units is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

[0084] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the present embodiment.

[0085] In addition, each functional unit in each embodiment of the present application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit.

[0086] If the function is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present application, or the part that contributes to the prior art or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium, including a number of instructions for a computer device (which can be a personal computer, server, or network device, etc.) to perform all or part of the steps of each implementation method of the present application. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM), random access memory (RAM), disk or optical disk, etc., various media that can store program codes.

[0087] The above are only specific embodiments of the present application, but the protection scope of the present invention is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A midpoint potential control method based on a diode clamped three-level grid-connected inverter, characterized in that: The method comprises: Generate an initial three-phase modulation wave according to the collected three-phase current value and three-phase voltage value on the grid side of the diode-clamped three-level grid-connected inverter by a current loop closed-loop control method; Calculating the zero-sequence component required for midpoint potential balance according to the initial modulation waveform generated by the initial three-phase modulation wave, the three-phase current value and the collected voltage value of the voltage-dividing capacitor on the DC side of the diode-clamped three-level grid-connected inverter; Superimposing the zero-sequence component required for the midpoint potential balance with the initial modulation waveform to obtain a final modulation wave; A driving signal matching each phase bridge arm in the diode-clamped three-level grid-connected inverter is generated according to the final modulation wave.

2. The method according to claim 1, characterized in that The method of generating an initial three-phase modulation wave according to the three-phase current value and the three-phase voltage value collected on the grid side of the diode clamped three-level grid-connected inverter by the current loop closed-loop control method includes: Collecting three-phase current values ​​and three-phase voltage values ​​on the grid side of the diode-clamped three-level grid-connected inverter, wherein the three-phase current values ​​and three-phase voltage values ​​are static alternating current quantities; The three-phase current value and the three-phase voltage value are transformed to obtain a two-phase current value and a two-phase voltage value, wherein the two-phase current value and the two-phase voltage value are rotating direct current values; Determine the two-phase voltage control signal according to the two-phase current value and the two-phase voltage value and the given two-phase current reference value by a current loop closed-loop control method; The two-phase voltage control signal is inversely transformed to obtain an initial three-phase modulation wave.

3. The method according to claim 2, characterized in that The three-phase current value and the three-phase voltage value are transformed to obtain the two-phase current value and the two-phase voltage value, including: Acquire phase information according to the three-phase voltage value through a phase-locked loop to obtain a phase angle; Clarke transformation and Park transformation are performed on the three-phase current values ​​and the three-phase voltage values ​​according to the phase angle to obtain two-phase current values ​​and two-phase voltage values.

4. The method according to claim 2, characterized in that: The method of determining the two-phase voltage control signal according to the two-phase current value and the two-phase voltage value and a given two-phase current reference value through the current loop closed-loop control method includes: Calculating the two-phase voltage reference value according to the difference between the two-phase current value and the given two-phase current reference value through the proportional-integral controller of the current loop; The two-phase voltage reference value is used as feedforward compensation, and the two-phase rotating coordinate axis control equation is decoupled to obtain a two-phase voltage control signal.

5. The method according to any one of claims 1 to 4, characterized in that: Before calculating the zero-sequence component required for midpoint potential balance according to the initial modulation waveform generated by the initial three-phase modulation wave, the three-phase current value and the collected voltage value of the voltage-dividing capacitor on the DC side of the diode-clamped three-level grid-connected inverter, the method further includes: Determine the maximum voltage value and the minimum voltage value of the three-phase voltage values; Determine the zero-sequence component of discontinuous pulse width modulation according to the distribution factor of the given total action time, the maximum voltage value and the minimum voltage value; The zero-sequence component of the discontinuous pulse width modulation is superimposed on the initial three-phase modulation wave to generate an initial modulation waveform.

6. The method according to any one of claims 1 to 4, characterized in that: The calculating of the zero-sequence component required for midpoint potential balance according to the initial modulation waveform generated by the initial three-phase modulation wave, the three-phase current value and the collected voltage value of the voltage-dividing capacitor on the DC side of the diode-clamped three-level grid-connected inverter comprises: Acquiring the capacitance value of the voltage-dividing capacitor on the DC side of the diode-clamped three-level grid-connected inverter and collecting the voltage value of the voltage-dividing capacitor; Determining the difference in initial charge amounts of the voltage-dividing capacitors according to the difference in voltage values ​​of the voltage-dividing capacitors and the capacitance value of the voltage-dividing capacitors; Determine the current flowing into the midpoint according to the three-phase current value and the switching function of the bridge arm switch tube in the diode clamped three-level grid-connected inverter; According to the current flowing to the midpoint, respectively calculate the average charge flowing into the midpoint before and after the zero-sequence component is injected; The zero-sequence component required for midpoint potential balance is calculated based on the difference between the average charge flowing into the midpoint before and after the zero-sequence component is injected, and the difference in the initial charge amount of the voltage-dividing capacitor.

7. The method according to any one of claims 1 to 4, characterized in that: The step of generating a driving signal matching each phase bridge arm in the diode clamped three-level grid-connected inverter according to the final modulation wave comprises: Comparing the final modulated wave with two carrier waves of the same phase in the diode-clamped three-level grid-connected inverter; If the final modulated wave is greater than the upper carrier of the two carriers, a first drive signal is generated, and the first drive signal is used to drive one of the switch tubes of the upper bridge arm corresponding to the upper carrier to turn on; if the final modulated wave is less than or equal to the upper carrier of the two carriers, a second drive signal is generated, and the second drive signal is used to drive one of the switch tubes of the upper bridge arm corresponding to the upper carrier to turn off; If the final modulated wave is greater than the lower carrier of the two carriers, a third drive signal is generated, and the third drive signal is used to drive the other switch tube of the upper bridge arm corresponding to the upper carrier to turn on; if the final modulated wave is less than or equal to the lower carrier of the two carriers, a fourth drive signal is generated, and the fourth drive signal is used to drive the other switch tube of the upper bridge arm corresponding to the upper carrier to turn off; The switch tube of the lower bridge arm corresponding to the lower carrier and the switch tube of the upper bridge arm corresponding to the upper carrier are complementarily turned on.

8. A midpoint potential control device based on a diode clamped three-level grid-connected inverter, characterized in that: The device comprises: An initial modulation wave generating unit, used for generating an initial three-phase modulation wave according to the collected three-phase current value and three-phase voltage value of the grid side of the diode clamped three-level grid-connected inverter through a current loop closed-loop control method; A zero-sequence component calculation unit, used to calculate the zero-sequence component required for midpoint potential balance according to the initial modulation waveform generated by the initial three-phase modulation wave, the three-phase current value and the collected voltage value of the voltage-dividing capacitor on the DC side of the diode-clamped three-level grid-connected inverter; A final modulation wave generating unit, used for superimposing the zero-sequence component required for the midpoint potential balance with the initial modulation waveform to obtain a final modulation wave; A drive signal generating unit is used to generate a drive signal matching each phase bridge arm in the diode clamped three-level grid-connected inverter according to the final modulation wave.

9. An electronic device, characterized in that: include: Processor and memory; The processor is connected to a memory, wherein the memory is used to store a computer program, and the processor is used to call the computer program to execute the method according to any one of claims 1 to 7.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, wherein the computer program includes program instructions, and when the program instructions are executed by a processor, the method according to any one of claims 1 to 7 is executed.

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